REACTIVE TRANSPORT OF SOLUTES AND ISOTOPES IN THE EUTAW AQUIFER, WESTERN ALABAMA

Water chemistry and quality of the Eutaw Formation in west-central Alabama has raised much concern in recent years because the aquifer has been exploited for large quantities of drinking water. Despite its importance as a major source of water supply, the sources of trace metals, the nature of water-rock interaction, fresh water recharge rates, and the influence of microbial processes on water chemistry remain poorly understood. Preliminary data indicate that high metal concentrations correlate with elevated alkalinity levels and pH values in groundwater. Elevated Fe²⁺, Mg²⁺, and Sr²⁺ concentrations may be derived from bacterial iron and manganese reduction as Fe-reducing bacteria metabolize organic matter such as lignite found in Cretaceous coastal plain sediments (Penny et al., 2003). Our field data also indicate that the chemical composition of groundwater evolves as it moves deeper into the subsurface and may also be influenced by physical mixing of Na-Cl and Ca-Mg-HCO₃ types of groundwater. Sequential peaks of Ca²⁺, Mg²⁺, K⁺, and Na²⁺ along the flowpaths indicate that separation of ions may be driven by cation exchange between groundwater and clay minerals like glauconite, which is abundant in the Eutaw Formation. It is thus hypothesized that the elevated alkalinity (and subsequent high metal concentrations) is potentially produced in two ways, by subsurface bacterial iron and manganese reduction, and/or inorganically by water-sediment interaction. Carbon isotopic signatures (¹³C/¹²C ratios) of groundwater and authigenic carbonate minerals (siderite and rhodochrosite) are used to quantitatively test hypothesized geochemical reactions (i.e., bacterial Fe reduction and cation exchange) that control bulk water chemistry, and to what extent microbial activity controls metal mobility. Reaction paths modeling of bacterial Fe³⁺ and Mn⁴⁺ reduction indicates that metal concentrations in coastal plain aquifers are highly influenced by biotransformation of Fe and Mn-oxide and carbonate minerals. A basin hydrology model Basin2 (Bethke et al., 1993) is used to simulate the transport of ³⁶Cl in the Eutaw aquifer to interpret residence time of groundwater. The results show that ³⁶Cl/Cl ratios are mainly affected by natural decay of ³⁶Cl and presence of Cl in old groundwater.