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A Former Beacon Station No.419 October 13, 1998 Stockton California Groundwater Monitoring Report Hydrocarbons as gasoline (TPHg) utilizing EPA Method 8015, for the volatile aromatics benzene, toluene, ethylbenzene, total xylenes (BTEX) by EPA Method 8020, and for fuel additives such as, methyl tertiary-butyl ether (MTBE) utilizing EPA Method 8260. Laboratory results are also summarized in Table 1. Copies of the Iaboratory reports and chain-of-custody are included as Attachment C. Results Water-level data collected by Doulos on September 8, 1998 was used to construct the Groundwater Contour Map (Figure 2). The average depth-to-water for the wells was approximately 33 to 34 feet below sea level. Water levels have remained at approximately the same depth as in June 1998. The flow direction is generally toward the southwest, with a magnitude of 0.00 16 foot/foot. Data collected from this sampling event was used to construct a Benzene Isoconcentration Map (Figure 3). Detectable concentrations of benzene were reported in the water samples collected from four of the seven wells sampled. No detectable concentrations of benzene were reported in wells MW-5, MW-7 and MW-9. Dissolved benzene concentrations have increased in well MW-6 and have decreased in well MW-8 since the June 1998 sampling event. The groundwater that was sampled from vapor extraction wells VW-1 and VW-4 indicated detectable concentrations of benzene at 36,000 parts per billion (ppb) and 19,000 ppb, respectively. No detectable concentrations of MtBE were reported in the water samples collected from wells MW-5 through MW9 during this sampling event. Low concentrations of MtBE were reported in the water samples collected from vapor wells VW-1 and VW-4 at concentrations of 0.7 ppb and 1.4 ppb, respectively. No detectable concentrations of other fuel oxygenates were reported in water samples collected during this sampling event, except for concentrations of methanol in wells VW-I and VW-4 at 61 ppb and 74 ppb, respectively. 98gmrsr.g3 Job No. 1419.41 2 HORIZON ENVIRONMENTAL INC.