SITE INFORMATION AND CORRESPONDENCE_FILE 2

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437 Miner Avenue Page 2 In 1978 depth to first water at this site was about 70 feet bgs. Depth to first water historically at this site has fluctuated as much as 45 feet as documented in the San Joaquin County Flood Control maps. EHD agrees with the ENSR Conceptual Site Model Figure 3-7 that shows the source area at the site. Since the release of petroleum was from the underground storage tanks that were removed in 1967 the depth to water at the time of the release is unknown. It does not appear that contaminants in the groundwater below 50 feet in the source area were reducing until the ozone system began operation. Groundwater contaminant concentrations used in the Mann-Kendall Analysis test, included in the Conceptual Site Model, were from December 2000 to February 2005. This data was used to evaluate trends that were summarized in Table 3-4 as Pre-Sparge and Post Sparge trends for various wells. Ozone sparging began in December 2000 and was expanded in February 2002, so the analysis was of the entire operating period of the ozone system. EHD does not agree with the Pre-Sparge and Post Sparge terms used in Table 3-4. Actually, the trends support that the ozone system has reduced groundwater contaminant concentrations. EHD does not understand the statement in the Conceptual Site Model that the graphs in Appendix A "do not show an increasing rate of contaminant degradation following installation or expansion of the ozone sparging system", since the only graphs included in Appendix A appear to be of depth to water in NP1-22 over time. Graphs of TPH-g and benzene concentrations were not included in Appendix A. Soil samples collected from borings U-C and U-D (source area)from 16.5 feet bsg and 31.5 feet bgs had high concentrations of TPH-g. There are high PID readings on the U-C and U-D boring logs from 6 to 30 feet bgs. This soil contamination has never been remediated and may be continuing to leach contaminants into the groundwater. Without evaluating the leaching of the soil contamination into the groundwater, the determination of a stable plume cannot be made. The residual contaminant mass in soil and its potential for further degradation of groundwater must be addressed. Also, the affect of the contamination on future use of the property in relation to vapor intrusion into buildings must be evaluated. A soil vapor extraction (SVE) test was conducted in 2000 and Gettler-Ryan did not recommend further SVE operation. The feasibility of soil excavation as a remedial alternative should be considered. The A zone contaminant plume for this site is defined down gradient by U11 and defined up gradient for the Vintage Car Wash site by their MW7 (screened 30 to 60 feet). The B zone contaminant plume for this site is not defined down gradient by U20 (screened from 65 to 75 feet) and may commingle with the Vintage Car Wash B zone plume documented by their MW2 and MW3 (screened from 50 to 75 feet). The C zone contaminant plume for this site is not defined down gradient by U21 (screened from 110 to 120 feet). The C zone contaminant plume for the Vintage Car Wash site has down gradient well MW11 (screened from 109 to 114 feet) that is not screened as deep as U21. The Vintage Car Wash site corrective action is overseen by the Central Valley Regional Water Quality Control Board (CVRWQCB) and EHD has been informed that further investigation has been required for that site. EHD agrees that there is a downward vertical component to the hydraulic gradient, that the vertical extent of the groundwater contaminant plume has not been defined and that a deeper screened monitoring well should be installed in the area of U-20/U-21. In addition, installation of additional wells down gradient of U20 and U21 for better definition of this sites B zone and C zone contaminant plumes must be evaluated.