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Groundwater Flow and Contamination 1
Introduction to Groundwater Flow
Hydraulic rock properties
hydraulic conductivity
effective porosity specific storage
Hydraulic parameter measurement
water pressure measurement
hydraulic conductivity measurement
typical hydraulic conductivity values
Groundwater resources
confined and unconfined aquifers
aquitards and aquicludes
complex aquifer systems
storativity and specific yield
modelling aquifer flow
Groundwater Flow and Contamination 2
Hydraulic and Piezometric Head
In hydraulics, the hydraulic (or energy) head in an incompressible
fluid is given by:
where u is the fluid velocity.
In porous media velocities are always very slow so the term u2/2gis
omitted and the piezometric head is given by:
Hydraulic (or piezometric ) heads are generally expressed in relation to
the mean sea level as with topography.
Pressure headH, is defined by
hu
g
p
gz= + +
2
2
hp
gz= +
H h zp
g= =
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Groundwater Flow and Contamination 3
Measurement of Water Pressure
Saturated Medium
(1) Piezometer
Hole drilled in ground fitted with perforated tube. The water levelgives the elevation of the water table (the point where the pressure is
equal to atmospheric)
Beneath the free surface pressure generally increases with depth
(2) Pressure Gauge - Required for low permeability formations
Electric pressure transducers
Hand operated pressure gauge: A tube with a porous point is
hammered into the ground. Air is injected by a foot pump. A rubber
membrane is held in place at the base of the air tube by the water
pressure. When the air pressure exceeds the water pressure, air comesout of the tube outlet (which can be placed in water to make it
visible!)
Groundwater Flow and Contamination 4
Darcys Law for Saturated Flow - Reminder
water
sandstone
cross-sectionalarea of pipe,
h
A
Q
constant water level
L
h
LhKQ
dxdhKq
=
==
Kq
:dimensionsin threeor
Aor
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Groundwater Flow and Contamination 5
Hydraulic conductivity - depends not only on the rock type, but also on
fluid viscosity and density.
A requirement to separate the hydraulic conductivity into that p art derived
from the rock and that from the fluid, has lead to the introduction of intrinsic
permeability, k, which is entirely dependent on the rock properties
where is the fluid viscosity, is the fluid density andgis the acceleration
due to gravity.
Intrinsic permeability is essentially a function of the diameter of the pore
throats that provide interconnected flow pathways through the rock; the largerthe square of the mean pore diameter, the higher the intrinsic permeability.
gkK
=
Intrinsic Permeability
Groundwater Flow and Contamination 6
Anisotropic Permeability in a
Sedimentary Formation
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Groundwater Flow and Contamination 7
Laboratory Measurement of Permeability
Medium to high permeability
constant head permeameter (Darcys Experiment)
falling-head permeameter
Low permeability medium
Apply large pressure differences with the help of pumps and measure
pressures up and down flow for different values ofQ.
Q
h
Groundwater Flow and Contamination 8
Typical Hydraulic Conductivity Values for Different
Rock Types (m/s)
UNCONSOLIDATED SEDIMENTS
coarse gravels 10-1-10-2
sands and gravels 10-2-10-5
fine sands silts loess 10-5-10-9
clay, shale, glacial till 10-9-10-13
HARD ROCKS
dolomitic limestones 10-3-10-5
weathered chalk 10-3-10-5
unweathered chalk 10-6-10-9
limestone 10-5-10-9
sandstone 10-4-10-10
granite, gneiss, compact basalt 10-9-10-13
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Groundwater Flow and Contamination 9
Specific Storage
Specific storage, SS, represents the volume of water that a fully
saturated porous rock will absorb or expel per unit volume per unit
change in head.
It is usually expressed in units m-1.
Specific storage depends on:
the elasticity of the water
the elasticity of the rock matrix structure
the elasticity of the rock
Groundwater Flow and Contamination 10
Groundwater Flow Equation
z
x
y
qx(x) qx(x+x)
qy(y)
qy(y+y)
q z(z)
qz(z+z)
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Groundwater Flow and Contamination 11
Groundwater Resources
Groundwater resources are described in terms of:
aquifers
unconfined (or phreatic)
confined
aquitards
aquicludes
Groundwater resources tend to have a much greater lateral extent than
vertical extent
aquifer modelling is generally two dimensional with groundwater
flow assumed to be horizontal
Groundwater Flow and Contamination 12
Aquifers
Aquifer A layer, formation or group of formations of permeable rocks,
saturated with water and with a degree of permeability that allows
economically profitable amounts of water to be withdrawn.
Typical Unconfined or Phreatic
Valley Aquiferground surface
lakes
water table
vertical exaggeration approximately 100
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Groundwater Flow and Contamination 13
Confined Aquifers
flowing artesian well
confined aquifer
aquiclude
aquiclude
water table
upland recharge area
The water table or phreatic surface, is the level to which the water
rises in a well
in an unconfined aquifer, this is the depth at which you reach water
in a confined aquifer the aquifer is under pressure, so this may even
be above the ground surface ...
Groundwater Flow and Contamination 14
Low Permeability Media
aquitard
a less permeable saturated geological formation
aquiclude
an almost zero permeability saturated geological formation
aquifuge
a saturated geological formation that neither contains nor transmits
significant quantities of water
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Groundwater Flow and Contamination 15
Aquitards & Aquicludes
An Unconfined Aquifer
System
saturated
unsaturated
unsaturated
saturated
AQUICLUDE
perchedwater table
mainwater table
AQUIFER
AQUIFER
Groundwater Flow and Contamination 16
Aquifers
AquiferA layer, formation or group of formations of permeable
rocks, saturated with water and with a degree of permeability that
allows economically profitable amounts of water to be withdrawn.
Unconfi ned (or Phreatic) Aquif ers: Aquifer where the piezometric
surface coincides with the free surface of the aquifer which is overlainby unsaturated zone
Valley aquifers in humid zones: Recharge occurs across the whole
surface, outlets are the low points in the topography such as sp rings and
rivers. The aquifers behave like underground watersheds.
Valley aquifers in arid zones: Recharge happens in short heavy
bursts. Water infiltrates through the river/waddy beds. The water table
is higher beneath the river than elsewhere contrary to what happens in
humid zones.
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Groundwater Flow and Contamination 17
Alluvial aquifers: Unconfined aquifers situated in alluvial deposits
along the course of a stream. The water is in equilibrium with the
stream which continually drains and recharges it. At the entrance of an
alluvial plain, the water level in the stream is higher than that in the
aquifer, as the plain narrows at the downstream end this causes the
water table in the aquifer to rise often causing marshy areas.
Perched aquifers: These lie on an impermeable layer and are not
connected to a stream which feeds or drains them They often result in
spring lines and provide what is termed a perched water table
Confi ned Aqui fers:An aquifer is said to be confined if it is overlain
by a formation with low permeability and if the hydraulic head of the
water it contains is higher than the elevation of the upper limit of the
aquifer.
Groundwater Flow and Contamination 18
Combined Aquifer System
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Groundwater Flow and Contamination 19
Modelling Aquifer Flow
Groundwater flow in large aquifers is generally assumed to be two-
dimensional and horizontal
In phreatic aquifers, the movement of the water table must be
accounted for
dewatering and resaturation of porous media
hence redevelopment of the storage term in the groundwater flow
equation
the concepts of storativity and specific yield are introduced
Groundwater Flow and Contamination 20
Storativity
The storativityorstorage coeff icient, S, represents the volume of
water that a porous rock will absorb or expel per unit surface area per
unit change in head.
It is a dimensionless quantity and is expressed as either a percentage
or fraction.
There are two basic components of storativity: specif ic storage, which refers to the compressibility of the water and
rock
specif ic yieldwhich describes the quantity of water drained from the
aquifer as the water table drops.
The specific storage of a porous medium is small in comparison to the
specific yield and only becomes significant in the case of confined
aquifers, where the rock matrix remains fully saturated.
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Groundwater Flow and Contamination 21
Specific Storage - Confined Aquifers
Q
clay layer
clay layer
sandstone aquifer
b
Where the aquifer is fully saturated,storativity is the amount of water
released due to specific storage, SSb
where b is the depth of the aquifer.
Specific storage (m-1) depends on:
the elasticity of the water
the elasticity of the rock matrixstructure
The elasticity of the rock
Groundwater Flow and Contamination 22
At the free water surface, the water table can move in response to
pressure changes and either falls, releasing water from pore space,
or rises, saturating new pores within the rock.
this is termed specific yield, Sy and represents the volume of water
that can be drained under gravity per unit surface area, per unit drop
in head.
In a phreatic aquifer, storativity, S, describes the release of water due
to both compressibility and desaturation of the rock matrix:
where h is the head in the phreatic aquifer
S S hS y S= +
Specific Yield
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Groundwater Flow and Contamination 23
Groundwater Flow Equation
h
x
y
qx(x) qx(x+x)
qy(y)
qy(y+y)
Groundwater Flow and Contamination 24
Recharge and Discharge Areas
In unconfined aquifers, some characteristics are common to most
recharge areas and some to most discharge areas.
Recharge areas:
topographical high places
deep unsaturated zone between the water table and the land surface
Flow lines tend to diverge from recharge areasDischarge areas:
topographical lows
water table is found either close to, or at, the ground surface
flow lines tend to converge, unless the discharge area is large, such as
the sea
physical manifestations of the groundwater take the form of a spring,
seep, lake, stream or presence of vegetation common in wet areas
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Groundwater Flow and Contamination 25
Springs
Groundwater Flow and Contamination 26
Groundwater Interaction with Lakes and Wetlands
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Groundwater Flow and Contamination 27
Naturally Varying Flow Regimes
Groundwater Flow and Contamination 28
Groundwater Flow Patterns in Homogeneous Aquifers
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Groundwater Flow and Contamination 29
A One-Lake System with a Homogeneous Anisotropic
Aquifer
Groundwater Flow and Contamination 30
A One-Lake System with a High Conductivity Layer
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Groundwater Flow and Contamination 31
Summary
Three important hydrogeological rock properties:
Hydraulic conductivity (or permeability), porosity and storativity
Groundwater flow governed by:
Darcys Law
Conservation of water mass groundwater flow equation
Groundwater resources
aquifers are economically viable groundwater resources that are either under
pressure, confined, or with a free surface, unconfined.
aquitards, aquicludes and aquifuges are all low permeability formations
aquifer systems are complex and may comprise several layers of aquifers
separated by low permeability formations
groundwater flow in large aquifers is generally assumed to be horizontal
storativity and specific yield are used to model two dimensionaltransient flow
groundwater can both drain and feed surface water features