(1) All exposed mineral seams remaining after mining must be covered with a minimum of 4 feet of the best available non-toxic and non-combustible material, unless otherwise approved by the department upon demonstration by the operator that a lesser cover depth will afford the same protection against combustion and other undesirable properties or effects of the mineral seam.
(2) Acid, acid-forming, toxic, toxic-forming, combustible, or other undesirable waste materials or fly ash identified by the department that are exposed, used, or produced during mining or mineral preparation must be covered in accordance with ARM 17.24.501(2) with the best available nontoxic and noncombustible material. The method and site of final disposal must be approved by the department. If necessary, these materials must be tested to determine necessary mitigations to neutralize acidity, to nullify toxicity, to prevent water pollution and sustained combustion, or to minimize adverse effects on plant growth and land uses. If necessary to protect against upward migration of salts or exposure by erosion, to provide an adequate depth for plant growth or to otherwise meet local conditions, the department may specify thicker amounts of cover using non-combustible and non-toxic material or the use of special compaction and isolation techniques to prevent contact of these materials with ground water. Acid, acid-forming, toxic, toxic-forming or other deleterious materials must not be buried or stored in proximity to a drainage course so as to cause or pose a threat of water pollution.
(3) Wastes must not be used in the construction of embankments for impoundments.
(4) Wastes from a strip mine may not be disposed of in a waste disposal structure that is located on the surface of the ground.
(5) Whenever waste is temporarily impounded:
(a) the impoundment must be designed and certified, constructed, and maintained:
(i) in accordance with ARM 17.24.603, 17.24.639, and 17.24.642, using current prudent-design standards; and
(ii) for structures meeting the criteria of 30 CFR 77.216(a) , to safely discharge the 6-hour, probable maximum precipitation (PMP) or greater event;
(b) the impoundment must be designed, and when operational must be managed, so that at least 90% of the water stored during the design precipitation event can be and is removed within the 10-day period following the event;
(c) spillways and outlet works for coal impounding structures must be designed to provide adequate protection against erosion and corrosion;
(d) inlets must be protected against blockage; and
(e) the impoundment may not include acid, acid-forming, toxic, or toxic-forming waste.
(6) Coal waste impoundments must not be retained as a part of the approved postmining land use.
(7) If any examination or inspection discloses that a potential hazard exists at a waste disposal site, including a temporary waste impoundment, the department must be informed promptly of the finding and of the emergency procedures formulated for public protection and remedial action. If adequate procedures cannot be formulated or implemented, the department must be notified immediately. The department shall then notify the appropriate agencies that other emergency procedures are required to protect the public.
(8) Wastes may be disposed of in underground mine workings, but only in accordance with a plan approved by the department and mine safety and health administration under ARM 17.24.901(1) (e) through (g) , and (2) .