Reactive transport of trace elements and isotopes in the Eutaw coastal plain aquifer, Alabama

Abstract

We integrate groundwater geochemistry, mineralogy, and numerical modeling techniques to study the reactive transport of heavy metals and isotopes in the Eutaw coastal plain aquifer, Alabama. Geochemical data show that the elevated concentrations of Fe, Mn, and Sr can be correlated with high pH and alkalinity. These geochemical correlations suggest that that elevated metal concentrations may be derived from bacterial iron and manganese reduction. Geochemical modeling of bacterial Fe(III) and Mn(IV) reduction shows that the biotransformation of iron and manganese minerals could control mobility and concentrations of Fe and Mn in coastal plain aquifers. Petrographic, SEM, and EDAX studies of sediments show the formation of biogenic siderite, rhodochrosite, and pyrite in sediments associated with Fe- and Mn-rich groundwater. Rhodochrosite and siderite occur together as spheroids ( approximate to 1.0 mm) in which rhodochrosite forms the center and siderite forms an outer rind, which is consistent with the redox sequence of mineral precipitation predicted by the geochemical modeling. The low delta C-13 ratios of siderite and rhodochrosite ( - 14.4 to - 15.4 parts per thousand, PDB) and groundwater DIC (- 20.6 to - 14.1 parts per thousand, PDB) imply carbon of biogenic origin. Higher DIC-delta C-13 levels are found to be correlated with elevated Fe and Mn concentrations and high pH values of groundwater. This unexpected result implies novel carbon isotopic fractionation processes associated with bacterial Fe( III) reduction. We used Cl-36/Cl ratios of groundwater and isotope transport modeling to calculate the residence time of regional groundwater in the Eutaw aquifer. The calculation considering the natural decay only would yield Cl-36 levels that are significantly higher than field data, suggesting that a significant mixing with Cl-rich, older groundwater at depth is an important reason for substantial Cl-36 depletion along flow path.