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Chevron Products Company Page 4 Chevron Service Station 9-4275 2905 Benjarrun Holt Drive,Stockton California May 20 2002 Influences in the upper water bearing zone in response to the Municipal Well No 10 operation have not been measured Trends in upper water bearing zone analyte concentrations are . representative of Monitoring well MW-6 and are presented in Attachment E Groundwater Analytical Results Groundwater samples were collected from the identified water-bearing zone or depth intervals showing distinct pore pressure Three to four depth-discrete groundwater samples per boring were submitted for analysis Groundwater samples were generally collected at depths of 10- to 12- feet bgs, 24- to 25-feet bgs, 35- to 40-feet bgs, and 68- to 78-feet bgs CPT groundwater analytical data is presented in Table 1 Laboratory analytical reports and chain-of-custody record are included in Attachment F Groundwater analytical results are summan zed below TPHg was detected in two of the 17 groundwater samples at concentrations of 73 micrograms per liter (gg/L) (CPT-5 at 37 ft bgs) and 130 gg/L (CPT-3 at 40 ft bgs), Ethylbenzene was detected in three of the 17 groundwater samples at concentrations ranging from 0 61µg/]L (CPT-2 at 24 ft bgs) to 2 3 µg/L (CPT-1 at 25 ft bgs), Xylenes were detected in three of the 17 groundwater samples at concentrations ranging from 0 88 µg/L (CPT-5 at 37 ft bgs) to 1 8 gg/L (CPT-1 at 25 ft bgs), MtBE was detected in six of the 17 groundwater samples at concentrations ranging from 0 59 µg/L (CPT-4 at 10 ft bgs) to 2 6 pg/L (CPT-5 at 37 ft bgs) Municipal Well No. 10 Sampling In response to an unqualified detection of MtBE in Municipal Well No 10 during the 4th quarter 2000 monitoring event, split samples were submitted to Sequoia Analytical and Kiff Analytical LLC during the March 2, 2001 sampling event The groundwater sample submitted to Kiff Analytical LLC reported MtBE at a concentration of 0 57 µg/L The Sequoia Analytical groundwater sample reported MtBE below laboratory detection limits (<2 5 µg/L) Groundwater samples submitted from April 19, 2001 to August 21, 2001 report MtBE below laboratory detection limits (<0 50 µg/L) Historical groundwater analytical results for Municipal Well No 10 are presented in Table 2 Estimated Mass Flux/Mass Discharge To calculate the mass flux/mass discharge of MtBE the following assumption were made • To conservatively estimate the mass of MtBE mobilized in the subsurface a transect-wide concentration of 2 6 µg/1 was assumed from 10 to 40 feet bgs, • Concentrations of MtBE are assumed to remain stable throughout the transect Because concentrations typically decrease exponentially with respect to distance, a linear relationship is likely conservative, and I 1Chevron1942751REPORTS12001 CPT SA and Muni sampling Rev A doc