CORRESPONDENCE_2014-2015 - Laserfiche WebLink

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Modeling Approach The overall approach includes setting up the target groundwater mass flux based on groundwater threshold concentrations. The thresholds will be the more conservative of the California public health goals (PHG) or maximum contaminant level (MCL), if available. The VLEACH model will be used to back -calculate soil and soil vapor concentrations that would be needed to create the selected groundwater mass flux. The model will be run with the lower boundary open, which allows downward vapor diffusion into the saturated zone. Partitioning from vapor to groundwater is a very slow process that is orders of magnitude smaller than vapor diffusion. Therefore, with the lower boundary open the VOCs entering the groundwater from the vadose zone cell will be overestimated by the model. At the water table boundary, downward advection of dissolved contaminant in soil water is the likely dominant transport mechanism of contaminants to the water table. VLEACH modeling will give conservative estimates of the SVSLs for the modeled VOCs. The transport mechanisms considered in VLEACH involve soil vapor transport and liquid phased advection through infiltration. Model input parameters will be grouped into site parameters, soil parameters, chemical parameters and computational parameters. The water table is encountered generally greater than 200 feet bgs adjacent to Module "I". The soil concentration profile above the water table will be conceptualized as a 200 -foot soil column with a uniform soil contamination concentration. The soil column will to be one (1) square foot for simplicity of calculation. Each target groundwater concentration will be selected based on the more conservative of the California PHG or MCL. The VLEACH output will be reported in terms of contaminant mass flux to groundwater in grams/year. The target mass flux will be calculated by assuming that contaminants only mix with the top 1 -foot of the saturated zone, and that groundwater is flowing at 80 ft./year, based on the approximate groundwater flow velocity calculated at the landfill. Because VLEACH runs in a forward mode, the threshold soil concentration will be back - calculated by varying the value of each soil contaminant's concentration, such that the maximum mass flux to groundwater over the simulated time period of 30 years will equal the target mass flux rate. Soil vapor concentration values will be calculated from the modeled soil concentrations based on the following equation from the VLEACH User's Guide (Ravi and Johnson, 1997): Cg = KHMT / [0+(O-0)KH +Kdpb] Where: Cg = Vapor concentration of VOC in gas phase in µg/m3; KH = Henry's Law Constant (dimensionless); MT = total mass of contaminant in soil (µg/kg); 0 = volumetric water content (percent); � = soil porosity (percent); Foothill Sanitary Landfill 15 Department of Public Works, Solid Waste Division Revised Engineering Feasibility Study County of San Joaquin — December 19, 2014