Geology Reference
In-Depth Information
new technologies to try to better bal-
ance withdrawal with recharge rates.
Another excellent example of what
we might call defi cit spending with re-
gard to groundwater took place in Cali-
fornia during the drought of 1987-1992.
During that time, the state's aquifers
were overdrawn at a rate of 10 mil-
lion acre-feet per year (an acre-foot is
the amount of water that covers 1 acre,
1 foot deep). In short, during each year
of the drought, California was with-
drawing more than 12 km
3
of ground-
water more than was being replaced.
Unfortunately, excessive depletion of
the groundwater reservoir has other
consequences, such as subsidence
involving sinking or settling of the
ground surface (discussed in a later
section).
Water supply problems certainly
exist in many areas, but on the posi-
tive side, water use in the United States
actually declined during the fi ve years
following 1980 and has remained
nearly constant since then, even though the population has
increased. This downturn in demand resulted largely from
improved techniques in irrigation, more effi cient industrial
water use, and a general public awareness of water problems
coupled with conservation practices. Nevertheless, the rates
of withdrawal of groundwater from some aquifers still ex-
ceeds their rates of recharge, and population growth in the
arid to semiarid Southwest is continuing to put signifi cant
demands on an already limited water supply.
Stalactites
Columns
Stalagmites
◗
Figure 13.12
Cave Deposits Stalactites are the icicle-shaped structures hanging from a
cave's ceiling, whereas the upward-pointing structures on the fl oor are stalagmites. Columns
result when stalactites and stalagmites meet. All three structures are visible in Shenandoah
Caverns, Virginia, USA.
105
100
Wyoming
South Dakota
Nebraska
t
r
40
Kansas
Colorado
The excessive pumping of groundwater in coastal areas has re-
sulted in
saltwater incursion
such as occurred on Long Island,
New York, during the 1960s. Along coastlines where perme-
able rocks or sediments are in contact with the ocean, the fresh
groundwater, being less dense than seawater, forms a lens-
shaped body above the underlying saltwater (
Oklahoma
New
Mexico
Figure 13.14a).
The weight of the freshwater exerts pressure on the underlying
saltwater. As long as rates of recharge equal rates of withdrawal,
the contact between the fresh groundwater and the seawater
remains the same. If excessive pumping occurs, however,
a deep cone of depression forms in the fresh groundwater
(Figure 13.14b). Because some of the pressure from the
overlying freshwater has been removed, saltwater forms a
cone of ascension
as it rises to fi ll the pore space that formerly
contained freshwater. When this occurs, wells become con-
taminated with saltwater and remain contaminated until
recharge by freshwater restores the former level of the fresh-
groundwater water table.
Saltwater incursion is a major problem in many rapidly
growing coastal communities. As the population in these
◗
35
Texas
High
Plains
aquifer
boundary
Explanation
Declines greater than 15m
Between 15m decline and
3m rise
Rises greater than 3m
0
50
100 Kilometers
◗
Figure 13.13
High Plains Aquifer The geographic extent of the
High Plains aquifer and changes in water level from predevelopment
through 1993. Irrigation from the High Plains aquifer is largely
responsible for the region's agricultural productivity.
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