The dynamic nature of the processes occurring within wetlands is often illustrated by the presence of flocs, which are multicomponent assemblages of microbes, minerals, and non-living organic matter. Abundant orange and reddish-brown flocs have been repeatedly observed at multiple locations along Tims Branch at the Savannah River Site. This site is representative of many riparian wetlands found in humid regions of the Southeast that have C rich soils and high Fe content. However, it is unique in that it received large amounts of contaminant metals and uranium as a result of previous industrial-scale manufacturing of nuclear fuel and target assemblies. Analysis of these flocs by ICP-OES and XAFS spectroscopy revealed that the flocs contained high levels of Fe (9.7%)—in the form of ferrihydrite (83%) and lepidocrocite (17%) as determined by Fe K-edge EXAFS spectroscopy—P (2.7%), S (1.2%), and Al (0.4%) on a dry mass basis. The flocs contained 320 ppm U and preliminary U L<sub>III</sub>-edge EXAFS analysis indicates that it is U(VI) in the form of a U oxyhydroxides phase. The concentration of U in floc material was 5.5 times greater than the highest concentration of acid-extractable U in streambank sediments collected at the same time as the floc, indicating the potential importance of flocs in controlling U transport at the site. The stability of flocs varies over time and is dependent on a multitude of physical, chemical, and microbiological processes within the floc and the surrounding water; however, the specific factors controlling the frequency and locations of floc formation and dispersion in Tims Branch are unknown and the focus of current studies. In addition, flocs can undergo microbially mediated cycling of redox active elements such as Fe and S; the effects of which on U speciation within the flocs in situ is likewise unknown, as is the fate of U when the flocs degrade/disperse; however, preliminary microcosm studies indicate that floc-associated U(VI) is reduced to U(IV) when flocs are exposed to anoxic conditions. The results of our studies of Tims Branch flocs suggests that they represent a significant, previously unidentified reservoir of potentially mobile U. Given that iron flocs are frequently observed in a broad range of wetland environments, our studies of iron floc formation in Tims Branch and their impact on U speciation and transport expands our understanding of their role in the speciation and cycling of trace elements in wetlands.

Iron flocs as biogeochemically dynamic reservoirs of uranium in Tims Branch wetlands, Savannah River Site

2019 Fall National Meeting

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