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' 17 May 1999 AGE-NC Project No 96-0249 Page 8 of 9 8020 at a concentration of 18 µg11, MTBE was detected in ground water samples collected Ifrom MW-4 and MW-6 utilizing EPA Method 8260 at concentrations as high as 38 µg/1 No other fuel-oxygenate analytes were detected above laboratory reporting limits ' • Laboratory data indicated that the greatest concentrations of TPH-g and TPH-d in ground water are near MW-3 The distribution of known dissolved TPH-g and TPH-d are depicted in Figure 13 The horizontal and vertical extent of impacted ground water is not known ' • Benzene was detected in samples collected from wells MW-1 through MW-6 at concentrations exceeding the State of California Maximum Contaminant Level (MCL) for drinking water of 10 ppb The area with the highest concentrations of dissolved benzene is located near the area of MW-3 • Concentrations of toluene exceeded the MCL of 150 ,ugll at well MW-3 • Concentrations of xylenes exceeded the MCL of 1,750 ,ug/l at well MW-3 I • Grab ground water samples collected from the domestic drinking water well that services the Three B's Truck Plaza (DW-1) did not contain analyte concentrations at or exceeding laboratory reporting limits 5 0. RECOMMENDATIONS Based upon data collected during March and April 1999, AGE recommends • Further assessment of petroleum hydrocarbon-impacted soil and ground water utilizing Geoprobe soil borings, recommended soil boring locations are depicted in Figure 2 Data derived from additional hydropunch soil borings will provide a more efficient and cost effective approach for determining the number and locations of additional monitoring well(s) • Installation of additional ground water monitoring wells southwest,west,north and northeast of the foriner UST area(based on results of the Geoprobe soil bonngs) in order to assess and monitor the horizontal extent of off-site dissolved petroleum hydrocarbons (See Figure 2) Consideration should be given towards dual-completing additional ground water monitoring wells to assess hydrocarbon-impacted ground water at deeper levels I • Installation of at least one deeply screened ground water monitoring well near the former UST area to assess and monitor the extent of vertical hydrocarbon-impact • Analysis of permeability, porosity, density, moisture content and total organic content by a Geotechnical Testing Laboratory during the next phase of soil sample collection The results Advanced GcoEnvironinentai,Inc