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Former Chevron Service Station No. 98632 575 West Grant Line Road Tracy, California 95376 September 5, 2014 Page 3 of 4 Except for 650 cubic yards of impacted soil that were removed in 1994 and the unquantified effects of natural attenuation, no other method of remediation has been implemented at this site. CRA concluded that decreasing contaminant concentrations in groundwater demonstrate lack of sufficient secondary source to change the (concentration) trends; regarding impact to groundwater the EHD agrees, but in respect to soil gas, this does not appear to be the case. Soil samples collected at approximately six feet below surface grade (bsg) from monitoring well borings MW-2, MW-3 and MW-4 (therefore between five and ten feet bsg) contained high concentrations of benzene that exceeded the acceptable concentrations of benzene for residential, commercial and utility workers as found in Table 1 of the LTCP checklist. These three wells are not near each other, nor are the wells located in the areas of the former excavation. This indicates that impacted soil is not limited to the excavated former underground storage tank (UST) area or exploratory trenches or the former used-oil tank/former station building which once contained a sump and hoist; instead the impacted soil may represent several localized more intensely impacted areas or a potentially widespread impacted area still present on the site. Based on high concentrations of contaminants of concern detected in soil gas samples collected at the site in 2009 and confirmed in soil gas samples collected in 2010, it is clear that high concentrations of contaminants of concern are present in soil gas under much of the site despite the remediation by excavation that occurred over fifteen years ago and the effects of natural attenuation. Concentrations of total petroleum hydrocarbons as gasoline (TPH-g) as high as 120,000,000 micrograms per cubic meter (pg/m3) were reported in soil gas samples collected from vapor point VP-1, and a TPH-g concentration of 22,000,000 pg/m3 in the soil gas sample collected from vapor point VP-4 may represent a health hazard that has not been quantified. Benzene at concentrations as high as 26,000 pg/m3, and ethylbenzene at concentrations as high as 120,000 pg/m3, greatly exceed concentrations of 280 pg/m3 and 3,600 pg/m3, respectively, established in the LTCP as low-risk for a commercial site with similar conditions for vapor intrusion issues. The occurrence of the portable office trailer that lies between these two soil gas sampling locations raises concern regarding a potential vapor intrusion problem. The EHD concludes that the 'secondary source' has not been removed to the extent practicable at this site and that vapor intrusion remains a potential, 'unquantified' hazard and risk to human health. The EHD also notes that oxygen concentrations are less than four percent in soil gas samples collected from VP-1 and VP-4, and that a number of soil samples collected 5 feet bsg or shallower contained more than 100 mg/kg TPH, indicating that there is no bioattenuation zone. CRA addresses the petroleum vapor intrusion to indoor air impediment to closing this site by stating that the crawl space beneath the trailer would likely permit any upward migrating vapor to dissipate, and if the site were to be redeveloped, CRA states that unspecified responsible parties could address or mitigate potential vapor intrusion concerns. Stating that soil gas intruding the crawl space under the office building is likely to be dispersed to nonhazardous concentrations does not necessarily make it so; with known high benzene and TPH concentrations in soil gas, this should be demonstrated with site-specific data and appropriate modeling. The EHD does not consider it technically justifiable to close this site unless the potential vapor intrusion concerns posed by the high concentrations of contaminants of concern in soil gas have been shown to pose an acceptable numeric level of health risk and hazard with site-