Midway Landfill is a 60-acre former gravel quarry in Kent, Washington that accepted municipal solid waste, demolition materials, wood waste, and industrial wastes from 1966 to 1983. EPA added it to the National Priorities List in June 1986. The site is organized into two operable units: a sitewide unit and a remedial unit. The primary contamination concerns are three chemicals found in groundwater at the remedial unit: 1,2-dichloroethane, chloroethene (also called vinyl chloride), and manganese. Health risks come from three main pathways: ingesting or touching contaminated groundwater, and breathing contaminated landfill gas.
Cleanup began under a consent decree with the State of Washington signed in May 1990. Between 1990 and 1992, workers installed an active gas control system, filled and graded the landfill surface, placed a cap over the site, and built stormwater detention and diversion systems. EPA declared construction complete in September 2000, when it also issued a Record of Decision selecting institutional controls, groundwater monitoring, and onsite containment as the long-term remedy. The site reached sitewide ready for anticipated reuse status in January 2007. More recently, construction for the Federal Way Link Extension light rail began on the east perimeter in 2020, and the Washington Department of Transportation is using part of the eastern perimeter for an I-5 road-widening project.
Human exposure is currently under control, and contaminated groundwater has been stabilized in its original area with no unacceptable discharge to surface water. Required land-use restrictions are in place. However, a 2015 five-year review deferred a protectiveness determination pending additional groundwater sampling for 1,4-dioxane. That sampling was performed, and 1,4-dioxane was detected in groundwater. A fourth five-year review in 2020 documented those findings, and groundwater monitoring remains ongoing. EPA completed a fifth five-year review on August 20, 2025. Remedial action for the remedial unit has not yet started, and the site has not been deleted from the National Priorities List.
Community members can get involved through EPA's five-year review process. EPA reviews Superfund sites every five years when contaminants remain on site to confirm the cleanup continues to protect human health and the environment. For questions, contact the EPA staff assigned to this site. The Washington Department of Ecology can be also contacted for additional information, including details about the Consent Decree Amendment and Cleanup Action Plan Amendment.
