Human and Ecological Risk Assessment - Earthjustice

Appendix DMINTEQA2 Nonlinear Sorption Isothermsis documented in U.S. EPA (2000, 2001a). The evaluation of the Norris and Hubbard commentsresulted in suggested revisions in the MINTEQA2 modeling strategy, as described in U.S. EPA(2001b).Based on a review of available information on CCW leachate composition and ananalysis of the potential effects of this composition on metals mobility, EPA (U.S. EPA, 2001b)also determined that if MINTEQA2 is to be used at CCW sites, leachate from CCW facilitiesshould be studied to look for trends in composition, especially with regard to the concentrationsof constituents that may• Contribute to elevated groundwater pH• Compete with the contaminant metal for sorption sites and thus result in reduced metalsorption (e.g., Ca, Mg, SO 4 , other metals)• Complex with the contaminant metal so that the metal is less likely to be sorbed (e.g.,SO 4 , CO 3 , organic ligands)• Precipitate with the contaminant metal (e.g., SO 4 , CO 3 ).D.3 MINTEQA2 Modeling Revisions for CCW Risk AssessmentMany of the suggested revisions from U.S. EPA (2001b) were implemented in theMINTEQA2 modeling for the current CCW risk assessment. Some of the suggested revisionswere not implemented, either because they were not applicable (e.g., organic carbon assumptionswere not changed, because CCW leachate has negligible organic carbon) or because models ordata were not adequate to carry forth the recommendation. These revisions are discussed ingreater detail in U.S. EPA (2003c).In addition to revising the MINTEQA2 model, EPA compiled leachate characteristicsinto the CCW constituent database (see Appendix A) and statistically analyzed these data toidentify three chemically distinct CCW leachate types: conventional CCW (including ash andflue gas desulfurization [FGD] sludge), codisposed CCW and coal cleaning wastes, and fluidizedbed combustion (FBC) waste. Leachate concentration ranges for major ions (e.g., Ca, SO 4 , Mg,Na, Cl, etc.) and pH were developed for each of these waste types and were used to representCCW leachate during MINTEQA2 modeling.As needed, sorption reactions were included for those CCW constituents known toundergo significant sorption. Including elevated concentrations of leachate constituents and theircorresponding sorption reactions in the MINTEQA2 model allowed for full competition with thecontaminant metal for sorption sites. The metal solubilizing effect through complexationbetween the contaminant metal and dissolved ligands was also included, as was the potential formetal precipitation. Because precipitation of the metal can serve to attenuate the transportableconcentration, the equilibrium fraction in all three phases (dissolved, sorbed, and precipitated)were stored and made available for use by EPACMTP. The precipitated fraction was used todevelop a solubility limit that was used during EPACMTP modeling (U.S. EPA, 2003c).April 2010–Draft EPA document. D-3