SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Larry's Auto Page 2 308 North Grant Street August 5, 2011 Stockton, California 95202 • Groundwater is approximately 30 feet below surface grade (bsg) and there exists a "non-permeable" floor and 30 feet of soil in the area where concentrations of contaminants of concern have historically been the highest (near MW-5) making the threat from vapor intrusion from impacted groundwater appear to be an unreasonable assertion; • Shallow soil contamination above 15 feet bsg is very limited with the exception of approximately 400 milligrams per kilogram (mg/kg) in a ten-foot sample (collected from MW-8). • Volatile hydrocarbons have not been detected in the shallow soil; • Soil vapor extraction is actively mitigating the impacted soil so soil vapor sampling within the shallow soil is not warranted; • The entire site is covered with a roadway surface; • The main building on-site is an auto repair shop that stores small volumes of gasoline and operates while one-third of the building is open to the atmosphere; • The second building associated with this site is a parts warehouse where a ten-ton forklift powered by diesel operates daily. AGE concluded by stating that "AGE would like to evaluate the soil vapor with an approach which minimizes the threat, yet does not waste effort and funds to sample for a non-threat problem." The EHD is unclear what AGE means by "to evaluate the soil vapor with an approach which minimizes the threat". Apparently AGE believes it is not necessary to collect soil gas samples; the EHD will concur with this position if it can be demonstrated satisfactorily using site data that there is minimal risk from vapor intrusion. The EHD has reviewed historical soil analytical data collected at this site. Soil samples were not collected and analyzed for chemicals of concern at depths less than 25 feet bsg during the installation of monitoring wells MW-1 through MW-4, MW-7, MW-9 and vapor wells VW1 through VW-4; and no soil samples were collected for chemical analysis during the installation of extraction wells EW-1, EW-2 and monitoring well MW-5. Soil samples were collected at 15 feet bsg from borehole VW5 and high concentrations of contaminants of concern were detected in the soil samples. The EHD could find only two boreholes, MW-6 and MW-8, where soil samples were collected at depths_of 5 and 10 feet bsg during the advancement of the boring. Analytical results were non-detect for soil samples collected from MW-6, located approximately 75 feet cross-gradient of the former underground storage tanks (USTs) located beneath the sidewalk on the north side of Miner Avenue. TPH-d was detected at 440 mg/kg in soil samples collected at 10 feet bsg in borehole MW-8 advanced within the former UST area beneath the sidewalk on the north side of Miner Avenue. MW-5 is approximately 60 feet down-gradient of the former USTs located beneath the sidewalk of Miner Avenue. During routine sampling events an EHD inspector has been inside the warehouse which is an older building that has a cement floor with many cracks and the potential threat from vapor intrusion may well exist since the former USTs were located within ., ten feet of this warehouse and the absence of intensely impacted soil directly underneath the building has not been demonstrated. The EHD agrees that soil gas samples should not be collected in the area where the soil vapor extraction system is currently in operation; however, the EHD does direct you to