Superfund Information Systems: Site Profile

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Cleanup Activities

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The 60-acre Midway Landfill site is located in King County, Washington. The former gravel quarry operated as a municipal solid waste landfill from 1966 to 1983. The landfill accepted demolition materials, wood waste and industrial wastes. Landfilling activities contaminated groundwater and air with hazardous chemicals. Following cleanup, operation and maintenance activities and monitoring are ongoing.

The City of Seattle completed a Remedial Investigation/Feasibility Study (RI/FS) pursuant to a consent decree with the State, signed in May 1990. The initial response (concluded by November 1992) included an active gas control system; landfill surface filling and grading; landfill cap installation; storm water detention pond to collect runoff from the surface of the landfill; and a storm water diversion for the Linda Heights Park drain. EPA designated this site as "construction complete" in September 2000. A Record of Decision for Midway was issued by EPA with Ecology’s concurrence in September 2000. The remedy included continuing the interim actions , added groundwater monitoring, and added institutional controls to alert any future buyers of the restirctions on the property.

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What Has Been Done to Clean Up the Site?

The site is being addressed through federal and potentially responsible party (PRP) actions.

EPA has conducted several five-year reviews of the site’s remedy. These reviews ensure that the remedies put in place protect public health and the environment, and function as intended by site decision documents. In the most recent review, EPA deferred making protectiveness determination until additional information can be obtained by sampling groundwater for 1,4-dioxane. Following EPA’s deferral, the site’s potentially responsible party (PRP) performed the required sampling and detected 1, 4-dioxane in groundwater samples. However, no one is using the groundwater; therefore, the exposure pathway is not complete. Following these discoveries, EPA determined that the site’s remedy is protective in the long term.

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What Is the Current Site Status?

The site’s long-term remedy included a gas control system; landfill surface filling and grading; stormwater detention, landfill capping and stormwater diversion; groundwater monitoring; and institutional controls. Remedy construction took place between 1990 and 2000. Groundwater monitoring is ongoing.

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Emergency Response and Removal

Cleanup has also included removal actions, or short-term cleanups, to address immediate threats to human health and the environment. Actions included covering the landfill with silt and fine sand and installing and operating gas migration control wells.

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