Puget Sound sediment cleanup remedy effectiveness retrospective

The monitoring of completed large-scale sediment remediation projects has revealed mixed effectiveness at reducing risks, thus highlighting uncertainties regarding whether and which remedial measures are necessary to achieve protective goals. To support valid statements about changes over time and the overall effectiveness of sediment remedial action, robust pre- and postremediation monitoring is required with sufficient time points, reference sites, and biological metrics. The five completed Puget Sound sediment remediation case studies reviewed here (Bellingham Bay, St. Paul Waterway, Eagle Harbor, Hylebos Waterway, and Sinclair Inlet) employed particularly robust remedy effectiveness monitoring programs that spanned decades, revealing common lessons for improving remediation outcomes. First, although sediment remediation can play an important role in reducing contaminant exposure in areas with higher sediment concentrations, at lower levels, sediment links with fish tissue concentrations diminish. As water column exposure from diverse sources becomes predominant, remediating sediments with lower concentrations yields proportionately less risk reduction. Second, timely monitoring of effective source controls achieving substantial (i.e., >80%) contaminant source load reductions as well as large-scale capping projects have revealed rapid changes in Puget Sound surface sediment concentrations and biological recovery metrics with an average recovery half-time of 1.6 ± 0.8 years. The weight of evidence suggests that natural recovery of Puget Sound surface sediments is significantly accelerated by exchange across the sediment-water interface from benthic organism feeding behaviors, porewater flux, and tide-generated currents. As a result, effective source controls in Puget Sound have rapidly improved surface sediment quality and achieved more significant risk reductions than broadscale sediment remediation. Going forward, comprehensive Puget Sound source control efforts that incorporate robust monitoring in an adaptive management framework are the best way to achieve protective remediation objectives. These lessons may apply more broadly across similar complex urban aquatic ecosystems. Integr Environ Assess Manag 2024;20:1355-1365. © 2024 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).