We integrate groundwater geochemistry, microbiology, and numerical modeling techniques to study the origin of elevated salinity and chemical evolution of groundwaters in the coastal plain aquifers of Alabama. Our field data indicate that chemical composition of groundwater evolves by various geochemical and microbial processes as it moves deeper into the subsurface. Sequential peaks of Ca 2+,Mg 2+,K +, and Na + along flow paths indicate that separation of ions may be driven by cation exchange. Microbial-mediated reactions are important for the formation of several discrete hydrochemical zones containing Fe 2+,Mn 2+,Sr 2+, and SO 4 2 rich groundwaters. Elevated Fe 2+,Mn 2+, and Sr 2+ concentrations may be derived from bacterial iron and manganese reduction. High sulfate concentrations observed a short distance from the recharge may be partly explained by microbial sulfur oxidation and nitrate reduction (denitrification). The presence of denitrifying and sulfur-oxidizing bacteria in water further supports these reactions. Major ion compositions and dD and d18O values are used to determine the source of salinity and the nature of mixing of different groundwaters. Three water types were identified; thes
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