Multiple Tracer Testing Approaches for Improved Groundwater Flow and Reactive Transport Modeling Input Parameters

At many U.S. Department of Energy Office of Legacy Management (LM) uranium mill tailings sites, initial groundwater flow and solute transport modeling was completed 20–25 years ago using field data that included hydraulic head information and a few aquifer pumping tests. Because the uranium tailings were either removed or placed in capped disposal cells, and the simulations assumed limited uranium sorption, these models did not consider ongoing uranium contaminant sources. Currently, several sites have uranium concentrations that exceed the prior model predictions, which has led to the identification of uranium sources in the subsurface that were not previously identified. In order to update the past models, tracer testing techniques are being considered to provide more accurate information on groundwater flow velocities, flow directions, ongoing uranium sources, uranium sorption potential, and dual-porosity issues related to aquifer grain coatings and cements.

Tracer testing will be completed in the summer or fall of 2017 using multiple tracers and techniques at LM’s Grand Junction, Colorado, Site, which is located on a shallow alluvial aquifer adjacent to the Gunnison River. This tracer testing will include the use of borehole dilution, push-pull, and cross-hole techniques that focus on identified areas with elevated solid-phase uranium concentrations. The test data will be analyzed to estimate groundwater flow directions, velocities, and uranium transport parameters (sorption and dual-porosity influences) and be compared to previous estimates. This presentation will discuss how these multiple tracer tests were completed, interpreted, and then used to provide updated input parameters for revising the site groundwater flow and reactive transport models (conceptual and numerical).