Application of Model Abstraction Techniques to Simulate Transport in Soils (NUREG/CR-7026)

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Publication Information

Manuscript Completed: June 2010
Date Published: March 2011

Prepared by:
Y. Pachepsky¹, T. Gish1, A. Guber¹
A. Yakirevich², M. Kouznetsov²
M. Van Genuchten³,
T. Nicholson⁴, R. Cady⁴

¹United States Department of Agriculture
Agricultural Research Service
Environmental Microbial and Food Safety Laboratory
Hydrology and Remote Sensing Laboratory
Beltsville, MD 20705

²Department of Environmental Hydrology & Microbiology
Zuckerberg Institute for Water Research
Blaustein Institutes for Desert Research
Ben-Gurion University of the Negev
Sede Boqer Campus, 84990, Israel

³Department of Mechanical Engineering, COPPE/LTTC
Federal University of Rio de Janeiro, UFRJ
Rio de Janeiro, RJ CEP 21945-970, Brazil

⁴U.S. Nuclear Regulatory Commission
Office of Nuclear Regulatory Research
Washington, DC 20555-0001

T.J. Nicholson, NRC Project Manager

NRC Job Code N6235

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

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Abstract

Successful understanding and modeling of contaminant transport in soils and groundwater is a precondition of risk-informed predictions of the subsurface contaminant transport. Exceedingly complex models of subsurface transport are often inefficient. Model abstraction is a methodology for reducing the complexity of a simulation model while maintaining the validity of the simulation. The objective of this work was to use model abstraction techniques to characterize and understand flow and transport in soils in the presence of shallow groundwater. We developed two case studies by carrying out two types of field tracer experiments at the USDA-ARS OPE3 Beltsville field site, and applying a sequence of model simplifications based on the HYDRUS software family and MODFLOW. Soil moisture, soil water potential, tracer concentrations in groundwater, groundwater levels, and weather data, along with ground penetration radar surveys, electric resistivity monitoring, and dilution tests complemented borehole log data and laboratory hydraulic measurements to characterize soil heterogeneity. The invoked series of model abstractions showed the important role of subsurface heterogeneity in the vadose zone and groundwater, and substantial improved the conceptualization of the subsurface. Results of this study provide techniques to aid the NRC licensing staff in their review of a licensee's abstraction of complex transport models, and to help confirm the acceptability of model abstraction assumptions used in performance assessments.