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r _ i A-2 BOREHOLE DRILLING Boreholes are drilled by a licensed drilling subcontractor, using a truck-mounted drilling rig' equipped with nominal 6-inch-diameter hollow-stem augers. Boreholes are drilled by continuous coring to a predetermined depth below the ground surface, or togroundwater, whichever is encountered first. Borehole depths are based on site conditions, including but not limited to conditions such as depth to water, topography, and depth to bedrock. Borehole depths also may be governed by obtaining two readings -on a pid, or equivalent instrument, which are less than 50 parts per million above the background reading, in which case the borehole is terminated. At the end of the drilling and sampling operations, boreholes are immediately backfilled from bottom to top with a bentonite/cement slurry pumped through a tremie pipe. To prevent cross contamination during drilling, all equipment is steam cleaned prior to and between use at each borehole. GROUNDWATER MONITORING AND EXTRACTION WELLS Monitoring Well Drilling. Groundwater monitoring wells are drilled by the method described in the section entitled Borehole Drilling, with the exception that they are drilled to a depth of 15 or 20 feet below the water table or piezometric surface, depending upon known conditions and upon unanticipated conditions encountered during drilling. 'The borehole is then completed as a monitoring well by the method described below. Monitoring Well Completion. Groundwater monitoring wells are completed with polyvinyl chloride casing (PVC) screen and casing. The off-site wells will be completed using 2-inch- diameter PVC. A PVC cap is installed at the bottom of each well. The extraction well will be completed with 6-inch-diameter Schedule 40 PVC casing. The well screen typically is 0.020-inch slotted flush-threaded PVC, extending from a depth of approximately 15 or 20 feet below the water table or piezometric surface to 5 to 10 feet above. The well design is intended to accommodate seasonal water-level fluctuations within the slotted interval. The well design may be modified in the field during the drilling operation, depending g upon conditions encountered during drilling. In no case does the monitoring well penetrate clay zones greater than 5 feet thick, which might allow downward migration of contaminants into lower water-bearing zones. Blank casing extends from.the top of the perforated section to the ground surface (or above, in the case of above-ground completion). The annular space surrounding the well screen and casing is packed with sand that is compatible with the well screen, from the bottom of the screen to approximately 2 feet above the top of the screen. The well is then bailed or surged to settle the filter' pack, and more sand is added as necessary to return the level to a height of 2 feet above the top of the screen. A 2- to