Redox Controls over the Stability of U(IV) in Floodplains of the Upper Colorado River Basin

Aquifers in the Upper Colorado River Basin (UCRB) exhibit persistent uranium (U) groundwater contamination plumes originating from former ore processing operations. Previous observations at Rifle, Colorado, have shown that fine grained, sulfidic, organic-enriched sediments accumulate U in its reduced form, U­(IV), which is less mobile than oxidized U­(VI). These reduced sediment bodies can subsequently act as secondary sources, releasing U back to the aquifer. There is a need to understand if U­(IV) accumulation in reduced sediments is a common process at contaminated sites basin-wide, to constrain accumulated U­(IV) speciation, and to define the biogeochemical factors controlling its reactivity. We have investigated U­(IV) accumulation in organic-enriched reduced sediments at three UCRB floodplains. Noncrystalline U­(IV) is the dominant form of accumulated U, but crystalline U­(IV) comprises up to ca. 30% of total U at some locations. Differing susceptibilities of these species to oxidative remobilization can explain this variability. Particle size, organic carbon content, and pore saturation, control the exposure of U­(IV) to oxidants, moderating its oxidative release. Further, our data suggest that U­(IV) can be mobilized under deeply reducing conditions, which may contribute to maintenance and seasonal variability of U in groundwater plumes in the UCRB

Physico-Chemical Heterogeneity of Organic-Rich Sediments in the Rifle Aquifer, CO: Impact on Uranium Biogeochemistry

The Rifle alluvial aquifer along the Colorado River in west central Colorado contains fine-grained, diffusion-limited sediment lenses that are substantially enriched in organic carbon and sulfides, as well as uranium, from previous milling operations. These naturally reduced zones (NRZs) coincide spatially with a persistent uranium groundwater plume. There is concern that uranium release from NRZs is contributing to plume persistence or will do so in the future. To better define the physical extent, heterogeneity and biogeochemistry of these NRZs, we investigated sediment cores from five neighboring wells. The main NRZ body exhibited uranium concentrations up to 100 mg/kg U as U­(IV) and contains ca. 286 g of U in total. Uranium accumulated only in areas where organic carbon and reduced sulfur (as iron sulfides) were present, emphasizing the importance of sulfate-reducing conditions to uranium retention and the essential role of organic matter. NRZs further exhibited centimeter-scale variations in both redox status and particle size. Mackinawite, greigite, pyrite and sulfate coexist in the sediments, indicating that dynamic redox cycling occurs within NRZs and that their internal portions can be seasonally oxidized. We show that oxidative U­(VI) release to the aquifer has the potential to sustain a groundwater contaminant plume for centuries. NRZs, known to exist in other uranium-contaminated aquifers, may be regionally important to uranium persistence

Reducing Conditions Influence U(IV) Accumulation in Sediments during <i>In Situ</i> Bioremediation

This study presents field experiments conducted in a contaminated aquifer in Rifle, CO, to determine the speciation and accumulation of uranium in sediments during in situ bioreduction. We applied synchrotron-based X-ray spectroscopy and imaging techniques as well as aqueous chemistry measurements to identify changes in U speciation in water and sediment in the first days follwing electron donor amendment. Limited changes in U solid speciation were observed throughout the duration of this study, and non-crystalline U(IV) was identified in all samples obtained. However, U accumulation rates strongly increased during in situ bioreduction, when the dominant microbial regime transitioned from iron- to sulfate-reducing conditions. Results suggest that uranium is enzymatically reduced during Fe reduction, as expected. Mineral grain coatings newly formed during sulfate reduction act as reduction hotspots, where numerous reductants can act as electron donors [Fe(II), S(II), and microbial extracellular polymeric substances] that bind and reduce U. The results have implications for identifying how changes in the dominant reducing mechanism, such as Fe versus sulfate reduction, affect trace metal speciation and accumulation. The outcomes from this study provide additional insights into uranium accumulation mechanisms in sediments that could be useful for the refinement of quantitative models describing redox processes and contaminant dynamics in floodplain aquifers

Speciation and Reactivity of Uranium Products Formed during <i>in Situ</i> Bioremediation in a Shallow Alluvial Aquifer

In this study, we report the results of <i>in situ</i> U­(VI) bioreduction experiments at the Integrated Field Research Challenge site in Rifle, Colorado, USA. Columns filled with sediments were deployed into a groundwater well at the site and, after a period of conditioning with groundwater, were amended with a mixture of groundwater, soluble U­(VI), and acetate to stimulate the growth of indigenous micro­organisms. Individual reactors were collected as various redox regimes in the column sediments were achieved: (i) during iron reduction, (ii) just after the onset of sulfate reduction, and (iii) later into sulfate reduction. The speciation of U retained in the sediments was studied using X-ray absorption spectroscopy, electron microscopy, and chemical extractions. Circa 90% of the total uranium was reduced to U­(IV) in each reactor. Noncrystalline U­(IV) comprised about two-thirds of the U­(IV) pool, across large changes in microbial community structure, redox regime, total uranium accumulation, and reaction time. A significant body of recent research has demonstrated that noncrystalline U­(IV) species are more suceptible to remobilization and reoxidation than crystalline U­(IV) phases such as uraninite. Our results highlight the importance of considering noncrystalline U­(IV) formation across a wide range of aquifer parameters when designing <i>in situ</i> remediation plans