SITE INFORMATION AND CORRESPONDENCE

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N%w Den's Auto Repair Page 2 308 South EI Dorado Street August 30, 2011 Stockton, California 95203 to target the high concentrations of petroleum hydrocarbon contaminants detected at this depth interval. Upon meeting the CVRWQCB requirements for a WDR permit, please submit a boring permit application, and application/inspection fees to the EHD for the four well installations. Although significant concentrations of petroleum hydrocarbon contaminants have been detected in shallow soil collected near the former underground storage tanks (USTs) and in groundwater collected from monitoring well MW-213, screened between 25 and 45 feet bsg, this pilot study does not address petroleum hydrocarbon contaminants detected in these areas. Based on the results from this pilot study, it may be necessary to redesign the ozone system to effectively address these areas of contamination. AGE states that the ozone injection rates, frequency, and duration will be based on the manufacturer's recommendations and adjusted based on site conditions during the pilot test. Include in the pilot study report of findings an estimate of the total ozone mass expected to be required to effectively remediate the petroleum contaminants at this site, taking into consideration the anticipated radius of influence of injected ozone based on site lithology and depth of sparging points, and the loss of ozone to subsurface reactions unrelated to petroleum contamination oxidation. Such reactions are evident at this site by the increasing concentrations of total and hexavalent chromium, bromate, vanadium, molybdenum, selenium, antimony, and arsenic reported by AGE in Ozone Bench Scale Test Report, dated 26 October 2010. During the advancement of the four ozone injection sparge wells, AGE proposes to select soil samples for analysis of total petroleum hydrocarbons as gasoline (TPH-g); benzene, toluene, ethylbenzene and total xylenes (BTEX); methyl tertiary-butyl ether (MTBE), 1,2- dichloroethane (1,2-DCA) and general minerals. During the pilot study, AGE proposes to collect groundwater samples monthly from MW-1A and MW-2A and analyze for TPH-g, BTEX, MTBE, 1,2-DCA, and metals. AGE also proposes to analyze groundwater from select wells for chromium, bromate and bromide, based on the well locations related to the ozone sparge wells, and monthly monitor groundwater from MW-1 and MW-2 for dissolved oxygen, oxygen reduction potential and depth to water. The EHD directs that all minerals and metals required by the CVRWQCB for their WDR permit be included in the analytical list and all groundwater constituents be reported below the maximum contaminant levels (MCLs) established by the California Environmental Protection Agency. The EHD directs that a human health risk assessment be performed. Residual contaminant concentrations should be compared to Environmental Screening Levels (ESLs) established by the San Francisco Bay Regional Water Quality Control Board, for gross contamination and direct exposure, and California Human Health Screening Levels (CHHSLs) published by the Office of Environmental Health Hazard Assessment, California Environmental Protection Agency. Residual contaminant concentrations that exceed any screening levels should then be evaluated in a Tier 2 risk assessment to determine if the site will qualify as a low-risk site. In addition, the potential risk from vapor intrusion should be evaluated. If additional data are needed to prepare a reliable vapor intrusion evaluation, submit a work plan to collect soil gas samples to the EHD by 31 October 2011.