Update: The boil water advisory was lifted Wednesday Oct. 8 at noon.

E. coli contaminations have been prompting Mercer Island’s City Council and staff to look for answers to lingering questions about the city’s water supply.

The first query is about when the ‘boil water’ advisory – the second in one week – will be lifted.

“The question everyone is asking is: where’s the light at the end of the tunnel… (or) end of the pipe,” said Deputy Mayor Dan Grausz during a Council study session on Oct. 6.

The city’s press release from Oct. 7 says that the agencies are “nearing a decision point” after five days of clear samples. The State Department of Health (DOH) has the authority to lift the advisory, though City Manager Noel Treat noted it’s a judgment call and not a precise exercise.

To lift the advisory, the city must work on a plan to stabilize chlorine levels in its reservoirs, do more thorough flushing throughout the distribution system, inspect high-hazard cross connection control sites and finish inspections of pressure reducing valves and water pipe vaults, according to Clark Halvorson, director of the DOH’s Office of Drinking Water.

Crews have made progress on these directives, and should be done Oct. 7 or 8. They have already boosted chlorine to nearly double customary concentrations, inspected 150 underground vaults and visited and ruled out 25 high-risk cross connections.

To maintain consistent chlorine residuals in the tanks, the city is looking into installing a mixer system. Maintenance Director Glenn Boettcher said he found a contractor in California that could deliver the equipment in a few weeks.

No problems were found in the reservoirs, according to dive teams that were sent into the tanks on Saturday to inspect them. The question remains of how E. coli was found on both the North and South-end of the Island, Grausz said, if the water coming out of the tanks is clean.

“This is the biggest puzzle in the whole mix,” Treat said. “What we’ve heard in working with the DOH is that it doesn’t necessarily make a lot of sense. It’s a hard one to figure out.”

The second question is about what the city can do to protect its water supply in the future.

“Long-term package is getting those mixers in place. Long-term strategy is for the city of Mercer Island to identify if they want to have their own ability to inject chlorine and not have to rely on SPU (Seattle Public Utilities),” Halvorson said.

Mayor Bruce Bassett suggested the city hire an outside consultant to test the water chemistry, look at the data and identify causes that may have been missed. The DOH suggested a list of firms, and the city picked Confluence WTIC. The consultation cost is estimated to be $10,000 to $15,000.

The question with the murkiest answer is about what caused the contamination. Derek Pell, a drinking-water engineer for the DOH, said it’s common to never find the source.

Development services inspectors visited all active construction sites on the Island and didn’t note any problems, Boettcher said.

There were questions about whether an incident at the high school – in which mortar dropped into the sewer line caused a sewage backup – could have been the source of the contamination, but there seems to be no connection, said Terry Smith, utilities operation manager. The school district had its backflow device retested, and it passed.

After inspections of city facilities revealed no abnormalities, crews started looking into cross connections and residential backflow prevention devices. Twenty-four devices were identified as risks, and another few hundred are technically out of compliance, Boettcher said.

There may be some devices that the city doesn’t know about, Bassett said, though Pell said the city has the “authority and responsibility” to make sure those devices are tested annually.

Pell said that there are hypotheses about what’s going with the pipes, involving possible corrosion and biofilms – which may form inside drinking water distribution systems similar to the way in which plaque forms on teeth. Biofilms exist even in healthy systems. He said Mercer Island’s situation is “new ground” for the DOH, which usually only finds E. coli issues in unchlorinated systems.

“I believe your staff is doing a really great job of trying to find it, but 98 percent of your facilities are buried,” Pell said. “If you can’t find it and fix it, then you need to have a second barrier in place to protect the public health.”

That second barrier is a robust disinfection process.

Pell said that the city should have higher chlorine residuals to take on contaminants, but that there’s no universal standard for the amount of chlorine that should be in a system. There are industry standards for procedures for disinfecting, he said, which involve “aiming to maintain a certain amount of chlorine residual for a certain amount of time in order to kill a certain kind of organism,” and are dependent on pH and temperature.

“You can do that pretty clearly in a pipe loop in a lab,” Pell said. “If you’re out in the field with 50-year-old cast iron pipes, you are working with some approximations.”

Bassett said that just boosting the chlorine levels could be “masking the problem” if there are issues with cross connections or other aspects of the system.

“It’s more complicated than just saying if we get to a chlorine level of 1.2, problem solved,” he said.