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Tuesday, July 28, 1998
Bacteria count continued last week
No warnings because levels
By Keith Rogers
Review-Journal
Runoff tainted with high levels of bacteria continued to flow down Las Vegas Wash and into Lake Mead after a second round of storms last week, water quality officials reported Monday.
The levels of fecal coliform bacteria measured in the lake's Las Vegas Bay on Wednesday had decreased from those measured July 20, but at 40 feet below the surface they still exceeded the state's safety standard for body contact at two sampling points.
No warnings have been issued, however, because bacteria levels at the surface, where people swim, are within the safety guideline of 200 cell counts per 100 milliliters of water.
The counts in the lake early Wednesday after a storm the previous day were 24,000 and 5,400 at two points about 40 feet below the surface. But by 5 p.m. that day those levels had tapered to 9,200 and 1,300 at the respective sampling locations.
In all there are three sampling points within a two mile stretch of Las Vegas Bay. The sampling point farthest from where Las Vegas Wash enters the lake showed 110 cell counts at 46 feet below the surface Wednesday evening.
It takes about 48 hours to analyze and calculate cell counts in the samples, according to officials at the Las Vegas Environmental Division Laboratory.
Fecal coliform counts in Las Vegas Wash, which is already posted with body contact warnings, had decreased from at least 240,000 on July 20 to 160,000 early Wednesday morning. That sampling point, beneath the North Shore Road bridge, is about one mile upstream from where the wash empties into the lake.
"You'd expect a decline with time," said biologist Larry Paulson, a member of the scientific team conducting the storm water monitoring experiment for the Lake Mead Water Quality Forum -- a multi-agency consortium with citizen representatives.
"It's just a question of how high it will go," he said about fecal coliform cell counts.
"The longer we go between major storms, the higher the fecal count is likely to be after the storm," he noted.
That may be because bacteria along channels that feed Las Vegas Wash tend to build up over time until storm runoff flushes it into the wash.
The wash empties into Lake Mead about six miles upstream of the Las Vegas Valley's water supply intakes at Saddle Island.
Fecal coliforms are discharged by humans, pets and wild, warm-blood animals. The presence of fecal coliforms is an indication that organisms, including diarrhea-causing E. coli bacteria, might also be present.
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