Increases in the chloride concentration have been observed in the water from numerous wells in northeastern Florida. Possible sources of the mineralized water include modern sea water intrusion, unflushed Pleistocene seawater, connate water in aquifer sediments, or mineralized water (brine) from formations beneath the Floridan aquifer system.

Water samples were collected from 53 wells tapping various aquifer zones including: the Fernandina permeable zone (FPZ), the Upper zone of the Lower Floridan aquifer (UZLF), the Upper Floridan aquifer (UFA), and both the UFA and the UZLF. Water samples were analyzed for major ions, trace constituents, and isotopes of carbon, oxygen, hydrogen, sulfur, strontium, chlorine, and boron. Major-ion concentrations vary as much within the UZLF and the UFA as between these two zones. Simple models of mixing between fresh ground water and either modern seawater or water from the FPZ as a mineralized end member show that many water samples from the UZLF and the UFA are enriched in bicarbonate, calcium, magnesium, sulfate, fluoride, and silica and are depleted in sodium and potassium (as compared to concentrations predicted by mixing). Chemical mass-balance models of mixing and reactions between a hypothetical initial seawater and aquifer minerals cannot account for the observed water chemistry in a few wells, implying a source other than seawater, either ancient or modern.

Chemical and isotopic data indicate that the most likely source of mineralized water in most UZLF and UFA wells is mineralized water from the underlying FPZ that moves upward along joints, fractures, collapse features, or other structural anomalies. Most of the water samples collected are a mixture of more than 99 percent fresh water with less than one percent mineralized water. The small fraction of mineralized water present in those samples makes identification of the source water more difficult. Ion enrichment/depletion data, along with mass-balance models and chlorine isotope data, indicate that for low-chloride samples, simple dilution of seawater can explain observed water chemistry. However, for higher-chloride water samples, another source of mineralized water, possibly a brine, is likely.