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BOREHOLE DRILLING <br /> Boreholes are drilled by a licensed drilling subcontractor, using a truck-mounted drilling rig <br /> equipped with nominal 6-inch-diameter:hollow-stem augers. Boreholes are drilled by continuous <br /> coring to a predetermined depth below the ground surface, or to groundwater, whichever is <br /> encountered first. Borehole depths are based on site conditions, including but not limited to <br /> conditions such as depth to water, topography, and depth to bedrock. Borehole depths also may <br /> be governed by obtaining two readings on a photoionization detector, or equivalent instrument, <br /> which are less than 50 parts per million above the background reading, in which case the <br /> borehole is terminated. <br /> At the end of the drilling and sampling operations, boreholes are immediately backfilled <br /> from bottom to top with a bentonite/cement slurry pumped through a tremie pipe. <br /> To prevent cross contamination during drilling, all equipment is steam cleaned prior to and <br /> between use at each borehole. <br /> Soil sampling and monitoring procedures during borehole drilling are described in a separate <br /> appendix. <br />