Phase II Documentation Overview of Corrective Action Unit 98: Frenchman Flat, Nevada Test Site, Nye County, Nevada, Revision 1

The U.S. Department of Energy (DOE), National Nuclear Security Administration Nevada Site Office (NNSA/NSO) initiated the Underground Test Area (UGTA) Subproject to assess and evaluate radiologic groundwater contamination resulting from underground nuclear testing at the NTS. These activities are overseen by the Federal Facility Agreement and Consent Order (FFACO) (1996, as amended March 2010). For Frenchman Flat, the UGTA Subproject addresses media contaminated by the underground nuclear tests, which is limited to geologic formations within the saturated zone or 100 meters (m) or less above the water table. Transport in groundwater is judged to be the primary mechanism of migration for the subsurface contamination away from the Frenchman Flat underground nuclear tests. The intent of the UGTA Subproject is to assess the risk to the public from the groundwater contamination produced as a result of nuclear testing. The primary method used to assess this risk is the development of models of flow and contaminant transport to forecast the extent of potentially contaminated groundwater for the next 1,000 years, establish restrictions to groundwater usage, and implement a monitoring program to verify protectiveness. For the UGTA Subproject, contaminated groundwater is that which exceeds the radiological standards of the Safe Drinking Water Act (CFR, 2009) the State of Nevada’s groundwater quality standard to protect human health and the environment. Contaminant forecasts are expected to be uncertain, and groundwater monitoring will be used in combination with land-use control to build confidence in model results and reduce risk to the public. Modeling forecasts of contaminant transport will provide the basis for negotiating a compliance boundary for the Frenchman Flat Corrective Action Unit (CAU). This compliance boundary represents a regulatory-based distinction between groundwater contaminated or not contaminated by underground testing. Transport modeling simulations are used to compute radionuclide concentrations in time and space within the CAU for the 1,000-year contaminant boundary. These three-dimensional (3-D) concentration simulations are integrated into probabilistic forecasts of the likelihood of groundwater exceeding or remaining below the radiological standards of the Safe Drinking Water Act (CFR, 2009) defined as the contaminant boundary. Contaminant boundaries are not discrete predictions of the location or concentration of contaminants, but instead are spatial representations of the probability of exceeding Safe Drinking Water Act radiological standards. The forecasts provide planning tools to facilitate regulatory decisions designed to protect the health and safety of the public