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 mis-quantified 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 Stokes' Law 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
“reflect” back 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, 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.
Environmental Practice 9:96–111 (2007)