URGENT ADVISORY: Sewer Lakeline Blockage, Request to Cease Wastewater Discharge

Final Update: Sewer Lakeline Blockage

Lifting of Wastewater Restrictions for Affected Properties

Issued: 02/24/2025, 8:15 p.m.

City of Mercer Island crews have successfully cleared the blockage in the underwater sewer lakeline. The "Cease Wastewater Discharge" advisory is now LIFTED for all properties. Pump Station 10 is back up and running. You may now resume normal usage of showers, toilets, dishwashers, and washing machines.

Property owners who experience any sewer issues overnight should contact the Non-Emergency Line at 425-577-5656. City of Mercer Island Customer Service is available at 206-275-7600 during normal business hours: 8:30 a.m. to 5 p.m. Monday – Friday.

Please note that the Public Health – Seattle & King County advisory to avoid contact with Lake Washington in the impacted area remains in effect for the next seven days as a standard safety precaution.

The City of Mercer Island sincerely thanks the affected property owners for their patience and cooperation throughout the day as crews worked to resolve this issue.

UPDATE: Sewer Lakeline Blockage

Issued: 02/24/2026, 6 p.m.

City of Mercer Island crews are continuing to work this evening to clear the blockage in the underwater sewer lakeline on the north end of Mercer Island.

Some progress has been made since the last update. City crews worked this afternoon to “draw down” pressure on this section of the sewer lakeline by removing wastewater where it had built up due to the blockage. This pressure draw down allowed City crews to open an access point to the sewer lakeline near Covenant Living at the Shores. Opening this access was essential as it put the sewer equipment closer to the likely blockage location. Jetting and clearing from this location will continue into the evening.

At the same time, a second crew has secured a longer, 1,000-foot high-pressure water line and they are currently working westward from an access point near Pump Station 11. They continue to scour the line to free any debris. Engineers onsite have reported that some debris has broken loose and have observed improved flow in the sewer lakeline, although it is still limited and the primary blockage remains.

Because the blockage has not cleared, all property owners between Luther Burbank Park, North Mercer Way, and 97th Ave SE must continue to cease all wastewater discharge. Please STOP any activities that result in wastewater discharge, including running washing machines and dishwashers, and taking showers/baths. Try to minimize toilet flushing. Any discharge into a residence’s private sewer line may result in a back-up and sewage overflow into the residence until the blockage is cleared.

Ceasing wastewater discharge in the affected area is helping City crews resolve this issue faster. The community’s rapid response today has helped limit new wastewater from entering the system allowing for pressure to be successfully drawn down. Thank you for doing your part to help us.

The Mercer Island Community & Event Center (MICEC) is open daily from 8:30 a.m. to 9:00 p.m. for residents needing restroom or shower facilities.

The next update will be published at 10:00 am on Wednesday, February 25 or sooner if the blockage is cleared.

For general questions/concerns until 6:30 p.m. today only, 2/24/2026, contact Customer Service at 206-275-7600. Normal hours are 8:30 a.m. to 5 p.m. Monday – Friday.

After 6:30 p.m.: Call the Police Non-Emergency Line at 425-577-5656 to report sewage backups or other related issues. They will notify after-hours Public Works staff.

URGENT UPDATE: Sewer Lakeline Blockage Status

Issued: 02/24/2026, 3 p.m.

Current Situation: Blockage Remains

City crews and contractors are continuing work to clear the blockage in the underwater sewer lakeline between Pump Station 10 and Pump Station 11 on the north end of Mercer Island. Properties in the area (denoted on the map) are currently unable to discharge wastewater to the sewer system.

Technical Progress:

Crews are working from both ends of the impacted lakeline section to clear the blockage. The work is difficult and time-consuming given that the sewer line is underwater, with limited access points compared to traditional sewer lines located on land. Based on current progress, it is unlikely the obstruction will be cleared today, which means properties in the affected area will continue to be impacted.

Pressure relief from the north: Crews are currently "drawing down" wastewater in the sewer system from the north end of the blockage. This relieves the pressure inside the sewer line, which is necessary to aid crews working to clear the blockage.
Scouring the sewer line from the south: Crews are also working from the south end of the blockage using a high-pressure water jet nozzle (scouring head) to clean the interior of the pipe, clear debris, and draw down the wastewater. Crews report that some debris has been removed from the line, but the main blockage remains. Response teams are rigging longer hoses to reach deeper into the lakeline and continue work to clear the debris.
Pump station shutdown, access near the midpoint of the line: City crews have temporarily shut down valves at Pump Station 10 to drain a portion of sewer line between the station and Covenant Living at the Shores, which is necessary to clear the line so that crews can access the blockage. Once the blockage is cleared, the station will return to normal service.

Continued Request: Cease All Wastewater Discharge

Because the blockage remains, all property owners between Luther Burbank Park, North Mercer Way, and 97th Ave SE must continue to cease all wastewater discharge. Please STOP any activities that result in wastewater discharge, including running washing machines and dishwashers, and taking showers/baths. Try to minimize toilet flushing. Any discharge into a residence’s private sewer line may result in a back-up and sewage overflow into the residence until this blockage is cleared.

Ceasing wastewater discharge in the affected area is helping City crews resolve this issue faster. The community’s rapid response today has helped limit new wastewater from entering the system. Thank you for doing your part in addressing this issue.

Resident Support & Resources

We recognize the significant burden this situation places on property owners in the impacted zone.

Restrooms & Showers: The Mercer Island Community & Event Center (MICEC) is open daily from 8:30 a.m. to 9:00 p.m. for residents needing restroom or shower facilities.
Avoid Lake Washington Water: Due to a discharge of sewage into Lake Washington associated with this incident, Public Health - Seattle & King County has issued an advisory for persons and pets to avoid the waters of Lake Washington in the impacted area (see map by scrolling to the bottom of this page).

Stay Informed

The City is building a Direct Contact List for affected property owners.

To Opt-In: Please email your name and address to customerservice@mercerisland.gov. Note: This list is a public record and may be subject to disclosure requests.

Reach the City of Mercer Island

For general questions/concerns until 6:30 p.m. today only, 2/24/2026, contact Customer Service at 206-275-7600. Normal hours are 8:30 a.m. to 5 p.m. Monday – Friday.
After 6:30 p.m.: Call the Non-Emergency Line at 425-577-5656 to report sewage backups or other related issues.
Next Update: The City anticipates the next update will be available at 6:00 pm and will be posted to the City’s website.

Updated: 02/24/2026, 11:00 a.m.

Area Affected: Properties located in North Mercer Island, from Luther Burbank Park to 97th Ave SE. (see map; scroll for list of affected addresses) Text "SewerFeb2026" to 38276 to be added to a text alert notification system for urgent updates.

Urgent Action Required

Due to a blockage in the underwater sewer lakeline between Pump Station 10 and Pump Station 11 on the north end of Mercer Island, properties in the area denoted on the map are currently unable to discharge wastewater to the sewer system. The City requests all property owners in the affected area immediately cease all wastewater discharge. Every gallon of wastewater discharged—whether from showering, laundry, dishwashing, or flushing—fills the limited capacity remaining in the sewer pipes serving these properties. Because there is currently no outlet, this wastewater has nowhere to go and may begin backing up.

The sewer pipe serving these properties is likely equipped with check valves designed to prevent backflow from the City sewer line. These valves are likely closed due to the blockage in the lakeline. This means that the wastewater from these properties is being “held” in the sewer lines serving the property and has nowhere to go. Once these sewer lines are completely full, wastewater will back up into homes.

Please immediately STOP any activities that result in wastewater discharge, including running washing machines and dishwashers, and taking showers/baths. Try to minimize toilet flushing.

The water distribution system that provides drinking water to your home is NOT affected or tied to the sewer system. Your water is safe to drink.

What The City of Mercer Island Is Doing

City crews are working around the clock to address this complex situation:

Emergency Relief: The City has contracted third-party Vactor trucks to pump out excess wastewater from the system to create temporary capacity while crews are working to clear the blockage.
Access Challenges: The blockage is located within the submerged lakeline with no manhole access, making repairs difficult. City crews have determined the blockage is in a section of the lakeline east of Covenant Shores and have narrowed it to a 1,000-foot section of the line. City crews are working this morning to access the lakeline from an on-shore access point with vactor equipment to remove the blockage.
Facility Closures: For the safety of the public and to reduce the load on this section of the sewer system, all restrooms at Luther Burbank Park are currently closed.

Status Updates: The City understands the severe impact of this situation on property owners and will provide updates as the work progresses to clear the blockage. Another update will be provided as soon as the blockage is cleared or by 3 p.m. if the City is unable to make progress on clearing the line. If you experience a sewer issue at your property or have immediate concerns, please contact our Customer Service team at 206-275-7600.

We appreciate your immediate cooperation in ceasing wastewater discharge at your property while City crews work to resolve this emergency.