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When drilling a well, the new borehole may connect two aquifers (fracture zones, lithologic horizons) with different heads (pressures, piezometric levels). This causes the water to flow from the area of higher pressure to lower pressure through the open borehole, or if not grouted properly, along the outside of the casing. The water may move up or down the borehole. If the water comes up and exits the hole, then it is artesian When drilling creates a pathway for two aquifers to interact it should be removed because of potential contamination of the two aquifers. This is done with proper casing and grouting of the well hole.