sensitivity of ground water to contamination in lawrence ... · graphic information system. thirty...

Sensitivity of Ground Waterto Contamination in LawrenceCounty, South Dakota

Water-Resources Investigations Report 00-4103

Prepared in cooperation with Lawrence Countyand the City of Spearfish

U.S. Department of the InteriorU.S. Geological Survey

Inside cover: Photographs of downstream sequence of the Madison Limestone streamflow-loss zone inSpearfish Canyon. Photographs by I.E. Arlton

Front cover: Photograph of Madison Limestone streamflow-loss zone in Spearfish Canyon.Photographs by I.E. Arlton

1 2

3 4

U.S. Department of the Interior U.S. Geological Survey

Sensitivity of Ground Water to Contamination in Lawrence County, South Dakota

By Larry D. Putnam

Water-Resources Investigations Report 00-4103

Prepared in cooperation with Lawrence County and the City of Spearfish

U.S. Department of the Interior

Bruce Babbitt, Secretary

U.S. Geological Survey

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.

Rapid City, South Dakota: 2000

For additional information write to:

District Chief U.S. Geological Survey 1608 Mt. View Road Rapid City, SD 57702

Copies of this report can be purchased from:

U.S. Geological Survey Branch of Information Services Box 25286 Denver, CO 80225-0286

CONTENTS

Abstract.................................................................................................................................................................................. 1Introduction ........................................................................................................................................................................... 1

Purpose and Scope....................................................................................................................................................... 1Description of Study Area ........................................................................................................................................... 2Method of Analysis ..................................................................................................................................................... 7

Ground-Water Regions and Hydrogeologic Settings ............................................................................................................ 9Sensitivity of Ground Water to Contamination ..................................................................................................................... 16

Hydrogeologic Units.................................................................................................................................................... 16Criteria for Delineation...................................................................................................................................... 17Designated Units................................................................................................................................................ 18

Recharge Rate.............................................................................................................................................................. 30Criteria for Delineation...................................................................................................................................... 32Designated Units................................................................................................................................................ 37

Depth to Water............................................................................................................................................................. 37Criteria for Delineation...................................................................................................................................... 39Designated Units................................................................................................................................................ 39

Land-Surface Slope ..................................................................................................................................................... 41Criteria for Delineation...................................................................................................................................... 41Designated Units................................................................................................................................................ 41

Streamflow Recharge................................................................................................................................................... 41Criteria for Delineation...................................................................................................................................... 43Designated Units................................................................................................................................................ 43

Use of Ground-Water Sensitivity Map .................................................................................................................................. 48Relating Sensitivity Units to Hydrogeologic Information........................................................................................... 48Limitations of Sensitivity Map .................................................................................................................................... 49

Summary................................................................................................................................................................................ 50Selected References............................................................................................................................................................... 51Supplemental Information ..................................................................................................................................................... 53

FIGURES

1. Map showing location of study area ........................................................................................................................... 32. Map showing hydrogeologic units of Lawrence County ............................................................................................ 53. Stratigraphic section for Lawrence County, South Dakota......................................................................................... 64. Schematic showing simplified hydrogeologic setting in study area ........................................................................... 75. Map showing excessively to well-drained soil classifications in Lawrence County .................................................. 106. Map showing most common hydrologic group soil classifications in Lawrence County .......................................... 117. Map showing hydrogeologic settings in Lawrence County........................................................................................ 128. Map showing hydrogeologic-unit sensitivity groups in Lawrence County ................................................................ 299. Map showing selected drainage basins and average precipitation in or near study area ............................................ 31

10. Map showing recharge-rate sensitivity groups in Lawrence County.......................................................................... 3811. Map showing depth-to-water sensitivity groups in Lawrence County ....................................................................... 4012. Map showing land-surface-slope sensitivity groups in Lawrence County ................................................................. 42

Contents III

TABLES

1. Characteristics of hydrogeologic settings in study area ......................................................................................... 132. Sensitivity ranges for types of aquifer media ......................................................................................................... 173. Sensitivity ranges for types of unsaturated media .................................................................................................. 184. Sensitivity ratings for range categories of aquifer hydraulic conductivity ............................................................. 185. Sensitivity characteristics of hydrogeologic units .................................................................................................. 196. Base flow within drainage basin above station 06422500...................................................................................... 327. Estimated recharge rate for hydrogeologic units as a percent of precipitation....................................................... 338. Sensitivity ratings for range categories of recharge rate......................................................................................... 379. Sensitivity rank for designated recharge-rate groups ............................................................................................. 37

10. Sensitivity ratings for range categories of depth to water ...................................................................................... 3911. Sensitivity rank for designated depth-to-water groups ........................................................................................... 3912. Sensitivity rank for designated land-surface-slope groups ..................................................................................... 4113. Characteristics of drainage areas upstream from streamflow-loss zones ............................................................... 4414. Data pertaining to annual yield for selected gaging stations in Lawrence County ................................................ 48

PLATES

[Plates are in pocket]

1. Map showing sensitivity of ground water to contamination in Lawrence County, South Dakota2. Map showing drainage areas upstream from potential streamflow-loss zones and locations of

selected streamflow-gaging stations in Lawrence County, South Dakota

CONVERSION FACTORS AND VERTICAL DATUM

Temperature in degrees Fahrenheit (° F) may be converted to degrees Celsius (° C) as follows:

° C = (° F - 32) / 1.8

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.

Water year: In U.S. Geological Survey reports, water year is the 12-month period, October 1 through September 30. The water year is designated by the calendar year in which it ends; thus, the water year ending September 30, 1994, is called the "1994 water year."

Multiply By To obtain

acre 4,047 square meteracre 0.4047 hectare

cubic foot per second (ft3/s) 0.02832 cubic meter per second foot (ft) 0.3048 meter

foot per day (ft/d) 0.3048 meter per daygallon per minute (gal/min) 0.06309 liter per second

inch (in.) 2.54 centimeterinch (in.) 25.4 millimeter

inch per year (in/yr) 25.4 millimeter per yearmile (mi) 1.609 kilometer

square mile (mi2) 259.0 hectaresquare mile (mi2) 2.590 square kilometer

IV Contents

Sensitivity of Ground Water to Contamination, Lawrence County, South DakotaBy Larry D. Putnam

ABSTRACT

Ground-water supplies in Lawrence County, South Dakota, can be contaminated by agricul-tural, urban, suburban, commercial, and industrial land uses. To address this issue, the U.S. Geological Survey in cooperation with Lawrence County and the City of Spearfish mapped the sensitivity of ground water to contamination in Lawrence County.

Sensitivity of ground water to contamina-tion was determined by delineating hydrogeologic settings with common hydrogeologic characteris-tics as described in the DRASTIC method, developed by the U.S. Environmental Protection Agency and the National Water Well Association. Within the framework of 11 hydrogeologic settings, sensitivity to contamination was ranked for six intrinsic hydrogeologic characteristics: (1) aquifer media, (2) unsaturated media (3) hydraulic conductivity, (4) recharge rate, (5) depth to water, and (6) land-surface slope. The rating conventions of DRASTIC were modified to provide a relative ranking of hydrogeologic char-acteristics without assignment of a combined numerical score. Soil characteristics were not included as a map layer because detailed digital data were not available; however, the general distribution of two soil characteristics were shown.

A total of 956 polygons were delineated and assigned a sensitivity-unit code that represented unique groups of sensitivity rank for the six intrin-sic hydrogeologic characteristics. The polygons were created by overlaying and intersecting maps that describe the geology, precipitation, land-

surface elevation, and depth to water using a geo-graphic information system. Thirty drainage areas upstream from potential streamflow-loss zones were delineated to describe an additional mecha-nism of transport of potential contamination. The sensitivity of ground water to contamination was presented on a 1:100,000-scale map with code and label explanations. Limitations of the sensitivity map are described to facilitate appropriate use of the map as a screening tool to compare sensitivity to contamination.

INTRODUCTION

Ground-water supplies in Lawrence County, South Dakota, can be contaminated by agricultural, urban, suburban, commercial, and industrial land uses. Population growth in Lawrence County continues to increase both the demand from and the potential for contamination to aquifers in Lawrence County. Areas sensitive to contamination can be identified and described in ways useful to natural-resource managers, regulatory policy makers, and educators. In an effort to provide this information, the U.S. Geological Survey (USGS), in cooperation with Lawrence County and the City of Spearfish conducted a study to delineate and describe sensitivity of ground water to contamination in Lawrence County.

Purpose and Scope

The purpose of this report is to present maps that describe the sensitivity of ground water to contamina-tion in Lawrence County. Hereinafter in this report, the word “sensitivity” refers to the sensitivity of ground

Introduction 1

water to contamination. Evaluation of potential con-taminant characteristics or land-use practices were not included in the sensitivity delineations. Mechanisms of potential contamination considered were limited to those that can occur near the land surface and did not include deep subsurface injection. Ground water, for the purposes of this report, refers to the water in the uppermost saturated rocks and sediments that can be yielded in usable quantities to a well or spring.

The study area was subdivided into map areas that delineate regions with relative ranking of intrinsic hydrogeologic sensitivity. Map areas showing sensi-tivity were designated and delineated by sequentially evaluating six intrinsic hydrogeologic characteristics: aquifer media, unsaturated media, hydraulic conduc-tivity, recharge rate, depth to water, and land-surface slope. Transport of a potential contaminant was assumed to occur from infiltration of precipitation from the land surface to the uppermost saturated rocks.

An additional mechanism of transport of poten-tial contaminants is streamflow loss (infiltration). Runoff from precipitation on drainage basins upstream from streamflow-loss zones affects the sensitivity of map areas containing the losing stream. Drainage basins upstream from potential loss zones were delineated and described; however, no attempt was made to produce a relative rating of hydrogeologic sensitivity that included both mechanisms of potential contaminant transport.

The products developed by this study are two 1:100,000-scale maps of Lawrence County showing the areal distribution of sensitivity units and delineated drainage basins above potential streamflow-loss zones. Tables of descriptive information are associated with the map units by the identifying codes and labels.

Description of Study Area

The study area, Lawrence County, consists of about 800 mi2 in the northwestern part of the topo-graphically distinct Black Hills and adjacent foothills and plains to the north (fig. 1). Land-surface altitudes range from greater than 6,000 ft above sea level in the southwestern part of the county to less than 4,000 ft in the plains to the north, resulting in an orographically induced microclimate characterized by generally greater precipitation and lower temperatures at the higher altitudes. The highest altitude is near 7,000 ft in the southwestern corner of the county, and the lowest

2 Sensitivity of Ground Water to Contamination in Lawrence C

altitude is near 3,000 ft in the northeastern corner of the county. Average (30-year) annual precipitation is 22.2 in. at Spearfish and 29.0 in. at Lead. Average annual precipitation is less than 20 in. in the northern and southeastern parts of the county based on precipi-tation stations outside the study area (U.S. Department of Commerce, 1994). The average (30-year) annual temperature is 46.5° F at Spearfish and 44.3° F at Lead (U.S. Department of Commerce, 1994).

A general classification of land use and land cover in Lawrence County can be determined from aerial photographs. A USGS land-use and land-cover classification system user’s guide (U.S. Geological Survey, 1986) describes the use of data compiled at 1:250,000 scale from high-altitude aerial photographs to make these classifications. The digital data for Lawrence County indicates that about 70 percent is forest land and about 25 percent is agriculture or range land. Most of the agriculture and range land is located in the northern part of the county with some small areas located within the forest land, which predominantly is in the southern part of the county. Most of the forest land is evergreen forest with about 5 percent mixed evergreen and deciduous forest.

The Black Hills National Forest, established in 1907 and managed by the U. S. Forest Service (USFS), constitutes about 53 percent of Lawrence County. Privately held lands that were excluded from the USFS land included active mining claims, foothills ranches, and meadows and bottom lands along numerous streams.

Gold mining was an important industry in the early development of the area. Placer mining, small surface pits, and shallow underground mines were common through the late 1800’s. This was followed by large-scale underground mining by Homestake Mining Company in Lead and the development of smaller mines. In the 1980’s, several new large-scale open-pit gold mines were developed utilizing heap-leach recovery methods for low-grade ores.

The largest component of the agriculture industry is cattle production, with a majority of the crop land used to produce cattle feed. Private land and USFS land in the higher elevations is used for summer pasture. The timber industry primarily consists of har-vesting ponderosa pine, which is hauled to local processing mills. Tourism is an important industry and is based on the mountain scenery, recreational opportunities, and cultural history.

ounty, South Dakota

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104˚

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103˚30'

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T. 6 N.

T. 5 N.

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Base from U.S. Geological Survey digital line graph, 1:100,000:Belle Fourche, 1983; Rapid City, 1977Universal Transverse Mercator projection, zone 13North American Horizontal Datum 1927

Figure 1. Location of study area.

APPROXIMATE EXTENT OF BLACK HILLS AREA, REPRESENTED BY GENERALIZED OUTER EXTENT OF INYAN KARA GROUP

EXPLANATION

1 2 3 4 5 6 MILES

1 2 3 4 5 6 KILOMETERS0

0

Introduction 3

The population of the county was about 21,000 in 1990 with the primary urban areas being the City of Spearfish and the Deadwood-Lead area, with popula-tions of about 8,000 and 3,000, respectively. Suburban areas have developed near the urban areas and on plots of private land within the forested, higher elevations of the county.

Geologic formations of the study area have been grouped into hydrogeologic units and mapped for the Black Hills area based on similarity in hydraulic properties (Strobel and others, 1999). A generalization of that map (fig. 2) shows the surface exposures of the hydrogeologic units for the study area labeled with the stratigraphic interval abbreviations for the grouped geologic formations (fig. 3). A simplified schematic diagram of the hydrogeologic setting in the study area is shown in figure 4.

Precambrian igneous and metamorphic rocks extend from near Lead to the southeastern corner of the study area. On the west, north, and east is a layered series of sedimentary rocks including limestones, sand-stones, and shales that extend concentrically outward from the Precambrian core. The bedrock sedimentary formations typically dip away from the uplifted Black Hills at angles that approach 15 to 20 degrees near the outcrops (Carter and Redden, 1999a, 1999b, 1999c), and decrease with distance from the uplift to about 2 degrees in the northeastern part of the county. Several centers of igneous activity exist across the central part of the study area that contain shallowly emplaced sills, laccoliths, dikes, and plugs (Lisenbee, 1985). Unconsolidated units include alluvium, collu-vium and gravel deposits. Mapped alluvial deposits along streams generally are widest in the northern part of the study area, where stream gradients generally are smallest. Gravel deposits in the northern part of the study area also include paleochannels, pediments, and stream terraces along former flood plains (Strobel and others, 1999).

The Inyan Kara, Minnekahta, Minnelusa, Madison, and Deadwood sedimentary bedrock aquifers are major aquifers (Strobel and others, 1999) and are extensively utilized for domestic, community, and agricultural water supplies within Lawrence County. These five important aquifers are recharged from pre-cipitation and/or from streamflow infiltration on the outcrops. Alluvial deposits and fractured Precambrian rocks also are widely used as sources of ground water within the county.


Unconsolidated deposits of colluvium and gravel, that are intermittently saturated, could supply usable quantities of water to wells during part of the year. The White River aquifer is a minor aquifer where saturated (Strobel and others, 1999). Most of the White River Group in the study area has been eroded away and only isolated areas remain. The Tertiary intrusive units are considered relatively impermeable; however, the permeability varies greatly depending on frac-turing. Sills, dikes, and plugs could provide usable quantities of water in localized areas.

Some layers within the sedimentary confining units and semiconfining units are utilized locally as sources of water. The Spearfish and Opeche confining units supply limited quantities of water to wells in local areas. The Ordovician semiconfining unit could supply limited quantities of water to wells from fractures.

The Minnekahta, Minnelusa, Madison, and Deadwood aquifers are collectively confined by the underlying Precambrian units and the overlying Spearfish confining unit. Individually the aquifers are separated by minor confining layers, and contain some relatively impermeable layers within them. Leakage between aquifers is extremely variable (Peter, 1985; Greene, 1993, Klemp, 1995). The Inyan Kara aquifer is used extensively. A series of Cretaceous shales acts as the upper confining unit to the Inyan Kara aquifer. Artesian conditions generally exist within the afore-mentioned aquifers, where they are confined from above. Similarly, artesian springs that originate from confined aquifers are common around the periphery of the Black Hills.

Streamflow yields in Lawrence County, the average annual volume of streamflow divided by the drainage basin area, generally are largest in the highest elevations where precipitation is largest. Numerous streams originate from headwater springs discharging from the Paleozoic formations in the northwestern part of the study area (fig. 2). Many streams generally lose all or part of their flow as they cross outcrops of the Minnekahta Limestone, Minnelusa Formation, Madison Limestone, and Deadwood Formation. Most losses occur on outcrops of the Minnelusa Formation and Madison Limestone with minimal losses to the Deadwood Formation and generally small losses to the Minnekahta Limestone (Hortness and Driscoll, 1998). Karst features of the Madison Limestone are respon-sible for a large part of the Madison Limestone’s capacity to accept streamflow recharge. Large springs occur in many locations downgradient from the stream-flow-loss zones providing an important source of base flow in many streams beyond the periphery of the Black Hills (Rahn and Gries, 1973).

ounty, South Dakota

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Figure 2. Hydrogeologic units of Lawrence County.

EXPLANATION

1 2 3 4 5 6 MILES


0

AlluviumColluviumGravel depositsWhite River aquiferTertiary intrusive unitsCretaceous-sequence confining unitInyan Kara aquiferJurassic-sequence semiconfining unit

Spearfish confining unitMinnekahta aquiferOpeche confining unitMinnelusa aquiferMadison aquiferOrdovician-sequence semiconfining unitDeadwood aquiferPrecambrian igneous and metamorphic units

WATER BODY

HYDROGEOLOGIC UNITS


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terb

edde

d lim

esto

ne a

nd s

ands

tone

at b

ase.

Mas

sive

ligh

t-co

lore

d lim

esto

ne.

Dol

omite

in p

art.

Cav

erno

us in

upp

er p

art.

Pin

k to

buf

f lim

esto

ne.

Sha

le lo

cally

at b

ase.

Buf

f dol

omite

and

lim

esto

ne.

Gre

en s

hale

with

silt

ston

e.M

assi

ve to

thin

-bed

ded

buff

to p

urpl

e sa

ndst

one.

Gre

enis

h gl

auco

nitic

sha

le fl

aggy

d

olom

ite a

nd fl

atpe

bble

lim

esto

ne c

ongl

omer

ate.

San

dsto

ne, w

ith c

ongl

omer

ate

loca

lly a

t the

bas

e.

Sch

ist,

slat

e, q

uart

zite

, and

ark

osic

grit

. In

trud

ed b

y di

orite

, met

amor

phos

ed

to

amph

ibol

ite, a

nd b

y gr

anite

and

peg

mat

ite.

PE

RM

IAN

PE

NN

SY

LVA

NIA

N

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SIS

SIP

PIA

N

P P

mM

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ELU

SA

FO

RM

AT

ION

MA

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ON

(P

AH

AS

AP

A)

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ES

TO

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EN

GLE

WO

OD

FO

RM

AT

ION

MD

m

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DE

VO

NIA

NW

HIT

EW

OO

D (

RE

D R

IVE

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FO

RM

AT

ION

WIN

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RM

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ION

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AD

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OD

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ION

UN

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ED

ME

TA

MO

RP

HIC

AN

D IG

NE

OU

S R

OC

KS

OC

d

pCu

OR

DO

VIC

IAN

CA

MB

RIA

N

PR

EC

AM

BR

IAN

PALEOZOICMESOZOICCENOZOIC

Mod

ified

from

info

rmat

ion

furn

ishe

d by

the

Dep

artm

ent o

f Geo

logy

and

Geo

logi

cal E

ngin

eerin

g, S

outh

Dak

ota

Sch

ool o

f Min

es a

nd T

echn

olog

y(w

ritte

n co

mm

un.,

Janu

ary

1994

)

3 0-1

50

---

1T

he s

ubsu

rfac

e th

ickn

ess

was

mod

ified

from

sev

eral

ref

eren

ces

to p

rovi

dea

rang

e th

at w

as th

e m

ost s

peci

fic to

the

stud

y ar

ea.

2D

eWitt

and

oth

ers,

198

9.3

Rob

inso

n an

d ot

hers

, 196

4.4

Kyl

lone

n an

d P

eter

, 198

7.5

Thi

ckne

ss e

stim

ated

from

sub

trac

ting

surf

aces

cre

ated

from

str

uctu

re c

onto

urs

of M

adis

on L

imes

tone

and

Min

nelu

sa F

orm

atio

n to

ps (

Car

ter

and

Red

den,

199

9ban

d C

arte

r an

d R

edde

n, 1

999a

).6

Thi

ckne

ss e

stim

ated

from

sub

trac

ting

surf

aces

cre

ated

from

str

uctu

re c

onto

urs

of D

eadw

ood

For

mat

ion

and

Mad

ison

Lim

esto

ne to

ps (

Car

ter

and

Red

den,

199

9can

d C

arte

r an

d R

edde

n, 1

999b

). T

he s

ubsu

rfac

e th

ickn

esse

s of

the

Mad

ison

Lim

esto

ne g

reat

er th

an 7

00 fe

et a

re in

the

nort

heas

t par

t of t

he s

tudy

are

a.

Inte

rbed

ded

sand

ston

e, li

mes

tone

, dol

omite

, sha

le, a

nd a

nhyd

rite.

6 Sensitivity of Ground Water to Contamination in Lawrence County, South Dakota

Figure 4. Simplified hydrogeologic setting in study area.

Potentiometric surfaceof aquifer in Madison Limestone

Flowingwell

Spring conduit

Alluviumaquifer

Englewood

Formation

OpecheFormationDip of sedimentary rocks exaggerated

Precambrian m

etamorphic

and igneous rocks

Deadw

ood

Inyan Kara Group

Thicknesses not to scale

Madison Lim

estoneM

innelusa

Minnekahta

Limestone

Cave

Formation

Formation

Formation

Watertable

Central Black Hills

Plains

HeadwaterSprings

LimestonePlateau

Spearfish

Method of Analysis

The method of aquifer sensitivity mapping used for this study was based on DRASTIC (Aller and others, 1987), with modifications derived from Hearne and others (1995) and Davis and others (1994). Selected parts of each method were combined and modified based on the availability of data and hydro-logic characteristics of the study area. Additional modifications were made based on studies comparing DRASTIC scores and observed ground-water quality.

The DRASTIC method, developed by the U.S. Environmental Protection Agency and the National Water Well Association, permits the ground-water pollution potential of any hydrogeologic setting in the United States to be systematically evaluated with existing information. The system is designed to be used as a screening tool and not as a site assessment

methodology. According to the documentation for DRASTIC (Aller and others, 1987, p. 11), “the con-cepts of the system were developed assuming the use of a pollutant having the mobility of water that is intro-duced at the surface, and carried towards the ground water by recharge from precipitation.”

The DRASTIC method analyzes characteristics that affect the potential for ground-water pollution. Each letter in the acronym stands for a characteristic that is rated over the mapped area in terms of contribu-tion to the “Pollution Potential Index” (Aller and others, 1987). The seven layers are combined using a geographic information system (GIS), and the point ratings for each layer are added together to give the total pollution potential index. In DRASTIC, the “D” stands for depth to water, “R” is for recharge rate, “A” is for aquifer media, “S” is for soil media, “T” is for

Introduction 7

topographic slope, “I” is for unsaturated (vadose) zone impact, and “C” is for aquifer hydraulic conductivity.

Large DRASTIC ratings suggest greater pollu-tion potential; thus, they generally are higher for shallower depth to water, larger (more rapid) recharge rate, more porous aquifer media, more transmissive soil media, less steep topographic slope (which yields less runoff), less unsaturated zone impact with less res-idence time, and higher aquifer hydraulic conductivity. In DRASTIC, each term’s importance can be adjusted by introducing a weighting factor that is multiplied by each term’s rating. For example, conductivity and aquifer media might typically be weighted more than topographic slope.

Hearne and others (1995) described the vulnera-bility of the uppermost ground water to contamination in the greater Denver, Colorado, area by combining the hydrogeologic characteristics aquifer media, unsatur-ated zone impact, and hydraulic conductivity. A letter code was assigned to each unique combination of the DRASTIC categories determined for the three hydro-geologic characteristics. GIS map layers were con-structed for depth to water, soil media, and topographic slope. The recharge rate consisted of a single DRASTIC category for the study area; therefore, this category was not included as a map layer. The map layers were overlain and intersected using a GIS to pro-duce polygons with a unique code consisting of a letter designation followed by three numbers (vulnerability response unit). Tables and text describing hydrologic characteristics associated with the digits of the code were developed to compare vulnerability response units. To simplify the final map, areas smaller than 100 acres were eliminated from digital maps by merging the small areas with adjacent polygons. The rating conventions from the DRASTIC method were adapted for each individual map layer, but relative weight between the layers and total pollution potential index were not calculated.

KARSTIC, a modification of the DRASTIC method developed by Davis and others (1994), was used to map ground-water vulnerability in the Rapid Creek Basin upstream from Rapid City, South Dakota (located southeast of the study area). In KARSTIC, the “K” stands for karst sinkholes with associated surface recharge, “A” is for aquifer media, “R” is for net recharge rate, “S” is for soil media, “T” is for topo-graphic slope, “I” is for unsaturated zone impact (includes the depth to water factor in DRASTIC as a multiplier), and “C” is for aquifer hydraulic conduc-tivity. A major difference between DRASTIC and


KARSTIC was introduced when Davis and others (1994) multiplied the k factor times an f factor for fracture development, and multiplied the i factor for unsaturated zone impact times the d factor for depth to ground water. This produced KARSTIC pollution potential indices that are, at selected places, 10 times greater than would have been determined for the same location using the DRASTIC method. Davis and others (1994, p. 73) contended that, “… in terms of relative pollution potential, clearly these larger values for sink-hole areas are indicative of pollution vulnerability that is at least an order of magnitude greater than most set-tings in alluvial aquifers or in consolidated aquifers with intergranular porosity.”

Scoring, weighting, and combining sensitivity characteristics into a single pollution potential index can be misleading because of the different transport characteristics of potential contaminants and the uncer-tainty in the relative importance of hydrogeologic parameters. A review of studies examining observed pesticide concentrations and predictions of pesticide occurrence and behavior in ground water by Barbash and Resek (1996, p. 395) concluded “… even if an assessment scheme incorporates all of the variables that control the transport and fate of contaminants of interest in the subsurface, its predictive power can be diminished--or eradicated--if the values selected for individual parameters or the relative weights assigned to them are inappropriate.”

A method described by Hearne and others (1995) was selected as a model for this study because of the similarity to the hydrogeologic setting in Lawrence County. The method uses comparative descriptions of unique sensitivity characteristics without weighting or combination into a single combined numerical score. Reviews of DRASTIC scores compared to observed concentrations of nitrates and agricultural pesticides in wells (Korterba and others, 1993; U.S. Environmental Protection Agency, 1993; Barbash and Resek, 1996; Rupert, 1998) found that many DRASTIC scores did not correlate well with observed ground-water con-tamination. Some of the concerns raised by these studies are discussed in the sections that describe the individual map layers.

The first step in the method used by this study was to subdivide the study area into a geographic framework of regions, which were further subdivided into settings with generally similar hydrogeologic characteristics. These regions and settings are discussed in the following section.

ounty, South Dakota

Because of the significant differences in sensi-tivity characteristics of the geologic formations, the hydrogeologic-unit map (collectively representing aquifer media, unsaturated media, and hydraulic con-ductivity) was selected as the first descriptive map layer within this framework. This map layer was followed by map layers for recharge rate, depth to water, and land-surface (topographic) slope.

The soil category was not included as a map layer because digital soils data were available only at the 1:250,000 scale. STATSGO (State soil geographic database) data are not detailed enough to make inter-pretations at the county level (U. S. Department of Agriculture, 1994). The U.S. Department of Agricul-ture, Natural Resources Conservation Service, currently is compiling 1:24,000-scale digital soils data in Lawrence County that would be needed to complete this sensitivity map layer. A generalized description of soil properties was prepared from STATSGO data to show the general distribution of two soil properties that have been used to predict contamination potential. Statistical correlations between STATSGO soil proper-ties and measured contaminant concentrations (Rupert, 1998, 1999) identified two soil properties as predictors of contamination in a regional study of a highly trans-missive basalt aquifer. Soil drainage showed a positive correlation with nitrate concentrations in the aquifer, and hydrologic soil group showed a positive correlation with the herbicide atrazine in the aquifer. Soil drainage categorizes the frequency and duration of wet periods of the soil, and hydrologic group categorizes soil infil-tration rates (U.S. Department of Agriculture, 1994).

Soil drainage categories range from excessively drained to very poorly drained with five intermediate categories. Most of the soils in Lawrence County are classified in the three categories ranging from exces-sively to well drained (fig. 5).

Hydrologic group soil categories include infiltra-tion rates of high, moderate, slow, and very slow with high being the most sensitive to contamination and very slow the least sensitive. The most common category in Lawrence County (fig. 6) is moderate. The southern part of the study area has more moderate infiltration rates and the northeastern part of the county has more very slow infiltration rates. An analysis of sensitivity related to these soil properties also should consider additional information such as the thickness of the soil.

Streamflow loss is an important source of recharge to several major aquifers in the study area. A map layer describing potential streamflow-loss zones

and the drainage areas upstream from these loss zones was added for consideration of this additional mecha-nism of ground-water contamination. The KARSTIC method (Davis and others, 1994) describes karst features associated with the streamflow-loss zones as a significant factor in evaluating ground-water sensitivity to contamination. A numerical rating that could rela-tively rank streamflow-loss characteristics was not made because of the lack of information on karst flow patterns.

The DRASTIC method was designed to generate pollution potential estimates for any area of 100 acres or larger. This study included a similar process of spatial generalization. If small polygons less than 100 acres were created in overlaying and intersecting map layers, the small polygons were merged with adja-cent polygons. The selection of polygons to which the smaller ones were merged was based on similarity to the hydrogeologic-unit map, followed in priority by the recharge-rate, the depth-to-water, and the land-surface-slope (topographic-slope) maps.

GROUND-WATER REGIONS AND HYDROGEOLOGIC SETTINGS

The United States has been divided into 15 ground-water regions (Heath,1984). South Dakota includes four ground-water regions. Two of these regions, Western Mountain Ranges and Non-Glaciated Central, are present in Lawrence County. Because the sensitivity of ground water may be highly variable within a ground-water region, each region was sub-divided into hydrogeologic settings, hereinafter referred to as settings (table 1), which were modified from the DRASTIC manual (Aller and others, 1987). Eleven settings were delineated in the study area and are shown as a range of colors on figure 7 and plate 1. The delineation of the areas was based on topography, precipitation patterns, and the hydrogeologic units map (Strobel and others, 1999), which describes the hydro-geologic characteristics of grouped units. The charac-teristics of each of the 11 settings (table 1) shows the wide range hydrologic and geologic factors that control the occurrence and movement of ground water in the study area.

Ground-Water Regions and Hydrogeologic Settings 9

T. 7 N.

T. 6 N.

T. 5 N.

T. 4 N.

T. 3 N.

T. 2 N.

R. 1 E. R. 2 E. R. 3 E. R. 4 E. R. 5 E.

103˚30'

104˚

103˚52'30"103˚37'30"103˚45'

44˚30'

44˚15'

44˚22'30"

Spea

rfish

Spearfish

Cre

ek

Cre

ek

Cre

ek

False

Creek

Creek

Creek

Creek

Creek

Bot

to

m

Whi

tewood

Whi

teta

il

Litt

leIr

on

REDWATER

RIVER

Robison

Higgins

Gulch

Gulch

Squaw

Creek

Creek

Creek

Annie Creek

Bea

rG

ulch

Creek

Crow

Bear

Butte

Boxelder

Creek

Little ElkNorth

Fo

rk

Creek

Fork

South

Rapid

Rapid

Creek

Elk

Bea

ver

90 14

14A

14A

14A

385

385

9014

85

85

85

85

34

34

Spearfish

Whitewood

Deadwood

LeadCentralCity

Nemo

LAW

RE

NC

E C

OU

NT

Y 95

95

95

74

74

9494

94

9999

95

89

100

100

100

98100 96

85

95


Figure 5. Excessively to well-drained soil classifications in Lawrence County.

SOIL DRAINAGE CLASSIFICATION--Modified from U.S. Department of Agriculture (1994). Number represents the percent of soil classifications in delineated area that are excessively or well drained.

EXPLANATION

1 2 3 4 5 6 MILES


0


Spea

rfish

Spearfish

Cre

ek

Cre

ek

Cre

ek

False

Creek

Creek

Creek

Creek

Creek

Bot

to

m

Whi

tewood

Whi

teta

il

Litt

leIr

on

REDWATER

RIVER

Robison

Higgins

Gulch

Gulch

Squaw

Creek

Creek

Creek

Annie Creek

Bea

rG

ulch

Creek

Crow

Bear

Butte

Boxelder

Creek

Little ElkNorth

Fo

rk

Creek

Fork

South

Rapid

Rapid

Creek

Elk

Bea

ver

90 14

14A

14A

14A

385

385

9014

85

85

85

85

34

34

Spearfish

Whitewood

Deadwood

LeadCentralCity

Nemo

LAW

RE

NC

E C

OU

NT

Y

103˚30'

104˚

103˚52'30"103˚37'30"103˚45'

44˚30'

44˚15'

44˚22'30"89

89

89

82

82

4141

41

61

61

89

40

68

68

82

50 40100

76


Figure 6. Most common hydrologic group soil classifications in Lawrence County.

HYDROLOGIC GROUP SOIL CLASSIFICATION--Modified from U.S. Department of Agriculture (1994). Number represents most common group as percent of total. Shade represents most common group.

EXPLANATION

1 2 3 4 5 6 MILES


0

Class B--Moderate infiltration rate

Class C--Slow infiltration rate

Class D--Very slow infiltration rate

89


T. 7 N.

T. 6 N.

T. 5 N.

T. 4 N.

T. 3 N.

T. 2 N.

R. 1 E. R. 2 E. R. 3 E. R. 4 E. R. 5 E.

Spea

rfish

Spearfish

Cre

ek

Cre

ek

Cre

ek

False

Creek

Creek

Creek

Creek

Creek

Bot

to

m

Whi

tewood

Whi

teta

il

Litt

leIr

on

REDWATER

RIVER

Robison

Higgins

Gulch

Gulch

Squaw

Creek

Creek

Creek

Annie Creek

Bea

rG

ulch

Creek

Crow

Bear

Butte

Boxelder

Creek

Little ElkNorth

Fo

rk

Creek

Fork

South

Rapid

Rapid

Creek

Elk

Bea

ver

9014

14A

14A

14A

385

385

9014

85

85

85

85

34

34

Spearfish

Whitewood

Deadwood

LeadCentralCity

Nemo

LAW

RE

NC

E C

OU

NT

Y

103˚30'

104˚

103˚52'30"103˚37'30"103˚45'

44˚30'

44˚15'

44˚22'30"


Figure 7. Hydrogeologic settings in Lawrence County.

EXPLANATION

1 2 3 4 5 6 MILES


0

HYDROGEOLOGIC SETTINGS BY GROUND-WATER REGIONWestern Mountain Ranges Ground-Water Region Nonglaciated Central Ground-Water Region

Mountain Slopes-East Igneous Domes

Mountain Sopes-North

Alluvial Mountain Valleys-EastAlluvial Mountain Valleys

Limestone Plateau

Alternating Sandstone, Limestone, and Shale-Thin Soil

Unconsolidated or Semi-consolidated Deposits

River Alluvium without Overbank Deposits

Mountain Flanks

Alluvial Mountain Valleys-North


Tab

le 1

. C

hara

cter

istic

s of

hyd

roge

olog

ic s

ettin

gs in

stu

dy a

rea

Hyd

rog

eolo

gic

sett

ing

Tota

l are

a(s

qu

are

mile

s)

Ave

rag

ela

nd

-su

rfac

e al

titu

de1

(fee

t)

Ave

rag

ela

nd

-su

rfac

e sl

op

e1

(per

cen

t)

Ave

rag

e p

reci

pit

atio

n2

(in

ches

per

yea

r)D

escr

ipti

on

Wes

tern

Mou

ntai

n R

ange

s G

roun

d-W

ater

Reg

ion

Mou

ntai

n Sl

opes

-Eas

t13

1.2

5,57

115

.121

.1T

his

setti

ng, l

ocat

ed in

the

sout

heas

tern

and

sou

th-c

entr

al p

art o

f th

e st

udy

area

, is

char

acte

rize

d by

ste

ep s

lope

s, th

in s

oil c

over

, and

fra

ctur

ed b

edro

ck

cons

istin

g m

ostly

of

igne

ous

and

met

amor

phic

roc

ks.

Wel

l yie

lds

typi

cally

ar

e lim

ited,

alth

ough

in lo

cal a

reas

the

hydr

aulic

con

duct

ivity

cou

ld b

e hi

gh

due

to f

ract

urin

g. T

hick

er w

eath

ered

zon

es m

ay d

evel

op lo

cally

pa

rtic

ular

ly o

n ta

lus

slop

es w

ith p

erch

ed w

ater

com

mon

. T

he to

pogr

aphi

c tr

end

of th

ese

slop

es is

to th

e ea

st in

the

rain

sha

dow

of

the

mou

ntai

ns, a

nd

lim

ited

rain

fall

is d

eriv

ed f

rom

moi

stur

e-la

den

prev

aili

ng w

este

rly

win

ds.

Bec

ause

of

the

orog

raph

ic e

ffec

t of

the

Bla

ck H

ills,

the

nort

hern

par

t of

this

se

tting

rec

eive

s m

ore

rain

fall

than

is ty

pica

l for

this

set

ting.

Gro

und-

wat

er

leve

ls a

re e

xtre

mel

y va

riab

le.

Mou

ntai

n Sl

opes

-Nor

th25

.25,

460

31.9

28.2

Thi

s se

tting

, loc

ated

on

the

nort

hern

slo

pe o

f th

e up

lift,

is s

imil

ar to

Mou

ntai

n Sl

opes

-Eas

t exc

ept p

reci

pita

tion

gene

rally

exc

eeds

that

whi

ch f

alls

on

east

ern

slop

es.

Rec

harg

e re

mai

ns r

elat

ivel

y lo

w, h

owev

er, b

ecau

se o

f th

e st

eep

slop

es a

nd lo

w p

rim

ary

poro

sity

of

the

rock

. In

this

par

ticul

ar s

ettin

g,

incr

ease

d fr

actu

ring

may

occ

ur in

are

as a

long

the

flan

ks o

f ig

neou

s in

trus

ions

.

Allu

vial

Mou

ntai

n V

alle

ys-E

ast

4.9

5,50

513

.122

.3T

his

setti

ng, l

ocat

ed in

eas

t slo

ping

val

leys

wit

hin

the

mou

ntai

n sl

opes

-eas

t se

tting

, is

char

acte

rize

d by

thin

allu

vium

and

bou

lder

s de

rive

d fr

om th

e su

rrou

ndin

g st

eep

slop

es.

Soil

cove

r ge

nera

lly is

gra

vel s

ized

. G

roun

d w

ater

may

be

obta

ined

from

the

allu

vial

dep

osits

and

als

o fr

om th

e fr

actu

res

in th

e un

derl

ying

bed

rock

, whi

ch a

re g

ener

ally

in d

irec

t hyd

raul

ic

conn

ectio

n w

ith th

e al

luvi

um.

Thi

s ty

pe o

f se

tting

may

be

expe

cted

alo

ng

mos

t of

the

stre

ams

with

in th

e M

ount

ain

Slop

es-E

ast s

ettin

g; h

owev

er,

som

e co

ntig

uous

map

pabl

e ar

eas

may

be

less

than

100

acr

es a

nd

cons

eque

ntly

may

not

be

show

n on

the

map

.

Allu

vial

Mou

ntai

n V

alle

ys-N

orth

.44,

718

23.5

28.6

Thi

s se

tting

, loc

ated

with

in th

e M

ount

ain

Slop

es-N

orth

set

ting,

is s

imila

r to

th

e A

lluvi

al M

ount

ain

Val

leys

-Eas

t set

ting.

Gro

und-

wat

er le

vels

are

ty

pica

lly s

hallo

wer

bec

ause

of

high

er a

mou

nts

of p

reci

pita

tion

and

subs

eque

ntly

gre

ater

rec

harg

e. T

his

type

of

setti

ng m

ay b

e ex

pect

ed a

long

m

ost o

f th

e st

ream

s w

ithin

the

mou

ntai

n sl

opes

-nor

th s

ettin

g; h

owev

er,

som

e co

ntig

uous

map

pabl

e ar

eas

may

be

less

than

100

acr

es in

man

y ar

eas

and

cons

eque

ntly

may

not

be

show

n on

the

map

.


Non

-Gla

ciat

ed C

entr

al G

roun

d-Wat

er R

egio

n

Igne

ous

Dom

es11

1.6

5,46

724

.825

.9T

his

setti

ng, l

ocat

ed a

cros

s th

e ce

ntra

l par

t of

the

stud

y ar

ea, i

s ch

arac

teri

zed

by ig

neou

s in

trus

ive

rock

s su

rrou

nded

by

dipp

ing

sedi

men

tary

roc

ks.

Fold

ing

and

faul

ting

of th

e se

dim

enta

ry r

ocks

alo

ng th

e fl

anks

of

the

dom

es

may

incr

ease

the

perm

eabi

lity

and

the

rech

arge

rat

e of

thes

e un

its.

Wel

l yi

elds

are

ext

rem

ely

vari

able

dep

endi

ng o

n th

e de

gree

of

fold

ing

and

faul

ting

and

the

type

of

rock

. W

here

few

fra

ctur

es e

xist

in th

e ig

neou

s do

mes

, wel

l yie

lds

are

very

low

or

non-

exis

tent

; how

ever

, wel

l yie

lds

typi

cally

are

gre

ater

alo

ng th

e fr

actu

red

area

s on

the

flan

ks o

f th

e do

mes

. Se

dim

enta

ry r

ock

outc

rops

, hyd

raul

ical

ly c

onne

cted

to th

e co

nfin

ed

regi

onal

aqu

ifer

s, c

ould

hav

e re

lativ

ely

larg

e re

char

ge r

ates

bec

ause

of

incr

ease

d fr

actu

ring

and

rel

ativ

ely

larg

e pr

ecip

itatio

n am

ount

s.

Lim

esto

ne P

late

au15

1.4

6,17

720

.424

.6T

his

setti

ng, l

ocat

ed in

the

sout

hwes

tern

par

t of

the

stud

y ar

ea, i

nclu

des

mos

t of

the

area

ref

erre

d to

loca

lly

as th

e “L

imes

tone

Pla

teau

.” T

his

setti

ng h

as

mor

e re

lief

than

the

mod

erat

e an

d va

riab

le to

pogr

aphi

c re

lief

desc

ribe

d in

th

e “S

olut

ion

Lim

esto

ne s

ettin

g” in

DR

AST

IC (

Alle

r an

d ot

hers

, 198

4).

The

lim

esto

ne is

cha

ract

eriz

ed b

y a

netw

ork

of s

olut

ion

open

ings

whe

re th

e lim

esto

ne h

as b

een

part

ially

dis

solv

ed a

long

bed

ding

pla

nes

and

frac

ture

s.

Soil

usua

lly is

thin

or a

bsen

t, bu

t whe

re p

rese

nt c

omm

only

is a

cla

yey

loam

. R

echa

rge

usua

lly is

hig

h be

caus

e in

filtr

atio

n of

pre

cipi

tatio

n oc

curs

eas

ily

thro

ugh

the

frac

ture

s an

d so

lutio

n op

enin

gs.

Ret

urn

of re

char

ge in

to s

urfa

ce

wat

er c

ours

es c

an b

e hi

gh.

Wat

er le

vels

typi

cally

are

mod

erat

ely

deep

ex

cept

nea

r su

rfac

e di

scha

rge

poin

ts.

Mou

ntai

n Fl

anks

168.

15,

025

18.8

23.1

Thi

s se

tting

, loc

ated

on

the

mou

ntai

n fl

anks

alo

ng th

e w

este

rn, n

orth

-cen

tral

, an

d ea

ster

n pa

rts

of th

e st

udy

area

, is

char

acte

rize

d by

mod

erat

ely

dipp

ing,

fr

actu

red,

con

solid

ated

sed

imen

tary

roc

ks.

Soil

usua

lly is

thic

ker

than

on

mou

ntai

n sl

opes

. G

roun

d w

ater

is o

btai

ned

from

the

perm

eabl

e se

dim

enta

ry ro

cks

or fr

om fr

actu

res

in th

e se

dim

enta

ry ro

cks.

The

rec

harg

e ra

te is

hig

her

in th

e w

este

rn a

nd n

orth

-cen

tral

par

ts b

ecau

se o

f gr

eate

r pr

ecip

itatio

n. T

he m

ount

ain

flan

ks s

erve

as

rech

arge

are

as f

or a

quif

ers

that

ar

e co

nfin

ed in

adj

acen

t are

as.

Tab

le 1

. C

hara

cter

istic

s of

hyd

roge

olog

ic s

ettin

gs in

stu

dy a

rea–

Con

tinue

d

Hyd

rog

eolo

gic

sett

ing

Tota

l are

a(s

qu

are

mile

s)

Ave

rag

ela

nd

-su

rfac

e al

titu

de1

(fee

t)

Ave

rag

ela

nd

-su

rfac

e sl

op

e1

(per

cen

t)

Ave

rag

e p

reci

pit

atio

n2

(in

ches

per

yea

r)D

escr

ipti

on


Non

-Gla

ciat

ed C

entr

al G

roun

d-Wat

er R

egio

n—C

onti

nued

Allu

vial

Mou

ntai

n V

alle

ys5.

04,

626

20.4

23.7

Thi

s se

tting

, loc

ated

in n

arro

w v

alle

ys w

ithin

the

igne

ous

dom

es a

nd m

ount

ain

flan

ks s

ettin

g, is

cha

ract

eriz

ed b

y th

in a

lluvi

um a

nd b

ould

ers

that

ove

rlie

s fr

actu

red

bedr

ock

of s

edim

enta

ry o

r ig

neou

s in

trus

ive

orig

in.

Surf

icia

l de

posi

ts h

ave

typi

cally

wea

ther

ed to

a s

andy

loam

. G

roun

d w

ater

is

obta

ined

fro

m s

and

and

grav

el la

yers

, whi

ch c

omm

only

are

in d

irec

t hy

drau

lic c

onne

ctio

n to

und

erly

ing

bedr

ock.

Thi

s ty

pe o

f se

tting

may

be

expe

cted

alo

ng m

ost o

f th

e st

ream

s w

ithin

the

igne

ous

dom

es a

nd m

ount

ain

flan

ks s

ettin

gs; h

owev

er, s

ome

cont

iguo

us m

appa

ble

area

s m

ay b

e le

ss th

an

100

acre

s in

man

y ar

eas

and

cons

eque

ntly

may

not

be

show

n on

the

map

.

Alte

rnat

ing

Sand

ston

e,

Lim

esto

ne a

nd S

hale

-T

hin

Soil

138.

43,

577

11.1

20.1

Thi

s se

tting

, loc

ated

in th

e no

rthe

rn p

art o

f th

e st

udy

area

, is

char

acte

rize

d by

lo

w to

mod

erat

e to

pogr

aphi

c re

lief.

Rel

ativ

ely

thin

loam

y so

ils o

verl

ie

hori

zont

al o

r sl

ight

ly d

ippi

ng a

ltern

atin

g la

yers

of

frac

ture

d, c

onso

lidat

ed

sedi

men

tary

roc

ks.

Gro

und

wat

er p

rim

arily

is o

btai

ned

from

san

dsto

ne

laye

rs o

r fr

actu

res

alon

g be

ddin

g pl

anes

or

vert

ical

fra

ctur

es.

Rec

harg

e ge

nera

lly

is m

oder

ate

to lo

w.

Shal

e or

cla

yey

laye

rs o

ften

for

m c

onfi

ning

la

yers

and

whe

re s

uffi

cien

t rel

ief

is p

rese

nt, p

erch

ed g

roun

d-w

ater

zon

es o

f lo

cal i

mpo

rtan

ce o

ften

are

dev

elop

ed.

Unc

onso

lidat

ed o

r Se

mi-

Con

solid

ated

Dep

osits

26.1

3,59

46.

420

.5T

his

setti

ng is

cha

ract

eriz

ed b

y m

oder

atel

y lo

w to

pogr

aphi

c re

lief

and

inte

rbed

ded

depo

sits

that

con

sist

of

laye

rs o

f cl

ay, s

ilt, a

nd s

and.

Alth

ough

so

ils a

re ty

pica

lly lo

amy

or s

andy

, rec

harg

e is

lim

ited

beca

use

of m

oder

ate

prec

ipita

tion

and

high

eva

potr

ansp

irat

ion.

Gro

und-

wat

er le

vels

typi

cally

ar

e le

ss th

an 5

0 fe

et.

Riv

er A

lluvi

um w

itho

ut

Ove

rban

k D

epos

its38

.93,

436

4.6

19.9

Thi

s se

tting

is c

hara

cter

ized

by

low

topo

grap

hic

relie

f an

d de

posi

ts o

f al

luvi

um a

long

str

eam

val

leys

. G

roun

d-w

ater

leve

ls ty

pica

lly a

re s

hallo

w.

Infi

ltrat

ion

of p

reci

pita

tion

is r

apid

but

lim

ited

by th

e am

ount

of

prec

ipita

tion.

Int

erac

tion

betw

een

the

stre

am a

nd th

e gr

ound

-wat

er s

yste

m

is s

igni

fica

nt.

1 Ave

rage

land

-sur

face

alti

tude

and

slo

pe w

ere

dete

rmin

ed f

rom

U.S

. Geo

logi

cal S

urve

y di

gita

l ele

vatio

n m

odel

s (D

EM

’s)

with

30-

met

er r

esol

utio

n. F

or m

ore

deta

iled

info

rmat

ion

desc

ribi

ng D

EM

’s,

see

U.S

. Geo

logi

cal S

urve

y (1

993)

dig

ital

ele

vati

on m

odel

use

r’s

guid

e.2 T

hese

val

ues

wer

e in

terp

rete

d fr

om p

reci

pita

tion

rec

ords

in a

nd a

roun

d th

e st

udy

area

. Fo

r m

ore

info

rmat

ion

desc

ribi

ng th

is in

terp

reta

tion,

see

the

“Rec

harg

e R

ate”

sec

tion

and

figu

re 9

.

Tab

le 1

. C

hara

cter

istic

s of

hyd

roge

olog

ic s

ettin

gs in

stu

dy a

rea–

Con

tinue

d

Hyd

rog

eolo

gic

sett

ing

Tota

l are

a(s

qu

are

mile

s)

Ave

rag

ela

nd

-su

rfac

e al

titu

de1

(fee

t)

Ave

rag

ela

nd

-su

rfac

e sl

op

e1

(per

cen

t)

Ave

rag

e p

reci

pit

atio

n2

(in

ches

per

yea

r)D

escr

ipti

on


The descriptions of the settings listed in the DRASTIC method (Aller and others, 1987) were modified because of the unique orographic effects on weather patterns around the northern perimeter of the Black Hills and the small area of the Black Hills in relation to other mountain ranges. The “Solution Limestone” setting description (Aller and others, 1987) was modified to represent the limestone plateau in the southwestern part of the study area, which has similar hydrologic characteristics. The “Metamorphic/Igneous Domes and Fault blocks” setting was modified to describe areas where igneous intrusive rocks are the most common rock type. The outline of settings follows lines on the 1:100,000-scale hydrogeologic-unit map of the Black Hills (Strobel and others, 1999) with some dividing lines added that delineate precipita-tion and topographic changes. This map layer provided the framework for the analysis of the hydrogeologic-unit, recharge-rate, depth-to-water, and land-surface-slope map layers.

SENSITIVITY OF GROUND WATER TO CONTAMINATION

Sensitivity to contamination represents the potential for ground water to be contaminated based on intrinsic hydrogeologic characteristics. Land-use practices or contaminant characteristics are not included. The terms vulnerability and susceptibility generally have evolved to refer to evaluations that may include land-use practices and/or contaminant charac-teristics, in addition to the intrinsic hydrogeologic char-acteristics. Mapping the sensitivity of ground water to contamination includes overlaying and combining map layers with attributes that relatively rank intrinsic hydrogeologic characteristics in relation to potential contamination. GIS ARC/INFO (Environmental Systems Research Institute, Inc., 1992) software was used to compile and intersect these map layers. The resulting map delineates areas having particular sensi-tivity characteristics within a specific hydrogeologic setting.

Hydrogeologic Units

The hydrogeologic-unit combines ratings for the characteristics of aquifer media, unsaturated media, and hydraulic conductivity into one code because the ratings primarily are determined by the geologic


materials of the aquifer and the unsaturated zone. Aquifer media refers to the consolidated and unconsol-idated rocks that compose the uppermost aquifer (or other hydrogeologic unit). Unsaturated media includes the rocks and sediments above the water table that are unsaturated or partially saturated. The unsaturated material can be the same as the aquifer media; however, in some instances the unsaturated material may include a different material or more than one type of material.

Aquifer media affects the rate at which a contam-inant can move in an aquifer and the attenuation of potential contaminants by processes such as adsorp-tion, ion exchange, and dispersion. Adsorption attaches ions and molecules to sediment particles and is a function of surface area and ionic charge. Ion exchange is controlled by ion size and charge, and occurs more frequently on smaller particles such as clays. Dispersion is the mixing of a potential contami-nant with ground water as the water moves. In a fine-grained homogeneous medium, the flowpaths of water change directions frequently by dodging around the particles, and a contaminant generally spreads out in an elliptical pattern in the direction of the ground-water gradient (Freeze and Cherry, 1979). In aquifer material with preferential flowpaths in fractures or solution con-duits in a limestone, the extent and shape of the mixing pattern can be significantly different. In a limestone with solution conduits, a contaminant could move a great distance with very little attenuation. Where frac-tures and conduits are highly interconnected, there is more opportunity for a potential contaminant to spread. Generally, limestones, unconsolidated sands and gravels, and sandstones are more sensitive than shales, clays, and unfractured igneous and metamorphic rocks because they typically import less adsorption and ion exchange and more dispersion. Adsorption and ion exchange are more likely in sands and gravels and sandstones with greater clay content or interbedded shale layers.

Unsaturated media affects sensitivity in many of the ways described for aquifer media; however, there are some significant differences. The presence of air in the unsaturated zone results in different biological and chemical processes that can break down or modify a potential contaminant. The potential for these pro-cesses to attenuate a potential contaminant generally increases with residence time (Hearne and others, 1995). Retardation of the movement of potential contaminants by adsorption and tortuous flowpaths can affect the chemical and biological degradation processes. Fractured media generally produce rapid

ounty, South Dakota

flowpaths that significantly reduce residence time. In sands and gravels, and sandstones, the distance from the land surface to the water table significantly affects the residence time. Generally, karst limestones, weathered and fractured rocks, sands and gravels, and sandstones in the unsaturated zone are more sensitive. Silts, clays, and shales are less sensitive.

Hydraulic conductivity, which is a function of the interconnection of void spaces in the aquifer media, is a bulk measurement of the ability of an aquifer to transmit water. Hydraulic conductivity affects the sensitivity of an aquifer because the transport of a con-taminant usually is a function of the bulk movement of water. Generally, unconsolidated sands and gravels are more sensitive to contamination because they have high hydraulic conductivities. Unfractured igneous and metamorphic rocks and shales are less sensitive because they have very low hydraulic conductivities. Layers of silts, clays, and shales within sands and gravels and sandstones can reduce the sensitivity of sands and gravels in the saturated zone.

Criteria for Delineation

Hydrogeologic and geologic maps were used to delineate and evaluate aquifer media and unsaturated media. The 1:100,000-scale hydrogeologic-unit map of the Black Hills area (Strobel and others, 1999) was the primary map for the delineation of hydrogeologic units. The 1:100,000-scale hydrogeologic map, modi-fied from several geologic maps at various scales ranging from 1:24,000 to 1:250,000, grouped some geologic units together based on similar hydraulic properties or unit thickness. The generalizations included in the hydrogeologic map were appropriate for the ground-water sensitivity map at the same scale.

An examination of well information in the study area indicated that at least some portion of all the exposed areas delineated on the hydrogeologic map were capable of yielding usable quantities of water to wells. Therefore, the shallowest ground water at any location was assumed to occur within the unit repre-sented on the hydrogeologic-unit map (fig. 2). The thicknesses of most of the exposed hydrogeologic units range from zero near the contact with the underlying unit, to the entire thickness of the unit near the contact with the overlying unit. Because of the dipping strata, the entire unit may not always be saturated. Also, some hydrogeologic units include layers of different geologic materials; however, the hydrogeologic units were not subdivided based on this variable saturation or

material. The sensitivity of the aquifer media and the unsaturated media was rated by evaluating the charac-teristics of the entire unit. Some hydrogeologic units were subdivided by intersection with the hydrogeo-logic settings map layer.

Geologic maps at more detailed scales (Van Lieu, 1969; DeWitt and others, 1989; Lisenbee, 1991a, 1991b, 1991c, 1991d, 1991e, 1991f, 1991g; Lisenbee and Redden, 1991a, 1991b) were consulted when rating aquifer media, unsaturated media, and hydraulic conductivity. Typically, data describing the depth to water were not available or the depth to water was highly variable in a large part of the study area; there-fore, the thickness of the unsaturated media evaluated was based on a range of depths to water estimated from well log information, thickness of the geologic forma-tions, and the setting. The ranges of ratings described in Aller and others (1987) were considered in deter-mining the relative sensitivity ratings for aquifer media and unsaturated media in the study area (tables 2 and 3).

Hydraulic conductivity of the aquifer media was estimated from aquifer tests (Greene, 1993; Greene and others, 1998), ground-water flow models (Downey, 1986; Kyllonen and Peter, 1987), and ranges of hydraulic conductivity for geologic materials (Freeze and Cherry, 1979). The categories of hydraulic conductivity proposed by Aller and others (1987) were used as a guide in determining sensitivity ratings for hydraulic conductivity (table 4).

Table 2. Sensitivity ranges for types of aquifer media

[Modified from Aller and others, 1987, table 6, p. 22]

Type of aquifer mediaSensitivity

rating1

Karst limestone 9-10

Basalt 2-10

Sand and gravel 4-9

Bedded sandstone, limestone, and shale sequences

5-9

Massive limestone 4-9

Weathered metamorphic and igneous rocks

3-5

Metamorphic and igneous rocks 2-5

Massive shale 1-3

1Higher rating indicates higher sensitivity to contamination.

Sensitivity of Ground Water to Contamination 17

Designated Units

A total of 45 hydrogeologic units (table 5) were designated within the 11 hydrogeologic settings. Each hydrogeologic unit was identified by an alphabetic character beginning with capital letters and continuing with lowercase letters. The hydrogeologic units were sorted in descending order based on the relative sensi-tivity rating from most sensitive to least sensitive. The primary sort was based on unsaturated media, with a secondary sort based on aquifer media, and a tertiary sort based on hydraulic conductivity. The sort order was based on reports (U.S. Environmental Protection Agency, 1992; Rupert, 1998) that evaluate the

Table 3. Sensitivity ranges for types of unsaturated media


Type of unsaturated mediaSensitivity

rating1

Karst limestone 8-10

Basalt 2-10

Sand and gravel 6-9

Sand and gravel having significant silt and clay

4-8

Bedded sandstone, limestone, and shale 4-8

Sandstone 4-8

Metamorphic and igneous rocks 2-8

Limestone 2-7

Silt and Clay 2-6

Shale 2-51Higher rating indicates higher sensitivity to contamination.

Table 4. Sensitivity ratings for range categories of aquifer hydraulic conductivity


Hydraulic conductivity(feet per day)

Sensitivityrating1

More than 270 10

13-270 8

90-130 6

40-90 4

13-40 2

Less than 13 11Higher rating indicates higher sensitivity to contamination.


measured presence of pesticides and nitrates in relation to DRASTIC scores for each characteristic (sub-scores). A national survey of pesticides in drinking water wells (U.S. Environmental Protection Agency, 1992) found that “… higher DRASTIC subscores for impact of the vadose zone were found to reflect a greater likelihood of pesticide detections in rural and domestic wells.” These reports also noted that the DRASTIC subscores for aquifer media and hydraulic conductivity did not correlate positively with the presence of pesticides and nitrates. Aquifer media and hydraulic conductivity may be more significant in eval-uating sensitivity to highly toxic point sources of con-tamination than indicated by studies evaluating nonpoint sources of contamination. Nonpoint sources by definition are dispersed; therefore, correlation of water-quality samples with changes in aquifer media may not be evident. The DRASTIC method of rating these parameters, which relies on best professional judgment, provides a means for ranking these hydro-geologic characteristics. Relative sensitivity ratings for aquifer media ranged from 2 to 10, for unsaturated media from 2 to 10, and for hydraulic conductivity from 1 to 10.

To provide a generalized representation of the sensitivity of the 45 hydrogeologic units, three groups (fig. 8) were created based on the relative sensitivity rank. The 15 hydrogeologic units with the highest sensitivity rank (A, B, C, D, E, F, G, H, I, J, K, L, M, N, O) include those units that consist of mostly lime-stones, alluvium, unconsolidated sands and gravels, and some sandstones. The 15 hydrogeologic units with the middle sensitivity rank (P, Q, R, S, T, U, V, W, X, Y, Z, a, b, c, d) include those that consist of mostly sand-stones and fractured crystalline rocks. The 15 hydro-geologic units with the lowest sensitivity rank (e, f, g, h, i, j, k, l, m, n, o, p, q, r, s) include units consisting of mostly shales or units with interbedded shale layers. Comments in table 5 include information describing the hydrogeologic unit such as location, well yield, and permeability characteristics.

Estimates of recharge can be made with water budget equations if the uncertainty of estimates of other equation components is small. The equation, “ground-water recharge rate = precipitation rate - evapotranspi-ration rate - runoff rate - change in ground-water storage” may be simplified by neglecting components whose value approaches zero. Over a long period of time, the average change in storage may approach zero.

ounty, South Dakota

T

able

5.

Sen

sitiv

ity c

hara

cter

istic

s of

hyd

roge

olog

ic u

nits

[mi2 , s

quar

e m

iles;

ft,

feet

; gal

/min

, gal

lons

per

min

ute;

ft/

d, f

eet p

er d

ay; -

-, n

o da

ta]

Hydrogeologic unit1

Hyd

rog

eolo

gic

sett

ing

Tota

lsu

rfac

ear

ea o

fu

nit

(mi2 )

Geo

log

icu

nit

s

Su

b-s

urf

ace

thic

knes

so

fg

eolo

gic

un

it(f

t)

Un

satu

rate

d a

nd

aqu

ifer

med

ia2

Rel

ativ

e se

nsi

tivi

ty

rati

ng

Co

mm

ents

3,4

Aquifermedia

Unsaturatedmedia

Hydraulic conductivity

AIg

neou

s D

omes

22.9

Mad

ison

L

imes

tone

Eng

lew

ood

Form

atio

n

5 350-

1,00

0

5 40-7

5

Lim

esto

ne, m

ore

dolo

miti

c w

ith d

epth

. G

ener

ally

mas

sive

, ka

rstic

, and

ca

vern

ous

in u

pper

pa

rt.

Dol

omiti

c lim

esto

ne.

1010

8T

his

part

of

the

Mad

ison

Lim

esto

ne o

utcr

op is

loca

ted

acro

ss th

e ce

ntra

l pa

rt o

f th

e st

udy

area

nea

r in

trus

ive

rock

s. I

nven

tori

ed w

ell y

ield

s in

the

regi

onal

Mad

ison

aqu

ifer

are

hig

hly

vari

able

with

the

larg

est g

roup

ra

ngin

g fr

om 1

0 to

200

gal

/min

and

a s

igni

fica

nt n

umbe

r of

wel

ls

betw

een

400

and

1,75

0 ga

l/min

. Pe

rmea

bilit

y is

mai

nly

from

enh

ance

d so

lutio

n op

enin

gs, w

hich

occ

ur p

redo

min

antly

in th

e up

per

200

ft o

f th

e fo

rmat

ion,

7 and

fra

ctur

es.

Add

ition

al f

ract

urin

g is

pos

sibl

e du

e to

ig

neou

s in

trus

ive

activ

ity.

Hyd

raul

ic c

ondu

ctiv

ity

dete

rmin

ed f

rom

aq

uife

r te

sts

of w

ells

with

larg

er y

ield

s ar

e in

the

rang

e of

200

ft/d

.7,8

R

apid

mov

emen

t of

pote

ntia

l con

tam

inan

ts w

ith

little

atte

nuat

ion

thro

ugh

solu

tion

open

ings

or

frac

ture

s in

the

aqui

fer

med

ia a

nd

unsa

tura

ted

med

ia is

like

ly.

Litt

le w

ell i

nfor

mat

ion

is a

vail

able

for

the

Eng

lew

ood

Form

atio

n, a

nd h

ydro

geol

ogic

cha

ract

eris

tics

wer

e as

sum

ed

to b

e si

mila

r to

the

over

lyin

g lo

wer

Mad

ison

Lim

esto

ne.

BL

imes

tone

Pl

atea

u10

9.0

Mad

ison

L

imes

tone

Eng

lew

ood

Form

atio

n

6 450-

650

5 40-7

5

Lim

esto

ne, m

ore

dolo

miti

c w

ith d

epth

. G

ener

ally

mas

sive

, ka

rstic

, and

ca

vern

ous

in u

pper

pa

rt.

Dol

omiti

c lim

esto

ne.

1010

6T

his

part

of

the

Mad

ison

Lim

esto

ne o

utcr

op, l

ocat

ed w

ithin

the

Spea

rfis

h C

reek

dra

inag

e ba

sin,

incl

udes

a la

rge

part

of L

imes

tone

Pla

teau

set

ting.

In

vent

orie

d w

ell y

ield

s w

ithin

the

unco

nfin

ed o

utcr

op a

rea

are

less

than

20

gal

/min

. Pe

rmea

bilit

y is

fro

m e

nhan

ced

solu

tion

open

ings

, whi

ch

occu

r pr

edom

inan

tly in

the

uppe

r 20

0 ft

of

the

form

atio

n7 and

fra

ctur

es.

Gro

und-

wat

er f

low

is to

war

ds s

prin

gs d

isch

argi

ng to

Spe

arfi

sh C

reek

an

d R

apid

Cre

ek, a

nd to

the

regi

onal

Mad

ison

aqu

ifer

. R

apid

mov

emen

t of

pot

entia

l con

tam

inan

ts w

ith li

ttle

atte

nuat

ion

thro

ugh

solu

tion

open

ings

or

frac

ture

s in

the

aqui

fer

med

ia a

nd u

nsat

urat

ed m

edia

is

like

ly.

CM

ount

ain

Flan

ks21

.5M

adis

on

Lim

esto

ne

Eng

lew

ood

Form

atio

n

5 350-

1,00

0

5 40-7

5

Lim

esto

ne, m

ore

dolo

miti

c w

ith d

epth

. G

ener

ally

mas

sive

, ca

vern

ous

in u

pper

pa

rt.

Dol

omiti

c lim

esto

ne.

1010

6T

his

part

of

the

Mad

ison

Lim

esto

ne o

utcr

op is

loca

ted

in th

e so

uthe

aste

rn

and

sout

hwes

tern

par

t of

the

stud

y ar

ea, n

ear

Spea

rfis

h C

reek

, and

a

smal

l are

a in

the

east

-cen

tral

par

t of

the

stud

y ar

ea.

Gro

und-

wat

er f

low

ge

nera

lly is

rad

ially

out

war

d fr

om th

e ce

ntra

l par

t of

the

Bla

ck H

ills.

Se

nsit

ivit

y ch

arac

teri

stic

s ar

e si

mila

r to

uni

t A w

ithou

t the

add

ition

al

frac

turi

ng d

ue to

igne

ous

intr

usiv

e ac

tivity

.


DL

imes

tone

Pl

atea

u20

.3M

adis

on

Lim

esto

ne

Eng

lew

ood

Form

atio

n

6 450-

650

5 40-7

5

Lim

esto

ne, m

ore

dolo

miti

c w

ith d

epth

. G

ener

ally

mas

sive

, ka

rstic

, and

ca

vern

ous

in u

pper

pa

rt.

Dol

omit

ic li

mes

tone

.

910

4T

his

part

of

the

Mad

ison

Lim

esto

ne o

utcr

op is

loca

ted

east

of

Spea

rfis

h C

reek

. T

he y

ield

s of

the

few

inve

ntor

ied

wel

ls lo

cate

d w

ithin

the

hydr

ogeo

logi

c un

it ra

nge

from

2 to

200

gal

/min

. G

roun

d-w

ater

flo

w

dire

ctio

ns a

re r

elat

ed to

topo

grap

hy a

nd s

urfa

ce d

rain

age

patte

rns.

The

ar

ea is

not

dir

ectly

con

nect

ed to

the

regi

onal

Mad

ison

aqu

ifer

but

doe

s co

ntri

bute

bas

e fl

ow to

hea

dwat

ers

of S

pear

fish

, Whi

tew

ood,

Elk

, and

R

apid

Cre

eks.

Hyd

raul

ic c

ondu

ctiv

ity p

roba

bly

is le

ss th

an d

eter

min

ed

for

high

er p

rodu

cing

wel

ls in

the

regi

onal

aqu

ifer

due

to li

mite

d sa

tura

tion

and

eros

ion

of th

e m

ore

perm

eabl

e up

per

part

s of

the

form

atio

n. M

ovem

ent o

f po

tent

ial c

onta

min

ants

wit

h lit

tle a

ttenu

atio

n th

roug

h so

lutio

n op

enin

gs o

r fr

actu

res

in th

e aq

uife

r m

edia

and

un

satu

rate

d m

edia

is p

ossi

ble.

EM

ount

ain

Flan

ks27

.1M

inne

kaht

a L

imes

tone

9 35-5

0M

assi

ve la

min

ated

cr

ysta

lline

lim

esto

ne.

89

4T

his

part

of

the

Min

neka

hta

Lim

esto

ne o

utcr

op is

loca

ted

in a

n ea

st-w

est

dire

ctio

n ac

ross

the

nort

h-ce

ntra

l par

t of

the

stud

y ar

ea.

Mos

t in

vent

orie

d w

ell y

ield

s ra

nge

from

2 to

200

gal

/min

with

som

e w

ell

yiel

ds a

s m

uch

as 5

00 g

al/m

in.

Perm

eabi

lity

pri

mar

ily is

fro

m f

ract

ures

w

ith s

ome

solu

tion

open

ings

. E

stim

ates

of

hydr

aulic

con

duct

ivity

fro

m

prod

uctio

n w

ell d

ata

rang

e fr

om le

ss th

an 1

0 to

60

ft/d

. V

ertic

al

mov

emen

t in

the

unsa

tura

ted

zone

is r

apid

with

littl

e at

tenu

atio

n of

po

tent

ial c

onta

min

ants

.

FA

ltern

atin

g Sa

ndst

one,

L

imes

tone

, an

d Sh

ale

.6M

inne

kaht

a L

imes

tone

1035

-50

Mas

sive

lam

inat

ed

crys

talli

ne li

mes

tone

.8

94

Thi

s pa

rt o

f the

Min

neka

hta

Lim

esto

ne o

utcr

op is

exp

osed

in a

nar

row

ban

d ar

ound

Elk

horn

Pea

k.

GA

lluvi

al

Mou

ntai

n V

alle

ys-E

ast

4.9

Allu

vium

9 0-50

Cla

y, s

ilt, s

and

and

grav

el.

79

10T

his

allu

vium

is lo

cate

d in

the

sout

hern

par

t of

the

stud

y ar

ea w

ithin

m

ount

ain

slop

es-e

ast.

Par

ticle

siz

e pr

obab

ly is

larg

er th

an m

ost a

lluvi

um

beca

use

of h

igh

stre

am v

eloc

ities

. Si

mila

r al

luvi

um is

like

ly a

long

all

stre

ams

in th

is s

ettin

g al

thou

gh th

e ar

ea m

ay n

ot b

e sh

own

due

to s

ize

and

map

-sca

le li

mita

tions

. Sa

tura

tion

is v

aria

ble

depe

ndin

g on

st

ream

flow

con

ditio

ns.

HA

lluvi

al

Mou

ntai

n V

alle

ys-N

orth

0.4

Allu

vium

9 0-50

Cla

y, s

ilt, s

and

and

grav

el.

79

10T

his

allu

vium

is lo

cate

d in

the

cent

ral p

art o

f th

e st

udy

area

with

in

Mou

ntai

n Sl

opes

-Nor

th.

Sens

itivi

ty c

hara

cter

istic

s ar

e si

mila

r to

uni

t G.

Tab

le 5

. S

ensi

tivity

cha

ract

eris

tics

of h

ydro

geol

ogic

uni

ts–C

ontin

ued

[mi2 , s

quar

e m

iles;

ft,

feet

; gal

/min

, gal

lons

per

min

ute;

ft/d

, fee

t per

day

; --,

no

data

]Hydrogeologic unit1

Hyd

rog

eolo

gic

sett

ing

Tota

lsu

rfac

ear

ea o

fu

nit

(mi2 )

Geo

log

icu

nit

s

Su

b-s

urf

ace

thic

knes

so

fg

eolo

gic

un

it(f

t)

Un

satu

rate

d a

nd

aqu

ifer

med

ia2

Rel

ativ

e se

nsi

tivi

ty

rati

ng

Co

mm

ents

3,4

Aquifermedia

Unsaturatedmedia



IA

lluvi

al

Mou

ntai

n V

alle

ys

5.0

Allu

vium

9 0-50

Cla

y, s

ilt, s

and

and

grav

el.

78

10T

his

allu

vium

is lo

cate

d ar

ound

the

peri

met

er o

f th

e B

lack

Hill

s up

lift i

n th

e so

uthe

rn a

nd c

entr

al p

art o

f th

e st

udy

area

with

in th

e Ig

neou

s D

omes

an

d M

ount

ain

Flan

ks s

ettin

gs.

Sim

ilar

allu

vium

is li

kely

alo

ng s

trea

ms

in th

is s

ettin

g al

thou

gh th

e ar

ea m

ay n

ot b

e sh

own

due

to s

ize

and

map

-sc

ale

limita

tions

. Sa

tura

tion

is v

aria

ble

depe

ndin

g on

str

eam

flow

co

nditi

ons.

JR

iver

Allu

vium

w

ithou

t O

verb

ank

Dep

osits

38.9

Allu

vium

9 0-50

Cla

y, s

ilt, s

and

and

grav

el.

77

10T

his

allu

vium

is lo

cate

d al

ong

stre

ams

in th

e no

rthe

rn p

art o

f the

stu

dy a

rea

with

gen

tler t

opog

raph

ic re

lief t

han

in th

e m

ount

ain

setti

ngs.

Inv

ento

ried

w

ell y

ield

s ra

nge

from

10

to 6

0 ga

l/min

. S

atur

atio

n is

mor

e co

nsis

tent

al

ong

the

pere

nnia

l str

eam

s.

KU

ncon

solid

ated

an

d Se

mi-

Con

solid

ated

D

epos

its

26.1

Gra

vel

depo

sits

9 0-60

Cla

y, s

ilt, s

and

and

grav

el.

75

6T

hese

dep

osits

are

loca

l aqu

ifer

s w

here

sat

urat

ed a

nd g

ener

ally

con

sist

of

terr

aces

and

for

mer

flo

od p

lain

s lo

cate

d ad

jace

nt to

rec

ent a

lluvi

al

depo

sits

. In

vent

orie

d w

ell y

ield

s ra

nge

from

10

to 1

00 g

al/m

in.

Sa

tura

tion

of th

e un

it va

ries

wit

h th

e al

titud

e of

the

unit

in r

elat

ion

to

adja

cent

allu

vium

and

hyd

rolo

gic

cond

ition

s.

LM

ount

ain

Flan

ks84

.6M

inne

lusa

Fo

rmat

ion

5 350-

650

Cro

ss s

trat

ifie

d sa

ndst

one,

lim

esto

ne,

dolo

mite

, and

sha

le.

Anh

ydri

te a

t dep

th.

Low

er p

art o

f th

e fo

rmat

ion

is

inte

rbed

ded

limes

tone

and

sha

le.

75

4T

his

part

of

the

Min

nelu

sa F

orm

atio

n ou

tcro

p is

loca

ted

east

to w

est a

cros

s th

e ce

ntra

l par

t of

the

stud

y ar

ea a

nd n

orth

to s

outh

alo

ng th

e so

uthw

este

rn p

art o

f th

e st

udy

area

. In

vent

orie

d w

ell y

ield

s ar

e hi

ghly

va

riab

le w

ith th

e la

rges

t gro

up r

angi

ng f

rom

10

to 2

00 g

al/m

in, a

co

nsid

erab

le n

umbe

r be

twee

n 20

0 an

d 70

0 ga

l/min

, and

a f

ew w

ells

as

muc

h as

1,7

00 g

al/m

in.

Perm

eabi

lity

is f

rom

the

prim

ary

poro

sity

of

sand

ston

e la

yers

. Se

cond

ary

poro

sity

is f

rom

fra

ctur

es a

nd d

isso

lved

an

hydr

ite c

olla

pse

feat

ures

. T

he lo

wer

par

t of

the

form

atio

n, w

ith

inte

rbed

ded

shal

es, g

ener

ally

is c

onsi

dere

d a

conf

inin

g la

yer7

with

hi

ghly

var

iabl

e le

akag

e to

and

fro

m th

e M

adis

on L

imes

tone

due

to

frac

ture

s an

d br

ecci

atio

n. H

ydra

ulic

con

duct

ivity

is v

aria

ble

with

val

ues

of a

bout

50

ft/d

det

erm

ined

fro

m a

quif

er te

sts

at p

ublic

sup

ply

wel

ls.8

Som

e at

tenu

atio

n of

pot

entia

l con

tam

inan

ts is

pos

sibl

e w

ith

mov

emen

t th

roug

h sa

ndst

one;

how

ever

, con

side

rabl

e va

riab

ility

is p

roba

ble

due

to

frac

ture

s an

d co

llaps

e fe

atur

es.

Tab

le 5

. S

ensi

tivity

cha

ract

eris

tics

of h

ydro

geol

ogic

uni

ts–C

ontin

ued

[mi2 , s

quar

e m

iles;

ft,

feet

; gal

/min

, gal

lons

per

min

ute;

ft/

d, f

eet p

er d

ay; -

-, n

o da

ta]

Hydrogeologic unit1

Hyd

rog

eolo

gic

sett

ing

Tota

lsu

rfac

ear

ea o

fu

nit

(mi2 )

Geo

log

icu

nit

s

Su

b-s

urf

ace

thic

knes

so

fg

eolo

gic

un

it(f

t)

Un

satu

rate

d a

nd

aqu

ifer

med

ia2

Rel

ativ

e se

nsi

tivi

ty

rati

ng

Co

mm

ents

3,4

Aquifermedia

Unsaturatedmedia



MA

ltern

atin

g Sa

ndst

one,

L

imes

tone

, an

d Sh

ale

1.2

Min

nelu

sa

Form

atio

n

5 350-

650

Cro

ss s

trat

ifie

d sa

nd-

ston

e, li

mes

tone

, do

lom

ite, a

nd s

hale

. A

nhyd

rite

at d

epth

. L

ower

par

t of

the

for -

mat

ion

is in

terb

edde

d li

mes

tone

and

sha

le.

75

2T

his

part

of

the

Min

nelu

sa F

orm

atio

n ou

tcro

p is

loca

ted

in th

e no

rthe

aste

rn

part

of

the

stud

y ar

ea.

Ero

sion

of

the

mor

e pe

rmea

ble

uppe

r pa

rts

of th

e fo

rmat

ion

prob

ably

dec

reas

es th

e hy

drau

lic c

ondu

ctiv

ity a

nd v

ertic

al

mov

emen

t of

a po

tent

ial c

onta

min

ant i

n th

e un

satu

rate

d zo

ne c

ompa

red

to u

nit L

.

NIg

neou

s D

omes

28.9

Dea

dwoo

d

Form

atio

n

1030

0-50

0Sa

ndst

one,

sha

le,

limes

tone

, and

loca

l co

nglo

mer

ate

at th

e ba

se.

75

1T

his

part

of

the

Dea

dwoo

d Fo

rmat

ion

outc

rop

is lo

cate

d ea

st to

wes

t acr

oss

the

cent

ral p

art o

f th

e st

udy

area

. In

vent

orie

d w

ell y

ield

s ar

e hi

ghly

va

riab

le w

ith th

e la

rges

t gro

up le

ss th

an 5

0 ga

l/min

, sev

eral

bet

wee

n 50

an

d 30

0 ga

l/min

, and

a f

ew w

ells

as

muc

h as

1,1

00 g

al/m

in.

Wel

ls

gene

rally

are

loca

ted

near

to o

r on

the

outc

rop.

Per

mea

bilit

y is

fro

m th

e pr

imar

y po

rosi

ty o

f sa

ndst

one

laye

rs a

nd s

econ

dary

por

osity

fro

m

frac

ture

s. T

his

part

of

the

outc

rop

is n

ear

intr

usiv

e ig

neou

s bo

dies

and

of

ten

incl

udes

sill

s, w

hich

incr

ease

the

frac

turi

ng.

The

gro

und-

wat

er

flow

pat

tern

s ar

e ve

ry c

ompl

ex b

ecau

se o

f th

e in

trus

ive

bodi

es a

nd

incl

ude

a m

ixtu

re o

f lo

cal a

nd r

egio

nal g

roun

d-w

ater

flo

w s

yste

ms.

M

ovem

ents

of

pote

ntia

l con

tam

inan

ts c

ould

be

high

ly v

aria

ble

beca

use

of th

e co

mpl

ex f

ract

urin

g pa

ttern

s.

OL

imes

tone

Pl

atea

u13

.3D

eadw

ood

Fo

rmat

ion

1030

0-50

0Sa

ndst

one,

sha

le,

limes

tone

, and

loca

l co

nglo

mer

ate

at th

e ba

se.

65

1T

his

part

of

the

Dea

dwoo

d Fo

rmat

ion

outc

rop

is lo

cate

d al

ong

Spea

rfis

h C

reek

and

the

head

wat

ers

of E

lk, B

oxel

der,

and

Rap

id C

reek

s. G

roun

d w

ater

cou

ld b

e cl

ose

to th

e su

rfac

e in

par

ts o

f th

is a

rea

beca

use

of th

e in

tera

ctio

n be

twee

n th

e st

ream

s an

d th

e D

eadw

ood

aqui

fer.

Man

y of

the

smal

ler

trib

utar

ies

flow

ing

acro

ss th

is a

rea

have

per

enni

al f

low

bec

ause

of

spr

ings

dis

char

ging

fro

m th

e M

adis

on o

r D

eadw

ood

aqui

fers

.

PIg

neou

s D

omes

.5M

inne

lusa

Fo

rmat

ion

6 400-

550

Cro

ss s

trat

ifie

d sa

ndst

one,

lim

esto

ne,

dolo

mite

, and

sha

le.

Anh

ydri

te w

ith

dept

h. L

ower

par

t of

the

form

atio

n is

in

terb

edde

d li

mes

tone

and

sha

le.

55

1T

hese

sm

all a

reas

of

Min

nelu

sa F

orm

atio

n ou

tcro

p, lo

cate

d on

the

east

ern

and

wes

tern

edg

es o

f th

e ce

ntra

l par

t of

the

stud

y ar

ea, a

re is

olat

ed f

rom

th

e re

gion

al M

inne

lusa

aqu

ifer

. E

rosi

on o

f th

e m

ore

perm

eabl

e up

per

part

of

the

form

atio

n pr

obab

ly d

ecre

ases

hyd

raul

ic c

ondu

ctiv

ity a

nd th

e m

ovem

ent o

f po

tent

ial c

onta

min

ants

in th

e un

satu

rate

d zo

ne.

No

inve

ntor

ied

wel

l inf

orm

atio

n is

ava

ilabl

e fo

r the

uni

t and

sat

urat

ion

coul

d be

lim

ited

beca

use

of d

isch

arge

to th

e su

rrou

ndin

g M

adis

on a

quif

er,

whi

ch h

as a

low

er a

ltitu

de.

Tab

le 5

. S

ensi

tivity

cha

ract

eris

tics

of h

ydro

geol

ogic

uni

ts–C

ontin

ued

[mi2 , s

quar

e m

iles;

ft,

feet

; gal

/min

, gal

lons

per

min

ute;

ft/d

, fee

t per

day

; --,

no

data


Hyd

rog

eolo

gic

sett

ing

Tota

lsu

rfac

ear

ea o

fu

nit

(mi2 )

Geo

log

icu

nit

s

Su

b-s

urf

ace

thic

knes

so

fg

eolo

gic

un

it(f

t)

Un

satu

rate

d a

nd

aqu

ifer

med

ia2

Rel

ativ

e se

nsi

tivi

ty

rati

ng

Co

mm

ents

3,4

Aquifermedia

Unsaturatedmedia



QL

imes

tone

Pl

atea

u0.

4M

inne

lusa

Fo

rmat

ion

6 450-

550

Cro

ss s

trat

ifie

d sa

nd-

ston

e, li

mes

tone

, do

lom

ite, a

nd s

hale

. A

nhyd

rite

with

de

pth.

Low

er p

art o

f th

e fo

rmat

ion

is

inte

rbed

ded

lim

e-st

one

and

shal

e.

55

1T

his

smal

l are

a of

Min

nelu

sa F

orm

atio

n ou

tcro

p, lo

cate

d w

ithin

the

Lim

esto

ne P

late

au, i

s is

olat

ed f

rom

the

regi

onal

Min

nelu

sa a

quif

er.

Ero

sion

of

the

mor

e pe

rmea

ble

uppe

r pa

rt o

f th

e fo

rmat

ion

prob

ably

de

crea

ses

hydr

aulic

con

duct

ivity

and

the

sens

itivi

ty c

hara

cter

istic

s of

the

unsa

tura

ted

zone

. N

o in

vent

orie

d w

ell i

nfor

mat

ion

is a

vaila

ble

for

the

unit

and

satu

ratio

n co

uld

be li

mite

d be

caus

e of

dis

char

ge to

the

surr

ound

ing

Mad

ison

aqu

ifer

, whi

ch h

as a

low

er a

ltitu

de.

RM

ount

ain

Flan

ks15

.1D

eadw

ood

Fo

rmat

ion

1030

0-50

0Sa

ndst

one,

sha

le,

limes

tone

, sha

le, a

nd

loca

l con

glom

erat

e at

th

e ba

se.

55

1T

his

part

of t

he D

eadw

ood

Form

atio

n ou

tcro

p is

loca

ted

in th

e so

uthe

aste

rn

part

of

the

stud

y ar

ea, a

nd a

sm

all a

rea

is lo

cate

d ne

ar S

pear

fish

Cre

ek.

Perm

eabi

lity

is f

rom

the

prim

ary

poro

sity

of

sand

ston

e la

yers

and

se

cond

ary

poro

sity

fro

m f

ract

ures

. T

he g

ener

al d

irec

tion

of g

roun

d-w

ater

flo

w in

the

sout

heas

tern

are

a is

to th

e ea

st, a

nd a

vera

ge

prec

ipita

tion

is le

ss th

an o

n th

e ot

her

Dea

dwoo

d Fo

rmat

ion

outc

rops

.

SM

ount

ain

Slop

es-E

ast

2.6

Dea

dwoo

d

Form

atio

n

1030

0-50

0Sa

ndst

one,

sha

le,

limes

tone

, sha

le, a

nd

loca

l con

glom

erat

e at

th

e ba

se.

55

1T

his

part

of

the

Dea

dwoo

d Fo

rmat

ion

outc

rop

is lo

cate

d w

ithin

the

Prec

ambr

ian

rock

s in

the

sout

hern

par

t of

the

stud

y ar

ea.

Perm

eabi

lity

is

from

the

prim

ary

poro

sity

of

sand

ston

e la

yers

and

sec

onda

ry p

oros

ity

from

fra

ctur

es.

Som

e of

the

area

s m

ay b

e is

olat

ed f

rom

the

regi

onal

D

eadw

ood

aqui

fer

with

gro

und-

wat

er f

low

pat

tern

s de

term

ined

by

loca

l to

pogr

aphy

and

hyd

raul

ic c

onne

ctio

n to

und

erly

ing

rock

s.

TM

ount

ain

Slop

es-E

ast

125.

1Ig

neou

s an

d m

etam

or-

phic

roc

ks

--Sh

ist,

slat

e, q

uart

zite

, am

phib

olite

, dio

rite

, gr

anite

, and

pe

gmat

ite.

45

1T

his

hydr

ogeo

logi

c un

it, lo

cate

d in

the

sout

heas

tern

and

sou

th c

entr

al p

art

of th

e st

udy

area

is a

loca

l aqu

ifer

whe

re s

atur

ated

. W

ells

are

loca

ted

on

or v

ery

clos

e to

the

outc

rop.

Mos

t inv

ento

ried

wel

l yie

lds

rang

e fr

om 5

to

20

gal/m

in w

ith s

ome

wel

l yie

lds

up to

60

gal/m

in.

The

dep

th o

f wel

ls

gene

rally

are

less

than

200

ft.

Per

mea

bilit

y is

fro

m f

ract

urin

g th

at

decr

ease

s w

ith d

epth

. G

roun

d-w

ater

flo

w p

atte

rns

are

com

plex

and

va

riab

le.

UM

ount

ain

Slop

es-N

orth

22.0

Igne

ous

and

met

amor

- ph

ic r

ocks

--Sh

ist,

slat

e, q

uart

zite

, am

phib

olite

, dio

rite

, gr

anite

, and

pe

gmat

ite.

45

1T

his

hydr

ogeo

logi

c un

it, lo

cate

d in

the

cent

ral p

art o

f th

e st

udy

area

, is

a lo

cal a

quif

er w

here

sat

urat

ed.

Wel

ls a

re lo

cate

d on

or

very

clo

se to

the

outc

rop.

Mos

t inv

ento

ried

wel

l yie

lds

are

less

than

20

gal/m

in w

ith o

ne

repo

rted

yie

ld o

f 60

gal

/min

. T

his

wel

l was

in a

n ar

ea w

here

num

erou

s si

lls h

ave

pene

trat

ed th

e Pr

ecam

bria

n ro

cks.

Per

mea

bilit

y is

fro

m

frac

turi

ng th

at d

ecre

ases

with

dep

th.

Gro

und-

wat

er f

low

pat

tern

s ar

e co

mpl

ex a

nd v

aria

ble.

Tab

le 5

. S

ensi

tivity

cha

ract

eris

tics

of h

ydro

geol

ogic

uni

ts–C

ontin

ued

[mi2 , s

quar

e m

iles;

ft,

feet

; gal

/min

, gal

lons

per

min

ute;

ft/

d, f

eet p

er d

ay; -

-, n

o da

ta]

Hydrogeologic unit1

Hyd

rog

eolo

gic

sett

ing

Tota

lsu

rfac

ear

ea o

fu

nit

(mi2 )

Geo

log

icu

nit

s

Su

b-s

urf

ace

thic

knes

so

fg

eolo

gic

un

it(f

t)

Un

satu

rate

d a

nd

aqu

ifer

med

ia2

Rel

ativ

e se

nsi

tivi

ty

rati

ng

Co

mm

ents

3,4

Aquifermedia

Unsaturatedmedia



VM

ount

ain

Flan

ks1.

3Ig

neou

s an

d m

etam

or-

phic

roc

ks

--Sh

ist,

slat

e, q

uart

zite

, am

phib

olite

, dio

rite

, gr

anit

e, a

nd

pegm

atite

.

45

1T

his

hydr

ogeo

logi

c un

it, lo

cate

d in

the

sout

heas

tern

par

t of

the

stud

y ar

ea

and

surr

ound

ed b

y th

e D

eadw

ood

Form

atio

n, is

a lo

cal a

quif

er w

here

sa

tura

ted.

An

inve

ntor

ied

wel

l yie

ld w

as 5

gal

/min

. Pe

rmea

bilit

y is

from

fr

actu

ring

that

dec

reas

es w

ith d

epth

. G

roun

d-w

ater

flo

w p

atte

rns

are

com

plex

and

var

iabl

e.

WM

ount

ain

Slop

es-E

ast

2.0

Col

luvi

um9 0-

50Ta

lus

and

othe

r de

bris

as

soci

ated

with

mas

s w

astin

g.

64

4T

his

hydr

ogeo

logi

c un

it, lo

cate

d w

ithin

the

Prec

ambr

ian

rock

s in

the

sout

hern

par

t of

the

stud

y ar

ea, c

ould

sup

ply

limite

d qu

antit

ies

of w

ater

w

here

sat

urat

ed.

No

info

rmat

ion

is a

vaila

ble

to d

escr

ibe

wel

l yie

lds

or

hydr

aulic

con

duct

ivity

. C

onsi

dera

ble

vari

atio

n in

sen

sitiv

ity

char

acte

rist

ics

is li

kely

dep

endi

ng o

n th

e pa

rtic

le s

ize

and

sort

ing

of th

e de

posi

ts.

XIg

neou

s D

omes

.6C

ollu

vium

9 0-50

Talu

s an

d ot

her

debr

is

asso

ciat

ed w

ith m

ass

was

ting.

64

4T

his

hydr

ogeo

logi

c un

it, lo

cate

d on

the

slop

es o

f ig

neou

s do

mes

in th

e w

este

rn p

art o

f th

e st

udy

area

, pro

babl

y w

ould

hav

e lim

ited

satu

ratio

n be

caus

e of

the

land

-sur

face

slo

pe.

YM

ount

ain

Flan

ks.3

Col

luvi

um9 0-

50Ta

lus

and

othe

r de

bris

as

soci

ated

with

mas

s w

astin

g.

64

4T

his

hydr

ogeo

logi

c un

it, lo

cate

d in

the

east

-cen

tral

par

t of

the

stud

y ar

ea,

coul

d su

pply

lim

ited

quan

titie

s of

wat

er w

here

sat

urat

ed.

No

info

rmat

ion

is a

vaila

ble

to d

escr

ibe

wel

l yie

lds

or h

ydra

ulic

con

duct

ivity

. C

onsi

dera

ble

vari

atio

n in

sen

sitiv

ity c

hara

cter

isti

cs is

like

ly d

epen

ding

on

the

part

icle

siz

e an

d so

rtin

g of

the

depo

sits

.

ZA

ltern

atin

g Sa

ndst

one,

L

imes

tone

, an

d Sh

ale

35.6

Inya

n K

ara

Gro

up:

Fall

Riv

er

Form

atio

n an

d L

akot

a Fo

rmat

ion

6 10-2

00

6 30-3

00

Sand

ston

e an

d ot

her

clas

tic r

ocks

.6

41

The

Iny

an K

ara

Gro

up o

utcr

op is

loca

ted

in th

e no

rthe

aste

rn p

art o

f th

e st

udy

area

. In

vent

orie

d w

ell y

ield

s in

the

Inya

n K

ara

aqui

fer

mos

tly a

re

less

than

50

gal/m

in, w

ith a

few

wel

ls b

etw

een

50 a

nd 2

00 g

al/m

in.

The

m

ajor

ity

of p

erm

eabi

lity

is f

rom

the

prim

ary

poro

sity

of

the

sand

ston

e.

aIg

neou

s D

omes

52.6

Tert

iary

in

trus

ive

igne

ous

rock

s

--U

ndif

fere

ntia

ted

intr

usiv

e ig

neou

s ro

cks

incl

udin

g rh

yolit

e, la

tite,

tr

achy

te, a

nd

phon

olite

.

44

1T

his

hydr

ogeo

logi

c un

it, lo

cate

d in

the

cent

ral p

art o

f th

e st

udy

area

, in

clud

es m

ost o

f th

e ce

nter

s of

igne

ous

activ

ity in

the

nort

hern

Bla

ck

Hill

s in

Sou

th D

akot

a.11

Inv

ento

ried

wel

l yie

lds

com

plet

ed in

intr

usiv

e ro

cks

gene

rally

are

less

than

20

gal/

min

. W

ell y

ield

s an

d hy

drau

lic

cond

uctiv

ity in

the

mas

sive

dom

es p

roba

bly

wou

ld b

e lo

w.

Lar

ger

wel

l yi

elds

cou

ld b

e ex

pect

ed w

here

the

outc

rop

is e

xpos

ed a

s sh

allo

w s

ills

intr

udin

g th

e D

eadw

ood

Form

atio

n. T

he li

thol

ogy

for

seve

ral o

f th

e w

ells

in th

is a

rea

that

are

iden

tifie

d as

Dea

dwoo

d aq

uife

r w

ells

incl

ude

laye

rs o

f in

trus

ive

rock

s. G

roun

d-w

ater

flo

w p

atte

rns

are

com

plex

and

va

riab

le.

Tab

le 5

. S

ensi

tivity

cha

ract

eris

tics

of h

ydro

geol

ogic

uni

ts–C

ontin

ued

[mi2 , s

quar

e m

iles;

ft,

feet

; gal

/min

, gal

lons

per

min

ute;

ft/d

, fee

t per

day

; --,

no

data


Hyd

rog

eolo

gic

sett

ing

Tota

lsu

rfac

ear

ea o

fu

nit

(mi2 )

Geo

log

icu

nit

s

Su

b-s

urf

ace

thic

knes

so

fg

eolo

gic

un

it(f

t)

Un

satu

rate

d a

nd

aqu

ifer

med

ia2

Rel

ativ

e se

nsi

tivi

ty

rati

ng

Co

mm

ents

3,4

Aquifermedia

Unsaturatedmedia



bM

ount

ain

Slop

es-E

ast

.7Te

rtia

ry

intr

usiv

e ig

neou

s ro

cks

--U

ndif

fere

ntia

ted

intr

usiv

e ig

neou

s ro

cks

incl

udin

g rh

yoli

te, l

atite

, tr

achy

te, a

nd

phon

olite

.

44

1T

hese

two

hydr

ogeo

logi

c un

its a

re lo

cate

d w

ithin

the

Pre

cam

bria

n ro

cks

in

the

sout

h-ce

ntra

l par

t of t

he s

tudy

are

a. N

o w

ell i

nven

tory

info

rmat

ion

is

avai

labl

e.

cM

ount

ain

Slop

es-N

orth

3.0

Tert

iary

in

trus

ive

igne

ous

rock

s

--U

ndif

fere

ntia

ted

intr

usiv

e ig

neou

s ro

cks

incl

udin

g rh

yoli

te, l

atite

, tr

achy

te, a

nd

phon

olite

.

44

1T

his

hydr

ogeo

logi

c un

it is

loca

ted

with

in th

e Pr

ecam

bria

n ro

cks

in th

e ce

ntra

l par

t of

the

stud

y ar

ea.

Lar

ger

wel

l yie

lds

gene

rally

are

fou

nd

adja

cent

to P

reca

mbr

ian

rock

s in

trud

ed w

ith s

hallo

wly

pla

ced

sills

, la

ccol

iths,

dik

es, a

nd p

lugs

. G

roun

d-w

ater

flo

w p

atte

rns

are

com

plex

an

d va

riab

le.

dIg

neou

s D

omes

2.8

Whi

tew

ood

Form

atio

n

Win

nipe

g Fo

rmat

ion

9 0-15

0

9 0-11

0

Lim

esto

ne a

nd

dolo

mite

.

Shal

e w

ith in

terb

edde

d si

ltsto

ne.

44

1T

his

hydr

ogeo

logi

c un

it is

loca

ted

in n

arro

w b

ands

adj

acen

t to

Dea

dwoo

d Fo

rmat

ion

outc

rops

and

intr

usiv

e ro

cks

in th

e ea

st- t

o w

est-

cent

ral p

art o

f th

e st

udy

area

. T

he c

ombi

ned

Whi

tew

ood

Form

atio

n an

d W

inni

peg

Form

atio

n is

con

side

red

a se

mic

onfi

ning

uni

t in

the

stud

y ar

ea.

No

wel

l in

vent

ory

info

rmat

ion

is a

vaila

ble

for

the

unit.

The

ove

rlyi

ng M

adis

on

aqui

fer

or th

e un

derl

ying

Dea

dwoo

d aq

uife

r w

ould

be

mor

e lik

ely

sour

ces

of w

ater

. The

Whi

tew

ood

Form

atio

n ex

tend

s as

far

sou

th a

s th

e in

ters

ectio

n of

Spe

arfi

sh C

reek

and

Hig

hway

85

with

onl

y fe

w p

lace

s w

here

the

Win

nipe

g Fo

rmat

ion

exte

nds

fart

her

sout

h.12

Perm

eabi

lity

from

fra

ctur

ed li

mes

tone

, dol

omite

, and

silt

ston

e co

uld

prod

uce

smal

l qu

antit

ies

of w

ater

.

eM

ount

ain

Flan

ks.3

Whi

tew

ood

Form

atio

n

Win

nipe

g Fo

rmat

ion

9 0-15

0

9 0-11

0

Lim

esto

ne a

nd

dolo

mite

.

Shal

e w

ith in

terb

edde

d si

ltsto

ne.

44

1T

his

hydr

ogeo

logi

c un

it is

loca

ted

in th

e pa

rt o

f th

e Sp

earf

ish

Cre

ek

drai

nage

bas

in ju

st a

bove

the

City

of

Spea

rfis

h. T

he u

nit i

s ex

pose

d al

ong

Spea

rfis

h C

reek

and

its

trib

utar

ies.

Tab

le 5

. S

ensi

tivity

cha

ract

eris

tics

of h

ydro

geol

ogic

uni

ts–C

ontin

ued

[mi2 , s

quar

e m

iles;

ft,

feet

; gal

/min

, gal

lons

per

min

ute;

ft/

d, f

eet p

er d

ay; -

-, n

o da

ta]

Hydrogeologic unit1

Hyd

rog

eolo

gic

sett

ing

Tota

lsu

rfac

ear

ea o

fu

nit

(mi2 )

Geo

log

icu

nit

s

Su

b-s

urf

ace

thic

knes

so

fg

eolo

gic

un

it(f

t)

Un

satu

rate

d a

nd

aqu

ifer

med

ia2

Rel

ativ

e se

nsi

tivi

ty

rati

ng

Co

mm

ents

3,4

Aquifermedia

Unsaturatedmedia



fL

imes

tone

Pl

atea

u8.

1W

hite

woo

d Fo

rmat

ion

Win

nipe

g Fo

rmat

ion

9 0-15

0

9 0-11

0

Lim

esto

ne a

nd

dolo

mite

.

Shal

e w

ith in

terb

edde

d si

ltsto

ne.

44

1T

his

hydr

ogeo

logi

c un

it is

loca

ted

in th

e up

per

part

of

the

Spea

rfis

h C

reek

dr

aina

ge b

asin

and

the

head

wat

ers

of E

lk, B

oxel

der,

and

Rap

id C

reek

in

the

sout

hern

and

cen

tral

par

ts o

f th

e st

udy

area

. T

he p

art o

f th

e un

it ex

pose

d al

ong

Spea

rfis

h C

reek

and

its

trib

utar

ies

is a

nar

row

ban

d lo

cate

d pa

rt w

ay u

p th

e ca

nyon

wal

l abo

ve th

e D

eadw

ood

Form

atio

n ou

tcro

p, w

hich

is e

xpos

ed a

t the

bot

tom

of

the

cany

on.

Mos

t of

thes

e ar

eas

have

ver

y st

eep

slop

es c

ausi

ng m

ost p

reci

pita

tion

that

fal

ls o

n th

e ou

tcro

p to

run

off

.

gM

ount

ain

Flan

ks2.

2W

hite

woo

d Fo

rmat

ion

Win

nipe

g Fo

rmat

ion

9 0-15

0

9 0-11

0

Lim

esto

ne a

nd

dolo

mite

.

Shal

e w

ith in

terb

edde

d si

ltsto

ne.

44

1T

his

hydr

ogeo

logi

c un

it, lo

cate

d in

the

sout

heas

tern

cor

ner

of th

e st

udy

area

, dip

s to

the

east

, and

the

gene

ral d

irec

tion

of g

roun

d-w

ater

flow

is to

th

e ea

st.

hA

ltern

atin

g Sa

ndst

one,

L

imes

tone

, an

d Sh

ale

16.5

Mor

riso

n Fo

rmat

ion

Unk

papa

Sa

ndst

one

Sund

ance

Fo

rmat

ion

Gyp

sum

Sp

ring

Fo

rmat

ion

9 0-15

0

9 0-27

5

9 250-

475

9 0-12

5

Inte

rbed

ded

shal

e,

sand

ston

e, a

nd

gyps

um

53

1T

his

hydr

ogeo

logi

c un

it, lo

cate

d in

the

nort

heas

tern

par

t of

the

stud

y ar

ea,

is a

sem

icon

fini

ng u

nit w

ith p

arts

of

the

unit

used

as

an a

quif

er lo

cally

. R

epor

ted

wel

l yie

lds,

pri

mar

ily f

rom

wel

ls c

ompl

eted

in th

e Su

ndan

ce

Form

atio

n, r

ange

fro

m 5

to 7

0 ga

l/min

. Pe

rmea

bili

ty m

ostl

y is

fro

m

sand

ston

e la

yers

with

in th

e un

it.

iM

ount

ain

Slop

es-E

ast

.8W

hite

Riv

er

Gro

up

100-

150

Sand

ston

e, c

lays

tone

, an

d si

ltsto

ne w

ith

chan

nel f

illin

gs a

nd

limes

tone

lens

es.

33

1T

his

part

of

the

Whi

te R

iver

Gro

up o

utcr

op, l

ocat

ed in

the

east

-cen

tral

par

t of

the

stud

y ar

ea, i

s a

smal

l are

a w

ithin

Pre

cam

bria

n ro

cks

that

has

not

be

en r

emov

ed b

y er

osio

n. N

o w

ell i

nven

tory

info

rmat

ion

is a

vaila

ble.

Pe

rmea

bilit

y m

ostly

is f

rom

san

d an

d gr

avel

lens

es a

nd f

ract

ured

lim

esto

ne.

Ben

toni

te w

ithin

vol

cani

c as

h de

posi

ts c

ould

lim

it ve

rtic

al

mov

emen

t of

cont

amin

ants

.

jM

ount

ain

Slop

es-N

orth

.2W

hite

Riv

er

Gro

up

100-

150

Sand

ston

e, c

lays

tone

, an

d si

ltsto

ne w

ith

chan

nel f

illin

gs a

nd

limes

tone

lens

es.

33

1T

his

part

of

the

Whi

te R

iver

Gro

up o

utcr

op, l

ocat

ed in

the

cent

ral p

art o

f th

e st

udy

area

is a

sm

all a

rea

in a

val

ley

of P

reca

mbr

ian

rock

s.

Tab

le 5

. S

ensi

tivity

cha

ract

eris

tics

of h

ydro

geol

ogic

uni

ts–C

ontin

ued

[mi2 , s

quar

e m

iles;

ft,

feet

; gal

/min

, gal

lons

per

min

ute;

ft/d

, fee

t per

day

; --,

no

data


Hyd

rog

eolo

gic

sett

ing

Tota

lsu

rfac

ear

ea o

fu

nit

(mi2 )

Geo

log

icu

nit

s

Su

b-s

urf

ace

thic

knes

so

fg

eolo

gic

un

it(f

t)

Un

satu

rate

d a

nd

aqu

ifer

med

ia2

Rel

ativ

e se

nsi

tivi

ty

rati

ng

Co

mm

ents

3,4

Aquifermedia

Unsaturatedmedia



kIg

neou

s D

omes

3.3

Whi

te R

iver

G

roup

100-

150

Sand

ston

e, c

lays

tone

, an

d si

ltst

one

wit

h ch

anne

l fill

ings

and

lim

esto

ne le

nses

.

33

1T

his

part

of

the

Whi

te R

iver

Gro

up o

utcr

op is

loca

ted

in th

e ce

ntra

l par

t of

the

stud

y ar

ea n

ear

igne

ous

intr

usiv

e ro

cks.

No

wel

l inv

ento

ry

info

rmat

ion

is a

vaila

ble.

lL

imes

tone

Pl

atea

u.3

Whi

te R

iver

G

roup

100-

150

Sand

ston

e, c

lays

tone

an

d si

ltst

one

wit

h ch

anne

l fill

ings

and

lim

esto

ne le

nses

.

33

1T

his

part

of

the

Whi

te R

iver

Gro

up o

utcr

op, l

ocat

ed in

the

east

-cen

tral

par

t of

the

stud

y ar

ea e

ast o

f Sp

earf

ish

Cre

ek, i

s a

smal

l are

a ov

erly

ing

Mad

ison

Lim

esto

ne a

nd D

eadw

ood

Form

atio

n ou

tcro

ps.

mM

ount

ain

Flan

ks4.

1W

hite

Riv

er

Gro

up

100-

150

Sand

ston

e, c

lays

tone

an

d si

ltst

one

wit

h ch

anne

l fill

ings

and

lim

esto

ne le

nses

.

33

1T

his

part

of

the

Whi

te R

iver

Gro

up o

utcr

op is

loca

ted

near

Mad

ison

L

imes

tone

and

Min

nelu

sa F

orm

atio

n ou

tcro

ps a

long

the

nort

hern

m

ount

ain

flan

ks in

the

cent

ral p

art o

f th

e st

udy

area

.

nA

ltern

atin

g Sa

ndst

one,

L

imes

tone

, an

d Sh

ale

0.3

Whi

te R

iver

G

roup

100-

150

Sand

ston

e, c

lays

tone

an

d si

ltst

one

wit

h ch

anne

l fill

ings

and

lim

esto

ne le

nses

.

33

1T

his

part

of

the

Whi

te R

iver

Gro

up o

utcr

op is

loca

ted

near

the

Inya

n K

ara

Gro

up o

utcr

op in

the

nort

heas

tern

par

t of

the

stud

y ar

ea.

oA

ltern

atin

g Sa

ndst

one,

L

imes

tone

, an

d Sh

ale

42.8

Spea

rfis

h Fo

rmat

ion

9 375-

800

Silty

sha

le in

terb

edde

d w

ith s

ands

tone

, si

ltsto

ne, a

nd s

pars

e lim

esto

ne.

Low

er

part

con

tain

s m

assi

ve

gyps

um.

33

1T

his

hydr

ogeo

logi

c un

it, lo

cate

d in

an

east

-wes

t dir

ectio

n ac

ross

the

nort

hern

par

t of

the

stud

y ar

ea, i

s a

conf

inin

g un

it th

at s

uppl

ies

wat

er to

se

vera

l wel

ls.

Mos

t rep

orte

d w

ell y

ield

s ar

e le

ss th

an 2

0 ga

l/min

with

a

few

bet

wee

n 20

and

100

gal

/min

and

one

wel

l yie

ldin

g 30

0 ga

l/min

. T

hick

ness

ran

ges

from

375

-450

ft a

long

the

east

ern

side

of

the

Bla

ck

Hill

s up

lift t

o 70

0-80

0 ft

on

the

nort

hwes

tern

fla

nk o

f th

e up

lift.12

Pe

rmea

bilit

y is

fro

m p

orou

s le

nses

and

sec

onda

ry p

oros

ity a

ssoc

iate

d w

ith g

ypsu

m s

olut

ion

open

ings

. A

ttenu

atio

n of

pot

entia

l con

tam

inan

ts

by s

ilts

and

shal

e is

pro

babl

e.

pM

ount

ain

Flan

ks2.

4Sp

earf

ish

Form

atio

n

9 375-

800

Silty

sha

le in

terb

edde

d w

ith s

ands

tone

, si

ltsto

ne a

nd s

pars

e lim

esto

ne.

Low

er

port

ion

cont

ains

m

assi

ve g

ypsu

m.

32

1T

his

hydr

ogeo

logi

c un

it is

isol

ated

are

as o

f th

e Sp

earf

ish

Form

atio

n th

at

have

not

ero

ded

away

. T

he p

art o

f th

e un

it lo

cate

d on

the

wes

tern

sid

e of

th

e st

udy

area

slo

pes

to th

e no

rthw

est.

The

par

t of t

he u

nit l

ocat

ed o

n th

e ea

st s

ide

of th

e st

udy

area

slo

pes

to th

e no

rthe

ast.

Lim

ited

satu

ratio

n in

th

ese

two

area

s is

like

ly.

No

inve

ntor

ied

wel

l inf

orm

atio

n is

ava

ilabl

e.

Tab

le 5

. S

ensi

tivity

cha

ract

eris

tics

of h

ydro

geol

ogic

uni

ts–C

ontin

ued

[mi2 , s

quar

e m

iles;

ft,

feet

; gal

/min

, gal

lons

per

min

ute;

ft/

d, f

eet p

er d

ay; -

-, n

o da

ta]

Hydrogeologic unit1

Hyd

rog

eolo

gic

sett

ing

Tota

lsu

rfac

ear

ea o

fu

nit

(mi2 )

Geo

log

icu

nit

s

Su

b-s

urf

ace

thic

knes

so

fg

eolo

gic

un

it(f

t)

Un

satu

rate

d a

nd

aqu

ifer

med

ia2

Rel

ativ

e se

nsi

tivi

ty

rati

ng

Co

mm

ents

3,4

Aquifermedia

Unsaturatedmedia



qA

ltern

atin

g Sa

ndst

one,

L

imes

tone

, an

d Sh

ale

41.1

Bel

le F

ourc

he

Shal

eM

owry

Sh

ale

New

cast

le

Sand

ston

eSk

ull C

reek

Sh

ale

9 150-

850

9 125-

230

9 0-10

0

9 150-

270

Shal

e, li

mes

tone

, and

sa

ndst

one.

22

1T

his

hydr

ogeo

logi

c un

it, lo

cate

d in

the

nort

heas

tern

cor

ner

of th

e st

udy

area

, is

a co

nfin

ing

unit.

The

New

cast

le S

ands

tone

may

yie

ld w

ater

to

wel

ls lo

cally

whe

re s

atur

ated

. A

sin

gle

wel

l in

the

Bel

le F

ourc

he S

hale

pr

oduc

ed 2

0 ga

l/min

. V

erti

cal m

ovem

ent o

f po

tent

ial c

onta

min

ants

is

limite

d by

sha

le a

nd b

ento

nite

laye

rs.

rM

ount

ain

Flan

ks9.

2O

pech

e Sh

ale

9 25-1

50Si

ltsto

ne a

nd s

hale

with

lo

cal g

ypsu

m a

nd

anhy

drite

at t

he to

p.

22

1T

his

hydr

ogeo

logi

c un

it is

loca

ted

in n

arro

w b

ands

bet

wee

n th

e M

inne

lusa

Fo

rmat

ion

and

Min

neka

hta

Lim

esto

ne a

cros

s th

e no

rth-

cent

ral p

art o

f th

e st

udy

area

. T

he O

pech

e Fo

rmat

ion

is a

con

fini

ng u

nit t

hat p

rodu

ces

wat

er to

wel

ls in

isol

ated

are

as.

Mos

t rep

orte

d w

ell y

ield

s ra

nge

from

2

to 2

0 ga

l/min

with

a f

ew w

ells

as

muc

h as

75

gal/m

in.

Perm

eabi

lity

is

from

por

ous

lens

es, s

olut

ion

feat

ures

, and

/or

frac

ture

s. A

ttenu

atio

n of

po

tent

ial c

onta

min

ants

by

silt

and

shal

e is

pro

babl

e. D

rain

age

from

the

area

cou

ld r

echa

rge

the

Min

neka

hta

aqui

fer.

sA

ltern

atin

g Sa

ndst

one,

L

imes

tone

, an

d Sh

ale

.3O

pech

e Sh

ale

9 25-1

50Si

ltsto

ne a

nd s

hale

with

lo

cal g

ypsu

m a

nd

anhy

drite

at t

he to

p.

22

1T

his

hydr

ogeo

logi

c un

it is

loca

ted

in a

nar

row

rin

g be

twee

n th

e M

inne

lusa

Fo

rmat

ion

and

Min

neka

hta

Lim

esto

ne a

roun

d a

peak

in th

e no

rthe

aste

rn

part

of

the

stud

y ar

ea n

ear

Polo

Cre

ek.

Dra

inag

e fr

om th

e ar

ea c

ould

re

char

ge th

e M

inne

kaht

a aq

uife

r.

1 The

se h

ydro

geol

ogic

uni

ts w

ere

sort

ed in

des

cend

ing

orde

r ba

sed

on r

elat

ive

sens

itiv

ity r

atin

gs w

ith th

e pr

imar

y so

rt b

ased

on

unsa

tura

ted

med

ia, t

he s

econ

dary

sor

t on

aqui

fer

med

ia, a

nd th

e te

rtia

ry

sort

on

hydr

aulic

con

duct

ivity

.2 S

umm

ariz

ed f

rom

Str

obel

and

oth

ers

(199

9).

For

mor

e de

tail

ed d

escr

ipti

ons

of th

e ge

olog

ic u

nits

, see

ref

eren

ces

list

ed in

foo

tnot

es 6

thro

ugh

12.

3 Hyd

raul

ic c

ondu

ctiv

ity w

as e

stim

ated

fro

m p

rodu

ctio

n w

ell d

raw

dow

n, p

rodu

ctio

n ra

te, w

ell d

iam

eter

, and

an

assu

med

sto

rage

coe

ffic

ient

usi

ng th

e no

n-eq

uili

briu

m e

quat

ion

(The

is, 1

935)

.4 W

ell i

nfor

mat

ion

com

pile

d fr

om U

.S. G

eolo

gica

l Sur

vey

Gro

und-

Wat

er S

ite I

nven

tory

Sys

tem

.5 Se

e fi

gure

3.

6 Kyl

lone

n an

d P

eter

, 198

7.7 G

reen

e, 1

993.

8 Gre

ene

and

othe

rs, 1

998.

9 Dew

itt a

nd o

ther

s, 1

989.

10R

obin

son

and

othe

rs, 1

964.

11L

isen

bee,

198

5.12

Gri

es a

nd M

artin

, 198

5.

Tab

le 5

. S

ensi

tivity

cha

ract

eris

tics

of h

ydro

geol

ogic

uni

ts–C

ontin

ued

[mi2 , s

quar

e m

iles;

ft,

feet

; gal

/min

, gal

lons

per

min

ute;

ft/d

, fee

t per

day

; --,

no

data


Hyd

rog

eolo

gic

sett

ing

Tota

lsu

rfac

ear

ea o

fu

nit

(mi2 )

Geo

log

icu

nit

s

Su

b-s

urf

ace

thic

knes

so

fg

eolo

gic

un

it(f

t)

Un

satu

rate

d a

nd

aqu

ifer

med

ia2

Rel

ativ

e se

nsi

tivi

ty

rati

ng

Co

mm

ents

3,4

Aquifermedia

Unsaturatedmedia



T. 7 N.

T. 6 N.

T. 5 N.

T. 4 N.

T. 3 N.

T. 2 N.

R. 1 E. R. 2 E. R. 3 E. R. 4 E. R. 5 E.

Spea

rfish

Spearfish

Cre

ek

Cre

ek

Cre

ek

False

Creek

Creek

Creek

Creek

Creek

Bot

to

m

Whi

tewood

Whi

teta

il

Litt

leIr

on

REDWATER

RIVER

Robison

Higgins

Gulch

Gulch

Squaw

Creek

Creek

Creek

Annie Creek

Bea

rG

ulch

Creek

Crow

Bear

Butte

Boxelder

Creek

Little ElkNorth

Fo

rk

Creek

Fork

South

Rapid

Rapid

Creek

Elk

Bea

ver

9014

14A

14A

14A

385

385

9014

85

85

85

85

34

34

Spearfish

Whitewood

Deadwood

LeadCentralCity

Nemo

LAW

RE

NC

E C

OU

NT

Y

103˚30'

104˚

103˚52'30"103˚37'30"103˚45'

44˚30'

44˚15'

44˚22'30"


Figure 8. Hydrogeologic-unit sensitivity groups in Lawrence County.

EXPLANATION

1 2 3 4 5 6 MILES


0

HYDROGEOLOGIC-UNIT GROUPSArea that includes the group of 15 hydrogeologic units with the highest relative sensitivity ratings (A-O)

Area that includes the group of 15 hydrogeologic units with the middle relative sensitivity ratings (P-Z, a-d)Area that includes the group of 15 hydrogeologic units with the lowest relative sensitivity ratings (e-s)


Recharge Rate

Infiltration of precipitation through the soil and the unsaturated zone to the water table is a common mechanism of ground-water recharge. Recharge rate represents the amount of water that penetrates the land surface and reaches the water table per unit area and unit time. Recharge water can transport a potential contaminant from the land surface to the water table. Large volumes of recharge water or frequency of recharge events increase the mobility of potential con-taminants. Generally, the larger the recharge rate the greater the sensitivity; however, in some instances, dilution of contaminants may diminish the impact of larger recharge rates.

Previous estimates of recharge from infiltration of precipitation on outcrops in the northern Black Hills range from 1 to 7 in/yr. Recharge commonly is expressed as a percent of precipitation. A recharge rate of 6.77 in/yr, or 31 percent of precipitation, and an evapotranspiration rate of 15.2 in/year was estimated by Rahn and Gries (1973) from analysis of spring dis-charge from carbonate basins where no surface runoff was observed. Kyllonen and Peter (1987) estimated recharge rates of 7 in/yr or 32 percent of precipitation for the Madison and Minnelusa aquifers and 1 in/yr or 6 percent of precipitation for the Inyan Kara aquifer in describing the hydrogeology of the northern Black Hills. Kyllonen and Peter (1987) assumed average precipitation that would fall on the outcrop of 22 in/yr for the Minnelusa Formation and Madison Limestone and 16 in/yr for the Inyan Kara Group.

Streamflow records at two USGS gaging stations were analyzed to further characterize the range of plausible recharge rates. The recharge rate for the car-bonate basin upstream from USGS gaging station 06408700 (fig. 9) examined by Rahn and Gries (1973) was assumed to represent the highest recharge rate as a percent of precipitation in the study area. The exposed bedrock in the drainage basin above station 06408700, located near the southwest edge of the study area, is about 96 percent Madison Limestone. An analysis of the basin with a longer period of record than that used by Rahn and Gries (1973) was made to refine the esti-mate of recharge rate. The average (1961-90) precipi-tation for the basin was 22.8 in., and the average water year 1983-97 runoff at the gaging station was 9.0 in/yr (U.S. Geological Survey, 1984-98). Runoff was assumed to be entirely ground-water discharge because surface-water runoff is not observed in the basin. This lack of surface-water runoff is documented by daily


streamflow records at station 06408700 (water years 1983-97), which shows gradual changes in discharge with a minimum flow of 3.1 ft3/s and a maximum flow of 8.5 ft3/s. Locations of ground-water divides could be different than the extent of the surface drainage basin. Assuming a range in the area contributing to ground-water flow of 20 percent greater than or less than the drainage basin area, a plausible recharge rate to the Madison aquifer ranges from 7.2 to 10.8 in/yr, or 32 to 47 percent of precipitation.

Analysis of streamflow records for a drainage basin with mostly igneous and metamorphic rocks was made to provide a plausible range for the lower recharge rates as a percent of precipitation in the study area. The bedrock in the basin above USGS gaging station 06422500 (fig. 9) is about 83 percent igneous or metamorphic rocks, 10 percent Deadwood Formation, 5 percent Madison Limestone, and 2 percent other rocks. Average (1961-90) precipitation on the basin (fig. 9) was 20.1 in/yr, and average runoff (water years 1967-96) at the gaging station was 2.4 in/yr. In the cen-tral part of the Black Hills with steeply sloping igneous and metamorphic rocks, a large part of the precipitation that becomes recharge reappears in the drainage basin as springflow. Ground-water recharge that reappears as base flow in the basin, estimated from analysis of streamflow-gaging records at station 06422500 (table 6), averages about 3.8 ft3/s (0.5 in/yr) or about 2.5 percent of precipitation. Runoff or springflow from the Madison Limestone and Deadwood Formation within the basin is unknown; however, a large part of the Madison Limestone and Deadwood Formation is along the east edge of the basin where the strata are dip-ping to the east. Most ground-water recharge in these areas probably flows to the regional system with little contribution to base flow. Therefore, the estimate of base flow from igneous and metamorphic rocks prob-ably is slightly higher than 2.5 percent of precipitation.

Because of uncertainty in evapotranspiration rates, applying the water budget formula does not produce accurate estimates of relatively small recharge rates that contribute to regional ground-water flow from igneous and metamorphic rocks. Permeability of the igneous and metamorphic rocks decreases with depth (Freeze and Cherry, 1979); therefore, an assump-tion was made that regional ground-water flow in igneous and metamorphic rocks probably was less than the base flow in the basin. A plausible range of recharge rates for igneous and metamorphic rocks was assumed to be between 3 and 6 percent of precipitation.

ounty, South Dakota

T. 7 N.

T. 6 N.

T. 5 N.

T. 4 N.

T. 3 N.

T. 2 N.

R. 1 E. R. 2 E. R. 3 E. R. 4 E. R. 5 E.

103˚30'

104˚

103˚52'30"103˚37'30"103˚45'

44˚30'

44˚15'

44˚22'30"

Spea

rfish

Spearfish

Cre

ek

Cre

ek

Cre

ek

False

Creek

Creek

Creek

Creek

Creek

Bot

to

m

Whi

tewood

Whi

teta

il

Litt

leIr

on

REDWATER

RIVER

Robison

Higgins

Gulch

Gulch

Squaw

Creek

Creek

Creek

Annie Creek

Bea

rG

ulch

Creek

Crow

Bear

Butte

Boxelder

Creek

Little ElkNorth

Fo

rk

Creek

Fork

South

Rapid

Rapid

Creek

Elk

Bea

ver

9014

14A

14A

14A

385

385

9014

85

85

85

85

34

34

Spearfish

Whitewood

Deadwood

Lead

Nemo

18

20

22

24

26

26

24

22

20

18

16

28

FortMeade

PactolaDam

BuskalaRanch

Lead

Deadwood

Spearfish

BelleFourche

LAW

RE

NC

E C

OU

NT

Y

442745103434500

442343103363900

441859103385600441832103523200

441852103594800

441810104062300

441207104012700

440756103450300

06392900

06408860

442745103434500

442343103363900

441859103385600441832103523200

441852103594800

441810104062300

441207104012700

440756103450300

06392900

06408860

Figure 9. Selected drainage basins and average precipitation in or near the study area.

LINE OF EQUAL ANNUAL PRECIPITATION--Shows 1961-90 average. Interval is 2 inches per year

STREAMFLOW GAGING STATION--Gaging station name and station identification number. Boundary describes selected drainage basin above gaging staion

PRECIPITATION STATION WITH 30-YEAR RECORD--U.S. Department of Commerce (1994)

PRECIPITATION STATION WITH 30-YEAR AVERAGE INTERPOLATED FROM 1989-96 PRECIPITAITON RECORDS--U.S. Geological Survey (1976-98). Number is station identification number

EXPLANATION

1 2 3 4 5 6 MILES


0

06422500Boxelder Creeknear Nemo

06408700Rhoads Forknear Rochford


06408860

Lead

20


Table 6. Base flow within drainage basin above station 06422500

Water year

Estimatedbase flow

(cubic feet persecond)

Mean dischargeat station

(cubic feet persecond)

1967 4.0 27.7

1968 4.2 8.7

1969 2.3 8.6

1970 3.5 22.1

1971 4.0 27.9

1972 3.5 55.1

1973 5.5 17.9

1974 3.0 6.0

1975 3.0 15.2

1976 5.5 20.7

1977 5.0 18.7

1978 4.1 22.0

1979 3.5 8.0

1980 2.3 5.4

1981 2.2 4.1

1982 3.0 10.8

1983 6.0 24.5

1984 6.1 23.2

1985 2.6 6.6

1986 3.4 12.3

1987 3.0 10.2

1988 1.4 4.6

1989 1.3 3.8

1990 1.4 5.6

1991 3.2 13.5

1992 3.6 6.9

1993 4.0 24.0

1994 5.2 18.5

1995 6.2 51.4

1996 7.4 42.9



Based on the range of plausible recharge rates as a percentage of precipitation, the hydrologic setting, unsaturated media, average precipitation, land-surface slope, and land-surface altitude, a comparative estimate of recharge rate as a percent of precipitation was made for each hydrogeologic unit (table 7). Recharge rate is strongly influenced by the unsaturated media. The general order of influence of unsaturated media on recharge rate in descending order was fractured lime-stone, unconsolidated sands and gravels, sandstones, colluvium, shales with interbedded sandstones, igneous and metamorphic rocks, and shales. Recharge rates, expressed in inches per year, increase with increased precipitation; however, evapotranspiration rates also increase, making the increase in recharge with increased precipitation nonlinear. Greater relative slopes decrease the recharge rate. The land-surface altitude affects recharge rates because of lower temper-atures and less evapotranspiration at higher altitudes. A relative ranking of the hydrogeologic units based on the above parameters was used as a guide to estimate a recharge rate as a percent of precipitation for each hydrogeologic unit. These data were converted to a GIS grid data set for the study area and multiplied by the grid of precipitation data (fig. 9) to estimate the spa-tial distribution of recharge in inches per year.

Because of uncertainty in estimating recharge rates, five broad categories of recharge rate were pro-posed by Aller and others (table 8). Studies comparing recharge volumes and pesticide concentrations in wells have shown a diversity of results; however, pesticide fluxes appear to be directly related to the amount of recharge entering the subsurface (Barbash and Resek, 1996). The inconsistencies in comparisons of recharge rate and contaminant concentrations are likely related to dilution, lag periods in observed concentrations in wells, and contaminant degradation. The recharge rates in the study area were within the first four recharge-rate groups with recharge rates less than 10 in/yr (table 9).

ounty, South Dakota

Tab

le 7

. E

stim

ated

rec

harg

e ra

te fo

r hy

drog

eolo

gic

units

as

a pe

rcen

t of p

reci

pita

tion

Hyd

ro-

geo

log

icu

nit

Hyd

rog

eolo

gic

sett

ing

Geo

log

ic u

nit

s U

nsa

tura

ted

med

ia1

Ave

rag

ep

reci

pit

atio

n(i

nch

es p

erye

ar)

Ave

rag

esl

op

e2

(per

cen

t)

Ave

rag

ela

nd

-su

rfac

e al

titu

de2

(fee

t ab

ove

sea

leve

l)

Rec

har

ge

rate

(per

cen

t o

fp

reci

pit

atio

n)

AIg

neou

s D

omes

Mad

ison

Lim

esto

ne

Eng

lew

ood

Form

atio

n

Frac

ture

d lim

esto

ne a

nd d

olom

itic

limes

tone

wit

h en

hanc

ed s

olut

ion

open

ings

.D

olom

itic

lim

esto

ne.

25.7

28.2

5,06

433

BL

imes

tone

Pla

teau

Mad

ison

Lim

esto

ne

Eng

lew

ood

Form

atio

n

Frac

ture

d lim

esto

ne a

nd d

olom

itic

limes

tone

wit

h en

hanc

ed s

olut

ion

open

ings

.D

olom

itic

lim

esto

ne.

25.6

18.8

6,27

333

CM

ount

ain

Flan

ksM

adis

on L

imes

tone

Eng

lew

ood

Form

atio

n

Frac

ture

d lim

esto

ne a

nd d

olom

itic

limes

tone

wit

h en

hanc

ed s

olut

ion

open

ings

.D

olom

itic

lim

esto

ne.

22.7

21.2

5,68

33 28

DL

imes

tone

Pla

teau

Mad

ison

Lim

esto

ne

Eng

lew

ood

Form

atio

n

Frac

ture

d lim

esto

ne a

nd d

olom

itic

limes

tone

wit

h en

hanc

ed s

olut

ion

open

ings

.D

olom

itic

lim

esto

ne.

25.1

24.4

5,96

931

EM

ount

ain

Flan

ksM

inne

kaht

a L

imes

tone

Frac

ture

d lim

esto

ne.

22.8

13.1

4,08

418

FA

ltern

atin

g Sa

ndst

one,

L

imes

tone

, and

Sha

leM

inne

kaht

a L

imes

tone

Frac

ture

d lim

esto

ne.

21.5

13.7

3,82

917

GA

lluvi

al M

ount

ain

Val

leys

-E

ast

Allu

vium

Cla

y, s

ilt, s

and

and

grav

el.

22.3

13.2

5,50

521

HA

lluvi

al M

ount

ain

Val

leys

-N

orth

Allu

vium

Cla

y, s

ilt, s

and

and

grav

el.

28.6

23.5

4,71

830

IA

lluvi

al M

ount

ain

Val

leys

Allu

vium

Cla

y, s

ilt, s

and

and

grav

el.

24.5

20.5

4,62

627

JR

iver

Allu

vium

with

out

Ove

rban

k D

epos

itsA

lluvi

umC

lay,

silt

, san

d an

d gr

avel

.19

.94.

63,

436

19

KU

ncon

solid

ated

and

Sem

i-C

onso

lidat

ed A

quif

ers

Gra

vel d

epos

itsC

lay,

silt

, san

d an

d gr

avel

.20

.16.

43,

594

18

LM

ount

ain

Flan

ksM

inne

lusa

For

mat

ion

Cro

ss s

trat

ifie

d sa

ndst

one,

lim

esto

ne, d

olom

ite,

and

shal

e w

ith

frac

ture

s an

d br

ecci

atio

n.24

.119

.25,

300

22


MA

ltern

atin

g Sa

ndst

one,

L

imes

tone

, and

Sha

leM

inne

lusa

For

mat

ion

Cro

ss s

trat

ifie

d sa

ndst

one,

lim

esto

ne, d

olom

ite,

and

shal

e w

ith f

ract

ures

and

bre

ccia

tion.

21.8

22.0

4,12

017

NIg

neou

s D

omes

Dea

dwoo

d Fo

rmat

ion

Sand

ston

e, s

hale

, lim

esto

ne, a

nd lo

cal

cong

lom

erat

e at

the

base

.26

.222

.75,

633

16

OL

imes

tone

Pla

teau

Dea

dwoo

d Fo

rmat

ion

Sand

ston

e, s

hale

, lim

esto

ne, a

nd lo

cal

cong

lom

erat

e at

the

base

.22

.720

.15,

955

14

PIg

neou

s D

omes

Min

nelu

sa F

orm

atio

nC

ross

str

atif

ied

sand

ston

e, li

mes

tone

, dol

omite

, an

d sh

ale

with

fra

ctur

es a

nd b

recc

iatio

n.24

.424

.84,

914

22

QL

imes

tone

Pla

teau

Min

nelu

sa F

orm

atio

nC

ross

str

atif

ied

sand

ston

e, li

mes

tone

, dol

omite

, an

d sh

ale

with

fra

ctur

es a

nd b

recc

iatio

n.24

.914

.56,

495

22

RM

ount

ain

Flan

ksD

eadw

ood

Form

atio

nSa

ndst

one,

sha

le, l

imes

tone

, and

loca

l co

nglo

mer

ate

at th

e ba

se.

19.5

21.4

4,95

910

SM

ount

ain

Slop

es-E

ast

Dea

dwoo

d Fo

rmat

ion

Sand

ston

e, s

hale

, lim

esto

ne, a

nd lo

cal

cong

lom

erat

e at

the

base

.22

.08.

15,

890

11

TM

ount

ain

Slop

es-E

ast

Igne

ous

and

met

amor

phic

ro

cks

Frac

ture

d an

d w

eath

ered

igne

ous

and

met

amor

phic

ro

cks.

21.3

15.3

5,56

75

UM

ount

ain

Slop

es-N

orth

Igne

ous

and

met

amor

phic

ro

cks

Frac

ture

d an

d w

eath

ered

igne

ous

and

met

amor

phic

ro

cks

28.1

32.1

5,43

87

VM

ount

ain

Flan

ksIg

neou

s an

d m

etam

orph

ic

rock

sFr

actu

red

and

wea

ther

ed ig

neou

s an

d m

etam

orph

ic

rock

s.19

.313

.84,

843

5

WM

ount

ain

Slop

es-E

ast

Col

luvi

umTa

lus

and

othe

r de

bris

ass

ocia

ted

with

mas

s w

astin

g.19

.315

.75,

445

12

XIg

neou

s D

omes

Col

luvi

umTa

lus

and

othe

r de

bris

ass

ocia

ted

with

mas

s w

astin

g.24

.534

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406

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YM

ount

ain

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ksC

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vium

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s an

d ot

her

debr

is a

ssoc

iate

d w

ith m

ass

was

ting.

26.4

13.5

4,17

215

Tab

le 7

. E

stim

ated

rec

harg

e ra

te fo

r hy

drog

eolo

gic

units

as

a pe

rcen

t of p

reci

pita

tion–

Con

tinue

d

Hyd

ro-

geo

log

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nit

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rog

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gic

sett

ing

Geo

log

ic u

nit

s U

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ted

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n(i

nch

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rag

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op

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(per

cen

t)

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rag

ela

nd

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titu

de2

(fee

t ab

ove

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leve

l)

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har

ge

rate

(per

cen

t o

fp

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pit

atio

n)


ZA

ltern

atin

g Sa

ndst

one,

L

imes

tone

, and

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leIn

yan

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a G

roup

(Fal

l Riv

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orm

atio

n an

d L

akot

a Fo

rmat

ion)

Sand

ston

e an

d ot

her

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tic r

ocks

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659

11

aIg

neou

s D

omes

Tert

iary

intr

usiv

e ig

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s ro

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Intr

usiv

e ig

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s ro

cks.

25.6

24.7

5,59

36

bM

ount

ain

Slop

es-E

ast

Tert

iary

intr

usiv

e ig

neou

s ro

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cM

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Slop

es-N

orth

Tert

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intr

usiv

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s ro

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28.7

31.9

5,63

26

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s D

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tew

ood

Form

atio

nW

inni

peg

Form

atio

nL

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tone

and

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omite

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ale

with

inte

rbed

ded

silts

tone

.25

.527

.25,

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5

eM

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ksW

hite

woo

d Fo

rmat

ion

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nipe

g Fo

rmat

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esto

ne a

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olom

ite.

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e w

ith in

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teau

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atio

nW

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with

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rbed

ded

silts

tone

.24

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.65,

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gM

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ksW

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d Fo

rmat

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rmat

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esto

ne a

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ite.

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e w

ith in

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edde

d si

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18.8

26.4

4,92

33

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g Sa

ndst

one,

L

imes

tone

, and

Sha

leM

orri

son

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nkpa

pa S

ands

tone

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ance

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mat

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sum

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ing

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ands

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sum

.20

.917

.73,

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5

iM

ount

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ast

Whi

te R

iver

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upSa

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ysto

ne a

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iltst

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with

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nnel

fi

lling

s an

d lim

esto

ne le

nses

.25

.09.

05,

467

4

jM

ount

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orth

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te R

iver

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upSa

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with

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nnel

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lling

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d lim

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ne le

nses

.29

.020

.35,

331

4

kIg

neou

s D

omes

Whi

te R

iver

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upSa

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iltst

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with

cha

nnel

fi

lling

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d lim

esto

ne le

nses

.26

.519

.95,

065

4

Tab

le 7

. E

stim

ated

rec

harg

e ra

te fo

r hy

drog

eolo

gic

units

as

a pe

rcen

t of p

reci

pita

tion–

Con

tinue

d

Hyd

ro-

geo

log

icu

nit

Hyd

rog

eolo

gic

sett

ing

Geo

log

ic u

nit

s U

nsa

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ted

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ia1

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rag

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reci

pit

atio

n(i

nch

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erye

ar)

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rag

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cen

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rag

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nd

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rfac

e al

titu

de2

(fee

t ab

ove

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leve

l)

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har

ge

rate

(per

cen

t o

fp

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pit

atio

n)


lL

imes

tone

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teau

Whi

te R

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27.1

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6,13

64

mM

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ksW

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roup

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ston

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lays

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esto

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.616

.74,

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imes

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leW

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.23

.511

.84,

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4

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imes

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leSp

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ish

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atio

nSi

lty s

hale

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rbed

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with

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dsto

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d sp

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esto

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er p

ortio

n co

ntai

ns

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sive

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sum

.

21.1

9.1

3,65

82

pM

ount

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ksSp

earf

ish

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with

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d sp

arse

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Low

er p

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n co

ntai

ns

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sive

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sum

.

24.0

10.3

4,11

33

qA

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g Sa

ndst

one,

L

imes

tone

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leSk

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reek

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dsto

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owry

Sha

leB

elle

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rche

Sha

le

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e, li

mes

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18.4

7.9

3,32

12

rM

ount

ain

Flan

ksO

pech

e Sh

ale

Silts

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and

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le w

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cal g

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he to

p.23

.423

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336

1

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ith lo

cal g

ypsu

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drite

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p.21

.717

.03,

890

1

1 Des

crip

tion

was

mod

ifie

d fr

om c

ombi

ned

unsa

tura

ted

and

aqui

fer

med

ia d

escr

iptio

n in

tabl

e 5.

2 Ave

rage

land

-sur

face

alti

tude

and

slo

pe w

ere

dete

rmin

ed f

rom

U.S

. Geo

logi

cal S

urve

y di

gita

l ele

vati

on m

odel

s (D

EM

’s)

with

30-

met

er r

esol

utio

n. F

or m

ore

deta

iled

info

rmat

ion

desc

ribi

ng D

EM

’s,

see

U.S

. Geo

logi

cal S

urve

y (1

993)

dig

ital e

leva

tion

mod

el u

ser’

s gu

ide.

3 A r

echa

rge

rate

of

20 p

erce

nt o

f pr

ecip

itatio

n w

as u

sed

for

a sm

all p

art o

f th

is h

ydro

geol

ogic

uni

t tha

t was

loca

ted

in th

e so

uthe

aste

rn c

orne

r of

the

stud

y ar

ea b

ecau

se th

e pr

ecip

itat

ion

amou

nts

wer

e co

nsid

erab

ly le

ss th

an th

e av

erag

e fo

r th

is h

ydro

geol

ogic

uni

t.

Tab

le 7

. E

stim

ated

rec

harg

e ra

te fo

r hy

drog

eolo

gic

units

as

a pe

rcen

t of p

reci

pita

tion–

Con

tinue

d

Hyd

ro-

geo

log

icu

nit

Hyd

rog

eolo

gic

sett

ing

Geo

log

ic u

nit

s U

nsa

tura

ted

med

ia1

Ave

rag

ep

reci

pit

atio

n(i

nch

es p

erye

ar)

Ave

rag

esl

op

e2

(per

cen

t)

Ave

rag

ela

nd

-su

rfac

e al

titu

de2

(fee

t ab

ove

sea

leve

l)

Rec

har

ge

rate

(per

cen

t o

fp

reci

pit

atio

n)


Designated Units

The distribution of the designated recharge-rate groups is shown in figure 10. Recharge-rate group 1 included most of the Madison Limestone outcrop except areas with lower precipitation on the flanks of the Black Hills uplift. Some alluvium in areas with higher precipitation amounts also were included in this group. The average precipitation rate for recharge-rate group 1 was 24.8 in/yr.

Recharge-rate group 2 included most of the alluvium and terrace gravels, part of the Minnekahta Limestone outcrop, most of the Minnelusa Formation outcrop, the part of the Madison Limestone outcrop with lower precipitation rates, and parts of Deadwood Formation outcrop where the precipitation rate was high. The average precipitation rate for the Minne-kahta Limestone outcrop was 23.9 in/yr, Minnelusa Formation outcrop was 24.1 in/yr, the Deadwood Formation outcrop was 26.9 in/yr.

Table 8. Sensitivity ratings for range categories of recharge rate

[Modified from Aller and others, 1987, table 5, p. 21. <, less than]

Recharge rate(inches per year)

Sensitivityrating1

More than 10 9

7 to <10 8

4 to <7 6

2 to <4 3

0 to <2 1

1Higher rating indicates higher sensitivity to contamination.

Table 9. Sensitivity rank for designated recharge-rate groups

[<, less than]

Recharge-rate group

Recharge rate(inches per year)

Sensitivityrank

1 7 to <10Highest

to

lowest

2 4 to <7

3 2 to <4

4 0 to <2

Recharge-rate group 3 included most of the Inyan Kara Group outcrop except for the northern part of the study area that receives lower precipitation amounts. Parts of alluvium and terrace gravels, the Minnekahta Limestone outcrop, the Minnelusa Forma-tion outcrop, and Deadwood Formation outcrop with lower precipitation amounts were included in this group. A small part of Madison Limestone outcrop, located in the southeastern corner of the study area, is included in this group because the average precipitation was 17.1 in/yr. A small part of the igneous and meta-morphic rocks near igneous intrusions were included in this group where the average precipitation rate was greater than 28 in/yr.

Recharge-rate group 4 includes all of the igneous intrusive rocks, rock units that are made up predomi-nately of shale, and most of the igneous and metamor-phic rocks. The parts of the Deadwood Formation outcrop and the Inyan Kara Group outcrop included in this group had an average precipitation rate of less than 18 in/yr.

The recharge-rate map (fig. 10) was intersected and combined with the hydrogeologic-unit map. Small polygons created when the coverages were intersected were merged with the largest adjacent polygon main-taining the polygon boundaries of the hydrogeologic-unit map.

Depth to Water

Depth to water, the vertical distance that water and a potential contaminant must move to reach the water table, affects the amount of contact with sedi-ments in the unsaturated zone and the travel time to the water table. Generally, there is less sensitivity with greater depth to water. Barbash and Resek (1996, p. 283) reviewed several matrix diffusion studies that show pesticide concentrations or detection frequencies are more likely to be observed as the travel time to the water table decreases. The characteristics of the unsat-urated media vary widely in Lawrence County depending on the extent of fracturing. In highly frac-tured settings, the contact time with the unsaturated media could be very short even at greater depths. Therefore, evaluating the significance of the depth to water categories needs to be considered in conjunction with the unsaturated media.


T. 7 N.

T. 6 N.

T. 5 N.

T. 4 N.

T. 3 N.

T. 2 N.

R. 1 E. R. 2 E. R. 3 E. R. 4 E. R. 5 E.

Spea

rfish

Spearfish

Cre

ek

Cre

ek

Cre

ek

False

Creek

Creek

Creek

Creek

Creek

Bot

to

m

Whi

tewood

Whi

teta

il

Litt

leIr

on

REDWATER

RIVER

Robison

Higgins

Gulch

Gulch

Squaw

Creek

Creek

Creek

Annie Creek

Bea

rG

ulch

Creek

Crow

Bear

Butte

Boxelder

Creek

Little ElkNorth

Fo

rk

Creek

Fork

South

Rapid

Rapid

Creek

Elk

Bea

ver

9014

14A

14A

14A

385

385

9014

85

85

85

85

34

34

Spearfish

Whitewood

Deadwood

LeadCentralCity

Nemo

LAW

RE

NC

E C

OU

NT

Y

103˚30'

104˚

103˚52'30"103˚37'30"103˚45'

44˚30'

44˚15'

44˚22'30"


Figure 10. Recharge-rate sensitivity groups in Lawrence County.

EXPLANATION

1 2 3 4 5 6 MILES


0

RECHARGE-RATE GROUPS--Recharge rate, in inches per year. Sensitivity ranking from highest to lowest.

Group 1, 7 to <10 inches Group 2, 4 to <7 inches

Group 3, 2 to <4 inches

Group 4, 0 to <2 inches



Seven depth-to-water categories were proposed by Aller and others (1987) (table 10). Studies com-paring depth to water and pesticide concentrations in wells have shown mixed results (Barbash and Resek, 1996). Druliner and others (1996) found a greater likelihood of contamination with atrazine in areas with shallow water tables in the High Plains aquifer in Nebraska. Rupert (1999) found no statistical differ-ence in nitrate concentrations in a fractured basalt aquifer in Wyoming and Idaho between depth to water from 0 to 100 ft and 101 to 300 ft; however, the differ-ences in concentrations in ground-water samples from depths of 0 to 300 ft and from 301 to 900 ft were statistically significant. Because of the level of detail in available data, the spatial variability in the unsatur-ated zone, and the complex topography, a modified system of evaluating depth to water was used in this study. The seven categories were grouped together into three broad quantitative categories and two qualitative categories because of the limited spatial distribution of water-level data in the outcrop areas (table 11).

Areas with steeply dipping strata of fractured rocks were not categorized for depth to water because of the large variations in topography and water levels. The shallower depth to water generally exists in stream valleys; therefore, areas with land-surface elevations less than 50 ft above streambed elevations were delin-eated (group 4, table 11) to make relative comparisons with areas greater than 50 ft (group 5, table 11) above streambed elevations. Other areas with highly variable depths to water included in group 5 were hydrogeo-logic units consisting mostly of shale with sandstone layers or lenses and gypsum with solution openings.

Designated Units

The distribution of depth-to-water groups is shown in figure 11. Depth-to-water group 1 includes most of the alluvium. Group 2 includes most of the terrace gravels, colluvium, Minnekahta Limestone out-crop, and parts of the Inyan Kara Group outcrop. Group 3 includes parts of the Inyan Kara Group out-crop. Group 4 includes valleys in the Mountain Slopes-East, Mountain Slopes-West, Igneous Domes, Limestone Plateau, and Mountain Flanks hydrogeo-logic settings. The depth to water within group 4 varies from 0 ft near springs to greater than 100 ft. Vertical movement of water is frequently through rock fractures with rapid travel times. In narrow canyons, the area

was too small to show in figure 11 and plate 1; how-ever, the depth to water should be considered less near stream channels than in topographically higher areas. Group 5 includes most of the mountain slopes-east, mountain slopes-west, igneous domes, limestone plateau, and mountain flanks hydrogeologic settings, in areas where land surface is greater than or equal to 50 ft above the streambeds. Also included in group 5 were outcrops of units that primarily consist of shale with highly variable depths to water.

The depth-to-water map was intersected and combined with the hydrogeologic-unit map and recharge-rate map. Small polygons created when the coverages were intersected were merged with the largest adjacent polygon.

Table 10. Sensitivity ratings for range categories of depth to water


Depth to water(feet)

Sensitivity rating1

0 to 5 10

5 to 15 9

15 to 30 7

30 to 50 5

50 to 75 3

75 to 100 2

more than 100 11Higher rating indicates higher sensitivity to contamination.

Table 11. Sensitivity rank for designated depth-to-water groups

[<, less than; >, greater than]

Depth-to-watergroup

Depth to water(feet)

Sensitivityrank

1 0 to <20 feetComparison

groupHighest

tolowest

2 20 to <50 feet

3 50 or >50

4 Highly variable (areas of fractured rock in stream valleys)1 Comparison

group

Highestto

lowest5 Highly variable (other

areas)1Areas with land-surface elevations less than 50 feet above stre-

ambed elevations were delineated to make relative comparisons with areas greater than 50 feet above streambed elevations.


T. 7 N.

T. 6 N.

T. 5 N.

T. 4 N.

T. 3 N.

T. 2 N.

R. 1 E. R. 2 E. R. 3 E. R. 4 E. R. 5 E.

Spea

rfish

Spearfish

Cre

ek

Cre

ek

Cre

ek

False

Creek

Creek

Creek

Creek

Creek

Bot

to

m

Whi

tewood

Whi

teta

il

Litt

leIr

on

REDWATER

RIVER

Robison

Higgins

Gulch

Gulch

Squaw

Creek

Creek

Creek

Annie Creek

Bea

rG

ulch

Creek

Crow

Bear

Butte

Boxelder

Creek

Little ElkNorth

Fo

rk

Creek

Fork

South

Rapid

Rapid

Creek

Elk

Bea

ver

9014

14A

14A

14A

385

385

9014

85

85

85

85

34

34

Spearfish

Whitewood

Deadwood

LeadCentralCity

Nemo

LAW

RE

NC

E C

OU

NT

Y

103˚30'

104˚

103˚52'30"103˚37'30"103˚45'

44˚30'

44˚15'

44˚22'30"


Figure 11. Depth-to-water sensitivity groups in Lawrence County.

EXPLANATION

1 2 3 4 5 6 MILES


0

DEPTH-TO-WATER GROUPS

Group 1, 0 to <20 Highestto

lowest

Highest

to

lowest

Group 2, 20 to <50

Group 3, 50 or >50

Group 4, highly variable, (areas of fractured rock in stream valleys)

Group 5, highly variable (other areas)

Comparisongroup

Comparisongroup

Depth to water, in feet Sensitivity rank


Land-Surface Slope

Land-surface slope influences the potential of a contaminant to remain on the land surface long enough to infiltrate into the subsurface. On gentler slopes, precipitation runoff is slow; therefore, a potential con-taminant is more likely to be moved towards the water table with the infiltration of precipitation. On steeper slopes, precipitation runoff is rapid; therefore, a potential contaminant is more likely to be moved away before infiltration occurs.


The five land-surface-slope categories proposed by Aller and others (1987) were used as the designated land-surface-slope groups (table 12). All groups are present in the study area, as topography ranges from steep mountain slopes to alluvial prairie flood plains. Statistical correlations of land-surface slope and contaminant concentrations or detections were not apparent in the reviewed literature. Statistical compar-isons are likely to be influenced by dilution and lag periods in observed concentrations in wells, as noted in the “Recharge Rate” section.

The percent-slope classification was determined from analysis of USGS digital elevation models (DEM). These electronic files provide the same coverage as standard USGS 1:24,000-scale map series 7.5-minute quadrangles. The data are stored as profiles in which the spacing of elevations along and between each profile is 30 meters. A DEM data user’s guide (U.S. Geological Survey, 1993) provides a more detailed description of the data sets. The 7.5-minute

Table 12. Sensitivity rank for designated land-surface-slope groups

[Modified from Aller and others, 1987, table 4, p. 21. <, less than; >, greater than]

Land-surface-slopegroup

Land-surface slope(percent)

Sensitivity rank

1 0 to <2

Highest

to

lowest

2 2 to <6

3 6 to <12

4 12 to <18

5 18 or >18

DEM’s were joined together to create a 30-meter resolution grid of elevation data for the study area. These data were used to calculate percent-slope data in a similar grid format and were grouped according to the land-surface slope groups (table 12).

Because of abrupt changes in slope in the moun-tainous terrain, many land-surface-slope areas were less than 100 acres or were too narrow to show on a 1:100,000-scale map. A GIS algorithm (Command file 1 in the “Supplemental Information” section at the end of this report) was used to merge these areas with adjacent areas.

Designated Units

The distribution of land-surface-slope groups is shown in figure 12. Land-surface-slope group 1 was in the northern part of the study area, and the most common rock type was alluvium and terrace gravels. Land-surface-slope group 2 was mostly in the northern part of the study area with some small areas in the southern part. Land-surface-slope group 3 was located mostly along the flanks of the Black Hills uplift and parts of the plains in the north. Land-surface-slope group 4 was mostly in the southern part of the study area. Land-surface-slope group 5 was mostly in the central and southern part of the study area. The land-surface-slope map was intersected and delineated with the previously delineated map layers.

Streamflow Recharge

Streamflow is an important source of recharge to the Minnekahta, Minnelusa, and Madison aquifers (Hortness and Driscoll, 1998). Because of the dipping strata with fractures, collapse features, and karst solu-tion openings, numerous streams lose all or part of their flow as they cross the outcrops of the formations making up these aquifers. Precipitation that falls on drainage areas upstream from these loss zones (pls. 1 and 2) could transport a contaminant to the water table by direct infiltration, but in many instances the runoff moving to a streamflow-loss zone is a more important mechanism of potential ground-water contamination. Water from streamflow loss is more likely to enter rapid flowpaths through the unsaturated and aquifer media because of the presence of solution openings in limestone or gypsum.


T. 7 N.

T. 6 N.

T. 5 N.

T. 4 N.

T. 3 N.

T. 2 N.

R. 1 E. R. 2 E. R. 3 E. R. 4 E. R. 5 E.

Spea

rfish

Spearfish

Cre

ek

Cre

ek

Cre

ek

False

Creek

Creek

Creek

Creek

Creek

Bot

to

m

Whi

tewood

Whi

teta

il

Litt

leIr

on

REDWATER

RIVER

Robison

Higgins

Gulch

Gulch

Squaw

Creek

Creek

Creek

Annie Creek

Bea

rG

ulch

Creek

Crow

Bear

Butte

Boxelder

Creek

Little ElkNorth

Fo

rk

Creek

Fork

South

Rapid

Rapid

Creek

Elk

Bea

ver

9014

14A

14A

14A

385

385

9014

85

85

85

85

34

34

Spearfish

Whitewood

Deadwood

LeadCentralCity

Nemo

LAW

RE

NC

E C

OU

NT

Y

103˚30'

104˚

103˚52'30"103˚37'30"103˚45'

44˚30'

44˚15'

44˚22'30"


Figure 12. Land-surface-slope sensitivity groups in Lawrence County.

EXPLANATION

1 2 3 4 5 6 MILES


0

LAND-SURFACE-SLOPE GROUPS-- Land-surface slope, as percent. Sensitivity rank from highest to lowest.

Group 1, 0 to <2Group 2, 2 to <6

Group 3, 6 to <12

Group 4, 12 to <18

Group 5, 18 or greater


Hortness and Driscoll (1998) documented streamflow-loss thresholds, in drainage basins in Lawrence County, that range from 0 to 50 ft3/s. The streamflow-loss threshold represents the maximum streamflow that could be lost when adequate stream-flow occurs. During low-flow conditions, many of these streams lose all of their flow crossing the Minnekahta Limestone, Minnelusa Formation, and Madison Limestone outcrops. Generally, the Madison Limestone outcrop is the upstream section, followed by the Minnelusa Formation and Minnekahta Limestone outcrops. Numerous small ungaged drainage basins probably lose most streamflow to these bedrock out-crops except during peak flows from large storm events. Extensive intrusive igneous domes exist in the central part of the study area. In several locations, the intrusive rocks form peaks within outcrops of sedimen-tary rocks that generally include the Deadwood Forma-tion and Madison Limestone. Runoff from these peaks probably provides streamflow recharge primarily to the Madison aquifer. Also, water is diverted from Spearfish Creek by a dam to an aqueduct leading to a hydroelectric power plant (pl. 2). Losses from the aqueduct could recharge the Minnekahta, Minnelusa, or Madison aquifers.


Drainage basins upstream from outcrops of the Minnekahta Limestone, Minnelusa Formation, or Madison Limestone were delineated using the DEM grid for the study area and GIS algorithms for the analysis of watersheds (Environmental Systems Research Institute, Inc., 1992). Some small tributaries were grouped together in a drainage area and combined with the main stream. Several drainage areas that con-tain igneous domes were identified by the name of the peak. Some drainage areas contribute to streams whose loss zones are outside the study area. These areas were identified by the main-stem stream that crosses a potential loss zone outside the study area. Drainage areas less than 100 acres were merged with an adjacent drainage area.

Designated Units

Thirty drainage areas were delineated in the study area (pls. 1 and 2) and ranged in area from 0.3 to 142.1 mi2 (table 13). The site numbers labeling the drainage areas (pls. 1 and 2) were ordered clockwise beginning in the northwest and grouped by drainage basins. Average precipitation, average percent slope,

and percent of exposed crystalline (igneous and meta-morphic) rocks were calculated for each drainage area for analysis and comparison. Combined loss thresh-olds (Hortness and Driscoll, 1998) for most of the larger streams crossing the three bedrock aquifer out-crops were listed for comparison with mean discharge (table 13).

Estimates of the annual yield (the total volume of runoff divided by the area of the drainage basin) and mean discharge were calculated for the drainage areas (table 13) by comparison with gaged drainage basins. Miller and Driscoll (1998) calculated annual yield as one of the statistics used to compare characteristics of drainage basins in the Black Hills. Two of the groups they compared, interior sedimentary basins and interior crystalline basins, include several gaging stations on streams within or including the drainage areas delin-eated upstream from potential streamflow-loss zones (pl. 2). A summary of the area and annual yield for the basins above these gaging stations (table 14) shows a range of annual yields for basins with mostly crystal-line rocks and for basins with mostly sedimentary rocks. Annual yields were estimated for the delineated drainage areas (table 13) based on comparisons with the yields for these gaged streams. Estimated mean discharge was calculated for each drainage area by multiplying the estimated annual yield by the area and dividing by time.

The comments in table 13 contain descriptive information related to particular drainage areas. The exposed rock in the Spearfish Creek Basin is mostly Madison Limestone. Streams within this basin generally are gaining flow from numerous headwater springs. Some streamflow-loss zones may be present within this drainage area, depending on topography, water levels, and precipitation cycles; however, the area generally is a discharge area for this part of the Madison Limestone outcrop. Some of the headwaters of other drainage areas also include outcrops of Madison Limestone (pl. 2) where streams generally gain flow.

Bear Gulch, Beaver Creek, False Bottom Creek, Bear Butte Creek, Elk Creek, and Boxelder Creek lose all of their flow during most of the days during the year because flow is generally less than the loss threshold. The flow in Rapid Creek tributaries is large in relation to the streamflow-loss threshold. Comparison of the mean discharge for smaller basins in relation to the length of the stream reach crossing the formation outcrops (pl. 2) indicates that most of these drainage areas probably lose most of their flow to these bedrock outcrops.


Tab

le 1

3.

Cha

ract

eris

tics

of d

rain

age

area

s up

stre

am fr

om s

trea

mflo

w-lo

ss z

ones

[mi2 , s

quar

e m

iles;

in.,

inch

es; f

t3 /s, c

ubic

fee

t per

sec

ond;

--,

no

data

]

Sit

e id

enti

fi-

cati

on

nu

mb

er

Nam

eA

rea

(mi2 )

Ave

rag

ep

reci

p-

itat

ion

(in

.)

Ave

rag

esl

op

e(p

erce

nt)

Per

cen

tig

neo

us

and

met

a-m

orp

hic

rock

s1

Lo

ssth

resh

old

2

(ft3 /

s)

Est

imat

ed

ann

ual

yiel

d(i

n.)

Est

imat

edm

ean

dis

char

ge

(ft3 /

s)

Co

mm

ents

Sand

Cre

ek B

asin

1Sa

nd C

reek

tr

ibut

arie

s0.

323

.617

.010

0--

3.5

0.1

Thi

s sm

all d

rain

age

area

, par

t of

an ig

neou

s do

me,

is in

the

very

upp

er p

art o

f th

e Sa

nd C

reek

Bas

in th

at e

xten

ds in

to

Sout

h D

akot

a an

d dr

ains

wes

t int

o W

yom

ing

befo

re

cros

sing

the

outc

rops

(M

inne

kaht

a L

imes

tone

, Min

nelu

sa

Form

atio

n, a

nd M

adis

on L

imes

tone

).

2B

ear

Gul

ch5.

523

.518

.581

43.

51.

4T

he p

redo

min

ant r

ocks

exp

osed

in th

e dr

aina

ge a

rea

are

igne

ous

rock

s w

ithin

the

Tin

ton

Dom

e. S

trea

mfl

ow

gene

rall

y is

less

than

the

com

bine

d lo

ss r

ate

for

the

outc

rops

(M

inne

kaht

a L

imes

tone

, Min

nelu

sa F

orm

atio

n,

and

Mad

ison

Lim

esto

ne).

Cro

w C

reek

Bas

in

3B

eave

r C

reek

8.5

24.3

17.0

729

3.5

2.2

Cha

ract

eris

tics

are

sim

ilar

to B

ear

Gul

ch.

4C

row

Pea

k.5

22.6

46.9

45--

3.5

.1T

his

area

, an

igne

ous

dom

e, d

rain

s m

ostly

tow

ards

Cro

w

Cre

ek w

ith s

ome

runo

ff d

rain

ing

tow

ards

Hig

gins

Gul

ch o

n th

e ea

st s

ide.

Mos

t run

off

prob

ably

is lo

st a

s fl

ow c

ross

es

the

Mad

ison

Lim

esto

ne a

nd M

inne

lusa

For

mat

ion

outc

rops

sur

roun

ding

the

dom

e.

Spea

rfis

h C

reek

Bas

in

5C

itade

l Roc

k1.

123

.928

.247

--3.

5.3

Cha

ract

eris

tics

are

sim

ilar

to C

row

Pea

k.

6Sp

earf

ish

Cre

ek

(abo

ve p

ower

pl

ant d

iver

sion

)

142.

125

.121

.46

3 235

52.3

Mos

t of

the

flow

is d

iver

ted

arou

nd th

e lo

ss z

one

by a

dam

an

d an

aqu

educ

t to

a hy

droe

lect

ric

pow

er p

lant

. T

he

dive

rsio

n ra

te is

bet

wee

n 11

5 an

d 13

5 ft

3 /s.2 T

he a

vera

ge

daily

flo

w (

wat

er y

ears

198

9-97

)4 exc

eede

d 11

5 ft

3 /s o

nly

7 pe

rcen

t of

the

days

.

7R

ubic

on G

ulch

(B

rida

l Vei

l Fal

ls)

1.4

26.2

26.0

6021

4.4

Con

flue

nce

with

Spe

arfi

sh C

reek

is lo

cate

d do

wns

trea

m fr

om

pow

er d

iver

sion

; thu

s, e

ntir

e fl

ow g

ener

ally

is lo

st.

8Sp

earf

ish

Cre

ek

(nea

r R

obis

on

Gul

ch

2.7

25.4

41.2

1621

51.

0D

risc

oll a

nd H

ayes

(19

95)

estim

ated

that

the

com

bine

d di

scha

rge

from

this

dra

inag

e ar

ea, R

ubic

on G

ulch

, and

le

akag

e fr

om th

e po

wer

pla

nt d

iver

sion

dam

ave

rage

d ab

out 3

ft3 /s

.

9Sp

earf

ish

Peak

(S

pear

fish

Cre

ek

trib

utar

ies)

1.1

24.9

28.7

794

.3T

his

drai

nage

are

a is

an

igne

ous

intr

usio

n in

the

Mad

ison

L

imes

tone

and

Min

nelu

sa F

orm

atio

n ou

tcro

ps d

rain

ing

in

seve

ral d

irec

tions

to th

e su

rrou

ndin

g be

droc

k. M

ost r

unof

f pr

obab

ly is

lost

to th

e M

adis

on L

imes

tone

and

Min

nelu

sa

Form

atio

n ou

tcro

ps.


Fal

se B

otto

m C

reek

Bas

in

10B

urno

Gul

ch2.

526

.429

.473

--5

0.9

Som

e st

ream

flow

loss

is li

kely

as

Bur

no G

ulch

cro

sses

ap

prox

imat

ely

0.75

mi o

f M

adis

on L

imes

tone

out

crop

and

1

mi o

f M

inne

lusa

For

mat

ion

outc

rop

befo

re jo

inin

g Fa

lse

Bot

tom

Cre

ek o

n th

e M

inne

lusa

For

mat

ion

outc

rop.

11Fa

lse

Bot

tom

Cre

ek7.

027

.429

.775

154

2.0

Mos

t of

the

stre

amfl

ow is

lost

to th

e th

ree

bedr

ock

aqui

fers

ex

cept

dur

ing

inte

nse

runo

ff e

vent

s. E

stim

ated

loss

th

resh

olds

are

4 f

t3 /s f

or th

e M

adis

on L

imes

tone

out

crop

, 7.

2 ft

3 /s f

or th

e M

inne

lusa

For

mat

ion

outc

rop,

and

abo

ut

7 ft

3 /s f

or th

e M

inne

kaht

a L

imes

tone

out

crop

.2

12Fa

lse

Bot

tom

Cre

ek

trib

utar

y.8

25.9

26.1

100

--4.

5.3

Thi

s ar

ea, a

n ig

neou

s in

trus

ion

with

in th

e M

adis

on L

imes

tone

an

d M

inne

lusa

For

mat

ion

outc

rops

, dra

ins

tow

ards

Fal

se

Bot

tom

Cre

ek.

Som

e of

the

stre

amfl

ow p

roba

bly

is lo

st to

th

e M

adis

on L

imes

tone

and

Min

nelu

sa F

orm

atio

n ou

tcro

ps

befo

re jo

inin

g Fa

lse

botto

m C

reek

.

13Te

tro

Cre

ek.7

26.5

25.6

37--

4.2

Mos

t of t

he fl

ow fr

om th

is a

rea

prob

ably

is lo

st a

s Te

tro

Cre

ek

cros

ses

appr

oxim

atel

y 2

mi o

f M

adis

on L

imes

tone

and

M

inne

lusa

For

mat

ion

outc

rops

bef

ore

join

ing

Fals

e B

otto

m

Cre

ek b

elow

the

Mad

ison

Lim

esto

ne a

nd M

inne

lusa

Fo

rmat

ion

outc

rops

.

14M

iller

Cre

ek2.

726

.330

.462

--4.

5.9

Mill

er C

reek

and

trib

utar

y st

ream

s or

igin

atin

g in

this

dra

inag

e ar

ea c

ross

abo

ut 1

to 1

.5 m

i of

Mad

ison

Lim

esto

ne a

nd

Min

nelu

sa F

orm

atio

n ou

tcro

ps b

efor

e jo

inin

g Po

lo C

reek

do

wns

trea

m.

15Po

lo C

reek

3.2

27.2

29.3

75--

4.5

1.0

Polo

Cre

ek a

nd tr

ibut

ary

stre

ams

orig

inat

ing

in th

is d

rain

age

area

cro

ss s

ome

Mad

ison

Lim

esto

ne o

utcr

op a

nd o

ver

1 m

i of

Min

nelu

sa F

orm

atio

n be

fore

join

ing

Fals

e B

otto

m C

reek

do

wns

trea

m.

Whi

tew

ood

Cre

ek B

asin

16Sl

augh

ter

Hou

se

Gul

ch.4

27.3

25.9

50--

4.5

.1M

ost o

f th

e st

ream

flow

fro

m th

is a

rea

prob

ably

is lo

st a

s Sl

augh

ter

Hou

se G

ulch

cro

sses

app

roxi

mat

ely

1 m

i of

Mad

ison

Lim

esto

ne o

utcr

op b

efor

e jo

inin

g W

hite

woo

d C

reek

.

17W

hite

woo

d C

reek

46.9

27.6

26.2

600

4.5

15.5

Sign

ific

ant l

osse

s do

not

occ

ur to

the

bedr

ock

outc

rops

alo

ng

Whi

tew

ood

Cre

ek.

Min

e ta

iling

s po

ssib

ly s

eal t

he lo

ss

zone

s.2

Tab

le 1

3.

Cha

ract

eris

tics

of d

rain

age

area

s up

stre

am fr

om s

trea

mflo

w-lo

ss z

ones

–Con

tinue

d

[mi2 , s

quar

e m

iles;

in.,

inch

es; f

t3 /s, c

ubic

fee

t per

sec

ond;

--,

no

data

]

Sit

e id

enti

fi-

cati

on

nu

mb

er

Nam

eA

rea

(mi2 )

Ave

rag

ep

reci

p-

itat

ion

(in

.)

Ave

rag

esl

op

e(p

erce

nt)

Per

cen

tig

neo

us

and

met

a-m

orp

hic

rock

s1

Lo

ssth

resh

old

2

(ft3 /

s)

Est

imat

ed

ann

ual

yiel

d(i

n.)

Est

imat

edm

ean

dis

char

ge

(ft3 /

s)

Co

mm

ents


Bea

r B

utte

Cre

ek B

asin

18B

ear

But

te

trib

utar

ies

(Bou

lder

Par

k)

3.5

26.3

15.5

1--

41.

0T

his

area

con

tain

s ab

out o

ne-t

hird

Min

neka

hta

Lim

esto

ne,

with

the

rem

aind

er p

redo

min

antly

sha

le u

nits

. T

hese

tr

ibut

arie

s jo

in B

ear

But

te C

reek

with

in th

e M

inne

lusa

ou

tcro

p. E

stim

ated

loss

thre

shol

ds a

re 4

ft3 /s

for

this

do

wns

trea

m r

each

.2

19B

ould

er C

reek

2.2

27.2

18.3

0--

4.6

Bou

lder

Cre

ek c

ross

es a

ppro

xim

atel

y 1

mi o

f M

adis

on

Lim

esto

ne o

utcr

op a

nd 2

mi o

f M

inne

lusa

For

mat

ion

outc

rop

befo

re jo

inin

g B

ear

But

te C

reek

.

20Tw

o B

it C

reek

6.8

27.4

25.8

49--

42.

0Tw

o B

it C

reek

cro

sses

app

roxi

mat

ely

1.5

mi o

f M

adis

on

Lim

esto

ne o

utcr

op b

efor

e jo

inin

g B

ould

er C

reek

.

21B

ould

er C

reek

tr

ibut

arie

s1.

226

.931

.510

0--

4.4

Thi

s ar

ea o

f in

trus

ive

igne

ous

rock

s in

clud

es s

ever

al s

mal

l dr

aina

ge c

hann

els

that

cro

ss a

bout

0.5

to 1

mi o

f m

ostly

M

inne

lusa

For

mat

ion

outc

rop

befo

re jo

inin

g B

ould

er

Cre

ek.

22L

ost G

ulch

1.5

26.8

29.7

80--

4.4

Dra

inag

e fr

om th

is a

rea

cros

ses

abou

t 2 m

i of

Mad

ison

L

imes

tone

and

Min

nelu

sa F

orm

atio

n ou

tcro

p be

fore

jo

inin

g B

ear

But

te C

reek

.

23B

ear

But

te C

reek

15.9

26.0

20.8

7212

3.8

4.4

Ave

rage

str

eam

flow

loss

5 was

4.3

ft3 /s

for

wat

er y

ears

19

89-9

7. D

urin

g th

is p

erio

d, th

e av

erag

e da

ily f

low

ex

ceed

ed th

e lo

ss th

resh

old

15 p

erce

nt o

f th

e da

ys.

24Pa

rk C

reek

3.2

24.7

24.2

20--

3.5

.8D

rain

age

from

this

are

a cr

osse

s ab

out 1

.5 m

i of

Mad

ison

L

imes

tone

out

crop

bef

ore

join

ing

Bea

r B

utte

Cre

ek.

Elk

Cre

ek B

asin

25E

lk C

reek

(no

rth

trib

utar

y)1.

023

.817

.129

--3.

5.3

Part

s of

this

trib

utar

y st

ream

flow

cou

ld b

e lo

st to

Mad

ison

L

imes

tone

out

crop

bef

ore

the

stre

am jo

ins

Elk

Cre

ek a

bove

th

e ar

ea o

f M

adis

on L

imes

tone

out

crop

cro

ssed

by

Elk

C

reek

.

26E

lk C

reek

22.3

24.2

15.3

5819

46.

6A

vera

ge s

trea

mfl

ow lo

ss6 w

as 7

.6 f

t3 /s f

or w

ater

yea

rs

1991

-97.

Dur

ing

this

per

iod,

the

aver

age

daily

flo

w

exce

eded

the

loss

thre

shol

d 18

per

cent

of

the

days

.

27M

eado

w C

reek

1.9

22.5

16.3

18--

2.5

.3D

rain

age

from

this

are

a cr

osse

s ab

out 0

.5 m

i of

Mad

ison

L

imes

tone

out

crop

bef

ore

join

ing

Elk

Cre

ek w

ithin

the

Mad

ison

Lim

esto

ne o

utcr

op.

Mos

t of t

he s

edim

enta

ry ro

ck

outc

rop

in th

e dr

aina

ge a

rea

is D

eadw

ood

Form

atio

n.

Tab

le 1

3.

Cha

ract

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Table 14. Data pertaining to annual yield for selected gaging stations in Lawrence County

Station number Station nameArea

(square miles)

Period ofrecord

(water years)

Annual yield(inches)

Interior Sedimentary Basins

06408700 Rhoads Fork near Rochford 7.95 1983-93 8.51

06430770 Spearfish Creek near Lead 63.5 1989-93 3.34

06430850 Little Spearfish Creek near Lead 25.8 1989-93 6.90

Interior Crystalline Basins

06422500 Boxelder Creek near Nemo 96.0 1967-93 2.17

06430800 Annie Creek near Lead 3.55 1989-93 3.10

06430898 Squaw Creek near Spearfish 4.07 1989-93 4.07

06436156 Whitetail Creek at Lead 6.15 1989-93 5.59

06437020 Bear Butte Creek near Deadwood 16.6 1989-93 3.77

Runoff from the areas between the Minnekahta and Minnelusa aquifer hydrogeologic units (pl. 2) probably recharges the Minnekahta aquifer. Most runoff from other small areas not delineated within the Minnekahta, Minnelusa, and Madison aquifer hydro-geologic units also becomes recharge to the adjacent hydrogeologic units.

USE OF GROUND-WATER SENSITIVITY MAP

The map of ground-water sensitivity to contami-nation (pl. 1) is intended to be used as a screening tool to compare the sensitivity of areas in Lawrence County to contamination. Each delineated area (sensitivity unit) on the map is identified by a color and a four-character alphanumeric code—one letter and three digits. Drainage areas upstream from potential stream-flow-loss zones also are delineated and described (pl. 2, table 13). The following sections describe how to relate the sensitivity code and the drainage area labels to hydrogeologic information. The limitations of the map also are discussed.

Relating Sensitivity Units to Hydrogeologic Information

The 11 hydrogeologic settings, which group areas with similar hydrologic and geologic characteris-tics, are identified by shades of color on plate 1. The name of the hydrogeologic setting is related to the

48 Sensitivity of Ground Water to Contamination in Lawrence

predominant rock type or physiographic location. More detailed information about the hydrogeologic settings is given in table 1.

The sensitivity map includes 956 sensitivity polygons that are labeled with identifying codes. The letter code identifies the hydrogeologic unit, which includes relative sensitivity ratings for aquifer media, unsaturated media, and hydraulic conductivity. The rel-ative sensitivity ratings were compiled on a scale of 1 to 10 with 10 being the most sensitive. The 45 hydrogeo-logic units are organized in descending order that are sorted on the sensitivity rating for unsaturated media, then aquifer media, and then hydraulic conductivity. The codes begin with an uppercase “A” and continue through lower case “s.” When the first 26 uppercase letters were used, the codes were continued with lower case letters. The explanation of alphabetic hydrogeo-logic-unit codes on the sensitivity map includes the rel-ative sensitivity ratings, the geologic units, and a description of the aquifer and unsaturated media. Com-ments in table 1 provide additional information describing characteristics unique to each hydrogeologic unit.

The first digit in the sensitivity-unit code repre-sents the recharge rate with group 1 the most sensitive and group 4 the least sensitive. The second digit in the sensitivity-unit code represents the depth to water. The depth-to-water group includes three quantitative groups and two qualitative groups. The three quantitative depth-to-water groups were ranked with group 1 the most sensitive and group 3 the least sensitive. The two qualitative groups were areas with highly variable depths to water. The stream valleys in the mountainous

County, South Dakota

areas were assumed to have shallower depths to water than adjacent areas and were included in group 4. Although the depth to water is highly variable in these areas, the delineation provides a basis for some relative comparisons to group 5. The third digit in the sensi-tivity-unit code represents the land-surface slope, which consists of five groups, with group 1 the most sensitive and group 5 the least sensitive.

Drainage areas above streamflow-loss zones were delineated on plates 1 and 2 with thick blue lines and a site identification near the upstream end of the streamflow-loss zone. The drainage areas listed on plate 1 also include estimates of streamflow-loss thresholds, if available, and the area of the drainage. Additional analysis and comments describing the drainage area and the loss zone are referenced to the site identification and listed in table 13.

Two areas were selected to describe examples of using the sensitivity map. Example area 1, in the part of section 3 in T. 6 N., R. 2 E. east of Interstate 90 located about 1 mi northeast of Spearfish, includes sensitivity polygons with three unique sensitivity-unit codes, h455, o451, o455. In area h455, the letter code indicates a semiconfining unit with interbedded sand-stone, limestone, and shale with a moderate sensitivity rank for aquifer media and a low sensitivity rank for unsaturated media, and hydraulic conductivity. The digit codes indicate that, with regard to sensitivity, the recharge-rate group is low, the depth-to-water group is highly variable, and the land-surface-slope group is low. In area o451, the letter code indicates Spearfish Formation with a low sensitivity rank for aquifer media, unsaturated media and hydraulic conductivity. The digit codes indicate that, with regard to sensitivity, the recharge-rate group is low, the depth-to-water group is highly variable, and the land-surface-slope group is high. Area o455 is similar to area o451 except the land-surface-slope group has a low sensitivity rank.

Example area 2, which is complicated by streamflow loss, is located in section 8 in T. 5 N., R. 2 E. west of Spearfish Creek located about 6 mi south of Spearfish, and includes sensitivity polygons with two unique sensitivity-unit codes, B155 and f455. In area B155, the letter code indicates Madison Lime-stone, with a high sensitivity rank for aquifer media and unsaturated media, and a moderately high sensitivity rank for hydraulic conductivity. The digit codes indicate that the recharge-rate group is high, the depth-to-water group is highly variable, and the land-surface-slope group is low. In area f455, the letter code indicates Whitewood and Winnipeg Formations, with a

low sensitivity rank for aquifer media, unsaturated media and hydraulic conductivity. The area also is within drainage areas upstream from potential stream-flow-loss zones beginning at sites 6 and 7. Most of the area is Madison Limestone, which produces very little surface runoff. However, runoff from the drainage area upstream from site 6 would contribute to a loss zone that usually has flow less than the loss threshold (table 13). The drainage area upstream from site 8 would contribute to a stream with larger flow, except that flow is usually directed to the hydroelectric power plant diversion.

A comparison of the sensitivity characteristics of the two areas indicate that example area 2 is much more sensitive than example area 1. The small part of example area 2 with the lower sensitivity characteris-tics also contributes runoff that recharges the Madison aquifer through streamflow loss. The recharge rate is much greater in example area 2. The depth to water in both areas is highly variable. The significance of the land-surface-slope groups in comparing these areas is not as important because of the large difference in hydrogeologic unit and recharge-rate groups.

Limitations of Sensitivity Map

Some limitations need to be considered when using the map. One of the most important limitations is that of scale. On a 1:100,000-scale map, a section of land, 5,280 ft on a side (640 acres), is described in an area less than 1-in. square; therefore, the hydrogeologic information includes numerous generalizations because of the complex geology in the study area. Modifying or overlaying the map at a scale that is more detailed than the original map is inappropriate and may be extremely misleading.

Available hydrologic data, such as hydraulic heads and descriptions of ground-water flowpaths, pro-vide a limited description of the complex hydrology. Predicting the fate and transport of a potential contam-inant is difficult because of fractured rocks and solution openings. This map assumes that a potential contami-nant has the mobility of water. Some potential contam-inants may have mobility characteristics different than water, such as oil, which floats on top of water, or some solvents, which may be more dense than water and sink. Also, the analysis of potential ground-water contaminants from nonpoint sources such as agricul-tural fields may require different considerations and approaches than a site-specific spill of a highly toxic substance.

Use of Ground-Water Sensitivity Map 49

The intended use of the sensitivity map is as a screening tool used in conjunction with other informa-tion and analysis. The map is not designed to replace the need for site-specific investigations or to be the sole criterion in making land-use decisions.

SUMMARY

Sensitivity of ground water to contamination in Lawrence County was mapped by subdividing the study area into map areas with relative ranking of intrinsic hydrogeologic sensitivity. Six intrinsic hydro-geologic characteristics, aquifer media, unsaturated media, hydraulic conductivity, recharge rate, depth to water, and land-surface slope were evaluated sequen-tially and overlaid to delineate polygons with compa-rable sensitivity characteristics. Streamflow loss, an additional mechanism of transport of potential contam-inants, was considered by delineating and describing drainage areas upstream from the potential streamflow-loss zones.

The method used for this study was a modifica-tion of the DRASTIC method (Aller and others, 1987). Hydrogeologic characteristics were evaluated and identified with a comparable code as described by Hearne and others (1995). A single weighted pollution potential index was not calculated. Streamflow-loss characteristics were considered based on the modifica-tion of DRASTIC by Davis and others (1994) that iden-tified increased pollution potential for karst sinkholes. The rating conventions of DRASTIC were modified to provide a relative ranking of hydrogeologic character-istics without assignment of a combined numerical score. The results of studies that compared observed water-quality and selected hydrogeologic characteris-tics at other locations were considered in the analysis of individual map layers. Additional modifications were made based on the availability of data. Soil character-istics were not included as a map layer because detailed digital data were not available; however, the general distribution of two soil characteristics were shown. A limited comparison of depth to water was made because of the highly variable depth to water in a large part of the study area.

The study area was subdivided into a framework of 11 hydrogeologic settings with unique hydrologic and geologic factors that control the movement of ground water. Within this framework, the hydrogeo-logic-units map layer was delineated by evaluating aquifer media, unsaturated media, and hydraulic


conductivity for sensitivity to contamination. A total of 45 hydrogeologic units were delineated and compara-tively ranked. The most sensitive hydrogeologic units included the limestones, alluvium, unconsolidated sands and gravels, and some sandstones. The least sen-sitive units included shales or units with interbedded shales.

The recharge-rate map delineated sensitivity areas by four recharge-rate groups ranging from 0 to 10 in/yr. The highest recharge-rate group (most sensi-tive) included most of the Madison Limestone outcrop except in the southeastern corner of the study area where precipitation rates are lowest. The lowest recharge-rate group (least sensitive) included mostly igneous intrusive rocks, shale units, and igneous and metamorphic rocks.

The depth-to-water map delineated sensitivity areas by five depth-to-water groups. The first three categories compared areas where data were available to estimate depth to water. The shallowest depth to water (most sensitive) included the alluvium. Group 3 (least sensitive of these three categories) included some of the sandstones. The depth to water in most of the study area was highly variable and was included in groups 4 and 5. Group 4 included delineated stream valleys, which generally would have shallower depth to water compared to adjacent areas with higher land-surface altitudes. Group 5 also included the remaining areas with highly variable depth to water.

The land-surface-slope map delineated sensi-tivity areas by five land-surface-slope groups that com-pared percent slope calculated from Digital Elevation Models. The smallest percent slopes (most sensitive) were in the northern part of the study area. The largest percent slopes (least sensitive) were in the south and central part of the study area.

Thirty drainage areas were delineated upstream from potential streamflow-loss zones ranging in area from 0.3 to 142.1 mi2. Mean discharge estimated for the drainage areas ranged from 0.1 to 52.3 ft3/s. Approximate streamflow-loss thresholds were avail-able for several of the loss zones. Comparisons of mean discharges with loss thresholds showed that runoff from most of these drainage areas during most of the year would become ground-water recharge.

The various map layers were overlain and inter-sected to produce a 1:100,000-scale map (pl. 1) showing 956 delineated polygons identified with a sensitivity-unit code, a letter code, and three digits. The letter code represents the hydrogeologic unit, with the first digit representing the recharge-rate group, the

ounty, South Dakota

second digit representing the depth-to-water group, and the third digit representing the land-surface-slope group. The hydrogeologic settings were color shaded. Drainage areas above streamflow-loss zones were out-lined and identified with a number referring to a table of information describing the drainage areas. A second plate showed the drainage areas in relation to the streamflow-loss zones, selected hydrogeologic units from Strobel and others (1999), and selected stream-flow-gaging stations. The sensitivity maps were designed as screening tools and do not replace the need for site-specific investigations.

SELECTED REFERENCES

Aller, L., Bennett, T., Lehr, J.H., Petty, R.J., and Hackett, G., 1987, DRASTIC: A standardized system for evaluating ground water pollution potential using hydrogeologic settings: U.S. Environmental Protection Agency, EPA/600/2-87/035, 455 p.

Barbash, J.E., and Resek, E.A., 1996, Pesticides in ground water—distribution, trends, and governing factors: Chelsea, Mich., Ann Arbor Press, Inc., 588 p.

Carter, J.M., and Redden, J.A., 1999a, Altitude of the top of the Minnelusa Formation in the Black Hills area, South Dakota: U.S. Geological Survey Hydrologic Investiga-tions Atlas HA-744-C, 2 sheets, scale 1:100,000.

———1999b, Altitude of the top of the Madison Limestone in the Black Hills area, South Dakota: U.S. Geological Survey Hydrologic Investigations Atlas HA-744-D, 2 sheets, scale 1:100,000.

———1999c, Altitude of the top of the Deadwood Forma-tion in the Black Hills area, South Dakota: U.S. Geological Survey Hydrologic Investigations Atlas HA-744-E, 2 sheets, scale 1:100,000.

Davis, A.D., Long, A.J., Nazir, M., and Xiaodan, T., 1994, Ground-water vulnerability in the Rapid Creek Basin above Rapid City, South Dakota: Final Technical Report prepared for the U.S. Environmental Protection Agency, Contract X008788-01-0, 78 p.

DeWitt, Ed, Redden, J.A., Buscher, David., and Wilson, A.B., 1989, Geologic map of the Black Hills area, South Dakota and Wyoming: U.S. Geological Survey Map I-1910, scale 1:250,000.

Downey, J.S., 1986, Geohydrology of bedrock aquifers in the Northern Great Plains in parts of Montana, North Dakota, South Dakota, and Wyoming: U.S. Geological Survey Professional Paper 1402-E, 87 p.

Driscoll, D.G., and Bradford, W.L., 1994, Compilation of selected hydrologic data, through water year 1992, Black Hills Hydrology Study, western South Dakota: U.S. Geological Survey Open-File Report 94-319, 158 p.

Druliner, A.D., Chen, H.H., and McGrath, T.S., 1996, Rela-tions of nonpoint-source nitrate and atrazine concentra-tions in the High Plains aquifer to selected explanatory variables in six Nebraska study areas: U.S. Geological Survey Water-Resources Investigations Report 95-4202, 51 p.

Environmental Systems Research Institute, Inc., 1992, ARC/INFO users guide—the geographic information system software: Redlands, Calif. [variously paged].

Freeze, R.A. and Cherry, J.A., 1979, Groundwater: Engle-wood Cliffs, N.J., Prentice-Hall, 604 p.

Greene, E.A., 1993, Hydraulic properties of the Madison aquifer system in the western Rapid City area, South Dakota: U.S. Geological Survey Water-Resources Investigations Report 93-4008, 56 p.

Greene, E.A., Shapiro, A.M., and Carter, J.M., 1998, Hydro-geologic characterization of the Minnelusa and Madison aquifers near Spearfish, South Dakota: U.S. Geological Survey Water-Resources Investigations Report 98-4156, 64 p.

Gries, J.P., and Martin J.E., 1985, Composite outcrop section of the Paleozoic and Mesozoic strata in the Black Hills and surrounding areas, in Rich, F.J., ed., Geology of the Black Hills, South Dakota and Wyoming (2d ed.): Alexandria, Va., American Geological Institute, p. 261-292.

Hearne, G.A., Wireman, Michael, Campbell, A.S., Turner, Sandy, and Ingersoll, G.P., 1995, Vulnerability of the uppermost ground water to contamination in the Greater Denver area, Colorado: U.S. Geological Survey Water-Resources Investigations Report 92-4143, 244 p.

Heath, R.C., 1984, Ground-water regions of the United States: U.S. Geological Survey Water-Supply Paper 2242, 78 p.

Hortness, J.E., and Driscoll, D.G., 1998, Streamflow losses in the Black Hills of western South Dakota: U.S. Geological Survey Water-Resources Investigations Report 98-4116, 99 p.

Klemp, J.A., 1995, Source aquifers for large springs in northwestern Lawrence County, South Dakota: Rapid City, South Dakota School of Mines and Technology, unpublished M.S. thesis, 175 p.

Koterba, M.T., Banks, W.S.L., and Shedlock, R.J., 1993, Pesticides in shallow groundwater in the Delmarva Pen-insula: Journal of Environmental Quality, v. 22, no. 3, p. 500-518.

Kyllonen, D.P. and Peter, K.D., 1987, Geohydrology and water quality of the Inyan Kara, Minnelusa, and Madison aquifers of the northern Black Hills, South Dakota and Wyoming, and Bear Lodge Mountains, Wyoming: U.S. Geological Survey Water-Resources Investigations Report 86-4158, 61 p.

Selected References 51

Lisenbee, A.L., 1985, Studies of the Tertiary intrusions of the northern Black Hills uplift, South Dakota and Wyoming; a historical review, in Rich, F.J., ed., Geol-ogy of the Black Hills, South Dakota and Wyoming (2d ed.): Alexandria, Va., American Geological Institute, p. 106-125.

———1991a, Geologic map of the Deadwood North quad-rangle, Lawrence County, South Dakota: South Dakota School of Mines and Technology, scale 1:24,000.

———1991b, Geologic map of the Lead quadrangle, Lawrence County, South Dakota: South Dakota School of Mines and Technology, scale 1:24,000.

———1991c, Geologic map of the Maurice quadrangle, Lawrence County, South Dakota: South Dakota School of Mines and Technology, scale 1:24,000.

———1991d, Geologic map of the Savoy quadrangle, Lawrence County, South Dakota: South Dakota School of Mines and Technology, scale 1:24,000.

———1991e, Geologic map of the Spearfish quadrangle, Lawrence County, South Dakota: South Dakota School of Mines and Technology, scale 1:24,000.

———1991f, Geologic map of the Deadman Mountain quadrangle, Lawrence County, South Dakota: South Dakota School of Mines and Technology, scale 1:24,000.

———1991g, Geologic map of the Tilford quadrangle, Lawrence County, South Dakota: South Dakota School of Mines and Technology, scale 1:24,000.

Lisenbee, A.L., and Redden, J.A., 1991a, Geologic map of the Deadwood South quadrangle, Lawrence County, South Dakota: South Dakota School of Mines and Technology, scale 1:24,000.

———1991b, Geologic map of the Sturgis quadrangle, Lawrence County, South Dakota: South Dakota School of Mines and Technology, scale 1:24,000.

Miller, L.D., and Driscoll, D.G., 1998, Streamflow charac-teristics for the Black Hills of South Dakota, through water year 1993: U.S. Geological Survey Water-Resources Investigations Report 97-4288, 322 p.

Peter, K.D., 1985, Availability and quality of water from the bedrock aquifers in the Rapid City area, South Dakota: U.S. Geological Survey Water-Resources Investiga-tions Report 85-4022, 34 p.

Rahn, P.H., and Gries, J.P., 1973, Large springs in the Black Hills, South Dakota and Wyoming: South Dakota Geological Survey Report of Investigations 107, 46 p.

Robinson, C.S., Mapel, W.J., and Bergendahl, M.H., 1964, Stratigraphy and structure of the northern and western flanks of the Black Hills uplift, Wyoming, Montana, and South Dakota: U.S. Geological Survey Profes-sional Paper 404, 134 p.


Rupert, M.G., 1998, Probability of detecting atrazine/desethyl-atrazine and elevated concentrations of nitrate in ground water in the Idaho part of the Upper Snake River basin: U.S. Geological Survey Water-Resources Investigations Report 98-4203, 32 p.

———1999, Improvements to the DRASTIC ground-water vulnerability mapping method: U.S. Geological Survey Water-Resources Fact Sheet FS-066-99, 6 p.

Strobel, M.L., Jarrell, G.J., Sawyer, J.F., Schleicher, J.R., and Fahrenbach, M.D., 1999, Distribution of hydrogeologic units in the Black Hills area, South Dakota: U.S. Geological Survey Hydrologic Investigations Atlas HA-743, 3 sheets, scale 1:100,000.

Theis, C.V., 1935, The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using ground-water storage: American Geophysical Union Transactions, v. 16, p. 518-524.

U.S. Department of Agriculture, 1994, State soil geographic data base (STATSGO): U.S. Department of Agricul-ture, Publication no. 1492, 114 p.

U.S. Department of Commerce, 1994, Climatological data for South Dakota, annual summary: Asheville, N.C. (issued annually).

U.S. Environmental Protection Agency, 1992, National survey of pesticides in drinking water wells, Phase II report: U.S. Environmental Protection Agency, EPA/579/09-91-020, 176 p.

———1993, A review of methods for assessing aquifer sen-sitivity and ground water vulnerability to pesticide con-tamination: U.S. Environmental Protection Agency, EPA/813/R-93/002, 147 p.

U.S. Geological Survey, 1967-75, Water resources data for South Dakota, 1966-74—part 1. Surface-water records (published annually).

———1976-98, Water resources data for South Dakota, water years 1975-97: U.S. Geological Survey Water-Data Reports SD-75-1 to SD-97-1 (published annually).

———1986, Land use land cover digital data from 1:250,000 and 1:100,000-scale maps: U.S. Geological Survey Data Users Guide 4, 36 p.

———1993, Digital elevation models data user’s guide (3d ed.): National Mapping Program, 47 p.

Van Lieu, J.A., 1969, Geologic map of the Four Corners quadrangle, Wyoming and South Dakota: U.S. Geological Survey Map I-581, scale 1:48,000.

ounty, South Dakota

SUPPLEMENTAL INFORMATION

Command File 1--Command file to create generalized slope categories and eliminate polygons with areas less than 100 acres.

grid

/***Group slope grid into categories (table 12).

rc10 = reclass(lawco_slp, reclass.tab)

/***Merge contiguous areas less than 10 acres by nibbling area from adjacent areas.

mask10 = setnull(zonalarea(regiongroup(rc10)) < 40469.5, 1) /*10 acres

n10 = nibble(rc10, mask10, dataonly)

/***Merge contiguous areas less than 20 acres by nibbling area from adjacent areas.

mask20 = setnull(zonalarea(regiongroup(n10)) < 80939, 1) /*20 acres

n20 = nibble(n10, mask20, dataonly)

***Merge contiguous areas less than 50 acres by nibbling area from adjacent areas.

mask50 = setnull(zonalarea(regiongroup(n20)) < 202347.5, 1) /*50 acres


***Merge contiguous areas less than 100 acres by nibbling area from adjacent areas.

mask100 = setnull(zonalarea(regiongroup(n50)) < 404695, 1) /*100 acres


/***Remove narrow sliver polygons by replacing the value in each cell with the majority /***of the values

within a circle with a 5 cell radius.

n100fm = focalmajority(n100, circle, 5, data)

***Merge contiguous areas less than 100 acres created in the step above (focalmajority) /***by nibbling area

from adjacent areas greater than 100 acres.

mask100fm = setnull(zonalarea(regiongroup(n100fm)) < 404695, 1) /*100 acres

slp_cat = nibble(n100fm, mask100fm, dataonly)

/***Remove temporary working coverages

&if [exists rc10 -grid] &then kill rc10 all

&if [exists n10 -grid] &then kill n10 all

&if [exists mask10 -grid] &then kill mask10 all







&if [exists n100fm -grid] &then kill n100fm all

&if [exists mask100fm -grid] &then kill mask100fm all

Command File 1 55

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