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Second Quarter 1998 Monitoring and Sampling Report and Closure Abstract 2 The Southland Corporation,4501 North Pershing Avenue,Stockton,CA July 2,9998 ' No separate-phase hydrocarbons(SP)were observed in any of the groundwater monitoring wells during this monitoring event A potenttometnc surface map was prepared using the groundwater elevation data collected from the five monitoring wells on June 15, 1998 (Figure 2) Based on the potentiometric surface ' map, groundwater flow Is toward the northeast at an approximate gradient of 0 004 foot per foot Groundwater monitoring results for June 15, 1998 and historical mondonng results are presented in Table 1 (Attachment 2) Field data sheets are included in Attachment 3 Prior to purging dissolved oxygen levels were recorded in each well Dissolved oxygen levels ranged from 1 20 milligrams per liter(mg/L) to well MW-3 to 210 mg/L in well MW-2 Low dissolved oxygen levels ' generally indicate depletion of oxygen through biodegradation Dissolved oxygen data for wells monitored on June 15, 1998 are presented in Table 1 ' Monitoring Well Sampling and Analytical Results On June 15, 1998,groundwater samples were collected from five 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 chain-of-custody record to state-certified Core Analytical Laboratory,to 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, DIPS, ETBE, and TAME using EPA method 8260 Analytical results for the June 15, 1998,sampling event and historical laboratory analytical results are summarized in Table 1 Highest dissolved benzene and TPH-G concentrations were reported in well MWA ' at 5,000 micrograms per liter(pg/L) and 52,000 pgA., respectively MTBE was detected in groundwater from four wells, MWA, MWAA,MW-3 and MW4 at concentrations of 21,000 pg/L,31 pg/L,25 pg/L and 2,700 pg/L, respectively Benzene and MTBE concentrations declined or remained at low to non-detectable ' concentrations in all wells except MWA TPH-G concentrations were non-detectable in MWA A and MW-2 but increased in the other three wells Historical groundwater monitoring and analytical data indicate that the dissolved hydrocarbon plume is restricted to an area between MW-1,near the pump island and UST pad, and MW-4 which is approximately 90 feet downgradtent(northeast) of MWA For the second quarter 1998, the downgradtent decline in benzene, MTBE and TPH-G concentrations between well MWA and MW-4 are 99 percent, 94 percent and 87 percent, respectively Wells MW-2 and MW-3,which are upgradient and ' cross-gradient of MW-1, historically have had non-detectable to low concentrations of benzene,MTBE and TPH-G The laboratory analytical reports for the groundwater samples are included in Attachment 4 FLUOR DANIEL GTI 1