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

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