Uncertainty and Sensitivity of Contaminant Travel Times from the Upgradient Nevada Test Site to the Yucca Mountain Area

Abstract

Yucca Mountain (YM), Nevada, has been proposed by the U.S. Department of Energy as the nation’s first permanent geologic repository for spent nuclear fuel and highlevel radioactive waste. In this study, the potential for groundwater advective pathways from underground nuclear testing areas on the Nevada Test Site (NTS) to intercept the subsurface of the proposed land withdrawal area for the repository is investigated. The timeframe for advective travel and its uncertainty for possible radionuclide movement along these flow pathways is estimated as a result of effective-porosity value uncertainty for the hydrogeologic units (HGUs) along the flow paths. Furthermore, sensitivity analysis is conducted to determine the most influential HGUs on the advective radionuclide travel times from the NTS to the YM area. Groundwater pathways are obtained using the particle tracking package MODPATH and flow results from the Death Valley regional groundwater flow system (DVRFS) model developed by the U.S. Geological Survey (USGS). Effectiveporosity values for HGUs along these pathways are one of several parameters that determine possible radionuclide travel times between the NTS and proposed YM withdrawal areas. Values and uncertainties of HGU porosities are quantified through evaluation of existing site effective-porosity data and expert professional judgment and are incorporated inmore » the model through Monte Carlo simulations to estimate mean travel times and uncertainties. The simulations are based on two steady-state flow scenarios, the pre-pumping (the initial stress period of the DVRFS model), and the 1998 pumping (assuming steady-state conditions resulting from pumping in the last stress period of the DVRFS model) scenarios for the purpose of long-term prediction and monitoring. The pumping scenario accounts for groundwater withdrawal activities in the Amargosa Desert and other areas downgradient of YM. Considering each detonation in a clustered region around Pahute Mesa (in the NTS operational areas 18, 19, 20, and 30) under the water table as a particle, those particles from the saturated zone detonations were tracked forward using MODPATH to identify hydraulically downgradient groundwater discharge zones and to determine the particles from which detonations will intercept the proposed YM withdrawal area. Out of the 71 detonations in the saturated zone, the flowpaths from 23 of the 71 detonations will intercept the proposed YM withdrawal area under the pre-pumping scenario. For the 1998 pumping scenario, the flowpaths from 55 of the 71 detonations will intercept the proposed YM withdrawal area. Three different effective-porosity data sets compiled in support of regional models of groundwater flow and contaminant transport developed for the NTS and the proposed YM repository are used. The results illustrate that mean minimum travel time from underground nuclear testing areas on the NTS to the proposed YM repository area can vary from just over 700 to nearly 700,000 years, depending on the locations of the underground detonations, the pumping scenarios considered, and the effective-porosity value distributions used. Groundwater pumping scenarios are found to significantly impact minimum particle travel time from the NTS to the YM area by altering flowpath geometry. Pumping also attracts many more additional groundwater flowpaths from the NTS to the YM area. The sensitivity analysis further illustrates that for both the pre-pumping and 1998 pumping scenarios, the uncertainties in effective-porosity values for five of the 27 HGUs considered account for well over 90 percent of the effective-porosity-related travel time uncertainties for the flowpaths having the shortest mean travel times to YM.« less