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Del Monte Foods—Research Center 205 North Wiget Lane(94598-2458) P.O. Box 9004 Walnut Creek, CA 94598-0904 Telephone: (925) 944-7300 FAX: (925)945-7453 June 16, 1999 Overnight Delivery Mr. Ronald Rowe LOP/Site Mitigation Unit E E San Joaquin County Public Health Services " Ems. Environmental Health Division 304 E. Weber Ave., Third Floor JUN 17 1999 Stockton, CA 95202 ENVIR0,NMEi IAL HEALTH Dear Mr. Rowe: PERMIT/SERVICES Subject: Quarterly Monitoring Results Report—February 1999 Enclosed are the Quarterly Monitoring Results for February 1999. The latest groundwater contour map and an updated tabulation of all monitoring data are included. The groundwater flow direction has continued to be in a south to southeasterly direction since October 1995. Monitoring Well samples were collected on February 10 and 11, and analyzed for BTEX, TPH as gasoline, and for MTBE, TBA, DIPS, ETBE, and TAME. The results were typical of what has been reported in the last several Quarterly Monitoring Reports except that TPH as gasoline was recorded in MW-5,MW-7S, and MW-71). As these wells do not have a history of i, contamination, and after discussions with the lab, these three wells and MW-2 were resampled on March 19, 1999. BSK Labs confirmed that the samples were cross-contaminated from lab equipment. The sample stations used to analyze samples from MW-5, MW-7S, and MW-7D previously contained samples with extremely high hydrocarbon contamination. A letter from BSK Labs to Del Tech explaining what happened is included in the Lab Data Sheets section of this report. The March 19 results for MW-5, MW-7S, and MW-7D were all ND. However,MTBE was detected in MW-2 by EPA Method 8020 at a concentration of 21 ug/l. This result was not confirmed by GC/MS (EPA Method 8260). Because MTBE has not been detected in this well in the 7 previous sampling events, the detection using EPA Method 8020 is anomalous, and appears to be a"false-positive"result. False-positives for MTBE commonly occur when analyzed by EPA Method 8020. This is particularly true when TPH is present in the groundwater sample, as is the case for MW-2. The groundwater sample collected from MW-2 on February 11, 1999 did not contain MTBE, and it was analyzed by both EPA Methods 8020 and 8260. The susceptibility of false-positives for MTBE by Method 8020 is well documented. A report published by Lawrence-Livermore Laboratory(June 1998) entitled: An Evaluation of MTBE Impacts to California Groundwater Resources, June 11, 1998,notes that EPA Method 8020 is sensitive to matrix effects and oxygenate analysis may be impacted significantly in the presence of moderate to high concentrations of gasoline. The report recommends using EPA Method 8260 when monitoring low concentrations of oxygenates. r