application of electromagnetic techniques in survey of contaminated groundwater at an abandoned mine...

Application of Electromagnetic Techniques in Survey of Contaminated Groundwater at an Abandoned Mine Complex in Southwestern Indiana, U.S.A. GLENN A. BROOKS Weston Designers/Consultants 100 Corporate North, Suite 101 Bannockburn, Illinois 60015, U.S.A. GREG A. OLYPHANT Department of Geology Indiana University Bloomington, Indiana 47405, U.S.A. DENVER HARPER Indiana Geological Survey 61t N. Walnut Grove Bloomington, Indiana 47405, U.S.A.

ABSTRACT / In part of a large abandoned mining complex, electromagnetic geophysical surveys were used along with data derived from cores and monitoring wells to infer sources of contamination and subsurface hydrologic connections between acidic refuse deposits and adjacent undisturbed geologic materials.

Electrical resistivity increases sharply along the boundary of

an elevated deposit of pyritic coarse refuse, which is highly contaminated and electrically conductive, indicating poor subsurface hydrologic connections with surrounding deposits of fine refuse and undisturbed glacial material. Groundwater chemistry, as reflected in values of specific conductance, also differs markedly across the deposit's boundary, indicating that a widespread contaminant plume has not developed around the coarse refuse in more than 40 yr since the deposit was created. Most acidic drainage from the coarse refuse is by surface runoff and is concentrated around stream channels.

Although most of the contaminated groundwater within the study area is concentrated within the surficial refuse deposits, transects of apparent resistivity and phase angle indicate the existence of an anomalous conductive layer at depth (>4 m) in thick alluvial sediments along the northern boundary of the mining complex. Based on knowledge of local geology, the anomaly is interpreted to represent a subsurface connection between the alluvium and a flooded abandoned underground mine.

Introduction

Geophysical techniques are increasingly being uti- lized to study the distribution of contaminants in shallow aquifers and to infer subsurface hydrologic connections in geologically complex areas (for example, Cartwright and McComas 1968; Warner 1969; Merkel 1972; Stollar and Roux 1975; Kelly 1976; Mazac and others 1987; Ebraheem and others 1990). Most of these studies have employed direct-current electrical resistivity methods (Zohdy and others 1974), but alternative techniques exist, including ground- penetrating radar and various other electromagnetic methods.

Water quality and hydrologic characteristics of various materials are being studied across a large abandoned mining complex in southwestern Indiana as part of ongoing investigations of reclamation feasibility. These studies have relied heavily upon point- specific information acquired from monitoring wells and drill cores. In conjunction with these conventional monitoring methods, electromagnetic geophysical methods using commercially available instruments were employed within a smaller study area. Informa-

tion is presented regarding the distribution of apparent conductivity, apparent resistivity, and phase angle, which are the geophysical parameters that were measured. The resulting patterns are interpreted to reflect variations in groundwater chemistry, as indicated by specific conductance (SpC). The observed distributions support inferences based on hydrologic characteristics of the study area and provide indirect evidence of a subsurface connection between an abandoned underground mine and the alluvial sediments of an adjacent floodplain.

Site Description

The abandoned Friar Tuck Mine complex is located in the northeast quarter of Section 36, T. 8 N., R. 8 W., Sullivan County, Indiana. Prior to mining, the area contained vegetated deposits of loess, till, and alluvium overlying Pennsylvanian bedrock. Extensive coal mining by underground and surface methods commenced during the 1930s, and much pyrite- bearing refuse (both coarse- and fine-grained) from a preparation facility was deposited at many sites scat-

Environ Geol Water Sci Vol. 18, No, 1, 39-47 © 1991 Springer-Verlag New York Inc,

40 G.A. Brooks et al.

EXPLANATION

coarse refuse Br ine refuse ~ spoil

t i l l

[]embankment [ ]Mud Creek floodplain

underground mine boundary jmh

Figure 1. Maps of the study area showing: (A) topography, locations of monitoring wells and of geophysical transects (T1-T12), and the location of cross section A-A' . The contour interval is 10 ft (about 3.05 m). (B) Surficial materials and concentrations of pyritic sulfur (in percent) in shallow samples (0-15 cm) and in deep (76-91 cm) samples (in parentheses). The approximate boundary of the underlying New Hope Mine in the Springfield coal is also shown. (C) Apparent resistivity across the deposit of coarse refuse and immediately adjacent materials. The contour interval is 10 ohm-re. The 5-ohm-m contour is also shown.

tered across an area of more than 3 km 2. These refuse deposits, together with voids of abandoned underground mines and deposits of disturbed material (referred to as "spoil") that were created by surface mining, are now sources and/or reservoirs of acidic contaminated groundwater and surface water. The entire area has been disturbed to depths as great as 24 m and is characterized by extreme heterogeneity of materials and hydrologic complexity.

As part of a reconnaissance study of the entire area, surface waters have been periodically sampled, and groundwater has been monitored through about 65 subsurface installations (monitoring wells and suction lysimeters), but such installations are incapable of pro- viding needed information regarding subsurface hydrologic pathways and local-scale variations in groundwater contamination.

Within the Friar Tuck Mine complex, a smaller study area (less than 14 ha) was identified where a variety of natural and disturbed materials are present (Fig. 1B) and which is a major source of acidic water. Electromagnetic techniques were applied there in an

attempt to determine the extent and distribution of groundwater acidity and to assess the hydrologic connections between an elevated deposit of coarse refuse (which is a local recharge area and a source of severely contaminated water) and neighboring deposits of undisturbed till, fine refuse, and the floodplain of Mud Creek.

Within the study area, coarse refuse, which is extremely heterogeneous with respect to rock types and grain sizes, had originally been deposited in the 1940s as a large lobate pile with a maximum thickness of about 27 m, but in the early 1970s, the weathered pile was graded and disturbed as part of an attempt at reclamation (which subsequently failed), so that its maximum thickness is now only about 6 m near the center (Fig. 2A). Surficial samples (0-15 cm in depth) of the refuse contain as much as 4.2 percent pyritic sulfur (by weight), although most of the surficial layer appears to have a pyrite content of 2 percent or less (Fig. 1B). The pyrite content of samples taken 76 to 91 cm below the surface is also about 4 percent (Fig. 1B), and a core taken near well 11 (Fig. 1A) indicated that the pyrite

Electromagnetic Survey of Contaminated Groundwater 41

South A I

Vertical exaggeration ~ 10x

North A'

coarse refuse

fine refuse

~i~ embankment

lit hified strata (Dugger Formation)

relatively impermeable

till

underground mine in Springfield Coal Member

PHASE ANGLE

till ~ • coarse refuse

embankment

Icoa'barii~ ' , L450

-70

\ -so

I-c-I

~t i l [~

APPARENT RESISTIVITY

~-~------coarse re luse-~-~ / ! P ,

I00 200 meters

:1 O0

0

• -~fine refuse--~

300 400 1

Figure 2. Profiles across the study area showing: (A) materials above the Springfield coal along cross section A-A ' ; (B) phase angle along transect T10; (C) apparent resistivity along transect T10. See Figure 1A for locations.

content at dep th is on the o rder of 10 percent by weight. As a consequence o f the high concentrat ion of pyrite and o f soluble salts derived f rom pyrite oxidation (Bayless and others 1989), the elevated pile is a source o f severely acidic, contaminated surface water and groundwater . T h e pile is bar ren of vegetation, ex- cept on some small scattered remnants of a thin (less than 0.6 m) soil cap that was applied dur ing the reclamation attempt.

In the coarse-refuse pile, specific conductance o f water in the u p p e r m o s t par t (0.5 m) of the unsaturated zone is as much as 46,000 ~mhos dur ing dry seasons. This near-surface pore water is f lushed downward in the relatively permeable material (Table

1, coarse refuse) dur ing wet seasons, and specific conductance of water f rom moni tor ing wells is as much as 20,900 ~mhos (Table 2, coarse refuse). T h e average water table is 6.6 m below the surface along the crest o f the pile.

T h e pile o f coarse refuse was emplaced on about 15 m of glacial till (Fig. 2A) that is overlain by a thin mantle of loess. T h e gamma- log of a nearby well emplaced in undis turbed till shows that the sequence con- sists primari ly of dense basal tills and heterogeneous ablation tills o f Illinoisan age (Fig. 3A). A thin (about 1 m thick) sand layer occurs below the uppe rmos t till sequence but may be laterally discontinuous, because it is not observed in some other wells in the vicinity. When


Table 1. Hydraulic conductivities of unconsolidated materials and refuse a

Saturated hydraulic Number of conductivity, Ko

Material determinations (cm/sec)

Till 6 1 x 10-6-1 x 10 -4 Alluvium 5 1 x 10-5-7 x 10 -4 Spoil 7 2 x 10-6-2 x 10 .3 Coarse refuse 13 1 x 10-5-7 x 10 -~ Fine refuse 7 4 x 1 0 - 6 - 1 x 10 -2

"Includes determinations from in situ slug tests, based on procedures outlined in Thompson (1987), and from laboratory tests of core samples using a falling-head technique.

compared with alluvial sediments (Fig. 3B), the relatively high values (counts per second) of the tills indicate that they are relatively dense and impermeable.

The south edge of the coarse-refuse pile is bounded by an ephemeral stream that has eroded into other- wise undisturbed loess and till (Figs. 1B and 2A). About 20 m south of the pile is the edge of a large abandoned surface mine that exploited the Hymera Coal Member (Dugger Formation) near its subcrop. In that area, about 9 m of surficial loess and till, as well as a few meters of uppermost bedrock, were disturbed and displaced (Fig. 2A). This moderately permeable spoil (Table 1, spoil) contains relatively little pyrite, is only moderately contaminated (Table 2, spoil), and is heavily forested.

Mud Creek, which receives contamination from a variety of upstream sources and which is bounded by constructed levees of silt and clay, is the principal perennial stream in the area (Fig. 1A). Water quality is variable with flow conditions (Table 2, Mud Creek). A monitoring well in the upstream vicinity of the study area was emplaced at a depth of 10 m in alluvial deposits near the creek bank; on average, groundwater quality (as indicated by pH and specific conductance) in the alluvium is relatively good, compared with Mud Creek itself (Table 2, Mud Creek alluvium). The gamma-log of a downstream monitoring well shows that coarse sands and gravels are present in the middle or lower part of the sequence, below a depth of 7.6 m (Fig. 3B). Those coarse sediments are probably very permeable. The relatively silty alluvium of Mud Creek is more permeable than adjacent upland deposits of till but less permeable than spoil and refuse (Table 1).

In contrast to coarse refuse, which was heaped in large piles, fine refuse was rejected from the nearby coal preparation facility in the form of a slurry that was directed into low-lying areas. Within the study area, the main deposit (about 120 m wide and as much as 3.6 m thick) of such fine refuse was created during

the 1930s along the original floodplain of Mud Creek (Figs. 1B and 2A). During the early 1950s, a smaller deposit (about 30 m wide and only 1 m thick) was emplaced between the northern edge of the coarse-refuse pile and a man-made embankment.

The fine-refuse deposits, which are barren of vegetation, receive surface drainage from the nearby pile of coarse refuse. In the weathered uppermost parts of the deposits (0-15 cm), the concentration of pyritic sulfur is less than 0.2 percent, but concentrations are as much as 4.5 percent in deep (71-96 cm) samples (Fig. 1B), where oxidation of pyrite is inhibited below the water table. The fine refuse has a wide range of hydraulic conductivity (Table 1), and depth to the water table in the main low-lying deposit ranges from 0.9 to 1.9 m below the surface. Groundwater within the main deposit is moderately acidic and contaminated (Table 2, well 5).

Much of the study area is underlain by an underground mine, the New Hope Mine, which was abandoned in 1948. The mine, which was developed in the Springfield Coal Member (Petersburg Formation), is part of more extensive workings that underlie more than 8 km 2. At a depth of about 17 m, the coalbed subcrops along the bedrock valley of Mud Creek (Fig. 1B), from which the mine is separated by a narrow (about 60 m) barrier of unmined coal (Fig. 2A). Groundwater in the abandoned workings is moderately contaminated (Table 2, underground mine). The static water level in two monitoring wells located about 1,100 m apart in the area south of the study area is about 157.6 m above sea level, and small seasonal fluc- tuations in water levels are perfectly synchronous, indicating that conduit flow exists throughout the abandoned workings.

Geophys ica l Methods

Two electromagnetic survey methods, referred to as terrain conductivity and very low frequency (VLF) resistivity, were used to survey parts of the study area. These methods yield measurements of induced electrical and magnetic fields, from which physical characteristics of the shallow subsurface can be inferred. The terrain conductivity technique uses a dipolar source to indicate the presence of conductors and provide information about their geometries. The VLF technique uses a distant plane-wave source and indicates variations in ground resistivity from differences in the am- plitude and phase of electrical and magnetic fields in the very low frequency range (3-30 kHz). The phase angle by which the horizontal electrical field leads the horizontal magnetic field indicates whether resistivity decreases o r increases with depth. A phase angle equal


Table 2. Values of pH and specific conductance (SpC) for waters in study area a

Range Average Range of SpC SpC

Site N of pH (~mho) (~mho)

Coarse refuse 30 1.4-5.9 4,850-20,900 Well 11 7 2.5-4.2 11,600-18,400 Well 12 7 1.8-4.2 14,400-20,000 Well 14 8 5.0-5.9 4,850-9,580

Fine refuse 67 2.0-6.8 370-11,300 Well 5 8 3.1-4.3 1,570-4,940

Till 22 3.6-7.6 780-16,300 Well 11A 7 3.6-6.1 11,900-16,300 Well 13A 4 6.5-6.9 1,060-1,540 Well 13B 7 5.9-7.0 780-1,140

Spoil 11 4.6-6.7 1,470-4,510

Underground mine 14 5.8-6.7 3,010-9,560

Mud Creek High flow

(>1.0 mNsec) 5 3.4-6.6 300-630 Low flow

(<0.2 mS/sec) 12 4.1-7.4 1,180-3,530

Mud Creek Alluvium 23 5.5 -6.8 770-3,670

14,270 14,590 18,300 6,870

3,090 2,770

4,890 13,600

1,230 940

3,310

7,600

522

2,280

1,990

aSome categories include analyses f r o m mon i to r i ng wells outside the immedia te s tudy area. (iV = n u m b e r o f analyses.)

a

50 0

1-

2

3.

4.

5. E £ 6

7

8

9

10-

1

1

counts per second

17o 19o

soil on loess

ablation till over basal till

J

;lit and fine sand <

ablation till over basal till

proglacial debris flow

bedrock

b 0

1-

2-

3

4

5. g

"~ 7

8-

9-

10-

1

counts per second

50 100

silt and clay embankment

soil on fine alluvium ~ 1 ~

and gravels j ~

bedrock not encountered

Figure 3. Gamma-logs of wells emplaced in: (A) undisturbed till in a monitoring well located about 0.3 km east of the study area, and (B) alluvial deposits along Mud Creek in a monitoring well located about 0.8 km west of the study area.

to 45 ° indicates a vertically homogeneous half-space within the skin depth of an electromagnetic measurement with the VLF; a greater phase angle indicates that resistivity decreases with depth, while a lesser phase angle indicates increasing resistivity with depth.

Terrain conductivity measurements were made using a commercially available instrument, the EM-34. A small transmitter coil is used to induce eddy currents in the ground, and a receiver coil, located several meters away from the transmitter, records the voltage


induced in it by the currents in the transmitter and the ground. The two effects can be separated because they are typically 90 ° out of phase with one another. Depth of exploration is determined by coil configuration and the distance between transmitting and receiving coils. An intercoil spacing of 10 m was used in the study area to provide nominal depths of penetration of 15 m in the vertical dipole configuration and 7 m in the horizontal dipole configuration. More detailed discussion of the application of terrain conductivity and resistivity is provided by Fitterman and Brooks (1989).

Remote radio transmission stations, which essen- tially represent grounded vertical electrical dipole wires that are several hundred meters long, are uti- lized as transmitter sources for the VLF resistivity method. Field measurements are made using a commercially available instrument, the EM-16R. In this study, naval communication transmitting antennas in Cutler, Maine (24.0 kHz), and Annapolis, Maryland (21.4 kHz), were used. Some advantages were derived from using the two different transmission stations: (1) At each field measurement site, at least one of the two transmission stations provided optimum null because of the local alignment of the primary magnetic field with the grid pattern; and (2) at least one of the stations was always transmitting during periods of field measurement. When both stations were operating, cross-checking measurements could be made at the different frequencies. Detailed analysis of electromagnetic wave propagation is discussed by Crossley (1982). More detailed descriptions of the VLF system are provided by Phillips and Richards (1975) and Geonics Limited (1979).

In the study area, the EM-34 was used at 10-m spacings along parts of three transects (north of the crest of the coarse-refuse pile along T1, T2, and T3, Fig. 1A), and the EM-16R measurements were made at 10-m spacings along 13 transects of varying lengths (T1-T13, Fig. 1A). The field data were used to derive geophysical cross sections and contour maps of terrain apparent conductivity, apparent resistivity, phase angle, tilt angle, and ellipticity. Cross sections showing apparent conductivity, apparent resistivity, and phase angle along transects T1 and T10, and a contour map of apparent resistivity are discussed below; data regarding tilt angle and elliptidty are not shown, but they support conclusions reached using the other geophysical parameters. When studied in conjunction with data derived from drill cores and monitoring wells, it is possible to evaluate separately lithologic and hydrochemical effects on the geophysical parameters. A complete presentation of all raw and processed data, as well as a discussion of the theoretical and practical

aspects of pertinent electromagnetic methods, is presented by Brooks (1990).

Results

Figure 1C shows the spatial distribution of apparent resistivity across part of the coarse-refuse pile, as derived from measurements using the EM-16R. The lowest values generally occur along the middle parts of the northern and southern flanks. We interpret this distribution to indicate those areas that contain the most contaminated groundwater, which is an interpre- tation consistent with chemical analYses of water from monitoring wells. Water from well 12, which is located in the middle of the northern flank, has greater specific conductance than water from wells 11 and 14, which are located along the crest of the pile (Table 2).

Along the northeastern edge of the coarse-refuse pile, values of apparent resistivity are intermediate between the low values measured across the coarse refuse (<10 ohm-m) and the high values measured across nearby deposits of undisturbed till (>30 ohm-m) (Fig. 1C). We interpret this transition to reflect very slow diffusion of contaminated groundwater from coarse refuse into undisturbed till. Because the coarse-refuse pile has existed for at least 40 yr, the narrowness (15 m) of the transition from low to high values is indirect evidence that the till is very impermeable that is, it lacks secondary permeability such as extensive fractures or interconnected macropores. Wells 13A and 13B, which are only 10 m from the edge of the refuse pile, contain groundwater that shows relatively little evidence of contamination (Table 2, wells 13A and 13B). Acidic subsurface drainage from the coarse-refuse pile is apparently being directed northwestward through more permeable fine refuse, even though the hydraulic gradient is steepest directly northward toward Mud Creek.

North of the crest of the coarse-refuse pile along transect T1 (Fig. 1A), values of apparent conductivity obtained using the EM-34 in the vertical-dipole configuration are consistently lower than values obtained in the horizontal configuration (Fig. 4). We interpret this to indicate that an electrically more conductive layer (the coarse-refuse pile) overlies a less conductive layer (original undisturbed till on which the refuse was deposited) and that contaminated water from the refuse has not deeply penetrated the underlying till. This in- terpretation is supported by VLF measurements of phase angle (less than 45 °, indicating increasing resistivity with depth) along transect T1. The acidic water obtained from well 11A (Table 2), which is screened in the uppermost 1.5 m of the till, may be a local phe-


1000]

f-horizontal dipole a loo

lo 5'o ~6o SOUTH NORTH Distance (m)

I:igure 4, Graph showing apparent conductivity along transect T 1, north of the crest of the coarse-refuse pile. Measure- ments were made in both the horizontal and vertical dipole configurations. See Figure 1A for location.

nomenon or may even indicate failure of the well seal and contamination by refuse water along the well an- nulus.

In the area north of the undisturbed till (Fig. 1B), phase angle measurements across the southern and central parts of the main fine-refuse deposit are less than 45 ° (Fig. 2B), indicating increasing resistivity with depth. Measurements of apparent resistivity exhibit a steady decrease northward, toward Mud Creek (Fig. 2C). We interpret these data to reflect a moderately contaminated upper layer of fine refuse overlying relatively uncontaminated alluvial deposits. Water samples from near the bottom of the main fine-refuse deposit (Table 2, well 5) can be characterized as moderately contaminated--that is, worse than groundwater within the coarse-refuse deposit (Table 2, coarse refuse) but better than water associated with Mud Creek and its alluvial deposits (Table 2, Mud Creek, Mud Creek alluvium). Pyrite is present in relatively small concentrations (Fig. 1B), but, because of the small thickness of the unsaturated zone, relatively little acidic water is probably being generated within the deposit itself. Much of the contamination may be derived from infiltration of surface drainage originating on coarse-refuse.

A striking anomaly exists in the northernmost part of the main fine-refuse deposit, immediately south of the Mud Creek levee. Values of phase angles increase northward, from 39 ° to 69 ° (Fig. 2B), indicating that the subsurface is becoming more conductive electrically in that direction. For those same stations, de- creasing values of apparent resistivity (Fig. 2C) also indicate that conductive groundwater is present in the subsurface between depths of 4 m (at the bottom of

the fine-refuse deposit) and 27 m (the maximum depth of sounding at the anomaly site). Anomalous values also were observed along transect T9, which is located 10 m to the east of transect T10, and along transect T13, which is a transect oriented perpendic- ular to transect T10 (Fig. 1A). Furthermore, on all transects across the anomaly, tilt angle and ellipticity (not shown) exhibit cross-over and inflection characteristics that substantiate the presence of a conductor at depth.

The source of the anomaly is uncertain. The nearest railroad (and any associated discarded steel rails) was located north of Mud Creek. Other metallic trash, such as discarded pipes, is also an unlikely source of the anomaly for the following reasons: (1) the steady decline (without any marked discontinuity) of apparent resistivity across the entire (120 m) width of the fine-refuse deposit (Fig. 2C), (2) the large width of the anomaly along transects T9 and T10 (about 20 m), and (3) the great length of the anomaly along transect T13 (exceeding 50 m) (Fig. 1A). A possible groundwater source of the anomaly is discussed below.

Across the Mud Creek floodplain (Fig. 1B), values of phase angle and apparent resistivity indicate only the presence of shallow, moderately contaminated water (Figs. 2B and 2C). This is consistent with analyses of surface water in Mud Creek itself and with water obtained directly from the alluvium at depths of 7.9-12.8 m (Table 1, Mud Creek, Mud Creek alluvium).

Discussion In coal-mining regions, piles of coarse refuse from

coal preparation typically represent recharge areas with a great potential for generating contaminated ground water and surface waters into the distant future. From the hydrochemical standpoint, the pre- ferred method of reclaiming such deposits is to bury them below the water table, but such burial is often economically or environmentally unfeasible. Conse- quently, such piles are commonly graded and capped with soil, or chemically treated (as with agricultural limestone, bactericides, or sewage sludge) and vegetated, but after reclamation, contaminated seeps often develop along the bases of the deposits.

Deposits of coarse refuse are very heterogeneous with respect to rock types, grain sizes, and chemistry. Furthermore, such deposits commonly are located in highly disturbed and hydrologically complex settings. It is difficult and expensive to gain detailed knowledge of such heterogeneous materials and complex hydrologic systems when only data from scattered point-spe-


cific installations (such as monitoring wells and suction lysimeters) is available, Application of geophysical methods before reclamation might allow improved detection of highly contaminated areas and hydrologic connections and thereby allow greater tailoring of reclamation by chemical amendments.

Within our study area, the cause of the occurrence of the most contaminated groundwater along the middle parts of the north and south flanks of the coarse,refuse deposit, as inferred from apparent resistivity (Fig. 1C), is unknown. The distribution of apparent resistivity contrasts with the distribution of pyrite, as determined by many analyses of surficial samples of the coarse refuse (Fig. 1B); pyrite concentrations are generally greatest along the crest of the deposit. While analyses of pyrite are essential for de- termining minimum application rates of agricultural limestone needed for reclamation by direct revegeta- tion (without a soil cap), additional measurements of apparent resistivity in the future might permit detailed evaluation of the long-term effects of such reclamation on groundwater chemistry.

As an additional example of insight into hydrologic complexity that is provided by the integration of geophysical methods with more traditional methods, con- sider the anomalous phase angles and apparent resisti- vities along the south bank of Mud Creek (Figs. 2B and 2C), discussed above. Based on observations of springs in the vicinity of the study area, we suggest that those anomalies may represent seepage of water from abandoned underground workings into the alluvium of Mud Creek.

About 0.8 km south of the study area, there is a perennial spring (spring 106) whose water is derived, in part, from the abandoned underground workings of the New Hope Mine in the Springfield coal (In- diana Geological Survey 1989). Spring 106 is located directly above the western boundary of the mine, where the workings may be as little as 18 m deep. The elevation of the potenfiometric surface associated with the mine (about 158 m above sea level) is higher than the spring's elevation (155 m above sea level).

In the vicinity of the study area, chemical analyses have been performed on waters from monitoring wells and springs associated with abandoned underground mines in three different coalbeds. Contamination of water from the New Hope Mine is intermediate between that of water associated with coarse refuse and fine refuse (Table 2, underground mine). Further- more, those analyses indicate the existence of anomalous sodium concentrations in water that has resided in workings of the Springfield coal (1,620-2,140 ppm), as well as in water from Spring 106 (422-884 ppm).

The topographic and geologic setting of the present study area is very similar to that of spring 106. The northern part of the study area lies directly above the northern boundary of the New Hope Mine, where the workings may be as little as 17 m deep (Fig. 2A). Moreover, the elevation of the potentiometric surface associated with the mine (about 158 m above sea level) is higher than the elevation of some undermined land and considerably higher than the bedrock surface.

Perhaps the geophysical anomaly that was de- scribed above represents subsurface seepage of contaminated water from the New Hope Mine into alluvial sediments of Mud Creek. Such seepage into permeable sands and gravels at depths greater than 5 m below the floodplain surface might occur either through the narrow barrier of unmined coal between the abandoned workings and the coalbed subcrop (Fig. 2A) or through undetected subsidence fractures in bedrock.

In the future, it may be possible to confirm the existence within the geophysicatly anomalous area of a subsurface seep from the flooded underground mine. I f sampling wells can be emptaced at different depths in the alluvium, then detection of anomalous sodium would provide such a confirmation. The utility of geophysical surveys, especially when conducted in conjunction with targeted chemical sampling, in the study of hydrologic pathways and mine subsidence would then be conclusively demonstrated.

Conclusions

Electromagnetic surveys in part of a large abandoned mining complex indicate that extremely acidic groundwater within a deposit of pyritic coarse-grained refuse has not percolated deeply into underlying glacial till nor migrated laterally into adjacent deposits of loess and fine-grained refuse during the past 40 yr. Acidic water leaves the area only by surface runoff and direct seepage into stream channels.

The distribution of apparent resistivity within the coarse refuse indicates that the most highly contaminated ground water occurs midway down the sides of the deposit, and this finding is supported by chemical analyses of water from monitoring wells.

Transects of apparent resistivity and phase angle across the deposit of fine-grained refuse indicate that the shallow surficial deposit is moderately contaminated and that contamination increases slightly toward the north (away from the coarse-refuse deposit). The values of apparent resistivity decrease markedly along the northern boundary of the mining complex, and phase angles, which become greater than 45 ° there, indicate that highly contaminated water might exist at


dep th in the alluvial sediments o f the s t ream that borders the site. T h e layer o f anomalously low resistivity may be evidence o f subsurface connection between the alluvial valley fill o f Mud Creek and a f looded, abandoned mine that exists within the project area.

Note Added in Proof

In October 1990, a moni tor ing well was installed at the center o f the electromagnetic anomaly near the Mud Creek levee (at the intersection of transects T10 and T 13, Fig. 1A). T h e screen was installed at depths o f 15 to 16 m in alluvium that underlies fine refuse (Fig. 2A). Water samples have an average p H of 6.9 and an average specific conductance o f 6,390 Ixmho; such values are consistent with those obtained f rom the New H o p e Mine (underg round mine, Table 2). T h e samples also contain high concentrations (> 1,000 mg L - l) o f sodium; such concentrations exceed values for g roundwate r samples f rom deposits o f coarse- and fine-refuse, spoil, and undis turbed geologic materials but are consistent with values for samples f rom the New H o p e Mine.

Acknowledgments

Geophysical equ ipment was loaned by the U.S. Geological Survey. Guidance concerning acquisition and interpretat ion o f geophysical measurements was given by D .V . Fit terman, U.S. Geological Survey, Geophysics Branch. Hydrologic investigations were par t o f reclamation feasibility studies funded by the U.S. Depa r tmen t o f the Interior, Office of Surface Mining, Abandoned Mine Lands P rogram through the State o f Indiana, Depar tmen t of Natural Re- sources, Division o f Reclamation. Interpretat ions of gamma-logs were provided by N. K. Bleuer, Indiana Geological Survey.

References Cited

Bayless, E. R., G.A. Olyphant, and D. Harper, 1989, Field studies of acid drainage on an unreclaimed coal refuse pile in southwestern Indiana: Geological Society of America Abstracts with Programs, v. 21, no. 6, p. 192.

Brooks, G.A., 1990, An application of electromagnetic techniques to delineate acidic contamination at an abandoned

coal mine in southwest Indiana: Unpublished MS Thesis, Indiana University, Bloomington, 211 p.

Cartwright, K., and M.R. McComas, 1968, Geophysical surveys in the vicinity of sanitary landfills in northeastern Illinois: Groundwater, v. 6, no. 5, p. 23-30.

Crossley, D.J., 1982, The theory of EM surface wave impedance measurements; in L. S. Collett and O. G. Jensen, eds., Geophysical applications of surface wave impedance measurements: Geological Survey of Canada, Paper 81-15, p. 1-17.

Ebraheem, A.M., M.W. Hamburger, E.R. Bayless, and N.C. Krothe, 1990, Application of earth resistivity methods to study groundwater contamination at the Wheatland reclamation site, southwestern Indiana: Groundwater, v. 28, no. 3, p. 361-368.

Fitterman, D. V., and G. A. Brooks, 1989, Geophysical inves- tigation of depth to saltwater near the Herring River (Cape Cod National Seashore), Wellfleet, Massachusetts: U.S. Geological Survey Open-file Report, 55 p.

Geonics Limited, 1979, Operating manual for the EM-16, VLF-EM, and the EM-16R:'Geonics Limited, Mississauga, Ontario, Canada, 80 p.

Indiana Geological Survey, 1989, Research and reclamation feasibility studies at the Friar Tuck Site, Sullivan and Greene Counties, Indiana: Second Annual Report (Part A) to the Indiana Division of Reclamation, Open-file Report, 145 p.

Kelly, W.E., 1976, Geoelectric sounding for delineating ground-water contamination: Groundwater, v. 14, no. 1, p. 6-10.

Mazac, O., W. E. Kelly, and I. Landa, 1987, Surface geoelec- trics for groundwater pollution and protection studies: Journal of Hydrology, v. 93, no. 3, p. 277-294.

Merkel, R. H., 1972, The use of resistivity techniques to delineate acid mine drainage in ground water: Groundwater, v. 10, no. 5, p. 38-42.

Phillips, W.J., and W. E. Richards, 1975, A study of the ef- fectiveness of the VLF method for the location of narrow mineralized fault zones: Geoexploration, v. 13, p. 215-226.

Stollar, R. L., and P. Roux, 1975, Earth resistivity surveys--a method for defining ground-water contamination: Groundwater, v. 13, no. 2, p. 145-150.

Thompson, D. B., 1987; A microcomputer program for in- terpreting time-lag permeability tests: Groundwater, v. 25, no. 2, p. 212-218.

Warner, D. L., 1969, Preliminary field studies using earth resistivity measurements for delineating zones of contaminated ground water: Groundwater, v. 7, no. 1, p. 9-16.

Zohdy, A. A. R., G. P. Eaton, and D. R. Mabey, 1974, Appli- cation of surface geophysics to groundwater investigations; in Techniques of water resources investigations; U.S. Geo- logical Survey, Book 2, Chapter D1, 116 p.

application of electromagnetic techniques in survey of contaminated groundwater at an abandoned mine...

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