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a. Monitoring Well Installation and Development The monitoring wells will be installed in each borehole by inserting 4-inch-inside- diameter, Schedule 40 polyvinyl' chloride (PVC) casing''with approximately 20 feet of 0.020-inch machine-slotted well screen beneath the blank casing. The slotted casing will extend 15 feet below the water table, .in order to accommodate for seasonal fluctuations. Screw-on bottom caps will be installed the bottom of each 'well.' The wells will be designed in the field by.the.on-site geologist.. To gravel pack each -well, No.. I.sand will be poured directly through the auger as are being gradually removed from .the borehole. The sand will extend to the augers Y g l� approximately 2 feet above the screen. A, 2-foot -bentonite pellet seal will be placed immediately above the gravel. pack and -the augers will then be totally- removed. The remaining annular space of' each well will be filled with cement. Each well will be completed with an at grade locking box. Typical monitoring well design is illustrated on Figure 4. Following installation of each well, the. well will be developed using a centrifugal pump. The well will be developed until relatively sediment-free water is .produced. m nt time will field conditions. Water. The length of develop vary depending a e y p g Aon produced during development and sampling will be contained' on site. ARCO will be responsible for the disposal of water .and any recovered petroleum product. Prior to and following development, the thickness of free product, if any, will be gauged. When the lead investigating agency deems that monitoring .groundwater quality is no longer necessary, ARCO will be responsible for .abandonment of the wells. The wells will be abandoned according to state water well codes (Bulletin 74-81, December 1981). Groundwater Sampling ` Following development of each well, a nitrogen gas-driven bladder pump, stainless steel bailer, or Teflon bailer will be used to evacuate at least three well volumes of water before samples are taken. A well volume is defined as the water in the well casing plus the water in the sand pack with an assumed porosity of 35 percent. The pH; electrical i conductivity (EQ, p EC), and temperature -of the water will be measured periodically during the evacuation of water. Samples will be taken when pH, .EC, and temperature measurements are stable. Stable measurements do not vary by. more than. 0.1 units. ' Samples will be . collected in 40-milliliter glass volatile organic- analysis '(VOA) bottles. Sample bottles will be filled directly from the discharge line of the bladder; pump or from the bailer, and will ' be filled to minimize any aeration of the :water. ' A Teflon-lined cap (septa) will be placed on the bottles, and the VOA bottles will be checked to ensure that no air bubbles remain within. All water samples will be labeled and stored as previously described in the soil samples section, and transported to the.laboratory. .