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Fortner Beacon Station No. 12474 November 14,2000 Stockton, California Ouarterly.Groundwater Monitoring Report Services-certified analytical laboratory (FLAP 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 fuel oxygenate methyl tert-butyl ether (MTBE) by Environmental Protection Agency (EPA) Method 826013. 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. The quarterly monitoring analytical program has been modified with approval from the San Joaquin County Environmental Health Division on 4 October 2000. Groundwater monitoring samples shall be analyzed for TPHg, BTEX and MTBE on a continuing quarterly basis. Groundwater samples collected for second quarter monitoring shall also be analyzed for the six other fuel oxygenates diisopropyl ether (DIPS), ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME), tert-butanol (TBA), methanol and ethanol, and the lead scavenger compounds 1,2-dichloroethane and 1,2-dibromoethane. 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 --31.1 feet mean sea level and represents an approximately 2.3-foot fall compared to that measured in June 2000. The groundwater flow direction beneath the site was generally to the south and southeast under a gradient of approximately 0.00 1. Benzene was reported in groundwater samples from monitoring wells MW-2, MW-6, MW- 10, MW-I1 and MW-12, and air-sparge well AS-I at concentrations ranging from 1.1 to 7500 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-2, MW-6 and air-sparge well AS-1, and increased in monitoring wells MW-10, MW-1 I and MW-12 compared to June 2000. Benzene concentration data from this sampling event were used to construct a benzene isoconcentration map as shown on Figure 3. j i Recommendations Horizon recommends continued quarterly monitoring at the site. A Problem Assessment - Report/Remedial Action Plan should be developed to define the problem and propose remedial actions. �i 474gmr00Q3.doc 1474.43 2 HORIZON ENVIRONMENTAL INC.