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r, Third Quarter 1999 Monitoring and Sampling Report 2 7-Eleven,Inc,Store 019976, 1399 North Main Street,Manteca,CA October 15, 1999 i 1 I steeper gradient of approximately 0 008 ft/ft(Figure 3), however, the gradient cannot be precisely determined due to the near linear alignment of the three deep wells Groundwater monitoring results are I w. presented in Table 1 Field data sheets are included in Attachment 3 3 Monitoring Well Sampling And Analytical Results On September 14, 1999, groundwater samples were collected from eight wells Before sampling, at least four-well-casing volumes of groundwater was removed from each monitoring well unless the well was purged dry The groundwater samples were collected using disposable bailers Groundwater samples were placed in analysis-specific containers with analysis-specific preservative The samples were then sealed using Teflon septum caps, labeled with the fob identification number, the sample number, date, time, and type of laboratory analysis required The samples were stored on ice and transported with a ` t chain-of-custody record to CORE Laboratories, in Anaheim, California Groundwater samples were analyzed for BTEX and TPH-G using Environmental Protection Agency(EPA) method 8020/modified 8015 and for the oxygenates TBA, MTBE, DIPE, ETBE and TAME using EPA method 8260 Laboratoryanalytical results for groundwater samples collected on September 14, 1999 and historical- yt 9 P results are summarized in Table 1 In water samples taken from the eight monitoring wells on September 14, 1999, benzene ranged from below method detection limit in wells MW-3, MW-4, MW-4A and MW-5A to 2,590 pg/L in well MW-2 TPH-G ranged from below method detection limit in wells MW-1A, MW-3, MW-4, MW-4A and MW-5A to 29,200 pg/L in well MW-2 MTBE ranged from below method detection limit in wells MWAA, MW-3, MW-4,MW-4A and MW-5A to 467 pg/L in well MW-5 No other oxygenates were detected above detection limits in any of the wells sampled The lateral and vertical r l extent of dissolved hydrocarbons and MTBE has been delineated upgradient(south) and crossgradient (east) of the former tank pit area based on non-detectable to low BTEX, TPH-G and MTBE concentrations in the groundwater samples collected from monitoring wells MW-3, MW-4 and MW-4A Decreasing hydrocarbon and oxygenate concentrations were detected in crossgradlent well, MWA which is located near the former UST pit and pump island (source area) Dissolved hydrocarbon and oxygenate concentrations remain low to non-detectable in the deep wells MWAA, MW-4A and MW-5A In r� downgradient well MW-5 (shallow) dissolved BTEX and TPH-G concentrations have decreased, but MTBE has increased since previous quarter Copies of the analytical reports and chain-of-custody forms are included in Attachment 4 A workplan for installation of an additional well, located north-northeast of the site has been submitted to the San Joaquin County Public Health Services (SJCPHS) t r Dissolved oxygen concentration was not measured in the monitoring wells on September 14, 1999 1997GQ39 RPT