Chloride concentrations in the coastal margin of the Floridan Aquifer, southwest Florida

The Floridan aquifer is the principal source of freshwater supply in southwest Florida. The freshwater part of the aquifer is underlain by saltwater in lower formations and is bounded on the west by saltwater in the coastal parts of the aquifer. A zone of mixing lies between the saltwater and freshwater. The saltwater and freshwater boundaries are defined by chloride concentrations of 19,000 milligrams per liter on the coastward side and 250 milligrams per liter on the landward side. Lateral intrusion of saltwater can occur along the coast in areas where the hydraulic gradient declines and upconing can occur in inland areas of heavy ground-water withdrawals. Safeguards against saltwater intrusion in these areas are necessary because flushing of the aquifer is a very slow process. A network of chloride monitor wells is proposed for the zone of mixing in the upper producing zone of the floridan aquifer. Seventy wells were selected at 54 sites to monitor chloride concentrations along the 250-milligram-per-liter line of equal chloride concentration. One-fourth of the wells are located in areas of increasing chloride concentrations or heavy ground-water withdrawals and the remaining wells are intended to improve delineation of the interface and to monitor long-term changes in chloride concentrations. (USGS)