Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk CharacterizationCCW risk analysis assumed that, after entering the aquifer, the leachate plume thoroughly mixeswith the ambient, uncontaminated groundwater. However, because this mixing zone is largely atthe periphery of the groundwater plume, thorough mixing may or may not occur at actual sites.The full mixing assumption results in higher receptor point concentrations for most metals,because metal sorption and precipitation tend to increase (i.e., Kd goes up) with higher pH andfull mixing tends to reduce the pH of CCW leachate, which is normally alkaline (i.e., assumingfull mixing results in a lower groundwater pH and lower sorption).To assess the effect of this simplifying assumption on the risk results, we compared twolandfill Monte Carlo simulations for coal ash waste containing As(III) and coal ash wastecontaining As(V): (1) the fully mixed aquifer assumption and (2) an assumption that no mixingoccurs in the aquifer and the leachate pH is the governing pH for Kd selection. These two metalspecies were selected because their sorption isotherm behavior with pH change differs; Kdsderived from As(III) isotherms tend to decrease as pH increases (which is typical of most metalspecies examined in the risk assessment), while Kds derived from As(V) isotherms tend toincrease with increasing pH.Percentiles of peak receptor well concentration from the As (III) and As (V) simulationswere selected and compared by calculating the percent change with mixing assumption asfollows:% Change=No Mix− CCFull Mix× 100CFull MixwhereC No Mix = Simulated peak receptor well concentration for a select percentile based on ano mixing assumption (mg/L)C Full Mix= Simulated peak receptor well concentration for a select percentile based on afully mixed assumption (mg/L)Table 4-29 compares the percent change in peak receptor well As (III) and As (V)concentrations between the well mixed and no mixing scenarios over a range of peak wellpercentiles. The results indicate that As(V) has a sensitivity to pH that leads to increasedreceptor well concentrations under the no mixing assumption (i.e., when the leachate pH is usedto determine Kd in the saturated zone) relative to the well-mixed assumption used in the riskassessment. These results suggest that a change in the complete leachate mixing assumptioncould raise the receptor well concentrations (and therefore risks) for metal constituents whoseKd values decrease with increasing pH.April 2010–Draft EPA document. 4-44