Wednesday, June 3, 2009

FIRESIDE CHAT: Hudson River PCB Dredging (b)

Bias in the US Environmental Protection Agency’s Baseline Health Risk Assessment Supporting the Decision to Require Dredging of PCB-Bearing Sediments from the Hudson River

Robert A. Michaels, Uriel M. Oko

The US Environmental Protection Agency’s (EPA) baseline Hudson River health risk assessment (HRA) is evaluated and found to be biased toward keeping polychlorinated biphenyls (PCBs) in sediments. The HRA systematically misquantified parameters, underestimating PCB movement from sediments to water and from water to air. The EPA excluded from its analysis all mono-and dichlorinated PCB congeners, which EPA subsequently estimated at one-third of total PCB mass in the river, and excluded dissolved and colloidal PCB.

The EPA included silt-adsorbed PCB, but overestimated the rate at which it would settle out of the water column by inappropriately basing the rate on StokesLaw for more massive spherical particles. Flat clay particles settle more slowly with a longer path length and residence time. The EPA omitted electrostatic charges on clay particles that separate them, preventing agglomeration and maintaining clay in suspension; they also assumed that particles never “reflectback into the water column after settling, likewise underestimating PCB concentrations in water.

Also omitted was PCB codistillation, in which PCBs at low bulk concentrations preferentially distribute to the air-water interface, accelerating PCB transfer from water to air. Indeed, EPA cited empirical data showing more rapid PCB water-to-air transfer, but reduced its effect on the HRA, by reducing the transfer coefficient by averaging in lower modeled PCB transfer coefficients that ignored codistillation. Finally, EPA omitted PCB release to the atmosphere from hot water in cooling towers in communities along the Hudson River. Water at cooling tower temperatures may release PCB into the air more than 10 times faster than rates determined from the surface of cold water and multiple orders of magnitude more rapidly than in EPA’s models. Together, EPA’s procedures reduced airborne PCB concentrations from above to below de minimis concentrations. This, in turn, eliminated the requirement for EPA’s HRA to quantify inhalation risks posed by airborne PCBs; the HRA, therefore, 'considered' airborne PCBs, but attributed zero health risk to them.


or see:

Michaels, RA.; and UM Oko. Bias in the US EPA baseline health risk assessment supporting the decision to require dredging of PCB-bearing sediments from the Hudson River. Environmental Practice (Cambridge University Press), 9(2):96-111, June 2007.

Copyright © 2009 by The Center for Health Risk Assessment and Management, a Division of RAM TRAC Corporation