Floridan Aquifer System Groundwater Availability Study

Radium Springs, Albany, Georgia: Photo by Alan Cressler, USGS
Radium Springs, Albany, Georgia: Photo by Alan Cressler, USGS

The hydrogeologic framework for the Floridan aquifer system (Williams and Kuniansky, 2015; Williams and Dixon, 2015) has been revised by incorporating new studies and data that have become available since the original framework established by the U.S. Geological Survey in the 1980s (Miller, 1986).

Adjustments were made to the internal boundaries of the Upper and Lower Floridan aquifers as well as the previously numbered middle confining units (MCUI–MCUVIII). Boundaries were mapped using results from local studies and regional correlations of lithstratigraphic and hydrogeologic units or zones. Additional zones within the aquifers have been incorporated into the framework to allow finer delineation of permeability variations within the aquifer system.

In the previous framework, discontinuous numbered middle confining units (MCUI–VII) were used to subdivide the system. Some of these individually numbered middle confining units overlapped each other vertically. Previously, where units overlapped, the least permeable rock unit within the middle part of the system was used to subdivide the system. In areas where less-permeable rocks do not occur within the middle part of the system, the system was previously considered one aquifer and named the Upper Floridan aquifer. Although the term “confining unit” is not totally abandoned within the revised framework, a new term “composite unit” is introduced for lithostratigraphic units that cannot be defined as either a confining or aquifer unit over their entire extent. This naming convention is a departure from the previous framework of the late 1980s in that stratigraphy is used to consistently subdivide the aquifer system into upper and lower aquifers across the State of Florida. Areas of differing hydraulic properties of composite units are delineated to indicate where the Upper and Lower Floridan aquifers behave as one aquifer system. The revised framework uses stratigraphic names for the composite units within the middle part of the Floridan aquifer system rather than numbers. Three laterally extensive units are used to consistently divide the Floridan aquifer system into the Upper and Lower Floridan aquifers; the Bucatunna Clay confining unit, the Lisbon-Avon Park composite unit, and the Middle Avon Park composite unit. This lithostratigraphic mapping approach does not change the concept of flow within the system.

The lateral extent of the updip boundary of the Floridan aquifer system has been modified to include some of the clastic facies which grade laterally into the Lower Floridan aquifer and have been previously included in either the Southeastern Coastal Plain aquifer system, the Floridan aquifer system, or both. The updip limit of the productive part of the Floridan aquifer system was also revised and generally coincides with the updip limit of the carbonate facies facies as previously used by Miller (1986) as the extent of the system.

Additionally, the extent and altitude of the freshwater-saltwater interface was been mapped to define the freshwater part of the flow system.

It should be noted that the revised framework is a regional work product intended for regional and subregional investigations (1,000 to greater than 10,000 square miles), rather than site-scale (less than 1 square mile) investigations intended for regulatory purposes.

The Floridan aquifer system is a principal aquifer of the United States and is one of the most productive aquifers in the world (Miller, 1990). In the northern part of the study area, the Floridan aquifer system overlies the Southeastern Coastal Plain aquifer system. In other parts of the study area, the Floridan aquifer system underlies several significant aquifers including the Biscayne aquifer in southeast Florida and the Sand and Gravel aquifer in the western Florida Panhandle, which are part of the surficial aquifer system (see diagram below). Throughout the rest of the study area (except where the Floridan aquifer system is unconfined), the Floridan aquifer system underlies the more extensive but less productive portions of the surficial aquifer system. The intermediate aquifer system also underlies the surficial aquifer system and overlies the Floridan aquifer system in several counties in southwest Florida.

Freshwater exchange between the Floridan aquifer system and the surficial and intermediate aquifer systems can significantly affect the water budget and groundwater levels of the Floridan aquifer system in some areas (Sepúlveda, 2002; Sepúlveda and others, 2012). Vertical exchange of freshwater between the Floridan and Southeastern Coastal Plain aquifer systems is small relative to the water budget of the Floridan aquifer system. In southeast Florida beneath the Biscayne aquifer, the Upper Floridan aquifer is brackish, well confined, and generally artesian, thus downward freshwater leakage is minimal.