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ate ° - - PDOSERVICES INCORPORATED • MPDS-UN5886-05 December 22, 1997 Tosco Marketing Company Environmental Compliance Department 2000 Crow Canyon Place, Suite 400 San Ramon, California 94583 Attention Mr Edward C Ralston RE Quarterly Data Report Unocal Service Station #5886 2701 West March Lane Stockton,„California Dear Mr Ralston This data report presents the results of the most recent quarter of 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 during this quarter 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 1 The ground water flow direction during the most recent quarter is shown on the attached Figure 1 Ground water samples were collected on December 1, 1997 Prior to sampling, the wells were each purged of between 4 and 9 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 three casing volumes were 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, benzene and TPH as diesel detected in the ground water samples 2401 Stanwell Drive suite 400 Concord CA 94520 TEL (510) 602 5120 FAX (510)689 1918