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Third Quarter 1998 Monitoring and Sampling Report and Closure Abstract 2 The Southland Corporation,4501 North Pershing Avenue Stockton CA October 2 1998 No separate-phase hydrocarbons (SP)were observed in any of the groundwater monitoring wells during this monitoring event A potentiometric surface map was prepared using the groundwater elevation data collected from the three monitoring wells on August 25, 1998 (Figure 2) Based on the potenfiometnc surface map,groundwater flow is toward the northwest at an approximate gradient of 0 04 foot per foot Groundwater monitoring results for August 25, 1998 and historical monitoring results are presented in Table 1 (Attachment 2) Field data sheets are included in Attachment 3 Pnor to purging,dissolved oxygen (DO) levels were recorded in each well Dissolved oxygen levels ranged from 1 0 milligrams per liter(mg/L) in wells MW-2 and MW-4 to 12 mg/L in well MW-3 Dissolved oxygen levels are included in Table 1 The overall DO distribution indicates overall high levels, and localized depletion in areas of higher hydrocarbon concentrations,which demonstrates ongoing intrinsic biodegradation Wells MW-1 and MW-1A were abandoned due to impending underground storage tank replacement, as ' approved by the San Joaquin County Public Health Services(SJCPHS) Well replacement following completion of service station upgrades will be addressed with the SJCPHS when appropriate Monitoring Well Sampling and Analytical Results On August 25, 1998, groundwater samples were collected from three monitoring wells Before sampling, at least 4 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 ichain-of-custody record to state-certified Core Analytical Laboratory, in Anaheim, California Groundwater samples were analyzed for BTEX and TPH-G using Environmental Protection Agency(EPA) methods 8020/modified 8015, and for oxygenates MTBE,TBA, DIPE, ETRE, and TAME using EPA method 8260 Analytical results for the August 25, 1998, sampling event and histoncal laboratory analytical results are summarized in Table 1 Highest dissolved benzene and TPH-G concentrations were reported in well MW-4 at 2 6 micrograms per liter(pg/L) and 1,800 pg/L, respectively MTBE was detected in groundwater from well MW-4 at concentrations of 1,220 Ng/L Benzene and MTBE concentrations declined or remained at low to non-detectable concentrations in all wells except MW-4 Hydrocarbon concentrations in well MW-4 have declined by about 50% from last quarter's results The downgradient extent of the plume is currently defined, based on non-detectable concentrations in well MW-3 The laboratory analytical reports for the groundwater samples are included in Attachment 4 i • 1 �- FLUOR DANIEL GTI