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Former Beacon Station No. 474 July 28, 2000 Stockton, California Groundwater Monitoring Report e Groundwater samples were collected from groundwater monitoring wells MW-1 through • MW-3. MW-5 through MW-10, MW-12. vapor well VW-1 and air-sparge well AS-1. Groundwater samples were submitted, under chain-of-custody documentation, to Kiff Analytical LLC (Kiff), a California Department of Health Services-certified analytical laboratory (ELAP 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 tert-butyl ether (MTBE), diisopropyl ether (DiPE), ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME) and tert-butanol (TBA), methanol and ethanol; and the lead scavenger compound 1,2-dichloroethane (EDC) by Environmental Protection Agency (EPA) Method 8260B. Laboratory results are summarized in Table 1. 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 as shown on 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 screened interval for the wells are noted in Table 1. Depth-to-groundwater measurements were subtracted from the surveyed TOC elevations (reported by previous consultants) to calculate groundwater elevations. The average groundwater elevation in the wells was approximately —28.8 feet mean sea level and represents an approximately one foot rise compared to that measured in March 2000. The groundwater flow direction was generally to the south and southeast under a gradient of approximately 0.002. Benzene was reported in groundwater samples from monitoring wells MW-2, MW-6, MW- 10, MW-12, air-sparge well AS-I and vapor well VW-1 at concentrations ranging from 0.85 to 23,000 micrograms per liter (parts per billion or ppb). With the exception of the groundwater sample from monitoring well MW-10, the other BTEX compounds were also detected in these groundwater samples. Benzene concentrations have decreased in monitoring wells MW-10 and MW-12 and increased in monitoring wells MW-2 and MW-6 and air-sparge well AS-1 compared to March 2000. Benzene concentration data from this sampling event were used to construct a benzene isoconcentration map as shown on Figure 3. No concentrations of the fuel oxygenates DIPE, ETBE, TAME or methanol were reported in any of the groundwater samples. The fuel oxygenate MTBE was reported in groundwater samples from monitoring well MW-2 and air-sparge well AS-1 at concentrations of 8.5 and 10 ppb, respectively. A TBA concentration of 17 ppb was reported in the groundwater sample from monitoring well MW-10. Ethanol was reported in groundwater samples from monitoring wells MW-3, MW-5, MW-7, MW-8 and MW-9 at concentrations ranging from 9.4 to 24 ppb, and in monitoring wells MW-12 and air-sparge well AS-1 at concentrations of 39,000 and 55,000 ppb, respectively. 474gmr00Q2.doc 1474.43 2 HORIZON ENVIRONMENTAL INC.