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Former Chevron Service Station No. 98632 September 5 , 2014 <br /> 575 West Grant Line Road Page 3 of 4 <br /> Tracy, California 95376 <br /> 1 . Except for 650 cubic yards of impacted soil that were removed in 1994 and the <br /> unquantified effects of natural attenuation , no other method of remediation has been <br /> implemented at this site. CRA concluded that decreasing contaminant concentrations in <br /> groundwater demonstrate lack of sufficient secondary source to change the <br /> (concentration) trends ; regarding impact to groundwater the EHD agrees, but in respect <br /> to soil gas, this does not appear to be the case. <br /> 2. Soil samples collected at approximately six feet below surface grade (bsg) from <br /> monitoring well borings MW-2, MW-3 and MW-4 (therefore between five and ten feet <br /> bsg) contained high concentrations of benzene that exceeded the acceptable <br /> concentrations of benzene for residential , commercial and utility workers as found in <br /> Table 1 of the LTCP checklist. These three wells are not near each other, nor are the <br /> wells located in the areas of the former excavation. This indicates that impacted soil is <br /> not limited to the excavated former underground storage tank (UST) area or exploratory <br /> trenches or the former used-oil tank/former station building which once contained a <br /> sump and hoist; instead the impacted soil may represent several localized more <br /> intensely impacted areas or a potentially widespread impacted area still present on the <br /> site. <br /> 3. Based on high concentrations of contaminants of concern detected in soil gas samples <br /> collected at the site in 2009 and confirmed in soil gas samples collected in 2010, it is <br /> clear that high concentrations of contaminants of concern are present in soil gas under <br /> much of the site despite the remediation by excavation that occurred over fifteen years <br /> ago and the effects of natural attenuation . Concentrations of total petroleum <br /> hydrocarbons as gasoline (TPH-g) as high as 120, 000, 000 micrograms per cubic meter <br /> (pg/m3) were reported in soil gas samples collected from vapor point VP-1 , and a TPH-g <br /> concentration of 22 , 000, 000 Ng/m3 in the soil gas sample collected from vapor point <br /> VP-4 may represent a health hazard that has not been quantified . Benzene at <br /> concentrations as high as 26, 000 pg/m3, and ethylbenzene at concentrations as high as <br /> i 20;000—pgfm3 greatly--exceed coneentratiorts—of--2-80—pg/m3 -and 3i600—pglm' <br /> i respectively, established in the LTCP as low-risk for a commercial site with similar <br />` conditions for vapor intrusion issues. The occurrence of the portable office trailer that <br /> lies between these two soil gas sampling locations raises concern regarding a potential <br /> vapor intrusion problem . The EHD concludes that the 'secondary source' has not been <br /> removed to the extent practicable at this site and that vapor intrusion remains a <br /> potential, 'unquantified' hazard and risk to human health. The EHD also notes that <br /> oxygen concentrations are less than four percent in soil gas samples collected from <br /> VP-1 and VP-4, and that a number of soil samples collected 5 feet bsg or shallower <br /> j contained more than 100 mg/kg TPH, indicating that there is no bioattenuation zone. <br /> 4. CRA addresses the petroleum vapor intrusion to indoor air impediment to closing this <br /> site by stating that the crawl space beneath the trailer would likely permit any upward <br /> migrating vapor to dissipate , and if the site were to be redeveloped, CRA states that <br /> unspecified responsible parties could address or mitigate potential vapor intrusion <br /> concerns. Stating that soil gas intruding the crawl space under the office building is <br /> likely to be dispersed to nonhazardous concentrations does not necessarily make it so; <br /> with known high benzene and TPH concentrations in soil gas , this should be <br /> demonstrated with site-specific data and appropriate modeling . The EHD does not <br /> consider it technically justifiable to close this site unless the potential vapor intrusion <br /> concerns posed by the high concentrations of contaminants of concern in soil gas have <br /> El been shown to pose an acceptable numeric level of health risk and hazard with site- <br /> f. <br /> F <br />