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R603 UPDATE-20 9/15/93 1.0 INTRODUCTION This update, STATUS UPDATE #20, presents data from July 29, through August 23 , 1993 collected at the site on August 18, 20 and 23 , 1993 . On July 29 , 1993 the REMOX Thermal Oxidation Unit was turned off and a heat exchanger was installed to preheat the vapor stream allowing for less 02 and natural gas augmentation. The unit was restarted on August 18, adjusted on August 20 and turned off for equipment check on August 23 , 1993 Lawrence Tank Testing (LTT) was performing once a week purging of selected wells and condenstate removal during this time. 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 monitor wells during sampling and monitoring 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; R6E; 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, ie . 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. The once a week well purging has been discontinued during this updated status (August 30, 1993) see Table 5 and Appendix B Vapor extraction creates a ground water low (cone of depression) beneath the site see Figures 4A - June 19, 4B - August 20, and 4C - August 23 , 1993 . Currently the ground water is in a discharge cycle, see Depth to Water measurements on Table 1 . As would be expected, changes in vacuum at the wells influences the shape of the potentiometric surface . This is noticeable in comparing the three ground water gradient figures supplied in this report . Figure 4A, are PAGE 1