Groundwater is the only significant source of potable water in Flagler County. Usable water occurs in the Upper Floridan aquifer, the intermediate population is expected to place stresses on the water resources of the county. Although rainfall averages almost 50 in/yr, most of the water leaves as evapotranspiration and streamflow. Less than 1 in/yr recharge may be occurring to the Upper Floridan aquifer, the highest yielding aquifer. The Upper Floridan aquifer consists of the Avon Park Formation, the Ocala Limestone, and the basal dolomitic limestone of the Hawthorne. Use of the Upper Floridan aquifer for public water supply is limited in most of the county because it contains marginally potable or brackish water. It is used extensively for agricultural irrigation. The intermediate aquifer system consists of thin, discontinues lenses of sand, shell, and limestone between clays overlying the Floridan aquifer system. The intermediate aquifer system is an important part of the public water supply of the county because of the good quality of the water. The intermediate aquifer system has variable yields because of the discontinuous lenses. The surficial aquifer system is composed of sand and shell with varying fractions of finer materials. Well yields are small in the west and central parts of Flagler County, but the surficial aquifer system is an adequate source of domestic supply on the barrier island. A zone of freshwater in the surficial aquifer system is very important in the Hammock area, being the local source of most domestic supply in the area. Changes in hydrologic conditions from the 1950 's include a long-term decline in water levels in the Upper Floridan aquifer coincident with lower-than-average rainfall and a greater seasonal fluctuation of water levels. Chloride concentrations of water in the Upper Floridan aquifer do not appear to have changed significantly, presently ranging from 7 to 3,700 mg/L. Development will place stress on the aquifers and may result in upconing of brackish water in pumping centers and in lateral saltwater intrusion in coastal areas. (Author 's abstract)