ENHANCED MASS REDUCTION OF URANIUM VIA ALKALINITY-DRIVEN DESORPTION AND GROUNDWATER FLUSHING

The concentration of uranium in groundwater can exceed the maximum contamination limit (MCL) at some Department of Energy Office of Legacy Management (LM) sites. Long-term groundwater monitoring data suggests that natural attenuation of uranium to below the MCL will likely be in excess of 100 years. Data from sediment cores indicate that uranium can be adsorbed to organic-rich materials and that desorption may be a slow process that contributes to persistent levels of uranium in groundwater. Therefore, enhancing desorption may facilitate groundwater flushing of uranium to help meet clean-up goals. Enhanced desorption of uranium is commonly conducted in the lab via acidification to quantify uranium on the solid phase. However, this practice is not transferable to the field due to obvious environmental concerns. Instead, enhanced desorption of uranium could be conducted in the field via increased alkalinity by the addition of NaHCO₃ with relatively less environmental concern.

In this study, columns were constructed with uranium-contaminated sediments from three boreholes at the LM site in Riverton, WY. The first two were organic-poor sandy sediments from the former tailings area (FTA), the third was organic-rich silty sediments down-gradient of the FTA. Synthetic water similar to groundwater near the FTA was used as the influent for the first two columns. Deionized water (DI) was used as the influent for the third column. Alkalinity was incrementally increased in three stages: 1000, 2500, and 5000 mg/L as CaCO₃. The effluent was analyzed for pH, alkalinity, uranium, and major ions. For the first two columns, 50 percent or more of the total mass of flushed uranium occurred before alkalinity was added and the remaining mass was flushed as alkalinity was increased. For the third column, approximately 30 percent of the total mass of flushed uranium occurred during the D.I.-only influent stage and the majority of mass was flushed as alkalinity increased. These results indicate that increased alkalinity can enhance flushing of uranium, likely due to desorption, from both organic-poor and -rich sediments. Therefore, utilizing increases in alkalinity to facilitate the desorption of uranium at the field scale has the potential to be an environmentally friendly approach to enhance groundwater flushing of uranium.