A volcanic eruption through the Yucca Mountain Repository, although highly improbable (1x10-8 per year) would be an event with a possible very high hazard. This study is to determine the effects on the critical population from erupted tephra carried along or through the Fortymile Wash drainage system, which is main drainage for the eastern slope of the Yucca Mountain area. The critical population (REMI) lives at a distance of approximately 20 km from the repository, on the alluvial fan/delta of the Fortymile Wash. Our study will evaluate ash redistribution resulting from wind or water erosion in order to determine the impact to the dose that the critical population would experience. In order to predict the dose amount we will determine the nature of ash redistribution area and how the ash is diluted through the drainage system. We purpose to calculate the volume of material that has been deposited within the past 50 years using Cesium-137, a bomb produced tracer, to determine the erosional and depositional patterns around Yucca Mountain. Most Cesium-137 studies to date have only covered a small (<5 square km) area. Initial results will be presented. This study will cover a much larger area of >1500 square km and require more than 200 samples. The Lathrop Wells volcanic cone, located 12 km south of Yucca Mountain, erupted 70,000 years ago and produced an extensive tephra sheet. This tephra sheet will be studied as an analog to the possible eruption through the repository to determine the nature of tephra dilution in drainages west and south of the cone. Samples have been collected at 100-ft intervals along the tephra sheet and along the ash filled drainage systems. Stratigraphy of the pits will be mapped and sampled to establish a transect along which ash is diluted from its source over a distance of > 2km. The values obtained from the ash dilution study, applying the scaling factor for the Fortymile wash system, will be used to calculate the amount of contaminated ash that would reach the critical population and impact the calculation of the dose.