The Rule of Diminishing Returns: A Large and Complex Sediment Remedy Feasibility Study in Portland Harbor, Oregon

摘要

Background/Objectives. The draft feasibility study (FS) for the Portland HarborSuperfund Site was recently submitted to U.S. Environmental Protection Agency (EPA)Region 10 for review. The draft FS represents more than 10 years of site investigationand evaluation to assist EPA in selecting a comprehensive contaminated sedimentremedy for this complex site, which covers 10 river miles and contains many contaminantsof potential concern. A baseline risk assessment identified that the highest riskswere to humans who consume fish tissue, as well as to some ecological receptors.Preliminary remediation goals for several bioaccumulative contaminants were derivedfrom target tissue levels in fish using a site-specific bioaccumulation model, whichresulted in very low remediation goals for some exposure scenarios. Consequently, a keyquestion for the draft FS was “To what extent can these low contaminant levels bereasonably addressed through a sediment remedy, and what potential low-level riskswould remain after remedy implementation due to the presence of residual contaminationand ongoing sources?” The draft FS answered this question by developing five alternatives,with several options under each alternative, that actively addressed progressivelylarger areas and volumes of sediment exceeding progressively lower contaminantremedial action levels.Approach/Activities. Using various evaluation tools, including a site-wide fate andtransport model, the draft FS found that long-term effectiveness (assessed through thenext 45 years) was very similar across the range of alternatives due to the importance oflow-level ongoing sources and relatively slow but steady progress of natural recovery ofthe harbor sediments. Low contaminant concentrations on incoming suspended sedimentdetermine the ultimate level of risk reduction achieved regardless of the alternative’sactive remedy area or amount of dredging. Every alternative is expected to achievesimilar long-term sediment residual concentrations within the range of background levels.Therefore, the draft FS concludes that institutional and engineering controls are the besttools to provide protectiveness from these residual concentrations.Results/Lessons Learned. The short-term effectiveness of the alternatives was consistentlydifferent, with the larger and more dredging-intensive alternatives creating greatershort-term impacts due to dredging-related resuspension/residuals, greater greenhousegas emissions, and a greater number of projected work accidents. The projected, timeaveraged,30-year sediment and fish tissue site-wide concentrations (a timespan similar torisk assessment exposure assumptions) under the larger-sized and more dredgingintensivealternatives would be substantially higher than under more focused sedimentremedial options. Based on these findings, the draft FS concludes that alternatives withlarger active remedy areas and more dredging not only provide diminishing returns of