Cross-Hole and Vadose-Zone Infiltration Tracer Test Analyses to Determine Aquifer Reactive Transport Parameters at a Former Uranium Mill Site (Grand Junction, Colorado)

Abstract

The U.S. Department of Energy Office of Legacy Management is responsible for the long-term care and maintenance of former uranium mill sites in the United States. Prior predictions of site flushing times (monitored natural attenuation) are not being met due to the presence of secondary contaminant sources associated with uranium-rich sediments in the vadose zone and organic-rich sediments near the water table below and near former mill tailings (tailings have been moved to a separate disposal site). Updated sitewide modeling for future releases of contaminants (including uranium) from these secondary sources to the groundwater need appropriate input parameters. To test field techniques, two cross-hole tracer tests and one infiltration tracer test were completed at a former uranium mill site in Grand Junction, Colorado. Reactive transport modeling was completed to derive physical and geochemical parameters. The observed data from saturated zone cross-hole tracer testing was adequately simulated using PHT-USG (reactive transport model) and PEST++ (calibration routine) with reasonable estimates of hydraulic conductivity, dispersion, effective porosity, cation exchange, calcite saturation index, and uranium sorption potential. The use of multiple layering in one cross-hole model was able to capture hydraulic conductivity variations with depth, which produced a double hump in the tracer concentrations. Estimated parameter values were very similar to prior estimates from column testing and single-well push–pull testing, except for a lower uranium sorption potential in one cross-hole test. This difference is likely due to the larger scale of the cross-hole testing including pathways with a lower uranium sorption potential. The infiltration testing released constituents from the vadose zone that can contribute to ongoing groundwater contamination. Modeling simulated the immediate release of these constituents to the water table similar to downward displacement of the existing residual porewater. Delayed drainage of the infiltration water was more difficult to simulate. However, the overall contaminant release concentrations from the vadose-zone secondary sources and ongoing groundwater contamination are adequately simulated for current site purposes. Additional details on vadose-zone processes may be needed if various remedial fluids are evaluated.