THE HYDROLOGIC, GEOCHEMICAL, AND MICROBIOLOGIC INTERACTIONS OF THE WESTERN EUTAW AQUIFER, ALABAMA

The Cretaceous Eutaw aquifer is a large coastal plain aquifer that covers an area of approximately 12,500 mi² in central Alabama. For over 150 years, the Eutaw aquifer has produced water supplies for much of central Alabama. Over 50 million gallons of water per day is pumped from the Eutaw aquifer reaching more than 650,000 people in 20 counties. The quality of drinking water supplied to this increasingly large population has been the concern of many in recent years. Although many have studied the geology of the coastal plain, elevated metal (iron, manganese, and strontium) content, water-sediment interaction, and influence of subsurface microorganisms on groundwater chemistry remain insufficiently understood. Preliminary data indicate that high alkalinity levels correlate with high metal concentrations in groundwater. It is hypothesized that the elevated alkalinity levels are potentially produced in two ways, by bacterial iron and manganese reduction, and/or inorganically by water-sediment interaction. These hypotheses are tested by groundwater chemistry, microbiology analysis and by quantifying cation exchange capacity of clay minerals in the aquifer. Our field data indicate that the 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 Na²⁺along the flow paths indicate that separation of ions may be driven by cation exchange. Elevated Fe²⁺, Mn²⁺, and Sr²⁺ concentrations may be derived from bacterial iron and manganese reduction. Basin-scale solute and isotope transport processes are modeled in a cross-section extending from the aquifer outcrops to the Gulf Coast. The modeling results show that the buried Jurassic Louann salt can significantly increase groundwater salinity in the shallow coastal plain aquifer by density driven advection and hydrodynamic dispersion.