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Former Beacon Station No. 12474 January19 2001 � I Stockton, California _ W W ,_Quarterly Groundwater Monitoring Report Groundwater samples were collected from groundwater monitoring wells MW-1 through MW-6, MW-8 through MW-12, vapor extraction wells VW-1 and VW-4, and air sparge well AS-1. Groundwater samples were submitted, under chain-of-custody documentation, to Kiff Analytical LLC (Kill), 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 fuel oxygenate methyl-tert-butyl ether (MTBE) 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 level and analytical data collected prior to 1998 are summarized in Tables I 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 (DIPE), ethyl-tert-butyl ether (ETBE), tert-amyl-methyl ether (TAME), tert-butanol (TBA), methanol and ethanol, and the lead scavenger compounds 1,2-dichloroethane (EDC) and 1,2-dibromoethane (EDB). Results f 1 Groundwater level data collected by Horizon was used to_construct the groundwater contour s 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 —30.2 feet mean sea level and represents an approximately 1-foot rise compared to that measured in September 2000. The groundwater flow direction beneath the site was generally to the south and southeast under a gradient of approximately 0.001. Concentrations of TPHg ranging from 96 to 190,000 micrograms per liter (parts per billion or ppb) were reported in monitoring wells MW-1, MW-2, MW-6, MW-9, MW-10, MW-11, MW-12, vapor extraction wells VW-1 and VW-4, and air sparge well AS-1. Benzene concentrations ranging from 2.0 to 29,000 ppb were reported in groundwater samples from monitoring wells MW-2, MW-6, MW-9, MW-10, MW-11, MW-12, vapor extraction wells VW-1 and VW-4, and air sparge well AS-1. With the exception of the groundwater sample from monitoring well MW-9, other BTEX compounds were also detected in these groundwater samples. With the exception of the groundwater sample from vapor extraction well VW-1, reported benzene concentrations represent decreases compared to September 2000. Benzene concentration data from this sampling event were used to construct a benzene isoconcentration map as shown on Figure 3. i 474gmrOOQ4.doc 1474.43 2 HORIZON ENVIRONMENTAL INC.