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Former Beacon Station 494 June 28, 1996 Stockton,California Quarterly Groundwater Monrtonn Groundwater samples were collected from the monitoring wells on May 30, 1996 Collected samples were submitted under chain-of-custody to Western Environmental Science & Technology, a Califorzua Department of Health Services-certified analytical laboratory (ELAN No 1346) Iocated to Davis, California Groundwater samples were analyzed for Total Petroleum Hydrocarbons as gasoline (TPHg) according to EPA Method 8015, benzene, toluene, ethylbenzene, and xylenes (BTEX) according to EPA Method 602 and Methyl-t-butyl ether (MTBE) according to Method 8020/602 Laboratory results are summarized on Table 1 Copies of the laboratory reports and chain-of-custody are included as Attachment D Laboratory result tables from sampling prior to April 1995 are listed in Attachment C Results Water-level data collected on May 30, 1996 were used to construct a Groundwater Contour Map (Figure 2) The groundwater elevation data indicates an overall groundwater flow direction toward the northeast and east with a gradient of approximately 0 016 to 0 02 Data collected before the start-up of the treatment system had shown a groundwater flow generally toward the northeast The gradient appears to have resumed its natural northward trend since groundwater extraction has ceased Water-level data collected on April 30, 1996 were not used to construct a Groundwater Contour Map due to insufficient drawdown during the purging phase of the sampling event As directed by the SJCPHS-EHD the purging drawdown level must exceed the depth of the upper portion of the well screen The water table has risen approximately 3 2 feet in the monitoring wells since the first quarter 1996 monitoring Concentrations of dissolved TPHg and BTEX have not been detected in samples collected from MW-1 and MW-2 for two consecutive quarters The sample collected from MW-3 revealed benzene at 0 93 ppb The sample collected from MW-4 revealed toluene at 0 78 ppb and xylenes at 1 3 ppb The sample collected from MW-5 revealed toluene at 0 72 ppb Also, no concentrations of MTBE were detected in any of the monitoring wells Based on the analytical results of the second-quarter monitoring activity, Horizon recommends closure The existing groundwater and vapor extraction wells at the site should be properly decommissioned . Report No 494gmr96 q2 Job No 1494 42 2 HORIZON ENVIRONMENTAL INC