Long-Term Risk From Actinides in the Environment: Modes of Mobility Page: 4 of 32
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Sound policies regarding cleanup activities and environmental management practices at
Department of Energy (DOE) facilities in arid or semi-arid environments requires
dynamic models for the terrestrial transport of soils contaminated with actinides. The
mobility of plutonium-contaminated soils is a particularly high priority issue.
Environmental factors can vary widely between these facilities, and thus the adequacy of
any Pu transport model depends greatly on site-specific parameter assessments. Of the
basic mechanisms for transporting Pu in the environment, wind and water erosion acting
on the soil surface are by for the most important. Vertical migration of Pu into soil
profiles is normally a relatively minor mechanism. On time scales of a few decades or
less, generally only the top few centimeters of soil profiles are subject to wind and water
erosion. Thus, Pu residing deeper in the soil profile is not immediately subject to
erosional processes. The purpose of this study was to quantify the vertical migration of
plutonium (using 134Cs tracer as a surrogate for Pu) in soils from the Waste Isolation Pilot
Plant (WIPP) and the Rocky Flats Environmental Technology Site (RFETS). This
information is crucial for developing credible, site-specific loss rate expressions for Pu
An assumption fundamental to the design of this study was that infiltrating precipitation
would be the primary driving force for the vertical migration of Pu. Other potential
driving forces such as bioturbation or freeze-thaw action were not considered due to
practical constraints on the scope and duration of the study. The effectiveness of
precipitation in driving vertical migration may be governed by factors such as soil type,
vegetation, and topography, all of which vpy widely between sites, and can vary
considerably within each site. Within the climatic regimes at either WIPP or RFETS, it is
relatively common for drought-like conditions to be punctuated by occasional large
precipitation events. Such extreme climatic sequences can result in large amounts of
erosion at the soil surface, but how they affect the vertical migration of soil particles or
colloids is not well known. This is an important issue for conservative actinide transport
modeling at these sites. The scope of this study was limited to examining vertical
migration as a function of climatically extreme sequences and soil type, both between and
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Shawki Ibrahim, Ph.D. Long-Term Risk From Actinides in the Environment: Modes of Mobility, report, May 24, 2001; United States. (digital.library.unt.edu/ark:/67531/metadc733911/m1/4/: accessed March 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.