Environmental Engineering Reference
In-Depth Information
Recharge
Water table
Stream or
lake
Groundwater
Rain water
Land
surface
Permeable
Artesian
well
Flowing
artesian
well
Recharge
zone
Impermeable
layer
Aquifer
Impermeable
layer
Figure 9.1
Illustration of an unconfined (upper) and a confined (lower) water aquifer.
From US Geological Survey (USGS), 2001.
Confined aquifers are deposits of water retained by porous rock that are sandwiched
between two layers of impermeable material. Rechargeable confined aquifers have an
area of the aquifer that is unconfined and serves as a recharge zone, which are often
located at elevated topographic locations and can consist of porous material, cracks in the
rocks, and sinkholes that capture rainwater directly or by way of runoff. At a lower topo-
graphic level from the recharge zone is the artesian zone where water is pressurized as a
result of the difference in level in reference to the recharge zone. If a well is drilled in the
artesian zone, pressure may be enough to bring the water to the surface without the need
of pumping.
In contrast, some confined aquifers do not have a recharge zone and the water contained
was trapped thousand or million years ago. These are called “fossil aquifers,” and the water
they contain is not a renewable resource. The practice of extracting water from these aquifers
is called “water mining” because it causes depletion of the resource. But for all practical
purposes, an aquifer does not necessarily have to have a nonrecharge zone to be considered
nonrenewable. Some aquifers have a low rate of recharge, so if this characteristic is combined
with high rates of extraction, eventually the aquifer will become dry at some point in time.
Aquifers vary in size, thickness, and depth. The area covered by aquifers can vary
from thousands to millions of square kilometers and can have one layer or multiple layers.
