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Ms.Lori Duncan January 11,2012 RE:2911 E.Fremont St, Stockton,CA Page 2 of 4 samples had different sources that the indoor contaminants were derived from the impacted soil underlying building. The EHD is actually impressed by how similar the contaminant ratios in each of the four samples resembles the others, despite a distance of approximately 300 feet between the easternmost outdoor sample and the westernmost indoor sample; could the similarity ofanalytical results among the four samples be due to a similarity of sources, such as parked vehicles slowly releasing hydrocarbons? �11¢`��4 ° L&P Response: L&P agrees that the number of samples is too few to make ch a statistical interpretdtton, and L&P does not claim in the report to have made a statistical interpretati n. L&P merely compared the ratios of the constituents in ambient air samples to those in the indoor airs ples,which is a common step in evaluating potential vapor intrusion risk at a given site(Cal/EPA-DTS Guidance Document,October 2011). Differences in ratios may infer different sources for the chemicals. L&P evaluated these ratios in a manner similar to analytical laboratories' use of relative percent difference (RPD) of matrix spike and matrix spike duplicate analyses for quality control. In this regard,most analytical laboratories consider an RPD of either 25%or 30%considered an acceptable range. When comparing the RPDs of the ratios of the two outdoor air samples, the RPDs range from 0.25% to 20.04%, which L&P considers within the acceptable range(<25%). Similarly,in comparing the RPDs of the ratios of the two indoor air samples,the RPDs range from 0.24%to 20.69%. To compare the ratios of the indoor air to outdoor air in an attempt to infer whether their sources differ,L&P calculated the RPDs of the average ratios of the two indoor samples to the average ratios of the two outdoor samples. In this evaluation,the RPDs of the averages ranged from 2.81% to 71.30%, with 9 of the 15 evaluated RPDs exceeding 25% (please see Table 1). Thus, L&P concluded that the chemicals in indoor air have a somewhat different source than the chemicals in ambient air. One would assume that if the source of contaminants from all sample locations are from parked vehicles, as postulated in your letter, then the RPDs between indoor and outdoor samples would all be within an acceptable range(<25%). Regardless of the source of the chemicals detected in the indoor and outdoor air samples collected at the site,the end result is that each sample contained low levels of volatile constituents that are known to exist both in the subsurface,as documented by soil samples collected at the site,and in ambient air due to the nature and location of the business(equipment rental yard with fuel tanks and repair bays on a busy surface street). 3. L&P stated that they modified the CHHSLs from the residential scenario to an indoor worker scenario by modifying the default parameters. The EHD notes that the CHHSLs and ESLs utilized in Table 3 of the report were Tier I values derived for commercial/industrial land use. The EHD was not party to modifying the input parameters and is not aware ofwhat the modified parameters were changed from and what authoritative source was used to set the modified parameters; please provide this information. Please also note how these values specifically were changed. L&P Response: L&P did not make any modifications to the CHHSLs,which are threshold concentrations established by Cal/EPA-OEHHA,below which are considered to pose no undue risk to site workers. As stated in the report, L&P conducted a Tier 1 risk evaluation by comparing detected concentrations of chemicals in both indoor air and outdoor air to the CHHSLs established by Cal/EPA-OEHHA. Since benzene in all ambient and indoor air samples exceeded the CHHSL for indoor air in a commercial setting, L&P progressed to a Tier 2 risk evaluation. The Tier 2 risk evaluation did not entail modifying any CHHSLs. The Tier 2 risk evaluation conducted by L&P entailed calculating site specific risk using 10759-LTR Response