Earthquake could cause flooding of Yucca Mountain repository
September 2, 1997
An earthquake in the vicinity of the proposed high-level nuclear
waste repository at Yucca Mountain could cause groundwater to surge up into
the storage area, according to a new study by two University of Colorado at
Boulder geophysicists.
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 published 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.
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The region near Yucca Mountain is
tectonically active, as there are several
recently active volcanic cones within
a few miles of the site.
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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.
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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.
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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 there are 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."
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