1999 Technical Reports
Groundwater Models in Support of NUREG/CR-5512
C. R. Cole, M. D. Williams, W. A. Perkins, M. D. White, P. D. Meyer
Pacific Northwest National Laboratory, Richland, WA
NUREG/CR-5621 PNNL-12050, Pacific Northwest National Lab, Richland, Wa, 1999.
Summary
This report describes an investigation of the modeling methodology laid out in NUREG/CR-5512, Volume 1, for determining site-specific radiological criteria for license termination. The basis of the NUREG/CR-5512 modeling approach is "prudently conservative" (as opposed to "worst case") scenarios using simple, "prudently conservative" models. Two levels of screening are provided for: Level 1 screening uses generic (default) parameter values and Level 2 screening substitutes site-specific parameter values for some of the generic parameters where appropriate. More realistic models that use site-specific data for nearly all inputs are required when NUREG/CR-5512 modeling is inappropriate. Four computer codes were developed: the standard model, an implementation of the three-compartment model of NUREG/CR-5512, Volume 1; an extended model that replaces the single-compartment vadose zone of the standard model with multiple compartments; and two hybrid codes constructed by replacing either the vadose zone or the aquifer compartment in the standard or extended NUREG/CR-5512 code with a more realistic numerical code, STOMP for vadose zone and CFEST for the aquifer.
Sensitivity analyses were used to determine which parameters must be "hard wired" to assure prudent conservatism and which can be varied with site data to extend the screening range. Infiltration rate (and irrigation rate for the residential scenario) was the parameter to which the models were most sensitive. Vadose zone depth was also consistently important. The analyses described in this report were used to conclude that the standard model is not adequate for Level 2 screening assessments because it does not adequately represent the effect of one of the most sensitive vadose zone parameters, vadose zone thickness. The extended model is adequate, however, for Level 2 screening. Comparisons between the models demonstrated the realistic conditions represented by the standard and extended models when using generic parameter values. Nonconservative results may occur when the standard and extended models when the radionuclides of concern have half-lives that are short relative to the expected travel time through the vadose zone. The results indicated that the conditions under which the NUREG/CR-5512 screening approach is not appropriate is quite simple. The NUREG/CR-5512 methodology is not appropriate if there already exists deep contamination at the site, if there are significant risks associated with contamination at the site (for example it overlies a sole source aquifer), or if the basic conceptual model and scenarios that are part of the NUREG/CR-5512 methodology are inappropriate (e.g., karst terrain, located in a flood plain, highly fractured bed rock system).
Anoxic Plume Attenuation in a Fluctuating Water Table System: Impact of 100-D Area In Situ Redox Manipulation on Downgradient Dissolved Oxygen Concentration
M. D. Williams, V. R. Vermeul, M. Oostrom, J. C. Evans, J. S. Fruchter, J. D. Istoka, M. D. Humphreya, D. C. Lanigan, J. E. Szecsody, M.D. White, T. W. Wietsma, C. R. Cole
Pacific Northwest National Laboratory, Richland, WA
aOregon State University, Corvallis, OR
PNNL-12192 (UC-2000) Pacific Northwest National Laboratory, Richland, WA
Summary
A treatability study is underway at the 100-D Area of the Hanford Site, 160 m from the Columbia River. The target contaminant for the treatability test is chromate (hexavalent chromium) in excess of 1,000 μg/L in groundwater. In Situ Redox Manipulation (ISRM) is an innovative treatment technology that establishes reducing conditions in an aquifer to treat redox-sensitive contaminants (e.g., hexavalent chromium, uranium, technetium, and chlorinated solvents) in groundwater. As a side effect, an anoxic plume is formed downgradient from the treatment zone. This report describes the results of a study on the fate of an anoxic groundwater plume in an unconfined oxidizing aquifer with a fluctuating water table.