Mont. Code Ann. § 75-11-515

The department may issue a cleanup order to any person who has discharged, deposited, or spilled any regulated substance into or onto any land or water in an unlawful or unapproved manner or who has discharged, deposited, or spilled any regulated substance into or onto any land or water so as to result in unlawful or unapproved disposal of a regulated substance. The order must direct the person to clean up and remove the regulated substance, to treat the regulated substance, or to take other actions as may be considered reasonable by the department.