Fire as a Long-Term Stewardship Issue for Soils Contaminated With Radionuclides in the Western U.S.

On both U.S. Department of Energy (DOE) and U.S. Department of Defense sites in the southwestern United States (U.S.), significant areas of surface soils are contaminated with radionuclides from atmospheric nuclear testing, and with depleted uranium, primarily from military training. At DOE sites in Nevada, the proposed regulatory closure strategy for most sites is to leave contaminants in place with administrative controls and periodic monitoring. Closure-in-place is considered an acceptable strategy because the contaminated sites exist on access-restricted facilities, decreasing the potential risk to public receptor, the high cost and feasibility of excavating contaminated soils over large areas, and the environmental impacts of excavating desert soils that recover very slowly from disturbance. The largest of the contaminated sites on the Tonopah Test Range in Nevada covers over 1,200 hectares. However, a factor that has not been fully investigated in the long-term stewardship of these sites is the potential effects of fires. Because of the long half-lives of some of the contaminants (e.g., 24,100 years for ²³⁹Pu) and changes in land-cover and climatic factors that are increasing the frequency of fires throughout the western U.S., it should be assumed that all of these sites will eventually burn, possibly multiple times, during the timeframe when they still pose a risk. Two primary factors are contributing to increased fire frequency. The first is the spread of invasive grasses, particularly cheatgrass (Bromus tectorum and Bromus rubens), which have out-competed native annuals and invaded interspaces between shrubs, allowing fires to burn easier. The second is a sharp increase in fire frequency and size throughout the western U.S. beginning in the mid-1980s. This second factor appears to correlate with an increase in average spring and summer temperatures, which may be contributing to earlier loss of soil moisture and longer periods of dry plant biomass (particularly from annual plants). The potential risk to site workers from convective heat dispersion of radionuclide contaminants is an immediate concern during a fire. Long-term, post-fire concerns include potential changes in windblown suspension properties of contaminated soil particles after fires because of loss of vegetation cover and changes in soil properties, and soil erosion from surface water runoff and fluvial processes.