On Nov. 3, city officials heard from a water utility consultant hired to find the source or potential causes of the contamination of the city’s water supply last month.

Melinda Friedman, founder and president of Confluence Engineering Group, presented a review of water system data to assess the E. coli and Total Coliform events that forced restaurants to close and residents to boil their water for weeks.

She said the contamination appears to be a “transitory event” and could not pinpoint a root cause, but did identify risks and vulnerabilities in the Island’s water system.

No problems were detected with water treatment by Seattle Public Utilities (SPU), sampling methods, storage facilities, main installations and repairs or leaking water mains.

“Your staff is very knowledgeable and dedicated to protecting public health,” she said. “Where they exist, shortcomings are due to lack of resources and funding relative to demands of managing an aging distribution system.”

Friedman explained that there are three factors that lead to bacteria issues: a pathway, a pathogen and a driving force.

There are four pathways by which “bugs get into the system,” she said, which are the water source or point of entry, distribution system itself, sampling procedure and purposeful contamination.

The first thought was to look at the city’s reservoirs, as all city water passes through them, though those were found to be clean by maintenance crews and a special dive team.

Friedman said that because subsequent sampling was overseen by SPU and because positive results occurred with different samplers, it seems unlikely that the cause was sample collection or site issues. The city conducted an assessment for purposeful contamination, and found no evidence of trespassing, vandalism, alarms or any credible threats.

Friedman said a major issue is with the city’s rate of replacing old or damaged pipes. The industry standard is to replace one percent of the pipeline every year, and Mercer Island replaces .4 percent.

The Island’s water infrastructure – comprised mostly of unlined cast iron pipe – is aging. Some pipes are 50 years old in places. The older pipes can cause conditions for a buildup of biofilm and tubercules, which can harbor bacteria.

That’s a problem for many utilities in this region, Friedman said.

“This is an issue for the city to deal with, but you’re not alone,” Friedman said. “This could have been anyone in the region that this happened to.”

Biofilm and pipe issues are impacting water quality, but aren’t likely the cause of the E. coli event, she said. Biofilm would have been the focus of the investigation if only Total Coliform was detected, but E. coli, a fecal indicator, was also found in many locations around the Island.

Friedman said on a positive note, the city averages about eight main breaks per year, systemwide. This is less than the industry goal of less than 15 breaks per year per 100 miles.

However, the city’s cross connection control program needs to be improved, and it needs to increase chlorine residual levels and water monitoring, documentation and data management.

Cross connections are any connections between potable water and a non-potable fluid. A control program, which calls for inspections to identify and eliminate cross connections and annual testing of backflow prevention devices, is required by the Environmental Protection Agency, state Department of Health and the city (ordinance A-38). But that ordinance hasn’t been updated since the 1980s, Friedman said.

There is also a need for additional staff, though Friedman praised the maintenance crews and other city staff for doing the best they could with the resources available.

There were other unusual distribution system bacteriological results on the days when Mercer Island got its hits on Sept. 24 and 25, Friedman said, which may be due to a significant rain event. There were seven Total Coliform positives in three separate systems in the Seattle area, one of which was Mercer Island.

Standing water was observed after that rainfall, which could be a problem. Still, the experts have not identified the specific cause of the contamination, or ‘smoking gun.’

As part of an expanded monitoring program, the city began collecting 18 samples per day on Oct. 3, but dropped down to nine samples per day starting Nov. 4. Since the lifting of the ‘boil water’ advisory, over 500 samples have shown that the water is safe.

The city has a goal to maintain chlorine residual of 0.5, but that has been raised to 1 since the E. coli event. Elevated levels will be maintained until the other risk factors are mitigated.

City staff will present an action plan to the Council at its next meeting on Nov. 17. The Utility Board meets at 7 p.m. on Thursday, Nov. 13.