Nearly 40 percent of the 828 nuclear explosions at the Nevada Test Site contaminated the groundwater with radioactive and chemical elements, according to a federal study.

The report by the U.S. Geological Survey says many of the contaminants "are highly toxic and are known to persist in the environment for thousands of years."But it says more research is needed to determine the direction and speed of the underground water flow so that scientists can figure out if and when the water will reach populated areas.

Although the groundwater level at the 1,350-square-mile Test Site is generally more than 800 feet below the surface, many of the explosions were detonated at depths near or below the water level to ensure that the radiation didn't escape into the atmosphere.

Water generally moves southwest through the Test Site toward such areas as Ash Meadows, Oasis Valley, Alkali Flat and Death Valley. Residents in these areas must rely on the groundwater because of the absence of any rivers or other surface water.

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More than 10 major wells have been drilled at the Test Site to provide water for drinking, industrial and waste uses. Regular testing hasn't detected any fission material but has discovered tritium. But the amounts didn't exceed federal standards for safe drinking water.

But test drilling at other locations inside the Test Site has found tritium and fission products where none was expected. In tests at Yucca Mountain, tritium was found to be in excess of those standards.

Groundwater has been used for drinking, livestock, agriculture and mining at sites south and west of the Test Site. These include numerous wells at Beatty, Springdale, Amargosa and Death Valley Junction.

The report by Randell Laczniak, James Cole, David Sawyer and Douglas Trudeau is meant to provide a fundamental understanding of the groundwater conditions so an "effective and efficient cleanup" of the water can start.

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