36.21.656 UNCONSOLIDATED FORMATIONS WITHOUT SIGNIFICANT CLAY BEDS
(1) The following special sealing methods may be required in situations where different techniques are necessary to protect aquifers.
(a) In drilled wells that penetrate an aquifer overlain by unconsolidated formations such as sand and gravel without significant clay beds, an unperforated well casing shall extend to at least one foot below the known seasonal low water table. An upper drill hole having a diameter at least three inches greater than the nominal size of the permanent casing shall extend to at least 25 feet below land surface.
(b) The annular space between the upper drill hole and the well casing shall be kept at least one-half full with bentonite slurry throughout the driving of the permanent casing into the aquifer. After the permanent casing is set in its final position, the remaining annular space shall be filled to land surface with appropriate sealing material (see Figure 3A at the end of this subchapter).
(c) If the oversized drill hole is extended to the same depth as the permanent casing, a suitable bridge shall be installed between the casing and the drill hole at a position directly above the production aquifer. The remaining annular space shall be completely filled and sealed to land surface with appropriate sealing material (see Figure 3B at the end of this subchapter).
(d) A suitable bridge is one that prevents the sealing material from dropping into the producing formations and reducing the output of the well.
(e) If temporary casing is used to maintain the oversized drill hole, the annular space shall be kept full with appropriate sealing material as the temporary casing is being withdrawn.
(2) For drilling with cable tool rigs, see ARM 36.21.654.
History: 37-43-202(3), MCA; IMP, 37-43-202(3), MCA; NEW, 1986 MAR p. 1699, Eff. 10/17/86; AMD, 1997 MAR p. 65, Eff. 1/17/97; AMD, 2010 MAR p. 1614, Eff. 7/16/10.