U.S. Water News Online
LAS VEGAS -- Ongoing drought has brought Lake Mead surface impurities closer to intakes for Las Vegas-area drinking water, prompting an expensive and ambitious proposal to draw cleaner water into water intakes.
The Southern Nevada Water Authority is developing a $5 million impermeable membrane -- an inverted L-shape plastic curtain -- to dip at least 100 feet beneath the lake's surface and stretch about 300 to 400 yards lengthwise.
``This is a very difficult kind of project,'' said Marcus Jensen, water authority engineering director. ``Very effective, but it's going to be challenging.''
The curtain could be in place by next summer, although another alternative could extend intakes downward toward the lake's bed. Engineers believe the curtain would be more cost-effective.
Similar technology has been used to protect the fishery of Northern California's Lewiston Lake, where warm water is shielded from cool water to protect maturing fish eggs.
It's a touchy issue for bureaucrats attempting to operate a multibillion-dollar water system during the region's worst recorded drought.
They say Lake Mead water remains safe for swimming and, after treatment, for drinking.
But surface pollutants have increased as the lake level has fallen 60 feet -- from 1,213 feet above sea level in January 2000 to 1,143. The lake could drop another 14 feet by next summer, officials say.
The water authority's intake tunnels are at 1,050 feet and 1,000 feet.
The drought has produced algae blooms, mucking up filters that cleanse water drawn through the towers, and reducing treatment plant capacity as much as 20 percent when filters are rinsed. The water authority must dip into limited groundwater supplies when the flow of lake water is limited.
The surface also has a mix of chemicals and organic materials -- primarily perchlorate and total organic carbons, which form trihalomethanes when mixed with chlorine.
Perchlorate, a jet fuel ingredient, was produced decades ago at a Henderson industrial plant and continues to slowly leach into the lake. Treatment facilities have been built to limit the perchlorate threat.
The recent opening of a water authority ozone treatment facility is expected to help reduce trihalomethane formation by limiting chlorine's catalytic role.
Warm summer weather boosts surface temperatures by as much as 20 degrees, increasing the interaction of chlorine with algae, pine needles and grass clippings swept into the lake.
On a recent morning, water authority lab technician John Corso and three colleagues made a weekly 10-minute boat ride to take water samples several hundred yards off of Saddle Island.
Around surrounding cliffs, they could see a six-story white ring of mineral deposits from when water levels were higher.
Lab tech Steven Lujan dropped plastic tubes to depths ranging from 33 to 188 feet to collect samples for testing for bacteria, perchlorate, trace metals and inorganic materials.
Lab boss Linda Blish said the falling lake levels have not sparked immediate health problems, but have raised concerns about new contaminants.
``You always have to watch what's going on in your water source,'' Blish said.
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