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Deep groundwater monitoring wells for the Colbert Landfill Superfund Project in Spokane County, Washington were constructed using 6.4-cm (2.5-in.) diameter Polyvinylchloride (PVC) well casing, instead of 5.1-cm (2.0-in.) diameter casing, to provide additional strength and a larger inner well diameter for groundwater sampling equipment. Two well screen/filter pack combinations were used to provide the convenience of preconstruction purchase of well construction materials, and the flexibility to adjust the well intake design for site-specific hydrogeologic conditions. High percent-solids bentonite grout was used for the annular seal in lieu of a bentonite pellet seal to avoid material bridging during installation. Well casing alignment was maintained by applying a tensile force to the casing during the placement of annular fill materials rather than using casing centralizers. Also, threaded temporary steel casing was used for borehole support (versus welded temporary steel casing) to reduce boring and installation time and expense. This monitoring well design and construction approach maintained construction quality standards and resulted in significant cost savings by eliminating common deep well construction problems.
monitoring well design, monitoring well construction, bentonite grout, well casing, well alignment
senior engineer, Landau Associates, Inc., Edmonds, WA