ENHANCED GROUNDWATER FLUSHING OF URANIUM VIA ALKALINITY-DRIVEN DESORPTION

Uranium concentrations in groundwater can exceed the maximum contamination limit (MCL) at some Department of Energy Legacy Management sites. Monitoring of groundwater data suggests that uranium concentrations may take 100’s of years to naturally attenuate to below the MCL. Data from sediment cores indicated 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 flushing of uranium to help meet clean-up goals. Enhancing desorption of uranium through acidification is a common lab practice but is not transferable to the field. Instead, enhanced desorption of uranium could be conducted in the field via increased alkalinity by the addition of sodium bicarbonate with relatively less environmental concern.

Initially, three columns with uranium-contaminated sediments were constructed. 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 to where a uranium plume has migrated. The column effluent was analyzed for pH, alkalinity, uranium, and major ions. In the organic-rich sediment, the majority of uranium was flushed as alkalinity increased. Four additional columns containing uranium contaminated sediments in the FTA have been completed. The four new columns continue to provide information on alkalinity driven desorption as well as investigate uranium that is bound to precipitated gypsumfound under a former uranium tailings pile. All seven columns will be analyzed using PHREEQC to quantify the desorption of uranium and gypsum dissolution. Preliminary results and modeling indicate that 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.