Hidden Valley Landfill (Thun Field)

Overview

Hidden Valley Landfill sits on 92 acres in Puyallup, Pierce County, Washington. It operated as a municipal solid waste facility from the mid-1960s until 1998. The U.S. EPA (Environmental Protection Agency) added it to the Superfund National Priorities List (NPL) in 1989. The site accepted solid, demolition, commercial, and industrial waste, along with liquids, heavy metal sludge, and other industrial materials during its operating years.

Groundwater is the affected medium at the site. Seven contaminants of concern have been identified there: 1,4-dichlorobenzene, chloride, iron, manganese, nitrate, sulfate, and total dissolved solids. The site also produced landfill gas containing methane and ammonia. EPA determined these substances pose an unacceptable risk to human health or the environment and targeted them for cleanup.

The landfill was closed in phases from 1989 to 2000. Cleanup actions included installing an impermeable cap, a landfill gas collection system, leachate management, and surface water and erosion controls. Three removal actions took place between 1989 and 1993. The final remedy was selected in 2000, and construction completed that same year. A consent decree was finalized in January 2004 and amended in 2015 when Washington's Department of Ecology revised the groundwater monitoring plan. An environmental covenant restricts release of remaining pollutants, and a fence keeps the public off the property.

The responsible parties are LRI and Pierce County, with oversight from the Washington Department of Ecology. LRI continues to operate groundwater and landfill gas treatment systems and monitor the site. A 2015 periodic review by Ecology found that cleanup actions are protective. Human exposure is currently under control across the entire site, and contaminated groundwater migration is stabilized with no unacceptable discharge to surface water. The site achieved "sitewide ready for anticipated reuse" status in September 2010. The landfill gas system has also been modified to generate electricity for an adjacent compost facility, and one business currently operates on site.

Community members with questions can contact EPA's Community Involvement Coordinator or Remedial Project Manager.