The Floridan aquifer system is made up of several Tertiary carbonate formations that are hydraulically connected to form a regional hydrologic unit; locally, however, there are significant differences in its water-bearing properties, water chemistry, and flow. Development has proceeded unevenly with large withdrawals concentrated in a few areas. For example, in northwest-central Florida, there is a very active near-surface flow system characterized by high permeability, high rates of recharge, very large spring flows, and insignificant development as of the early 1980's. In contrast, along the coastal strip from Savannah, Ga., to Jacksonville, Fla., the Floridan is deeply buried and prior to development had a very sluggish flow system. Currently, the heavy withdrawals along this strip have caused a regional decline in artesian head and significant changes to the flow system.

Specifically, the objectives of the Floridan aquifer-system study were: (1) provide a complete description of the hydrogeologic framework and geochemistry of the aquifer system, (2) define the regional flow system, and (3) to assess the effects of ground-water development on the system.

Computer simulation was used extensively to understand the flow system, and locally the computer models were used to evaluate the effects of increased development. The approach to the Floridan study described here is two-tiered; to focus on (and document) local flow systems while tying together in a regional analysis the individual segments of the aquifer system.

Prior to the study, there existed much information on the geologic framework and aquifer hydraulic characteristics. In several areas, flow models had been developed with predictive capability. However, the greatest amount of data and the modeling efforts tend to be clustered around a few major pumping centers. Data are scarce in some areas where there are large untapped ground-water supplies.

To meet the project objectives, initial efforts concentrated on the assembly and analysis of the vast amount of data on the Floridan. A series of regional hydrogeologic, geochemical, and potentiometric-surface maps was prepared (Miller, 1982a, b, c, d, and e; Sprinkle 1982a, b, c, and d; Johnston, Healy, and Hayes, 1981; and Johnston and others, 1980).

A data-collection program was undertaken to fill data gaps. This work involved a program of exploratory drilling, aquifer tests, seismic surveys (onshore and off-shore), selective geochemical sampling, and mass measurement of water levels and artesian pressures. A notable example of these activities was the collection of hydrologic and geochemical data from an abandoned oil exploratory well 55 mi offshore from the Florida coast (Johnston and others, 1982).

Computer simulation involved the design and calibration of a "coarse-mesh" regional model and four subregional models. The goal of the regional model was to understand major features of the regional flow system. The regional model design and simulation results of predevelopment conditions were described by Bush (1982). Four subregional models focus on the areas of greatest ground-water development. Preliminary reports describing model design and results of subregional predevelopment aquifer-system simulation have been prepared for three areas: southeast Georgia, including small adjacent parts of South Carolina and Florida (Krause, 1982); west-central Florida (Ryder, 1982); and east-central Florida (Tibbals, 1981).

The Professional Paper 1403 series consists of nine Professional Papers that describe various aspects of the geology, hydrology, and geochemistry of the Floridan aquifer system. Emphasis is placed on descriptions of regional and local ground-water flow systems. In particular, the differences between predevelopment and current conditions are discussed in order to document the effects of ground-water development.

Professional Paper 1403-A (this report) summarizes important aspects of the hydrogeologic framework, hydraulic properties of the aquifers, regional flow system, effects of ground-water development, and geochemistry, which are discussed in detail in Professional Papers 1403-B through 1403-I.

Professional Paper 1403-B (Miller, 1986) presents the hydrogeologic framework of the Floridan aquifer system. The Floridan was subdivided into regional aquifers and confining units that provide a generalized permeability distribution so that we could analyze the flow system by computer simulation and relate the water chemistry to the flow system. Locally the aquifers and confining units may coincide with formally named geologic formations and groups. However, regionally neither the top nor base of the Floridan, nor its component aquifers and confining units, coincide with geologic-formation contacts. Thus Professional Paper 1403-B is concerned not only with descriptions of the stratigraphic units and hydrologic units but also the relations between them. It also addresses how geologic structure and the distribution of sedimentary facies have affected the permeability distribution and provided controls on regional ground-water flow.

Professional Paper 1403-C (Bush and Johnston, in press) discusses ground-water hydraulics, including aquifer and confining-unit properties, and describes features of the regional flow system. Inferred conditions prior to development are discussed and are compared to documented current conditions that reflect the effects of pumping. This comparison is concerned primarily with long-term changes in water levels and with changes in the rates and distribution of recharge and discharge caused by development. This regional analysis relies heavily on the results of computer simulation of the regional flow system.

To address the hydrology in more detail and to investigate local water problems, the Floridan aquifer system was divided into five subareas on the basis of natural (predevelopment) ground-water divides. Figure 1 shows the five subareas, which are the subjects of Professional Papers 1403-D through H.

Professional Paper 1403-D (Krause and Randolph, in press) discusses the hydrology of the Floridan in southeast Georgia, including adjacent parts of South Carolina and northeast Florida. This part of the system is characterized by the largest area of artesian head decline within the Floridan. Differences between the relatively sluggish predevelopment flow system and the current heavily pumped system are emphasized. Computer simulation is used to evaluate various alternative development schemes for the future.

Professional Paper 1403-E (Tibbals, in press) summarizes the hydrology of the Floridan in east-central Florida. Currently (early 1980's), pumpage accounts for about one-third of the aquifer discharge; however, declines in the artesian head occur in only a few small areas. Computer simulation is used to evaluate the predevelopment and current flow systems as well as to evaluate the effects of future increases in pumpage. Large springs and drainage wells receive special attention in this report.

Professional Paper 1403-F (Ryder, 1986) describes the Floridan hydrology in west-central Florida. Withdrawals in this area are about 1 Bgal/d; however, the large springs in the northern part of this area still remain the dominant feature of the current flow system. Computer simulation is used to evaluate the effects of future additional development, which is expected to be largely new municipal wells rather than pumping by the phosphate mining industry (the largest user in the early 1980's).

Figure 1. Extent of the Floridan aquifer system, showing subareas whose hydrology is discussed in Professional Papers 1403-D through 1403-H. [larger version]

Professional Paper 1403-G (Meyer, in press) discusses the hydrology in south Florida, where the Floridan contains saline water and has not been significantly developed for water supply. The lack of development results in a scarcity of geologic and hydrologic data; therefore, this study involved deep test drilling, geochemical sampling, and offshore seismic surveys. This report attempts to bring together new and existing data into a new hydrologic analysis of the Floridan aquifer system in south Florida. Waste injection in the deeper parts of the Floridan, which contain seawater, is also discussed.

Professional Paper 1403-H (Maslia and Hayes, in press) describes the Floridan in southwest Georgia and the Florida panhandle. This report is primarily concerned with an analysis of two parts of these areas: the 15-county Dougherty Plain area of southwest Georgia, where seasonal withdrawals for irrigation are very large and the Fort Walton Beach area of the Florida panhandle, where light pumping has produced a deep widespread cone of depression.

A sixth subarea in northwest Florida and adjoining south Georgia (figure 1) is characterized by little ground-water development and limited available hydrologic data. No Professional Paper was written for this area; however, certain aspects of its hydrology are discussed in Professional Papers 1403-B, 1403-C, and 1403-I.

Professional Paper 1403-I (Sprinkle, in press) describes the geochemistry of the Floridan. The principal chemical processes that are operating in the ground-water flow system are explained using previously published and newly collected data. Quantification of the principal chemical reactions is if illustrated for major segments of the flow system.