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3 Groundwater monitoring wells will be installed in the boreholes. The wells will be completed with 4-inch-diameter , Schedule 40 polyvinyl chloride (PVC) flush-threaded casing (meeting County standards for ASTM F-480 casing) and will be screened with 0.020-inch slotted casing from approximately 5 to 30 feet below ground surface. The depth to groundwater is approximately at 13 feet. Blank casing will extend from the top of the perforated section to ground surface. The actual depths and screened interval will depend on conditions encountered during drilling. The wells will be packed with Number 20/40 sand from the bottom of the screen to approximately 2 feet above the top of the screen. A 2-foot seal of bentonite pellets will be placed on top of the sand pack. The well will be grouted with bentonite-cement slurry or neat cement from the top of the bentonite seal to the surface. All wells will be completed below grade, fitted with top and bottom PVC caps, and have locking water-tight covers installed. A standard well completion diagram is shown on Figure 2. Monitoring wells will not be completed opposite significant clay zones nor will monitoring wells penetrate significant clay zones that might allow for downward migration of contamination into lower aquifers. Sampling Procedures The following field procedures will be used during soil sampling at ARCO Station 2133. These methods will ensure proper identifi- cation of subsurface materials and contaminants. Soil Samples. Soil samples will be taken every 5 feet during drilling . Samples will be obtained using an 18-inch drive sampler . Three 2-inch-diameter by 6-inch-long brass tubes will be fitted into the drive sampler . The sampler will be driven into undisturbed soil to obtain the soil samples. Sample tubes will then be removed from the sampler , the ends visually inspected in the field to identify lithology, and capped with aluminum foil and plastic caps to provide an airtight seal. If visible hydrocarbons or odor are present, it will be noted. Before sealing, hydrocarbon vapor concentrations will be measured using a photoionization detector . Lithologic logs will be constructed for each well. Each sample will be labeled on site to show the date, project number , sample location, and depth interval. The sealed samples will be stored on ice in closed chests and protected from meltwater . If hydrocarbon odors or visible hydrocarbons are detected in a soil sample, selected soil samples will be analyzed for total petroleum fuel hydrocarbons, benzene, toluene, and xylene (BTX) (U.S. Environmental Protection Agency (EPA) Method 8015 , modified) . Samples will be delivered to Brown and Caldwell' s analytical laboratory in Emeryville within 48 hours of collection.