Friday, August 28, 2009

FIRESIDE CHAT: Liquid PCB Oils and Hudson River PCB Dredging

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The U. S. Environmental Protection Agency (EPA) reported at a public meeting in the Hudson River town of Fort Edward, New York on 19 August that GE clamshell dredges recently have started to encounter liquid PCB oils in dredging ‘prisms’. I attended that meeting along with my colleague, Dr. Uriel M. Oko. EPA's revelation constituted a virtually silent bombshell, as apparently none of the many media representatives in attendance took special note of it. It is worthy of note, however, because pure PCB oils must be viewed in an entirely different, more ominous context than river sediments harboring PCBs in the parts-per-million (ppm) concentration range. Pure liquid PCB oil concentrations can be expressed in ppm if necessary: 1,000,000 ppm. Mobilizing PCB oils via dredging is commensurately more serious than mobilizing sediments bearing PCBs in the low ppm range as originally anticipated.

As reported in a 28-August CNS News article published online (for which I also was interviewed; 1), EPA acknowledged that the project recently had discovered “sheens” of liquid PCB oil on the river surface, “an indication, the EPA said, that the river floor contained not only contaminated silt, but more potent pockets of pure PCB oil – and that the dredging is releasing the oil into the river water.” EPA, however, placed a relatively benign spin on this news: “It’s not really affecting dredging, that’s why we’ve taken those mitigation measures to, you know, to counteract the sheens.” Counteracting microscopically thin PCB ‘sheens’ visible on the river surface is very different from the real challenge: counteracting pools of liquid PCB oils of unknown, potentially large volume in trenches beneath.

PCB sheens on the river surface constitute yet another source of airborne PCBs not addressed in any EPA assessment of risks to health potentially posed by sediment dredging at PCB hotspots in the Hudson River. (See previous blog posts to learn of other potentially risk-posing sources of airborne PCBs associated with dredging.) Observation of these PCB sheens raises the question of whether they might be expected to arise from underlying sediments bearing PCBs in merely the ppm range, or whether they must originate from more massive pools of liquid PCB oils at the river bottom as suggested by EPA.

Can bottom sediments form surface sheens via some upward PCB migration process? Three factors suggest not:

--1. PCBs chemically bound to sediments for decades are unlikely to become unbound,

--2. If they did become unbound, they also would have to become concentrated from the low-ppm range at the river bottom to near purity at the surface, which is unlikely to occur in a flowing, turbulent river, and

–3. PCB oils are denser than water, so they would be expected to sink, not to rise from the bottom, unless physically forced upward, or lifted.

PCB liquids include more than 200 types, or congeners, varying in their degree and pattern of chlorination (with from one to 10 chlorine atoms per PCB molecule). Each congener has a unique density, but bulk density of commercial PCBs of the types disposed to the Hudson River (known as Aroclors) are reported to have a density of about 1.5 grams per milliliter (g/ml; 2), which is 1.5 times the density of water.

The most probable origin of PCB liquids forming surface sheens, in my view and apparently in EPA's view, is that they are being massively disrupted from pools of liquid PCB oils formed in sediment low points (depressions) following original disposal from land-based facilities or ships. These bottom pools gradually might have become covered over with debris and sediments. The onset of dredging may be forcing the liquids upward toward the surface as dredge jaws expose them and close around them. This process is visible for sediments (which have tightly-bound PCBs that would not be expected to form sheens), and reasonably would apply as well to liquid pools (which would be expected to form surface sheens).

Sediments and PCB oils that are not forced upward by closing dredge jaws might be retained within the dredge buckets, and physically lifted. These materials would be subject to leakage during their ascent to the surface and beyond. These dual processes of disruption by squishing and by lifting reasonably would be expected to generate PCB liquids and PCB sheens at the surface, as PCBs are chemically attracted to surfaces, including to the air-water interface in rivers.

EPA’s report of encountering PCB liquids seems, at least in retrospect, unsurprising given the history of past PCB disposal into the river in the form of liquid PCB oils from land-based facilities and from ships. Given this history, why did EPA adopt the clamshell method of dredging, thereby failing to prepare for this seemingly expectable eventuality? Its sobering actuality now casts further doubt on the wisdom of specifying clamshell dredging rather than vacuum dredging, or no dredging.

One objection to the liquid-pool hypothesis that must be addressed is the fact that PCB concentrations in downstream water samples have not revealed PCB oils or concentrations high enough to suggest their presence upstream. The heavier-than water density of PCBs, however, would be expected to cause them to hug the river bottom as they move downstream. They would not be expected to register in surface water samples taken five miles downstream.

EPA now appears to be faced with the quandary of whether to:

--1. mobilize PCB oils by continuing clamshell dredging, even though the dredge buckets cannot retain the oils efficiently,

--2. allow the PCB oils to be mobilized by river currents washing over the now-exposed pools,

--3. institute vacuum dredging, or

--4. stabilize the oils by covering them over again.

I wish I had the answer to this quandary, but for now suffice it to bring this issue to the light of public scrutiny… which EPA’s silent bombshell on 19 August did not seem to do.

Literature Cited

1. Brickley, Adam. EPA river clean-up uncovers pools of cancer-causing PCBs. Alexandria, Virginia; CNS News, http://www.cnsnews.com, 28 August 2009;

2. UN EP. Training manual for the preparation of a national environmentally sound management plan for PCBs and PCB-contaminated equipment in the framework of the implementation of the Basel Convention. Ch√Ętelaine, Switzerland, United Nations Environment Programme; Basel Convention Series/SBC No. 2003/01, ISBN : 92-1-158674-7, 103 pages, March 2003.


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