PREDICTING THE CONSEQUENCES TO THE CRITICAL POPULATION OF AN ERUPTION THROUGH THE YUCCA MOUNTAIN REPOSITORY AND REDISTRIBUTION OF VOLCANIC ASH ALONG FORTYMILE WASH, AND INTO THE AMARGOSA VALLEY, NEVADA

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 and into the Amargosa Valley, which is the main drainage for the eastern slope of Yucca Mountain. The critical population lives at a distance of approx 20 km from the repository, on the alluvial fan of Fortymile Wash. Our study evaluates ash redistribution through this drainage system in order to determine the impact to the critical population. We will determine the nature of the ash redistribution area and how the ash is diluted through the drainage system. We propose 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. Initial results will be presented. This study covers an area of >1500 square km (significantly larger than previous Cesium-137 studies). Because of the proximity to the location of nuclear air tests, the background (reference) Cesium-137 values are determined. 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 has been studied as an analog to a tephra apron resulting from an eruption through the repository to determine extent of tephra dilution (by non-tephra sediment) in drainages west and south of the cone. Samples have been collected at intervals along the first order ash filled drainages on the tephra sheet and along the second and third order drainages into which the tephra is carried. A transect has been established 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. Additional analog dilution studies have been done at Sunset Crater, AZ and in the Cima volcanic field, CA.