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
CONTINUED FROM PREVIOUS POST
This investigation adopts the methods of health risk assessment (HRA) and health impact assessment (HIA) to identify parameters used by EPA to assess baseline (non-dredging) health risks potentially posed by PCB in the Hudson River and, essentially, peer review their quantification based upon Agency documentation. Two criteria were used to determine whether effective insulation of science from politics was attained: 1. whether parameter values were estimated accurately and, 2. with respect to any parameters evaluated inaccurately, whether the direction of estimation error was mixed, or consistently overestimated or underestimated potential PCB transfer from Hudson River water to air. Overestimating the risk of transferring PCBs from sediments to water and water to air in the vicinity of Hudson River communities could contraindicate dredging, whereas the reverse error would be conducive to dredging.
The null hypothesis is absence of bias. This corresponds to finding a random distribution of errors, not to finding an absence of errors. Any finding of significant systematic error in either direction would constitute evidence of bias justifying rejection of the null hypothesis..
We determined whether each parameter examined was estimated correctly. If EPA's evaluation of a parameter was grossly inaccurate, we included it among parameters to be examined statistically to determine whether the distribution of the directions of mis-estimation was nonrandom. Each parameter that is estimated inaccurately must be overestimated or underestimated (otherwise it is accurate). If these outcomes can be assumed to be equally probable, then occurrence of each is associated with an equal expected probability of 0.5 (50 percent, or ‘fifty fifty’). If the parameters also are independent (mis-estimating one parameter does not affect estimation of another), then any two randomly selected parameters that are mis-estimated would have a 0.25 probability (P = 0.5 x 0.5 = 0.25) of being mis-estimated in a direction more permissive to dredging and, likewise, 0.25 would be the probability of the same two parameters being mis-estimated in a direction less permissive to dredging.
In general, the probability of mis-estimating all of n parameters consistently in a particular direction by chance alone is 0.5n where, for example, the probability of mis-estimating five out of five parameters in a direction permissive to dredging would be 0.03 (P = 0.55 = 0.03). When probabilities reach such low values, below the usual 0.05 criterion of scientific uncertainty, the null hypothesis of randomness is rejected. Speaking qualitatively, bias in the outcome of EPA's analysis (possibly unintentional) would be inferred.
Secondary methods also were applied. They are not a priori methods, and they are not described in detail here. Rather, they are the diverse methods typical of peer review, which most essentially consists of considering the scientific merit with which numerous methods were selected for use and applied in the original analyses supporting the dredging decision. Readers can judge for themselves whether or not we applied the methods of HRA, HIA, and peer review objectively.
EPA identification of parameters used in assessing potential risks posed by PCBs
The number of parameters on which the dynamics of PCBs entering the water column from sediments and entering the air from the water column are diverse and numerous, numbering in the hundreds or thousands. The number that are visible in any scientific explication of this issue depends upon the degree of detail with which the analysis is conducted. The parameters include initial concentrations of all 209 PCB congeners, from monochlorinated to decachlorinated biphenyls, in each medium, bulk amounts, areas involved, depths of water and sediments, as well as parameters describing the physical, chemical, and environmental degradation (such as half life), transformation (such as dechlorination), and other environmental dynamics (such as solubility, boiling point, volatilization, and vapor density) of these numerous congeners. The safety issue also encompasses toxicological parameters of each PCB congener. The full list of such parameters is too long to elucidate in detail here.
Fortunately, the present analysis requires no such highly detailed elucidation. The parameters that are of greatest concern here are those that are most susceptible to being overestimated or underestimated, especially if by a wide margin, or overlooked entirely. These are the parameters (unlike, say, molecular weights, which are known to a high degree of accuracy) whose estimated values substantially may depend upon who is doing the estimating. Quantification of these parameters can vary from liberal to conservative, depending upon whether the estimator has a (conscious or unconscious) agenda other than to conduct a purely scientific analysis… in short, a bias. Nine such estimated, determinative parameters that were (or should have been) used for technical analysis in the baseline HRA supporting the dredging decision were identified, as follows:
--1. Mobilization of sediment-borne PCBs in dredging. Sediment-borne PCBs will become mobilized by dredging. The amount mobilized depends upon the dredging method. Mobilization must be considered in assessing potential public health significance of PCB dredging;
--2. PCB congeners to be included in the analysis. All 209 PCB congeners should be included;
--3. Phases of PCBs to be included in the analysis. All phases should be included, most notably including PCBs that are adsorbed onto particles, molecular PCBs that are dissolved, and particulate PCBs that are colloidal;
--4. Precipitation of PCB-bearing sediment particles from the water column.Precipitation rates should be quantified realistically, as this parameter is important in determining the rate of PCB removal (to sediments), and the resulting PCB concentration in the water column;
--5. Electrostatic charges on PCB-bearing sediment particles in the water column.Clay sediment particles resuspended in water (as by dredging) tend to exhibit negative surface charges. Such particles are maintained in suspension by electrostatic interaction of the negative surface charges with cations (positive ions) in the water. This electrostatic charge configuration inhibits agglomeration. It should be accounted for because of its potential importance in inhibiting settling of clay particles and removal of adsorbed PCB from the water column;
--6. Reflection coefficient of precipitating PCB-bearing sediment particles. The reflection coefficient quantifies the tendency of particles, once settled out of the water column, to return to the water column as a result of ‘bouncing’. The reflection coefficient should be quantified, and is especially important for particles that are of low mass, or likely to be affected by currents, as in the Hudson River;
--7. PCB codistillation. Codistillation is a chemical process well documented for PCBs. It results from molecular attraction to surfaces. For PCBs these surfaces include the air-water interface in lakes and rivers. Entry of PCBs into air from water is significantly faster and more extensive in a given interval than would be the case if the same mass of waterborne PCBs were assumed to be distributed evenly throughout the water column (as quantified by the ‘bulk concentration’). Accurate estimation of waterborne PCB entry into the air that people will breathe requires quantification of PCB codistillation;
--8. Empirical measurement of airborne PCBs over PCB-contaminated waters.Empirical measurements, to the extent available, should be used for validating modeled relationships, such as models of PCB entry into the air from Hudson River water;
--9. Warm water sources of Hudson River PCB entry into the atmosphere. Warm water occurs at near-shore locations where cooling water is discharged from industrial facilities and, before discharge, in cooling towers supplied by Hudson River water. These sources of potential entry of PCBs into the atmosphere near population centers must be accounted for assessing potential public health significance of PCBs, and the possibly increased significance to public health if PCB dredging is undertaken.
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TO BE CONTINUED
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.