King County Cedar Hills Landfill

Facility Site ID: 2020 Cleanup Site ID: 7027

Site Location

Photo Provided by King County Solid Waste Division
Photo Provided by King County Solid Waste Division

The Cedar Hills Regional Landfill is one of the largest municipal solid waste landfills in the Pacific Northwest and serves King County, exclusive of the cities of Seattle and Milton. The landfill occupies 920 acres.

Responsible Agencies

  • Public Health – Seattle & King County (PHSKC) regulates the permitting, operation, and groundwater monitoring of the municipal solid waste landfill in accordance with Chapters 173-304 and 173-351 of the Washington Administrative Code (WAC) and Title 10 of the King County Board of Health Solid Waste Regulations. PHSKC regulates the leachate lagoons at the landfill in accordance with Section 173-350-330 of the WAC. The Washington State Department of Ecology supports PHSKC with the regulation of the landfill. PSHKC also responds to citizen complaints for odor, litter, and landfilling operations.
  • Puget Sound Clean Air Agency (PSCAA) regulates landfill gas emissions under a Title V operating permit as required by 40 Code of Federal Regulations 60, Subpart WWW and odor complaints associated with the landfilling, landfill gas, and leachate operations. PSCAA issues orders of approval to regulate emissions from the Bio Energy Washington (BEW) facility, which is located on site. BEW receives landfill gas emissions and produces pipeline quality natural gas.

Summary Of Landfill Development

Cedar Hills Regional Landfill began operations in 1965 and continues today. Municipal solid waste is disposed in landfill cells, including the South Solid Waste Area (subsequently removed/relocated), the Main Hill, the Southeast Pit, the Central Pit, Area 2/3, Area 4, Area 5, Area 6, Area 7, and Area 8. The landfill cells were permitted under evolving regulations and standards of practice.

The South Solid Waste Area, the Main Hill, and the Southeast Pit were constructed without bottom liners, which is consistent with the practice at the time. The remaining landfill cells were constructed with composite liners consistent with regulations in Chapters 173-304 (effective 11/27/1985) and 173-351 (effective 4/9/1994) of the WAC. Landfill gas extraction systems and engineered geomembrane covers were installed for landfill cells developed before 1985, consistent with current regulations.

Area 8 began accepting waste in July 2019 and is the only active landfill cell. Area 7 was closed with an intermediate cover in 2019 and the final cover is planned for 2021. Areas 5 and 6 were closed with intermediate covers in 2007 and 2009. Additional disposal is planned in Areas 5 and 6 between 2025 and 2027, depending on landfill settlement and the filling rate of Area 8. The remaining landfill cells are closed and will not receive additional solid waste under the current landfill permit.

Main Hill And East Perched Zone (Epz)

KCSWD is independently cleaning up the East Perched Zone on the east side of the Cedar Hills Regional Landfill Main Hill.

Closure and Post-Closure Activities

The Main Hill was closed with a temporary cover in the 1980s and a final geomembrane cover was constructed in 1991. Landfill gas is extracted from vertical and horizontal collectors within the refuse. A leachate collection system was installed retroactively for the Main Hill. The leachate collection system includes lateral toe collectors on the periphery of the refuse area and vertical extraction wells within the refuse.

Landfill gas has migrated from the Main Hill and impacted shallow, perched groundwater.

What is the East Perched Zone?

The East Perched Zone (EPZ) is an area of shallow groundwater east of the Main Hill occurring in compacted low-permeability glacial till and glacio-lacustrine deposits. Perched groundwater in the EPZ is not hydraulically connected to the underlying Regional Aquifer. The thickness of the unsaturated soils between perched groundwater and the regional aquifer ranges from 160 to 280 feet near the EPZ.

Cleanup Activities

KCSWD attempted to control groundwater contamination in the EPZ using a groundwater extraction system. KCSWD operated the system from 1993 to 2010 by extracting groundwater from 29 wells completed in the EPZ. Operations were terminated because of the system was ineffective in the low permeability geological formation.

KCSWD initiated cleanup activities in accordance with the Model Toxics Control Act (MTCA) in 2012 and is performing an independent cleanup action, with oversight from the Department of Ecology.

KCSWD completed an Agency Draft Remedial Investigation and Feasibility Study (RI/FS) for the EPZ in accordance with MTCA in December 2016, and Ecology provided comments in March 2017.

KCSWD is implementing a phased, interim action approach that incorporates components of the draft RI/FS alternatives for the EPZ, with Ecology approval. KCSWD prepared a Phase 1 Interim Action Completion Memo (September 25, 2020) to describe the completed Phase 1 interim activities and identify subsequent interim action phases.

The completed Phase 1 activities include:

  • The 29 extraction wells used in the ineffective groundwater extraction system were decommissioned and replaced with six new groundwater monitoring wells in May and June 2018. Also in June 2018, six new soil gas probes were constructed to evaluate the presence and attenuation of volatile organic compounds in EPZ soil.
  • Precision flow control devices were installed on numerous east Main Hill landfill gas extraction wells in December 2018. KCSWD optimized the quality of extracted landfill gas by sequentially adjusting the precision valves to increase the concentrations of methane and decrease the concentrations of oxygen. The Landfill Gas Optimization Assessment Update (September 1, 2020) summarizes the landfill gas optimization activities and provides recommendations for Phase 2 optimization and considerations for Phase 3 optimization.
  • The Phase 1 Interim Action Completion Memo (September 25, 2020) summarizes landfill gas and groundwater monitoring in the EPZ. Additionally, the Annual Reports for Cedar Hills Regional Landfill are available here: https://kingcounty.gov/depts/dnrp/solidwaste/facilities/documents.aspx#cedar_reports


Planned Phase 2 activities include:

  • KCSWD will continue to optimize the landfill gas system. Landfill gas from older parts of the landfill, including the Main Hill and Southeast Pit, are discharged and treated at the Migration Control Flare. Precision flow control devices will be installed on landfill gas extraction wells beyond the east Main Hill. By optimizing the quality of landfill gas extracted from the larger well field, KCSWD can increase the effectiveness of landfill gas control near the EPZ.
  • KCSWD plans to install additional landfill gas extraction wells within and beneath the refuse near the EPZ.  
  • KCSWD will continue to monitor groundwater quality in the EPZ.

South Solid Waste Area (Sswa)

The SSWA was an unlined landfill cell beneath the Bonneville Power Administration transmission lines that contained refuse from the late 1960s through the 1970s. A final cover was placed in 1989.

What was done at the South Solid Waste Area?

Solid waste was removed from the SSWA to facilitate the construction of Area 6, Area 8, the contaminated stormwater lagoon, and the southwest siltation pond.  

  • In 2003, approximately 350,000 cubic yards of soil and 100,000 cubic yards of refuse were excavated from the northern portion of the SSWA as part of Area 6 development. Approximately 152,000 cubic yards of refuse and soil were placed in Area 5.
  • In 2016, the remaining refuse in the SSWA was excavated as part of the development of the contaminated stormwater lagoon and the southwest siltation pond for Area 8 development. The extent of hazardous and non-hazardous contaminated soils were pre-characterized in April 2016, and the soils were excavated during the summer of 2016. The refuse and non-hazardous soils were disposed into Area 7, and any encountered hazardous soils were disposed offsite in a Subtitle C landfill.
  • The SSWA is no longer considered a refuse area.

What contamination remains from the South Solid Waste Area?

Contaminated soil remains beneath portions of the SSWA, between Area 8 and the contaminated stormwater lagoon. An impermeable cap was constructed over residual soil contamination in 2016. The cap includes a geosynthetic clay liner that prevents water from infiltrating through the contaminated soil and an overlying composite drainage net that drains water away from the contaminated soil.

Report An Environmental Issue

Northwest Region Office
Northwest Region Office
To report an environmental problem or concern, any time,  day or night, use the online reporting form.

Counties: Island, King, Kitsap, San Juan, Skagit, Snohomish, and Whatcom

Emailnwroerts@ecy.wa.gov
Phone: 206-594-0000

Requesting Accommodation

Ecology is committed to providing people with disabilities access to information and services. To request an ADA accommodation, contact Ecology by phone at 360-407-6831 or email at ecyadacoordinator@ecy.wa.gov. For Washington Relay Service or TTY call 711 or 877-833-6341.  Visit Ecology’s website for more information.

​If you speak a non-English language, free language services are available.

Technical Reports 24
Document Title Document Date Document Type
King County Cedar Hills Landfill - Area 7/8 Liner Tie-In, Potentially Contaminated Soils Excavation Technical Memorandum 2/8/2023 Interim Action Documents
King County Cedar Hills Landfill - Initial Investigation Report for May 2018 Leachate Release 6/18/2021 Initial Investigation Report
King County Cedar Hills Landfill - East Perched Zone Phase 1 Interim Actions Completion Memorandum 9/25/2020 Interim Action Documents
King County Cedar Hills Landfill - East Perched Zone Phase I Interim Actions - Fourth Round, August 2019 Soil Gas Sampling Technical Memorandum 9/2/2020 Interim Action Documents
King County Cedar Hills Landfill - East Perched Zone Phase I Interim Actions - Third Round, May 2019 Soil Gas Sampling Technical Memorandum 9/2/2020 Interim Action Documents
King County Cedar Hills Landfill - East Perched Zone Landfill Gas Optimization Assessment Update 9/1/2020 Interim Action Documents
King County Cedar Hills Landfill - East Perched Zone Landfill Gas Optimization Assessment 5/31/2019 Interim Action Documents
King County Cedar Hills Landfill - East Perched Zone Phase I Interim Actions - Second Round, February 2019 Soil Gas Sampling Technical Memorandum 5/30/2019 Interim Action Documents
King County Cedar Hills Landfill - East Perched Zone Phase I Interim Actions - Baseline, First Round Soil Gas Sampling Technical Memorandum 5/30/2019 Interim Action Documents
King County Cedar Hills Landfill - East Perched Zone Phase I Interim Actions - Infrastructure Upgrades Technical Memorandum 8/31/2018 Interim Action Documents
King County Cedar Hills Landfill - East Perched Zone Extraction Well Decommissioning Technical Memorandum 7/26/2018 Interim Action Documents
King County Cedar Hills Landfill - East Perched Zone Infrastructure Upgrades Work Plan 5/1/2018 Interim Action Documents
King County Cedar Hills Landfill - Ecology Comments on East Perched Zones Remedial Investigation and Feasibility Study, Agency Draft 3/27/2017 Site Specific Administrative Document - other (Administrative correspondence)
King County Cedar Hills Landfill - East Perched Zones Remedial Investigation and Feasibility Study, Agency Draft, Appendix E 12/1/2016 Remedial Investigation / Feasibility Study
King County Cedar Hills Landfill - East Perched Zones Remedial Investigation and Feasibility Study, Agency Draft, Appendix D 12/1/2016 Remedial Investigation / Feasibility Study
King County Cedar Hills Landfill - East Perched Zones Remedial Investigation and Feasibility Study, Agency Draft, Appendix C 12/1/2016 Remedial Investigation / Feasibility Study
King County Cedar Hills Landfill - East Perched Zones Remedial Investigation and Feasibility Study, Agency Draft, Appendices A, B, F, and G 12/1/2016 Remedial Investigation / Feasibility Study
King County Cedar Hills Landfill - East Perched Zones Remedial Investigation and Feasibility Study, Agency Draft 12/1/2016 Remedial Investigation / Feasibility Study
King County Cedar Hills Landfill - South Solid Waste Area Contaminated Soil Removal Report 10/1/2016 Interim Action Documents
King County Cedar Hills Landfill - South Solid Waste Area Draft Contaminated Soil Investigation Report 4/1/2016 Site Specific Technical Document - other
King County Cedar Hills Landfill - Response to Ecology's Comments on East Perched Zones Remedial Investigation and Feasibility Study Work Plan, Draft (December 2014) 4/3/2015 Site Specific Administrative Document - other (Administrative correspondence)
King County Cedar Hills Landfill - East Perched Zones Remedial Investigation and Feasibility Study Work Plan 4/1/2015 Remedial Investigation Work Plan
King County Cedar Hills Landfill - East Perched Zones Remedial Investigation and Feasibility Study Work Plan Appendices 4/1/2015 Remedial Investigation Work Plan
King County Cedar Hills Landfill - Ecology Comments on East Perched Zones Remedial Investigation and Feasibility Study Work Plan, Draft (December 2014) 3/3/2015 Site Specific Administrative Document - other (Administrative correspondence)
There may be more documents related to this site. To obtain documents not available electronically, you will need to make a public records request.

Places to see print documents

  • Solid Waste Program - Northwest Regional Office
    15700 Dayton Ave N
    Shoreline, 98133
    This location may only have print documents available during open comment periods.

Contaminants 13

Contaminant Type
Soil
Groundwater
Surface Water
Air
Sediment
Bedrock
Other Contaminant - Base/Neutral/Acid Organics C C S C
Halogenated Organics - Halogenated Organics C C S S
Metals - Metals Priority Pollutants S
Metals - Metals - Other S C S
Halogenated Organics - Polychlorinated biPhenyls (PCB) C C
Non-Halogenated Organics - Petroleum Products-Unspecified S S
Non-Halogenated Organics - Phenolic Compounds S S S
Non-Halogenated Organics - Non-Halogenated Solvents S S S S
Non-Halogenated Organics - Polycyclic Aromatic Hydrocarbons S C C S
Other Contaminant - Conventional Contaminants, Organic S S S S
Other Contaminant - Conventional Contaminants, Inorganic S S S S
Non-Halogenated Organics - Petroleum-Diesel RB B
Pesticides - Pesticides-Unspecified S S S
S
Suspected
C
Confirmed Above Cleanup Levels
B
Below Cleanup Levels
RA
Remediated-Above
RB
Remediated-Below
R
Remediated
This contaminant list was based on our best information at the time it was entered. It may not reflect current conditions at the site.