The safety of the proposed Nevada site has been debated for more than 10 years, primarily due to concerns about earthquakes and groundwater. Now it appears that one of those concerns could lead to a problem with the other.
In a study to be published next month in Environmental Geology, physics research associates John B. Davies and Charles Archambeau present their conclusions on what might happen if a significant earthquake struck the Yucca Mountain area. It is the first study to assess the impact of an earthquake on the area's groundwater levels.
Using computer modeling based on geological data, historical quakes and results from about 20 test wells, they showed that a magnitude 5 or 6 earthquake could raise the water table between 450-750 feet at the storage site. Because the repository would be only 600 to 800 feet above the present water table, "flooding could be expected to occur," they write.
The water table below the Yucca Mountain site is unusually deep, about 1,500 feet below the surface, Davies said. But within a 6-mile area north of the proposed storage facility the groundwater level rapidly rises to a more normal depth of about 600 feet.
The reason for this abrupt change in the water table is a cause for concern, Davies said.
Davies and Archambeau believe that the presence of open fractures underneath Yucca Mountain has allowed the water table to descend to unusually low depths, and that closed fractures to the north have resulted in a more normal water table level. The danger is that an earthquake of sufficient magnitude could cause the open fractures underneath the Yucca Mountain site to squeeze shut, forcing the water upward into the storage facility.
"If water hits the storage area it could cause a rapid corrosive breakdown of the containers and allow the plutonium to leak into the water table and the atmosphere," Davies said.
Historical evidence exists for earthquakes causing groundwater to squeeze upward and even erupt from the surface, the authors said, citing the magnitude 7 quake at Idaho's Borah Mountain in 1983 and the 7.3 quake at Montana's Hebgen Lake in 1959. Both quakes occurred in areas subject to similar geological forces as Yucca Mountain.
The inference also is supported by the relatively large water table changes resulting from a 5.6 magnitude earthquake at Little Skull Mountain near the proposed repository in 1992, they said. And the recent discovery by federal researchers that rainwater falling on top of Yucca Mountain has rapidly seeped 800 feet into its interior, presumably through cracks, also supports this hypothesis, Davies said.
"The low water table beneath Yucca Mountain . . . implies an open fracture system to large depths producing the low water-table levels and an environment which is particularly likely to produce fracture closing and major seismic pumping following an earthquake," they write.
The study of Davies and Archambeau was funded by the state of Nevada, which is opposed to the federal repository at Yucca Mountain.
A study by the U.S. Geological Survey provided an alternative explanation for the steep difference in water table levels north of the site. The USGS has hypothesized that a deep underground fault stretches for 60 miles between the two different water table levels and acts as a barrier. Davies and Archambeau doubted that explanation.
If an earthquake of magnitude 6 or greater were to strike, the authors' model predicts that the "wall" of water associated with the unusually steep gradient in the water table to the north of Yucca Mountain could shift southward and cause groundwater to rise 750 feet above present levels at the repository site. The containment is being designed for a minimum of 10,000 years and one or more earthquakes of that magnitude are a reasonable possibility during that time frame, the authors said.
The region near Yucca Mountain is tectonically active, as witnessed by several recently active volcanic cones within a few miles of the site, the authors said. There also is strong geologic evidence that large volume spring flows have occurred at or near the surface of Yucca Mountain as recently as a few thousand years ago.
"These historical geologic observations strongly imply that seismic pumping and major changes in the water table have occurred frequently in the past," Archambeau said. "Our work demonstrates what can be expected in the future and how dangerous that could be."