Abstract

The Floridan aquifer system in southeastern Florida consists of the Upper Floridan aquifer, the middle confining unit, and the Lower Floridan aquifer and ranges from Paleocene to Oligocene in age. The top of the rocks of Eocene age in the study area approximates the top of the Upper Floridan aquifer and coincides with an important flow zone.

An upper zone of brackish water and a lower zone of water with salinity similar to that of seawater are present in the Floridan aquifer system. The brackish-water zone is defined as that in which water has a dissolved-solids concentration of less than 10,000 milligrams per liter (chloride concentration less than about 5,240 milligrams per liter), and water in the saline-water zone has a dissolved solids concentration of about 35,000 milligrams per liter (about 18,900 milligrams per liter chloride concentration). The brackish-water and saline-water zones are separated by a transitional zone, typically 100 feet thick, in which salinity increases abruptly with depth. The base of the brackish-water zone lies within the Upper Floridan aquifer along the coast but extends into the middle confining unit inland. The brackish-water zone is as much as 1,200 feet thick inland, whereas the Upper Floridan aquifer is typically 500 to 600 feet thick. Chloride concentrations range from about 850 to 5,640 milligrams per liter and from about 1,410 to 3,330 milligrams per liter in the upper and lower intervals of the brackish-water zone, respectively.

The base of the brackish-water zone and the top of the saline-water zone were approximately determined mostly through the use of resistivity borehole geophysical logs. Changes in lithology or permeability do not usually control the position of the boundary between the brackish-water and saline-water zones.

Calculations of the depth of a brackish-water/saline-water interface using the Ghyben-Herzberg relation show good agreement between calculated and actual positions of the interface, indicating equilibrium between the zones.

Several areas of high salinity (chloride concentrations greater than 3,000 milligrams per liter) are present in the upper interval of the brackish-water zone near the coast, and in one of these areas in northeastern Broward County, salinity decreases with depth from the upper to lower interval. Available data indicate that in areas of high salinities, the Upper Floridan aquifer has relatively high transmissivity. The high salinities could be a result of seawater preferentially encroaching into zones of higher permeability in the Upper Floridan aquifer during Pleistocene high stands of sea level and incomplete flushing of the seawater by the present-day flow system.