Chloride concentrations in the surficial and intermediate aquifer systems in southwestern Florida indicate a general trend of increasing salinity coastward and with depth. There are some notable exceptions to this trend. Brackish water is present in the sandstone and mid-Hawthorn aquifers in several inland areas in Lee County. In an area near the coast in Collier County, the lower Tamiami aquifer contains freshwater, with brackish water present farther inland. Saline water is present in the lower Tamiami aquifer along the coast in Collier County, but water is brackish in the underlying mid-Hawthorn and Upper Floridan aquifers. The analyses of major ions, hydrogen and oxygen isotopes, and strontium isotopes indicate the primary sources of salinity are underlying aquifers and the Gulf of Mexico. Based on these data, much of the salinity is from upward leakage of brackish water from underlying aquifers. Discharge as diffuse upward leakage and artesian wells are two possible pathways of saltwater intrusion from underlying aquifers. Artesian wells open to multiple aquifers have been pathways of saltwater intrusion in the sandstone and mid-Hawthorn aquifers in much of Lee County. The source of brackish water in the lower Tamiami and mid-Hawthorn aquifers in Collier County may be natural diffuse leakage from underlying aquifers. The source of the saline water in the lower Tamiami aquifer in Collier County is apparently the Gulf of Mexico; it is unclear however, whether this saline water is residual water from former Pleistocene sea invasions or recent saltwater intrusion.