Long-term risk from actinides in the environment: Modes of mobility. 1998 annual progress report

Abstract

'The mobility of actinides in surface soils is a key issue of concern at several DOE facilities in arid and semiarid environments, including Rocky Flats, Hanford, Nevada Test Site, Idaho National Engineering Laboratory, and Los Alamos National Laboratory and the Waste Isolation Pilot Plant (WIPP). Key sources of uncertainty in assessing Pu mobility are the magnitudes of mobility resulting from three modes of transport: (1) wind erosion, (2) water erosion, and (3) vertical migration. Each of these three processes depend on numerous environmental factors and they compete with one another, particularly for actinides in very shallow soils ({approximately} 1 \265m). The overall goal of the study is to quantify the mobility of soil actinides from all three modes. The authors study is using field measurements, laboratory experiments, and ecological modeling to address these three processes at three DOE facilities where actinide kinetics are of concern: WIPP, Rocky Flats, and Hanford. Wind erosion is being measured with suite of monitoring equipment, water erosion is being studied with rainfall simulation experiments, vertical migration is being studied in controlled laboratory experiments, and the three processes are being integrated using ecological modeling. Estimates for clean up of soil actinides for the extensive tracts of DOE land range to hundreds of billion $ in the US Without studies of these processes, unnecessary clean-up of these areas may waste billions of dollars and cause irreparable ecological damage through the soil removal. Further, the outcomes of litigation against DOE are dependent on quantifying the mobility of actinides in surface soils. This report provides a summary of work for the first year of a 3-year project; subcontracts to collaborating institutions (Colorado State University and New Mexico State University) were not in place until late December 1997, and hence this report focuses on the results of the 5 months from January through May 1998. The major result to date is a review of literature on the potential for using soil concentrations of {sup 137}Cs and {sup 241}Am as tracers for plutonium in soil. Measurements of {sup 239}Pu contamination in the environment are expensive and time consuming, requiring radiochemical analysis and alpha spectroscopy. They evaluated the literature for measurements of {sup 137}Cs and {sup 241}Am, both of which are more cost-effectively measured by gamma spectrometry, as tracers for Pu in soil. Their results indicate that: significant positive correlation exists between Pu, Cs, and Am in soils and sediments at several locations including Rocky Flats, Los Alamos, and Hanford; atmospheric transport of Pu and Cs from worldwide fallout is essentially the same; the attachment of Pu and Cs to soil particles of various size is very similar; both Pu and Cs movement in the environment correlate well with soil and sediment particle movements; a significant correlation between Pu, Cs, and Am was found in soil as a function of depth, indicating similar vertical migration behavior (most of the activity of these radionuclides is confined to the top 10--20 cm of soil at virtually all locations); most Pu and Cs are strongly absorbed onto clay and organic matter in soils and there is essentially very little leaching of Pu, Am and Cs through soil columns. Based on the above information, they believe that {sup 137}Cs and {sup 241}Am are excellent tracers for both {sup 239}Pu and soil particle transport processes in clay, mineral bearing and/or organic soils. Therefore, Cs and Am would be good tracers for the proposed water erosion and vertical migration work, at least for both Rocky Flats and Hanford. The correlation between Pu and Cs may not be as strong in sandy soil (e.g. WIPP site), however, examination of more data is needed.'