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6 Site Geology I" Quaternary alluvial deposits immediately underlie the Tracy area. These deposits, which may be up to 350 feet thick, are made up of interbedded clay, silt, sand, and gravel. This information is based on U.S. Geological Survey Open-File Report 6433-02. Boreholes BC will supervise the drilling of three boreholes and the installation of a monitoring well in each of those boreholes at the site. Proposed borehole locations are shown on Figure 3. The boreholes will be advanced to 15 feet below the groundwater surface. La Depth to groundwater in this area is approximately 11 feet. The boreholes will be drilled using a truck-mounted drilling rig equipped with 6-inch-inside-diameter, 12-inch-outside- diameter hollow-stem augers. t. Sampling Procedures w. The following field procedures will be used during soil sampling at ARCO Station 2093. These methods will ensure proper identification 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 L,, 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, La plastic caps, and tape to provide an airtight seal. If visible hydrocarbons or odor are present, it will be noted. Before sealing each sample, hydrocarbon vapor concentrations will be measured using a photoionization detector. In addition, each sample will be exposed to ultraviolet light to detect any visible hydrocarbons. 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. Lithologic logs will be constructed for each boring. 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, ethylbenzene, and xylene (BTEX) using U.S_ Environmental Protection Agency (EPA) Methods 8015, modified, and 8020, respectively. Samples will be delivered to Eureka Laboratories, Inc. (Eureka), a California Depar-tment of Health Services (DHS) certified Lo laboratory located in Sacramento, within 48 hours of collection. Drill cuttings will be stockpiled on site and enveloped in polyfilm. A composite sample will be obtained from the stockpiled soil. This sample will be analyzed for total petroleum hydrocarbons (TPH) and BTEX for manifestation purposes. Lo a.