Use of bentonite pellets to seal water wells

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Zemansky, G.M. 2009 Use of bentonite pellets to seal water wells. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/45 16 p.

Abstract: Northland Regional Council (NRC) requested advice from GNS Science (GNS) with regard to the use of bentonite pellets instead of grout to seal water wells in various geologic settings. Available information on the topic within GNS and a search of relevant scientific and regulatory literature as well as literature available from bentonite manufacturers was conducted. This information was compiled and reviewed to prepare this report. The Environmental Standard for Drilling of Soil and Rock NZS44411:2001 requires the annular space between the casing and hole to be sealed from the surface to what ever depth necessary to prevent the movement of contaminants into the aquifer from the ground surface. Standard practice involves installation of a relatively short annular seal above the sand filter pack around the well screen. This is commonly prepared using granular bentonite or bentonite pellets or chips. The annular space above this “filter pack” seal is subsequently filled with grout. Three forms of grout are commonly used for this purpose: neat cement, cement-bentonite, and bentonite. However, it is also possible to use the same form of bentonite used to make the “filter pack” seal. A proper well seal must be relatively impermeable and form a good bond to the exterior of the well casing and interior of the borehole wall. The use of proper seal installation procedures is as important as the use of suitable materials. A satisfactory seal may be produced using any of these materials when installed using proper procedures. Similarly, it is possible to obtain a poor well seal with any of them if proper procedures are not used. Proper procedures, regulations specified in some jurisdictions, and manufacturer’s recommendations are discussed in this report. The most important factors in achieving a good seal with bentonite pellets are prevention of bridging when inserted into the annular space and ensuring adequate hydration. The latter is of particular importance when bentonite pellets are used above the water table within the unsaturated zone. (auth)