DRILLING, SAMPLING, AND CONSTRUCTION OF MONITORING WELLS UNDER FLOWING ARTESIAN CONDITIONS

Drilling and sampling in flowing artesian aquifers is significantly mo re complex than in unconfined aquifers, especially in settings such as dam sites and hazardous waste sites where flow of water from the aqui fer can cause safety or contamination problems. Successful drilling in flowing artesian conditions requires four items: properly weighted dr illing fluid, inflatable packers, surface casing cemented in place, an d driven casing. The primary control used to counteract the upward flo w of water and thus prevent flow at the surface is drilling fluid of s ufficient density. If the borehole begins to flow during drilling, the other three items are necessary to regain control of the flow The pac ker can be used to stop the flow so that drilling fluid can be replace d under no-flow conditions. The surface and driven casings prevent ero sion of the borehole walls if artesian flow occurs, and provide second ary locations for installation of a packer. When a boring has been dri lled to the desired depth, monitoring well materials may be placed in the hole. These materials consist of a well screen, an inflatable pack er, and riser pipe. The well materials are lowered through the drillin g fluid into the boring, and a packer attached to the well riser pipe is inflated against the confining bed above the aquifer. Drilling flui d is flushed from above the packer with clean water, which is subseque ntly displaced with cement grout using a tremie pipe. After the grout has hardened, the drilling fluid can be flushed from inside the well r iser and screen. The well can be developed by surging, followed by a p eriod of free flow to remove drilling fluid from the aquifer. These te chniques were used to successfully install a number of monitoring well s at a site in a river valley underlain by interbedded clay tills and outwash sand deposits of glacial origin. The deeper sand deposits were confined aquifers, with the clay tills acting as confining beds. Once the clay till was penetrated during drilling, flowing artesian condit ions would have occurred unless proper drilling techniques were used. Soil sampling and installation of monitoring well materials would not have been possible under flowing conditions, necessitating the special procedures and equipment for drilling, sampling, and well installatio n.