Geochemical, Mineralogical and Microbiological Characteristics of Sediment from a Naturally Reduced Zone in a [Naturally Occurring?] Uranium-Contaminated Aquifer

Uranium (Nuclear Minerals) / Related Environmental Issues / Environmental Impact - Cases, Nuclear Wastes (Used Fuel)

Kate Campbell, and 11 co-authors, reported in 2012 that localized zones or lenses of naturally reduced sediments have the potential to play a significant role in the fate and transport of redox-sensitive metals and metalloids in aquifers. To assess the mineralogy, microbiology and redox processes that occur in these zones, several cores from a region of naturally occurring reducing conditions in a U-contaminated aquifer (Rifle, CO) were examined.

Sediment samples from a transect of cores ranging from oxic/suboxic Rifle aquifer sediment to naturally reduced sediment were analyzed for U and Fe content, oxidation state, and mineralogy; reduced S phases; and solid-phase organic C content using a suite of analytical and spectroscopic techniques on bulk sediment and size fractions. Solid-phase U concentrations were higher in the naturally reduced zone, with a high proportion of the U present as U(IV). The sediments were also elevated in reduced S phases and Fe(II), indicating it is very likely that U(VI), Fe(III), and SO₄ reduction has occurred or is occurring in the sediment. The microbial community was assessed using lipid- and DNA-based techniques, and statistical redundancy analysis was performed to determine correlations between the microbial community and the geochemistry.

Increased concentrations of solid-phase organic C and biomass in the naturally reduced sediment suggests that natural bioreduction is stimulated by a zone of increased organic C concentration associated with fine-grained material and lower permeability to groundwater flow. Characterization of the naturally bioreduced sediment provides an understanding of the natural processes that occur in the sediment under reducing conditions and how they may impact natural attenuation of radionuclides and other
redox sensitive materials. Results also suggest the importance of recalcitrant organic C for maintaining reducing conditions and U immobilization.

The authors concluded that the lenses of higher U in the natural reduced zone exist on the border of highly variable redox conditions in the aquifer. Sample P103-10 was from the vadose zone/capillary fringe at the time of sampling, although it is seasonally saturated as the water table increases with Colorado River stage during peak runoff in the spring and early summer. The presence of significant amounts of U(IV) in P103-10 not only confirms the reduction of U(VI), but also suggests that the U(IV) in the naturally reduced zone is stabilized to re-oxidation when seasonally placed in more oxidizing conditions in the vadose zone. The lower permeability sediment and higher organic C content may have created a reducing microenvironment where U(IV) may be protected from the more oxidizing conditions. The lower permeability zones may create biogeochemically favorable conditions to stabilize reduced phases and associated constituents, such as U, in the alluvial aquifer.

Association with biomass and clays may also serve to stabilize the U(IV) to oxidation by providing a diffusive boundary for oxidants like dissolved O₂. In addition, the minerals closely associated with U(IV) may also play a stabilizing role (e.g., oxidative buffer capacity provided by Fe sulfide mineral phases). The accumulation of U(IV) in these localized lenses may also have implications for larger-scale aquifer loading of U; slow release from these lenses may explain the persistence of dissolved U in the aquifer long after natural flushing was predicted to decrease the concentration of U below applicable standards. Lenses of naturally reduced sediments may be common in alluvial aquifers and because of their capacity to accumulate and stabilize trace elements and redox-active phases, they are important for accurate estimation of natural attenuation capacity.

It has been demonstrated here that U(VI)-, Fe(III)- and SO₄- reduction driven by microbial processes stimulated by deposition of organic matter has likely occurred in localized zones of sediment in the Rifle aquifer. The conditions in the naturally reduced sediment are analogous to the artificially biostimulated conditions created upon injection of acetate; the reducing conditions created during biostimulation are designed to be favorable for long-term sequestration of U(IV). Characterization of the mineralogy and natural organic C in the zone of natural reduction suggests that addition of recalcitrant organic C may help to sustain low-level, long-term reducing conditions. By understanding these processes in the naturally reduced sediment in more detail, the overall design and long-term strategy for artificially stimulated reducing conditions can be optimized to maximize U removal from the groundwater and stabilize U(IV) products and other reduced phases to oxidation.