King County Cedar Hills Landfill

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 is located on a 920-acre parcel in Maple Valley.

• King County Solid Waste Division (KCSWD) operates the landfill in accordance with Title 10 of the King County Code and is the lead agency for ensuring that the landfill is in compliance with solid waste, air, and water quality regulations.

• Public Health – Seattle & King County (Public Health) regulates the development, operation, closure, and post-closure care 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 Code. Public Health regulates the leachate lagoons at the landfill in accordance with Section 173-350-330 of the WAC. Public Health, in partnership with KCSWD, respond to citizen complaints for odor, litter, and landfilling operations.

• Ecology’s Solid Waste Management Program supports Public Health in the regulation of the landfill and is the lead agency for implementing the state’s cleanup law, the Model Toxics Control Act (MTCA).
  

• ​Puget Sound Clean Air Agency (PSCAA) regulates the operation of the landfill gas collection and control system and emissions under a Title V operating permit as required by 40 Code of Federal Regulations 60, Subpart XXX. PSCAA issues orders of approval to regulate emissions from the landfill gas-to-energy facility, CHRGE, which converts landfill gas to pipeline quality natural gas. PSCAA responds to odor complaints associated with the landfilling, landfill gas, and leachate operations.

• King County Industrial Waste Program regulates the discharge of landfill leachate to the South Treatment Plant wastewater treatment plant in Renton, WA.

• Ecology’s Water Quality Program regulates the discharge of stormwater from the landfill in accordance with the Industrial Stormwater General Permit and the Construction Stormwater General Permit.

Cedar Hills Regional Landfill began operations in 1965 and continues today. Municipal solid waste is disposed in landfill cells, including the Main Hill, the Southeast Pit, the Central Pit, Area 2/3, Area 4, Area 5, Area 6, Area 7, and Area 8. Municipal solid waste that was disposed in the South Solid Waste Area was removed during development of Areas 6 and 8.

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. Areas 5 and 6 were closed with intermediate covers in 2007 and 2009. Additional disposal is planned in Areas 5 and 6 between 2026 and 2030. Work is underway to develop Area 9.

KCSWD’s development plans for the landfill are discussed on their website: https://kingcounty.gov/en/dept/dnrp/waste-services/garbage-recycling-compost/solid-waste-facilities/cedar-hills-development

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

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. The low permeability of the glacial till limits the recharge of perched groundwater into the deep 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 the system was ineffective in the low permeability geological formation.

KCSWD initiated cleanup activities in accordance with the MTCA in 2012 and is performing an independent cleanup action, with oversight from 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. 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.

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.

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