The Floridan aquifer system is one of the major sources of groundwater supplies in the United States. This productive aquifer system underlies all of Florida, southeast Georgia, and small parts of adjoining Alabama and South Carolina, for a total area of about 100,000 square miles. About 3 billion gallons of water per day were withdrawn from the aquifer system in 1980, and in many areas the Floridan is the sole source of freshwater.

The Floridan aquifer system is a sequence of hydraulically connected carbonate rocks (principally limestone with some dolomite) ranging in age generally from late Paleocene to early Miocene. The rocks vary in thickness from a featheredge where they crop out to more than 3,500 feet where the aquifer is deeply buried. The aquifer system generally consists of an upper aquifer and a lower aquifer separated by a lesspermeable confining unit of highly variable properties. In parts of north Florida and southwest Georgia, where little permeability contrast exists among the units, the Floridan is effectively one continuous aquifer. The upper and lower aquifers, named the Upper Floridan aquifer and the Lower Floridan aquifer, are defined on the basis of permeability and their boundaries locally do not coincide with those for either time-stratigraphic or rock-stratigraphic units.

Overlying much of the Floridan aquifer system are low-permeability clastic rocks. The lithology, thickness, and integrity of these rocks determine the degree of confinement and influence the distribution of natural recharge, discharge, and ground-water flow in the Floridan.

The permeability of the Floridan aquifer system is derived from small openings including fossil hashes and solution-widened joints as well as large cavernous openings in karst areas. Diffuse flow predominates where the small openings occur, whereas conduit flow may occur where large cavernous openings are. Transmissivities are highest (greater than 1,000,000 feet squared per day) in the unconfined karst areas of central and northern Florida. Lowest transmissivities (less than 50,000 feet squared per day) occur in panhandle Florida and southernmost Florida where the Upper Floridan aquifer is confined by thick clay sections. The hydraulic properties of the Lower Floridan aquifer are not well known; however, intervals of high transmissivity occur that have been attributed to paleokarst development.

Springs, nearly all of which occur in unconfined and semiconfined parts of the Upper Floridan aquifer in Florida are the dominant feature of the Floridan flow system. Before ground-water development, spring flow and point discharge to surface-water bodies were about 88 percent of the estimated 21,500 cubic feet per second total discharge, or about 19,000 cubic feet per second. Diffuse upward leakage, which occurs primarily in confined areas, accounted for the remaining 12 percent or about 2,500 cubic feet per second.

Most of the recharge necessary to sustain springflow and aquifer discharge to streams and lakes occurs relatively close to springs and areas of point discharge to surface-water bodies. Recharge to the Upper Floridan is highest, averaging 10-20 inches per year, in unconfined or semiconfined spring areas. The proximity of high recharge to high discharge implies a vigorous and well-developed shallow flow system in the unconfined and semiconfined parts of the Upper Floridan aquifer.

Ground-water flow is very sluggish in the parts of the aquifer system that are deeply buried and tightly confined, primarily southeast Georgia and northeast Florida, south Florida, and far-west panhandle Florida. Discharge to springs, streams, and lakes is practically nonexistent in the tightly confined areas and natural discharge occurs almost exclusively by diffuse upward leakage through thick overburden.

The regional flow system has not been appreciably altered by groundwater development. However, increasing pumpage that reached 3 billion gallons per day by 1980 has resulted in long-term regional water level decline of more than 10 feet in three broad areas: coastal Georgia, adjacent South Carolina, and northeast Florida; west-central Florida; and panhandle Florida. Saltwater encroachment as a result of pumping has occurred locally in coastal areas.

Pumpage from the Upper Floridan aquifer is supplied primarily by reduction of natural discharge and by increased recharge rather than by depletion of aquifer storage. About 20 percent is from reduced discharge to springs, streams, and lakes, about 20 percent is from reduced upward leakage, and about 60 percent is from increased recharge. Compared to predevelopment conditions, discharge to springs, streams, and lakes is reduced by less than 5 percent, upward leakage is reduced by about 30 percent, and recharge is increased by about 12 percent. Total recharge and, therefore, discharge increased from a predevelopment rate of 21,500 cubic feet per second to about 24,100 cubic feet per second by 1980.

A considerable area remains of the Floridan aquifer system where large ground-water supplies may be developed. This area is largely inland from the coasts and characterized by high transmissivity and minimal development prior to the early 1980's. The major constraint on future development probably is degradation of water quality rather than water-quantity limitations.