DIRECT RIVER WATER TO GROUNDWATER RECHARGE EXPERIMENTS TO ELUCIDATE MASS TRANSPORT MECHANISMS OF URANIUM

The concentration of uranium in groundwater downgradient of a former uranium mill tailings site persists above levels as predicted by a site-wide transport model. Uranium concentrations in groundwater within the floodplain of the Little Wind River in Riverton, Wyoming are higher than concentrations below the former mill tailings. It is hypothesized that the mass transfer of less mobile uranium in the unsaturated zone solid phase contributes to mobile uranium in the aqueous phase during periods of river water to groundwater recharge (flooding events). Thus, contributing to persistent levels of uranium.

To test this hypothesis, a 100-gallon solution of tracer-amended river water was injected into a single well and was tracked over several weeks in two downgradient transects of observation wells. Dilution-adjusted breakthrough curves will be analyzed to determine if uranium was mobilized from the solid- to the aqueous-phase. Breakthrough curves will also be modeled using a simple numerical approach (finite-difference) to characterize advection, dispersion, and possible source terms, e.g., desorption, re-oxidation, matrix diffusion, of uranium.