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First Quarter 1999 Monitoring and Sampling Report and Closure Abstract 2 The Southland Corporation,Store No 17334,4501 North Pershing Avenue,Stockton,CA A nI 15 1999 A potenbometnc surface map was prepared using the groundwater elevation data collected from the five monitoring wells on February 17, 1999(Figure 2) Based on the potenbometnc surface map,groundwater flow Is toward the northwest at an approximate gradient of 0 004 foot per foot ' Groundwater monitoring results for February 17, 1999 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 0 25 milligrams per liter(mg/L)in well MW-5B to 12 mg/L In well MW-2 Dissolved oxygen levels are included In Table 1 The overall DO distribution Indicates significant localized depletion In hydrocarbon source areas(the former underground storage tank pit) These data ' Indicate that intrinsic biodegradation of hydrocarbons and MTBE Is ongoing,and therefore likely to continue ' Monitoring Well Sampling and Analytical Results On February 17, 1999,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 theft 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 Laboratories,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, ETBE,and TAME using EPA method 8260 Analytical results for the February 17, 1999,sampling event and historical laboratory analytical results are summarized in Table 1 (Figure 2) Highest dissolved benzene and TPH-G concentrations were reported in well MW-5A at 4,400 micrograms per liter(pg/L)and 90,000 pg/L, respectively MTBE was detected in groundwater from wells MW-4 and MW-5A at concentrations of 4,300 and 4,800 pg/L respectively Dissolved benzene,TPH-G,and MTBE concentrations are ' plotted on Figure 2 The laboratory analytical reports are Included in Attachment 4 Dissolved benzene concentrations are low(11 pg/L)to non-detectable In all wells except source ' area well MW-5A,where concentrations have declined by about 60%from former levels TPH-G concentrations in well MW-5A have remained relatively stable 17&Uo wpd •