Unit 02b : Advanced Hydrogeology

Aquifer Mapping

Mapping Regional Flow Systems

• Hydrostratigraphy

• Hydrogeological Cross Sections

• Potentiometric Surfaces

• Water Table Maps

• Recharge & Discharge Areas

• Surface Water Interactions

Definition

Hydrostratigraphy is the identification of mappable-units on the basis of hydraulic properties (aquifer / aquitard) that have considerable lateral extent and that also form a geologic framework for a reasonably distinct hydrogeologic system.

MJR

Hydrostratigraphy

Sand & Gravel

Sand

Shale / Mudstone

Till

Clay

Stratigraphic Lithologic Hydrostratigraphic

Surficial Deposits Surficial Aquitard

Floral AquiferFloral Fm

Floral Aquitard

Empress Gp Empress Aquifer

Bearpaw Fm Bedrock Aquitard

Regional Hydrogeology

• The starting point for any regional hydrogeological characterization study is to establish the hydrostratigraphy by identifying mappable flow units and intervening aquitards.

• This is done using standard subsurface and surface mapping techniques based of the principles of sequence or genetic-unit stratigraphy.

• Emphasis is placed on the characterization of hydraulic properties of lithofacies within each genetic-unit on various scales using data from a wide range of sources including thin-sections, core studies, slug tests, DSTs and pump tests.

Aquifers and HSUs• Hydrogeologists have long noted that ground water flow

often does not conform to the boundaries of recognized stratigraphic units.

• Two terms, “aquifer” and “hydrostratigraphic unit,” are commonly employed to subdivide the subsurface into units more relevant to groundwater hydrology.

• The term “aquifer” is commonly defined for water supply usage in economic terms. In many areas, “aquifer” is defined by local laws and regulations which makes it difficult to use as a technical term.

• The term “hydrostratigraphic unit” (HSU) has been defined in a variety of ways in the literature, and does not currently have a formal definition

Maxey

• The term hydrostratigraphic unit was first proposed by Maxey (1964) for “bodies of rock with considerable lateral extent that compose a geologic framework for a reasonably distinct hydrologic system.”

• Maxey (1964) identified the need to define ground water units that are based not solely on specific lithologic characteristics but also included parameters “that apply especially to water movement, occurrence, and storage.”

Seaber• Seaber (1982; 1986; 1988) proposed a definition of

hydrostratigraphic unit as “a body of rock distinguished by its porosity and permeability,” which he considered more consistent with established stratigraphic nomenclature.

• With this definition, Seaber intended to accommodate the observation that a “hydrostratigraphic unit may occur in one or more lithostratigraphic units.”

• Seaber (1988) attempted to define a hydrostratigraphic unit that applied to all geological environments by focusing on the material properties of the rock or sediment.

Lawrence Livermore National Lab• LLNL define a hydrostratigraphic unit (HSU) as a

body of sediment and/or rock characterized by ground water flow that can be demonstrated to be distinct under both unstressed (natural) and stressed (pumping) conditions.

• Flow in an HSU is distinguishable from flow in all other HSUs.

• Using this definition, an HSU is not restricted to any particular geologic setting, which was the intent of Seaber, but is principally based on the properties of ground water flow, which is consistent with Maxey.

Aquifers, aquitards and HSUs

• An aquifer can be composed of one or more HSUs.

• Thick aquitards or aquicludes may be defined as HSUs based on their distinct groundwater flow characteristics.

• Thin aquitards or aquicludes that form significant, laterally continuous layers that limit hydraulic communication may be used to define HSU boundaries.

Hydrostratigraphic Maps

• Hydrostratigraphic maps and sections superficially look very similar to simple lithostratigraphic maps with two important differences:– Amalgamated HSUs have properties controlled by

the fabric of their components (eg high K lenses and fractures rather than a simple bulk average).

– Hydraulic anisotropies in otherwise similar materials may be classified as different HSUs (fracturing or strong transverse anisotropy may define a distinct unit).

Hydrostratigraphic Units

Unit #1: Isotropic Aquitard

Unit #2a: Anisotropic Aquitard Kv>>Kh

Unit #2b: Isotropic Aquitard

Unit #3: Isotropic Aquifer

Unit #4: Isotropic Aquitard

Unit #5a: Fractured Bedrock Aquifer

Unit #5b: Unfractured Bedrock Aquifer

Hydrostratigraphic Unit Analysis• Hydrostratigraphic Unit (HSU) analysis was developed

and implemented at Lawrence Livermore National Laboratory (LLNL).

• HSU analysis integrates chemical, hydraulic, geologic,and geophysical data to produce a comprehensive three-dimensional model of the subsurface based on fluid flow characteristics.

• It is the focus on fluid flow pathways that renders this management tool so effective for characterization and remediation activities.

• HSU analysis significantly enhances the basis of computer models of fluid flow and transport at contaminated sites.

HSU Analysis and DecisionsHSU assists decision making :(1) locating and designing

characterization boreholes and monitor wells,

(2) prioritizing the construction and phased startup of remediation systems,

(3) managing extraction of subsurface contaminants,

(4) finding sources of past contaminant releases,

(5) tracking the migration of subsurface contaminants,

(6) evaluating the subsurface effectiveness of the remediation systems.

Hydraulic Heads• Hydraulic heads vary in three

spatial directions and time, h(x,y,z,t).

• The time element can be removed if all measurements are made at the same time, h(x,y,z).

• A further dimension can be eliminated if piezometers are placed where the flow is largely two-dimensional, h(x,y)

• However, the resulting maps must be viewed as 2D projections of a 3D field.

Potentiometric Surface Maps

• Hydraulic heads from water wells and piezometers, from a single HSU, can be contoured on map.

• Such a map for a confined aquifer is called a potentiometric surface map.

• If the aquifer is confined, the surface is a water table map.

• These maps assume that flow in the HSU is horizontal and that there is no change in head with depth.

• Potentiometric surface maps are only valid for HSUs corresponding to aquifers.

Constructing Potentiometric Maps• In order to create a potentiometric surface map

numerous observations of the elevation of the surface must be made with wells and piezometers. Several assumptions must be satisfied:

1. The wells and piezometers have screens installed that are near the same elevation. A large variation in elevations may lead to problems since both horizontal and vertical gradients exist in aquifers.

2. There must have enough data points to draw a contour map. If only a few points are available it may only be possible to determine the general direction of flow and the hydraulic gradient.

3. Surface bodies of water such as streams, lakes, and springs provide information about the water table in unconfined aquifers.

Dakota Sandstone

Darton (1909) was responsible for one of the first potentiometric maps

for the Cretaceous Dakota Sandstone in South Dakota.

BlackHills

Flow Nets

• Equipotentials and flow lines form a network called flow net.

• For a flow net flow all tubes carry the same flow and there is no flow normal to flow lines.

• This means: Q = K1hA1/L1 = K2hA2/L2

K1

K2

h

h

h

h

h

A1

L1

A2

L2

Q

Q

Interpreting Flow Nets

• For a uniformly thick aquifer A1 = A2 and h is constant for contours at equal intervals.

• It follows that: K1 = L1

K2 L2 • Thus if K1 has been measured K2 can be

readily estimated for an isotropic but inhomogeneous aquifer.

Interpreting Potentiometric Surfaces • Increasing gradients

mean:– Increasing flux (in

recharge area)– Reducing Kh (aquifer

becoming less conductive)

– Reducing thickness (aquifer thinning)

Interpreting Potentiometric Surfaces

• Recharge and Discharge– Increasing flux (in

recharge areas)– Decreasing flux (in

discharge areas)– Flow has a downward

component in recharge areas

– Flow has an upward component in discharge areas

Discharge to Surface Water System

• Streams– Mainly discharge features

(some streams can lose flow )

• Springs– where the water table

intersects the ground surface associated with conductivity changes

• Seepages– Salinity increases for

seepages and springs in arid climates due to evaporation