SITE INFORMATION AND CORRESPONDENCE

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II r Remo Canepa Page 3 of 4 6230 Pacific Avenue March 5 , 2012 Stockton , CA and by TPHg and benzene (Figures 9 and 10) is the cross-gradient MW-2 , which is not impacted by MTBE . If MTBE figures prominently in the second release, this well data puts additional doubt on the value of the butane, pentane and isooctane data . • The EHD considers the carbon stable isotope analysis data presented to not be inconsistent with a possible two-release model , but the data might also be interpreted to show that the MTBE in MW-3 is simply older than that in SV-4, and represents a more advanced stage of isotopic fractionation . Intuitively, the fractionation processes would have longer to operate and yield an isotopically heavier MTBE , in the more distal MW-3 than in the more proximal SV-4. • The EHD does not see how the presence of highly volatile organic compounds in groundwater, but a lack of the same in soil gas samples , would support a conclusion that I, a second unauthorized release had occurred . f Similar to the last point, the EHD does not understand how the fact that the UST system appears to not be leaking at the present time demonstrates that there were two separate I, releases in the past, or any releases . ji II The lines of evidence presented by AGE may not convincingly demonstrate the occurrence of a N second release on the site from the current UST system , but none of them shows that a second occurrence had not occurred . During review of the site data , the EHD noted some data l relationships that may strengthen the inference of a second, release — although none of them is definitive either: • The first soil samples analyzed for MTBE , collected from SB- 1 in May 1998 , yielded detectable MTBE concentrations in samples collected between 25 feet below surface grade (bsg) to 70 feet bsg , with concentrations as high as 37 milligrams per kilogram j (mg/Kg ) at 35 feet bsg . MTBE detected in samples from 55 feet and 70 feet bsg could possibly be due to cross contamination from impacted groundwater. The MTBE concentrations in samples collected at 25 feet, 30 feet, 35 feet and 40 feet bsg exceeded the concentrations of total petroleum hydrocarbons quantified as gasoline (TPHg ) , and benzene , toluene , ethylbenzene and xylenes (BTEX) . Boring SB- 1 was advanced approximately 7 feet from vapor extraction well SV-4 . Soil boring B- 1 , advanced in 1993 about the same distance , but on the opposite side of SV-4 , generally had significantly higher TPHg and BTEX concentrations at the same or similar depths compared to SB-1 samples, the B-1 samples were not analyzed for MTBE. As MTBE is highly volatile and has less affinity for sorption to soil than TPHg and BTEX, the EHD infers from the TPHg and BTEX data that the soil vapor extraction (SVE) operation between March 1994 and June 1995 may have significantly reduced the contaminant mass in soil sampled by SB- 1 , and that the MTBE in the SBA soil samples may be due to a release from the UST system after the SVE system ceased operation in June 1995 . • The highest concentration of dissolved MTBE detected was 740 , 000 pg/L, encountered in SV-4 in July 2002 , approximately nine years after the UST system had been replaced . This was also after the initial SVE operation from March 1994 through June 1995 , and during intermittent operation of the SVE system restarted in 1999 . This is a remarkably high concentration after nine years and intermittent remediation by SVE , intuitively it demonstrates either a second release occurred or there have been very minimal effects of natural attenuation and intermittent active remediation . UAR Request Response Letter 0312