Hudson River PCB Dredging: Midcourse Assessment and Implications Regarding Possible Project Continuation Versus Termination

摘要

Phase 1 of Hudson River dredging fractionally reduced the amount of polychlorinated biphenyl (PCB)-contaminated sediment remaining in the river bottom, but greatly increased its downstream transport, as "clamshell" dredge buckets leaked more material than they retained. Clamshells were especially inefficient at capturing liquid PCBs and PCBs adsorbed to silt and fine sediment. Material that was removed, therefore, was relatively coarse, and contained relatively little PCB. Volume reduction was achieved only at the expense of massive mobilization of liquid, colloidal, and adsorbed forms of PCB, spreading contamination to previously uncontaminated river areas. We estimate that Phase 1 increased the surface area of contaminated riverbed by at least 3 orders of magnitude, from about 50 dredged acres to many square miles. PCB entry into ecosystems, water, and air depends upon the surface area of contaminated sediment newly exposed to scouring from the riverbed, causing further downstream transport, especially in high-flow events over years or decades. Indeed, Phase I contravened EPA's benefit criterion by mobilizing PCB at a higher rate than would occur naturally, without dredging, over the planned seven-year project duration. If dredging is to continue, therefore, Phase 2 must assure health risk acceptability and net benefit compared with no actiono dredging. Further, EPA's benefit criterion must be revised to account for impacts of PCB mobilization to ecosystems and air, not just resuspension to water. Finally, Phase 2 must increase the scope and duration of environmental monitoring to enable more reliable warning of harm in real time, and project evaluation over decades. We recommend that EPA consider for Phase 2 such technologies as coffer damming, vacuum dredging, and dredging within enclosures to minimize sediment resuspension; mobilization of liquid, colloidal, and adsorbed forms of PCB; and PCB entry into ecosystems and air. If Phase 2 cannot assure health risk acceptability and comply with a revised benefit criterion, we recommend that EPA terminate the dredging project.