Montana Department of Environmental Quality About Us Permitting & Operator Assistance Public Participation

Colstrip Ash Pond Cleanup

Program Overview

The Department of Environmental Quality is engaged in an enforcement action against Talen Energy (formerly PPL Montana) over groundwater contamination caused by leaking ash ponds at the Colstrip Steam Electric Stations. The steps Talen must take to address these problems are outlined in the Administrative Order on Consent, which both parties agreed to in 2012.

This webpage includes the documents, maps, and other analyses of the Colstrip site related to the AOC. Because this enforcement action involves complicated legal and environmental factors, DEQ has prepared a set of frequently asked questions about the Colstrip facility (FAQs). We recommend that anyone interested in DEQ's work at the Colstrip facility refer to the FAQs first.

To stay up to date with DEQ's work at the Colstrip facility, or with other questions, email us at: We will add you to an email list which receives updates on public notices and other major announcements about this project.

Related Links

Talen's CCR Website

More Information on the Montana Major Facility Siting Act

Colstrip Coal Ash Pond Cleanup Contacts

Main Program Email

Colstrip Project Officer
Environmental Science Specialist
Sarah Seitz (406) 444-6797

Waste Management Bureau Chief
Rick Thompson (406) 444-5345

Frequently Asked Questions

What is an Administrative Order on Consent (AOC)?

This AOC is an enforcement action taken by the DEQ under the Montana Water Quality Act and the Major Facility Siting Act to address groundwater contamination at the Colstrip Steam Electric Station (Colstrip Power Plant).You can read the entire AOC that DEQ and PPL agreed to here.

How does the AOC process work?

The Colstrip Power Plant has been divided into three areas: the Plant Site complex, the coal ash disposal ponds for Units 1 and 2, and the coal ash disposal ponds for Units 3 and 4. While the AOC mentions a fourth area to cover past pipeline spills, the areas for the Plant Site complex, the coal ash disposal ponds for Units 1 and 2, and the coal ash disposal ponds for Units 3 and 4 have been drawn large enough to include the pipeline spills.

The AOC lays out several steps that ultimately require PPL to remediate the groundwater contamination. Each step is subject to DEQ review and approval.

Step 1: First, Talen Energy prepared “Site Characterization Reports” for each of the three areas that describe the existing conditions, including the extent of the contamination. The reports also describe what has been done so far to address the contamination, and how effective those measures have been in remediating the contamination.

Step 2: Next, Talen prepared Cleanup Criteria and Risk Assessment Reports. These reports identified the constituents of concern (COCs) and constituents of interest (COIs) along with the cleanup criteria standards that PPL Montana will have to achieve in its remediation of the contamination.

Step 3: Talen must prepare Remedy Evaluation Reports, which will evaluate different options for remediation of the contamination for each of the areas: Plant Site Complex, Units 1 and 2 coal ash ponds, and Units 3 and 4 coal ash ponds. All three areas have completed Remedy Evaluation Reports.

Step 4: DEQ selected a Remedy based on each Remedy Evaluation Report. DEQ has selected a remedy for the Plant Site Complex and the Units 1 and 2 coal ash ponds. DEQ has conditionally selected a remedy for the Units 3 and 4 coal ash ponds.

Step 5: Talen is required to submit Remedial Design/Remedial Action Work Plans based on the selected remedies for each area. Currently, the Plant Site Complex is the only area with a final Remedial Design/Remedial Action Work Plan. Units 1 and 2 coal ash ponds will have a Remedial Design/Remedial Action Work Plan by May 2023. Units 3 and 4 are finalizing their Revised Remedy Evaluation Report and is moving into the Remedial Design/Remedial Action Work Plan.

Step 6: Talen will be required to implement the selected remediation based on the approved Remedial Design/Remedial Action Work Plan.

Step 7: At the completion of the remedies for each area, Talen will submit a Final Remedial Action Report that details the completion and compliance of the selected remedy. Since each remedial action will take more than a decade, Talen is required to submit annual reports on the remedial action implementation during the implementation of the selected remedies.

What is the current state of progress in the AOC process?

You can read the documents by clicking the Documents Accordions below.

What are the reasons for the length of this process?

The Colstrip site is a complex contamination site that requires thorough hydrogeological analysis. The coal ash disposal ponds are part of an evolving, dynamic industrial process. Each of the sites has a network of ponds of different construction and inactive filled or closed ponds.

The ponds are surrounded by a complicated network of thousands of monitoring, flushing wells, and capture wells. In certain areas, clean water is flushed through the ground and contaminated water is pumped back from the capture wells for treatment. The monitoring wells are individually monitored and sampled for water quality, usually twice a year.

The large number of wells is required in order to track the contamination in the complex groundwater system. (See next question for a more detailed summary of the geology surrounding this site.)

As a result of the geological complexities, it is difficult to accurately determine how far and fast contamination will spread. The site characterization and remedy evaluation reports include descriptions of the ash disposal ponds, the water chemistry of the ponds, pond leakage estimates, the geology and groundwater flow, the detailed assessment of the current contamination, and the assessment of the extent the monitoring and capture well network has stopped the spread of contamination.

Talen continues to use the site specific environmental and plume extent data to update and expand the remedial design and remedial action work plan during the remedy implementation. DEQ will continue to monitor the remedial design process and evaluate the remedial implementation on an annual basis.

What is the geology of the Colstrip site?

Past strip mining on the site has altered the shallow geology. The removed material above the coal is dumped back into the depressions left after the coal is mined. The “mine spoil” is highly variable in composition and resulting water quality.

In some areas, the bedrock is believed to be fractured. Fractures can direct groundwater in unpredictable directions.

The shallowest coal seam, Rosebud Coal, has naturally burned in the area in an irregular pattern. The burning has caused variable collapse of the overlying material and variable amounts of baking of the surrounding bedrock. The altered bedrock (“clinker”) can be extensively altered to a hard, highly fractured material.

Because of the formation process, several of the geological materials show large horizontal changes in the parameters that govern groundwater flow.

Has the public been involved in the AOC process?

Yes, DEQ has invited and accepted input from the public about the AOC between DEQ and PPL Montana (now Talen Energy). DEQ evaluates all the substantive comments submitted by people who are following this important issue.

DEQ also posts each report on its website for public review. Before approving or disapproving each new report, DEQ is also required to hold community meetings, so the agency can obtain public comments prior to issuing its approval or disapproval.

Members of the public may also provide written comments to DEQ by surface mail or electronic mail during public comment periods:

Montana DEQ
Attn: Sarah Seitz
P.O. Box 200901
Helena, MT 59620-0901

How does the public know that DEQ is considering public comments?

The AOC indicates that DEQ will respond to substantive public comments as part of its action on the report. Once DEQ approves or disapproves Talen’s report, DEQ’s responses to public comments will be posted on this website.

How can I be notified about public meetings?

If you wish to receive updates on future public meetings via email, please send your contact information to:

DEQ will set a public meeting date within ten days of the submission of major reports. Talen is required to publish the meeting announcement in the local paper and the Billings Gazette. DEQ also posts notices of the public meetings on its website at this page.

Why is the Colstrip site described as being contaminated?

As part of normal operations of the power plant, the ponds are filled with a mixture of water and coal ash. While in the ponds, the water acquires chemicals from the coal ash. Some of the water leaks from the ponds into the neighboring geological materials, contaminating the groundwater.

What is the nature of the contamination leaking from the ponds?

Elevated levels of certain chemicals (dissolved boron, dissolved chloride, and dissolved sulfate) have all been found in the groundwater around the ponds. Talen Energy uses the three chemical concentrations and the value of specific conductance (the measure of the water's ability to conduct electricity), collectively termed “indicator parameters,” to define the extent of the contamination.

Additional chemicals may be identified as contaminants when the Cleanup Criteria and Risk Assessment Reports are submitted by Talen Energy and reviewed by DEQ.

In what direction is the contamination moving?

In general, shallow groundwater moves horizontally under gravity downhill, typically following the slope of the land. In each of the three areas, the direction will be different:

  • At the Plant Site, contamination flows northwest towards the East Fork of Armells Creek because the ponds are built on the eastern side of the valley created by the creek.
  • At the site of the effluent ponds for Units 1 and 2, most of the contamination flows east towards the East Fork of Armells Creek because the ponds were built in a side valley that drained into the creek.
  • At the site of the effluent ponds for Units 3 and 4, the ponds were built in an elevated circular depression; groundwater flows away from the center in all directions.

How far has the contamination spread?

The extent of the contamination is represented in the maps from the groundwater modeling reports, the entirety of which can be found on the documents by clicking the Documents Accordions below. While many different chemical parameters have been tested in the area, for clarity, maps showing one of the indicator parameters, boron, are included below:

How fast is the contamination moving?

Even in areas that have much simpler geology than Colstrip does, the speed of groundwater is uncertain because it is influenced by highly complex underground geological factors. The geology at Colstrip can vary horizontally over distances of tens to hundreds of feet. Consequently, any estimate of the speed of the contamination’s movement would be inaccurate

Why are the ponds leaking?

The Major Facility Siting Act Certificate of Compliance that was issued in 1976 for Montana Power’s Colstrip Units 3 and 4 required the ash disposal ponds to be “sealed.” Compaction and the application of a clay liner was the conventional method of sealing ponds at the time the Certificate was issued. A concrete cutoff wall was also constructed around the perimeter of the area 3 and 4 ponds before ash began to fill the ponds. The Certificate was issued according to the technological and scientific standards of the time. You can read the entire Certificate and associated documents by following the links below:

While the Certificate contains “completely sealed” and “closed loop” language, water was anticipated to leak in small amounts from the ash disposal ponds at the time the Certificate was issued. The Certificate required the operator to collect the leaking water and return it to the ponds. In addition, if leakage was detected by monitoring wells, the Certificate required the operator to resort to more stringent measures than conventional sealing methods, up to and including installation of plastic liners.

Water leaking from the ash disposal ponds has been detected in monitoring wells. Talen Energy has constructed an expanding system of pump-back wells to capture the contaminated groundwater. Despite these efforts, groundwater contamination continued to migrate beyond the pump-back systems in some areas, prompting issuance of the AOC. All of the ponds into which Talen is currently disposing coal ash have been lined with geotextile liners.

Has the contamination entered the town of Colstrip drinking water supply?

The town of Colstrip receives its water from Castle Rock Lake (the Surge Pond) that is filled with water pumped from the Yellowstone River. Based on available sampling, there is no indication that the contamination has entered the public water supply.

Historically, private wells in the area have experienced contamination.

What is the rate of leakage from the ponds?

The following are estimates from the Site Characterization Reports:

  • At Units 1 and 2 the Stage 1 pond is estimated to leak at 8.47 gallons per minute. The Stage 2 pond is estimated to leak at 21.5 gallons per minute.
  • The lined ponds at Units 3 and 4 are estimated to leak at 1.4 gallons per minute. An estimated 277 gallons per minute are estimated to leak below or through the slurry cutoff wall surrounding all of the lined and unlined ponds for Units 3 and 4. This number also includes rain and snowmelt that falls within the perimeter of the slurry cutoff wall.
  • Approximately 60 gallons per minute are leaking from the ponds at the Plant Site.

Will the AOC process shut down the Colstrip Steam Electric Station power plant operated by Talen Montana LLC (Talen Montana)?

No, Talen Montana is continuing to operate the Colstrip Power Plant while the groundwater contamination is addressed under the AOC.

What happens if Talen Montana sells the power plant to another power company?

The terms of the AOC are binding on any company that purchases the Colstrip Power Plant from PPL Montana. Talen Energy is now the responsible entity.

What happens if Talen Montana closes the plant?

DEQ does not have bonding authority under the Major Facility Siting Act. Talen Energy, however, voluntarily agreed to post bonds at certain phases of the AOC process. Talen Energy's submission of the bonds was made a requirement of the AOC in 2012. Talen Energy in conjunction with the other Colstrip SES Plant Owners has submitted annual bonds since 2012 that cover the approved remedial actions and final closure plans under the AOC.

Is the pumping of the capture wells around the ash ponds affecting other landowners’ access to groundwater, as defined by water rights?

The DEQ does not determine water rights. The Department of Natural Resource and Conservation (DNRC) is responsible to determining water rights. Please contact the Water Rights Bureau, Water Resources Division, DNRC concerning the water rights issue.

What remediation efforts has Talen Energy undertaken to address the contamination?

To address the impacts associated with leakage from the Colstrip waste disposal ponds, Talen Energy has designed remedies for each of the AOC areas and implemented several measures over the years:

When a monitoring well indicates impacts from the waste disposal ponds, capture systems have been installed to contain the impacts.

Units 1&2 were shut down in 2020 and are no longer producing waste. Prior to shut down, the Units 1&2 area used a paste disposal system to reduce the available free water and therefore leakage at the Stage Two Evaporation Pond area.

At the operational Units 3&4 Evaporation Holding Pond area a paste disposal system and non-liquid (dry disposal) system, designed to reduce the free available water and leakage, has been implemented to address the source of the impacts. It provides for the material to be placed in the disposal ponds with minimal water so that it dries quickly and reduces the potential for leakage from the pond.

Concurrently with dry disposal, the sealing of the waste disposal ponds has been improved with the installation of synthetic liners, which incorporate a leachate collection system to capture any leaks at the pond and helps prevent impacts to the groundwater.

Forced evaporation systems have been installed at the ponds to reduce the amount of water in the waste disposal ponds. This reduction in water reduces potential for leakage from the ponds.

In 2020 Talen started a full-scale clean-water flushing and capture system at the Plant Site area to address groundwater contamination. In 2023, Talen is starting the small-scale clean-water flushing and capture system at the Units 1&2 pond area and a full-scale flushing and capture system at the Units 3&4 EHP area to address existing groundwater contamination.

Talen Montana continues to design and implement DEQ-approved remedial actions to address the source contamination from the coal ash ponds and the groundwater contamination. The Remedy Evaluation Reports and Remedial Design/Remedial Action Work Plans for each AOC area are included in the documents below.

What efforts has Talen Montana undertaken to evaluate if beneficial use or reuse of coal ash (also known as coal combustion residual) is appropriate for the Colstrip SES coal ash material?

Talen Montana is not required to address beneficial use of coal ash material or coal combustion residual (CCR) under the AOC, Coal-Fired Generating Unit Remediation Act or MFSA certificate; however, Talen Montana has gathered information and talked to multiple beneficial use companies and researchers about the feasibility of beneficial use of the CCR/coal ash material. Talen’s efforts to examine beneficial use are outlined in the following summary document from 2023: Talen Montana CCR Beneficial Use Summary


The Montana Legislature passed the Coal-Fired Generating Unit Remediation Act (75-8-105, Montana Code Annotated) in 2017.

Water Feasibility Study - Colstrip SES (November 2022)

As a part of the AOC, Talen Energy is required to submit analyses of the groundwater contamination at the Colstrip site. These reports contain the results of groundwater testing performed by Talen in each of the three areas defined in the AOC: at the plant site itself, and at the two major pond complexes. The following are the final approved versions of the site reports.

Units 1 and 2 Release Report (December 2021 CVID 23695)

Plant Site Reports (July 2015)

Units 1 and 2 Site Report (October 2017)

Units 3 and 4 Site Report (October 2017)

In 2015, DEQ requested that Talen Energy revise the Background Screening Levels for the Site. These levels are important for determining the extent of the contamination and some of these levels may be designated as cleanup criteria. The following is the final approved version of the BSL report.

As part of the AOC, Talen must submit reports for each of the three designated areas that identify and summarize feasible remedial alternatives , and provide pros and cons of each alternative. Talen must identify a preferred remedy and demonstrate that risks from constituents of interest will be substantially mitigated by the plan.

Plant Site

Units 1 & 2

Units 3 & 4

As part of the AOC, Talen must submit reports for each of the three designated areas that identify the cleanup criteria for the constituents of interest, and provide and assessments of the risk posed by these constituents of interest to human health and the environment.

Plant Site

Units 1 & 2

Units 3 & 4

Plant Site

Plant Site Remedy Evaluation

*Subsequent annual remedy evaluation reporting will be included in the Annual Hydrologic Reports

Units 1 and 2