Uranium (U) released from the M-Area at the Department of Energy Savannah River Site into Tims Branch, a seasonal wetland and braided stream system, is estimated to be 43,500 kg between 1965 and 1984. The motivation for this work is the uranium’s persistence in the wetland for decades, where it is estimated that 80% of the U currently remains in the Tims Branch wetland. U has begun to incorporate into wetland iron (Fe) and carbon cycles, associating with local Fe mineralogy and deposits of rich wetland organic matter (OM). The objective of this work is to characterize the chemical phases responsible for sequestration or mobilization of U, Fe, and C in a riparian wetland system and to understand the partitioning and lability of uranium incorporated into natural Fe and C cycles. Born from the observation of Fe-OM flocs under specific hydrologic conditions, it is hypothesized that the mobilization of Fe-OM flocs drive U transport from the wetland as U incorporates into the wetland cycles. This work first investigates the lateral distribution of U within the wetland and identifies key hotspots where U has accumulated due to hydrologic and geochemical controls. With knowledge of these hotspots, work sought to identify relationships of U to Fe and OM throughout the wetland in the water column and as a function of depth at the hotspots. Next, these relationships to Fe and OM were investigated further via parallel extraction of redox-preserved cores in oxic and anoxic atmospheres. This study determined that OM concentrations coupled with OM compound diversity heavily impact metal sequestration and availability in wetland sediments. Deep sediment layers with relatively non-labile OM accumulate metals naturally, as isotopic ratio evidence indicates that sequestration of natural U is occurring in addition to accumulation of anthropogenic depleted U. With an improved understanding of the metal inventories in the wetland, a seasonal study of metal transport allowed for initial estimates of seasonal U and Ni transport associated with Fe-OM flocs. Fluxes of U in Fe-OM flocs can be conservatively estimated to be of minimal risk to water quality, as worst-case assumptions and measurements pre- and post-storm determine that flocs would have mobilized only 132 kg U over the last 60 years. Currently, stream sediments are now hypothesized to be the primary driver of U transport from the wetland, but these U fluxes only amount to roughly 20 kg per year, based on stream flowrate data collection and stream water sampling
