GROUNDWATER GEOCHEMISTRY, ISOTOPE HYDROLOGY, AND MICROBIOLOGY OF COASTAL PLAIN AQUIFERS IN CENTRAL-SOUTH ALABAMA

We integrate groundwater geochemistry, microbiology, and numerical modeling techniques to study hydrologic and biochemical processes in the coastal plain aquifers in central-south Alabama. Data collected along two flow paths in the Eutaw and Tuscaloosa aquifers includes major ions, trace elements, stable and radioactive isotopes, and the type of microorganisms. Chemical composition and redox potential of groundwater evolves by biochemical processes as it moves deeper into the subsurface. Three water types were identified, including carbonate groundwater, groundwater associated with evaporates, and groundwater of meteoric origin. Significantly higher alkalinity and pH of groundwater in western Alabama correspond to the parallel spikes in Fe, Mn, and Sr concentrations. These correlations and the presence of the iron reducing bacteria Pseudomonas mendocina support that elevated Fe, Mn, and Sr concentrations are derived from bacterial iron and manganese reduction. Cl/Br and oxygen and hydrogen isotope ratios are used to determine the source of salinity in groundwaters. Both isotope and Cl-Br trends indicate the mixing of remnant evaporated seawater (close to halite saturation) with meteoric water. Groundwater age differences and flow velocities were calculated using 36Cl/Cl ratios. Calculated groundwater flow velocities in the Eutaw and Tuscaloosa aquifers are 0.2 m/yr and 0.15 m/yr, respectively. We developed two basin-scale hydrologic transport models in a cross section extending from the aquifer outcrops to the Gulf Coast. The first model evaluates the extent or depth to which freshwater could invade the aquifer since the emergence of the coastal plain. Although the predicted salinity distribution pattern generally agrees with the observed trend, the salinity level from Choctaw County to the coastal region is too high (ranges up to 180,000 ppm) to be explained by the seawater model. Our second solute transport model 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 stable isotope signatures which indicate that salinity of the groundwater could be derived from seawater that has been evaporated beyond halite saturation.