Butte Mine Flooding Operable Unit (BMFOU)

This Operable Unit (OU) includes contaminated groundwater in the flooded underground mine workings below the city of Butte, along with the contaminated water in the Berkeley Pit. The remedy selected for this OU includes water treatment at the Horseshoe Bend Water Treatment Facility. This treatment plant adds lime to the water to precipitate out heavy metals. The heavy metal sludge is discharged to the Pit, and the clean water is used at Montana Resources. Should the mine not be able to use the treated water, it will then be discharged to Silver Bow Creek. The discharged water will be constantly monitored and must meet strict water quality standards. This site will be maintained in perpetuity.


Comprehensive Environmental Response, Compensation & Liability Act of 1980, which is also known as Superfund or Superfund Law.

Contaminants of Concern (COC)

Broadly defined, contaminants of concern are chemical elements or compounds that are known to cause a threat to human health, or animal and plant life. In the Clark Fork Watershed, the contaminants of concern are arsenic, cadmium, copper, lead, mercury and zinc. Arsenic, cadmium, and lead are particularly concerning for human health, while copper and zinc can affect aquatic and terrestrial life.

Consent Decree (CD)

Consent Decree is the legal term for a settlement agreed to by court order. In a Consent Decree, the dispute between the parties is agreed to without admission of guilt. In the case of Superfund, the companies or owners are referred to as “potentially” responsible parties, rather than responsible parties as there is no admission of wrong-doing. In Superfund, the Consent Decree negotiations are the final step for settlement. The CD defines the methods, and extent of cleanup, as well as future monitoring plans. The CD is informed by the remedial investigation, feasibility studies, the Record of Decision, and any ROD modifications completed prior to the CD negotiations.

Controlled Groundwater Area (CGWA or CGA)

Controlled groundwater areas are identified in order to limit groundwater use for protection of human health, or to protect water right holders from overuse of groundwater resources. The Montana Department of Natural Resources and Conservation used its authority to designate a CGA in Butte to limit residents’ access to contaminated aquifers. The Butte CGA encompasses three Superfund groundwater areas: Butte Mine Flooding, Butte Priority Soils and Montana Pole and Treatment Plant. The program is administered by the Butte-Silver Bow Water Quality District.

Explanation of Significant Differences (ESD)

This legal term applies to the process that the EPA and PRP’s utilize if there are significant changes to the Record of Decision. The ESD gives rationale for the changes that were made. The ESD is sent to the Federal Department of Justice for review. The information and data included in the ESD are reviewed by a federal judge. The public is allowed to send written comment letters to the judge regarding the changes proposed in the ESD, however, the ESD is not subject to the usual public comment period.

Feasibility Study (FS)

Once the remedial investigation is complete, and the nature and extent of contamination are understood, the EPA evaluates various remedy options for technical, environmental, and cost concerns. The feasibility study guides the conceptual design and construction process. Typically, both the EPA and the responsible party will conduct feasibility studies. These studies are completed prior to the Record of Decision.

National Priorities List (NPL)

The National Priorities List is often interchanged with the term “Superfund Sites.” The list refers to identified sites that have known releases of hazardous substances/pollutants/contaminants throughout the United States. As of July 2022, there were 1,175 active sites on the NPL, 41 proposed new sites and 431 deleted sites.

Operable Unit (OU)

Large acreage superfund sites, such as Butte’s site, are often broken into smaller management sections, or operable units. The operable units typically are divided by geographic constraints and/or nature of the remedial action needed.

Potentially Responsible Party (PRP)

This term is defined in CERCLA or Superfund Law as the company and/or individual owners who are potentially responsible for contamination at a Superfund site.

Protective Water Level (PWL)

Previously known as the Critical Water Level (CWL), this term applies only to the Butte Mine Flooding Operable Unit. Set by the EPA and Montana DEQ, the Protective Water Level, elevation 5,410  feet is well below the elevation where contaminated groundwater could escape the cone-of-depression surrounding the Berkeley Pit and contaminate Silver Bow Creek. It might be better referred to as the Safe Water Level.

Record of Decision (ROD)

This is the legal term for the public document created by the involved agencies that explains the clean-up plan and outlines the remedies to be utilized.

Record of Decision Modification (RODMOD) or ROD Amendment

The Record of Decision can be modified if there are fundamental changes that the parties would like to implement. For example, new data and/or new technology may become available to the parties after a remedial action has been agreed to. If the new data and/or technology fundamentally changes the way that the remedy is handled, the parties can request a modification. The public is allowed a comment period for the ROD modifications. The EPA is required to accept comments on the proposed changes and respond to public comment prior to modification approval.

Remedial Investigation (RI)

These investigations are intended to determine the nature and extent of contamination. The RI includes sampling soils, groundwater and streams. Often, both the responsible party and the EPA will conduct remedial investigations. The Record of Decision is based on the RI results and the follow-up feasibility study.

Remedial Investigation/Feasibility Study (RIFS)

In some literature and discussions, the remedial investigation and feasibility studies are lumped together and referred to as one study. Often, the remedial investigation and feasibility studies are conducted concurrently.

Settling Defendant (SD)

Under CERCLA, a Settling Defendant (SD) is a Potentially Responsible Party (PRP) that enters into a Consent Decree (CD) with the United States and agrees to legal terms and conditions which may include financial responsibility for environmental liabilities.  The SDs for the BMFOU site are Atlantic Richfield Company, and the MR Group (i.e. Montana Resources, Inc., Montana Resources LLC, and Dennis Washington).

Technical Impracticability (TI)

This term refers to waiving standards when it is impracticable from an engineering perspective to meet a standard. A TI can be invoked where the cost of cleanup is inordinately costly. The remedy must still protect human health and the environment. The vast majority of TIs address groundwater contamination. In Butte, a TI evaluation concluded that it would cost between $11 and $13 billion to clean up groundwater, with little certainty of the effectiveness of the cleanup. On that basis, the BMFOU was granted a TI waiver. Human health will be protected by not allowing residents to use the mine water. The environment will be protected by pumping and treating groundwater as it approaches the protective water level. Treated water will meet water quality discharge standards.