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Work Plan for Additional Sit 7Cssessment • ARCO Facility No. 6100 Tracy,California Page 3 monitoring events. On three events, vapor extraction well VW-1 had product sheen present on the water surface. Chemical analyses of water samples collected in February 2001 from vapor extraction wells VW-2 through VW-6 and monitoring well MW-1 indicate the presence of petroleum hydrocarbons as well as methyl tertiary butyl ether (MTBE). PROJECT OBJECTIVE AND APPROACH The objectives of the additional subsurface investigation are: • Further assess the subsurface conditions beneath and adjacent to the site, including detailed logging of the subsurface stratigraphy and identification of hydraulically transmissive material; • Assess the extent of free phase petroleum hydrocarbons; • Assess the vertical and lateral extent of total petroleum hydrocarbons and MTBE southeast of the dispensers and UST's, and southeast of the site; • Assess the vertical and lateral extent of oxygenates, specifically MTBE, in the vicinity of the dispensers and USTs. The following scope of work was developed based on our current understanding of the site, extent of impact, recent monitoring and sampling data, and discussions with PHS/EHD. Delta is proposing the following activities to accomplish the objectives outlined above: • Advance six soil borings (4 onsite and 2 offsite [13-33 through B-38j) to depths of approximately 120 to 140 feet bgs to assess the extent of liquid-phase hydrocarbons and to further characterize the subsurface stratigraphy. Continuous soil sampling to be performed using mud rotary drilling methods; • Conduct wireline geophysical logging within the six borings drilled by mud rotary to provide a continuous log of the subsurface stratigraphy and for correlation with collected samples; • Advance six soil borings adjacent to the mud-rotary borings using an air-rotary casing-hammer drill rig ,or rig with similar capabilities, and convert them to 4- inch diameter groundwater monitoring wells (MW-2 through MW-7); • Advance two onsite soil borings (B-39 and B-40) to approximately 100 feet bgs to evaluate the extent of oxygenates in the vicinity on the USTs and fuel dispensers; • Develop, sample, and survey the newly installed wells. CAARC0\610MEPORTS\6100 workplan 05-22-01.doc