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03/27/02 WED 13 39 FAX 19788582355 GROUNDWATER ia 003I Third quarter MI Mo 14onng and Sampling Report and Pre-Closure Sampling Workprtln 2 7-Eleven,Inc,Store#7 7647 1048 West Yosemite Avenue Manteca,CA OGtaber 4 2001 Groundwater Monitoring Data Before monrtorinG,the well caps were opened to allow the water ir1 Mach well to stabilize to atmospheric pressure The D- VV was measured to the nearest 0 01 toot from trill-top of the weft casing using an interface probe Groundwater monitoring results for the quarter are presented In Ta de 1 (Attachment 2) Historical data is Included as'Tabl 31A A potenbornetric surface map was prepared lasing the quarterly groundwater elevation data (Figure 2),t rhrch indicates that as of the date of the samplinr I went,groundwater flow was toward the north at a gradie tit of approximately 0 007 Rift Field data sheets are included in Attachment 3 Monitoring WeiI Sampling and Analytical Results Groundwater w mples were collected from the three monrtonng w ilts Dissolved oxygen (DO) concentrations were measured in the three wells prior to sampling The grcurid)*oter sampies were Collected using disposable balls Irs Groundwater samples were placed in anslyvA-speafic containers with analysis-specific preservative Tie samples were then sealed using Teflon septum caps, labeled with the fob identification number,the sa tlple number,date,time, and type of laboratory gni alysts required The samples were stored on ice and train ported with a chain-of-custody record toa.state-e�trtitred laboratory Groundwater samples were analyzed for BTEX and TPH-G using BPA:methods 80211m adifted 5415,and for the oxygenates MTBE,TEA, D PE,ETRE, and TAME using EPA method 8250 Dissolved oxygen data indicate retattvefy uniform,generally snael otic conditions Dissolved oxygen levels ranged betwet n about 0 9 and 10 milligrams per liter(mg/L) Laboratory an ilybcat results for quarterly groundwater sampleG t Ire summanzed in Table 1 Dissolvett benzene level i ranged between 0 74 and 214 micrograms per Mar(ug/L) Dissolved TPH-G concentrations were not date fed Dissolved MTSE concentrations were datec0d in wells MW-2 and MW-3 at levels of between 36 end 5 ug/L. The other oxygenates were not detectiA, as has been the case for a natal of= quarterly sampling events(Table 1A) ' 1 Normally,groundwater flow is to the northwest,with MW-3 lying furthest downgradient from the former tanWpurnp isl and locations MTBE concentrations in well MW 3 have declined since the first quarter 2000, when levels v rete at 103 t4V1.Low and non-detectable di'ssohed gasoline-constituent concentrations with � declining dissolved oxygen levels(MW-3)indicate that natural attenuationtbiodegradatlon is occurring ,The laboratory an alyGeal reports for the groundwater samples are iti eluded in Attachment 4 I .r I i I I i I j I 121756qA rpt I I i i t 11