SIMULATION OF U(VI) TRANSPORT IN A HETEROGENEOUS SAND AND GRAVEL AQUIFER

Estimating the migration of U(VI) in groundwater is an important part of assessing the risk from uranium associated with mining activities, ore processing and waste disposal sites. Field and modeling studies were conducted to evaluate the effects of variable chemical conditions and spatial variability of hydraulic conductivity and adsorption properties on migration of an existing U(VI) plume in a shallow alluvial aquifer at a former U(VI) mill located near Naturita, CO. The groundwater at the site had U(VI) that ranged from 0.01 to 21µM, pH values that ranged from 6.5 to 7.7 and alkalinities that ranged from 2.5 to 20 meq/L. Slug tests and small-scale tracer tests indicate that the hydraulic conductivity varied by at least a factor of 10. The aquifer sediments consisted of cobbles, gravels and sands and some clay-sized material and had a specific surface area that ranged by a factor of 4. Laboratory experiments found that 85% of the adsorption capacity was contributed by the <3mm fraction that represented only 15% of the aquifer sediments. U(VI) adsorption and transport were simulated using a semi-empirical surface complexation model (SCM) determined independently in the laboratory. Contaminated sediments from the site had K_D values that varied by a factor of 16 but application of the surface complexation model demonstrated that the aqueous geochemistry accounted for most of the observed variability. U(VI) transport at the field site was simulated using the laboratory derived SCM and a two-dimensional multicomponent reactive transport model that was calibrated to the observed U(VI) by estimating the duration and composition of the source term. With a simple source term and uniform hydraulic and geochemical properties, the observed U(VI) and alkalinity concentrations were reproduced satisfactorily. These simulation results will be compared with simulations that account for the observed heterogeneity of hydraulic conductivity and the observed variability of the adsorption site concentration.