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CA ARNG Field Quality Assurance Project Plan 8.0 BOREHOLE/WELL ABANDONMENT In the event that a well or vapor piezometer was not constructed properly or no longer meets performance criteria, the wells will be abandoned by filling the well with a bentonite cement grout.More importantly, in addition to the well casing being physically sealed,hydraulic integrity of subsurface geologic materials should be restored to a preconstruction condition.The focus of decommissioning activities should be to effectively seal subsurface geologic materials and prevent cross-contamination of aquifers. 8.1 Borehole Abandonment 8.1.1 Upon completion or termination,each soil boring will be filled with a neat,non-shrinking,cement grout. The cement grout will consist of a mixture of Type H Portland cement,or equivalent,with 3 pounds of powdered nonbeneficiated bentonite and 7 gallons of water per 94-pound sack of cement; grout materials will be weighed or metered.Grouting procedures differ for boreholes created by different drilling techniques. 8.1.2 Overdrilling of the wells may be performed to remove casing and well construction materials from the borehole prior to grouting for abandonment. A hollow-stem auger will be used to drill out existing 2- inch and 4-inch diameter casing.The size of the augers (inside diameter[IIID])will be at least 2 inches larger in diameter than the ID of the casing.The full depth of the wells will be drilled out. After all casing has been removed,the augers will be rotated at the bottom of the borehole to remove all well construction debris to create a satisfactory seal between the new grout and the borehole wall. Grout will then be placed in the borehole(through the augers)using a tremie pipe. 8.1.3 For HSA borings,the grout is placed through the augers in the boring,additional grout being added as the augers are removed.Grout is placed until it rises a few inches above the ground surface.The grout column will settle into the boring as it compacts and-dries. Any water within 4he borings will be floated out by the grout as it is brought to ground level. In borings greater than 20 feet deep,a tremie pipe with a maximum outside diameter of 2 inches is lowered to the bottom.As the grout is pumped, the tremie is positioned so it is below the top of the grout rising in the boring.The bottom of the drill pipe is always positioned below the top of the grout seal to prevent possible"caving"from the sides of the borehole.The volume of grout required to seal a borehole is calculated using the formula provided in the Borehole Abandonment Form(Figure 8-1 at the end of the section). 8.1.4 The location of the borehole is marked by barricades or traffic cones until the grout settlement is complete and the grout is flush with the ground surface. Approximately 24 hours after grout placement, the grout will be checked for any settling and additional grout added to fill any depressions.In paved areas,the grout will be finished below final grade and an asphalt or concrete patch will be used to match the pavement surface. 8.1.5 For CPT boreholes, grout is pumped through a flexible hose after the sounding rods are removed from the boring.Approximately 0.75 gallons of grout are generally required to seal 10 feet of penetrometer boring.Grout is continuously placed from total depth to the ground surface. All CPT locations are checked within 24 hours to observe if grout has settled; if it has,additional grout is added to the CPT hole.The location is then checked after each 24-hour period until the grout remains flush with the ground surface. field QAPP.DOC 8-1 May 2000