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Project FAULTLESS, having an announced yield of between 200 and 1,000 kilotons, was detonated at the Central Nevada Test Area (CNTA) - (MAP) in emplacement borehole UC-1 on January 19, 1968, at a depth of 3,200 feet below ground surface in zeolitized tuff. FAULTLESS was designed to study the behavior and characteristics of seismic signals generated by nuclear detonations and to differentiate them from seismic signals generated by naturally occurring earthquakes and also to evaluate the usefulness of the site for higher-yield nuclear tests that could not be safely tested at the Nevada Test Site (NTS). As a result of the test, radioactive contamination of the deep bedrock around the shot cavity occurred at the CNTA. Groundwater is the most likely transport medium for the deep contamination, however, because of the depth of the contamination (in excess of 3,200 feet) and the remoteness of the site, exposure to humans is unlikely. As part of subsurface characterization work, hydrogeologic investigations are being conducted by the U. S. Department of Energy (DOE) under the Federal Facility Agreement and Consent Order (FFACO). The FFACO outlines a process to insure that the DOE and/or the U. S. Department of Defense (DoD), under the regulatory authority and oversight of the NDEP, identify sites of potential historic contamination, thoroughly investigate these sites, and implement corrective actions based on public health and environmental considerations. The strategy for the subsurface is to characterize groundwater flow and contaminant transport through numerical modeling utilizing site-specific hydrologic, hydrogeologic, and geochemical data. The contaminant of focus is tritium, because, based on presently available data, it is the most conservative (i.e., remains in solution) and therefore the most mobile of the potential radiological contaminants. The overall objective of the investigation is to define the hydrologic boundaries encompassing groundwater resources that may be unsafe for domestic or municipal use. Exploratory drill holes and hydrologic characterization wells installed in the years following the Project FAULTLESS test were used to collect information on the regional groundwater flow system. Existing data were used to build a three-dimensional numerical groundwater flow and transport model in 1999, which was developed to predict the future movement of radionuclide migration in groundwater from the CNTA and allow hydrologists, site managers, and regulators to develop a better understanding of the groundwater system and assess future work and monitoring requirements. The transport of the radioactive contaminants in groundwater is related to groundwater flow. Contaminants such as tritium are generally conservative and the most mobile of the potential radiological contaminants. Some contaminants move more slowly than the bulk of the groundwater because they are subject to retardation through processes such as sorption and matrix diffusion along the flow path. All of of these processes are incorporated into the contaminant transport model. The CNTA flow model was developed by the Desert Research Institute (DRI) using stochastic (probabilistic - involving random variables) methods. The model domain is aligned in a north-south orientation and is approximately twice the size of the UC-1 land withdrawal area with its center over the UC-1 emplacement hole. The vertical extent of the model is 4430 feet below ground surface. The domain is discretized into a grid of cubic cells with edge dimensions of 164 feet. Three hydrogeologic categories included in the model are defined based on lithology, electrical resistivity, and hydraulic conductivity: Quaternary alluvium, Tertiary volcanics having low hydraulic conductivity, and Tertiary volcanics having high hydraulic conductivity. The conceptual flow model includes the description of hydrogeologic features such as primary groundwater flow direction, boundary conditions, sources and sinks, and fracture locations. 
The numerical model simulates the conceptual model which includes southward flow through the alluvium and northward to northeastward flow in the deeper volcanic section. Strong vertically downward hydraulic gradients are present in the north part of the domain, while strong vertical gradients upward from the volcanics to the alluvium are present to the south. Groundwater recharge was not directly simulated in the model. The top of the model and the east and west faces are no-flow boundaries. The north, south, and bottom faces are specified-head boundaries and are based on the head relationships observed in the CNTA wells in Hot Creek Valley.
NDEP has been carefully reviewing the model predictions, which suggest that no significant radionuclide migration will occur beyond the immediate area of the Project FAULTLESS test. While the model indicates limited transport, large uncertainties were recognized in a number of important parameters. Reducing model uncertainty and increasing confidence in model predictions are key issues for the Off-Sites program. In 2000, a Data Decision Analysis (DDA) - (Document) was performed. The DDA was a cost-benefit analysis of additional potential characterization activities that might reduce model uncertainty. Based on sensitivity analyses, six model input parameters were identified as uncertain in terms of the model's ability to predict solute migration: 1) Specified head boundary conditions used in the flow model; 2) The spatial distribution of the underlying welded tuff unit; 3) Effective porosity; 4) Sorption coefficients; 5) Matrix diffusion coefficient; and 6) Geochemical release function for the nuclear glass dissolution. The optimal activities would represent the greatest uncertainty reduction per unit cost. Results of the DDA indicated that a new hydrogeologic characterization well(s) would significantly decreased the uncertainty in the prediction of the contaminant boundary, however, this activity was also the most expensive task. Other optimal activities included the barometric test and matrix diffusion analogues which were more cost effective but had less of an impact on uncertainty reduction. The DDA showed that while the uncertainty in the input parameters is large, the predicted uncertainty in the contaminant boundary was relatively small. The uncertainty parameters tested in the DDA were subsequently incorporated into the Contaminant Transport Model - (Document) as stochastic parameters rather than the earlier deterministic parameter values used. Work Currently in Progress: DOE completed the groundwater flow and contaminant transport modeling, and NDEP reviewed the work and concurred with the modeling results. Current efforts are underway to create a combined Corrective Action Decision Document (CADD)/Corrective Action Plan (CAP) for the CNTA subsurface (See also FFACO Appendix VI - Corrective Action Strategy, Section 3, for descriptions of the CADD and CAP). As a first step toward developing the corrective action plan, DRI proposed a methodology for Model Validation - (Document) for the CNTA. The combined CADD/CAP for the CNTA is due in July 2004. |