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Summary of the Hydrology of the Floridan Aquifer System In Florida and In Parts of Georgia, South Carolina, and Alabama
By Richard H. Johnson and Peter W. Bush
Professional Paper 1403-A
The storage coefficients calculated from aquifer tests for the Upper Floridan range from a low of 1 x 10-5 to a high of 2 X 10-2 with most values in the 1 x 10-3 to 1 x 10-4 range. In the Floridan aquifer system, reported storage coefficients bear no discernible relation to thickness of aquifer tested on a regional basis. The higher values, 1 X 10-2 to 1 X 10-3, reflect the semiconfined nature characteristic of some parts of the system, such as southwest Georgia, where the aquifer is very close to land surface. The higher values indicate that some of the water from aquifer storage comes from dewatering of the aquifer rather than totally from compression of the aquifer skeleton and expansion of water. Where the confining unit on the Upper Floridan is thin or nonexistent, the Upper Floridan together with the surficial sand aquifer overlying it can behave as a single aquifer. The response to pumping may involve dewatering only in the overlying sands or it may also involve dewatering of the Upper Floridan depending upon pumping rates.
The areal distribution of the storage coefficient of the Upper Floridan could not be developed from transient simulation due to the lack of steady-state initial conditions and historical pumping and associated water-level data. However, transient simulation provided insight into the relative importance of storage in different hydrogeologic areas. Depending on hydrogeologic conditions and the estimated value of storage coefficient, the time required from the start of a new pumping period for the system to reach a new steady-state condition can range from days to years. The time needed from the start of a new pumping period for the system to reach steady state in confined areas depends on the fraction of water pumped that must come from aquifer storage. If the water necessary to sustain a given pumping rate is readily available from vertical leakage (induced recharge) or from adjacent areas within the aquifer (diversion of natural discharge), then only a small part of the water pumped must come from aquifer storage, and a steady-state condition will be achieved relatively quickly. Thus, leaky, high-transmissivity areas are relatively quick to reach equilibrium, and conversely, tightly confined, low-transmissivity areas, which of necessity are more dependent on water from aquifer storage when pumped, are relatively slow to reach equilibrium.
The difference in time required to reach equilibrium can be illustrated by contrasting the aquifer's response to pumping in a low-transmissivity, tightly confined area near Fort Walton Beach, Fla. (where transmissivity and leakage coefficient are 2,000 ft2/d and 5.4 X 10 per day, respectively) with a more transmissive, less tightly confined area in Polk County, Fla. (where transmissivity and leakage coefficient are 130,000 ft2/d and 2.8 X 10 per day, respectively). Simulation shows a relatively low dependence on water from aquifer storage in Polk County, whereas proportionately much more water must come from storage near Fort Walton Beach. Thus the system reaches steady state quickly (a few weeks) at Polk County but slowly (more than a year) near Fort Walton Beach.
U.S. Department of the Interior, U.S. Geological Survey
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