Molecular controls over uranium biogeochemistry in the upper Colorado River Basin: A regional perspective
Résumé
Organic-rich anoxic sediments at DOE’s Rifle, CO site contain relatively high concentrations of uranium. Based on sediment characteristics, we speculated that these ‘naturally reduced zones’ (NRZs) are common and accumulate uranium at similar contaminated sites across the upper Colorado River Basin (CRB). To test this hypothesis, we sampled NRZs at 4 additional sites along a 700 km north-south transect of the upper CRB: Grand Junction and Naturita, CO; Shiprock, NM; and Riverton, WY. This work confirmed our hypothesis and showed that NRZs are also important reservoirs for nutrients and biogeochemical critical elements (BCE), including C, N, S, and Fe. Sulfate-reducing conditions are required for uranium accumulation, suggesting at least a strong indirect control of sulfide on U(VI) reduction. Indeed, the nominal oxidation state of water-soluble soil organic carbon was found to be correlated to sediment sulfide concentration, suggesting that sulfide plays a major role in poising the redox conditions of NRZs regionally.
To better understand molecular controls over uranium behavior in NRZs, we performed controlled microcosm experiments designed to mimic sulfate-reducing conditions in NRZs and to more clearly define the potential roles of organic functional groups as uranium binding sites. U(IV) was found to be dominantly associated with surfaces of particulate organic carbon and to exhibit local molecular structure consistent with sorbed complexes. An important implication of this finding is that U(IV) will be readily mobilized in the presence of complexing agents and oxidants. Such conditions are common in NRZ sediments, which experience large variations in saturation state and redox conditions throughout the annual winter (base flow) / summer (meltwater discharge) cycle within the upper CRB.
In ensemble, these studies suggest new conceptual and process models for uranium and BCE biogeochemical behavior that are unprecedented in their regional scale, detail, and awareness of variable hydrologic conditions. By studying anoxic sediment systems from a regional perspective, we have gained important insights into the range and intensity of processes that are likely to be important at individual sites.