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0 0 TECHNICAL MEMORANDUM Evaluation of Bed Ash Disposal Forward Landfill San Joaquin, California November 1, 2016 Page 9 should be noted that virtually any initial contaminant concentration may be assumed. Using the initial concentration, MULTIMED calculates a concentration at the receptor well. A dilution attenuation factor (DAF) is then calculated as ratio of the concentration at the receptor well and the initial contaminant concentration. • Thickness of Unsaturated Zone. Site data indicate the depth to groundwater below the landfill is on the order of 60 to 80 feet below the native ground surface. For the purposes of analysis,the unsaturated zone was conservatively assumed to be about 31.5-feet-thick to account for cell construction excavation and possible fluctuations in groundwater elevation.5 Under steady-state conditions, the thickness of the vadose zone is irrelevant because time and attenuation are not considered. • Unsaturated Zone Vertical Hydraulic Conductivity and Air-Entry Pressure Head. The vertical hydraulic conductivity of the unsaturated zone was assumed to be approximately equal to the average horizontal hydraulic conductivity of the Victor Formation divided by 10. The air-entry pressure head of the vadose zone soil was assumed to be zero. This is conservative because soil suction does not have to be overcome for migration to occur (the air-entry value has no effect on steady-state flow rates). • Residual Water Content (Bound Moisture) and van Genuchten Coefficients. These values were selected based on MULTIMED model documentation for the CCL (silty clay) and the underlying native deposits(conservatively assumed to be sand). Under transient conditions, these values affect the time required for a wetting front to pass through the unsaturated zone to the underlying groundwater. They have no effect on steady-state analyses. • Percent Organic Content and Biological Decay Coefficient. In general, the percent organic material affects retardation of adsorption of organic constituents and the biologic decay coefficient is also more applicable to organics. For the purposes of this analysis, the percent of organic material and the biological decay coefficient were assumed to be zero. s The 2-foot-thick CCL was included as part of the vadose zone because the assumed locus of leakage would be directly on top of the clay and the clay is not likely to be saturated.