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King County believes cleaning up the Lower Duwamish is the right thing to do.

The County has worked for decades to restore the waterway by investing in habitat restoration, water quality improvement, pollution controls and sediment cleanups. These investments in pollution control and CSO reduction projects have successfully removed decades of contamination from some of the waterway's most contaminated areas.

Duwamish Waterway timeline

The Duwamish is cleaner today than it has been in nearly a century. Highlights of King County's efforts to restore the waterway to health include:

1958: Voters created Metro (later to merge with King County) a regional, wastewater agency, and authorized Metro to build and operate regional sewage treatment and water quality facilities to address the pollution of Lake Washington and other local waters

1958: Duwamish River water quality studies: Metro begins multiple water quality studies in the Green/Duwamish River.

1963: Metro finds that levels of dissolved oxygen in Green/Duwamish River to be dangerously low for fish. Summer levels of dissolved oxygen in the Green/Duwamish River were far below minimum requirements for fish because of discharges of untreated wastewater and stormwater over the previous 50 years.

1964: A 17-year Green/Duwamish River water quality study begins: Metro begins 17 years of continuous data collection on water quality.

1965: Sewage and stormwater from along the Green/Duwamish River was sent to a sewage treatment plant as Metro completes South Plant in Renton. Treated wastewater from the plant was then discharged into the Green River.

1967: Metro completes the East and West Marginal Way Interceptor Sewers, which diverts industrial wastewater from the Duwamish to the West Point Treatment Plant.

1968: Voters approve spending $1.4 million to separate the storm and wastewater systems and improve drainage. Separated sewers and improved drainage reduced the volume of stormwater and wastewater discharged directly to the waterway.

1969: Metro launches the Industrial Waste Pretreatment Program regulating industries to remove pollution before discharging their business wastewater into the sewer system.

1969: The Diagonal Way Treatment Plant was closed, eliminating its discharge of partially treated wastewater to the Lower Duwamish Waterway.

1972: The federal Clean Water Act becomes law; it is the cornerstone of surface water quality protection in the United States.

1973: The Kent lagoons are closed and wastewater is transferred to the South Treatment Plant in Renton, no longer going into the Green/Duwamish.

1976: Studies showed that dissolved oxygen concentrations in the Green/Duwamish River had greatly improved since the 1960’s when Metro began improvements on the Duwamish. Dissolved oxygen concentrations had increased by a factor of 4 since the 1960s.

1977: The Auburn lagoons were closed and wastewater was transferred to the South Treatment Plant in Renton, no longer going into the Green/Duwamish.

1978: Metro’s first water comprehensive quality plan – Metro issues the first comprehensive water quality plan for the Cedar and Green River basins.

1980: Metro joins the first program in the country to target consumer products as a source of toxic chemicals. Metro and area jurisdictions begin planning the Household Hazardous Waste Disposal Program.

1980: Duwamish dissolved oxygen concentrations continued to improve - increasing from 4 milligrams per liter (mg/L) to 7 mg/L since the 1960’s.

1983: Metro issued the Duwamish Clean Water Plan, addressing pollution problems in the lower river.

1985: Elliott Bay Action Program: EPA and Ecology, working with City of Seattle, Metro, the Port of Seattle, and others began the Elliott Bay Action Program, a large multi-year comprehensive program focusing on toxic sediment contamination, its sources, and solutions for cleanup in Elliott Bay and the Lower Duwamish Waterway. This program was part of the Urban Bay Action Program of the multi-agency Puget Sound Estuary Program.

1986: Metro began work on a new outfall for the South Treatment Plant in Renton so that treated wastewater discharges could be diverted from the Green River into Elliott Bay.

1986: A $1.1-billion secondary treatment/combined sewer overflow (CSO) control plan: Metro amended the Comprehensive Sewerage Plan by adopting a $1.1-billion secondary treatment/combined sewer overflow control plan to help reduce the impacts of a growing population in the central Puget Sound area.

1987: Metro and the City of Seattle completed the Hanford separation project, reducing combined sewage (a mix of sewage and stormwater) flows to the Diagonal Way storm drain (and into the Duwamish River) by two-thirds.

1990: Dissolved oxygen concentrations in the Duwamish had increased to 10 times the levels of the 1960s, greatly improving fish health.

1991: Elliott Bay/Duwamish Restoration Program (EBDRP) is established: A federal consent decree established the program, with the City of Seattle and King County required to spend approximately $24 million on habitat restoration, sediment cleanup, and source control projects.

1992: Metro’s Lander Sewer Separation Project added 1.4 million gallons of storage capacity to further reduce combined sewer overflows into the Lower Duwamish.

1994: Metro merged with King County, and the King County Wastewater Treatment Division was created.

1996 – 1998: King County undertook an extensive study: “King County Combined Sewer Overflow Water Quality Assessment for the Duwamish River and Elliott Bay,” to better understand the dynamics of the estuary and the impacts of CSOs relative to other pollution sources. The study team included scientists, planners, engineers, and other professionals; a stakeholder committee (composed of representatives from local communities, businesses, environmental organizations, tribal governments, and agencies); and a national peer review panel.

1999: King County’s Regional Wastewater Services Plan is approved – including extensive public involvement and review, the 30-year plan was approved for work to control many county CSOs in Seattle, including five that required control in the Lower Duwamish Waterway. Five of those projects began in 2012.

1999: Norfolk combined sewer overflow sediment remediation project: King County removed 5,190 cubic yards of contaminated sediment near the county’s Norfolk CSO. The project was monitored for a period of five years. The EPA, Ecology, King County and City of Seattle worked together on source control, project planning and design for the project.

1999 – 2000: Hamm Creek Habitat Restoration Project: together with the U.S. Army Corps of Engineers, King County created 2,300 feet of new riparian stream bed and channel for Hamm Creek. EBDRP designed and monitored a one- acre estuarine marsh with fish-passable connection to the Duwamish Waterway. The enhanced freshwater Hamm Creek channel featured meanders, fish pools, and large woody debris. The intertidal habitat was planted with native estuarine marsh vegetation in spring 2000.

2000: Herring's House Park and Intertidal Habitat Restoration Project. This project was completed in 2000, except for monitoring, which continues. The work included removal of mill structures, a shoreline dock, and contaminated sediments. A 1.8 acre intertidal bay was created, with fringing emergent vegetation in an intertidal zone. Riparian vegetation was also planted to create a riparian buffer.

1999 – 2000: North Wind's Weir Restoration Project: In a project at Cecil B. Moses Park, a one-acre basin was created to provide off-channel habitat for out-migrating salmonids. Emergent and riparian vegetation was planted.

2000: The Lower Duwamish Waterway Group was formed to investigate Duwamish sediments: The Boeing Company, the City of Seattle, King County, and the Port of Seattle (the Lower Duwamish Waterway Group or LDWG) completed a voluntary agreement to begin investigation of the Lower Duwamish Waterway sediments, toward an ultimate waterway cleanup plan.

2005: King County's Diagonal/Duwamish dredging removed 66,000 cubic yards of contaminated sediment from the Lower Duwamish. The project was monitored for a period of seven years.

2006: Turning Basin No. 3. Restoration Project. This project is located on the former Kenco Marine Services property. An old building, dock, and grounded barges were removed. Fill material was removed to push back the shoreline. Marsh and riparian vegetation were planted. The project was completed in April 2006, with monitoring for intertidal habitat success conducted through 2015.

2006: The Brandon Street Regulator Station was upgraded to reduce overflows from the combined sewer overflow. A project for full control began in 2013

2007: King County reduced LDW combined sewer overflows from 834 million gallons per year in 1990 to about 78 million gallons in 2007.

2007: Remedial Investigation was submitted to EPA and Ecology and released for public review. The final Remedial Investigation was approved in 2009.

2007: Baseline ecological and human health risk assessments were finalized for the LDW.

2007: The City of Seattle completed habitat restoration south of the Duwamish substation on the west side of the Upper Turning Basin.

2008: The City of Seattle, The Boeing Company, and King County signed an agreed order with Ecology to conduct a Remedial Investigation and a Feasibility Study for North Boeing Field, which drains to Slip 4.

2009: A Draft Feasibility Study was submitted to EPA and Ecology and released for public review. The final Feasibility Study was approved in 2012. 

2009: Construction began by the City of Seattle to clean and replace the Georgetown Steam Plant flume that leads to Slip 4.

2011: The City of Seattle completed the Slip 4 cleanup and habitat restoration.

2011: King County began the $19 million Rainier Valley Wet Weather Storage Facility (formerly known as Hanford #1 CSO control) project to control overflows to the Diagonal/Duwamish outfall. The project was completed in 2018.

2012: King County updated the Combined Sewer Overflow Control Plan, which refined the CSO projects to control the remaining CSOs in the waterway. This included two projects at a cost of more than $158 million, beginning in 2013; and two projects in the East and West Waterways at a cost of $332 million, beginning in 2016.

2013: King County completed an Equity Impact Review   on the cleanup alternatives being considered for the waterway to help inform the County’s position on a preferred cleanup. The review assessed effects of the cleanup which will likely inform selected ‘determinants of equity’ for those who live and work in the LDW adjacent to the cleanup activities and those who depend on or utilize the river for fishing and recreation.

2013: The U.S. Environmental Protection Agency (EPA) announced its proposed cleanup plan for the Lower Duwamish Superfund site in February 2013. The public comment period closed on June 13, 2013. The Lower Duwamish Waterway Group (LDWG) submitted a detailed set of comments reflecting the group's key goals for the cleanup.

2014: The EPA issued the Final Cleanup Plan (Record of Decision)  in November 2014. The Record of Decision (ROD) directs cleanup actions and offers plans for source control and long-term monitoring in the Lower Duwamish Waterway.

2014: King County began the siting process for its Georgetown Wet Weather Treatment Station (formerly known as the "Brandon/Michigan CSO Control Project"), which will keep up to 70 million gallons of untreated stormwater and sewage out of the Lower Duwamish River on rainy days. The treatment station is scheduled to be operational in late 2022. 

2014: Lower Duwamish Waterway Group began a Fisher Survey to better understand the habits of community members who fish in the river, specifically what they are eating and how they use their catch. The information will be used in the Superfund Cleanup process to develop tools to protect people eating fish. The study was completed in 2016.

2014: Lower Duwamish Waterway Group began a pilot study to test the effectiveness of activated carbon as a tool to clean up historic contamination in Lower Duwamish Waterway sediment. Study results plus tribal and community input will help the EPA decide if this technology could be used to reduce contamination levels in the Lower Duwamish Waterway. The activated carbon was placed in the waterway at the end of 2016 and monitored through 2020.

2014: King County adopted its Source Control Implementation Plan. The five-year plan identifies all the actions the County took in 2014-2018 to implement to control sources from its properties and drainage systems, regulate discharges to the combined sewer, and provide technical assistance to businesses.

2015: Boeing completed the Plant 2 sediment cleanup and habitat restoration.

2015: The City of Seattle and Port of Seattle completed the Terminal 117 sediment cleanup. The habitat restoration was completed in 2020. This marks the completion of all the Early Action cleanups by the Lower Duwamish Waterway Group, which reduced the PCB sediment concentrations in the waterway by half – a big step to completing the cleanup.

2016: Baseline sampling and other pre-design studies began. The Lower Duwamish Work Group study established baseline conditions in water, sediment, and seafood following the early actions; and evaluated the effectiveness of those cleanups, natural recovery of sediments, and source control towards achieving the cleanup goals. The data from these sampling and studies informed the cleanup design.

2017: Duwamish Estuary Water Quality assessment and monitoring study was published. This report summarizes existing information on water quality, sediment chemistry, and fish/shellfish tissue in the Duwamish Estuary, described current conditions, identified long-term trends, and reviewed compliance with appropriate environmental quality standards.

2019: The Lower Duwamish Waterway Group began cleanup design for the upper third of the waterway. Extensive sampling was conducted to gather the information needed to refine the areas needing action and develop designs for the cleanup. Once design is complete in 2023, the cleanup will begin simultaneous with design for the rest of the cleanup.

2019: King County updated its Source Control Implementation Plan. The next five-year plan identifies all the actions the County will take in 2018-2023 to implement to control sources from its properties and to its drainage systems, regulate discharges to the combined sewer, and provide technical assistance to businesses.