Transient redox regimes in a shallow alluvial aquifer

Using dialysis cells, sediment analysis and systematic ground water sampling we are investigating transient redox gradients in a shallow aquifer at the Rio Calaveras research site located in the Jemez Mountains of northern New Mexico. Hydrochemical data show that the dominant redox potentials shift spatially and temporally from O2/H2O2 during spring to a Fe3+/Fe2+, Mn4+/Mn2+ and SO42−/HS− system with the onset of summer. In autumn, both iron and sulfate reduction processes are more pronounced in the upper meter of the water table a condition that persists until spring snow melt infiltration. Infiltration of spring snow melt transports dissolved oxygen to the top of the aquifer where it reacts with Fe2+ and HS− and shifts the redox potentials from moderately reducing to moderately oxidizing in the upper regions of the aquifer. Redox is strongly controlled by inputs of organic carbon, the primary reductant in the system. Low molecular weight organic acids (acetate, formate, propionate and oxalate) are vertically zoned with a greater abundance in the upper meter of the aquifer. Organic acids, derived from organic-rich sediments in the aquifer, are transported from the overlying vadose zone reservoir providing a substrate for heterotrophic bacteria that reduce the terminal electron acceptors (TEAs) O2, MnO2(s), Fe(OH)3(s) and SO42−. We postulate that two central mechanisms are primarily responsible for transient redox gradients during an annual cycle: (1) bacterially mediated reduction of manganese, iron and sulfate shifts redox to moderately reducing conditions during autumn and (2) transport of molecular oxygen to the top of the water table during infiltration events oxidizes Fe2+ and HS− diminishing their concentrations and shifting redox towards more oxidizing conditions.