Unit 02b : Advanced Hydrogeology
Aquifer Mapping
Mapping Regional Flow Systems
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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
Stratigraphic
Surficial Deposits
Floral Fm
Lithologic
Hydrostratigraphic
Clay
Surficial Aquitard
Sand
Floral Aquifer
Till
Floral Aquitard
Empress Gp
Sand & Gravel
Empress Aquifer
Bearpaw Fm
Shale / Mudstone
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 geneticunit 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 Decisions
HSU 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
Black
Hills
Darton (1909) was responsible for one of the first potentiometric
maps for the Cretaceous Dakota Sandstone in South Dakota.
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 = K1DhA1/L1 = K2DhA2/L2
Q
Dh
A1
Dh
K1
L1
Dh
Dh
Dh
A2
K2
L2
Q
Interpreting Flow Nets
• For a uniformly thick aquifer A1 = A2 and
Dh 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