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Former Beacon Station No 419 April 20,2000 Stockton California Groundwater Monitoring Re ort Groundwater samples were collected from the ten groundwater monitoring wells MW-1 through MW-10 and air-sparge well AS-1 The screened interval for the wells are noted on Tables 1 and la This was the initial analytical sample collected from air-sparge well AS-1 Vapor extraction wells VW-1 and VW-2 were dry Groundwater samples were submitted under chain-of-custody (COC) documentation to Kiff Analytical (Kiff), a California Department of Health Services-certified analytical laboratory (ELAN 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 tertiary butyl ether (MTBE), di-isopropyl ether (D1PE), ethyl-tertiary-butyl ether (ETBE), tertiary- amyl methyl ether (TAME) and tertiary butyl alcohol (TBA), methanol and ethanol, and the lead scavenger compounds 1,2-dichloroethane (ethylene dichloride or EDC) and 1,2- dibromoethane (ethylene dibromide or EDB) by Environmental Protection Agency (EPA) Method 8260B Laboratory results are summarized in Tables 1 and la Copies of the laboratory data sheets and chain-of-custody reports are included as Attachment C Results Groundwater level data collected by Horizon was used to construct the Groundwater Contour Map (Figure 2) The average depth-to-water for the wells was 45 5 feet below surface grade (bsg) The average groundwater level represents an approximately 16-foot • rise compared to that measured in December 1999 The groundwater flow direction was generally toward monitoring well MW-9 under a gradient of approximately 0 002 foot/foot Benzene was detected in groundwater samples from monitoring wells MW-3, MW-6, MW-9 at concentrations ranging from 29 to 160 micrograms per liter (parts per billion or ppb) and from air-sparge well AS-1 at a concentration of 7,600 ppb Other BTEX compounds were also detected in these groundwater samples Benzene concentrations have increased in monitoring wells MW-3 and MW-6 and decreased in monitoring well MW-9 compared to December 1999 No previous analytical data are available for air-sparge well AS-1 Benzene concentration data from this sampling event were used to construct a Benzene Isoconcentration Map (Figure 3) Concentrations of the fuel oxygenates DIPE, ETBE, or TAME were not detected for any of the groundwater samples this quarter Concentrations of MTBE were detected in groundwater samples from monitoring wells MW-2, MW-8, MW-9 and air-sparge well AS- 1 at 10, 2 0, 0 55 and 3 4 ppb, respectively Concentrations of MTBE have decreased in monitoring wells MW-2 and MW-9, while increasing in monitoring well MW-8 compared to December 1999 Concentrations of TBA were reported in wells MW-7 and MW-9 Concentrations of TBA have decreased in monitoring wells MW-7 and remained stable in MW-9 compared to December 1999 Concentrations of ethanol were detected in groundwater samples from monitoring wells MW-3, MW-6, MW-9 and air-sparge well AS- . 1 at 10, 20, 8 8 and 12,000 ug/L, respectively A methanol concentration of 380 ppb was detected in the groundwater sample from air-sparge well AS-1 A 114191431419QMR00 Q) doc Job No 1419 43 2 HORIZON ENVIRONMENTAL INC.