Friday, April 3, 1998 | 10:42 a.m.
Bacteria levels in Lake Mead may pose a threat to swimmers and the Las Vegas Valley's drinking water supply.
David James, a UNLV associate engineering professor, in a study released Thursday reported that lake water at lower depths violated water quality standards twice in 1996 and once in 1997 for fecal coliform, a common organism indicating the presence of bacteria.
Lake Mead, which is a multiple-use recreation area, also is the source of 90 percent of Southern Nevada's drinking water.
After reviewing water quality data collected from Lake Mead over five years, James presented his findings at the Lake Mead Water Quality Forum to federal, state and local water officials who are trying to make the lake safe for swimmers as Southern Nevada's population continues to boom.
Asked for his perspective on the findings, James said, "If you're a water skier, you're alright; if you're a scuba diver, you might have something to worry about."
Swimmers exposed to high levels of bacteria can suffer from a range of maladies including diarrhea and skin irritations.
The cause of the bacteria levels found in Lake Mead continue to puzzle scientists like James.
James said one possible explanation for the high bacteria levels could be traced back to waste water discharged from the three treatment plants -- Las Vegas, Clark County and Henderson -- which release treated water into the Las Vegas Wash.
The City of Las Vegas wastewater treatment plant after 1992 began removing chlorine -- which is toxic to fish -- contained in its discharges, said Debra Bolding, head of the plant's laboratory. Chlorine kills bacteria, but in the lake it poses a threat to fish, so the wastewater plants are removing chlorine after treatment.
"Any flowing water without a toxic disinfectant will have higher bacteria counts," Bolding said. Without chlorine, the discharged wastewater could carry bacteria that start to grow again in the Las Vegas Wash.
James looked at water sampled from 1992 through 1997 to see how wastewater treatment plants and runoff affected the lake and the wash.
Other theories for the lake's bacteria levels include rising water levels, contamination from birds, animals and people living beside the lake.
Bacteria counts don't violate the water quality standards when the Southwest suffers a drought, but the lake level has jumped 40 feet since 1995, James said. Such a drastic increase in water could block sunlight from reaching the lower water, allowing bacteria to grow. Sunshine kills bacteria when the lake level is lower, James said.
Bacteria also could enter the Las Vegas Wash and the lake from birds, animals, shallow ground water infiltration or people living beside the lake, James said.
James said he will continue to sample the wash and the lake this summer in an effort to identify bacteria sources.
In a related study released Thursday, the U.S. Geological Survey said a summary of samples collected at 20 national sites from 1992 through 1996, shows pesticides from large cities such as Las Vegas are contaminating both surface and ground waters. A preliminary report was released in November 1996.
USGS scientists expected farming area runoff to contain pesticides, but the high levels found from urban areas surprised them, said Hugh Bevans, a USGS hydrologist in Carson City and author of the Nevada study.
More pesticides were found in the Reno area than Southern Nevada.
"Everyone was a little surprised at the big city results," he said.
Common pesticides used by homeowners as well as by commercial companies include diazinon and malathion. Diazinon was found in one of every two samples taken from the Las Vegas Wash and malathion in one of every four samples, Bevans said.
Pesticides such as DDT and DDE, polychlorinated biphenyls (PCBs), dioxins and furans were found in the water, sediment and bottom-feeding carp in the Las Vegas Wash and Bay. Dioxins and furans are produced during the manufacturing of pesticides and PCBs and are commonly found in treated waste water.
The USGS detected pesticides in 14 percent of drinking water wells in Las Vegas and 44 percent of the wells in Reno.
Substances found in solvents, called volatile organic compounds, appeared in half of the shallow wells in both Las Vegas and Reno and in 45 percent of the drinking water wells. These substances, however, were not detected in rural areas.
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