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CAP Addendum:Former Fuee*Mfg Station, 7500 West Eleventh Street, Tracy, CA-W Page 7 2.2.1.4 Dissolved Contaminants in Groundwater Although, as noted above, the mass of dissolved contaminants in groundwater beneath the area of the site affected by LNAP has been accounted for by adding the mass of LNAPL to the mass of contaminants computed from the analyses of soil, it is useful to make an estimate of the mass of dissolved contaminants in the groundwater. As was the case used for developing an estimate of the mass of fuel hydrocarbons in soil, the mean concentration of fuel hydrocarbons dissolved in groundwater in the area of the site affected by LNAPL can be developed by consideration of the sub-area that is located between Monitoring Wells MW-13 and MW-7 and the sub-area between Monitoring Wells MW-7 and MWFP-3. As can be seen by examination of Table 2, the sum of the concentrations of TPHd and TPHg in groundwater in Monitoring Well MW-13 in July 2007 was 4,300 µg/L. The equivalent numbers for Monitoring Wells MW-7 and MWFP- 3 are listed below. )V)// Monitoring Concentration of Co,3r75-;'%7/1 75 a^7 p Sa�7f 9/ r Well Contaminants in GW K5 boa 5c5� MW-13 4,300 µg/Lo MW-7 89,000 µg/L MWFP-3 0.00 µg/L* * Although a groundwater sample was not recovered from Monitoring Well MWFP-3 in July 2007, it is reasonable to assume, based on analyses of samples recovered previously, that the concentrations of fuel hydrocarbons in that well was, for all practical purposes, 0.00 µg/L. Given those values, the mean concentration of fuel hydrocarbons in the sub-area between Monitoring Wells MW-13 and MW-7 is 46,650 gg/L. The equivalent value for the sub- area between Monitoring Wells MW-7 and MWFP-3 is 44,500 µg/L. The mean concentration in the whole area is 50,575 µg/L. Taking the porosity of the soil to be 0.3, and noting that the dimensions of the soil volume in the sub-area between Monitoring Wells MW-13 and MW-7 are 78 ft. from north to south, 80 ft. from east to west and 15 ft. in depth, the volume of groundwater in that zone is 28,080 ft3 or 795,137 L. Then, using the estimated concentration of 46,650 �tg/L for fuel hydrocarbons in groundwater in that sub-area, the mass of dissolved fuel hydrocarbons is 37.09rams. Applying the same set of calculations to the sub-area between Monitoring Wells MW-7 and MWFP-3 yields a mass of 16.78 grams. Therefore, adding the estimated mass of dissolved fuel hydrocarbons in groundwater in the sub-areas yields: ESTIMATED TOTAL MASS OF DISSOLVED CONTAMINANTS IN GROUNDWATER=54 GRAMS /I