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�7 CHAPTER 2 FIELD METHODS During the week of April 20 , 1987, four groundwater monitoring wells were installed at the locations shown on Figure 2-1. Soil boreholes were' drilled by PC Exploration, Inc. , with a truck-mounted F drilling rig equipped with 4-inch--inside-diaineter and 8-inch-outside- diameter hollow-stem augers. PC ' Exploration, Inc. , has a C-57 contractors license. Boreholes were advanced to depths of approximately 35 feet, or about 20 feet below the groundwater surface. Soil samples for litho-- logic classification and potential laboratory analyses were collected at 5-foot intervals by using an 18-inch-long by 2-inch-diameter j Modified California type sampler with 6-inch-long brass tube liners. The drive sampler was driven ahead of the auger into undisturbed soil. Each borehole was logged from the . samples taken at 5-foot intervals, using the Unified Soil Classification system. Borehole logs are in Appendix D. Soil samples were' screened in the field for hydrocarbon vapors with, a photoi-onization' detector . The -ends of the tube were visually inspected to describe litnology, then covered with aluminum foil, plastic caps, and tape to pr�ovi-de an air-tight seal.. These samples were stored on ice from the time of collection through delivery to Brown and -Caldwell'.s (BC) Emeryville laboratory. Pending a results of the analytical tests, cuttings from each borehole were containerized and sealed in 55-gallon drums and temporarily stored on site . A monitoring well was theninstalled in each borehole by inserting 2-inch-inside-diameter , Schedule ; .. 40 polyvinyl chloride (PVC) casing with approximately 22 feet of . 0. 020-inch slotted well screen (0. 020-inch slots) attached to the !bottom. The perforated casing was installed. in an interval that ranges from approximately 16 feet below 'the water table to 6 feet above the water table in G order to accommodate water table fluctuations and to detect free- floating product. Wet ' sands' , accumulating in the auger and heaving into the annulus near the bottom of the hole, made installation of the PVC difficult. Some boreholes, therefore, were drilled deeper than the anticipated depth' of the wells. A bottom plug was installed at the bottom of the perforated interval. To gravel pack the well, coarse aquarium sand was poured directly through the augers as the augers were gradually removed from the borehole. A bentonite seal was placed immediately above the gravel pack and the augers were then removed from the borehole. The BROWN AND CALDWELL r CONSULTING ENGINEERS �4 I