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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 transmissivity of the Upper Floridan aquifer varies by more than three orders of magnitude as a result of the wide variation in hydrogeologic conditions. The conditions that most affect transmissivity are the degree of solution development in the aquifer and, to a lesser extent, the aquifer thickness. High transmissivities usually occur in the areas having less confinement because circulation of flow helps to develop solution openings in the aquifer. Table 3 illustrates the combinations of these hydrogeologic characteristics that produce the variations in transmissivity for the geographic areas underlain by the Upper Floridan.
The low values of transmissivity (less than 50,000 ft2/d) occur in the Florida panhandle and southernmost Florida (where the aquifer is confined by thick clay sections and contains thick sections of low-permeability limestone) and in the updip areas of Alabama, Georgia, and South Carolina (where the aquifer is thinnest). Transmissivities are highest (greater than 1,000,000 ft2/d) in the karst areas of central and northern Florida, where the aquifer is generally unconfined or semiconfined.
The areal distribution of transmissivity of the Upper Floridan aquifer is shown on figure 2. The map portrays the most probable ranges of transmissivity based on values derived from 114 aquifer tests, computer simulation, and geology. A tabulation of the aquifer tests, including method of analysis and source of test data, is presented in Professional Paper 1403-C. At sites where test wells are fully penetrating, the field-test values and the model-derived values generally are in agreement. However, where test wells do not fully penetrate the Upper Floridan, the field-test values are generally less than the model-derived numbers. The field-test data tend to be concentrated in the areas of heavy withdrawals. Where there has been little or no ground-water development, the transmissivity estimates used to prepare figure 2 are based primarily on model calibration. This includes the area of very large spring flows in central and northwest Florida. Within this area, simulation indicates transmissivities ranging from 250,000 ft2/d to as much as 10,000,000 ft2/d. An appraisal of the reliability of the transmissivity map based on the availability of aquifer-test data and the sensitivity of a regional flow model to transmissivity is presented in Professional Paper 1403-C.
The distribution of transmissivity shown on figure 2 is closely related to the degree of confinement of the Upper Floridan. Comparison of figure 2 with plate 2, which shows confined and unconfined conditions for the Upper Floridan, indicates that the confined areas generally have lower transmissivity than semiconfined or unconfined areas. All of the very high transmissivity area (greater than 1,000,000 ft2/d) and much of the high-transmissivity area (250,000 to 1,000,000 ft2/d) occurs where the aquifer is either unconfined or semiconfined.
The very high transmissivity areas are characterized by the extensive development of solution features in the carbonate rock. The development of these features is related to the geologic history, and is discussed further in Professional Paper 1403-B and has been described in detail by Stringfield (1966). Where there is extensive karst development, the permeability distribution is extremely complex, with marked differences in transmissivity occurring in short distances. For example in a flow-net analysis of the Silver Springs drainage area, Faulkner (1973, p. 95) calculated transmissivities varying by more than three orders of magnitude: 11,000 to 25,000,000 ft2/d for individual cells within the 92-mi2 area of his flow net.
The low values of transmissivity (less than 50,000 ft2/d) occur in the Florida panhandle, southernmost Florida, and the updip areas of Alabama, Georgia, and South Carolina. In the updip areas, the decreased transmissivity results simply from thinning of the aquifer. However, the development of karst in the out-crop area of southwest Georgia causes a sharp increase in transmissivity just downdip from the featheredge of the aquifer. The low transmissivity in the thick downdip sections of the Florida panhandle and southernmost Florida results from fades changes in the carbonate rock. As discussed in Professional Paper 1403-B, the aquifer in these areas contains large amounts of micritic limestone that has very low permeability.
Areal variations in the transmissivity of the Lower Floridan aquifer cannot be defined because of a lack of aquifer test data. The digital flow models provided little basis for improving initial estimates of transmissivity, inasmuch as the models were insensitive to changes in transmissivity of the Lower Floridan. In southeast Florida, the Lower Floridan contains a cavernous unit termed the "Boulder zone" (p1.1) that is increasingly being used for injection of treated sewage and industrial wastes. Aquifer tests in the Boulder zone suggest a transmissivity in excess of 3,000,000 ft2/d (Meyer, 1974; Singh and others, 1983).
U.S. Department of the Interior, U.S. Geological Survey
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