Residents of 500 homes in North Bend’s Riverbend community are boiling their water after unsafe levels of E. coli bacteria were found in their private water supply.

The King County Department of Health found E. coli in a water sample taken at a Riverbend residence on Wednesday, Aug. 12. After results were tested, a warning went out to residents on Friday, Aug. 14.

The system was chlorinated, and clean results came after a test on Tuesday, Aug. 18. If tests show a fully clean sample from tests Wednesday, the advisory could be lifted as early as Thursday, Aug. 20.

Water System Manager Roger Lillejord said Wednesday that the source of the bacteria remained a mystery.

Water from the private water system’s well is clean, and nothing was found in the tanks. Lillejord said it would take a major leak in the one of the system’s eight miles of pipes to create the conditions needed for a backflow.

“It’s challenging, because there’s no evidence,” Lillejord said. “Everybody’s house is a potential source.”

Riverbend does not normally chlorinate its water, and Lillejord said that residents are typically more than satisfied with the water.

The system was built in 1982, and had a one-day contamination event in 2002.

“Prior to that, the water’s always tested well,” Lillejord said.

Water is tested twice per month in the system.

Lillejord is frustrated by the inability to point to a known source of the contamination.

“It prolongs the length of time to deal with the problem,” he said. “We’ve been on it 18 hours a day.”

Customers, he said, have been supportive and understanding.

“They know we’re doing everything we can,” he said.

• E. coli bacteria are bacteria whose presence indicates that water may be contaminated with human or animal wastes. Microbes in these wastes can cause short-term effects, such as diarrhea, cramps, nausea, headaches, or other symptoms. They may pose a special health risk for infants, young children, some of the elderly, and people with severely compromised immune systems.