Study finds little threat in ground water's taint

A U.S. Geological Survey report disclosed widespread chloroform contamination in the Las Vegas Valley's shallow ground-water supplies but not at a level considered harmful to humans.

Chloroform, a product of water chlorination during treatment, was found in 68 percent of the Las Vegas samples. Only two of the local samples turned up volatile organic compounds such as solvents and gasoline components at concentrations greater than federal water standards.

The contamination is not considered an immediate threat because local drinking water is drawn from deep aquifers rather than from shallow ground water. Both levels of water are separated by thick layers of impervious rock and clay.

But Michael Lico, a USGS hydrologist in Carson City who compiled the research from 1993 to 1995, said the contamination could become a problem if shallow water reaches deep aquifers through wells that are abandoned or damaged.

"The biggest concern is having a layer of contaminated water sitting on top of the drinking water supply," Lico said. "But the levels of contamination we found were well below (unsafe) drinking standards."

Southern Nevada Water Authority spokesman J.C. Davis attributed the high chloroform readings to runoff from lawn over-watering. The runoff includes high concentrations of chlorine used to treat the water.

But he added that recent water authority studies have found no contamination of the valley's deep aquifers, some of which reach depths of 400 feet. Most of the shallow water tables are 25 to 100 feet below ground.

As a precaution, the water authority has a ground water citizens advisory task force that is auditing all known active and abandoned wells in its service area.

"We're hoping to get a full accounting of every well in the region," Davis said.

The 24-page report compiled by Lico is titled, "Quality of Ground Water Beneath Urban and Agricultural Lands in the Las Vegas Valley and the Carson and Truckee River Basins, Nevada -- Implications for Water Supply." The report is available for inspection at the USGS office at 6770 S. Paradise Road and at the Las Vegas Library at 833 N. Las Vegas Blvd.

The report also may be purchased from the USGS Branch of Information Services, Box 25286, Denver, Colo. 80225-0286 (toll free at 1-800-435-7627). The USGS Internet website address for Nevada water studies is www.nv.wr.usgs.gov /.

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