Several Mercer Island beaches and public beach access areas will be closed for the next week due to a sanitary sewer overflow at a sewer pump station off of East Mercer Way.

According to the city, its sewer utility workers identified the overflow and quickly halted the discharge into Lake Washington at 9:30 a.m. on April 9.

The city is warning the public that the water is possibly contaminated with sewage.

While conducting a routine check of the city’s Supervisory Control and Data Acquisition (SCADA) system at about 7:30 a.m. on April 9, the city said that workers determined that a blockage occurred between the city’s sewer catch basin and a sewer pump station at around 3 p.m. on April 8.

Public Works Deputy Director Alaine Sommargren said the city was dealing with a pretty severe blockage.

“It looks like it was a mechanical failure in one of the inlets to the pump station and it was compounded by debris which was getting blocked in the line,” she said. “They actually weren’t able to clear that until the afternoon.”

The city explained in a social media post that, “The blockage caused a sewer catch basin to surcharge and emergency relief overflow into a 12-inch diameter pipe, which outfalls into Lake Washington approximately 145 feet offshore. The city estimates the overflow rate was 37.5 gallons per minute over an 18-hour period, resulting in an estimated total discharge of approximately 40,500 gallons of combined stormwater and wastewater.”

After the city swiftly reported the situation to Public Health – Seattle & King County and the state Department of Ecology, public health department officials recommended the week-long closure of Luther Burbank Beach, Fruitland Landing, Mercer Island Boat Launch, SE 40th Street Landing and Bellevue’s Enatai Beach. Mercer Island’s staff contacted the city of Bellevue, which has posted signage and released a public notice.

These spots will be shuttered from now until 3:30 p.m. on April 16.

“Signs will be posted in these locations for at least seven full days to warn the general public against coming into contact with water possibly contaminated with sewage,” said the city, whose staffers posted the signage on the night of April 9. They read: “High bacteria levels in this water may cause illness. No swimming.”

Using door hangers, the city is also notifying the Mercerwood Shore Club, Covenant Shores and residents of homes along the eastern shoreline included in the potential area of impact.

Sommargren said the city’s two-fold response to the situation includes notifying the public of the potential impacts to the water and working closely with the public health department.

“We generally follow their guidelines really closely. They tend to be more on the conservative side, which is good. We want people to avoid potential contact with contaminated water,” said Sommargren, who added that the city doesn’t know if anyone was directly affected by the possible contamination.

The largest closure is a portion of the Luther Burbank Park shoreline, which will be shuttered from the south edge of the park up to where the boiler building stands.

Fruitland Landing, which is another pump station site, is currently closed to the public because of construction. “We want to make sure that the construction workers aren’t touching the water, either,” Sommargren said.

The public health department recommended the week-long closure, according to Sommargren, who added that the public health experts have done testing to understand what the dissipation rate is in these scenarios. The experts have conservatively calculated how long the city needs to wait before the water is safe.

Since the situation arose, Sommargren said city staff has been tirelessly working on identifying the issue and figuring out how to correct it.

For the last couple of years, the city has been working on updating its SCADA system, which allows staff to remotely check out the status of the city’s water and sewer collection systems. The water system element has been completed and they’re at the tail end of implementing the sewer portion.

With the alarm system turned on, “It alerts the city right away (that) there could be an issue here, there could be a blockage, something happening. It will automatically let our staff know you need to check this out, and then they can research what’s happening out here. That vastly increases our response time,” Sommargren said.

The city has also implemented a catch-basin monitoring system to detect the level of waste in the basin. Sommargren said that the sewer pump station off of East Mercer Way tends to become overwhelmed with stormwater runoff and the overflow was perhaps caused by the heavy rain and hail on April 8.

That same station experienced an overflow in the fall of 2023 following a massive storm event.

“Having SCADA now with these systems allows us to really look at very accurate flow data in a way that we weren’t able to before. It provides us with a lot of information we can actually work with to upgrade our pump stations to meet current needs and current demands on the Island,” Sommargren said.