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Beacon Station No 3641 January 15, 2001 Stockton California Quarterly Groundwater Monitoring Report 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), and the fuel oxygenate methyl tert-butyl ether (MTBE) 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 TOC elevations to calculate static groundwater elevations Calculated groundwater elevations for monitoring wells MW-6, MW-7 and MW-8 were used to construct groundwater elevation contours because the water table intersected the screened intervals in only those wells The average groundwater elevation was approximately 55 5 feet mean sea level, approximately 4-foot higher compared to that measured in September 2000 The groundwater flow direction was generally southeastward under a gradient of approximately 0 003 Concentrations of TPHg were reported in groundwater samples from monitoring wells MW- 2 and MW-6, vapor extraction wells VW-1, VW-3, VW-5 and VW-7, and sparge well SW- 1 Relatively low concentrations of TPHg of 56 to 140 micrograms per liter (parts per billion or ppb) were reported in monitoring wells MW-2 and MW-6 Concentrations of TPHg ranging from 3700 to 21,000 ppb were reported in 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- 5 and MW-8, and vapor extraction wells VW-1, VW-3, VW-5 and VW-7 Relatively low benzene concentrations of 42 and 0 56 ppb were reported in monitoring wells MW-5 and MW-8, respectively Benzene concentrations ranging from 220 to 4000 ppb were reported in vapor extraction wells VW-1, VW-3, VW-5 and VW-7 in the UST area With the exception of vapor extraction well VW-7, reported benzene concentrations have decreased compared to September 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 Recommendations Horizon recommends quarterly monitoring and evaluation of groundwater quality trends continue concurrently with development of a Problem Assessment Report and Corrective Action Plan as recommended in the Horizon document Results of Additional Subsurface Investigation dated June 15, 2000 64 t gmrOOQ4 doc 1641 43 2 HORIZON ENVIRONMENTAL INC