INSIGHTS FROM PROCESS-LEVEL MODELING OF CONTAMINANT TRANSPORT FROM URANIUM MILL TAILINGS

Uranium mill tailings site licensees have used geochemical modeling to determine the effect of contaminant transport on risk to the public health and safety. Various transport models, submitted in license amendment requests to the Nuclear Regulatory Commission, rely on the chemical, isotopic, and hydrologic characteristics of the site. Generally, these license amendment requests propose and justify application of alternative concentration limits of contaminants in groundwater at the point of compliance (POC), which can be a well or wells, located just downstream of the mill tailings pile, monitored for evidence of contaminant leakage from the tailings to the uppermost aquifer. Fate and transport models have used generic (literature-derived), or site-specific sorption coefficients to estimate retardation of contaminants that migrate from the tailings toward the point of exposure (POE) in the one-thousand year period of compliance. The POE is the location where water, depending on its quality, can be used for drinking (human consumption), livestock, or agriculture. Alternatively, process-level models use groundwater compositions from monitored wells, and mineralogic information to assign initial chemical conditions along the expected path of contaminant migration. Then, simulations of contaminant transport can be performed that include speciation, precipitation/dissolution, and sorption/desorption. Contaminant concentration versus time and distance are produced.

This study compares the results of modeling contaminant transport from a mill tailings site using a process-level approach versus a fate and transport approach. The effect of the choice of the modeling approach on risk is addressed. Alternative selections of monitored site characterization parameters important to performance are considered.