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Work Plan for Additional Site 7{ssessment • ARCO Facility No.6100 Tracy,California Page 4 GROUNDWATER MONITORING WELLS-Assessment of Liquid-Phase Hydrocarbons and Characterization of Hydrogeology In an effort to better characterize the soil stratigraphy and hydrogeology beneath and adjacent to the site, and to further evaluate the extent of liquid-phase hydrocarbons currently present in monitoring well MW-1, Delta proposes to advance six soil borings (4 onsite and 2 offsite) to depths of approximately 120 to 140 feet bgs. Analytical results of soil samples collected from MW-1 indicate that petroleum hydrocarbons are predominantly present at depths between 105 and 125 feet bgs. One of the onsite borings (B-38) will also provide information on the extent of oxygenates detected in selected vapor extraction wells near the dispensers. The borings will be drilled using mud-rotary drilling methods, which allows collection of continuous soil cores and enables the use of various geophysical logging tools that measure several properties of the subsurface material. This data will be used to identify and correlate hydraulically connected layers or units that possibly enable water and/or liquid-phase hydrocarbons to migrate vertically and laterally beneath and adjacent to the site. The proposed locations were selected based on our understanding of the current site conditions, the location of possible source areas for both groundwater and gasoline hydrocarbons, and the preferential flow associated with the local stratigraphy. Three of the proposed onsite borings (B-33 through B-35) will be located along the southern portion of the site, perpendicular to the dip direction observed in local bedding. Borings B-33 and B-34 are also located downdip of the dispensers and USTs. Boring B-35 is positioned between MW-1 and the water supply well, which may be a possible source of water to the shallow subsurface material given the construction of the water well and potentiometric elevation of the water in the well. The two offsite borings (B-36 and B- 37) will be located on the property southeast of Patterson Pass Road, downdip of MW-1. Figure 2 presents the proposed locations. Following the completion of the six mud rotary borings and review of the geophysical logs, groundwater monitoring wells will be installed at the locations. Because the use of drilling mud typically causes the sidewalls of the borehole to become less permeable and often impedes the flow of groundwater, the wells will be completed in new soil borings advanced adjacent to the mud-rotary borings. These borings will be drilled using an air- rotary casing-hammer drill rig or rig with similar capabilities. The borings will be converted to 4-inch diameter groundwater monitoring wells (MW-2 through MW-7). The actual completion depth and screen interval will be based on the results of the geophysical logs and conditions encountered during drilling operations (degree of impact and soil type). The monitoring well screen interval will target potentially permeable material. Q\ARC0\6100\REPORTS\6100 workplan 05-22-01.doc