Topical Report - Evaluation of the Potentially Adverse Condition "Evidence of Extreme Erosion During the Quaternary Period" at Yucca Mountain, Nevada

The potentially adverse condition identified at 10 CFR 60.122(c)(16), evidence of extreme<br/>erosion during the Quaternary Period, has been determined to not be present at Yucca<br/>Mountain. A literature search for typical hillslope erosion rates in the U.S. and the world was<br/>performed to establish a range of typical values for erosion rates. Low to moderate erosion<br/>rates in the U.S. were identified to range from 2 to 50 centimeters per thousand years (cm/ka)<br/>in semiarid environments. Long-term average hillslope erosion rates established for Yucca<br/>Mountain were determined to be 0.19 cm/ka. The Yucca Mountain rates were established<br/>utilizing cation ratio dating of rock varnish on colluvial boulder deposits to establish age<br/>control and by measuring hillslope denudation and hillslope channel incision marginal to<br/>these boulder deposits. The geologic record examined for Yucca Mountain area hillslopes<br/>extend from 170 ka to about 1400 ka, and represents the longest geologic record obtained to<br/>date from hillslopes in the southwestern United States. Based upon a comparative evaluation<br/>of Yucca Mountain hillslope erosion with erosion rates in other analogous geologic and<br/>climatic regimes, DOE has concluded that consistent with NRC staff use and in keeping with<br/>the plain meaning of the term extreme, extreme erosion has not occurred at Yucca Mountain<br/>during the Quaternary Period. Therefore, the potentially adverse condition of evidence of<br/>extreme erosion during the Quaternary Period identified at 10 CFR 60.122(c)(16) does not<br/>exist at Yucca Mountain.