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s .. 4 Similar trends for TPH-gasoline in monitor well MW-2 also indicate WQO being reached in six years. Figure 8 shows the TPH-gasoline trend in MW-2, while figure 9 gives greater resolution on the y-axis "results" scale by shortening the time frame. The greatest time frame for WQO attainment at the subject site is limited by TPH-gasoline in well MW-5.. As shown in figure 10, WQO attainment is expected in twelve.years. ' 8.2 Gasoline Contaminant Mass in Groundwater JJW calculated the mass of gasoline remaining in the groundwater at 0.09 kilograms (kg). The only two monitor wells that reported gasoline values in the recent June 2009 sampling event were MVV-2 and MW-5, both at the top of the groundwater table (figure 4). The area of the plume, as represented in figure 4, is 4,540 square feet for the 50. µglL to 500 µglL concentration contour lines, and 735 square feet from the 500 µglL contour to the center, ' estimated at 1000 µg/L of gasoline. Assuming that the concentrations trend downward with depth, as is typical, trending from the surface concentration down to 50 µg/L, the laboratory reporting limit, over an average depth of 10 feet, the volumes of petroleum contaminated groundwater of the inner and outer sections would be 45,400 cubic feet, or 1,285.,000 liters, and 7,350 cubic feet, or 208,000 liters, respectively. For the outer section, averaging the concentration by adding 50 µglL and 500µglL and dividing by 2, yields an average concentration of 275 µg/L over the area. Averaging the concentration as it decreases with depth yields a total average volumetric concentration in the outer section of 162.5 µg/L. ( [275µg/L+ 50µg/L] 12 ). Similarly, the inner section is calculated to have an average volumetric concentration of 400 µg/L. The outer section volume of 1,285,000 liters times the average concentration of 162.5 µg/L yields a gasoline mass in the outer section of 0.21 kg. The inner section volume of 208,000 liters times the average concentration of 400 µglL yields a gasoline mass in the inner section of i' 0.08 kg, for a total of 0.29 kg of gasoline. 100% lithologic porosity is assumed, the gasoline E� remaining in the contaminant plume would be 0.09 kg. While there are a number of undeterminable variables that could affect the calculations, these calculations should yield a good approximation. 9.0 SENSITIVE RECEPTOR SURVEY A sensitive receptor survey was performed by PC Toxic, at the request of the SJCEHD, in October 1998. No sensitive receptors were noted within 2000 feet of the subject site. A copy of the survey report is included in Appendix C for reference. 10.0 CONCLUSIONS The petroleum hydrocarbon contaminant plume is highly stable 'and not migrating off-site. The location of the contaminant plume has essentially not moved since the groundwater monitor wells were first installed over ten years ago, in park 'due to the relatively flat groundwater gradient and clay content common in the subsurface lithology. Representative j soil boring logs can be found in.appendix D for reference. The plume is well defined in its areal and vertical extent and petroleum hydrocarbon analytical ' values have shown a steady decline since groundwater monitor well installation. 8 JJW GEOSCIENCES Inc.