Uranium Retention in a Bioreduced Region of an Alluvial Aquifer Induced by the Influx of Dissolved Oxygen

Abstract

Reduced zones in the subsurface represent biogeochemically active hotspots enriched in buried organic matter and reduced metals. Within a shallow alluvial aquifer located near Rifle, CO, reduced zones control the fate and transport of uranium (U). Though an influx of dissolved oxygen (DO) would be expected to mobilize U, we report U immobilization. Groundwater U concentrations decreased following delivery of DO (21.6 mg O/well/h). After 23 days of DO delivery, injection of oxygenated groundwater was paused and resulted in the rebound of groundwater U concentrations to preinjection levels. When DO delivery resumed (day 51), groundwater U concentrations again decreased. The injection was halted on day 82 again and resulted in a rebound of groundwater U concentrations. DO delivery rate was increased to 54 mg O/well/h (day 95) whereby groundwater U concentrations increased. Planktonic cell abundance remained stable throughout the experiment, but virus-to-microbial cell ratio increased 1.8-3.4-fold with initial DO delivery, indicative of microbial activity in response to DO injection. Together, these results indicate that the redox-buffering capacity of reduced sediments can prevent U mobilization, but could be overcome as delivery rate or oxidant concentration increases, mobilizing U.

Complexation by Organic Matter Controls Uranium Mobility in Anoxic Sediments

“…This behavior has implications for U mobility in the subsurface. Previously, oxidation of U­(IV) has been posited as a mechanism by which U is released in contaminated anoxic subsurface environments. , However, depending on the redox-buffering capacity of the sediment, U­(IV) can persist during influxes of oxidants, such as O 2 and NO 3 – . ,, An additional mechanism of U mobilization from anoxic sediments is therefore needed to explain persistent elevated groundwater concentrations. We suggest two possibilities: first, adsorbed U­(IV), whether complexed by OM or clay mineral surfaces, could be mobilized by desorption; in particular, by HCO 3 – /CO 3 2– , which solubilizes nonmineral forms of U­(IV) .…”

Stability of Floodplain Subsurface Microbial Communities Through Seasonal Hydrological and Geochemical Cycles

“…However, as noted above, the oxidation of soil layers occurred several weeks after the water table dropped below them, whereas the onset of reducing conditions was almost immediate upon water saturation during the spring flood (Figure 1). This hysteresis effect indicates that the TRZ is considerably more resilient to oxidation than to reduction, as has been noted at other similar sites (Noël et al, 2017a(Noël et al, ,b, 2019Dwivedi et al, 2018;Pan et al, 2018). The buffering capacity against re-oxidation has been explained by a combination of slow diffusivity and high oxygen consumption rates driven by microbial respiration of organic C (Pan et al, 2018) and abiotic oxidation of reduced species, such as iron monosulfides, FeS (Noël et al, 2017a(Noël et al, ,b, 2019.…”

“…This hysteresis effect indicates that the TRZ is considerably more resilient to oxidation than to reduction, as has been noted at other similar sites (Noël et al, 2017a(Noël et al, ,b, 2019Dwivedi et al, 2018;Pan et al, 2018). The buffering capacity against re-oxidation has been explained by a combination of slow diffusivity and high oxygen consumption rates driven by microbial respiration of organic C (Pan et al, 2018) and abiotic oxidation of reduced species, such as iron monosulfides, FeS (Noël et al, 2017a(Noël et al, ,b, 2019. This is likely also the case for the Riverton TRZ where the HCl-extractable Fe(II) increased after the flood (Supplementary Figure S4), indicating an accumulation of "reactive" Fe(II) minerals, such as FeS.…”

“…Noël et al (2017bNoël et al ( , 2019 suggested that the onset of seasonal oxidizing conditions in TRZs could be fast enough to drive major changes in the chemical form and stability of U trapped within. However, recent research shows that oxidative release of U by seasonal increases in oxygen (Bone et al, 2017;Pan et al, 2018) and nitrate reduction (Nolan and Weber, 2015) is impeded by bioavailable organic carbon (C) in suboxic sediments, especially TRZs. Thus, rapid oxidant consumption and subsequent anaerobic respiration in TRZs link C cycling and U distribution and mobility.…”

Export of Organic Carbon from Reduced Fine-Grained Zones Governs Biogeochemical Reactivity in a Simulated Aquifer

“…Shallow alluvial aquifers constitute important groundwater resources that are highly influenced by hydrologic changes and anthropogenic activities (e.g., irrigation, fertilizer and pesticide applications, mining-related activities). − Alluvial aquifers also typically exhibit a multitude of sediment interfaces as a result of changing deposition conditions during their formation. Subsurface interfaces generate sharp gradients in physicochemical properties (e.g., redox conditions, mineralogical and organic matter composition, hydraulic conductivity) and, as a consequence, have a disproportionately large influence on biogeochemical processes. − Previous studies have shown that groundwater chemistry in shallow alluvial aquifers is strongly influenced by the presence of organic-rich, fine-grained, and predominantly anoxic layers or lenses in otherwise coarse-grained and predominantly oxic aquifers. − This effect has primarily been attributed to anaerobic microbial activity occurring in the fine-grained, organic-rich zones and subsequent export of reduced aqueous products to the coarse-grained aquifer. − Recent field- and laboratory-based experiments suggest that reducing conditions develop in the coarse-grained zones downstream of the fine-grained lenses or layers. Sustained development of anoxic conditions in the aquifer is beyond what can be explained by the export of reduced aqueous products and their downstream abiotic reactions.…”