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6 Report on Additional Site Assessment March 5 2001 7-Eleven Inc,Stare#19976 1399 N Main St Manteca,California 3.4 Groundwater Monitoring Well installation/Surveying Groundwater monitoring well MW-7 was installed on November 1,2000 (Figure 2) The groundwater ' monitoring well was constructed of 2-inch-diameter, Schedule 40 polyvinyl chloride casing with flush threads, and 0 020-inch-slot well screen The well is screened from 15 feet to 30 feet below grade Sand occupies the annular space from the bottom to about two feet above the top of the well screen ' The sand pack is sealed with about 2 feet of bentonde pellets saturated in place with neat-cement grout to about 10 inches below grade The wellhead is protected at grade by a locking,water-tight well cap and a traffic-rated street box(with watertight bolted lid) set in concrete Well construction details are on the drill log (Appendix A) ' The top of casing and nm of the new well box was surveyed by Morrow Surveying for horizontal position and elevation relative to mean sea level datum on January 5,2001 (Appendix B) 1 35 Groundwater Monitoring and Sampling Quarterly gauging information was collected by measuring depth to water in each well to the nearest 0 01 foot from the top of the well casing, using an interface probe All wells were sampled except well MW-7,which had not been installed as of the date of the quarterly sampling event' Field data sheets are included in Appendix C Potentiometric surface maps for the quarter were prepared using the groundwater elevation data (Figures 3 and 3A) (Table 3) Prior to groundwater purging, dissolved oxygen was measured in each well using a field instrument Dissolved oxygen data generally indicate mainly anearobic conditions, possibly due to intrinsic biodegradation Dissolved oxygen readings range between 7 1 milligrams per liter(mg/L)in well MW-4, located upgradient of the dissolved hydrocarbon plume,to between 0 3 and 0 9 mg/L in the other wells ' (Table 3) Subsequent to monitoring, approximately four well volumes of water were removed from each monitoring I well to ensure stabilization of pH, electrical conductivity, and temperature The purge water was stored In DOT-approved 55-gallon drums The groundwater samples were collected using disposable bailers, then transferred to 40-millilder glass vials filled to zero headspace and acidified to a pH of 2 The samples were sealed using Teflon&septum caps, labeled, placed on ice in an insulated container, and transported under chain-of-custody to a state-certified laboratory The groundwater samples were ' analyzed for BTEX, and TPH-G using EPA Methods 8020/modified 8015, and for TBA,MTBE, DIPE, ETBE, and TAME by EPA Method 8260 (Table 3) ' 19976 4SA