PART 10
Aquifers and aquitards
Definitions
Aquifer
-
geological formation which contains and yields water.
-
saturated, permeable geologic unit which can transmit significant quantities of
water.
Aquitard -
saturated, permeable geologic unit which cannot transmit significant quantities of
water (but can transmit small quantities). Also called a semi-pervious formation or
leaky formation.
Aquiclude -
geologic formation which may contain water, but is incapable of transmitting
water.
Aquifuge -
geologic formation which neither contains nor transmits any water.
In reality, there are no aquicludes and aquifuges and these terms are no longer used.
Types of aquifers
(1) Unconfined aquifer is one whose upper boundary is the water table, i.e., where pressure is
zero (p=0).
Look ate the total head h: h = z + p/γ
At the top of the aquifer, htop = ztop + ptop/γ
but because ptop = 0, we have:
htop = ztop
which means that if the head (h) increases, groundwater table rises.
Hydrogeology, 431/531 - University of Arizona - Fall 2007
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Aquifers and aquitards
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There are four types of unconfined aquifers:
(a) valley aquifer
VALLEY AQUIFER
water table
vadose zone
yyyyyy
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phreatic
aquifer
bedrock
divide
Sources of water: infiltration of rain water;
surface water bodies;
lateral influx.
Sink of water:
rivers.
Water table reflects topography.
Saturated zone - below water table.
Unsaturated (vadose) zone - above water table.
Examples: High Plains aquifer (Ogallala Formation), Coastal Plains aquifer (Atlantic, Gulf).
Hydrogeology, 431/531 - University of Arizona - Fall 2007
Dr. Marek Zreda
Aquifers and aquitards
81
(b) valley aquifer in arid zones
Surface recharge is negligible because of high evapotranspiration rates. Only in valleys, rivers
may carry water from mountains and recharge the aquifer.
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VALLEY AQUIFER (IN ARID ZONES)
flooded
wadi
water table
divide
Here, contrary to valley aquifers (in humid, temperate climatic zones), water table is highest
beneath rivers.
Examples: North African aquifers (Nubian Aquifer), aquifers in the American Southwest (Arizona, New Mexico).
Hydrogeology, 431/531 - University of Arizona - Fall 2007
Dr. Marek Zreda
Aquifers and aquitards
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(c) alluvial aquifer
Along streams. Usually in equilibrium with the stream, i.e.,
alternately drains and recharges streams along their length
and at different times.
Example: Rhine River
Rhine River
Stream may be either gaining water from the aquifer or losing water to the aquifer.
Str
eam
losing part
(upstream)
gaining part
(downstream)
red = equipotentials
green = flow directions
yyyy
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yyyy
(d) perched aquifer
Located on impermeable lenses or discontinuous layers.
perched water table
clay lens
sand
water table
Hydrogeology, 431/531 - University of Arizona - Fall 2007
Dr. Marek Zreda
Aquifers and aquitards
83
(2) Confined aquifer is one in which the top of the saturated zone is confined (bounded) by an
aquitard, i.e., at the top of the aquifer, pressure is not zero (ptop ≠ 0).
htop = ztop + ptop/γ
thus,
htop ≠ ztop
which means that if the head (h) increases, the pressure (p) also increases.
In a confined aquifer, the piezometric head (or water level in an observation well, or a piezometer) is higher than the upper boundary of the aquifer.
If the head is higher than the surface elevation, the aquifer is artesian.
Piezometric surface - a conceptual, imaginary (!) surface joining the water levels in all piezometers in the aquifer. In a phreatic aquifer, it was the water table and it had a physical meaning.
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recharge
area
piezometric
surface
well
discharge
area
artesian
well
Examples of artesian aquifers: the Great Artesian Basin in Australia, Milk River aquifer in
Alberta (Canada).
Hydrogeology, 431/531 - University of Arizona - Fall 2007
Dr. Marek Zreda
Aquifers and aquitards
84
Water seeks the most efficient way to lose potential (or energy). Therefore, in isotropic and
homogeneous media, water flows in the direction perpendicular (normal) to the equipotential
lines, i.e., normal to the piezometric (potentiometric) contours.
Isotropic
aquifer
Anisotropic
aquifer
h1
h2
no preferred
direction
h3
h1
h2
_
q
h3
preferred
direction
_
q
Flow line (along the direction of specific discharge q) is a compromise between the direction of
applied gradient J and the preferred direction (or the direction of least resistance to flow).
Hydrogeology, 431/531 - University of Arizona - Fall 2007
Dr. Marek Zreda