PLEASANTON -- East Bay residents looking to beat the heat this weekend will have fewer options for cooling off, thanks to the closures of two popular swimming lakes due to toxic bacteria buildups.

Extremely high levels of E. coli prompted the complete closure on Friday of Pleasanton's Shadow Cliffs Lake to swimmersuntil further notice, East Bay Regional Park District officials said.

Lifeguards told swimmers to leave the water after test samples taken from the lake's north beach showed levels of the harmful bacteria at about four times the state's daily maximum standard. District officials had closed portions of the lake's south beach on Wednesday, but allowed visitors to swim in other areas.

Shadow Cliffs Lake in Pleasanton (Staff File)
Shadow Cliffs Lake in Pleasanton (Staff File)

"We got the results back from the lab (Friday) and they were higher than we were comfortable with," said district water management supervisor Hal MacLean. "We're going with the numbers and if they remain high, we'll remain closed. It's day-to-day."

MacLean said it's the first time the district has ever had to close a beach due to bacteria, adding the district would re-sample the water every day until it is safe to reopen.

Readings from Thursday's tests showed E. coli at the lake's south beach at 6,100 E. coli colonies per 100 milliliters, and 1,000 at the north beach, according to the district. The state standard for daily maximum exposure is 235.

"We've said 'wow, this is high' before, but it's never been this bad," MacLean said.


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District officials suspect a combination of drought conditions, low water levels and animal waste are to blame for the high bacterial content.

"The geese are congregating more at the beach now that the lawns are dead," MacLean said. "We think that's it."

MacLean said the district intends to treat the lake next week with an oxidizer and direct geese away from the swim area in an attempt to lower the levels.

Residents won't find relief over at Oakland's Lake Temescal either. It remains closed to swimmers until at least Monday due to blooms of a toxic blue-green algae. The lake will reopen once the blooms subside.

"We're concerned about the health of our patrons," MacLean said. "It unfortunate that we have to do this especially with the hot weather but we think keeping people safe is more important than having those people get sick."

MacLean said that people experiencing signs of stomach distress, rashes, cramps or diarrhea after swimming should contact the Alameda County Environmental Health Department at 510-567-6700.

Contact Jeremy Thomas at 925-847-2184. Follow him at Twitter.com/jet_bang.