Human and Ecological Risk Assessment - Earthjustice

Appendix DMINTEQA2 Nonlinear Sorption Isothermsassumes a solubility limit has been reached and caps the concentration of the leachate enteringthe saturated zone at the water table to that limit.D.6 Sampled K d s from MINTEQA2 Isotherms by EPACMTPAs described above, a range of K d s from an isotherm were used by EPACMTP in eachunsaturated zone transport simulation. To simplify the presentation of K d here, an effective K dvalue was calculated and reported by EPACMTP. An effective K d was determined by firstestimating the value of the retardation factor, described in Equation D-1, as follows:R tWTd soilqwheret wtqd dsoil= time for leachate to reach the water table (yr)= seepage velocity of leachate through the unsaturated soil column (m/yr)= depth to the water table or length of the unsaturated soil column (m)Substituting this value for R in Equation D-1 and solving for K d yields an effective K dthat is based on the first arrival of the leachate front at the water table.Table D-1 presents selected percentiles of K d sampled from the MINTEQA2 isothermsfor every waste management modeling scenario conducted in the CCW risk assessment for thegroundwater pathway. Each scenario corresponds to a unique combination of waste type, metalspecies, waste management unit type, and subsurface domain (unsaturated zone or saturatedzone). The values presented for the saturated zone are taken from the set of actual K d values usedin each modeling scenario.April 2010–Draft EPA document. D-6