A modeling study of perched water phenomena in the unsaturated zone at Yucca Mountain

The purpose of this paper is to illustrate the use of a multiphase subsurface flow model to study perched water phenomena in vadose zones. Modeling studies are based on field investigations of the unsaturated zone (UZ) at Yucca Mountain, Nevada. Perched water data have been compiled, analyzed, and incorporated into a three-dimensional (3-D) UZ flow model developed to investigate perched water phenomena at Yucca Mountain. A conceptual model of perched water occurrences is discussed, and a series of comprehensive computer modeling studies on perched water at the site are presented. The model has been calibrated using perched water data observed in six boreholes, and reproduces water-perching conditions in the unsaturated zone of the mountain. Both steady-state and transient simulations have been conducted. The steady-state simulation results are in agreement with the observed perched water data, including water saturation and potential profiles and perched water elevations. Transient numerical pumping tests were performed using pumping testing data collected in the field from two boreholes. The numerical pumping test results match observed water levels collected during pumping and recovery periods.