Biogeochemical processes responsible for wetland attenuation of uranium
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A field study was conducted to understand the hydrological and biogeochemical processes responsible for contaminant uranium immobilization in a wetland located on the Savannah River Site, a nuclear fabrication facility in South Carolina, USA. Based on 900,000 spectra collected from mobile gamma detectors coupled with GPS systems, it was determined that 94% of the released uranium remains in the wetland more than 50 years after being released. Uranium existed primarily in five multi-hectare hotspots that coincided with areas that had slower stream flow. XAS analysis of 30 sediment cores indicated that most of the uranium remained in the more soluble form of U(VI) bound to organic matter and iron(III)-oxides. Under water-saturated conditions the uranium existed as an adsorbed reduced U(IV) complexes (i.e., not mineralized). Furthermore, uranium was found to be 11 times more concentrated in the rhizosphere (the soil zone influenced by the presence of roots), where organic carbon composition (FTICR-MS), microbial structure (q-PCR), and iron mineralogy (Mössbauer) was unique with respect to the bulk sediment. Together, these studies show that wetlands can be extraordinarily effective at binding and retaining uranium, thereby providing a natural barrier to the transport of uranium out of a watershed. However, significant anthropogenic or climatic changes to wetlands, such as those associated with flooding, forest fires, or land use, may disrupt the complex hydrological and biogeochemical balance necessary to maintain long-term immobilization of contaminants.
Melody Maloubier