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R603 UPDATE-18 7/09/93 i1 . 0 INTRODUCTION This update, STATUS UPDATE #18, presents data from May 18, through June 28, 1993 collected at the site on June 14, 22 , and 28, 1993 During this time, a thermal oxidation unit was operating and vapor extraction for soil and ground water remediation was taking place Lawrence Tank Testing (LTT) was also performing once a week purging of selected wells for ground water removal and treatment The permit to dump treated ground water to the sewer was received and presently we are batch dumping the treated water from the "ConVault" monthly This water is collected from the vapor recovery condensate dropouts and the twice weekly purging of monitor wells MW1, MW2, MW3 , MW8 and MW10 by Lawrence Tank Testing (LTT) pumping service We are presently evaluating the need for a continuous pump and treat system if it is determined that continuous pumping of ground water is needed, monitor wells MW1 and MW3 will be continuously pumped, see Appendix A - Sewer Discharge Report LOCATION The station is located at 6425 Pacific Avenue at the intersection of Douglas Road, in San Joaquin County, Stockton, California. The site lies in Section 21, T2N, RGE, MDB&M at approximately 12 feet above mean sea level (MSL) (see Figures 1, 2 , 3) The parcel is # 31 and can be located in Assessor' s Book 97 on page 41 (97-041-31) . On October 2, 1992 , a licensed surveyor with Nolte & Assoc surveyed all monitoring well locations and top of casing elevations to a known datum, City of Stockton Bench Mark Number 12 , elevation 11 . 74 ' above mean sea level 2 . 0 GROUND WATER LOCAL GROUND WATER FLOW Ground water flow direction in this region is to the west and towards south Stockton from the foothills of the Sierra Nevada Mountain Ranges, and towards the east from the Sacramento-San Joaquin Delta. At present, encroachment of saline waters from the Delta is impairing the quality of the ground water in the Stockton area Starting on December 1, 1992 , LTT has been performing twice a week purging of monitor wells MW1, MW2 , MW3 , MW8 and MW10 The twice a week well purging has been reduced to once a week in May, 1993 , see Table 4 and Appendix B . This purging is creating a ground water low (cone of depression) beneath the site near MW1 and MW2 , see Figures 4A - May 18 , 1993 and 4B - June 14 , 1993 GROUND WATER QUARTERLY SAMPLING . Ground water samples were collected on June 14 , 1993 , see Table 1 and Appendixes C - Laboratory Report and D - Field Notes . These samples were collected by a WEGE geologist and "Chain of Custody" PAGE 1