THE GRADUAL BURIAL OF YUCCA MOUNTAIN

The proposed high-level nuclear waste repository described in the 2008 DOE license application is located 200-250 m below the surface of Yucca Mountain (YM), a tilted fault block composed of 12-Ma densely welded tuff. On the basis of a stream-power erosion model Stüwe and others (2009) suggested that the repository horizon would be unroofed in 0.5-5.0 million years (m.y.). The model, which does not include local geomorphic or basin history, makes untenable assumptions that weathered debris is removed from the Amargosa Valley-Fortymile Wash closed basin and that baselevels lower through time. More than 40 boreholes logged adjacent to Yucca Mountain record more than 11 m.y. of basin-fill depositional history. In 8 boreholes well-dated Pliocene and late Miocene basalts, interstratified with basin fill alluvium, indicate long-term aggradation rates in adjacent Crater Flat basin of 35 m/m.y. and 41 m/m.y. to the east and south of YM. Scores of boreholes in the southern Amargosa Valley do not contain detritus from YM volcanic rocks; therefore, nearly all debris shed off YM is located within 30 km of the mountain. The general baselevel has risen, not declined, about 400 m since erosion began in the late middle Miocene. Recent cosmogenic dating by Cl-36, Ne-21, and Be-10 indicate a vertical rock-weathering rate of about 2 m/m.y. on bedrock surfaces. Horizontal erosion rate of cliffs and uppermost hillslopes is about 10 m/m.y. Dated segments of the main YM cliff have been preserved for more than 250 k.y. Small volumes of debris mantle YM hillslopes and U-series-dated soils on older colluvium are commonly of middle Pleistocene age and as old as ~1 Ma. Because long-term erosion and aggradation rates integrate multiple climate cycles we believe that these long-term rates are good predictors of future rates of similar geomorphic processes. Based on our data, during the next 5 m.y. the highest point on Yucca Mountain crest may become about 10 m lower vertically, upper valley walls may recede 50-100 m horizontally, and continued aggradation will likely raise the baselevel on three sides of the mountain by 150-200 m. Yucca Mountain is downwasting very slowly, continues to be buried by its own waste, and will continue to do so as long as the present closed basin exists.