INVESTIGATIONS OF SENSITIVITY AND UNCERTAINTY IN SOME HYOROLOGIC MODELS OF YUCCA MOUNTAIN AND VICINITY

Abstract

The uncertainty in travel time for water through unsaturated and saturated zones of Yucca Mountain and vicinity was determined by considering uncertainty associated with input parameters to the hydrologic models of these zones. A first-order analysis was used to investigate uncertainty in water travel time through the unsaturated zone at Yucca Mountain, based on an analytic solution for water flow. Results of the investigation of uncertainty for the unsaturated zone indicated that uncertainty in the percolation estimate contributed significantly more to uncertainty in travel time than uncertainty in estimates of hydraulic conductivity. Monte Carlo and first-order approaches were used to investigate uncertainty in ground-water travel time for different cases that varied in the treatment of the input parameters to the hydrologic model of the unsaturated zone. Comparison of the Monte Carlo and first-order estimates of mean ground-water travel and travel time uncertainty in the saturated zone demonstrates that the first-order approach underestimated both the mean and variance of travel time for all considered. This underestimation suggests that the Monte Carlo approach should be used to estimate mean travel and its variance as well as the probability density distribution of travel time. A sensitivity study of ground-water travel time in themore » standard zone indicated that the zones with smaller transmissivity produced the largest influence in travel time for most cases. 31 refs., 40 figs., 19 tabs.« less