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. r z�J P C7 O a DOSERVICES, INCQRPORATED � aafn MPDS-UN6981-16 November 17, 1997 Tosco Marketing Company Environmental Compliance Department 2000 Crow Canyon Place, Suite 400 San Ramon, California 94583 Attention Mr Edward C Ralston CEIVED RE Quarterly Data Report DEC 12 1997 Unocal Service Station #6981 IM-34Y; 4707 Pacific Avenue Stockton.- California Dear Mr Ralston This data report presents the results of the Fourth Quarter 1997 monitoring and sampling of the monitoring wells at the referenced site by MPDS Services, Inc RECENT FIELD ACTIVITIES *The monitoring wells that were monitored and sampled are indicated in Table 1 Prior to sampling, the wells were checked for depth to water and the presence of free product or sheen The monitoring data and the ground water elevations are summarized in Table i The ground water flow direction during this most recent quarter is shown on the attached Figure 1 Ground water samples were collected on October 15 and 16, 1997 Prior to sampling, the wells were each purged of between 7 and 37 gallons of water During purging operations, the field parameters pH, temperature, and electrical conductivity were recorded on the purging/sampling data sheets which are attached to this report Once the field parameters were observed to stabilize, and where possible, a minimum of approximately three casing volumes had been removed from each well, samples were then collected using a clean Teflon bailer The samples were decanted into clean VOA vials and/or one-liter amber bottles, as appropriate, which were then sealed with Teflon-lined screw caps, labeled, and stored in a cooler, on ice, until delivery to a state-certified laboratory MPDS Services, Inc transported the purged ground water to the Tosco Refinery located in Rodeo, California, for treatment and discharge to San Pablo Bay under NPDES permit ANALYTICAL RESULTS The ground water samples were analyzed at Sequoia Analytical Laboratory and were accompanied by properly executed Chain of Custody documentation The analytical results of the ground water samples collected to date are summarized in Tables 2 and 3 The concentrations of Total Petroleum Hydrocarbons (TPH) as gasoline and benzene detected in the ground water samples collected this quarter are shown on the attached Figure 2 Copies of the laboratory analytical results and the Chain of Custody documentation are attached to this report 2,101 siamyell C ve 5ui,e 300 Concorc, CA 94520 TEL c5 0)� 2 5 120 CAX (5 10) 659 1918