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Work Plan for Additional Site'Assessment • ARCO Facility No. 6100 Tracy,California Page 6 Task 2: Field Activities The following section has been subdivided into two parts. The first part addresses those activities associated with the on and offsite soil borings that will be drilled using mud- rotary and the installation of groundwater monitoring wells in adjacent boreholes. The second part addresses those activities associated with the drilling of the two onsite soil borings in the vicinity of the USTs and dispensers using hollow stem augers. Task 2a: Soil Borings (Mud Rotary Drilling) and Well Installation The following section details those activities associated with 1) drilling the six soil borings using mud rotary drilling methods and sample collection, 2) logging the borehole using downhole geophysical tools, 3) drilling adjacent borings and installing monitoring wells using air-rotary casing-hammer drilling methods, and 4) developing, sampling, and surveying the newly installed monitoring wells. The proposed locations of the soil borings and wells are shown on Figure 2. Soil Borings and Sample Collection The initial five feet of each boring will be advanced with a hand auger and/or posthole digger to reduce the possibility of damaging underground utilities. A mud-rotary drilling rig will be used to advance six exploratory soil borings (B-33 through B-37) to a depth of approximately 120 to 140 feet bgs. Non-petroleum based drilling mud will be used during the advancement of the soil borings. The drilling mud will have a density greater than the density of water to prevent water from flowing into the borehole and to keep the hole from collapsing. This method of drilling has been selected primarily because of its capability of being able to drill deep (>100 feet) through difficult material, to collect continuous cores for lithologic logging, and to accept downhole geophysical instruments. Because the use of drilling mud typically causes the sidewalls of the borehole to become semi-impermeable and often impedes the flow of groundwater, these borings will not be converted to groundwater monitoring wells. Instead, new soil borings will be drilled adjacent to the mud-rotary borings using an air-rotary casing-hammer rig or a drilling rig with similar capabilities and completed as monitoring wells. Continuous sampling will be conducted to the total depth explored. Select soil samples will be extracted from the continuous core using pre-cleaned brass tubes. The ends of the tube from each sample will be lined with TeflonTm sheets, capped, and sealed. Each sample will then be labeled, placed in a resealable plastic bag and stored in an ice-chilled cooler. The samples will remain chilled until relinquished to a state-certified analytical laboratory. Strict chain-of-custody procedures will be followed from the time the samples are collected until the time the samples are relinquished to the laboratory. Additional soil from the sample point will be placed in a resealable plastic bag and screened for volatile organic compounds using field instrumentation equipped with a C:W2CO\6100\RFPORTft100 workplan 05-22-01.doc