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Fourth Quarter Post-Remediation Monitoring and Sampling Report and Second Request for Closure 2 The Southland Corporation Store No.20304 455 Grant Line Road Tracy,CA December 11 1998 Joe it Separate-phase hydrocarbons were not detected in any of the monitoring wells. Water table elevations, calculated from the October 15, 1998 monitoring data,were used to prepare a potentiometric surface map (Figure 3). The groundwater flow direction is generally toward the north at a hydraulic gradient between �.. 0.0015 and 0.005 ft/ft. Groundwater monitoring results from the monitoring and sampling event are presented in Table 1, and historical results are summarized in Table 1A(Attachment 2). Field data sheets are included in Attachment 3. rr.. Purge Water Transportation and Disposal �.. The purge water was stored onsite in Department of Transportation (DOT) approved 55-gallon drums pending characterization and proper disposal. Approximately 20 gallons of purge water was generated during this sampling event and will be disposed of by a licensed waste hauler during the early portion of the next quarter. Waste hauler documentation will be forwarded upon receipt. &0 Monitoring Well Sampling and Analytical Results rr.. Groundwater samples were collected from wells MW-4, MW-5, and MWA 6. Dissolved oxygen (DO) concentrations were measured in the three wells prior to purging. Before sampling, at least four well- casing volumes of groundwater was removed from each monitoring well, unless the well was purged dry. Field measurements of temperature, conductivity, and pH were recorded during well purging. The groundwater samples were collected using disposable bailers and placed in analysis-specific sample containers with analysis-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 Core Laboratories in Anaheim, California. The groundwater samples were analyzed for BTEX, MTBE and TPH-G using Environmental Protection Agency(EPA) methods 8020/modified 8015, and the oxygenates, MTBE, TBA, DIPE, ETBE and TAME using EPA method 8260. Dissolved oxygen data taken from three monitoring wells indicate depletion of oxygen levels due to intrinsic biodegradation. Dissolved oxygen readings taken ranged from 0.35 mg/L in MW-16 to 5.75 mg/L in MW-4. High dissolved oxygen levels have been typical historically in upgradient well MW-4,which historically has had non-detectable levels of dissolved hydrocarbons. Lower dissolved oxygen levels are generally more typical in wells within and downgradient of the dissolved hydrocarbon plume. Dissolved oxygen data for �.. wells monitored on October 15, 1998 are presented in Table 1. Dissolved benzene concentrations were detected in groundwater samples from monitoring wells MW-4 and MW-16 at 0.942 micrograms per liter(pg/L) and 18.8 pg/L respectively; and TPH-G was detected in monitoring well MW-16 at 1,500 pg/L. MTBE was not detected in any of the wells sampled. Dissolved hydrocarbon concentrations continued to decline in MW-16 (source area) and remain at very low to non- detectable levels in all Other wells sampled. The dissolved hydrocarbon plume remains defined and �+ restricted to a very limited area near MWA 6. Analytical results from the groundwater samples collected on October 15, 1998 are presented in Table 1, and historical analytical results are summarized in Table 1A. The laboratory analytical reports and the chain-of-custody records for this sampling event are included in r.. Attachment 4. 20304g4.rpt #" FLUOR DANIEL GTI . n 4