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f ATC Associates Inc. - - (YVATC 1117 Lone Palm Avenue, Suite 201 Modesto, California 95351 209-579-2221 fax: 209-579-2225 'a I measurement associated with MW3 and the approximate depth of the deepest screened interval associate with the groundwater monitoring well network (MW I through MW3) . ATC assumes the geometric shape of the TPHg impacted groundwater plume to be a sub-spherical shape cut in l half along it longest axis. As a result, ATC used the formula "Volume = (4/3 *7r * r, * r2 * t)/2" to calculate the estimated volume for each contour interval, where rc = 3 . 14159, r, = longest measured radius of ellipse in feet, r2 = radius of ellipse in feet measure perpendicular to r, , and r3 l = average thickness of the water column in feet. The estimated volume of TPHg impacted soil and groundwater is approximately 51 ,616.32 11:3. The estimated mass was calculated for each contour interval using the following formula: ' I : I Mass (Ibs) = Volume (ft) * 28.31 LAO * Average Concentration (ug/L) * 2.205e" lbs/ug * effective porosity After reviewing boring logs from previous subsurface investigations and referencing the described soil types to their respective published values for effective porosity, ATC estimated an effective porosity value of 20 percent. ) Based on the third quarter 2008 groundwater sampling results, the mass of TPHg remaining in groundwater is estimated to be 10.64 lbs. J 3.0 EFFECTIVENESS OF REMEDIATION SYSTEM As of the end of the third quarter 2008, ATC has estimated that approximately 8,811 .95 lbs of TPHg have been removed by the soil vapor extraction system and approximately 10.292 lbs of — TPHg have been removed by the groundwater treatment portion of the remediation system. The j predominant mass of TPHg is located within the saturated zone soil. These systems have been operating at the site since December 19, 2002, Although they have not run continuously. Assuming these systems were to continue to operate as they are currently configured, the groundwater treatment system would have to operate for at least 6 more years to remove the remaining amount of TPHg in groundwater and the SVE/AS system would have to operate for at least 19 more years to remove the estimated remaining amount of TPHg from the vadose zone and saturated zone soil. Both of these zones were considered since the majority of TPHg impacted saturated zone soil are located between 25 and 30 feet bgs and static depth to water measurements tend to fluctuate between approximately 21 and 29 feet bgs. Although unlikely, the SVE system has removed enough TPHg mass to account for the estimated impact to vadose - zone soil. Some of the removed mass could be attributed to volatilization of petroleum hydrocarbons in groundwater during air sparging and from the fluctuation of groundwater beneath the site exposing portions of the saturated zone. Additionally, the stingers placed inside !—I the extraction wells create a cone of depression in MW1 , VWl , and VW2 exposing saturated zone soil while the SVE sytem is operating, f . r � S:EnviM=ental\62596\Reports\Mms Calm 2009.doc 6 u