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N Remo Canepa Page 3 of 5 <br /> 6230 Pacific Avenue May 18 , 2011 <br /> Stockton, CA <br /> the above graph , DTW is shown increasing downward so the profile resembles the actual effect <br /> of the groundwater level in the well). The EHD interprets this to show that the higher that <br /> groundwater is above the screen interval in MW-1 (50 to 70 feet below the top of the well <br /> casing ), the lower TPHg concentrations become in the water monitored in the well. In contrast, <br /> SV-4 is screened 8 to 48 feet below the top of the well casing , so groundwater is usually within <br /> the well screen; here as the DTW increases (goes downward ) the concentrations also decrease <br /> (note the trends of DTW and TPHg concentrations in SV-4 from approximately June 2002 to <br /> January 2008). The EHD interprets this to indicate that as the groundwater level drops, <br /> groundwater is in contact with less of the contaminated soil and so becomes less contaminated <br /> too. The MW-3 data has less of a relationship to DTW, probably because it is not in the <br /> immediate area of contaminated soil and there is considerable lag time before any effect is <br /> evident due to the well 's distance from the source area; indeed initially ( 1994) MW-3 usually did <br /> not contain detectable concentrations of TPHg even though MW-1 was intensely impacted . <br /> By reversing the DTW scale (on the right side) on the graph , the close relationship of TPHg <br /> concentrations in MWA to DTW is very evident as can be seen below, where the DTW trend <br /> almost directly overlies the TPHg trend in MWA : <br /> DTW vs TPHg <br /> 1000000 60 <br /> x <br /> 100000 50 <br /> a <br /> 10000 x 40 <br /> N -f- MW-1 TPHg <br /> 0 3D -6- MW-4 TPHg <br /> 1000 -K SV4TPHg <br /> �6230 Pacific DTW <br /> 0 <br /> 10020 <br /> v o <br /> 10 10 <br /> 0 <br /> 1 0 <br /> 9n 9y cp a1 ge 99 00 P^ o`Y `1 oA a0 Ac �'1 Ae 99 <br /> s so 0, so )� �0c 3Psp �4 std Sic S S V slc <br /> Samping Dates <br /> AGE interpreted the concentration trends of MTBE in various wells as indicating an <br /> unauthorized release from the current UST system since its installation in 1993. The EHD has <br /> researched the use of MTBE in gasoline and found that commercial use of MTBE in gasoline <br /> started in 1979; in 1981 , up to 10% MTBE was approved for usage in gasoline and commonly <br /> comprised 2% to 9% of premium grade gasoline, but less than 1 % of regular grade. In 1990 , the <br /> Clean Air Act Amendment required use of federal oxygenated gasoline or federal reformulated <br /> gasoline. It can be concluded that MTBE was in use prior to the UST system replacement in <br /> UAR Request Response and Directive Letter 0511 <br />