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