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Evaluation of NaturalAttenuation: 7500 West Eleventh Street, Tracy, CA. Page 29 <br /> be calculated by using a quantitative mass balance methodology. Uncontaminated <br /> groundwater flowing down-gradient across the source area of contamination transports <br /> r--� � g � p <br /> contaminants into the down-gradient portion of the plume. In the case of a stable or <br /> i- shrinking plume, the rate at which that material is lost from the source area is, by the law <br /> of conservation of mass, equal to the rate at which the processes of natural attenuation <br /> within the plume are removing contaminants from the groundwater. <br /> Water passing through the source area transports components of fuel hydrocarbons from <br /> two zones the smear zone and the zone of contaminated water beneath the smear zone. <br /> The smear zone is created by the rise and fall of the groundwater table. That zone, at least <br /> for several years after the fuel leak occurs, is affected by LNAPL or high concentrations <br /> of dissolved contaminants trapped in the soil pore space. The second zone from which <br /> flowing groundwater displaces contaminants from the source area is the zone between the <br /> bottom of the smear zone and the maximum depth to which groundwater is affected by <br /> j contamination. Research has shown that the vertical extent of contamination below the <br /> smear zone is no more than 10 ft. at sites where the soil down to that depth is a uniform, <br /> porous medium (Chiang, C.Y., et. al. 1989, Robbins, G.A. and Martin-Hayden 1991). <br /> However, the vertical depth of contamination may be restricted by an aquitard or <br /> aquiclude present at a depth less than 10 ft. below the smear zone. ASsit happens, at the <br /> Navarra Site, the distance from the bottom of the smear zone to the bottom of the <br /> contaminated aquifer is about 10 ft. <br /> The dissolved BTEX-loading rate of groundwater passing through the source area of the <br /> plume is the sum of BTEX-loading rate passing through the smear zone and the <br /> contaminated zone of the plume beneath the smear zone. If the BTEX-loading rate due to <br /> the smear zone is BTEXSZ and the BTEX-loading rate in the portion of the plume deeper <br /> E than that zone is BTEXb,,, the total BTEX-loading of groundwater passing through the <br /> source area is BTEXt,,t, which is calculated as follows: <br /> r.< BTEXt,,t= BTEXSZ+ BTEXb,, (Equation 2) <br /> BTEXSZ can be computed from: <br /> BTEXSZ = C',x QSZ (Equation 3) <br /> where: <br /> Csz = theoretical stability of BTEX in groundwater in contact with gasoline <br /> components. <br /> That value is approximately 100 mg/L (0.01 lb/ft), based on immiscible <br /> partitioning (Shin W.Y, et al 1988). <br /> 1 ;t QSz, is the rate of groundwater flow through the smear zone and is computed <br /> from: <br /> sic <br />