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Former Beacon Station 494 September 12, 1995 Stockton,Califonna Quazterly Groundwater Morutonn Groundwater samples were collected from the monitoring wells on August 15, 1995 Collected samples were submitted under chain-of-custody to Western Environmental Science & Technology, a California Department of Health Services-certified analytical laboratory (ELAN No 1346) located in Davis, Cahfonua Groundwater samples were analyzed for Total Petroleum Hydrocarbons as gasoline (TPHg) according to EPA Method 8015 and for benzene, toluene, ethylbenzene, and xylenes(BTEX) according to EPA Method 602 Laboratory results are summarized on Table 1 Copies of the laboratory reports and chain-of-custody are included in Attachment D Laboratory results tables from sampling prior to April 1995 are shown in Attachment C Results Water-level data were used to construct a Groundwater Contour Map (Figure 2) The groundwater elevation data reveal a circular depression in the groundwater surface centered toward MW-3 The data implies that the groundwater flow is radial at a gradient of approximately 0 17 to 0 26 toward the pumping well MW-3 Data collected before the start-up of the treatment system had shown a groundwater flow generally toward the northeast The change in gradient indicate that groundwater beneath the site is being affected by groundwater extraction from MW-3 Groundwater levels have decreased 0 82 feet in the monitoring wells • since the previous momtonng event This reflects the seasonal decline in groundwater A Benzene Isoconcentration Map (Figure 3) was also prepared Dissolved benzene concentrations of 0 64, 1 1, and 0 70 parts per billion(ppb) were detected in samples collected from morutonng wells MW-1, MW-3, and MW-5, respectively Concentrations of benzene decreased in monitoring well MW-2 and MW-3 and increased this quarter in MW-1 and MW-5 Groundwater extraction from MW-3 has produced mixed results during the past year As shown in Table 1, TPHg and benzene concentrations have declined rather consistently in MW-2 In contrast, concentrations have fluctuated up an down in MW-3 and MW-5, and this is the first time in several quarters that hydrocarbons have been detected in MW-1 Although concentrations are low,the system has been unable to completely remove all hydrocarbons from the groundwater Based on companson to similar sites elsewhere, we believe that the system has reached its effective limits and that attempts to reduce concentrations further would be unsuccessful Therefore, we recommend that the groundwater pumping be discontinued If subsequent groundwater analytical results indicate no increase in the levels of dissolved hydrocarbons in the groundwater we recommend groundwater monitoring be discontinued and the monitoring wells destroyed Report No 494gmr95 q3 Job No 1494 31 2 HORIZON ENVIRONMENTAL INC