County public health officials have warned residents at Nacimiento Lake not to drink water from the reservoir without purifying it because of a break in a sewer line that is submerged in the lake.

Penny Borenstein, county public health officer, said in a news release late Tuesday that the Oak Shores Wastewater Treatment Plant’s collection system developed a break in a submerged sewer line.

“Due to this apparent breach in the sewer line, thousands of gallons of lake water have entered the treatment plant,” she said. “Public works staff has reduced the flow through the compromised section of the sewer line and are actively attempting to locate the breach. The wastewater treatment plant is currently able to handle the increased inflow of sewage and lake water.”

Residents who get their domestic water from the lake — Nacimiento Water Co. at Oak Shores, Lake Nacimiento Resort, and the North Shore Boat and Ski Club — should use bottled water or boil the lake water for a minimum of three minutes, Borenstein said.

Domestic uses of water are drinking, cooking and bathing.

In addition, “The public is ordered to specifically avoid swimming and other water contact activities for the time being,” Borenstein said.