Long-Term Institutional Management of U.S. Department of Energy Legacy Waste Sites (2000) / Chapter Skim
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Appendix G Mathematical Models Used for Site Closure Decisions
Appendix G Mathematical Models Used for Site Closure Decisions
Pages 149-158
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From page 149... ...
Assessments of the hazards posed by sites where contamination will remain into the distant future are known as risk assessments orperformance assessments. The term performance assessment usually refers to evaluation of the extent to which an engineered system satisfies predetermined design or performance criteria.
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From page 150... ...
; · a leaching model that describes how contamination passes from the solid phase into the aqueous phase; · a vadose zone model that describes how contamination moves downward from the point of disposal toward the water table, or upward with some contaminants such as radon; · an air dispersion model that describes the transport of dust that blows off contaminated soil, or of gaseous contaminants such as radon; and · an ecological risk model that evaluates how the contamination affects ecosystems. Most commonly, models used at DOE remediation sites involve direct exposure, hydrologic transport, and risk.
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From page 151... ...
Vadose Zone Models Virtually all existing multimedia risk assessment models view fluid flow and radionuclide transport in the vadose zone as moving vertically downward at a uniform and steady rate. Though many recognize that this conceptual model is oversimplified, it is often defended as being conservative, in that mathematical models based upon it overpredict contaminant concentrations at receptor locations and associated risks.
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From page 152... ...
For example, all existing multimedia models consider a single source for each surface water pathway. At large DOE sites like ORNL and the Savannah River Site (SRS)
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From page 153... ...
Groundwater Transport Models in principle, the Monte Carlo method of uncertainty analysis should be easy to implement in conjunction with a risk assessment methodology of the kind just described. The only potential obstacle for such implementation is the large amount of computer time that may be required to repeat detailed hydrologic model simulations many times so as to generate a meaningful statistical sample of equally likely random flow and transport solutions.
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From page 154... ...
Two wells completed in the vicinity of the TYBO underground nuclear test site on Pahute Mesa, at the Nevada Test Site, were sampled as they were pumped. The sampling revealed the presence of plutonium, in association with colloids, at significant concentrations in well ER-20-5 #1, 278 m west of TYBO at a depth of 860 m, and at very small concentrations in a deeper aquifer penetrated by well ER-20-5 #3, 30 m south of #1 at a depth of 1,309 m (Kersting et al., 1999~.
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From page 155... ...
This is particularly important in using multimedia models, whose input and output variables frequently are not directly observable. In order to assess potential risks and hazards from residual contaminants under various cleanup and land/water use scenarios, one should ideally have detailed information about their nature, quantity and location; the manner and rate at which they could be mobilized to migrate toward human and/or ecological receptors; the pathways and rates of their migration; their concentrations at receptor locations; associated doses to receptors; and their effects on receptor health.
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From page 156... ...
, which are based on a limited menu of highly simplified conceptual models, are often used (for screening as well as more advanced investigative purposes) with generic parameters and inputs rather than with site-specific data, and are virtually never compared against actual site conditions.
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From page 157... ...
1995. Multimedia Environmental Pollutant Assessment System (MEPAS)
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