Gas Works Sediment Area

Floyd|Snider has assisted the City of Seattle (City), through Seattle Public Utilities (SPU), with environmental investigations and remedy planning for sediments in the Gas Works Sediment Area, located offshore of Gas Works Park. Gas Works Park is the former location of manufactured gas, tar refining, and fuel handling facilities, and is located on the north shore of Lake Union, a heavily developed urban lake north of downtown Seattle. This site is highly visible, with intense public and stakeholder interest, due to its location and use as a public park. 

The investigation and remediation of Lake Union sediments offshore from the Gas Works Uplands has been addressed by the City and PSE. Floyd|Snider has assisted the City since 2002 with developing a Remedial Investigation/Feasibility Study (RI/FS) for the cleanup of the Gas Works Sediment Area. This work includes field investigation, scientific evaluation, engineering design, aquatic habitat mitigation, and regulatory negotiations and permitting. Before initiating the RI/FS, Floyd|Snider performed environmental investigations and advanced remedy planning for the Gas Works Sediment Area. Prior to the execution of an Agreed Order between the Washington State Department of Ecology (Ecology), the City, and PSE, Floyd|Snider worked with PSE's consultants to compile existing sediment data, correct errors within Ecology's SEDQUAL database relevant to site data, and delineate a technically defensible Area of Investigation offshore of the park. Floyd|Snider also supported City and PSE efforts to develop methods and strategies for coordinating technical work between the Eastern Study Area, which is led by PSE, and the Western Study Area, led by the City. The Agreed Order formalized this separation into two study areas.

Floyd|Snider developed a Current Situation Report and RI/FS Work Plan that identified data gaps and highlighted the technical and political complexities of the Western Study Area. These complexities include the presence of free-phase tars in the sediments, overlapping sediment impacts from adjacent facilities, and intense public interest due in part to the site's central location and use as a park. Floyd|Snider provided oversight for implementation of the RI/FS Work Plan in Summer 2005. The sediment investigation included 16 surface grabs, 22 environmental cores, and geotechnical sampling and testing. Floyd|Snider then produced a Data Report for submittal to Ecology. Floyd|Snider completed a screening level evaluation of Gas Works Park conveyance systems. This evaluation included a video survey of all accessible conveyance structures within and adjacent to Gas Works Park, and sampling of available and accessible catchbasin solids. Due to changes in the determination of performing party status, Floyd|Snider's role has recently changed on the project to become a support role to the City.


  • Remedial Investigation/Feasibility Study
  • Contaminant Fate and Transport Analysis (DNAPL)
  • Source Control
  • Agency Coordination and Negotiation
  • Remedy Design and Remedial Cost Estimation

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