Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterizationdistributions, which are regularly used in the probabilistic national risk analyses conducted byEPA. Therefore, the contribution of the exposure factors to the variability in risk was notparticularly useful for the primary purposes of the sensitivity analysis, to better understandsources of uncertainty in the CCW risk results and to help focus regulatory development onsensitive variables that can be addressed through the RCRA regulatory process.The outputs from the sensitivity analysis were goodness-of-fit values for the regressionmodels and the relative importance of each input parameter in determining environmentalconcentrations across different WMU, waste type, and constituent scenarios. The goodness-of-fitvalues of the regression models were moderate to very good for the drinking water pathway(R 2 =0.53–0.90) and good to very good for fish consumption (R 2 =0.76–0.90). In general, thedrinking water pathway had more input parameters that were significant (seven) than the fishconsumption pathway (three). The most sensitive parameters for most (over 70 percent) of thedrinking water scenarios 10 evaluated were parameters impacting groundwater flow:• Infiltration rate within the WMU footprint• Leachate concentration from the WMU• Aquifer hydraulic conductivity and groundwater gradient (i.e., groundwater velocity).For many (over 30 percent) of the scenarios, including those corresponding to stronglysorbing contaminants (i.e., metals with high soil/water partition coefficients), sorption and traveltime parameters are also important, including• Adsorption isotherm coefficient• Depth to groundwater• Receptor well distance.For the fish consumption pathway, only three variables were consistently significantacross scenarios:• Infiltration rate within the WMU footprint• Leachate concentration from the WMU• Waterbody flow rate.Additional detail on how the CCW sensitivity analysis was conducted can be found inU.S. EPA (2009b). In terms of the model inputs, the sensitivity analysis found that the mostconsistent drivers of the risk results were constituent concentration in waste leachate (i.e., thesource term for the risk assessment and infiltration rate through the WMU), which is largelycontrolled by the liner conditions and, to a lesser extent, soil type and (for landfills only)precipitation. These variables and their uncertainties are discussed in the following section.10Scenarios represent unique combinations of WMU, waste type, chemical, exposure pathway, and receptor.April 2010–Draft EPA document. 4-35