Cleaning up: How a light rail project keeps a landfill site on track

Grassy field, fencing on left, interstate with cars driving on right of field.

A grassy meadow covers the Midway Landfill, next to the I-5 freeway southbound lanes between Seattle and Tacoma.

How do you build light rail and widen a freeway along the edge of a well-managed former municipal waste landfill? Very carefully! 

And, with plenty of cooperation among public agencies.

We’re working to do exactly that with Sound Transit, the Washington State Department of Transportation (WSDOT), and Seattle Public Utilities (SPU).

The site: a former landfill

The City of Seattle’s 60-acre Midway Landfill site adjoins the west side of I-5 in Kent. The landfill closed in 1983. Under our oversight, and review by the U.S. Environmental Protection Agency, SPU:

  • Covered the landfill with an engineered, multilayered waterproof cap and a top layer of grass.
  • Installed a gas extraction system to control methane generated by material in the landfill.
  • Controlled surface water.
  • Fenced the landfill to limit access to the site.

These protective elements have been in place since 1992. SPU continues to monitor groundwater quality and landfill gas at the site, under a legal agreement with us, last updated in 2006.

Transportation projects

Map of the landfill, red border surrounding landfill, shows interstate to right and roads and buildings to left.

Sound Transit will build light rail on the landfill's east side,
along I-5's southbound lanes.

Two planned south King County transportation projects will run half a mile through the eastern edge of the landfill. Sound Transit plans to extend its Link light rail line from Angle Lake to Federal Way. WSDOT plans to widen a stretch of I-5 as part of its SR 509 Completion Project.  Preparation of the landfill for the two projects will be combined into a single project called FWLE/SR 509 Midway. FWLE stands for Federal Way Link Extension.
FWLE/SR 509 Midway will bring changes to the site. We’re developing legal agreements and engineering plans with all three parties — SPU, WSDOT and Sound Transit — to ensure that the site’s environmental and public health and safety measures continue to function during and after construction.

The combined project will involve excavating some of the landfilled municipal waste, and will affect the landfill cap, the gas collection system, and surface water monitoring network. Because of this, we’re requiring:
  • Transfer of excavated municipal waste to an authorized landfill.
  • Restoration of disturbed portions of the landfill cap and other infrastructure.
  • Documentation of all required work for our review and approval.

Some of the site’s land ownership will change. Sound Transit will acquire part of a strip — for the light rail tracks — that now belongs to WSDOT and will assume responsibility for maintaining that portion of the landfill cap. Seattle will continue to operate surface water controls, the gas extraction system, and the ongoing monitoring program.

Plans and agreement available

We’re asking the public to review and comment on four documents that will govern how the FWLE/SR 509 project will be built, and how the site will be reconfigured:
  • Consent Decree Amendment:  update to our existing legal agreement with SPU.
  • Prospective Purchaser Consent Decree:  new legal agreement between us and Sound Transit.
  • Cleanup Action Plan Amendment:  describes actions we will require to maintain the integrity of the site’s protective elements during and after the proposed construction.
  • Public Participation Plan: explains how people can participate in the cleanup process.

Comments invited; public meeting planned

Image of grass field, fencing to landfill on right, interstate with cars driving on left.

Looking south across the Midway Landfill where it adjoins I-5.

The comment period begins on Jan. 27 and concludes on Feb. 25, 2020. Comment online or mail comments to:
Mark Adams
Site Manager; Dept. of Ecology
3190 160th Ave. SE 
Bellevue WA, 98008-5452
We’re also inviting the public to a meeting and hearing. We, SPU, Sound Transit and WSDOT will give a short presentation and answer questions about the site and the projects. There will be time for giving oral comments.
  •  7 p.m. to 9 p.m. Feb. 11, 2020
  • Des Moines Elementary School, 23801 16th Ave. S.
  • Interpreter services will be available in Spanish, Korean, and Somali.

 We’ll review and respond to all comments received. We expect the new plans and agreements to be in place by mid-2020.