Mont. Code Ann. § 75-10-737
Voluntary action to preclude remedial action by department
75-10-737 . Voluntary action to preclude remedial action by department. If a party has elected to undertake an approved voluntary cleanup and is diligently proceeding to implement the plan, the department may not, except as provided in 75-10-712 , take remedial action under 75-10-711 with regard to those releases or threatened releases of hazardous or deleterious substances that are addressed by the approved voluntary cleanup plan.