Through the Environmental Management Science Program (EMSP), DOE’s Office of Environmental Management (EM) and Office of Science (SC) collaborate to fund basic research to solve intractable problems that threaten the successful closure of DOE sites. As one of the programs within the Office of Science and Technology, EMSP ensures that OST’s projects cover the full spectrum of R&D. EMSP’s Web site is at http://emsp.em.doe.gov.

Over the last 50 years, nuclear R&D programs have resulted in releases of plutonium to both on- and off-site locations at several western DOE facilities. Most plutonium and other actinides are tightly bound to soil particles, but the particles themselves are subject to redistribution. The major driver of risks to human health and the environment for DOE facilities in arid and semiarid environments is the mobility of these actinides in surface soils. Research indicates that actinide redistribution is driven primarily not by chemical processes, but by the physical and biological processes associated with ecosystem dynamics. Surface disturbances that cause changes in vegetation and upper soil profiles, such as fire, drought, tilling, and burrowing animals, are critical processes for determining long-term mobility of actinides in the environment.

Actinide mobility is an issue at Rocky Flats and Hanford, and there is increasing concern at other DOE facilities in dry regions about the effect of environmental disturbances that increase mobility. At all of these facilities, uncertainty in actinide mobility hinges on the relative roles of three modes of transport: wind erosion, water erosion, and vertical migration, each of which depends on multiple, interrelated environmental factors. Because it is a fundamental factor in remediation decisions and long-term stewardship strategies, understanding actinide mobility is an Environmental Management Science Program priority in the health, ecology, and risk category.


The vast majority of actinide contaminants at DOE sites are of low concentration and sequestered in soils, so it’s worth asking not how, but whether to clean them up. Physical removal of the soils is costly, presents significant health risks to workers, virtually destroys the contaminated ecosystem, and requires a licensed disposal site. If the long-term risk from actinides in surface soils were known to be sufficiently low, contaminants might better be left in place; but this option would require assessments that took into account low-frequency, high-impact disturbances that change thresholds between significant and nonsignificant transport of soil contaminants. Until sound technical data and knowledge are available to accurately address long-term fate and effects of soil actinides, scientific, regulatory, and public confidence in cleanup decisions will be limited.


A recently concluded EMSP project, led by Los Alamos National Laboratory’s David Breshears with participation from researchers at Colorado State University and the Carlsbad Environmental Monitoring and Science Center of New Mexico State University, is providing DOE with data and tools to improve risk assessments, cut cleanup costs, and facilitate technology transfer. After first developing advanced measurement techniques for assessing the three pathways, the team assembled new data on each pathway based on site-specific field and laboratory measurements at three major DOE facilities and developed multipathway, multisite assessments of mobility based on existing long-term transport models. The results were integrated in new assessment tools that will provide the basis for incorporating strong technical information into the cleanup decision-making process.

Measurement of low levels of plutonium-239 contamination in the environment requires radiochemical analysis and alpha spectrometry, which are expensive, time-consuming, and destructive. In contrast, gamma spectrometry is cost-effective, rapid, and capable of measuring activity levels in situ. The project team determined that cesium-137 and americium-241, two radionuclides readily measured via gamma spectrometry counting, could reliably be used as tracers for plutonium in soil and applied this finding to their studies of the three transport modes. Initial studies focused on Hanford, Rocky Flats, and the Waste Isolation Pilot Plant, three semiarid DOE sites that differ in climate, soils, vegetation, and actinide sources.


Risk assessments often estimate wind erosion based on averages of reported data, which can obscure variations of several orders of magnitude. Average values have a large degree of uncertainty and may not be accurate for other DOE sites. The LANL-led research team quantified wind erosion rates using spatially distributed, finely time-resolved aerosol measurements and correlated them with meteorological and ground-cover conditions. Project results show that episodic, high-wind events disproportionately increase resuspension.

Measurements also yielded strong evidence that wind erosion rates from sites disturbed by fire or overgrazing are significantly greater than those from undisturbed sites. The results demonstrate the importance of low-frequency thresholds and disturbances in ground cover in determining actinide transport by wind erosion.


Water erosion is a second major process affecting contaminant transport, but major knowledge gaps exist. Intense convection thunderstorms often play a major role in generating runoff and erosion in these environments. Soil types, vegetation, surface slope, and the amount and intensity of rainfall are key factors governing runoff, erosion, and associated transport of actinides; but quantitative data were lacking on specific sites and the hydrologic effect of site disturbance on the process. This study created simulated rainstorms at different levels of initial soil water content to measure erosional losses of sediment and an actinide surrogate from disturbed (burned) and control field plots located near three DOE sites.

Results highlighted the large effect of burning as a disturbance on contaminant transport and mobility via runoff and erosion. Average erosion rates at both WIPP and Rocky Flats were about three times higher from disturbed plots than from control plots, and activity readings from disturbed plots exceeded those from paired control plots by factors of 3–13. Soil texture also had a pronounced effect on runoff and erosion, making associated actinide transport strongly site-specific. Contrasts in vegetation and soil composition between Rocky Flats and WIPP caused radionuclide transport to differ by a remarkable factor of 12.


Because plutonium solubility is extremely low under normal environmental conditions, vertical migration of actinides must result from downward transport of soil particles through either cracks or the soil matrix itself. Project researchers used an ingenious new measurement system on soil columns collected from the field to investigate vertical migration as a function of soil type, water percolation, and wetting/drying cycles that promote cracks in soil profiles.

Soil columns were subjected to a series of wetting/drying cycles in a drying oven that could induce soil cracking within weeks or months, accelerating a process that might occur only annually in the field. Soil cracking was far more pronounced in certain soils, probably due to higher clay content. Collectively, the results highlight the impact of low-frequency events, such as soil cracking, on vertical migration.


The researchers used existing models of long-term transport to conduct a preliminary assessment for each of the three modes of transport. Data for the three field sites were used to parameterize and run models and to compare predictions with measured field data. Water erosion is most sensitive to slope angle of the land and soil texture; wind erosion is most sensitive to range vegetation type and amount; and vertical migration is most sensitive to soil texture and land contour. The relative importance of the three pathways was evaluated to predict average long-term responses for a total of seven DOE sites in arid and semiarid sites.

The project team also developed a transport model for simulating dynamic contaminant transport in soils. The model provides a framework for creating code in which the soil is represented by a matrix of adjoining soil columns, each subdivided into layers. The design is easy to modify to represent a transport process, add new transport pathways, or customize for site-specific processes. The code lays the groundwork for future modeling efforts to fully couple all three pathways and incorporate low-frequency, high-impact contaminant transport events.


The project’s measurement and assessment results offer several payoffs for DOE. Field studies demonstrate that disturbances that reduce ground cover, such as fire or heavy grazing, can increase wind and water erosion by more than two orders of magnitude. These results clarify the need to factor disturbance events and recovery rates into long-term assessment of actinide mobility. Research found that infrequent, extreme climatic and disturbance events greatly increase transport rates on all three pathways relative to long-term averages, highlighting the need to account for extremes in climate and disturbance that may be brief but contribute most to long-term risks. Models indicate that the relative importance of actinide transport by wind erosion, water erosion, and vertical migration differs within sites and from site to site by more than an order of magnitude. These results—some of the first multipathway, multisite estimates for DOE facilities—can be used to set priorities to improve risk assessments and remediation.

An important theme emerging from this project is that each of the three pathways exhibits threshold responses to a suite of environmental conditions—precipitation intensity, wind velocity, wetting-drying cycles, surface heterogeneity (vegetation and ground cover), and disturbances that impact the surface heterogeneity—resulting in nonlinear increases in contaminant transport. These results highlight the importance of finer-scale processes that could dominate the overall risk estimates associated with the long-term mobility of actinides; but these processes are not generally evaluated in concert and their threshold response is largely unconsidered. Improved risk assessment for addressing remediation, litigation, and long-term stewardship will require a more mechanistic understanding and predictive capability of these processes.

Quantifying the thresholds that determine changes in the rates of soil actinides transport from wind erosion, water erosion, and vertical migration over longer time frames provides the basis for a scientifically defensible risk assessment. Such an assessment may justify leaving the contaminants in place, saving taxpayers billions of dollars. At a minimum, a sound technical basis for cleanup decisions can help increase stakeholder confidence in selected strategies and in plans for long-term site stewardship.

For more information on this research, contact principal investigator David Breshears, Los Alamos National Laboratory, (505) 665-2803, daveb@lanl.gov.