Testing at Shadow Cliffs Regional Recreation Area on July 16 showed enough E. coli in the lake near both the left and right lifeguard chairs to warrant a yellow light warning on the Park District website, www.ebparks.org.
"It's actually just slightly elevated levels in there but this is not unexpected in the summer season," said East Bay Regional Park District spokeswoman Emily Hopkins.
The water is tested for E. coli and fecal matter weekly April through October and twice a month November through March, Hopkins said. Swimming is allowed year-round.
"There is an increased health risk for swimming and wading. Bacteria results do NOT meet the State Health Standards," states the website.
The beach would be closed if conditions worsen, and a red light warning would be given. If the water quality improves, the beach will receive a green light to indicate bacteria testing results are within the State Health Standard Limits.
"Beach water contact may cause illness," the warning explains. "Do not swallow water. Shower and towel dry after water contact. There is always some risk with beach water contact. Water quality conditions can change at any time."
For more information call (510) 544-2328.