Lake probe

Thursday, Nov. 18, 1999 | 11:56 a.m.

BOULDER CITY -- Federal scientists have taken a three-dimensional snapshot of a polluted plume of water that streams seven miles through Lake Mead, from the Las Vegas Wash to Hoover Dam, to understand how fast and how deep the pollutants flow.

Federal, state and local scientists are trying to measure what is in the plume, where it goes within the lake and how fast it moves.

Lake Mead is the primary source of drinking water for the Las Vegas Valley.

The plume consists of the Las Vegas Valley's runoff, ground water that bubbles to the surface and treated sewage from local plants and the Basic Management Industrial complex in Henderson, all of which flows through the wash into the lake.

Next to the Colorado River, which created Lake Mead, the Las Vegas Wash contributes the most water to the lake. Other tributaries are the Virgin and Muddy rivers.

Harmful bacteria, insecticides, heavy metals, toxic chemicals and algae all flow in the plume that shoots from the wash across the Las Vegas Bay, then spreads out as the Colorado River water mixes with it, the scientists found.

The Bureau of Reclamation's latest study has discovered pollutants reached Saddle Island, where the drinking water intake for the Las Vegas Valley is located, in two days. However, the scientists found, the plume floated like a raft about 30 feet beneath the surface of the lake this summer. The intake pipe was built 150 feet below the surface.

As the water has cooled this autumn, the plume had dropped to about 90 feet, the scientists said, but that is normal and there is plenty of fresh river water diluting its contents.

Bureau of Reclamation scientists James LaBounty and Tracy Vermeyen of the agency's Water Resources Research Laboratory in Denver used Doppler radar devices last August to probe the plume. They measured its temperature, speed, depth, width and direction.

While Doppler has been used to track storms, the smaller devices used in Lake Mead helped the scientists track the plume through the complex layers of the lake, LaBounty said. The Bureau of Reclamation hopes to return to the lake next year to repeat the measurements.

"With this technology, you can follow a single drop of water through the reservoirs," LaBounty said Wednesday after a technical meeting at the bureau's Boulder City office.

The scientists faced complications from July's floods during their expedition. The first flood on July 8 struck the Las Vegas Wash at 16,000 cubic feet per second, equal to a 125-year event. The valley funneled 2.8 billion gallons of water into the wash after two thunderstorms joined over the western Las Vegas Valley, flooding all of the washes. Then the rain moved east, adding to the floodwaters.

Vermeyen said that the Doppler picked up evidence of a plume from the flood as well as the polluted plume they were targeting. The flood water and its sediment pushed about three miles from the wash into Las Vegas Bay, where it settled at the bottom.

The Doppler also found that the intake valves for the valley's drinking water as well as the intake pipes at Hoover Dam had an effect on both on the contaminated plume 30 feet below the surface and the flood debris sitting at the bay's bottom. Those intakes bend the plumes, scientists found, but there was no evidence that parts of either plume had been sucked into the pipes.

The three-dimensional plume picture could not discriminate between soil particles and bits of living cells such as algae. Further studies on water quality will continue to analyze what the polluted plume carries. "This is a snapshot," LaBounty said.

LaBounty, who has been studying the plume for a decade, said it is important for scientists to keep track of it for the future of the valley's water quality.