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Three-dimensional variably saturated flow and multicomponent biogeochemical reactive transport modeling, based on published and newly generated data, is used to better understand the interplay of hydrology, geochemistry, and biology controlling the cycling of carbon, nitrogen, oxygen, iron, sulfur, and uranium in a shallow floodplain. In this system, aerobic respiration generally maintains anoxic groundwater below an oxic vadose zone until seasonal snowmelt-driven water table peaking transports dissolved oxygen (DO) and nitrate from the vadose zone into the alluvial aquifer. The response to this perturbation is localized due to distinct physico-biogeochemical environments and relatively long time scales for transport through the floodplain aquifer and vadose zone. Naturally reduced zones (NRZs) containing sediments higher in organic matter, iron sulfides, and non-crystalline U(IV) rapidly consume DO and nitrate to maintain anoxic conditions, yielding Fe(II) from FeS oxidative dissolution, nitrite from denitrification, and U(VI) from nitrite-promoted U(IV) oxidation. Redox cycling is a key factor for sustaining the observed aquifer behaviors despite continuous oxygen influx and the annual hydrologically induced oxidation event. Depth-dependent activity of fermenters, aerobes, nitrate reducers, sulfate reducers, and chemolithoautotrophs (e.g., oxidizing Fe(II), S compounds, and ammonium) is linked to the presence of DO, which has higher concentrations near the water table.

Field-scale biostimulation and desorption tracer experiments conducted in a uranium (U) contaminated, shallow alluvial aquifer have provided insight into the coupling of microbiology, biogeochemistry, and hydrogeology that control U mobility in the subsurface. Initial experiments successfully tested the concept that Fe-reducing bacteria such as Geobacter sp. could enzymatically reduce soluble U(VI) to insoluble U(IV) during in situ electron donor amendment (Anderson et al. 2003, Williams et al. 2011). In parallel, in situ desorption tracer tests using bicarbonate amendment demonstrated ratelimited U(VI) desorption (Fox et al. 2012). These results and prior laboratory studies underscored the importance of enzymatic U(VI)-reduction and suggested the ability to combine desorption and bioreduction of U(VI). Here we report the results of a new field experiment in which bicarbonate-promoted uranium desorption and acetate amendment were combined and compared to an acetate amendment-only experiment in the same experimental plot. Results confirm that bicarbonate amendment to alluvial aquifer sediments desorbs U(VI) and increases the abundance of Ca-uranyl-carbonato complexes. At the same time, the rate of acetate-promoted enzymatic U(VI) reduction was greater in the presence of added bicarbonate in spite of the increased dominance of Ca-uranylcarbonato aqueous complexes. A model-simulated peak rate of U(VI) reduction was ~3.8 times higher during acetate-bicarbonate treatment than under acetate-only conditions. Lack of consistent differences in microbial community structure between acetatebicarbonate and acetate-only treatments suggest that a significantly higher rate of U(VI) reduction in the bicarbonate-impacted sediment may be due to a higher intrinsic rate of microbial reduction induced by elevated concentrations of the bicarbonate oxyanion. The findings indicate that bicarbonate amendment may be useful in improving the engineered bioremediation of uranium in aquifers. * Concentration/enrichment within the injection tank. ** Tank #2 injection was initially started on 9-Sept-10; however, a closed injection valve prevented flow from the tank; injection was restarted on 13-Sept-10, as indicated.

[1] Subsurface simulation is used to build, test, and couple conceptual process models to better understand the persistence of uranium concentrations above federal drinking water standards in a 0.4 km by 1.0 km groundwater plume beneath the 300 Area of the U.S. Department of Energy's Hanford Site in eastern Washington State. At this location, the unconfined aquifer and the variably saturated lower vadose zone sediments are subject to significant variations in water levels driven by diurnal, weekly, and seasonal fluctuations in the Columbia River stage. In the near-river aquifer, uranium-contaminated sediments in the highly transmissive Hanford formation are subject to high groundwater velocities, daily flow reversals, and exposure to river water. One-and two-dimensional simulations of variably saturated flow and reactive transport based on laboratory-derived models of uranium sorption are used to assess the representation of uranium transport processes in the vadose zone-aquifer-river system. The simulations show that the various frequencies of river stage fluctuation are capable of driving significant inland transport above the average water table, which is in contrast to the net groundwater flow to the river. Inclusion of a rate-limited uranium mass transfer process model is notably more important to the timescales of the river stage-driven groundwater flow than for vadose zone flow driven by natural recharge. Spatially and temporally variable solution chemistry from the dynamic exchange of river water and groundwater in the unconfined aquifer is shown to significantly alter uranium mobility as represented by a multicomponent uranium surface complexation model.

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