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Beacon Station No 3641 October 30, 2000 Stockton, California _ _ _ Quarterly Groundwater Monitoring Report Groundwater samples were collected from all eight monitoring wells, the sparge well and four of the eight vapor wells The groundwater samples were submitted, under chain-of- custody documentation, to Kiff Analytical (Kiff), a California-certified analytical laboratory (ELAP No 2236) located in Davis, California The groundwater 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), tert-butanol (TBA), methanol and ethanol, and the lead scavenger compounds 1,2-dichloroethane (EDC) and 1,2-dibromoethane (EDB) by Environmental Protection Agency (EPA) Method 8260B The laboratory data sheets and the chain-of- custody document are included as Attachment C Results Groundwater level data collected by Doulos were used to construct a groundwater contour map (Figure 2) Depth-to-groundwater measurements were subtracted from the surveyed TQC elevations to calculate static groundwater elevations Calculated groundwater elevations for monitoring well MW-1 and sparge well SW-1 were not used to construct groundwater elevation contours because the screened intervals are approximately 12 feet below the water table The average groundwater elevation was approximately 51 5 feet mean sea level, approximately 6-foot lower compared to that measured in June 2000 The groundwater flow direction was generally west with southwest flow in the eastern portion of the site and northwest flow in the southwestern portion of the site The groundwater gradient across the site was approximately 0 002 Concentrations of TPHg were reported in groundwater samples from monitoring wells MW- 1, MW-2, MW-4, through MW-8, vapor extraction wells VW-1, VW-3 and VW-7, and sparge well SW-1 Relatively low concentrations of TPHg ranging from 120 to 170 micrograms per liter (parts per billion or ppb) were reported in monitoring wells MW-1, MW-4, MW-6 and MW-8 Concentrations of TPHg ranging from 940 to 85,000 ppb were reported in monitoring wells MW-2 and MW-5, vapor extraction wells VW-1, VW-3 and VW-7, and sparge well SW-1 The highest TPHg concentrations were reported in groundwater samples from wells in the UST area Benzene concentrations were reported in groundwater samples from monitoring wells MW- 1, MW-2, MW-4, MW-5 and MW-8, vapor extraction wells VW-1, VW-3 and VW-7, and sparge well SW-1 Relatively low benzene concentrations ranging from 0 73 to 95 ppb were reported in monitoring wells MW-1, MW-4 and MW-8, and sparge well SW-1 Benzene concentrations ranging from 800 to 16,000 ppb were reported in monitoring wells MW-2 and MW-5, and vapor extraction wells VW-1, VW-3 and VW-7 in the UST area With the exception of vapor extraction wells VW-3 and VW-7, reported benzene concentrations have increased compared to June 2000 Benzene concentration data were used to construct a benzene isoconcentration map (Figure 3) Generally, benzene concentration data from wells screened below the water table were not used to construct the benzene isoconcentration contours 641 gmr00Q3 doe 1641 43 2 HORIZON ENVIRONMENTAL INC.