MILL-RELATED URANIUM IN THE UNSATURATED ZONE: LONG-TERM RELEASE AND WHAT TO DO NEXT

The U.S. Department of Energy Office of Legacy Management is the long-term custodian of multiple legacy uranium mill sites in the semiarid western United States. At three sites where tailings were relocated to disposal cells, prior conceptual models considered the tailings removal as a complete removal of the uranium source that contaminated underlying groundwater. However, elevated uranium concentrations in the groundwater greater than transport model predictions led to the identification of persistent secondary sources. Measured solid-phase uranium above background has been found in the unsaturated zone (1) underneath the former tailings and (2) downgradient of the former tailings areas above the shallow uranium plumes in areas with high evapotranspiration rates.

Uranium release rates in these secondary source areas were determined using laboratory column tests under saturated conditions that indicate uranium release rates follow an exponential decay curve. Field-scale infiltration tracer tests confirm the release of uranium from the unsaturated zone at concentrations well above groundwater standards. Using mass balance calculations, assumed recharge rates, and contaminant transport modeling, subsequent groundwater contamination can continue for hundreds to thousands of years. These estimates are highly uncertain, as actual unsaturated zone flow is dependent upon highly variable and unpredictable recharge conditions (e.g., flooding, large rain events, snowmelt, evapotranspiration, climate change, and so on). Regardless, these estimates indicate long time frames for release of uranium from the unsaturated zone.

With the recognition of uranium in the unsaturated zone as a long-term source and using initial estimates of release rates, site reactive transport models can simulate multiple scenarios to evaluate future site compliance strategy options. Such modeling has limitations that require mechanistic simplification. However, given the challenges with the time frames involved, modeling provides a first step in guiding future decision-making processes.