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6 GtnlogtcalTcckxtcslxc Page 14 Revised Site Conceptual Model Report j Project No.507.2 April 3,2008 skewed by the extremely high data point in boring MW-9-(3;090,000,mg/kg and the non- detect points (I mg/kg) within the irregular ellipsoid. Note: The WJH boring log includes,the following description for soil samples from MW-9 at 10, 15 and 20 feet bgs, respectively: "No odors or staining", "No odors or visible contamination", "Petroleum odor noted but no visible staining". Therefore, GTI believes ` the laboratory data of 1,600,000 mg/kg, 1,120,00 mg/kg and 3,090,000 mg/kg, respectively f are probably an error and the units;should be u /k rather than the reported mg/kg. In spite P Y , g g , r of this anomaly between the field observations and lab data GTI does use the original data as { presented using the geometric mean methodology to temper the results. a 2.6.2 Groundwater Plume r +` The February,11, 2008 groundwater' sampling data were used to define the groundwater TPH- G plume. Figure G3 in Appendix G illustrates the TPH-G concentration in the aquifer with zone numbers assigned to each contour interval. Figure G4 in Appendix G shows the groundwater plume projected onto cross section B-B'. i The total mass of contaminant in groundwater at the site was determined by first calculating s the volume of,water within each aquifer zone contour. GTI used CAD software to determine the area (in square feet) within each contaminant contour line (zone) in Figure G3. The area was then multiplied by the thickness of the impacted aquifer (estimated 21.5 feet) to produce , the volume of zone in cubic feet. The volume (in cubic feet) of each zone was then multiplied by'a porosity value of 45.83% (average porosity for MW-7 @ 20', MW-10 @ 25' and MW-10 @30') to obtain the total volume of water in each zone. This value was then converted to liters and then multiplied by the median contaminant value in mg/l within the contour zone.':This produces the mass of contaminant within each contour. As shown in Table G2 in Appendix G there is approximately 64 gallons (or 181 kg) of TPH- G in groundwater at the site. ` Total Mass Estimate The total TPH-G at the site in soil and groundwater combined is estimated to be 1.,722 gallons (or 4,842 kg). 2.7 Groundwater Benef chd Uses The Regional,Water Quality Control Board-Central Valley Region's Basin Plan designates the beneficial uses of groundwater in the San Joaquin Valley as domestic, municipal, industrial and agricultural supply. >Documentation of the non-use of the area's groundwater was completed in September 2000 when GTI performed a well survey for a 2,000-foot radius around the site. The purpose of, the survey was to locate any municipal, domestic, or agricultural wells within the 2,000-ft radius and to determine, if possible, the construction of these wells. The well survey data is included as Appendix H. y 1