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Ground-water levels in the
upper Floridan aquifer in 1980; also shown is the ground-water discharge (natural and pumping) in 1980.
tour lines that are distorted or uneven indicate appreciable discharge to springs and rivers and are characteristic of areas such as northwest-central Florida where the aquifer is at or close to land surface, and relatively smooth contour lines are characteristic of areas such as southern Florida and indicate that the aquifer is more deeply buried and thus is insulated from surface stresses.
The change in the potentiometric surface caused by pumping depends on the rate and duration of the pumping and the hydrogeologic setting. Regional long-term declines in water levels greater than 20 feet are shown on the illustration (stippled areas). As of 1980, about 640 million gallons per day, mostly for industrial use, were being withdrawn from the coastal strip of southeastern Georgia-northeastern Florida. As a result, the troughlike depression of the 1940's in coastal Georgia and northeastern Florida has spread inland to become the largest area of significant water-level change in the Floridan aquifer system. Although this water-level depression is relatively shallow, it is widespread because the permeable limestone is overlain by several hundred feet of clay that slows down the recharge rate. Thus, the area over which the recharge occurs must be large.
In west-central Florida, about 900 million gallons a day is pumped from the Floridan aquifer. The phosphate industry and irrigation account for the major part of that pumpage. As a result, a regional depression exists in the potentiometric surface southeast of Tampa. Although the pumping rate is greater in the west-central area of Florida than it is in the area of southeastern Georgia-northeastern Florida, the area of influence is smaller because the thin layers of sand and clay overlaying the Floridan aquifer permit the aquifer to be readily recharged.
In the Fort Walton Beach area of panhandle Florida, pumping of about 15 million gallons per day has caused a regional depression in the potentiometric surface. This relatively low pumping rate has caused a regional depression because, in this area, the ability of the Floridan aquifer to transmit water to the center of pumping is about 100 times less than that in westcentral Florida or in southeastern Georgia-northeastern Florida. Steep cones of depression in water levels due to pumping are characteristic in areas where an
aquifer's ability to transmit water is low.
To estimate rates of recharge and discharge and to determine how these rates are affected by various pumping conditions, a computer simulation of the Floridan's flow system was developed by bringing together everything that was known about the Floridan's hydraulics and hydrogeology. This computer simulation of the regional flow system showed that before ground-water development began about 21,000 cubic feet per second were discharged from the Floridan aquifer. Of that amount, about 19,000 cubic feet per second, or 90 percent of the discharge, flowed to springs or discharged to rivers. The remainder discharged as areal seepage in coastal areas. Because discharge from the flow system balanced recharge to the system, simulated predevelopment recharge was also about 21,000 cubic feet per second. The average predevelopment recharge rate was about 4 inches per year over the area where recharge occurred, and the average predevelopment rate of areal seepage was about 0.6 inch per year over the area where areal seepage occurred.
As previously mentioned, most of the natural flow (large-discharge springs and highest rates of recharge) is in areas where the aquifer is at or close to land surface (areas shown by hatch marks on the accompanying map). Only about 13 percent of the regional predevelopment discharge occurred where the aquifer is hundreds of feet below the land surface, although that part of the aquifer system includes about 50 percent of the total area of the aquifer.
Of today's total Floridan aquifer discharge of about 23,500 cubic feet per second, about 75 percent is discharge to springs and rivers, 7 percent is areal seepage, and 18 percent is pumpage. Groundwater development has reduced spring flow and discharge to rivers by less than 5 percent, and discharge by areal seepage in coastal areas has been reduced by about 30 percent. Of the amount pumped, about 20 percent is salvaged from spring flow and discharge to rivers, and 20 percent is from reduced areal seepage in coastal areas. The remaining 60 percent of the pumpage is from additional recharge that is induced bacause pumping has lowered water levels in the aquifer.
Today's average recharge rate is about 4 inches per year, and the average rate of areal seepage over the discharge areas is
about 0.5 inch per year. These averages are virtually unchanged from predevelopment values because ground-water development has enlarged the area over which recharge occurs and has shrunk the area where loss by areal seepage occurs.
In summary, the major part of the flow system today is largely unchanged from predevelopment conditions. Large
discharge springs are still the dominant feature of the system. Although pumping has caused recharge rates to increase locally, the greatest recharge still occurs near the springs. Even after development, ground-water flow remains sluggish in areas where the aquifer is deeply buried relative to flow in areas where the aquifer is close to the land surface.
Similar ground-water studies are being con-
York, New Jersey, Pennsylvania, Dela-
Colorado, Kansas, Missouri, Nebraska,
Oregon, and Idaho;
Nebraska, New Mexico, Oklahoma, South
Dakota, Texas, and Wyoming;
England States, New York, and the glaci-
and northeastern Ohio;
North and South Dakota, and Wyoming;
Minnesota, Missouri, and Wisconsin;
parts of Georgia, Alabama and South
South Carolina, Georgia, Alabama, and
Arizona, New Mexico, Colorado, and
New Mexico, and Wyoming; and
Alabama, Arkansas, Kentucky, Louisiana,
Although the primary responsibility of the U.S. Geological Survey is to conduct investigations within the limits of the United States and its territories, international activities have been an important component of U.S. Geological Survey operations for more than four decades. Such activities have included technical assistance to other countries and international organizations, scientific cooperation with earth resources agencies abroad, exchange of scientists and training of participants, representation of the Survey or the U.S. Government in international commissions and associations, and assessment of mineral and energy resources of foreign countries. During 1983, the Survey was involved in a wide range of activities in each of these categories.
In undertaking international activities, the Survey has four principal objectives: • To help achieve domestic research ob
jectives through the comparative
study of scientific phenomena abroad
and in the United States. • To contribute toward foreign policy ob
jectives and to provide support for the international programs of other Federal
agencies. • To obtain information about existing and
potential foreign resources of interest
to the United States. To develop and maintain contacts with
counterpart institutions and programs to facilitate scientific cooperation and exchange.
Major Programs and
During 1983, Survey technical assistance was extended to 25 nations under bilateral assistance programs and to several regional or worldwide organizations (see table).
International scientific activities conducted by the U.S. Geological Survey in 1983, listed by country or region
Technical Assistance Activities
- Mineral resources exploration; modernization of Bangladesh
Coal resources project development.
Earthquake hazards reduction.
Earthquake hazards reduction.
Geothermal resources; seismic hazards-San Miguel Island, Azores.