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L ' IWO Work Plan for Additional Assessment ARCO Facility No.2186 Stockton.California Page 5 encountered groundwater. Additional sample collection will target significant changes in soil type and permeability. The wells will be installed to a depth of approximately 45 feet below first encountered groundwater, which is anticipated to occur between 33 and 36 feet bgs. The well screen will consist of 20 feet of 0.02-inch factory slots and will target the saturated zone identified beneath the site beginning at approximately 65 feet bgs. 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. Task 3: Groundwater Sampling 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. The newly installed groundwater monitoring wells will be placed on the quarterly monitoring program adhering to CRWQCB guidelines. To assess the groundwater quality of the first encountered groundwater in the vicinity of the possible source area, the three shallow vapor extraction wells (completed to 36 feet bgs) located around the USTs and downgradient of the dispensers will be sampled. The elevation of the top of each vapor extraction well casing will be surveyed by a California licensed surveyor. Depth to water in the vapor extraction wells is anticipated to be approximately 34 feet bgs, based on groundwater elevations obtained from the monitoring wells during the second quarter groundwater sampling event (June 2000). Depth to water in the vapor extraction wells will be recorded and a groundwater gradient and flow direction map will be generated. Task 4: Laboratory Analysis Soil and groundwater samples will be analyzed by a State-certified laboratory. The soil and groundwater samples will be analyzed for total petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, and xylenes, and for oxygenates (including Methyl Tertiary Butyl Ether [MtBE], Ethyl Tertiary Butyl Ether [EtBE], Di-isopropyl Ether [DIPE], Tertiary Amyl Methyl Ether [tAME], Tertiary Butyl Alcohol [tBA], and 1,2-dichlorethane (1,2-DCA), using Environmental Protection Agency(EPA) Method 8260.