Influence of Carbon and Microbial Community Priming on the Attenuation of Uranium in a Contaminated Floodplain Aquifer

Abstract

The capacity for subsurface sediments to sequester radionuclide contaminants, such as uranium (U), and retain them after bioremediation efforts are completed is critical to the long-term stewardship of re-mediated sites. In U bioremediation strategies, carbon amendment stimulates bioreduction of U(VI) to U(IV), immobilizing it within the sediments. Sediments enriched in natural organic matter are naturally capable of sequestering significant U, but may serve as sources to the aquifer, contributing to plume persistence. Two types of organic-rich sediments were compared to better understand U release mechanisms. Sediments that were artificially primed for U removal were retrieved from an area previously biostimulated while detrital-rich sediments were collected from a location never subject to amendment. Batch incubations demonstrated that primed sediments rapidly removed uranium from the groundwater, whereas naturally reduced sediments released a sizeable portion of U before U(VI)-reduction commenced. Column experiments confirmed that U release persisted for 65 pore volumes in naturally reduced sediments, demonstrating their sink-source behavior. Acetate addition to primed sediments shifted the microbial community from sulfate-reducing bacteria within Desulfobacteraceae to the iron-reducing Geobacteraceae and Firmicutes, associated with efficient U(VI) removal and retention, respectively. In contrast, Geobacteraceae communities in naturally reduced sediments were replaced by sequences with similarity to Pseudomonas spp. during U release, while U(VI) removal only occurred with enrichment of Firmicutes. These investigations stress the importance of characterizing zones with heterogeneous carbon pools at U-contaminated sites prior to the determination of a remedial strategy to identify areas, which may contribute to long-term sourcing of the contaminants.

Efficacy of acetate-amended biostimulation for uranium sequestration: Combined analysis of sediment/groundwater geochemistry and bacterial community structure

“…The close relationship identified between the uranium removal efficiency and the abundance of sulfate-reducing bacterial gene markers supports the conclusion that sulfate reduction can be effective for immobilizing aqueous uranium. This conclusion is consistent with those of former field-scale investigations carried out at multiple uranium-contaminated sites (Cardenas et al, 2008;N'Guessan et al, 2008;Lezama-Pacheco et al, 2015;Mouser et al, 2015). Although aqueous U(VI) was shown to form Ca-UO 2 -CO 3 complexes in the slightly alkaline groundwater used in this study (e.g., Williams et al, 2011), the high uranium-removal efficiency (up to 95%) observed in the amended D08 sediments indicated that the presence of such aqueous ternary complexes had little effect on the uranium bioavailability.…”

Hierarchical Bayesian method for mapping biogeochemical hot spots using induced polarization imaging

“…In addition to these two studies, Mouser et al . [] found unique microbial characteristics within the NRZs, such as the abundance of iron‐reducing Geobacteraceae communities. Recent modeling studies suggested that NRZs have a significant impact on subsurface carbon flux to the atmosphere as well as to the river [ Arora et al ., a, b].…”

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

“…In surface horizons, seasonal changes in communities were observed at the onset of reducing conditions following snowmelt with the appearance of Firmicutes in Topsoil and Clay layers (Supplementary Figure S7). This could be relevant for U cycling; a column experiment with U-contaminated soil taken from the Rifle, CO, site showed that U(VI) removal only occurred when there was an enrichment of Firmicutes relative to other taxa (Mouser et al, 2015).…”

“…The importance of microorganisms in mediating key biogeochemical processes in soils is well-documented. Particularly in floodplain soils that host TRZs and other organic-rich layers, microorganisms have been shown to produce compounds that control redox states, and determine solubility and mobility of contaminants, including metals such as Fe and U (Davis et al, 2006;Mouser et al, 2015). Previous work has indicated that these microorganisms may be distributed based on the geochemical environment in which they reside and its respective seasonal changes (Hug et al, 2015;Danczak et al, 2016a;Nelson et al, 2019), and also that microbial metabolisms in groundwater-influenced soils are interconnected and interdependent (Wrighton et al, 2014;Anantharaman et al, 2016).…”