Chloride and total hardness concentrations of water from the upper part of the Floridan Aquifer in St. Johns County, Florida

Data collected from 1976 to 1979 indicate large water-quality variations in the upper 300 feet of the Floridan aquifer in St. Johns County, Florida. Chloride concentrations range from less than 10 milligrams per liter in the northwestern part of the county to more than 1,000 milligrams per liter in the southeast and extreme southwest. Total hardness concentrations range from 110 milligrams per liter in the northwest to 1,700 milligrams per liter in the southeast. In most of the county, high chloride and total hardness concentrations indicate the presence of saline water that entered the aquifer during the Pleistocene Epoch and has not been flushed out. Water quality in the aquifer has also been influenced by submarine discharge in the southeast and by heavy pumpage for irrigation in the southwest. High total hardness concentrations not accompanied by high chloride values in the western part of the county may be structurally controlled or may indicate the presence of gypsum evaporite beds in the upper 300 feet of the aquifer. (USGS)