Human and Ecological Risk Assessment - Earthjustice

Appendix DMINTEQA2 Nonlinear Sorption Isothermsand (3) a scientifically defensible approach, given significant uncertainties with respect to thetrue impacts of leachate pH on the subsurface. Two modifications to the EPACMTP wereconsidered: (1) determine the governing pH in the soil column (either the pH of the leachate orthe native soils); and (2) determine the pH of the saturated zone as a result of the infiltratingleachate.The approach selected for determining the governing pH of the soil column (unsaturatedzone) beneath the waste management unit (WMU) compares the operational life of the WMU(the duration of leaching) to an estimate of the first arrival time of the contaminant front at thewater table (a surrogate for the residence time of the contaminant in the soil column). If theoperational life of the WMU is relatively long compared to the time required for the contaminantto migrate to the water table, there is a high likelihood that the leachate permeates the soilcolumn and that the pH environment is governed by the leachate. Conversely, a relatively shortoperational life and retarded contaminant migration would favor ambient soil pH conditions. Ananalysis of the relationship between operational life and travel time indicated that a ratio ofapproximately 5 (operational life over travel time) would, in many cases, result in a balancedselection of cases where leachate pH governs versus cases where soil pH governs overapproximately 10,000 Monte Carlo iterations.For each iteration of EPACMTP, the operational life was compared to a travel-timeestimate based on a K d averaged from isotherms selected based on the leachate pH and soil pH.If the ratio was greater than 5, the pH of the leachate was assumed to govern, and the pH of theleachate was used to select the isotherm for transport in the unsaturated zone. If the ratio was lessthan 5, the soil pH was used to select the isotherm.In the saturated zone, the impacts of leachate pH were handled using a simplehomogeneous mixing calculation. The volume of leachate released from the WMU was mixedwith the volume of the aquifer that was likely to be impacted by a plume. The resulting mixedpH was used to select the isotherm for transport in the saturated zone with one limitation: incarbonate environments, the mixed pH in the aquifer was not allowed to drop below a pH of 6.Such acid conditions would likely result in significant dissolution of the soil matrix.Metal Solubility Limits. As mentioned above, each sorption isotherm comprisesequilibrium concentrations of the three contaminant phases (dissolved, sorbed, and precipitated)over a range of total concentration values. An examination of the change in the dissolved-phaseconcentrations relative to changes in the total concentration in any isotherm reveals solubilitybehavior for that contaminant: if the dissolved component does not change with increasing totalconcentration, a solubility limit has been achieved. If, however, the dissolved componentincreases along with the total concentration, then there is capacity for more dissolved mass in thegroundwater or soil porewater.EPACMTP uses this information (contained in each isotherm file) to determine if asolubility limit should be imposed in the saturated zone. Once an isotherm has been selected(after pH considerations have been addressed), the equilibrium states corresponding to the threehighest total concentrations are examined. If the dissolved concentration changes more than onetenth of one percent over the last three points, then EPACMTP assumes there is no solubilitylimit. If the change in dissolved concentration is less than one tenth of one percent, EPACMTPApril 2010–Draft EPA document. D-5