SURFACE INFILTRATION OF RIVER WATER TO GROUNDWATER TO SIMULATE PERIODIC FLOODING EVENTS AT A URANIUM-CONTAMINATED SITE

A series of natural river flooding events have occurred over the last three decades at a uranium-contaminated site situated along the Little Wind River, Riverton, Wyoming, USA. Field monitoring data suggested that there was a positive correlation between flooding events and elevated concentrations of uranium in groundwater. It was hypothesized that these flooding events mobilize uranium from the floodplain vadose zone and transport it to the shallow and unconfined aquifer via dissolution of uranium-bearing precipitates above the water table.

To test this hypothesis, a controlled river flooding event was conducted by infiltrating 2000 gallons of river water with added solute tracers into the vadose zone followed by groundwater sampling in a series of down-gradient monitoring well transects. Data from this experiment will be analyzed by comparing the breakthrough curves of the added non-reactive solute tracers and the potentially reactive solutes, e.g., uranium, sulfate, calcium, etc. Dilution-adjusted breakthrough curves will also be generated to determine if uranium was added to the aqueous phase; preliminary data analysis suggested that this is indeed the case. Reactive transport modeling of the dilution-adjusted breakthrough curves will be conducted to test if dissolution of uranium-bearing precipitates is in fact the predominant mechanism controlling the mobility of uranium during flooding events.