Seattle DOT Mercer Parcels
800 Mercer St, Seattle, WA 98109

Seattle DOT Mercer Parcels – Aerial view of Site and parcels.
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This Site is part of a multi-property transaction associated with the Mercer Mega-Block Development project in Seattle’s South Lake Union neighborhood. The Seattle DOT Dexter Parcel Site is also part of the property transaction and development project. Please see the web page for Seattle DOT Dexter Parcel for details and documents related to that site. 

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Ecology is hosting a public comment period from December 6, 2021 – January 19, 2022 to get input on the following cleanup documents for the Seattle DOT Mercer Parcels Site:

Prospective Purchaser Consent Decree (PPCD) – legal agreement that directs the Prospective Purchaser to implement cleanup of the Site in accordance with the Cleanup Action Plan (below) once it has completed its purchase.  

Remedial Investigation (RI) – describes contamination at the Site. To view RI appendices, please click the 'View Electronic Documents' link to the right.

Focused Feasibility Study (FFS) – evaluates and recommends a permanent cleanup action as the preferred alternative.

Draft Cleanup Action Plan (dCAP) – describes Ecology’s chosen cleanup plan and specifies cleanup standards and requirements for the Site.

State Environmental Policy Act – Determination of Non-Significance (SEPA) – evaluation and determination that the proposed cleanup at the Site is not likely to cause harm.

State Environmental Policy Act - checklist

Public Participation Plan (PPP) –  describes how Ecology will engage with the community and keep the public informed.

Submit your comments

View our Factsheet (English)

Online public meeting

On January 6, 2022, Ecology held an online public meeting to present on the cleanup projects.  You can view the slides for this presentation here:

Public presentation slides

Current view of Site looking northeast from the intersection of Mercer St and Dexter Ave N.
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The Seattle DOT Mercer Parcels Site is located in the South Lake Union neighborhood of Seattle. The property where the Site is located (Property) consists of two land parcels totaling 2.35 acres in size and is bounded to the north by Roy St, to the south by Mercer St, to the east by 9th Ave N, and to the west by Dexter Ave N. The Property contains the Site.

Site History

Past uses of the Property included auto repair and wrecking shops, fueling and service stations, a sign painting and retail painting business, a soap and chemical works, and private residences. During the 2010s, construction occurred to reroute the Broad Street underpass and facilitate Mercer Corridor improvements. In 2017, an investigation conducted as part of this project discovered contamination in soil and groundwater beneath the Property. More recently, the Property was a construction staging area. It is now vacant. 

Image of proposed redevelopment. Credit: Alexandria Real Estate Equities, Inc.
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Future development plans include constructing two 13-story towers separated by a right-of-way. The two separate towers will share a below-grade parking garage. The buildings will be occupied by biotech and life science companies, with commercial space and public amenities on the ground level. Redevelopment is expected to begin in 2022 and is expected to be completed by late 2024.


Investigations at the Site found the following contaminants in soil and/or groundwater:

•Gasoline-range petroleum hydrocarbons

•Diesel-range petroleum hydrocarbons



These contaminants are present at levels that exceed allowable standards under the state’s cleanup law, the Model Toxics Control Act (MTCA), and will be addressed during the Cleanup Action.

Other hazardous substances are present on the Property at levels that exceed MTCA cleanup standards but are not considered constituents of concern for the Site. These substances include carcinogenic polycyclic aromatic hydrocarbons (cPAHs), arsenic, and chlorinated solvents, and are associated with separate contaminated sites in the vicinity of the Property. The proposed cleanup action for the Site and planned redevelopment of the Property will concurrently address portions of these other sites.


The selected cleanup action for the Site meets standards for permanence, protectiveness, implementability, public concerns, compliance with cleanup standards and state and federal laws, and ability to provide compliance monitoring. While nearby sites that may impact this redevelopment are being investigated and remediated, vapor mitigation measures beneath the new structures will be installed to protect human health and the environment. The selected cleanup action for the Site consists of the following:

•Excavating all contaminated soil within the planned redevelopment excavation required for construction of the new buildings to an approximate elevation of at least 7.75 feet (approximately 26 to 51 feet  below ground surface or bgs).

•Hauling the excavated contaminated soil to an off-site facility for treatment and/or disposal.

•Performing compliance monitoring.

View of the Property showing historical locations of structures and streets, areas of contamination, and the proposed area of excavation. Petroleum contamination is in red. cPAHs and arsenic contamination are in orange. Lead contamination is in gray.
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The Seattle DOT Mercer Parcels is part of the South Lake Union neighborhood, which is located next to downtown Seattle and bordered on the north by Lake Union. This area has a long history of industrial and commercial activities which caused contamination of the soil and groundwater. Currently, this area is undergoing extensive redevelopment as new buildings are constructed for commercial and residential use. Ecology is working with property owners to clean up contamination present at the Site and other nearby sites to reduce impacts to human health and the environment. 


The Model Toxics Control Act (MTCA; Chapter 70.105D RCW is Washington’s environmental cleanup law). It provides requirements for contaminated site cleanup and sets standards that protect human health and the environment. Ecology enacts the MTCA and oversees cleanups. The MTCA site cleanup process is completed in steps (see graphic below) over a variable timeline.

Please click on the photo for a larger view


Map showing site location as King County, WA SITE INFORMATION


View Electronic Documents

Cleanup Site Details Report

Facility Site ID: # 27913

Cleanup Site ID: 14784

Seattle, King County

Status: Cleanup Started   Get definitions of Status terminology

Tena Seeds
Site Manager

Document Repositories:

Northwest Regional Office
15700 Dayton Ave N
Shoreline, 98133