(1) The
following special sealing methods may be required in situations where different
techniques are necessary to protect aquifers.
(2) 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 1 foot below the known seasonal low water
table. An upper drill hole having a diameter at least 3 inches greater than the
nominal size of the permanent casing shall extend to at least 18 feet below
land surface.
(3) The
annular space between the upper drill hole and the well casing shall be kept at
least 1/2 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 sub-chapter) .
(4) 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 sub-chapter) .
(5) A
suitable bridge is one that prevents the sealing material from dropping into
the producing formations and reducing the output of the well.
(6) 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casing is being withdrawn.
(7) For
drilling with cable tool rigs, see ARM 36.21.654.