SITE INFORMATION AND CORRESPONDENCE

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Former Cheaper Store Number 37 Page 2 15 East Grant Line Road April 1, 2011 Tracy, California 95376-2720 The mobile laboratory reported high concentrations of TPHg and significant concentrations of benzene, ethylbenzene, and MTBE in soil gas collected from SG2 and SG3. 1,1-DFA was not detected in any of the samples analyzed by the mobile laboratory. The off-site stationary laboratory reported high concentrations of TPHg in soil gas samples SG2 and SG3 and significant concentrations of TPHg, and BTEX in soil gas samples SG1 and SG4. Significant concentrations of benzene, MTBE, and 1,1-DFA were also detected in soil gas sample SG2. ATC completed a Tier 1 soil gas evaluation using only the data from the stationary laboratory. Benzene concentrations detected in soil gas sample SG2 exceeded the California Human Health Screening Levels (CHHSLs) for Indoor Air and Soil Gas for commercial/industrial land use, published by the Office of Environmental Health Hazard Assessment, California EPA; however, ATC did not consider this high benzene concentration to be a representative value "N• J i since the leak check compound, 1,1-DFA, was detected at 870,000 micrograms per cubic meter (Ng/m3) in soil gas from sample SG2. Also, TPHg concentrations in soil gas samples SG2, SG3 and SG4, and ethylbenzene concentrations in soil gas sample SG3 exceeded the Environmental Screening Levels (ESL) values listed in Table E. Indoor Air and Soil Gas (Vapor Intrusion Concerns) for commercial/industrial land use, established by the California Regional Water Quality Control Board, San Francisco Bay Region. In view of these results, ATC completed a Tier 2 Johnson and Ettinger Model of the soil gas data using the Department of Toxic Substances Control (DTSC) Vapor Intrusion Guidance Version 2.0, dated December 2004. Again, ATC omitted the soil gas data for SG2 from the Tier 2 modeling. Results from the calculated hazard quotient value did not exceed the target level of 1.0, nor did the cumulative incremental risk value exceed lifetime cancer risk of one-in-a-million (10-6); therefore, ATC suggested that human health and/or the environment do not appear to be at risk from soil gas. The EHD is concerned that the elimination of the soil gas data for SG2 from the Tier 2 modeling may result in inappropriate interpretation of the data. Prior to determining the effects of soil gas to human health and the environment, the EHD directs that the data for soil gas sample SG2, analyzed by the mobile laboratory, be incorporated into the Johnson and Ettinger Model. Since both the mobile and stationary laboratories reported similar concentrations of TPHg, benzene and MTBE in soil gas sample S132, the EHD believes it appropriate to use this mobile laboratory data to evaluate the health risk posed by soil gas at this site. Submit these results to the EHD by 25 May 2011. ATC recommends performing additional characterization of impacted soil in the source area by advancing multiple soil borings "to evaluate the residual impact to vadose zone soil including the capillary fringe." ATC proposes to utilize the data to prepare mass calculations and provide an explanation for the declining trend in petroleum hydrocarbon concentrations in groundwater. By letter dated 16 April 2010, the EHD had directed that an estimate of the contaminant mass in soil and groundwater be prepared and an evaluation of a relationship between groundwater elevations to groundwater concentrations of chemicals of concern be performed. The EHD is uncertain that it is necessary to advance additional soil borings since it appears that MTBE concentrations in groundwater have declined significantly [to less than 6 micrograms per liter (Ng/L)] in impacted wells MW-1, MW-4, MW-5 and MW-8. With groundwater currently at approximately 8 feet bsg, and with screen tops at 5 to 6 feet bsg in all eight monitoring wells, the EHD is of the opinion that it is unlikely that residual impacted soil