The Underground Test Area Project of the Nevada Test Site: The Role of Modeling, Monitoring and Institutional Controls in Establishing Regulatory Protection of the Public - 10349

The Underground Test Area (UGTA) Subproject is an Environmental Restoration activity under the U.S. Department of Energy, National Nuclear Security Administration Nevada Site Office Environmental Management Project at the Nevada Test Site. Models of groundwater flow and contaminant transport are used as the central part of an integrated strategy to guide regulatory decisions to protect the public from underground contamination of groundwater by radionuclides produced during nuclear weapons testing. A revised UGTA strategy was developed to better promote scientific discovery during the iterative process of data gathering and modeling, to fully acknowledge uncertainty in modeling studies, and to incorporate risk-informed investigations. The complexity and uncertainty in the hydrogeologic settings of corrective action units (CAUs) on the Nevada Test Site constrains the reliability of model forecasts of radionuclide migration over the next 1,000 years. Risk for UGTA is defined as the likelihood and consequences of future public exposure to contaminated groundwater and is managed through a tripartite combination of characterization and modeling studies, monitoring, and institutional control of areas of contaminated groundwater. Model development for flow and transport studies is designed to investigate the effects of statistical and structural uncertainty on multiple alternative representations of contaminant transport used to develop ensembles of contaminant boundaries. The negotiated metric for defining a contaminant boundary perimeter is the 95 th percentile of exceeding the radiological standards of the Safe Drinking Water Act. Model evaluation is conducted during model development, through systematic sensitivity and uncertainty analysis, and during initial monitoring studies designed to test model results/forecasts. The goal of model evaluations is to gain regulatory acceptance that the modeling studies can be used for their intended purpose: aiding and guiding regulatory decisions. Model application uses model results to negotiate compliance boundaries between contaminated and uncontaminated groundwater, to achieve CAU closure, and to design a long-term closure monitoring network. Periodic evaluations during closure monitoring are used to evaluate whether the correction action design remains valid.