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Ms.Lori Duncan <br /> November 15,2011 <br /> RE:2911 E.Fremont St,Stockton,CA <br /> Page 6 of 14 <br /> contains color copies of photographs taken during excavation,backfill, and restoration activities. <br /> CONTAMINANT MASS ESTIMATE CALCULATIONS <br /> 211.23 tons (approximately 170 cubic yards) of mostly contaminated soil was removed from the <br /> excavation and transported to Forward Landfill. The concentrations of TPHg detected in confirmation <br /> soil samples collected during the excavation,as well as soil samples collected form borings located within <br /> the excavation boundaries, were used to estimate the mass of contaminants present at the site and the <br /> mass of contaminants removed through excavation. <br /> Figures 4 and 5 present soil sample locations,concentrations of TPHg in soil samples,and the estimated <br /> extent of TPHg contamination in soil. Table 1 summarizes the analytical results of soil samples collected <br /> during pre-excavation activities as well as soil samples collected from the excavation floor and sidewalls. <br /> The estimated mass of contaminants present in the soil prior to and after excavation activities using the <br /> contours depicted in Figures 4 and 5. Estimated mass was calculated using the average concentrations of <br /> samples within a give"contour zone"using two assumptions regarding the"shape" of the contaminant <br /> plume. Assuming a cylindrical shape throughout the entire depth interval of documented soil <br /> contamination results in a maximum estimate of mass,while assuming each zone of contamination has a <br /> spherical shape results in a minimum estimate of mass. <br /> Based upon the above assumptions and the concentrations of soil samples collected within and below the <br /> area of excavation,an estimated mass of between approximately 280 and 380 pounds of gasoline(TPHg) <br /> was present in soil prior to excavation,and a roximately 125 to 190 ounds of residual asremains <br /> in the subsurface after excavation activities. Based uponese estima es,excavation activities resulted in <br /> the removal of between 49%and 55%of the soil contamination. The configuration of the site(inside the <br /> building) prevented further excavation at the site. Contaminant mass calculations are included in <br /> Attachment 4. <br /> DISCUSSION OF SOURCE REMOVAL <br /> Prior to excavation, contaminated soils in the vadose zone were present in the immediate vicinity of the <br /> former clarifier to a depth of approximately 33-34 feet bgs. The source removal excavation succeeded in <br /> removing the former clarifier as well as approximately 170 cubic yards of impacted soil from the upper <br /> vadose zone. Due to the location of the former clarifier and impacted soil inside the warehouse,however, <br /> not all impacted soil was accessible for removal. Approximately 49-55% of the impacted soil was <br /> removed, leaving approximately 125 to 190 pounds of residual gasoline in place. The remaining <br /> impacted soil sits at a depth between approximately 18.5 and 33-34 feet below the ground surface, <br /> primarily within fine-grained silt. The potential for residual contamination in the soil to leach to <br /> groundwater is minimal due to the presence of silt and clay between the base of the contaminated zone at <br /> 34 feet and the documented water table at a depth of approximately 55 feet. Groundwater samples <br /> collected from soil borings at the site contained no detectable petroleum hydrocarbons, although low <br /> 10759-EXC SUMMARY LTR <br />