Chemical profiles of groundwater in the Upper Floridan aquifer in southwest Florida vary between an inland and a coastal site. At the inland site, sulfate concentrations increase with depth, and chloride concentrations remain low. At the coastal site, sulfate concentrations are high and constant, but chloride concentrations increase with depth. The sulfate source at the inland site is gypsum dissolution, whereas at the coastal site, the sulfate source is predominantly gypsum dissolution overprinted by saltwater mixing. Mass transfer modeling in conjunction with isotopic data was used to evaluate hypothetical flow paths within the aquifer. Dedolomitization reactions (gypsum and dolomite dissolution with calcite precipitation) dominate along flow paths where sulfate concentration increase. The distribution of gypsum in the aquifer indicates that a deeply circulating flow path controls the chemical evolution of groundwater between the two sites. Fresh groundwater moves into deep zones in the aquifer where gypsum is found. Farther along the flow path in the discharge area, this groundwater moves upward to shallower depths prior to the saltwater mixing zone. Saltwater mixing becomes more pronounced with both proximity to the coast and with increasing depth. These results underscore the importance of evaluating vertical, as well as lateral, flow paths when quantifying geochemical reactions in a chemically stratified aquifer. ¿ American Geophysical Union 1995.