Saturday, February 6, 2010

FIRESIDE CHAT: Hudson River PCB Dredging: The Case of the Shifting Sediment Mobilization Definition

In my 4-February post, I called attention to a major discrepancy between sediment mobilized in Phase I of GE’s EPA-mandated Hudson River PCB dredging project vs. the much smaller amount of sediment mobilization measured and reported by GE and EPA. Sediment mobilization as reported refers to the amount that is ‘resuspended’ by dredging, and monitored miles downstream of the Phase I dredging area, almost all of which is near Roger’s Island. As EPA also has reported, however, most dredged sediment falls back to the river bottom in the trench or near the spot from which it was dredged initially. Thus, the preponderance of mobilized sediment remains on the river bottom, still mobile, but unrecorded in GE or EPA sediment mobilization data... hence the ‘sediment mobilization discrepancy’.

The sediment mobilization discrepancy is more than merely a difference between a measured and an actual parameter value. Rather, it is a fundamental inconsistency in EPA’s past justification of the need to dredge vs. the Agency’s current characterization of the performance of the dredging project now that it is underway. The need for dredging was justified by the mobility of sediments in PCB ‘hotspots’ requiring, according to EPA, their removal by dredging. Indeed, a small but persistent trickle of buried PCB moving downstream was documented from some 27 PCB hotpsots.

In contrast, in the new context of actual dredging, EPA dramatically altered its concept of mobility. Mobility in the dredging project now is quantified by the lightest-weight fractions of PCB-tainted sediments measured in the water at the Thompson Island Dam about five miles downstream of Phase I dredging, and further downstream at Lock 5 and still further downstream at Waterford (see EPA’s figure, below).

Don’t be confused by EPA’s altered terminology, referring to mobilized sediment as ‘resuspension’ estimates. These estimates reflect near-term mobilization, and ignore the fact that all of the sediment that falls back to the riverbed also is ‘mobilized’, in the original sense of that term as used by EPA to justify dredging. That is, it is mobilized because it can re-enter the water column whenever turbulent conditions arise, and it can enter riverine ecosystems. It can be transported by migrating organisms, such as fish and birds, and it can enter the air in communities directly from the water, or from heated sources of water (cooling towers) used to cool industrial processes, such as in factories and power plants.

In past posts I estimated that 80 percent of sediment disrupted by dredging was mobilized rather than transferred successfully to barges. In my 4-February post I used GE’s ‘bucket files’ (1), recording dredge bucket hauls on onboard computers on each dredge platform, to better quantify this estimate. The best number that I came up with is 74 percent for the five-cubic-yard capacity bucket typically used, but that excludes sediment that is disrupted by dredge buckets when they crash to the bottom but fail to close due to obstructions such as boulders or construction debris. The 80-percent estimate therefore looks pretty good, though I can quantify only a 74-percent estimate, and only approximately as explained in my 4-February post.

The mobilized fraction of sediment, therefore, amounts to about 211,000 cubic yards of sediment, which is about 100 million kilograms, assuming a dredged sediment density of about 0.6 compared with water. Yet, EPA’s figure reports 388 kg at Thompson Island, 226 kg at Lock 5, and 122 kg at Waterford. Ignoring the obvious double-counting, EPA reports 736 kg of mobilized ‘resuspended’ sediment, which most would agree is less than 100 million kg. Thus, EPA’s figures exclude nearly all mobilized sediment… EPA simply ignores it in evaluating the performance of the dredging project, notwithstanding that the mobility of sediment was a central rationale for dredging in the first place.

Literature Cited

1. GE. Phase I Evaluation Report: Hudson River PCBs Superfund Site. Draft report prepared for General Electric Company (Albany, New York) by: Anchor QEA (Glens Falls, New York) and Arcadis (Syracuse, New York); 191 pages plus tables, figures, and appendices; Appendix G, Table G-1, 44 pages; January 2010.

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