Human and Ecological Risk Assessment - Earthjustice

Appendix DMINTEQA2 Nonlinear Sorption IsothermsD.4 MINTEQA2 Modeling for CCW Risk AssessmentThe expected natural variability in K d for a particular metal was represented during theMINTEQA2 modeling effort by varying the input parameters that most impact K d : groundwatertype (carbonate or noncarbonate), pH, concentration of aquifer sorbents, composition andconcentration level of CCW leachate, and concentration of the contaminant metal. The naturalpH range for the two groundwater types was sampled from a range of 7 to 8 for carbonateaquifers and 4 to 10 for noncarbonate aquifers (U.S. EPA, 2003c).In addition, CCW leachate ranges from acidic (pH < 2) to highly alkaline (pH > 12), andit can impact unsaturated zone and groundwater pH. To account for this possibility, the CCWleachate/ groundwater system was equilibrated at a series of pH values that spanned the range ofexpected variability in mixed CCW leachate-groundwater systems (U.S. EPA, 2003c).To account for the variability in the sorption capacity of soil and aquifer materials, thesoil and groundwater systems were equilibrated with various concentrations of two commonlyoccurring natural sorbents: ferric (iron) oxyhydroxide (FeOx) and particulate organic matter(POM). CCW leachate can include elevated concentrations of inorganic constituents such ascalcium, sulfate, sodium, potassium, and chloride, which may reduce sorption of metals due tocompetition for sorption sites or complexation with metals in solution. To account for this effect,these leachate components were added to the MINTEQA2 model inputs at concentrationsrepresentative of the three CCW waste types (conventional CCWs, codisposed CCW and coalcleaning wastes, and FBC wastes). This new MINTEQA2 master variable is termed leachate“richness” or ionic strength (U.S. EPA, 2003c).The results of each MINTEQA2 model run were compiled as the equilibrium distributionof the contaminant metal among dissolved, sorbed, and precipitated fractions for each metalconcentration, and were saved in a separate file indexed with the settings of all variables used todefine the system. These files were produced for all possible values for the variables defining thesystem, and were compiled into a database of indexed K d values for use in the EPACMTP fateand transport model (U.S. EPA, 2003c).D.5 EPACMTP Modeling Revisions to Accommodate MINTEQA2 UpdatesEPA updated EPACMTP to support the new system variable (leachate ionic strength) forisotherm selection, to address issues regarding the impacts of leachate pH on ambient soil andaquifer pH, and to address issues regarding solubility limits for metals in solution. A briefdescription of these model changes are discussed below, with more detail provided in U.S. EPA(2003d).Ionic Strength. A new system or “master” variable was added to include ionic strengthas a key for choosing the representative isotherm from the database for both the unsaturated andsaturated zones.Leachate Effects on Geochemical Environment. These effects were addressed inEPACMTP under the following constraints: (1) no significant impairment of the computationalefficiency for probabilistic applications; (2) data requirements limited to readily available data;April 2010–Draft EPA document. D-4