King County Denny Way CSO

Last updated: 2/11/2026

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The comment period we held from January 12 - February 10, 2026 for this Site is now closed. We are reviewing and responding to the the comments we received and may make changes to the documents below. Final version of the below documents will be uploaded to this webpage.  

The King County Denny Way Combined Sewer Overflow site is located along the shoreline of Elliott Bay next to Myrtle Edwards Park in Seattle. Combined sewer overflows (CSOs) are locations where stormwater and sewage mix and are discharged during heavy rainfall events. In the early 2000s, the CSO at this location was closed, and a partial cleanup (known as an Interim Action) was completed; however, additional cleanup is still needed. 
We invited you to review the following documents for the Denny Way CSO site (Site): 

• Agreed Order: This new legal agreement between Ecology and King County requires King County to study the contamination in a remedial investigation, evaluate cleanup options in a feasibility study, and develop a draft cleanup action plan for the Site. 
• Public Participation Plan: A document explaining how people can participate in the cleanup process.

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If you would like to be added to the mailing or email list for this site, contact Meredith Waldref (contact information to the left). 

From the late 1800s to the mid-1900s, sewers discharged raw sewage and stormwater to an outfall at Denny Way. After decades of untreated discharges caused concerns about high fecal bacteria levels in Elliott Bay, major construction projects eventually eliminated all untreated wastewater flows into the bay. 

In the 1960s, the Municipality of Metropolitan Seattle (now King County) built the present system that included treatment plants which collected, transported, and treated wastewater in the greater Seattle area. This helped reduce the amount of wastewater and stormwater discharges to the Denny Way outfall area but did not eliminate them. In the 1970s and 1980s, improvements were made to reduce pollutants entering the combined sewer from sewage and stormwater.

In the early 2000s, a joint effort between King County and Seattle Public Utilities to further control CSO discharges resulted in construction of two new outfalls in the Denny Way CSO area. One of the new outfalls handles treated discharges (on average eight times a year) and the other outfall handles untreated discharges (on average once per year) when there is too much stormwater in the system.

Contamination 
The following contaminants were found in sediment above state cleanup levels, caused by discharges from the CSO: 

• Polychlorinated biphenyls (PCBs)
• Polycyclic aromatic hydrocarbons (PAHs)
• Cadmium
• Copper
• Silver
• Lead
• Mercury

You can learn more about these contaminants by looking at the chart located at the bottom of this webpage. 

Over the past 35 years, King County has performed many investigations and several cleanup actions to understand and decrease sediment contamination levels at this Site. 

• 1990: King County and the U.S. Army Corps of Engineers (USACE) capped three acres of contaminated sediment at the Site with three feet of clean sand. The cap was monitored for 10 years, and monitoring results showed that some areas around the cap required additional cleanup.
• 2007: Ecology and King County entered into a legal agreement called an Agreed Order to perform a partial cleanup called an Interim Action. This interim action was completed in 2008 and removed contaminated sediment which was then replaced by a mixture of clean sand and gravel. This cleanup was purposely scheduled to happen after the 2005 removal of the shoreline outfall . 
• 2007-Present: King County has conducted detailed monitoring of the Site to determine next steps.

Once the new Agreed Order is finalized after this public comment period, King County will be required to do the following: 

• Draft a Remedial Investigation Report: Using the many previous investigations of the Site, King County will prepare a draft report that finds out what contamination exists, where it is, and how it might impact living things. Ecology may require more data collection if there are gaps in our understanding of the Site. 
• Prepare for an Interim Action (if required): It may be necessary to perform an additional partial cleanup. If so, King County will prepare a plan and conduct the partial cleanup.
• Draft a Feasibility Study Report: This will compare options to keep the contamination from harming people or the environment and weighs benefits versus costs of each cleanup option.
• Draft a Cleanup Action Plan: This plan describes Ecology’s selected cleanup option, sets cleanup standards that will protect living things, schedules next steps, and sets requirements for monitoring and maintenance.