Long-Term Risk From Actinides In The Environment: Modes Of Mobility

The mobility of actinides in surface soils remains a key issue of concern at several DOE facilities in arid and semiarid environments, including Rocky Flats, Hanford, Nevada, Idaho, and Los Alamos. Over the last 50 years, nuclear research and development programs have resulted in releases of plutonium to both on-site and off-site locations. Most of this plutonium and other actinides are currently in soils where it is tightly bound to soil particles (Watters et al. 1983), but these particles themselves are subject to redistribution. Research indicates that actinide redistribution is driven primarily by physical and biological processes associated with ecosystem dynamics, rather than by chemical processes. Actinide mobility is a high visibility issue at Rocky Flats and Hanford due to pending litigation and clean-up decisions. The potential for redistribution has lead plaintiff groups to sue the DOE and its contractors at Rocky Flats and Hanford by claiming that past releases of plutonium have occurred and that these releases have exposed off-site human populations to large amounts of plutonium with consequent negative health risks to humans (Goble 1996). Plaintiffs also claim that these exposures will continue to occur because of chronic releases from contaminated soil from on-site sources (Goble 1996, Smallwood 1996a, b). The concern about past and potential releases of contaminants from Rocky Flats appears to have reduced property values in nearby communities (Flynn et al. 1998). In addition, public groups focusing on past and potential off-site transport from Rocky Flats are expressing concern about increased erosion-driven transport associated with disturbances such as fire. A central issue in arguing these cases has been whether the plutonium presently in soils is immobile (Litaor et al 1996) or whether it is subject to transport by biological, physical, or chemical processes. Plaintiffs claim that wind erosion of soil is producing in large chronic releases of plutonium to off-site locations (Goble 1996, Smallwood 1996a, b). While past and current air and soil monitoring data would suggest that these releases are very unlikely (Price 1989, Dirkes and Hanf 1996), definitive scientific data are not available to defend a position on the long-term spatial changes in the plutonium distribution in soils due to biological, physical, or chemical processes, including effects of site disturbances. Past data reflect site management practices (e.g., fire suppression, erosion control) and climatic conditions that may not be relevant to sites in the future (Swetnam et al. 1999). There is now evidence that climate change is likely to impact the return frequencies of extreme events such as droughts, fires, and floods, in many areas (Olsen et al. 1998). The key source of uncertainty in assessing actinide mobility is the relative importance of transport by: (1) wind erosion, (2) water erosion, and (3) vertical migration: 4 VERTICAL M IGRATION Each of these three processes depends on several environmental factors and they compete with one another. A scientific assessment of the long-term risks associated with actinides in surface soils depends on better quantifying each of these three modes of mobility. The objective from our EMSP study was to quantify, the mobility of soil actinides by wind erosion, water erosion, and vertical migration at three semiarid sites where actinide mobility is a key technical, social and legal issue. Our EMSP project was the first to evaluate all three factors at a site. Our approach has been to investigate both short- and long-term issues based on field and lab studies and model comparisons. Our results demonstrate the importance of incorporating threshold responses into a modeling framework that accounts for environmental factors and natural disturbances that trigger large changes in actinide mobility