IN-SITU FLOODING EXPERIMENTS TO ELUCIDATE MASS TRANSPORT MECHANISMS OF URANIUM IN GROUNDWATER: A CASE STUDY OF RIVERTON, WYOMING

The observed concentrations of uranium in the aquifer at a uranium-contaminated site in Riverton, Wyoming, USA were substantially higher than the regulatory standards and maximum concentrations predicted by a site-wide and long-term flow and transport model. A series of natural river flooding events are occurring over the last three decades from the Little Wind River. It is hypothesized that flooding events are concomitant with high levels of uranium leading to uranium dissolution, re-oxidation, and desorption in the saturated zone. The main objective of the project is to understand the mechanisms controlling the uranium concentration in groundwater during flooding events. A series of controlled river flooding events have been conducted at a small-scale experimental site located on the floodplain of the river and down-gradient of the former uranium tailings site in Summer 2020. The events have been conducted via surface infiltration of river water traced with halides and fluorobenzoates in the saturated zone of the site with increased alkalinity to characterize dissolution, re-oxidation, and desorption of uranium. The results will be calculated using the mass balance equation and will be visualized by interpolating the point data in ArcMap to show the migration of tracer elements from one well to another.