Human and Ecological Risk Assessment - Earthjustice

Appendix DMINTEQA2 Nonlinear Sorption IsothermsFor each metal, the resulting set of isotherms was tabulated into a supplementary inputdata file for use by the EPACMTP model, hereafter referred to as an “empirical nonlinearisotherm.” In the fate and transport modeling for a particular metal, EPACMTP was executed,and the national probability distributions for these five master variables formed the basis for theMonte Carlo selection of the appropriate adsorption isotherm.In modeling metals transport in the unsaturated zone, EPACMTP uses a range of K dvalues from the nonlinear sorption isotherms. However, in modeling metals transport in thesaturated zone, EPACMTP selects the lowest from all available K d values corresponding toconcentrations less than or equal to the maximum water table concentration. For more details seethe EPACMTP Technical Background Document (U.S. EPA, 2003b). This simplification in thesaturated zone is required for all solution options and is based on the assumption that, afterdilution of the leachate plume in groundwater, the concentrations of metals will typically be in arange where the isotherm is approximately linear. However, this assumption may not be validwhen the metal concentrations in the leachate are exceedingly high. Although EPACMTP is ableto account for the effect of the geochemical environment at a site on the mobility of metals, themodel assumes that the geochemical environment at a site is constant and not affected by thepresence of the leachate plume. In reality, the presence of a leachate plume may alter the ambientgeochemical environment.D.2 Previous CCW Metals Modeling EffortIn a previous risk assessment for fossil fuel combustion wastes (FFCWs) conducted in1998 (U.S. EPA, 1998), sorption isotherms generated using MINTEQA2 were used inEPACMTP to account for metal partitioning. However, these isotherms were not calculatedspecifically for use in FFCW modeling—they had been computed using MINTEQA2 in 1995 foruse in modeling support for the Hazardous Waste Identification Rule (HWIR).The disposal scenario for HWIR was the industrial Resource Conservation and RecoveryAct (RCRA) Subtitle D nonhazardous waste landfill. In fact, the MINTEQA2 modeling thatproduced the isotherms had originally been designed to represent municipal solid waste landfills,and leachate from those landfills had been sampled so that appropriate forms of leachate organicacids at various concentrations could be included in the modeling. For the HWIR analysis, thescenario was changed to industrial Subtitle D, and only the isotherms corresponding to lowconcentrations of the leachate organic acids were used for HWIR modeling. The same isothermswere used in the 1998 FFCW risk assessment. As in the HWIR modeling, only the isothermscorresponding to the lowest setting of leachate organic carbon were used.In 1999, EPA received review comments concerning the use of the industrial Subtitle Dmetal partitioning isotherms in the 1998 risk assessment. The most comprehensive review wasprepared by Charles Norris and Christina Hubbard on behalf of the Environmental Defense Fundand other environmental advocacy groups (Norris and Hubbard, 1999). The Norris and Hubbardreport criticized the 1998 risk assessment for using MINTEQA2 isotherms designed for adifferent scenario (nonhazardous industrial landfills). Norris and Hubbard also offered 20specific criticisms on the input parameters and other factors involved in the MINTEQA2modeling. EPA responded by evaluating each of these criticisms through review and assessmentof MINTEQA2 input values, model sensitivity tests, and consultations with experts. This reviewApril 2010–Draft EPA document. D-2