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Groundwater (Elevation Data Depth to groundwater was obtained from the existing groundwater monitor wells at the site using a water level indicator graduated to 0 01 foot The depth to groundwater measurement was converted to a groundwater elevation for each well and used to construct a potentiometric surface map (Figure 2), and calculate the local groundwater flow direction and hydraulic gradient The groundwater potentiometric surface elevation averaged approximately 72 59 feet below ground surface (bgs) during this monitoring event The groundwater flow direction at the site is directed toward the southwest with a hydraulic gradient of approximately 0 04 (ft /ft) A summary of data regarding groundwater levels measured through July 14, 1997, is presented in Table 1 Copies of field observations documenting work performed at the site is presented in Attachment I Groundwater Analytical Results The distribution of groundwater sample analytical results obtained from monitor wells at the site on July 14, 1997, is presented on Figure 2 and summarized in Table 2 Chemical analytical results indicate the highest concentrations of TPHg and benzene are in the vicinity of monitor wells MW-1, MW-3 and MW-5 TPHg concentrations in groundwater samples ranged from below the laboratory method detection limit in monitor wells MW-7 and MW-10 to 55,200 parts per billion (ppb) in monitor well MW-1 Benzene concentrations ranged from below the laboratory method detection limit (<0 5 ppb) in monitor wells MW-7 and MW-10 to 1,470 ppb m MW-1 At the request of the San Joaquin County Public Health Services, Environmental Health Division (SJCPHS), two groundwater samples were submitted for analysis of MTBE by EPA Method 602 MTBE • concentrations ranged from 7 9 ppb in a sample collected from monitor well MW-10, to less than detectable levels in a sample collected from monitor well MW-1 Attachment II contains photocopies of the laboratory analytical reports and the chain-of-custody form Waste Ulandfing Purge water generated during groundwater development and sampling was transported by Integrated Waste Management, Inc to Mckittnck Waste Treatment Site, in Mckittrnck, California, for treatment and disposal FORTHCOMING ACTIVITIES During the fourth quarter of 1997, SECOR will conduct additional well monitoring, purging, and sampling activities The results of these activities will be summarized in a quarterly monitoring report to be submitted to you in January 1998 A work plan to conduct soil vapor extraction testing dated June 26, 1997 has been submitted to the SJCPHS for approval The work plan will be implemented once approval has been granted and the information will be used to develop a remedial action plan SECOR LCKFRUym 191 2 May 6 1997 10710 OOI 0[