Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterizationwhat may occur with the management of CCW under actual scenarios, it is possible to comparethe results of these models to specific situations.The risk assessor needs to consider the importance of excluded variables on a case-bycasebasis, because a given variable may be important in some instances and not important inothers. A similar problem can occur when a model that is applicable under one set of conditionsis used for a different set of conditions. In addition, in some instances, choosing the correctmodel form is difficult when conflicting theories seem to explain a phenomenon equally well. Inother instances, EPA does not have established model forms from which to choose to addresscertain phenomena, such as facilitated groundwater transport.The models used in this analysis were selected based on science, policy, and professionaljudgment. These models were selected because they provide the information needed for thisassessment and because they are generally considered to reflect the state of the science. Eventhough the models used in this analysis are used widely and have been accepted for numerousapplications, they each retain significant sources of uncertainty. These limitations are welldocumented in the model development references cited in Section 3.Although the sources of model uncertainty in this assessment could result in either anoverestimation or an underestimation of risk, the models used in this assessment have beendeveloped over many years to support regulatory applications. As a result, they have beendesigned to be protective of the impacted populations that they represent. In other words, wheresimplifying assumptions are necessary, the assumptions are made in a way that will notunderestimate risk.Assumption of Clean Closure of Surface Impoundments. As described in Section3.5.1, the surface impoundment model treats a surface impoundment as a temporary wastemanagement unit with a set operational life. At the end of this life, clean closure is assumed; allwastes are removed and there is no further release of waste constituents to groundwater.Although this simplifying assumption is not consistent with the practice to close CCW surfaceimpoundments with wastes in place, and it limits the length of potential exposure, the peakannual leachate concentrations on which the CCW risk results were based are not likely to beaffected. Releases to groundwater are much higher during surface impoundment operationbecause the higher hydraulic head in an operating impoundment drives wastewater into theunderlying soil with greater force than infiltration through the impoundment cover after theimpoundment is closed. This higher head results in a greater flux of contaminants to groundwaterduring the active life of the surface impoundment, especially in unlined units. Thus, even if thepost-closure period were modeled, the corresponding results would not be as high as the peakannual leachate concentrations used in the analysis.Arsenic and Selenium Speciation. Because the models used in this assessment do notspeciate metals during soil or groundwater transport, arsenic and selenium speciation in thesubsurface is a significant groundwater modeling uncertainty in this analysis. Arsenic can occurin either a +3 (arsenic III) or +5 (arsenic V) oxidation state in groundwater, with arsenic IIIbeing the more mobile form. Selenium can occur in either a +4 (selenium IV) or +6 (seleniumVI) oxidation state in groundwater, with selenium VI being the more mobile form. Because thesoil and groundwater models assume one form for each model run, the risk results presented forApril 2010–Draft EPA document. 4-42