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Beacon Station No 641 vla} 10 2000 Stockton California Groundwater klonitonn,J Repos tt . field instrumentation during the purging process The -Visual characteristics of the groundwater were also noted during the purging process Groundwater samples were collected from all five monitoring wells, the sparge .%ell and four of the eight vapor wells Location of the wells and other pertinent site features ale shown on the attached Site Map (Figure 2) The collected samples were submitted under chain-of-custody documentation to Kiff Analytical (Kiff), a California-certified analytical laboratory (FLAP 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 fir,e fuel oxygenates methyl tert-butyl ether (MTBE), ditsopropyl ether (DIPE), ethyl tert-butt' ether (ETBE), tert-amyl methyl ether (TAME), tert-butanol (TBA), methanol and ethanol by Environmental Protection Agency (EPA) Method 8260B The laboratory data sheets and the chain-of-custody report are included as Attachment C Results Groundwater level data collected by Doulos were used to construct the Gioundwater Contour Map (Figure 3) The average depth-to-groundwater for the wells was 42 5 feet below surface grade (bsg) The average gioundwater level represents an appro\imately 2 foot rise compared to that measured in December 1999 The Groundwater flow dii ection was generally to the northeast under a gradient of approximately 0 001 foot/foot • Benzene was detected in groundwater samples from monitoring well MW-5 at 21 micrograms per liter (parts per billion or ppb) and in vapors wells VW-1, VW-3 VW-5 and VW-7 at concentrations ranging from 500 to 5,100 ppb Other BTEX compounds wel e also detected in groundwater samples from the vapor wells Benzene concentrations ha"e decreased in monitoring well 1\4W-5 and in vapor wells VW-1, VW-5 and VW-7 compared to December 1999 The benzene concentration detected in the groundwater sample from vapor well VW-3 increased slightly compared to December 1999 Benzene concentration data from wells screened in the interval of 40 to 55 feet bsg (vapor wells VW-1, VW-3 VW- 5 and VW-7) were used to construct a Benzene Isoconcentratlon Map (Figure 4) Groundwater samples from monitoring wells MW-1 and MW-5 (screened in the inters ats 60 to 80 and 50 to 75 feet bsg) located within the 500 ppb benzene contour contained benzene concentrations of <2 0 and 21 ppb, respectively Benzene concentrations in groundwater appear to be limited to depths of less than 60 feet bsg Concentrations of the fuel oxygenates DIPE, ETBE and TAME were not detected in any of the groundwater samples this quarter Concentrations of MTBE were detected in groundwater from all sampled wells at concentrations ranging form 0 91 to 14,000 ppb Concentrations of MTBE have decreased in all sampled wells except MW-1 and sparge well SW-1 compared to December 1999 Concentrations of TBA were detected in samples from monitoring wells MW-2, MW-4 and MW-5 at concentrations ranging from 1; to 72 ppb and represent concentration decreases compared to December 1999 641gnu00Q1 doc 1641 43 2 HORIZON ENVIRONMENTAL INC