Thursday, February 4, 2010

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

Sediment mobilization in Phase I of GE’s EPA-mandated Hudson River PCB dredging project is monitored miles downstream of the Phase I dredging area, almost all of which is near Roger’s Island. Available data, therefore, reflects mobilization of PCB-tainted sediment that enters the water column and remains waterborne over miles of downstream river flow. As EPA has reported, however, most of the 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 discrepancy between mobilized sediment vs. sediment mobilization measured and reported by GE and EPA.

Sediment that rests on the river bottom is termed ‘mobile’ because it can enter the riverine ecosystem and food chain, from which it can disperse further, such as in migrating fish and birds. These previously buried sediments, now mobile, also can re-enter the river flow in turbulent conditions, such as in storms. From the water, PCBs also may enter the air, and may be inhaled by people in affected communities. These outcomes constitute compelling reasons to monitor mobilization of dredged sediment, not just to learn about the near-term flow of the lightest-weight PCB fractions over the Thompson Island Dam and Troy Dam miles downstream, but also to learn about the long-term transport of all mobilized PCBs into ecosystems and the air, and their possible bioconcentration in organisms.

Design of the dredging project conspicuously omits holistic quantification of sediment mobilization that is needed. The evolution of technology, however, seems to have introduced a kind of ‘back-door’ way of quantifying sediment mobilization, at least approximately. Specifically, dredges used in the project are computer-controlled to keep them operating within defined dredging ‘prisms’ corresponding to PCB ‘hotspots’ slated for removal. Each time a dredge bucket descends to the river bottom, it can do either of two things: close, or remain opened. If the bucket remains opened, it still has massively disrupted and mobilized a significant amount of sediment, as these buckets weigh tons, and they are large. The five-cubic-yard capacity bucket typically used has two sets of jaws that are nearly 15 feet in length. These massive jaws crash to the river bottom violently, thereby mobilizing much sediment and removing none of it. Obstructions in the river bottom, such as boulders or construction debris, sometimes prevent closure. In those cases the dredge bucket may be crashed to the river bottom repeatedly before withdrawal unclosed. My guess is that closure fails to occur between one and 10 percent of the times that a bucket descends to the river bottom. These sediment mobilization events, unfortunately, are not recorded.

In most cases, presumably, the dredge bucket does close, at least partially, and those cases are recorded by onboard computers on each dredge platform. Also in those cases, even more sediment is disrupted and mobilized. Sediments within closing dredge bucket jaws are forced upward, with much material billowing out of the open tops of dredge buckets and into the downstream river flow (see photo above). The slurry of sediment and water that remains within the bucket, which typically leaks from the bottom, is released gradually to the water column and, upon emerging, drops back to the river surface (see photo below). As sediment is concentrated at the bottom of each dredge bucket, much if not most of the material that leaks back into the river is sediment (not just water).

GE’s dredge platform on-board computer records may be used to quantify the number of bucket closures. Indeed, GE published its ‘bucket files’ in its recent draft report (1) as Table G-1 in Appendix G. This information can be combined with GE’s separate records of the volume of material that was retained by the buckets, even after all these losses, and placed into barges. A summary of my analysis of this information is presented in the table below.

As the table indicates, the preponderance of dredged material was not barged, but mobilized. The exact fraction mobilized depends upon the capacity of the dredge buckets that were used. The dredge bucket capacity in turn depends upon the maximum capacity of each bucket and the depth of its bite into the sediment, both of which were variable because three bucket sizes were used, and sometimes they did not penetrate to their full design depth, for example if they struck a boulder or construction debris. The standard bucket capacity was five cubic yards with a design penetration depth of 18 inches. My understanding is that the five-cubic-yard bucket was used routinely, except in sensitive spots, say near shore, when a one-cubic-yard capacity bucket sometimes was used. The project also had one two-cubic-yard bucket. I further assume that the design penetration depth was typically the target depth, so the overall average bucket capacity probably was close to five cubic yards.

As the above table shows, Phase I dredging pulled out 286,006 cubic yards of sediment (actually a slurry of sediment and river water), which went into barges (topped by a layer of water). The dredge buckets closed 221,521 times, producing an average load of 1.29 cubic yards per bucketload that was transferred to the waiting barges (including the water). The amount of sediment that will be shipped by train to Texas will be less, because sediments must be dried before loading. Even using the larger figure as the estimate of barged ‘sediment’, however, the amount barged still is only 26 percent of an assumed five-cubic-yard average bucket capacity, or 32 percent of a four-cubic-yard capacity, or 43 percent of a three-cubic-yard capacity.

That means that the amount of sediment that was mobilized was 74 percent if the average bucket capacity is assumed to be five cubic yards, 68 percent if the average bucket capacity is assumed to be four cubic yards, and 57 percent if the average bucket capacity is assumed to be three cubic yards. Whatever assumption you choose, the preponderance of material was mobilized, not barged. Whatever assumption you choose, still more was mobilized when the dredge buckets descended but failed to close. Some of the mobilized sediment, however, might be dredged again, and a fraction removed from the river, if the sediment falls back to the river bottom within a ‘prism’ slated for future dredging.

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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