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Work Plan for Additional Sit�ssessment • ARCO Facility No. 6100 Tracy,California Page 8 attenuate gamma photons, high electron density (soil density) will yield lower gamma counts returned to the sonde. The neutron logs and the gamma-gamma logs will be compared during the interpretation of the logging. If saturated soils are encountered, the log comparisons may be able to determine if free product is present. Since gasoline has a lower density than water, soil that contains gasoline within the intergranular pore space may show a lower density than soil that contains water within the intergranular pore space. Monitoring Well Installation An air-rotary casing-hammer drill rig will advance six 10-inch diameter soil borings to depths of approximately 120 to 140 feet bgs adjacent to the mud-rotary soil borings. The borings will be located approximately 10 to 12 feet from the mud-rotary borings. Given the proximity of the borings to the mud-rotary borings, soil samples will not be collected. The new borings will be converted to 4-inch diameter groundwater monitoring wells. The actual completion depth and screen interval of the monitoring wells will be based on the results of the geophysical logs and conditions encountered during drilling operations. Based on our current understanding of the site and groundwater elevations recently collected from MW-1 and VW-1 through VW-6, the one monitoring well located onsite near the USTs and the dispensers (MW-7) may be completed at a much shallower depth than the other monitoring wells. Again, the actual well construction will be based on information gathered during field operations. Typically, the wells will be constructed using 20 feet of 0.02-inch diameter machine- slotted PVC well casing set to a depth approximately 10 feet above and 10 feet below first encountered groundwater. A filter pack of #3 sand will be placed in the annular space around the well screen. The sand will be placed to a maximum of 4 feet above the top of the well screen. The well will be surged for a minimum of 10 minutes to seat the filter pack prior to installing a sanitary seal. A maximum of four feet of bentonite pellets will be placed on top of the filter pack to provide a sanitary seal for the well. Cement grout will be placed in the space around the well from the top of the bentonite seal to the ground surface. A watertight locking cap will be installed over the top of the well casing and a traffic rated vault box will be mounted over the well. Well Development Sampling and Survey The monitoring wells will be developed using a surge block and groundwater pumping. Development will continue, to the extent practical, until the development water runs clean and pH and conductivity measurements stabilize. After well development and stabilization of the water levels, water levels and the quantity of well volumes removed from the well will be recorded. A minimum of 48 hours after the monitoring wells are developed, the wells will be purged and a groundwater sample will be collected. Samples C:W2Co\6100\REP0RTS\6100 workplan 05-22-01.doc