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455 Grant tine Road, Tracy, California January 4 31, 199 The Southland Corporation Page 4 �.' Separate-phase hydrocarbons were not detected in any of the monitoring wells. The water-table elevations calculated from the December 10, 1993 monitoring data were used to prepare a potentiometric surface map (Figure 2). The potentiometric surface elevation Increased an average of 0.77 foot since the previous quarter's monitoring event. The groundwater flow direction is toward the northeast with a hydraulic gradient of approximately 0.004 foot/foot. The historical and current water-level measurements are presented in Table 1. MONITORING WELL SAMPLING AND ANALYTICAL RESULTS On December 10, 1993, groundwater samples were collected from 13 monitoring wells (MW-1, MW-3 through MW-6, MW-8 through MW-15) and one groundwater extraction well. Monitoring wells MW-13 and MW--14 are sampled on an annual (December) basis because historically the analyses of water samples collected from these two wells have indicated BTEX and TPH-G concentrations below the method detection limits. MW-2 was not sampled because the well casing was clogged with tree bark. Before sampling, at least four well-casing volumes of groundwater was removed from each of the monitoring wells unless the wells were purged dry. Field measurements of temperature, conductivity, and pH were recorded during well purging (Attachment 1). The groundwater samples were collected using a Teflon® sampler and placed in analyses-specific sample containers with analyses-specific preservatives. The samples were then sealed and labeled with the job identification number, the sample number, date, time, and type of laboratory analysis required. The samples were stored on ice in an insulated container and transported under chain-of-custody manifest to GTEL Environmental Laboratories, Inc. in Concord, California. The groundwater samples were analyzed for BTEX, and TPH-G using EPA methods 8020/8015. Dissolved BTEX and TPH-G were detected in the groundwater samples collected from monitoring wells MW-1, MW-8, MW-9, MW-10 and the recovery well. The highest concentrations of benzene (12,000 parts per billion (ppbj) and TPH-G (43,000 ppb) were detected in the groundwater samples collected from well MW-10. Figures 3 and 4 illustrate the distribution of benzene and TPH-G concentrations, respectively. Review of the historical data shows that the BTEX and TPH-G concentrations, detected in the samples collected December 10, 1993, are within the same order of magnitude as previous results (Table 1). The laboratory analytical reports and the chain-of-custody records for the December 10, 1993 sampling event are included in Attachment 2. CONCLUSIONS AND RECOMMENDATIONS The potentiometric surface elevation as measured in the groundwater monitoring wells decreased an average of 0.77 foot since the previous quarter monitoring event. J GROUNDWATER iTECHNOLOGY