External Peer Review Team Report Underground Testing Area Subproject for Frenchman Flat, Revision 1

An external peer review was conducted to review the groundwater models used in the corrective action investigation stage of the Underground Test Area (UGTA) subproject to forecast zones of potential contamination in 1,000 years for the Frenchman Flat area. The goal of the external peer review was to provide technical evaluation of the studies and to assist in assessing the readiness of the UGTA subproject to progress to monitoring activities for further model evaluation. The external peer review team consisted of six independent technical experts with expertise in geology, hydrogeology,'''groundwater modeling, and radiochemistry. The peer review team was tasked with addressing the following questions: 1. Are the modeling approaches, assumptions, and model results for Frenchman Flat consistent with the use of modeling studies as a decision tool for resolution of environmental and regulatory requirements? 2. Do the modeling results adequately account for uncertainty in models of flow and transport in the Frenchman Flat hydrological setting? a. Are the models of sufficient scale/resolution to adequately predict contaminant transport in the Frenchman Flat setting? b. Have all key processes been included in the model? c. Are the methods used to forecast contaminant boundaries from the transport modeling studies reasonable and appropriate? d. Are the assessments of uncertainty technically sound and consistent with state-of-the-art approaches currently used in the hydrological sciences? 3. Are the datasets and modeling results adequate for a transition to Corrective Action Unit monitoring studies—the next stage in the UGTA strategy for Frenchman Flat? The peer review team is of the opinion that, with some limitations, the modeling approaches, assumptions, and model results are consistent with the use of modeling studies for resolution of environmental and regulatory requirements. The peer review team further finds that the modeling studies have accounted for uncertainty in models of flow and transport in the Frenchman Flat except for a few deficiencies described in the report. Finally, the peer review team concludes that the UGTA subproject has explored a wide range of variations in assumptions, methods, and data, and should proceed to the next stage with an emphasis on monitoring studies. The corrective action strategy, as described in the Federal Facility Agreement and Consent Order, states that the groundwater flow and transport models for each corrective action unit will consider, at a minimum, the following: • Alternative hydrostratigraphic framework models of the modeling domain. • Uncertainty in the radiological and hydrological source terms. • Alternative models of recharge. • Alternative boundary conditions and groundwater flows. • Multiple permissive sets of calibrated flow models. • Probabilistic simulations of transport using plausible sets of alternative framework and recharge models, and boundary and groundwater flows from calibrated flow models. • Ensembles of forecasts of contaminant boundaries. • Sensitivity and uncertainty analyses of model outputs. The peer review team finds that these minimum requirements have been met. While the groundwater modeling and uncertainty analyses have been quite detailed, the peer review team has identified several modeling-related issues that should be addressed in the next phase of the corrective action activities: • Evaluating and using water-level gradients from the pilot wells at the Area 5 Radioactive Waste Management Site in model calibration. • Re-evaluating the use of geochemical age-dating data to constrain model calibrations. • Developing water budgets for the alluvial and upper volcanic aquifer systems in Frenchman Flat. • Considering modeling approaches in which calculated groundwater flow directions near the water table are not predetermined by model boundary conditions and areas of recharge, all of which are very uncertain. • Evaluating local-scale variations in hydraulic conductivity on the calculated contaminant boundaries. • Evaluating the effects of non-steady-state flow conditions on calculated contaminant boundaries, including the effects of long-term declines in water levels, climatic change, and disruption of groundwater system by potential earthquake faulting along either of the two major controlling fault zones in the flow system (the Cane Spring and Rock Valley faults). • Considering the use of less-complex modeling approaches. • Evaluating the large change in water levels in the vicinity of the Frenchman Flat playa and developing a conceptual model to explain these water-level changes. • Developing a long-term groundwater level monitoring program for Frenchman Flat with regular monitoring of water levels at key monitoring wells. Despite these reservations, the peer review team strongly believes that the UGTA subproject should proceed to the next stage