King County Denny Way CSO

The site is located along the northeastern shoreline in Elliott Bay and is adjacent to Myrtle Edwards Park at the foot of Denny Way in Seattle, Washington. The Denny Way CSO regulator station is located at 3165 Alaskan Way. The Denny Way historical CSO outfall was located along a riprap bulkhead on the shoreline and was exposed during normal low tide.

The Denny Way Combined Sewer Overflow (CSO) in Seattle was constructed in the 1960’s when Metro (now King County) built the present system of interceptors and treatment plants that collected, transported and treated wastewater in the greater Seattle area. Until 2004, the discharge point of the Denny Way CSO was at the shoreline directly offshore of Myrtle Edwards Park. This outfall was exposed during normal low tide and frequently discharged directly across exposed intertidal sediment.

The proposed project site had been contaminated for nearly 40 years due to the operation of the CSO. During heavy rains, when flows exceeded the capacity of the sewer system, diluted sewage mixed with storm water was discharged from the Denny Way CSO. The site is no longer affected by the discharge due to the completion of a 2005 King County CSO control project, which has diminished the frequency of discharges, treated CSOs locally and relocated the CSO pipe further offshore.

Ecology and King County have developed a legal agreement called an Agreed Order to conduct an interim action to remediate contaminated sediments at this site. The interim action required by the Agreed Order will include dredging contaminated sediments, disposing of the dredged materials in an approved landfill, restoring the habitat, and monitoring sediment quality at the site.

The Public Participation Plan outlines and describes the tools and approaches that Ecology will use to inform the public about site activities, and it identifies opportunities for the community to become involved in this process.