Analysis - The Institute for Southern Studies

This RIA extracted only those results from the EPA 2009 risk assessment to either represent (a) conventional CCR (i.e., fly ash, bottom ash,
boiler slag, and flue gas desulphurization waste managed in the landfill or impoundment without mixing with other materials), or (b) CCR comanaged
with coal refuse. 113 Of these results, only those for trivalent arsenic were used. 114 For the primary analysis, it was assumed that all
arsenic was speciated in this manner. As noted in the EPA source data, arsenic III and arsenic V cancer risk results for unlined surface
impoundments that co-dispose CCR with coal refuse were not statistically different at the 90 th percentile, and these risks are likely to drive the
population risk estimates. A sensitivity analysis was conducted where all arsenic was assumed to be speciated in the arsenic V state. This
analysis is presented in Appendix K5. Finally, risks for both adult and child receptors were included so that each group would be accurately
represented. Once all of these data were collected they were sorted by CCR disposal unit and liner type.
This analysis reflects possible groundwater and surface water interactions that could affect the population risk estimates. In situations in which
the modeled distance to a surface water body was less than the modeled distance to a drinking water well EPA assumed that the groundwater
plume is fully intercepted by a surface waterbody. 115 To this end, EPA extracted the model inputs for the distance to groundwater wells and the
distance to surface waterbodies used in the EPA source, randomly selected from input distributions. 116 These two were then compared using a
logical test in Microsoft Excel. This test returned a 0 if the surface waterbody was closer than the drinking water well and 1 if it was not.
Thus, a 1 was a positive indication that the contaminant plume in that model run reached the groundwater well.
Finally, EPA extracted the exposure durations used in each model run from the EPA-ORCR 2009 CCR risk report to capture the fraction of the
individual’s lifetime risk that was experienced in a one-year period. EPA accomplished this by matching the probabilistic exposure duration
inputs, to their corresponding age category. Then, each probabilistic run was sorted to return the exposure duration of the adult and child age
category. These Monte Carlo data constituted a weighted approach for estimating individual human cancer risks. Population risk is typically
calculated by multiplying risk results by the affected population. Since there were thousands of equally valid model iterations, this RIA
assigned each of these risks an equal weight in its final population risks by using the average of these individual risks.
Individual risk estimation took into account the fact that the contaminant plumes might be intercepted by surface waterbodies by multiplying by
either 0 or 1 as identified above. Each of these risks was then divided by exposure duration to estimate the yearly cancer risk. 117 Once all of
these risks were calculated for a given WMU/liner type they were summed and divided by the number of iterations to give the average one year
increment of risk for that WMU/liner type at the peak risk. Thus, the final equation that was used for calculating average risks can be stated as:
113 Fluidized Bed Combustion waste results were not deemed appropriate for use for the reasons discussed in EPA “Human and Ecological Risk Assessment of Coal
Combustion Wastes,” Office of Resource Conservation & Recovery, August 2009.
114 A 1981 Oak Ridge National Laboratory study states “As (III) is likely to be the predominant arsenic species in ash pore water and groundwater.” Source: Turner, Ralph
R. “Oxidation State of Arsenic in Coal Ash Leachate,” Environmental Science & Technology, Vol.15, Number 9, September 2001.
115 Full interception will not occur in instances where the waterbody is shallow, the waterbody is man-made, or the facility is oriented perpendicular to the waterbody. This
simplifying assumption serves to minimize the influence of the model runs in which interception may have occurred, but was not reflected in EPA “Human and Ecological
Risk Assessment of Coal Combustion Wastes,” Office of Resource Conservation & Recovery, August 2009.
116 For further discussion of how these distributions were developed, see Appendix C of EPA “Human and Ecological Risk Assessment of Coal Combustion Wastes“ Office
of Resource Conservation and Recovery, Washington, DC, August 2009.
117 For further discussion of cancer risks and exposure durations, see Appendix K4 of this RIA.
116