Urban Park Cleanup

Former Manufactured Gas Plant – Seattle, WA

GWP from EastCRETE principals performed cleanup and restoration of an urban park and prepared an RI/FS for the lake sediment. The 25-acre public park was created in the mid-1970s and retained many of the former industrial structures.

The upland remedy included tar removal, soil capping, an air sparging system along portions of the shoreline to remove benzene from shallow groundwater that discharges to the lake, and upland and shoreline habitat restoration. The selected remedy allowed continued use of the park without significant disruption. Design of the remedy was negotiated with numerous stakeholders included Seattle Design Commission and Historic Landmarks Board. Features such as a Corten steel equipment enclosure were used to maintain the historical aesthetic, and the cap design maintained the land sculpture topography of the park. MTCA remediation levels for PAHs and benzene were negotiated such that the air sparging system met performance criteria in 2 years.

CRETE principals analyzed the range of cleanup approaches for sediment. Probabilistic cost modeling provided a likely range of cleanup costs which were used in insurance cost recovery negotiations.

CRETE was involved in preparation of a draft RI/FS for the cleanup and restoration of subaqueous sediment and the adjacent shoreline. Historical MGP, shipyard, and other industrial operations impacted the sediment with PAHs, metals, and other contaminants. CRETE principals helped develop soft-sediment capping solutions that are resistant to frequent boat wake erosion and offer habitat value. Cap design included fate and transport modeling of PAHs through sediment to determine cap thickness and in situ strength testing to determine a suitable method for placing cap materials on very soft sediment because traditional laboratory strength tests did not provide accurate in situ sediment strength data. Capping challenges also include consideration of sub-aqueous slope stability under both static and seismic conditions.

Work Performed

  • Regulatory negotiations – MTCASnapshot 1 (1-29-2013 9-21 AM)
  • Permitting – design commission, historic landmarks
  • Shoreline site cleanup and development integration
  • Construction management
  • Insurance cost recovery
  • Sediment RI/FS
  • Geotechnical analysis of sediments
  • Sediment cap design

Value Added

  • Negotiated remediation levels based on groundwater to surface water attenuation factors
  • Completed cleanup of the park during the winter and successfully reopened park for July 4th celebration
  • Incorporated design features, such as the use of corten steel, to satisfy the Design Commission and the Historic Landmarks Board
  • Successfully demonstrated the ability to cap very soft sediments
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