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Y Chevron if S A Products Company April 8, 1992 02320 2241 Page 3 a traffic-rated street box with a water-tight bolted lid Well construction details are shown in the drill logs in Appendix A The monitoring wells were developed by hand bailing approximately three to five well volumes of water to remove fine-grained sediments from the well and filter pack 34 GROUNDWATER MONITORING AND SAMPLING On March 11, 1992, groundwater monitoring was performed at the Chevron site and the Atlantic Richfield Company (ARCO) service station south of West Benjamin Holt Drive, in coordination with ARCO consultant Brown and Caldwell Monitoring wells were gauged again on March 17, 1992 during the required quarterly monitoring and sampling event (Table 1) The monitoring wells were gauged using an Oil Recovery Systems (ORS) INTERFACE PROBE Well Monitoring System that utilizes a dual optical sensor and conductivity probe to distinguish between water and petroleum liquids Prior to groundwater monitoring and sampling, the wellheads were surveyed (Appendix B) Quarterly groundwater sampling was conducted on March 17, 1992 and included the collection of groundwater samples from the two newly installed monitoring wells (MW-6 and MW-7) Prior to sampling, . the monitoring wells were purged until pH,temperature and total dissolved solids (TDS) readings stabilized Field measurements of pH, temperature and TDS are provided in Appendix B Groundwater samples were collected using a Teflon" sampler cleaned with an industrial detergent and distilled water Prior to the collection of the water samples, a rinsate blank from the Teflon" sampler and a trip blank were collected for quality assurance/quality control purposes Each sample was acidified, labeled, and placed on ice in an insulated container for delivery under chain-of-custody manifest to Superior Precision Analytical laboratories Groundwater samples were analyzed for TPH-G and BTEX by modified EPA methods 5030/8020/8015 All water generated during the purging process was transferred into a DOT-approved purge water trailer and transported to the Chevron refinery to Richmond, California for recycling 40 SITE CONDITIONS 41 HYDROGEOLOGY The site is located in the northern part of the San Joaquin Valley at an elevation of approximately three feet above mean sea level The valley fill consists of late Mesozoic through Eocene dominantly marine sedimentary rocks overlain by late Tertiary and Quaternary non-marine deposits In the Stockton area, the late Cenozoic deposits are approximately 3,000 feet thick and generally consist of alluvial gravel, sand, silt, and clay The non-marine sedimentary units are generally fresh-water-bearing in the area of the valley east and southeast of Stockton, but brackish or saline water occurs in these sediments in the delta�harea west 2241SUM1 RPT JL1041, GROUNMATER TECHNOLOGY INC