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FFL Workplan Addendum ARCO Facility No.5469 Stockton,California Page 2 approximately 10 to 15 feet from each other. As stated in the original work plan, the actual location of the monitoring wells will be based on accessibility and the location of underground utilities. Since each location will now contain two groundwater monitoring wells, soil sampling will only be conducted on one of the two wells. Collection of soil samples will be performed at 5-foot intervals during the advancement of the borings using a California-type, split-spoon sampler equipped with three precleaned brass tubes. The ends of the bottom-most, intact tube from each sample interval will be lined with Teflon-m 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. Soil contained in the remaining brass tubes will be screened for volatile organic compounds using field instrumentation equipped with a photoionization detector (PID). Delta will record PID readings, soil types, and other pertinent geologic data on the borehole log. Four soil samples will be submitted for chemical analysis from the two borings drilled to 70 feet bgs, the sample exhibiting the highest PID response, the sample from just above first-encountered groundwater, and one sample each from the water bearing zones. The samples will be analyzed The two additional wells will be installed to a depth of approximately 70 feet bgs, though the actual depth may be modified in the field based on subsurface conditions. The well screen will consist of 20 feet of 0.02-inch factory slotted PVC, installed from approximately 50 to 70 feet bgs. A filter pack of #3 sand will be placed in the annular space around the well screen from the bottom of the screen to a maximum of two feet above the top of the screen. Two feet of bentonite will be placed on top of the filter pack to provide a sanitary seal for the well. The borehole will be backfilled with neat cement up to surface grade. A traffic rated vault box will be placed over the well and a water-tight locking cap will be placed on the top of the well. This well completion is similar to the existing groundwater monitoring wells. The actual well construction may be modified in the field, based on conditions encountered at the time of the investigation. Development of the monitoring wells will consist of surging with a surge block and 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. The elevation of the top of each well casing will be surveyed by a California licensed surveyor. A minimum of 48 hours after the groundwater monitoring wells are installed, the wells will be purged and groundwater samples will be collected. The samples will be collected using a disposable bailer, transferred to laboratory-supplied glass vials, and placed 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 sample is collected until the time it is relinquished to the laboratory.