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CA A ZNG Field Quality Assurance Project Plan 8.2 Conductor Casing 8.2.1 Several drilling methods that can be used at CA ARNG facilities and are described in Section 4.0 of this field QAPP.With the exception of HSA,all of these methods involve installing a drill string of temporary threaded conductor casing or drill rods that effectively seal off successive lithologic intervals and water bearing zones as drilling proceeds.However,the configuration of the HSA flights on the outside of the hollow drill stem results in a boring diameter several inches larger than the hollow drill stem and can create a pathway for the vertical migration of perched water. 8.2.2 When using the HSA drilling method in an area with perched water, a temporary flush threaded PVC conductor casing is used to effectively seal off a perched water zone and allow drilling to proceed to greater depths.If perched water is contaminated and allowed to migrate vertically,deeper vadose zone soil and groundwater may be impacted.Uncontaminated perched water may leach contaminants present at greater depths within the vadose zone and provide a transport mechanism for contaminants to ground- water.The-installation of a temporary conductor casing has proven effective for reducing this risk and allows drilling activities to continue. 8.2.3 Once the total depth of the perched water has been identified,the augers should be withdrawn approximately 2 feet,and a minimum 2-foot thick seal of sodium bentonite pellets/chips emplaced at the total depth of the boring.This creates an effective seal prior to removing the augers and during reaming of the boring to a larger diameter. 8.2.4 The borehole should then be reamed using minimum a 10-5/8-inch OD auger(to allow emplacement of a minimum 8-inch OD conductor casing) to the bottom of the bentonite plug, and the conductor casing set.The conductor casing should be installed using centralizers to maintain equal spacing from the walls of the reamed borehole.An outer seal is then placed in the annulus of the -conductor casing using tremie pipe;consisting of either sodium bentonite pellets/chips or neat cement. When the conductor casing is set,and the bentonite pellets/chips have hydrated or neat cement has hardened,continue drilling through the interior of the conductor casing to total depth. 8.3 Grout Composition Common neat cement type#2 is made from limestone(or other materials high in calcium carbonate), clay,or shale, with some iron and aluminum oxides added if they are not present in sufficient quantity in the clay or shale.The dry materials are ground,mixed,and fed into a kiln at temperatures of 2,600 degrees Fahrenheit(°F)to 2,800°F.The principal compounds formed during this burning process are tricalcium aluminate,tetracalcium aluminoferrite,tricalcium silicate,and dicalcium silicate. 8.4 Grout Placement 8.4.1 The consistency of the grout should be sufficient to flow through the screen and into the filter pack.Additives may be used to prevent excessive loss of grout into the formation.The grout will be tremied from the bottom of each well by continuously withdrawing the tremie pipe as the grout is placed. The bottom of the tremie pipe will remain below the calculated grout level to prevent bridging of the grout.Displaced groundwater will be collected as it discharges above the top of the connector casing and pumped into a large-capacity storage tank. 8.4.2 Grouting will take place in several lifts.The initial lifts of grout will be allowed to set up before continuing perforation and pressure grouting of the shallower well sections.For this first stage of sealing, field QAPP.DOC 8-2 May 2000