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i 1 455 Grant Line Road, Tracy, California October 27, 1993 ~` The Southland Corporation Page 2 i t 9 GROUNDWATER MONITORING DATA ' Groundwater monitoring was performed on September 10, 1993. Before monitoring, the wellcaps were opened to allow the water in each well to stabilize to atmospheric pressure. The DTW was measured to the nearest 0.01 foot from the top of the well casing using an ORS Environmental ' Equipment INTERFACE PROBE Well Monitoring System. Separate-phase hydrocarbons were not detected in any of the monitoring wells. The water-table ' elevations calculated from the September 10, 1993 monitoring data were used to prepare a , potentiometric surface map (F=igure 2). Data from monitoring wells MW-8 and MW-12 Indicated ' anomalous groundwater elevations and therefore were not used to calculate the potentiometric groundwater surface. The potentiometric surface elevation as measured in the groundwater monitoring wells decreased an average of.approximately 1.0 foot since the previous quarter's ' monitoring event. The groundwater flow direction is toward the northwest with a hydraulic gradient of approximately 0.008 foot/foot. The historical and current water-level measurements are presented In Table 1. ' MONITORING WELL SAMPLING AND ANALYTICAL RESULTS r1 On September 10, 1993, groundwater samples were collected from 10 monitoring wells (MW-1, MW-3 through MW-6, MW-8 through MW-11, and 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 4 well-casing volumes of groundwater were 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 2). 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 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. The analytical results indicated that dissolved BTEX and TPH-G were detected in the groundwater ' samples collected from monitoring wells MW-1, MW-8, MW-9, and MW-10. The highest concentrations of benzene (16,000 parts per billion [ppb]) and TPH-G (42,000 ppb) were detected in ' the groundwater samples collected from well MW-10. Figure 3 illustrates the distribution of benzene and TPH-G concentrations. Review of the historical data shows that the BTEX and TPH-G concentrations, detected in the samples collected September 10, 1993, are within the same order of GROUNDWATER TECHNOLOGY 1