Human and Ecological Risk Assessment - Earthjustice

Section 2.0Problem Formulationand C.Data sources and collection methods for these variables may be found in Appendices BCCW constituent data in the CCW constituent database were used as a national empiricaldistribution of the concentrations of the constituents of concern in CCW landfill leachate andsurface impoundment porewater. Like the WMU database, the CCW constituent data includeWMU type and waste type, which enabled constituent concentrations to be assigned to the 181CCW sites by waste type and WMU type. The CCW constituent database was also used to assign(by waste type) the high, medium, and low leachate pH and ionic strength conditions needed toselect the appropriate subsurface sorption isotherms for each model run (see Appendix D).Because site-specific data were not readily available, national distributions were used topopulate the following variables by model run:• Distance to nearest drinking water well• Distance to nearest surface waterbody• Aquifer depth, thickness, gradient, and hydraulic conductivity (based on site-specifichydrogeologic setting)• Soil hydrologic properties (based on site-specific soil type).The data sources used to develop national distributions for these variables are describedin Appendix C. Human exposure factors, such as exposure duration and drinking water and fishconsumption rates, were also based on national distributions, which are provided in Appendix F.2.4.2 Model ImplementationAs a first step in the modeling process, the groundwater model (EPACMTP) read thesite-based data files to estimate the following for each model run:• Drinking water well peak concentration• Time to drinking water peak concentration• Peak surface water contaminant flux• Time to peak surface water contaminant flux.The groundwater model was run until contaminant concentrations at the receptor pointreturned to zero after the concentration peak or for the maximum simulation time of 10,000years, whichever came first.April 2010–Draft EPA document. 2-14