Environment News Service

AmeriScan: June 1, 1999


One of the great worries about using Yucca Mountain, Nevada as a permanent high-level nuclear waste repository is that even in the desert water will invade the storage chambers and dampen the containment vessels, releasing radioactive material into the environment. But now a new study by three U.S. Geological Survey scientists has found the site to have been dry for millions of years. The slow growth rates of calcite and opal minerals that coat fractures and cavities in Yucca Mountain attest to the mountain's hydrological stability say the researchers, who presented the results today at the spring meeting of American Geophysical Union in Boston. "There is no evidence at Yucca Mountain, based on the distribution of calcite and opal, that water has ever flooded the potential repository area," said James Paces, a USGS scientist from Denver, Colorado. Paces described cavities in the volcanic mountain's interior as being relatively free of deposits of calcite and opal. Where they are found these deposits are restricted mostly to the lower surfaces. "If water had filled the cavities, minerals would have been deposited on the walls and ceilings as well," Paces said. "Instead, our data indicate that the minerals formed from thin films of water flowing downward into open spaces." The long term hydrologic stability of Yucca Mountain, 100 miles northwest of Las Vegas, is an important factor in evaluating it as a potential site for storing nuclear waste. It is the only site being evaluated. The mountain is a thick accumulation of 11-to 13-million-year-old volcanic rocks, 1,600 to 2,00 feet of which are above the present water table. Because the USGS team knows how much water forms calcite and opal deposits over a given period of time, they were able to determine how much or how little water had seeped through the mountain.

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