Transcript Confined aquifer

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Geology and Groundwater
• Introduction
– Geology complexities are reflected in hydrogeology
– Geology is the basis for any groundwater
investigation
• Topics of the chapter:
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Aquifers and confining beds
Transmissive and storage properties of aquifers
Geology and hydraulic properties
Hydraulic properties of granular and crystalline
media
– Hydraulic properties of fractured media
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4.1 Aquifers and Confining Beds
• Aquifer:
A lithologic unit or a combination of lithologic
units capable of yielding water to pumped wells
or springs.
• Aquifer can cut across formations (independent
of geologic units)
• Confining Beds
units of low permeability that bound an aquifer
– Examples are unfractured igneous rock, metamorphic
rock, and shale, or unconsolidated sediments such as
clays
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Types of aquifers
• Confined aquifer (artesian):
bounded by low-permeability beds on both
sides (above and below)
• Unconfined (water-table):
water table forms upper boundary
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P= atm
P> atm
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UNCONFINED AQUIFER
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Confining beds
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ARTESIAN WELL
A well whose source of water is a confined
(artesian) aquifer. The water level in
artesian wells stands at some height
above the water table because of the
pressure (artesian pressure) of the aquifer.
The level at which water stands is the
potentiometric (or pressure) surface of the
aquifer. If the potentiometric surface is
above the land surface, the well is a
flowing artesian well.
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ARTESIAN WELL
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SPRING
A place where
ground water
naturally comes
to the surface at
the intersection
of the water
table and land
surface.
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Potentiometric surface,
water table maps
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Perched aquifer
Unconfined aquifer
developed above
regional water table
(lens) caused by a
low-permeability layer
Water table
Unconfined aquifer
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Types of confining beds
Aquifuge, Aquitard, Aquiclude
Not favored (used) anymore
• Aquifuge: ultimate low-k unit, essentially
impermeable. e.g., granite
• Aquitard: low-perm unit, capable of storing
water, transmitting water between adjacent
aquifers
• Aquiclude: confining bed
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4.2 Transmissive and Storage Properties
•
Two most important aquifer characteristics:
1. Ability to store groundwater
2. Ability to transmit groundwater
•
Transmissivity:
Ease with which water moves through an aquifer
(rate at which water is transmitted through a unit
width of aquifer under a unit hydraulic
gradient
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Transmissivity
T = Kb
T:
K:
b:
Transmissivity, units: [L2/T] e.g., m2/d
Hydraulic conductivity
aquifer thickness
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example
• What is the transmissivity of an aquifer
that has a thickness of 20 m and a
hydraulic conductivity of 15 m/d?
• T = Kb = 20*15 = 300 m2/d
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Storage
If water is removed from a groundwater
reservoir:
– Hydraulic head decreases - water level in
wells falls
– Fluid pressure decreases in the aquifer.
– Porosity decreases as the granular skeleton
contracts.
– The volume of water increases as the water
compresses.
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Storativity (coefficient of storage)
• Storativity (S): the volume of water that a
permeable unit will absorb or expel from
storage per unit surface area per unit
change in head.
• Storativity is a dimensionless property
S = L3/(L2 * L)
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Storativity (coefficient of storage)
The equations for estimating storativity are different for
confined and unconfined aquifers
• In an unconfined aquifer, the height of the hydraulic head
is shown by the water table.
• Thus, a change in hydraulic head results in either
increasing or decreasing saturation of the aquifer. A
large change in hydraulic head results in a large change
in the volume of water in the aquifer.
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Storativity, contd.
• In a confined aquifer, the height of the hydraulic head is
given by the potentiometric surface, which is usually
above the upper surface of the aquifer.
• Water can be added or removed from the aquifer without
affecting the saturation of the aquifer.
• The potentiometric surface can rise and fall, but as long
as it stays above the upper surface of the aquifer, there
is no change in saturation in the aquifer.
• In this case, the change in hydraulic head is being
accomplished by a change in pressure.
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Storativity, contd.
• The amount of water that is absorbed or
expelled from the aquifer is determined by the
changes in water volume and porosity that result
from the change in pressure.
• Because the changes in water volume and
porosity are relatively small, in a confined
aquifer a large change in hydraulic head does
not result in a large change in storage.
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Specific storage
• The amount of water per unit volume of a
saturated aquifer that is absorbed or
expelled due to changes in the
compression of the fluid and medium
caused by a change in hydraulic head is
called - specific storage (Ss).
• Ss has units of [1/L]
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• In a confined aquifer, the equation for storativity
becomes:
• where b is the thickness of the aquifer.
• In an unconfined aquifer, a change in hydraulic head
results in both a change in pressure in the saturated
portion of the aquifer, as well as a change in the
thickness of the saturated zone. In this case, storativity
equals:
• Sy is the specific yield of the aquifer - the amount of
water per unit volume that will drain from an aquifer
under the influence of gravity
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• Specific yield is usually several orders of
magnitude larger than h x Ss so that in all but
very fine grained units the h xSs component is
ignored and storativity is considered equivalent
to specific yield.
• The volume of water that will be drained from or
added to an aquifer as the head is raised or
lowered:
V = S A h
A: area overlying the aquifer
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Specific yield and Specific
Retention
• Specific yield: water released from aquifer
by gravity drainage
Sy =Vd/VT
• Specific Retention: water retained in
aquifer due to molecular attraction
Sr = Vr/VT
Sy + Sr = n
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Example
• A fully saturated soil sample weighs 105 g. After
the sample is drained by gravity, the weight of
the sample is 85 g. After the sample is ovendried, the sample weighs 80 g. The bulk density
of the wet soil is 1.65 g/cm3, and the density of
water is 1.0 g/cm3.
• Calculate Sy, Sr, porosity.
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Example 4.2
•
A confined aquifer is composed of dense,
sandy gravel with a thickness of 100 m
and a porosity of 20%.
1. Estimate the likely range for specific storage and
storativity.
2. For a total head drop of 100 m in an area of 1
x109 m2, how much water is released from the
storage?
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Geology and Hydraulic properties
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Hydraulic properties of geologic material
are related to rock type
material types to be examined:
1.
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3.
4.
5.
Unconsolidated sediments
Semi-unconsolidated sediments
Carbonate rocks
Sandstone rocks
Volcanic and other crystalline rocks
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Aquifers in unconsolidated sediments
• Blanket sand and gravel aquifers (alluvial)
– Medium to coarse sand and gravel
• Basin-fill aquifers (valley-fill, wadi-fill)
– Sand and gravel filling depressions formed by
faulting or erosion
• Aquifers in these materials are mainly
unconfined
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Unconsolidated
K depends on:
– grain size,
– mineral composition,
– Sorting
K (clay) < 3 x 10-4 m/d
K (coarse gravel) = 100 m/d
K (well sorted) > K (poorly sorted)
Most aquifer in western Saudi Arabia are of this type
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• Blanket sand and gravel aquifers
– E.g., fluvial deposits (alluvial aquifer):
long, narrow, thin aquifers
– Braided rivers
– Meandering rivers
– Alluvial fans
• Basin-Fill aquifers
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• Aquifers in semi-consolidated Sediments
– Sandstone aquifers
– Carbonate-Rock aquifers
• Enhancement of permeability and porosity
by dissolution
• Karst aquifers
• Basaltic and other Volcanic-Rock aquifers
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4.4 Hydraulic Properties of Granular and
Crystalline Media
• Do rocks keep original porosity and permeability?
• What geologic processes change hydraulic properties?
• Original porosity >30% in many deposits
– Porosity changes with depth (compaction)
– More clay, more loss of porosity
– More ss, less loss of porosity (resistance of compaction)
– Mineralogical alterations due to high T
– Cementation
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4.5 Hydraulic Properties of fractured Media
• Originally impermeable rocks can be good
aquifers due to fractures
• Fracture: a planar discontinuity in a rock
or cohesive sediment
• Joints: macro-fracturess, no movement
along plain
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4.5 Hydraulic Properties of fractured Media
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4.5 Hydraulic Properties of fractured Media
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Fracture described by
– Orientation
– Size
– Aperture (b): measure of width of fracture
opening
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Fracture set
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Fracture density: number of fractures
per volume
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Fracture frequency: number of fractures
intersecting a unit length of borehole
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Fracture spacing: distance between two
adjacent fractures
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4.5 Hydraulic Properties of fractured Media
Snow, 1968
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b
k 
12s
b

s
Example 4.4
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