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11 .- • In addition to the deep soil gas wells and monitoring thereof, the County is proposing to use the EPA's VLEACH Modeling software in Phase 1. VLEACH is a one-dimensional, finite difference model for making preliminary assessments of the effects on groundwater from the migration of VOCs through the vadose zone. The program models four (4) main processes: liquid -phase advection, solid -phase sorption, vapor -phase diffusion, and three-phase equilibration between the liquid, solid, and vapor phases. Numerous VOCs have been detected in the vadose zone at the FSL. The movement of LFG from the unlined base of Module "I" at FSL is believed to be the source of VOCs in soil voids within the vadose zone. It is proposed that VLEACH modeling will be conducted to estimate the VOC concentrations in the vadose zone that would result in an exceedance of groundwater regulatory levels for VOCs. The model provides a conservative threshold screening value to evaluate soil vapor concentration in the vadose zone since no thresholds exist for LFG VOCs in the vadose zone. The modeling work will develop protection of groundwater soil vapor screening levels (SVSLs) for 10 VOCs found in soil vapor in the vadose zone. Model Description VLEACH is a one-dimensional model that describes the movement of contaminants between three (3) phases: (1) as solute dissolved in liquid phase, (2) as a gas in vapor phase, and (3) as an adsorbed chemical in the solid phase. VLEACH estimates liquid phase advection and gaseous diffusion in the vapor phase to simulate vertical transport of chemicals through the vadose zone. The VLEACH Model predicts a mass flux of the chemical from the vadose zone to groundwater over time. The model uses the following assumptions: • Contaminant partitioning between phases follows linear relationships • Liquid, vapor, sorbed phases are in a state of equilibrium in each cell • Each chemical acts independently (i.e. chemical mixtures do not affect equilibrium concentrations) • Soil moisture content profile is constant and the vadose zone is in a steady state with respect to downward water flux or recharge rate • Liquid -phase dispersion is neglected • No mobile non -aqueous phase liquid (NAPL) is present in the vadose zone • Soil column from surface to the groundwater table is homogeneous with uniform soil properties, and • Chemical concentration is not subject to natural degradation in soils. The use of VLEACH to determine a SVSL is a very conservative approach as laboratory column studies indicate that VLEACH may overestimate the mass leached from soil to groundwater (Bong, 1999). Foothill Sanitary Landfill 14 Department of Public Works, Solid Waste Division Revised Engineering Feasibility Study County of San Joaquin — December 19, 2014