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Evaluation of Natural Attenuation: 7500 West Eleventh Street, Tracy, CA. Page 30 <br /> Qu-=KSZ x i x ASz (Equation 4) <br /> 1 where: <br /> i KSZ= the effective hydraulic conductivity of the smear zone <br /> i= groundwater gradient <br /> AS, = area of the smear zone as measured on a section orthogonal to the <br /> longitudinal axis of the plume. <br /> KSZ is less than Kb.,,, the hydraulic conductivity of the soil beneath the smear <br /> zone, because product trapped in the soil pores impedes the flow of <br /> groundwater through the soil in the smear zone. At the Navarra Site, KbSZ has <br /> ,J been reliably estimated to be 1.5 x 10-2 cm/sec (42.52 ft/day) (The San Joaquin <br /> Company 2002c). <br /> is<; <br /> In medium sand, similar to the permeable aquifer that is contaminated at the <br /> 7500 West Eleventh Street site, the ratio of KSZ to Kb,, has been estimated by <br /> Parker, et al(1987) to be 0.3, so that KSZ would be 12.76 ft/day. <br /> As noted in Section 2.0 above, the groundwater gradient is 0.003 ft/ft <br /> The thickness of the smear zone is the vertical distance between the highest <br /> groundwater elevation and the lowest groundwater elevation. As can be seen <br /> E by examination of Table 1, the maximum depth to groundwater in Monitoring <br /> Well MW-3 is 8.96 ft. and the minimum depth to groundwater in that well is <br /> 7.36 ft.; therefore the thickness of the smear zone is 1.60 ft. The width of the <br /> smear zone at section B-B' (see Figure 3 for location) is estimated to be 219 ft. <br /> This is based on the consideration that Monitoring Well MW-4, where <br /> groundwater is affected by significant concentrations of fuel hydrocarbons, is <br /> approximately 109 ft. east of Monitoring Well MW-3 (see Table 6). However, <br /> 196 ft. west from Monitoring Well MW-3, significantly elevated <br /> concentrations of contaminants are not present in Monitoring Well MW-2. <br /> Under these circumstances, it was judged conservative to assume that the <br /> smear zone would be symmetric about Monitoring Well MW-3, which is <br /> located at the point where the fuel hydrocarbons were released, so that the <br /> }:.. width of the smear zone has been estimated to be 219 ft, approximately twice <br /> the distance from Monitoring Well MW-3 to MW-4. From those dimensions, <br /> the area of the smear zone, ASZ, is 350.4 ft2. <br /> Applying those values to the parameters in Equation 3 above, <br /> ' Q.= 13.41 ft3/day <br /> i _ J <br /> t I <br /> sic <br />