Karst Aquifers: Upper Floridan and Biscayne Aquifers

The Floridan aquifer system (FAS) is a principal aquifer of the United States and is one of the most productive aquifers in the world. It covers approximately 100,000 square miles of the southeastern United States including all of Florida and parts of Georgia, Alabama, Mississippi, and South Carolina; however, the FAS in Mississippi is not used due to its high salinity and depth. Topography within the study area is relatively flat; altitudes range from sea level along the Atlantic Ocean and Gulf of Mexico coastlines to approximately 500 feet in central Georgia. Water-supply wells were first drilled in the late 1880s and currently the FAS is the primary source of drinking water for almost 10 million people. Water from the FAS is also used for industrial purposes and agricultural irrigation which accounts for nearly 50 percent of all withdrawals from the FAS.

The FAS consists of a thick sequence of Tertiary carbonate rocks that generally thickens seaward from the northern boundary of the system and is over 3,000 ft thick in south Florida. The top of the FAS is confined by late and middle Miocene series rocks of the upper confining unit (where present) and the bottom is confined by early Paleocene series rocks. From top to bottom, the major hydrogeologic units of the FAS are the Upper Floridan aquifer (UFA), middle confining and composite units, and Lower Floridan aquifer (LFA).

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The Floridan aquifer system behaves as one aquifer over much of its extent, though rocks of relatively lower permeability create hydrologic separation between the UFA and LFA sub-regionally. The majority of freshwater is contained in the Upper Floridan aquifer and is used for water supply. In south Florida, the Upper Floridan aquifer is brackish and used for purposes of reverse osmosis source water, blending with shallower fresh Biscayne aquifer groundwater, and aquifer storage and recovery. The Lower Floridan aquifer contains fresh to brackish water in northeastern Florida and Georgia, while in south Florida it is saline and used to dispose of effluent from wastewater treatment processes.

In the northern part of the study area the early Paleocene rocks underlying the FAS comprise part of the Southeastern Coastal Plain aquifer system (SECPAS). Vertical exchange of freshwater between the FAS and SECPAS likely is small, but in updip areas lower units of the FAS are hydraulically connected to clastic-equivalent units composing the upper part of the SECPAS and the degree of water exchange is a matter of debate. These clastic-equivalent units are included in the FAS framework by Williams and Kuniansky (2015) along with an updated extent of the most productive part of the system representing the predominantly carbonate facies (similar to the original extent of Miller, 1986).