Generalized maps showing variations in concentration of chlorides, sulfates, hardness, and dissolved solids in the Floridan aquifer have been prepared as part of a cooperative program with the Southwest Florida Water Management District. This report covers 10 counties and parts of 6 others comprising the District. Data used to develop the report were retrieved from the water-quality files of the U.S. Geological Survey. Considerable vertical and areal variation of chemical constituents was found in ground water of the Floridan aquifer. In general, ground water becomes more mineralized with increasing depth and with increasing distance from recharge areas due to solution of minerals from the aquifer. Ground water was also more mineralized with proximity to the coast, due to saltwater intrusion. (USGS)