The Seattle Municipal Landfill operated from 1968 to 1986 on a 90-acre former ravine in Kent, Washington, about 14 miles south of Seattle. The landfill accepted municipal garbage as well as industrial wastes, paint residues, and sludge. It was added to the National Priorities List (NPL) in 1990. The NPL is the federal government's list of the most serious hazardous waste sites in the country.
The landfill contaminated groundwater in two aquifers, the Sand Aquifer and the Recent Alluvium Aquifer. The Recent Alluvium Aquifer connects to the Green River, though the river itself showed no observed impacts. Surface water in nearby Midway Creek was also affected. The main health risks come from ingesting or touching contaminated groundwater or leachate, or breathing landfill gas.
The Washington Department of Ecology selected a long-term cleanup plan in 1993. That plan included an engineered cap to isolate waste, a gas collection system, carbon adsorption to treat vapors, drainage systems, leachate extraction, and institutional controls that restrict land use. The City of Seattle completed construction of these measures in 1995. Five-year reviews have been completed in 1998, 2003, 2009, and 2014 to check whether the cleanup continues to protect people and the environment. A 2003 review found the remedy protective in the short term, and additional treatment options for groundwater and surface water were being evaluated at that time.
The site reached "sitewide ready for anticipated reuse" status in August 2011, meaning cleanup goals have been met under current and foreseeable land uses. Human exposure is currently under control, and all required land-use restrictions are in place. However, contaminated groundwater is still migrating and has not yet stabilized. EPA continues to monitor the site to track conditions and confirm that the remedies are working. The site has not yet been deleted from the NPL.
Community members with questions can contact EPA's Remedial Project Manager or the Community Involvement Coordinator. For state-related questions, contact the Washington Department of Ecology.
