Kent Highlands Landfill

Kent Highlands Landfill (Site) is located at 23076 Military Road South, Kent, Washington.  The Site is situated on the eastern flank of the Des Moines upland, where it adjoins the Green River valley.  The landfill occupies a former natural ravine that extended about 2,500 feet from near the top of the upland down to the valley floor.  The Site is adjacent to the Green River.

The Seattle Solid Waste Utility started landfilling operations at the Site in 1968.  The landfill received mostly municipal garbage until 1983.  After 1983, the landfill also took in industrial wastes and construction debris.  Disposal operations terminated on December 31, 1986.  The City of Seattle was in the process of completing a closure plan at that time.

The Site was placed on the Federal National Priorities List (NPL) on August 30, 1990 for cleanup under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA).  CERCLA sites are known as Superfund sites.  Ecology is the lead agency for cleanup of the Site as stipulated by an agreement with Region 10 of the Environmental Protection Agency (EPA).  Accordingly, cleanup at this Site was implemented under the Model Toxics Control Act (MTCA) regulations, Chapter 173-340 Washington Administrative Code (WAC).

Cleanup activities at this Site were conducted under a Consent Order between Ecology and the City of Seattle, as executed on May 26, 1987.  Per the 1993 Cleanup Action Plan, the remedy included the following components: access control, site grading, landfill cover, stormwater conveyance, leachate control, and landfill gas control.

The cleanup actions were necessary because of high concentrations of landfill decomposition gas, and leachate with high specific conductance, high chemical oxygen demand, and high concentrations of ammonia and iron.  Major metals detected were iron, zinc, and manganese.  Volatile organic compounds detected were primarily ketones, aromatic hydrocarbons, and chlorinated hydrocarbons.  The primary semi-volatile organic compounds were low molecular weight polycyclic aromatic hydrocarbons, alkyl phenols, benzoic acid, and chlorinated benzene.  The presence of the volatile and semi-volatile organic compounds was consistent with the disposal of household products in the landfill.  Contaminants remaining at the Site exceed MTCA cleanup levels. 

The 2019 periodic review focused on three areas:
(1) activities undertaken by the City of Seattle in response to the previous periodic review,
(2) actions that need to be performed or modifications that need to be considered during the next five-year period, and (3) a protectiveness discussion.

 




The remedy required by the Cleanup Action Plan at the landfill remains protective of human health and the environment with respect to refuse encapsulation, landfill gas control, stormwater quality maintenance, and leachate capture. Vinyl chloride continues to exceed regulatory values in groundwater, which discharges to surface water (the Green River). There is no current use of the river for drinking water adjoining the landfill and vinyl chloride dissipates quickly in surface water. Therefore, the landfill failed the periodic review because of regulatory value exceedances, but additional information is necessary to determine if further remedial action is necessary. Ecology and the City of Seattle are continuing to monitor and manage the landfill and are discussing the next steps to address the exceedances.

The Environmental Covenant for the property is in place and appears to continue to be effective in protecting public health and the environment from exposure to hazardous substances by ingestion and direct contact, as well as protecting the integrity of the existing remedy.

Actions that need to be performed and modifications that need to be considered during the next five-year period are summarized in the complete periodic review document.