Tuesday, July 18, 2000 | 11 a.m.
After more than a year looking at the depths of Lake Mead, scientists have discovered that sediment pouring down the Las Vegas Wash is contributing more muck than what's coming down the Colorado River.
The sediments can give scientists an idea of how quickly Lake Mead's bottom is filling up with ooze from the Las Vegas Valley and offer a chemical fingerprint of the insecticides, pesticides, toxins and bacteria running into Southern Nevada's major drinking water supply.
"Everyone and their mom is doing sampling of the water, but no one is paying attention to the sediments," UNLV health physicist Mark Rudin said.
Shifting masses of the goo at the bottom of the bay and the lake could change the chemistry of the water, he said. The muck could release more contaminants into the water as surface runoff, treated sewage and floodwaters increase.
Before 1965, when Glen Canyon Dam formed Lake Powell in Arizona, sediment almost 100 feet thick flowed from the Colorado River into the Las Vegas Bay and backed up behind Hoover Dam.
Once Glen Canyon Dam plugged the river upstream, the Las Vegas Wash, carrying runoff and wastewater from the valley's 1,650-square-mile basin, became the primary source of sediment flowing into Lake Mead.
Recent floods have deposited from 1 to 9 feet of muck into the Las Vegas Wash, forming a delta in Las Vegas Bay.
This summer scientists from UNLV and the U.S. Geological Survey's marine geology team from Woods Hole, Mass., are collecting cores of that sediment with a $55,000 grant from the Bureau of Reclamation.
Mud stored in long plastic tubes tells the story. A lighter shade of brown from newer silt was dropped into the bay during the July 8, 1999, flood. An older, darker layer of older mud is built up beneath it. The tube ground to a halt when it hit coarse sand and gravel less than 3 feet into the sediment, oceanographer David Twichell said.
The murk enveloped divers Rick Rendigs and Ken Parolski at a depth of 60 feet during a sampling trip on Sunday, they said.
"It got really dark fast and very cold," Rendigs said. "We had to come up and get a light."
"Literally, it's like putting a bag over your head and trying to do something," Parolski said.
Parolski said the cloudy water near the lake's bottom appeared to be the plume of polluted water coming from the Las Vegas Wash, and his experience diving through it seemed to support theories of former Bureau of Reclamation scientist Jim LaBounty, who now works for the Southern Nevada Water Authority.
LaBounty, who has studied the plume for 10 years, predicted the runoff, which like a stream is cooler than the lake, would plunge deeper into the bay during the summer months when the surface water is heated by the sun.
Other scientists said the wash runoff would disperse more readily into the bay during the summer. But Parolski said the cloudy water he dove through near the bottom of the lake appeared to be the plume intact, not dispersing.
The sediments collected by Parolski and Rendigs will be analyzed in the laboratory once the sampling efforts are completed. How long that takes depends on the wind and weather.
Some day, Rudin said, the sediments could reveal how much radiation fell into Lake Mead during the above-ground nuclear weapons testing days.
Rudin has detected some radioactive fallout in the form of cesium-137 from earlier sediment samples. Cesium-137 is considered a fingerprint form of radiation, because it is easy to identify and can indicate how much radioactive material the sediment has absorbed.
He plans to test next for plutonium.
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