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Evaluation of Natural Attenuation: 7500 West Eleventh Street, Tracy, Cry. Page 9 <br /> 2.0 GROUNDWATER ELEVATIONS.AND CONTOURS <br /> To initiate the groundwater monitoring round and natural attenuation evaluation program <br /> conducted in February and March 2005, on February 24, 2005, the depth to groundwater <br /> in each of the 27 monitoring wells was measured using a conductivity probe. The water <br /> table elevations were computed relative to mean sea level (MSL). Those measurements <br /> and the computed groundwater table elevations are recorded in Table 1. <br /> The groundwater elevations presented in Table 1 were used to generate the groundwater <br /> '! contours shown on Figure 2. However, because, as is noted in Table 3, an apparent 0.02 <br /> ft. of floating product was detected in Monitoring Well MW-7, the groundwater elevation <br /> in that well was corrected for the purpose of drawing the groundwater contours to allow <br /> for the depression of the water table in the vicinity of that well by the hydrocarbon fuel <br /> products floating on the surface of the water. The correction was derived as follows: <br /> r Assume Specific Gravity of Diesel= 0.84 <br /> Assume Specific Gravity of Gasoline= 0.74 <br /> Assume relative proportions of diesel and gasoline in the floating product in Monitoring <br /> Well MW-7 are in the same ratio as the concentrations of diesel and gasoline in the <br /> sample of groundwater recovered from that well on March 9, 2005. They were, <br /> respectively, 18,000 µg/L and 15,000 [.g/L. (See Table 2.) <br /> ......... . <br /> Thus,the estimated specific gravity of the floating product in well MW-7 is: <br /> PP = 0.79 <br /> The depression in the elevation of the water table due to the presence of floating product <br /> F b given is <br /> g y <br /> W=T (pp/pw) (Equation 1) <br /> Where: <br /> '=-{ W = the depression of the water table <br /> T = the thickness of floating product in the monitoring well <br /> p,N — the specific gravity of the groundwater <br /> '- PP= the specific gravity of the floating product <br /> Thus, in the case of the floating product measured in well MW-7 on February 24, 2005: <br /> = ". W= 0.02 (0.7911.0) = 0.02 ft. <br /> :. sic <br />