30 CFR 816.14: Casing and sealing of drilled holes: Temporary.

Each exploration hole, other drill or boreholes, wells and other exposed underground openings which have been identified in the approved permit application for use to return coal processing waste or water to underground workings, or to be used to monitor ground water conditions, shall be temporarily sealed before use and protected during use by barricades, or fences, or other protective devices approved by the regulatory authority. These devices shall be periodically inspected and maintained in good operating condition by the person who conducts the surface mining activities.