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The Floridan aquifer is the principal source of freshwater in southwest Florida. The freshwater part of the aquifer is underlain by saltwater in lower formations and is bounded on the coast by saltwater in the coastal parts of the aquifer. Management of water resources in the coastal margin of the area is important because of the potential for saltwater intrusion in areas of groundwater withdrawals. Saltwater and freshwater are mixed in the coastal zone. The saltwater boundary on the Gulf of Mexico side of the mixed zone is defined by chloride concentrations of 19,000 milligrams per liter, equal to that of seawater. On the landward side, the freshwater boundary of the mixed zone is defined by chloride concentrations of 25 milligrams per liter, which generally represents background levels of inland waters. The freshwater boundary of the mixed zone is probably moving inland in some areas because of lowering of the potentiometric surface of the Floridan aquifer and increasing ground-water withdrawals along the coast. Areas that are most subject to increasing chloride concentrations lie immediately inland from the 250-milligram-per-liter line of equal chloride concentration, near centers of heavy pumpage. Saltwater is present in the coastal margin of the aquifer in Citrus, Hernando, Pasco, Pinellas, and Hillsborough Counties north of Tampa Bay. Saltwater is absent in the coastal margin, at least in the upper part of the aquifer, in central Hillsborough County and in Manatee and Sarasota Counties. Freshwater is present above the saltwater in the coastal margin of the aquifer in Citrus, Hernando, Pasco, Pinellas, and Hillsborough Counties. A mixture of freshwater and saltwater is present in all coastal margins with the mixed zone much thicker and wider in Sarasota County than in Citrus County. Seventy wells at 54 sites are proposed to monitor chloride concentrations along the 250-milligram-per-liter line of equal chloride concentration in the coastal margin of the Floridan aquifer. Sixteen of the sites are proposed for areas of increasing chloride concentrations and heavy ground-water withdrawals. The remaining wells are needed to improve understanding of chloride-concentration distribution and to monitor long-term changes in chloride concentrations in the coastal margin of the Southwest Florida Water Management District. INTRODUCTION Ground-water withdrawals in southwest Florida are increasing because of population growth, industrial expansion, and increasing agricultural irrigation. Although there is an abundance of potable ground water in inland areas, management of water resources in the coastal margin is necessary to protect aquifers containing freshwater from saltwater intrusion. The Floridan aquifer is the source of most water supplies in southwest Florida. Saltwater occurs in the aquifer in the vicinity of the coastline and withdrawal of freshwater from the aquifer can induce saltwater to move inland from the coast. As an increasing amount of water is withdrawn to meet demands, the likelihood of saltwater intrusion also is increased. From July 1977 to September 1980, the U.S. Geological Survey, in cooperation with the Southwest Florida Water Management District, studied the distribution of chloride concentrations along coastal southwest Florida (fig. 1) for the purpose of delineating the location of the zone of mixing between freshwater and saltwater and for selecting well locations to monitor chloride concentrations in the coastal margin. Results of the early part of this study were released in a report on the 1979 position of the 250-milligram-per-liter (mg/L) line of equal chloride concentration in the upper part of the Floridan aquifer (Causseaux and Fretwell, 1982). The secondary maximum contaminant level of chloride recommended for drinking water is 250 mg/L (U.S. Environmental Protection Agency, 1977). Purpose and Scope The purpose of the study was to design an effective network of wells for monitoring the location and movement of the saltwater-freshwater mixed zone in the Floridan aquifer in west-central Florida. This report presents proposed locations of wells for monitoring chloride concentrations in the coastal margin of the Southwest Florida Water Management District. Selection of well sites was based on ground-water movement, centers of ground-water withdrawals, areas of increasing chloride concentrations, and the 1979 position of the 250-mg/L line of equal chloride concentration in the upper part of the Floridan aquifer. Previous Investigations The ground-water resources of southwest Florida have been discussed in varying detail in numerous reports of the Florida Bureau of Geology, the U.S. Geological Survey, and private organizations. Matson and Sanford (1913) described the geology and ground-water resources of the study area in a statewide report. In reports of investigations of five Florida counties, Stringfield (1933a; 1933b) discussed the geography, geology, and ground-water conditions in parts of southwest Florida. He called attention to the problem of saline-water contamination of the freshwater-bearing units. He also discussed the geqhydrology of the study area in his regional reports (Stringfield, 1936; 1966). Cooke (1945) and Vernon (1951) described the geology of southwest Florida, and Parker and others (1955) named and described the Floridan aquifer. Cooper and others (1953) and Cherry and others (1970) treated the general hydrology of parts of southwest Florida.

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