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Additional Site Assessment and Second Quarter 1999 Monitoring and Sampling Report 6 7-Eleven Inc,Store#19976, 1399 N Main St,Manteca,California _ _ Jufy 15,1999 Prior to groundwater purging, dissolved oxygen was measured in each well using a field Instrument ' Dissolved oxygen data taken from seven of the eight monitoring wells indicate depleted oxygen levels due to Intrinsic biodegradation The dissolved oxygen reading in MW-4 was 6 5 mg/L Dissolved oxygen readings taken from the other seven wells ranged from 0 15 mg/L in MWA and MW-2 to 2 05 mg/L in MW-5A(Table 2) Subsequent to monitoring, approximately four well volumes of water were removed from each monitoring 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-milliliter 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 state-certified Core Laboratories in Anaheim, California The groundwater samples were analyzed for BTEX, and TPH-G using EPA Methods 8020/modified 8015, and for TBA, MTBE, DIPE, ETRE, and TAME by EPA Method 8260 ' All purge water generated during the monitoring and sampling event was stored in a Department of a Transportation (DOT)-approved 55-gallon drum pending characterization and proper disposal Approximately 112 gallons of purge water generated during this sampling event will be transported from the site during the third quarter 1999 Waste documentation will be forwarded once received 40 RESULTS ' 41 Geology and Hydrogeology The site is located in the middle of the northern San Joaquin Valley Shallow stratigraphic units in the vicinity of Manteca are mapped as Quaternary dune sand overlying the Quaternary Modesto Formation ' which consists dominantly of arkosic alluvium (California Division of Mines and Geology, 1991, Geologic Map of the San Francisco-San Jose Quadrangle, 1250,00) ' The materials encountered during recent drilling at the site consisted of sand and silty sand to approximately 10 feet BGS underlain by clayey silty and silt to approximately 25 feet BGS and fine to ' coarse sand, clayey silt and clayey silty sand to maximum depth of investigation at 66 feet BGS (Appendix A) Static groundwater level in the borings was approximately 20 feet BGS Groundwater flow, as mapped from data collected on June 6, 1999, is to the northeast at a hydraulic gradient of 0 015 foot per foot 1 ' 1997E3g2 sar