In situ porewater uranium concentrations in a contaminated wetland: Effect of seasons and sediment depth

- The studied wetlands were highly effective at sequestering groundwater uranium.
- A majority of the uranium was bound to organic matter in the sediment.
- In situ porewater uranium concentrations were monitored against redox potential over the course of a year and as a function of sediment depth.
- There appeared to be a critical redox potential, above which aqueous uranium concentrations increased sharply.
- Stewardship of contaminated wetlands needs to protect soil organic matter and hydrological status.

Previous studies have shown that Tims Branch wetlands on the Savannah River Site in South Carolina, USA is an effective environmental sink for sequestering the 44 tons of uranium (U) released into the system. The objective of this study was to evaluate over the course of a year, the fluctuations in sediment porewater U concentrations as a function of sediment depth, and the conditions and the extent that the contaminated wetlands acted as an environmental source for U. Sediment desorption tests indicated that U was strongly bound (Kd values were 2100-6900 L/kg), and sequential extraction experiments indicated that a majority of the U was associated with the readily oxidizable fraction (presumably, organic matter fraction). In situ porewater samples were collected using diffusion samplers that were placed in the contaminated wetlands and their uranium concentrations indicated that as much as 3 × 10−5 wt-% of the system U was in the mobile aqueous phase (federal maximum contaminant levels (MCL) = 0.03  μg/L U). Aqueous U concentrations were correlated to Eh (r = 0.422; n = 113; p ≤ 0.001). These data also suggested that there may be a critical Eh at ∼400 mV, above which aqueous U concentrations increased significantly (p ≤ 0.01) by more than an order of magnitude. These results have implications on the long-term stewardship of this contaminated system; sediment organic matter concentrations and wetland hydrology and plant vegetation need to be maintained in a manner that does not permit strong reoxidation of the system. This could be achieved by minimizing land-use changes or the occurrences of forest fires and ensuring that the system's hydrology is not greatly altered.