Shifting ground at nuclear waste site.

Yucca Mountain in Nevada--the nation's top candidate for a high-level radioactive waste repository--is on the move. This windswept ridge and its environs are shifting at least 10 times faster than geologists had expected, according to precise surveying measurements made with the Global Positioning System (GPS) satellites.

For nearly 2 decades, federal scientists have been studying Yucca Mountain to determine whether it would make a suitable underground burial site for radioactive waste from nuclear power plants and weapons facilities. Once tucked away inside the mountain, the waste must remain isolated for 10,000 years while its radioactive isotopes decay (SN: 11/1/97, p. 277).

To study ground movement around the proposed location, geologist Brian Wernicke and his colleagues took GPS readings at five sites situated along a 34-kilometer line that cuts across Yucca Mountain. Given the relatively short time frame of their study and the history of faults, the researchers expected to find no movement. Yet from 1991 to 1997, the two farthest stations moved apart roughly 1.7 millimeters per year, with smaller shifts between the middle stations, the researchers report in the March 27 Science.

"That is 10 to 100 times [the value] that you would derive from what's known about the seismic history of faults across Yucca Mountain," says Wernicke of the California Institute of Technology in Pasadena.

Researchers are puzzled about why the region is stretching so much faster now than it has over the last million years. One possibility is than the crust is undergoing temporary readjustments following a magnitude 5.4 earthquake that struck 20 km southeast of Yucca Mountain in 1992. Wernicke and his colleagues argue that this explanation is unlikely because the quake was relatively small and far away from many of the GPS sites.

Instead, they propose that magma movement deep in the crust could be driving the ground motion at Yucca Mountain. If so, the region could be passing through a geologically short period of activity, lasting roughly 100,000 years, when the rates of earthquakes and volcanic eruptions exceed the long-term average over millions of years.

Eruptions are of particular concern because a direct hit would blast radioactive material into the atmosphere. Yet past studies have concluded that the volcanic threat is minimal. Researchers have estimated a 1 in 10,000 chance that a volcanic eruption will disrupt the site of the proposed nuclear waste repository over the next 10,000 years, says Bruce M. Crowe, a geologist with Los Alamos (N.M.) National Laboratory.

Wernicke and his coworkers suggest that the true probability could be 10 times higher. Crowe says the impact of the new study remains uncertain. "I think [Wernicke] has jumped a little too quickly to the volcanism model to explain his interpretations." Both agree that a prudent plan would be to set up a network of GPS stations around Yucca Mountain to resolve how much the ground is shifting.