SITE INFORMATION AND CORRESPONDENCE_FILE 2

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437 Miner Street, Stockton Page 2 well log for U-17 it has a well screen from 110 to 120 feet. The sampling data sheets show depth to bottom of U-17 is 114.63 feet. It is apparent that there is a problem with well U-17. The EHD recommends you consult with a C-57 licensed well driller to see if the well can be cleared. There was discussion about reduction of chemical analyses and EHD pointed out that the EHD has concurred with previous ENSR recommendations for reduction of chemical analyses that had never been enacted (ENSR recommended discontinuing analysis for EDB, MTBE and MTBE metabolites. EHD responded in letters dated April 10, 2006 and March 20, 2007). Chevron/ENSR can submit a proposal for the frequency of sampling each well and the chemical analysis to be performed on the samples for EHD's review and approval. Chevron/ENSR suggested that the source of the contamination found in the water from wells U- 20 and U-21 and hydropunch HP-1 may be from Chet's Auto Repair or the Vintage Car Wash sites, and not from the former Unocal site. The EHD pointed out that Unocal and their consultants, Gettler Ryan and ENSR had previously accepted that the contamination found in the water from wells U-20 and U-21 were from the Unocal site. The EHD informed Chevron/ENSR that the location of the former underground storage tanks (UST's) at Chet's Auto Repair was near the street (area shown as "parking" on the site map on the northwest corner of Miner and American). The EHD also recollected that there were low concentrations of contaminants in the soil and groundwater at Chet's Auto Repair and that it was close to being ready for closure consideration. The EHD informed Chevron/ENSR that the Vintage Car Wash site is owned by the State of California and that the CVRWQCB is overseeing the investigation and remediation of the site. The EHD noted there may be differences in the chemical profiles of the groundwater sample results from wells U-20 and U-21 and other site wells. Chevron/ENSR stated they would compare the sample results from other wells to those from U-20 and U-21 to see if they could demonstrate that the contamination is from another source. The EMD notes that the chemical profile of impacted groundwater is one line of evidence that may, in conjunction with other lines of evidence, demonstrate the likely origin of the contaminants in the wells in question. There was discussion about the historical trends in groundwater contaminant concentrations. The EHD agreed that concentrations of contaminants have declined from natural attenuation and/or ozone sparging. The EHD informed Chevron/ENSR that in order to concur with closure of the site, the contaminant plume must be defined laterally and vertically, and that the CVRWQCB considers a contaminant plume defined when there is no detection of contaminants in the samples from the monitoring wells placed laterally and vertically away from the source area. In addition to the contaminant plume being defined, it has to be shown that the plume is stable and declining and it can be estimated that the contaminant concentrations will meet water quality objectives in a reasonable time. Since the meeting the EHD has learned that a reasonable time frame might be 10 years and that sites should be evaluated to see if additional remediation is necessary to expedite site clean up so that it will be completed within that reasonable time frame. In addition, after reviewing groundwater sample results, the EHD noted that since the ozone system was shut down in April 2005, it appears that contaminant concentrations are rising/rebounding in wells U-3, NPl-22 and NP2-22. This data doesn't support the conclusion that the plume is declining and stable. Additional remediation in the area of monitoring well U3, near the former location of the UST's removed in 1967, was discussed at the meeting. In a letter to UNOCAL in April 2006, EHD