Gas Works Stormwater Drainage Investigation

Floyd|Snider is assisting the City of Seattle (City), through Seattle Public Utilities (SPU), with source control and stormwater investigations in the vicinity of Gas Works Park. The source control and stormwater work is being conducted in cooperation with the Gas Works Sediment Area investigation and remedy planning. The source control and stormwater work will identify potential sources of recontamination of the sediments that will be remedied. 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.

Floyd|Snider is currently providing the scientific and engineering expertise to improve source control within this area. As part of this project, Floyd|Snider, in conjunction with SPU, prepared a Joint Source Control Evaluation (JSCE) Report that evaluated businesses and activities within the stormwater drainage basins that discharge into Lake Union within the vicinity of Gas Works Park. Preparing this report included working with SPU’s Business Inspection Program to inspect and identify potential sources of recontamination from businesses and provide recommendations and resources for business activity improvements. The JSCE Report also evaluated stormwater drainage basins within Gas Works Park, identified known areas of contamination, and provided recommendations for additional work.

Floyd|Snider is currently working with SPU on implementing the additional recommended work. This work includes the evaluation and investigation of the stormwater conveyance systems in the vicinity of Gas Works Park, including areas that have outdated as-built records and are over 90 years old. The stormwater conveyance system investigation has included video inspection of the conveyance system, collection analysis of solids from within the conveyance system, analysis of typical urban runoff, and the use of as-builts, smoke tests, and metal detectors to identify missing and buried structures and expose them. This work has not only assisted Floyd|Snider with its source control investigation, but has also allowed SPU to conduct necessary maintenance on the structures to provide unimpeded stormwater conveyance to the City.

Throughout this process, Floyd|Snider has worked with the Washington State Department of Ecology and Puget Sound Energy. Based on the results of the initial investigation, an additional investigation was planned for the winter of 2009/2010, providing further information to assess the potential of recontamination of the Gas Works Sediment Area.


  • Stormwater System Investigations
  • Catch Basin Sample Collection, Analyses, and Data Interpretation
  • Agency Coordination and Negotiation

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