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• BOREHOLE DRILLING <br /> Boreholes are drilled by a licensed drilling subcontractor using a truck-mounted drilling ng <br /> equipped with nominal 6-inch-diameter hollow-stem augers Boreholes are drilled by continuous <br /> c.onng to a predetermined depth below the ground surface, or to groundwater. whichever is <br /> encountered tirst 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 pans 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 /> trom bottom to top with a bentonite/cement slurry pumped through a tremie pipe. <br /> To prevent cross contamination during dnlling. all equipment is steam cleaned prior to and <br /> between use at each borehole <br /> Soil sampling and monitoring procedures during borehole drilling are descnbed in a separate <br /> appendix <br />