DETERMINATION OF IN SITU REACTION RATES AS A RESULT OF BIOSTIMULATION AT THE FIELD RESEARCH CENTER, OAK RIDGE, TN

Bioremediation of radionuclides such as uranium (U) and technetium (Tc) is challenging due to the fact they are typically found at very low, yet hazardous concentrations, whereas other redox-sensitive elements are typically much more abundant. Successful bioremediation must be accomplished in the presence of these competing terminal electron acceptors in the groundwater system. Push-pull tests provide valuable information on in situ chemical reaction rates in groundwater systems that are typically too complicated to duplicate by other experimental methods. We have measured major, minor, and trace elements as well as C, N, and S isotopes in a time series of groundwater samples from a biostimulation field experiment conducted by Professor Jack Istok at the Department of Energy's Oak Ridge Field Research Center. Nutrients were added to the system to increase the activity of the native microbial population to such a degree that the competitive terminal electron acceptors were depleted through a sequence of microbially mediated redox reactions, allowing for the reduction of U(VI) and Tc(VII) to the insoluble, less-mobile U(IV) and Tc(IV) species. The time series samples allow us to derive in situ nitrate, sulfate, uranium, and technetium reduction rates as well as reactive-mineral precipitation/dissolution rates.