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�y .� GEOMATRIX • Mr Jeff Wong San Joaquin County Department of Public Health Services August 10, 2004 Page 3 Reported analytical results for soil samples (Table 1) and a review of boring log data indicate that the area of soil potentially impacted by petroleum related constituents is present over an area of approximately 140 by 90 feet (Figure 2) and at depths from approximately 5 to 30 feet bgs During the time that groundwater monitoring was performed, the water table appears to have fluctuated significantly with reported depth-to-water measurements ranging between approximately 5 and 14 feet bgs The petroleum-affected soil thus likely is present within a "smear zone" both above and below the current water table The magnitude of the horizontal hydraulic gradient at the site is reported to be very low (i e , less than approximately 0 0001 feet per feet) with an apparent direction toward the south and southwest Groundwater samples collected from monitoring wells installed within the affected soil area (MW-2, MW-3, and MW-4) have been reported by analytical laboratories to be impacted by petroleum-fuel related analytes including total petroleum hydrocarbons in the diesel range (TPHd), total petroleum hydrocarbons in the gasoline range (TPHg), and one or more of the aromatic hydrocarbon compounds (benzene, toluene, ethylbenzene, and xylenes, collectively • referred to as BTEX, Table 3) Concentrations of TPHd, for example, have been reported at levels in groundwater samples collected from wells MW-2, MW-3, and MW-4 that exceed designated water quality objectives (WQOs) (e g , 100 micrograms per liter [pg/L] for TPHd) for sites in RWQCB Region 5 Methyl tertiary butyl ether(MTBE) has not been reported as detected in soil or groundwater samples collected from the site With the exception of the most recent(June 2004) groundwater sampling event and the initial sampling of well MW-1, groundwater samples collected from perimeter monitoring wells MW-1, MW-5, and MW-6 are not reported by the laboratory to have contained detectable concentrations of petroleum hydrocarbon constituents Monitoring well MW-1 was sampled a total of 15 times between the period of August 1998 to June 2002 Groundwater samples collected from this well are not reported to have shown any detections of TPHd, TPHg, or BTEX with the exception that TPHd was reported as detected at a concentration of 66 Ng/L in the samples collected in August 1998 Similarly, groundwater samples collected from wells MW-5 and MW-6 between June 2000 and June 2002 (nine sampling events) were not reported to have shown detections of TPHd, TPHg, or BTEX Laboratory results from the most recent groundwater sampling event in June 2004 do report the detections of TPHd in groundwater samples from monitoring wells MW-1, MW-5, and MW-6 (at concentrations of 59, 250, and 170 µg/l, respectively) These results do not appear consistent with previous results and are considered anomalous (See Table 3) Future groundwater sampling activities will be used to assess the validity of the most recent data 1 IProjcct19000s199631CAP Work Plan doc