Human and Ecological Risk Assessment - Earthjustice

Section 3.0Analysisconsolidation. Also, LDS flow rates from three geomembrane/geosynthetic clay lined-cells werenot used. For one cell, postclosure flow rates were very high, and were more than twice as highas those recorded during the cell’s operating period. Data were not used for two other cellsbecause of inconsistencies with the data for the 27 landfill cells used to develop composite linerinfiltration rates (U.S. EPA, 2002b). The composite liner infiltration rates were specified as anempirically distributed input to the landfill model (see U.S. EPA, 2003a).3.4.2.2 Source Depletion and Mass BalanceFor this assessment, the landfill source-term model represented releases from landfills asa finite source where the mass of a constituent in a landfill is finite and depleted over time byleaching. The landfill source-term model was set as a pulse source, where the leachateconcentration is constant over a prescribed period of time and then goes to zero when theconstituent is depleted from the landfill. A pulse source is appropriate for metals and otherconstituents whose sorption behavior is nonlinear. Because all but one (nitrate/nitrite) of theconstituents addressed in the full-scale analysis were metals, releases from landfills weremodeled as pulse sources.For a pulse source, basic mass balance considerations require leaching from the landfillto stop when all of the constituent mass has leached from the landfill. For the constantconcentration pulse source condition, the pulse duration is given byTSOURCCWASTE × DEPTH × FRACT × CTDENS= (3-1)CZERO × SINFILwhereTSOURC = Pulse duration (yr)CWASTE = Constituent concentration in the waste (mg/kg)DEPTH = Depth of landfill (m)FRACT = Volume fraction of the landfill occupied by the waste (unitless)CTDENS = Waste density (g/cm 3 )CZERO = Initial waste leachate concentration (mg/L)SINFIL = Annual areal infiltration rate (m/yr).The landfill source-term model uses the above relationship to determine the leaching duration.More details regarding the waste concentration and WMU parameters in Equation 3-1 areprovided below and in Appendices A and B.3.4.3 Landfill Model Input ParametersInput parameters required by the landfill source-term model are discussed below.Additional details on how data for these inputs were collected for the CCW risk assessment areprovided in Appendix A for leachate and waste concentrations and Appendix B for landfilldimensions and characteristics.• Landfill Area. The model uses landfill area to determine the area over which infiltrationrate occurs and, along with landfill depth and waste concentration, to calculate the totalApril 2010–Draft EPA document. 3-24