BEYOND ORDINARY AQUIFER TRACER TESTING: INCLUDING GEOCHEMICAL DATA FOR CONTAMINANT TRANSPORT PARAMETERS AT A LEGACY URANIUM MILL SITE (Invited Presentation)

The U.S. Department of Energy Office of Legacy Management conducted aquifer tracer testing at a legacy uranium pilot mill in Grand Junction, Colorado. Aquifer tracer testing is often used to characterize physical parameters, such as porosity, advection, and mechanical dispersion. However, this approach may also elucidate geochemical reactions that might influence contaminant transport, such as sorption/desorption, mineral precipitation/dissolution, and oxidation/reduction reactions. At the Grand Junction site, mill tailings have been removed, but the groundwater still has concentrations of uranium and vanadium that exceed background concentrations. Multiple tracers were used with single-well and multiple-well injection and extraction configurations to test and provide results for physical aquifer parameters. The injection fluids consisted of tracers added to water from the nearby Gunnison River to mimic seasonal intrusion of river water into contaminated areas and thus allow the interrogation of geochemical reactions that influence contaminant mobility. Changes in contaminant concentrations are compared to chloride (native conservative transport tracer) to remove any physical dilution effects from geochemical reactions. Using the observed redox conditions, uranium and vanadium sorption/desorption with potential mineral solubility controls were evaluated using PHREEQC (a geochemical modeling program). Mineral controls include uranium/vanadium minerals (e.g., carnotite and tyuyamunite), plus calcite and gypsum, which can influence uranium sorption/desorption by changing alkalinity concentrations. Geochemical modeling results indicate that uranium mobility is dominated by sorption/desorption reactions with minimal mineral controls; thus, providing transport parameters that can be used in a site reactive transport model for contaminant flushing predictions. This site does not have a well-defined contaminant plume due to a dispersed contaminant source and large seasonal fluctuations in groundwater flow directions. Thus, field-scale reactive transport parameters are difficult to determine without employing techniques such as the controlled “geochemical tracer test” presented in this study.