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Keith Rogers
December 01, 2000 — Levels of the rocket-fuel ingredient perchlorate reached a record high in Lake Mead drinking water in November, prompting water-quality officials to stop recharging supply wells with lake water as a precautionary measure.
Kay Brothers, resources director for the Southern Nevada Water Authority, said the authority's board members were told Thursday that a combination of factors — unseasonably warm temperatures on the lake's surface, the lowering of the lake's level and decreased Colorado River flows into the lake — had pushed perchlorate-tainted water close to the Saddle Island intakes.
The intakes are currently 146 feet below the lake's surface. State Environmental Protection Division Administrator Allen Biaggi said treated water from the lake is safe to drink, but consumers should be aware that the increased perchlorate levels are still within the range of 20 parts per billion to 40 parts per billion the
EPA is considering for a safe drinking water standard. A part per billion is equivalent to about one grain of sand in three residential swimming pools. The so-called reference dose being considered for the EPA standard is 32 parts per billion.
That would be less stringent than California's guideline of 18 parts per billion, a so-called action level that requires notifying health officials but not necessarily shutting down a tainted water supply. Studies are under way to determine how much perchlorate humans can consume without adversely affecting thyroid gland functions that control growth and metabolism.
The highest reading in November for perchlorate in Lake Mead water after treatment was 24 parts per billion, or 9 parts more than the previous high on June 17, 1999. The highest concentration of raw water from the lake before treatment has been about the same, 16 parts per billion, and the average for treated water since monitoring began in 1997 has been about 9 parts.
Brothers said, "What's happening is the thermocline is stuck in the general area of the intakes." Based on temperature, the thermocline is the middle water layer sandwiched between the top layer, which absorbs surface heat from the sun, and the bottom layer, which is a few degrees colder than the thermocline. In winter, the cooling effect of land surrounding large bodies of water can cause the thermocline to disappear.
"As it cools off, we expect the thermocline to go down and there will be more mixing with the water below," Brothers explained.
That, in turn, results in more mixing of surface water carrying pollutants entering the lake from Las Vegas Wash, a conduit for perchlorate and other pollutants. The compound, ammonium perchlorate, was produced at two plants near Henderson.
One of the companies, Kerr-McGee Chemical Corp., has been working with the state to keep the highly soluble, salty contaminant out of the lake. Since November 1999, Kerr-McGee has diverted a stream where perchlorate has seeped and has extracted some 50 tons of the perchlorate using an expensive, ion-exchange process.
Once the perchlorate levels decrease to 18 parts per billion, Brothers said the recharging of supply wells with treated lake water will resume. She said she doubts that the amount of perchlorate entering the lake from Las Vegas Wash is increasing because the stream that represents a major source of the pollutant is being cleaned. "We're hoping over time perchlorate will flush out of the lake completely," she said.
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