Human and Ecological Risk Assessment - Earthjustice

Section 3.0AnalysisIn the saturated zone, the EPACMTP model assumes that movement of chemicals isdriven primarily by ambient groundwater flow, which in turn is controlled by a regionalhydraulic gradient and hydraulic conductivity in the aquifer formation. The model does take intoaccount the effects of infiltration through the WMU, as well as regional recharge into the aquiferaround the WMU. Infiltration through the WMU increases the groundwater flow in all directionsunder and near the WMU and may result in groundwater mounding. This 3-dimensional flowpattern enhances the horizontal and vertical spreading of the contaminant plume. The effect ofrecharge (outside the WMU) is to cause a downward (vertical) movement of the contaminantplume as it travels along groundwater flow direction. In addition to advective movement with thegroundwater flow, the model simulates mixing of contaminants with groundwater due tohydrodynamic dispersion, which acts along the groundwater flow direction, as well as verticallyand in the horizontal transverse direction.To model sorption of CCW constituents in the unsaturated zone, soil-water partitioningcoefficients (K d values) for metal constituents were selected from nonlinear sorption isothermsgenerated from the equilibrium geochemical speciation model MINTEQA2 (U.S. EPA, 2001a).Chemicals with low K d values will have low retardation factors, which means that they willmove at nearly the same velocity as the groundwater. Chemicals with high K d values will havehigh retardation factors and may move many times slower than groundwater. As described inAppendix D, CCW-specific partition coefficients were developed with MINTEQA2 consideringCCW leachate chemistry, including the highly alkaline chemistries that are characteristic ofsome CCWs. Although a complete listing of all K d values available in the MINTEQA2 isothermsused in these analyses would not be practicable, Table D-1 presents a sampling of the K d valuesused.MINTEQA2 is a product of ORD, and like EPACMTP, has a long history of peer- andSAB-review during its development, use, and continued improvement for regulatory supportover the past two decades. These reviews largely focused on the use of MINTEQA2 to generatesorption isotherms for metals for EPACMTP, which is how it was used in the CCW riskassessment. Two of the more recent peer reviews include one for application within the 3MRAmodel (U.S. EPA, 1999d) and a review of its use and application to RCRA rulemaking andguidance support, including revisions made to the model to support IWEM and the CCWrulemaking efforts (U.S. EPA, 2003f). In the latter review, three experts found that the revisionsmade to the MINTEQA2 model were appropriate, but also suggested further improvements inhow the model addresses environments with highly alkaline leachate (such as CCW sites). Asexplained in Appendix D, these comments were addressed in this application of MINTEQA2 toCCW waste transport by the development of sorption isotherms that are specific to geochemicalconditions encountered in CCW landfills and surface impoundments.3.6.3 Model Inputs and Receptor LocationsEPACMTP requires information about soil and aquifer properties as model inputs. Forsoils, EPACMTP uses soil texture to generate consistent hydrological properties for theunsaturated zone model, and soil pH and organic matter to select appropriate sorptioncoefficients to model contaminant sorption in the soil. As described in Appendix C,Attachment C-2, site-specific soil texture, pH, and organic carbon data were collected aroundeach site from the STATSGO soils database. Similarly, the hydrogeological setting around eachApril 2010–Draft EPA document. 3-34