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Mr.Nuel Henderson,P.G., SJCEHD September 21,2005 ARCO Station 6100,Tracy,California Project No.U6100-15-1 Additional Site Assessment Work Plan Page 9 Given the significant number of soil borings previously completed, the findings from previous off-site borings, and our understanding of the distribution of petroleum hydrocarbons in the subsurface off-site, soil cores may not be collected from the upper 30 feet of subsurface soils. The remaining section of the boreholes (approximately 30 feet bgs to total depth) will be continuously cored in 2.5 or 5-foot intervals. Select soil samples will be extracted from the soil cores 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 photoionization detector (PID). Stratus will record PID readings, soil types, and other pertinent geologic data on the borehole log. Soil types will be classified on-site using the Unified Soil Classification System. A minimum of six soil samples from each boring will be submitted for chemical analysis, including the sample exhibiting the highest PID response, the sample from just above groundwater (if encountered), and a sample collected near the base of the boring. Monitoring Well Installation A truck mounted drill rig equipped with 10-inch diameter hollow stem augers will be used to advance separate boreholes to facilitate monitoring well installation. The initial five feet of each boring will be advanced with an air/water knife and vacuum extraction equipment to reduce the possibility of damaging any underground utilities that may be present near the work areas. The borings will be converted to 4-inch diameter groundwater monitoring wells. The precise well boring depth will be determined following completion of the exploratory soil borings. Soil samples will not be collected from the well borings. The screen interval of each monitoring well will be targeted at the Sand 2 stratum, using 15 to 20 feet of 0.01-inch diameter machine-slotted PVC well casing set to a depth approximately 10 feet above and 10 feet below first encountered groundwater (similar to off-site wells MW-1, MW-12, and MW-15). The bottom of the screen interval will be placed to intersect the contact between Sand 2 and the underlying silty unit. A filter pack of 92/12 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. If groundwater is present in the well boring, 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 chips will be placed on top of the filter pack and hydrated with clean water to provide a sanitary seal for the well. Cement grout will be placed in the space around the well from K:\Bp-Paco for URS\6100\Work Plans\arco6I00-workplan 09-2005.doc