Geology Reference
In-Depth Information
EXERCISE
16
Groundwater Overdraft
and Saltwater Intrusion
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INTRODUCTION
Groundwater accounts for about 25 percent of the
total fresh water used in the United States, and
untapped resources have great potential to meet
future needs. Even though vast amounts of ground-
water are available, in some areas pumping rates are
such that water levels have declined hundreds of
feet, wells have gone dry, the cost of pumping the
water has increased substantially, and water of poor
quality has been induced to flow into aquifers. Areas
of large irrigation systems, such as that part of the
Great Plains states underlain by the High Plains or
Ogallala aquifer, exemplify the problems of ground-
water overuse. Irrigation is the largest user of ground-
water, although areas of dense population and heavy
industry also consume large quantities.
Techniques have been devised to halt or reduce
the water-level decline in some water-short areas.
The most obvious method is to conserve water and
reduce pumping. In other instances it may be possi-
ble to divert surface water, including treated waste-
water, to infiltration basins, pits, or wells, which will
allow the water to percolate into the ground at a rate
that is considerably greater than that permitted by
natural conditions. These techniques are collectively
known as artificial recharge and have been used suc-
cessfully throughout the world.
In this exercise we examine an agricultural area
along the Mississippi River that has seen the ground-
water level decline for over 100 years and a south-
eastern urban coastal region with an even longer
period of declining water levels. In the rice-growing
Grand Prairie region (east of Little Rock, Arkansas),
the Mississippi River Alluvial Aquifer is predicted to
soon become useless and a deeper aquifer there is
declining as well. In the Savannah, Georgia, area,
declining water levels in the coastal Floridan Aquifer
are impacting the water supplies of that city
and nearby communities including those in South
Carolina.
The general objectives of this exercise are to
explore changes in water levels and flow directions in
overpumped aquifers, the environmental and eco-
nomic impacts of such changes, and responses to
groundwater overdraft in these two settings.
PART A. OVERUSE OF A GROUNDWATER
RESOURCE: GRAND PRAIRIE
REGION, ARKANSAS
The Grand Prairie region is in east-central Arkansas.
This region is characterized by low relief, which, in con-
junction with an extensive aquifer and warm climate,
provides an ideal setting for rice irrigation. In 1904 the
Mississippi River Alluvial Aquifer became the irrigation
source for rice farming in the district and since 1915 the
water table has declined about 1 ft/yr in some areas.
The configuration of the water level in the Alluvial
Aquifer in 1915 is shown in Figure 16.1. Concentrated
pumping of irrigation water from this sandy aquifer has
caused a substantial overdraft in the groundwater sup-
ply and a decline in water levels of several tens of feet.
The objective of Part A of this exercise is to examine
water-level and flowline changes due to overpumping
and the responses of the Grand Prairie region.
Groundwater plays a key role in rice production
here. As the water level declines, the cost of pumping
water increases. In the long run, pumping costs and
other farm operating costs could be greater than the
value of the crop. The economic impact of a declining
water level is obvious.
In order to evaluate the rate and areal extent of water-
level decline and to determine remedial measures, maps of
conditions in 1915 and 1954 were prepared and evaluated.
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