Memory Response: Exposure history dependence of microbial mediated transformations of substrates in groundwater

The flow, transport, and reactivity of dissolved-phase constituents in an unconfined and shallow aquifer were characterized, in situ, by utilizing the single-well push-pull test method. In the first study, the re-oxidation/mobility of uranium, in the presence of nitrate oxidant, was shown to be mitigated by preferential oxidation/mobilization of solid-phase, reduced, sulfur-bearing species. These results indicated that establishing conditions conducive to uranium reduction and the formation of reduced sulfur-bearing species can increase the efficacy of sustained uranium reduction/immobility in the presence of re-mobilizing oxidants. In the second study, the analytical solution to describe the one-dimensional displacement of the center of mass of a tracer during a push-pull test was expanded to account for displacement during the injection phase. The expanded solution improved the theoretical description of the displacement of a tracer during a push-pull test and the in situ application demonstrated an improvement for the estimation of effective porosity. In the third study, an analytical model was developed to describe the breakthrough of a potentially reactive solute due to non-reactive mixing and was applied to an in situ data set. The analytical model accurately predicted the breakthrough curve of nonreactive solutes and allowed for quantifying the rate and extent of reactive solute mass transfer and transformation. In the fourth study, the exposure history dependence of microbial mediated ethanol transformation was demonstrated to last up to six weeks in the absence of ethanol injections with no apparent enrichment of a select microbial community. This suggested that the predominant mechanisms of adaptation may exist at the enzymatic- and/or genetic-levels. In conclusion, the single-well push-pull test method was utilized and improved to characterize hydraulic parameters and processes, and microbial mediated transformations of substrates in groundwater.