Enhanced Natural Recovery/Activated Carbon Pilot Study of the Lower Duwamish Waterway
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The Lower Duwamish Waterway (LDW) Superfund Site is a five-mile-long, 440-acre, federally authorized navigation channel that has served the City of Seattle and regional port needs for more than 90 years.

The Lower Duwamish Waterway Group (LDWG) is conducting a pilot study to evaluate the potential effectiveness of an innovative remedial technology in the LDW. Geosyntec and Geosyntec staff (prior to joining the firm) were retained to conduct a pilot study to determine whether a thin-layer placement of sand (enhanced natural recovery, ENR) amended with activated carbon (AC) can be successfully applied to reduce the bioavailability of PCBs in remediated contaminated sediment, as compared to ENR without AC. The purpose of the treatments is to reduce the exposure of aquatic organisms to contaminants like PCBs. ENR+AC may have the potential added benefit of reducing the bioavailability of PCBs by actively sorbing contaminants from LDW sediment. Both remedial treatment options offer many advantages over more traditional approaches like dredging and capping, including cost effectiveness, demonstrated risk reduction, sustainability, and logistical effectiveness.

Geosyntec Scope of Services

Geosyntec staff have been working with LDWG, LDW stakeholders, and other team members to design and implement the pilot study. A key part of this work has included the design of a monitoring study involving a variety of measurements to gauge the placement, resilience, and performance of the two remedies over a 3-year monitoring period. As a part of the planned monitoring work, we designed and configured innovative porewater passive samplers to achieve the sensitive detection capabilities necessary to monitor low levels of PCBs in sediment and remedy layer porewater. We implemented the baseline and post-amendment porewater monitoring at over 400 locations, including a detailed data analysis and statistical evaluation to confirm the sampling design for later monitoring events. We also supported remedy design, evaluated the expected effectiveness of the selected design through advanced AC modeling, and addressed potential ecological concerns associated with the ENR and ENR+AC materials. We have presented to USEPA and other stakeholders, written monitoring and design documents, and key staff pioneered a green and sustainable remediation (GSR) analysis for contaminated sediment that recently led to a journal publication. During the next year, Geosyntec will continue to help plan and execute the monitoring programs, interpret remedy performance data, and communicate results to stakeholders.

Notable Accomplishments

Geosyntec's support in the remedial design (RD) and monitoring plan for the pilot study has helped the project stay on track and address numerous challenging questions from USEPA and other stakeholders. Using innovative fate modeling, we have demonstrated optimal configurations for the AC treatment that will enable reductions of PCBs to be detected within the three-year post remedy study period, reducing uncertainty and potentially costly and ineffective overapplication of expensive AC treatments. We reconfigured a passive sampling deployment device to meet the challenging deployment environment, saving additional equipment costs. Initial results from the Year 1 and Year 2 monitoring events have indicated that ENR and ENR+AC have reduced PCB availability by 70–90% from baseline, indicating a successful and effective remedy. Our team developed a peer-reviewed cost estimating tool specific to sediments that can be adapted to other sediment projects, and our GSR analysis was one of the first sustainability evaluations approved by USEPA for a sediment Superfund site.

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Project Summary

  • Location: Seattle, Washington
  • Client: Lower Duwamish Waterway Group (LDWG)
  • Project Practice Areas: Contaminated Sites, Water and Wastewater
  • Type of Facility: Sediment site
  • Services Provided: Passive sampling of PCBs in sediment porewater, Long-term remedial monitoring, Evaluation of remedial options, Pilot-scale remedial study design
  • Type of Work: Remedy evaluation
  • Governing Regulation: CERCLA