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Former Beacon Station No.474 April 20,2000 Stockton California Groundwater Monitoring Repo Groundwater samples were collected from groundwater monitoring wells MW-1 through MW-3, MW-5 through MW-12, vapor well VW-1 and air-sparge well AS-1. Vapor extraction wells VW-2 through VW-7 were dry. Monitoring wells MW-10 through MW-12 and air-sparge well AS-1 were installed and initially sampled in October 1999. The screened interval for the wells and analytical results of the initial sampling of monitoring wells MW-10 through MW-12 and air-sparge well AS-1 are summarized in Tables 1 and la. Groundwater samples were submitted under chain-o€-custody (COC) documentation to Kiff Analytical (Kiff), a California Department of Health Services-certified analytical laboratory (ELAN No. 2236) located in Davis, California. The samples were analyzed for Total Petroleum Hydrocarbons as gasoline (TPHg); the volatile aromatic compounds benzene, toluene, ethylbenzene and total xylenes (BTEX); the seven fuel oxygenates methyl tertiary butyl ether (MTBE), di-isopropyl ether (DIPE), ethyl-tertiary-butyl ether (ETBE), tertiary- amyl methyl ether (TAME) and tertiary butyl alcohol (TBA), methanol and ethanol; and the lead scavenger compounds 1,2-dichloroethane (ethylene dichloride or EDC) and 1,2- dibromoethane (ethylene dibromide or EDB) by Environmental Protection Agency (EPA) Method 8260B. Laboratory results are summarized in Tables 1 and Ia. Copies of the laboratory data sheets and chain-of-custody reports are included as Attachment C. Historical groundwater levels and analytical results prior to 1998 are summarized in Tables 1 and 2 in Attachment D. Results Groundwater level data collected by Horizon was used to construct the Groundwater Contour Map (Figure 2). Monitoring wells with substantially submerged well screens (wells MW-1, MW-2 and MW-3) were not used for groundwater contour construction. The average depth-to-water in the wells was 55.5 feet below surface grade (bsg). The average groundwater level represents an approximately 1.1-foot rise compared to that measured in December 1999. The groundwater flow direction was generally to the south and southeast under a gradient of approximately 0.004 foot/foot. Benzene was detected in groundwater samples from monitoring wells MW-2, MW-6, MW- 10, MW-11, MW-12, from air-sparge well AS-1 and from vapor well VW-1 at concentrations ranging from 0.80 to 23,000 micrograms per liter (parts per billion or ppb). Other BTEX compounds were also detected in these groundwater samples. Benzene concentrations have decreased in monitoring wells MW-2 and MW-6 and increased in vapor well VW-1 compared to December 1999. Benzene concentration data from this sampling event were used to construct a Benzene Isoconcentration Map (Figure 3). Concentrations of the fuel oxygenates DIPE, ETBE, or TAME were not detected in any of the groundwater samples. Concentrations of MTBE were detected in groundwater samples from monitoring wells MW-3, MW-7, MW-11, MW-12 and air-sparge well AS-I at 5.0, 0.62, 1.9, 59 and 11 ppb, respectively. Concentrations of MTBE have increased slightly in i" monitoring wells MW-3 and MW-7 compared to December 1999. Concentrations of TBA A:14741431474QMROOQ 1.doe Job No. 1474.43 2 HORIZON ENVIRONMENTAL INC.