Groundwater and microbial processes of Alabama coastal plain aquifers

Abstract

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 Ca2+, Mg2+, 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 Fe2+, Mn2+, Sr2+, and SO42− rich groundwaters. Elevated Fe2+, Mn2+, and Sr2+ 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 δD and δ18O values are used to determine the source of salinity and the nature of mixing of different groundwaters. Three water types were identified; these include carbonate groundwater, brines associated with evaporites, and groundwater of meteoric origin. Groundwater age differences and flow velocities were calculated using the 36Cl/Cl ratios. Calculated groundwater flow velocities within the Eutaw and Tuscaloosa aquifers are about 0.20 m/yr and 0.15 m/yr, respectively. We modeled basin‐scale hydrologic and solute transport processes in a cross section extending from the aquifer outcrops to the Gulf Coast. The modeling result shows that the buried Jurassic Louann Salt can significantly increase groundwater salinity in the overlying coastal plain aquifers by density‐driven advection and hydrodynamic dispersion. The modeling results are consistent with Cl/Br ratios and O/H isotope signatures, which indicate that salinity of the groundwater could be derived from seawater that has been evaporated beyond halite saturation. The predicted groundwater flow pattern reveals the mixing of meteoric water, carbonate groundwater (from the Ordovician Knox Group), and saline brines associated with the Louann Salt. The hydrologic model is consistent with the hydrochemical facies distribution in the Alabama coastal plain.