COMPARISON OF COLUMN LEACHING RATES ON CONTAMINANT RELEASE FROM RIVER FLOODPLAIN SEDIMENTS AT A FORMER URANIUM MILL SITE

The U.S. Department of Energy Office of Legacy Management is conducting column studies to evaluate the leachability of uranium and other constituents from river floodplain sediments that were formerly under uranium tailings. While the tailings at the Grand Junction, Colorado, legacy uranium pilot mill site have been removed, contaminant concentrations above background continue to persist in the groundwater. The understanding of this contaminant persistence is important for site management, and the selection of strategies that meet regulatory compliance. Groundwater flow rates associated with river stage are important in the understanding of contaminant mobility due to possible kinetic limitations of geochemical reactions. In addition, due to the location of the site sitting in the floodplain of the Gunnison River, the change in river stage directly influences the groundwater elevation, creating variations in unsaturated zone thickness. As a result, contaminated sediments may have different contact times with flowing groundwater. Sediments below the former tailings were leached using similar column leaching methods, but with different flowrates of influent groundwater, one being rapidly filled with a column pore volume each day and 24 hours of no flow, the other being continuous flow at a rate similar to groundwater flow velocities. These different flow rates are meant to be somewhat representative of unsaturated zone versus saturated zone flow conditions. For relatively mobile constituents influent flow rates are irrelevant when plotted by column pore volumes. For a contaminant that can sorb to the sediments (e.g., uranium), a column pore volume each day allows for slightly greater desorption than continuous flow, when plotted by column pore volumes. Overall, the results indicate that long equilibration times are not necessary when running these columns. However, actual contaminant release rates in the field are dependent on groundwater elevations, which are controlled by river stage. This results in contaminant concentrations in groundwater that persist through time as contaminants are stored and released from the unsaturated zone.