Frenchman Flat modeling studies

These final sections overview the results of flow and transport modeling studies and assessments of uncertainty for the Frenchman Flat CAU, the most developed of the CAU studies on the NNSS. There are three dominant features of all conceptual models of the Frenchman Flat basin (Fig. 26.4):

1. the high hydraulic heads in the CP basin northwest of Frenchman Flat (over 100 m higher heads than the Frenchman Flat basin; see Fig. 26.4),

2. the semi-perched condition of groundwater in the alluvial and volcanic aquifers with higher heads in these aquifers than the regional LCA,

3. the southeastward thinning of the volcanic section beneath the basin across Frenchman Flat.

These combined features support two inferential observations for the basin. First, groundwater flow in the alluvial and volcanic aquifers is likely hori­zontal across the basin from northwest to southeast (NNES, 2010a, b). Second, there is increased leakage downward into the LCA from the allu­vial and volcanic aquifers as the basal volcanic confining unit thins to the southeast and/or is offset by faults associated with the Rock Valley fault system. Particle track studies originating at locations of underground tests show southeast flow through the alluvial and volcanic aquifers changing to southwestward flow in the LCA following surface and subsurface faults associated with the basin structure (Bechtel Nevada, 2005; SNJV, 2006; NNES, 2010a, b). These observations are consistent with groundwater flow converging into and following faults of the Rock Valley fault system in southern Frenchman Flat (Fig. 26.8).

Modeling studies for the Frenchman Flat CAU combine steady state and transient source term studies, multiple alternative representations of the groundwater flow system, and probabilistic transport simulations. Source term models of radionuclide releases into groundwater were developed for

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26.8 Satellite photograph of the Frenchman Flat basin on the southeast edge of the NNSS showing the major structural features of the basin and directions of groundwater flow (large black arrows: regional flow system; large gray arrow: local flow in the alluvial and volcanic aquifers). The Rock Valley fault zone is a zone of echelon faults that form the Rock Valley fault system. The asterisks mark the location of ten underground nuclear tests; three in central Frenchman Flat and seven in the north part of the basin. The solid gray lines outline the edges of contaminated groundwater defined by the 95th percentile of exceeding the radiological standards of the Safe Drinking Water Act over 1,000 years. These contaminant boundaries are small (<500 m length and for some tests in alluvium, the contaminant boundaries are smaller than the asterisk symbol marking the test locations); the contaminant boundaries are larger for two tests where the underground cavity was in or near fractured volcanic rocks (two tests in the northern area) or where a 17-year radionuclide pumping experiment discharged contaminated groundwater on the surface (one test in the central area).

two settings. First, the radiological source term for underground tests in alluvium were calibrated, for both steady-state and transient models, to observed breakthrough of radionuclides at a pumping well located 91 m from the CAMBRIC test in the water table in alluvium (Tompson et al., 1999; Carle et al., 2007). Second, two underground tests in northern French­man Flat were conducted above the water table in or near fractured vol­canic rock, where the rock permeability and porosity is inferred to be enhanced from the effects of the test detonation (IAEA, 1998). Simplified source term models were developed for these tests that account for unsatu­rated and saturated flow and transport and test-induced changes in rock properties (NNES, 2010a, b).

Multiple steady state groundwater flow models were developed for the Frenchman Flat CAU (SNJV, 2006) that are calibrated to hydraulic heads and permeability data for hydrostratigraphic rock units, and attempt to account for conceptual model uncertainty. The evaluated components of conceptual (structural) model uncertainty include variability in boundary conditions and boundary fluxes, permissible alternative hydrogeological frameworks for the basin, including structure (faults and basin features), stratigraphic units within the basin, and alternative recharge models. The goal in developing flow models was not to identify a best-fit calibration or a best predictor flow model but instead to distinguish a range of alternative flow models that capture the range of variation in flow fields from paramet­ric and structural uncertainty. This range in groundwater flow was then used in transport simulations. Statistical metrics of goodness of fit of alternative groundwater calibrations did not provide useful information for discrimi­nating or screening groundwater flow models. Two alternative sets of data did provide useful information for categorizing results for calibrated flow models (SNJV, 2006). These include variability in particle track results, and variability in groundwater velocity and direction at test cavity locations using linear predictive uncertainty analysis from parameter estimation soft­ware (PEST; Doherty, 2007).

Monte Carlo transport simulations were conducted for underground tests at the two testing areas in Frenchman Flat (Fig. 26.4). Four flow models were combined with alternative sets of boundary conditions (boundary fluxes, hydrostratigraphic frameworks and recharge) to represent the vari­ability in the groundwater flow field (velocity and direction of flow at the test cavity). These flow conditions were established at the underground test cavities as the initial conditions for transport simulations sampling stochas­tic transport parameters using a streamline-based convolution transport code (Robinson et al., 2011). Radionuclide concentrations for 1,000 years of transport were post-processed to develop probabilistic forecasts of exceeding the radiological requirements of the SDWA (Fig. 26.8); the boundary of this representation denotes the limits of contaminated groundwater (contaminant boundary) defined as a 5% chance or less of exceeding the SDWA. There are two categories of contaminant boundaries: (1) small boundaries (<500 m maximum lateral distance) where the test cavity and transport are in the alluvial aquifer and (2) larger boundaries (>1600 m) where the source term and/or transport is in fractured volcanic rock. For the latter category (two underground tests), the contaminant boundaries extend slightly off the NNSS boundaries into adjacent Federal land (Fig. 26.8).

The contaminant boundaries of the central testing area of Frenchman Flat (Fig. 26.8) are complicated by two factors. First, the long-term pumping test for the CAMBRIC test discharged contaminated groundwater on the surface into a ditch that drained into the Frenchman Flat playa. Second, the discharged contaminated water in the drainage ditch infiltrated to the water table in concentrations that exceed the SDWA. This required transient models to account for the 17 years of continuous aquifer pumping and surface discharge of contaminated water and significantly extended the contaminant boundaries of the central testing area.

The contaminant boundaries depicted in Fig. 26.8 will be used for two regulatory decisions. First, the boundary geometries will be used to desig­nate surface use restriction areas where institutional controls will be imposed to restrict all drilling to potentially contaminated groundwater. Second, the contaminant boundaries and results of subsequent monitoring studies will be used by NDEP to identify a regulatory boundary designed to protect the public and environment from exposure to contaminated groundwater. The NNSA/NSO will be required to develop a plan to miti­gate potential impacts on the public, if radionuclides are detected at the regulatory boundary. The regulatory boundary has tentatively been identi­fied as the Rock Valley fault zone at the southern end of Frenchman Flat, the expected migration pathway to public access to groundwater south of the southern boundary of the NNSS.

The transport model for the Frenchman Flat CAU was accepted by NDEP following successful external peer review of the CAU studies (Navarro-Intera, 2010). This marks the first successful completion of the model development stage under the UGTA strategy and the initiation of the model evaluation stage for the Frenchman Flat CAU (USDOE, 2011).

26.4 Acknowledgments

This chapter is a snapshot of ongoing work for UGTA. This work is a multi­disciplinary cooperative effort conducted by scientists at the Desert Research Institute, the Lawrence Livermore National Laboratory, the Los Alamos National Laboratory, National Security Technologies, Navarro — Intera and the US Geological Survey. The chapter was improved through review comments provided by Bimal Mukhopadhyay, Joe Fenelon and Susan Krenzien. Nathan Bryant and Joe Fenelon assisted in the develop­ment of the chapter figures.

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