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Mr. Ron Rowe <br /> May 5, 1998 <br /> Page 2 <br /> although this relationship varies from time to time. The screened interval in MW3 makes this <br /> well currently better suited than MW7 to provide early indication of plume migration. <br /> 2) Monitor oxygenate additives at the furthest downgradient point available: <br /> (Sample wells MW11 and MW12) <br /> Wells MW11 and MW12 are the furthest downgradient of the former USTs and MW4 and had <br /> DIPE and/or TBA detected during both of the previous two sampling events. MTBE has not <br /> been detected in these wells during the three previous events. Sampling is recommended both <br /> to verify DIPE&TBA concentrations and to provide additional downgradient warning of MTBE <br /> migration. <br /> 3) Monitor oxygenates up and downgradient of onsite soil contamination: <br /> (Sample wells MWI and MW13) <br /> Oxygenates have not been detected to date in well MW13 (immediately downgradient of the <br /> former USTs and associated soil contamination) nor in the vapor wells installed within the area <br /> of soil contamination. Wells MW 1 and MW6 are both located essentially at the upgradient edge <br /> of the area of soil contamination. Of the two, MW 1 is better suited for representative sampling <br /> as the screen is not currently submerged. Differences in concentrations in wells MW1 and <br /> MW13 may provide important evidence concerning whether or not soil contamination from the <br /> former USTs is a source of oxygenates. <br /> We suggest reviewing the oxygenate data quarterly, in conjunction with data from adjacent sites, and <br /> modifying the sampling schedule as appropriate. <br /> If you have any questions or comments please contact the undersigned at your earliest convenience. <br /> Respectfully, ��RED GFGZ <br /> Groun ero Analysis,� CCS <br /> GREGORY P. <br /> STAHL <br /> Gregory P. Stahl, RG 5023 <br /> No. 5023 'f <br /> CA Certified Hydrogeologist No. 264 <br /> 9lF OF CAL\F�Q� <br /> cc: Mr. Pat Riddle, Esq. <br /> Mr. James DiGiorgio, SWRCB <br />