GROUNDWATER AND MICROBIAL PROCESSES IN COASTAL PLAIN AQUIFERS, ALABAMA

We integrate groundwater geochemistry, microbiology, and numerical modeling techniques to study the origin of trace metals and elevated salinity in the coastal plain aquifers in central-south Alabama. Significantly higher alkalinity and pH of groundwater correspond to the parallel spikes in Fe and Mn concentrations. These correlations and the presence of the iron-reducing bacteria Pseudomonas mendocina indicate that elevated Fe and Mn concentrations are derived from bacteria iron and manganese reduction. Reaction path modeling techniques were used to trace the biogeochemical reactions would accompany bacteria Fe (III) and Mn (IV) reduction in the coast plain aquifers. The simulation shows that Mn (IV) mineral pyrolusite in the initial system becomes thermodynamically unstable and transforms to a sequence of more stable manganese minerals at progressively lower oxidation states. Once reduction of Mn has nearly completed, hematite begins to dissolve to form more stable magnesite at very low oxidation states. This modeling results support that the Ostwald's Step Rule governs the biotransformation of iron and manganese minerals in coastal plain aquifers. Major ion and stable isotopic compositions are used to determine the source of salinity and chemical evolution of groundwaters. Three water types were identified, these include carbonate groundwater, groundwater associated with evaporites, and groundwater of meteoric origin. Measured 36Cl/Cl ratios indicate that groundwater flow velocities within the Eutaw and Tuscaloosa aquifers are about 0.2 m/yr and 0.15 m/yr, respectively. The result of basin-scale hydrologic modeling shows that the buried Jurassic Louann Salt can significantly increase groundwater salinity in the overlying coastal plain aquifers by advection and diffusion. 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 Louann Salt. The hydrologic model is consistent with the hydrochemical facies distribution in the Alabama coastal plain.