PHREEQC MODELING OF LABORATORY FLOW-THROUGH COLUMN DATA TO INVESTIGATE THE MECHANISMS OF URANIUM MOBILITY

Adsorption/desorption of uranium-bearing species in aquifers can play a major role in the mobility of uranium. In addition, precipitation/dissolution of uranium-bearing minerals may play an equally or more important role in the mobility of uranium, particularly at uranium mill tailings sites. It is hypothesized that uranium may become bound in precipitated gypsum that forms when low-pH tailings fluids are buffered. When those tailings are removed that gypsum can dissolve and uranium is mobilized. This hypothesis was tested by conducting four continuous flow-through column experiments with sediments from a former uranium mill tailings site. Along with testing gypsum dissolution, the alkalinity of the influent (background groundwater) was varied to enhance desorption to quantify the various mechanisms for uranium mobility. PHREEQC modeling of the results from the columns will be presented to elucidate the drivers of uranium mobility. Resulting insights are used to revise the site conceptual model with specific application to determining natural flushing rates.