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GROUND ZERO ANALYSIS, INC. r <br /> 1714 Main Street <br /> Fscalon, California 95320 <br /> Telephone: (209) 838-9888 <br /> Facsimile: (209) 838-9883 <br /> May 5, 1998 <br /> Mr. Ron Rowe <br /> San Joaquin County-Public Health Services <br /> Environmental Health Division <br /> 445 North San Joaquin Street <br /> P.O. Box 388 <br /> Stockton, CA 95201-0388 <br /> Subject: Knowles/Roek Property, 102 South Wilson Way, Stockton, California <br /> Recommendations for 8260 Analyses - Second Quarter 1998 <br /> Dear Mr. Rowe: <br /> Following up on our phone conversation I am writing to present recommendations for sampling <br /> groundwater for oxygenate additives by EPA method 8260. <br /> As we discussed, high concentrations of MTBE, in conjunction with a dramatic increase in hydrocarbon <br /> concentrations, appeared in MW4 in November, 1997. The presence of MTBE in this well was <br /> confirmed in December, 1997 and again in February, 1998. MTBE was not detected in this well when <br /> first sampled in July, 1997. This is interpreted as the encroachment of a plume of contamination <br /> originating offsite. Low levels of MTBE have also been twice detected in MW9, north of and generally <br /> crossgradient from the site. <br /> On the other hand, it is unclear whether or not diisopropyl ether (RIPE) or tertiary butyl alcohol (TBA) <br /> are consistently present in groundwater in the site area. It is unknown whether the source(s) may be <br /> onsite or offsite UST leaks or other sources (leaking sewers, industry, etc.) The initial round of 8260 <br /> analyses (11/97) reported relatively low concentrations of DIPE and/or TBA in most wells north, east <br /> and west of the former USTs. These constituents were not reported as present in most wells south of the <br /> former USTs nor in the vapor wells installed within the onsite gasoline source area. The second round <br /> of 8260 analyses (02/98) detected DIPE and TBA only in wells MW 11 and MW 12 although the detection <br /> limits were somewhat higher than during the initial sampling. <br /> To meet the objectives listed below while conserving financial resources, the following is suggested: <br /> 1) Monitor the MTBE plume in the vicinity of MW4: <br /> (Sample wells MW3 and MW4) <br /> Sample MW4 for oxygenate additives to gauge changes in concentration. Also sample MW3 in <br /> order to detect plume migration. Both MW3 and MW7 are generally downgradient of MW4 <br />