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Colstrip Steam Electric Station Coal Ash Ponds

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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 both DEQ and 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 facilityWe 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: We will add you to an email list which receives updates on public notices and other major announcements about this project.  

Questions about the AOC process in general are immediately below; for other questions, please click the following links: 

Questions Concerning the Colstrip Administrative Order on Consent (AOC) Process

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 must prepare “Site Characterization Reports” for each of the three areas that describe the existing conditions, including the extent of the contamination.  The reports must also describe what has been done so far to address the contamination, and how effective those measures have been in remediating the contamination.  You can read the Site Characterization reports at this link
Step 2: Next, Talen will prepare Cleanup Criteria and Risk Assessment Reports.  These reports will identify the standards that PPL Montana will have to achieve in its remediation of the contamination.  
Step 3: Finally, Talen must prepare Remedy Evaluation Reports, which will evaluate different options for remediation of the contamination. 
DEQ will use the Remedy Evaluation Reports to select a remediation plan for Talen Energy, who will be required to submit final designs based on that plan.  After DEQ approves the final plans, Talen will be required to implement the selected remediation.

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

Currently, the process is at Step 1; submission and evaluation of the Site Characterization Reports. The Site Characterization Reports for each of the three areas identified in the AOC have been submitted.  You can read them at this link.  
DEQ, with the assistance of a third party contractor, is currently conducting technical reviews of the Site Characterization Reports. According to the AOC, DEQ has the option of: 
1) Approving each report as written; 
2) Approving each report with conditions; or
3) Disapproving each report. 
DEQ has approved the Plant Site Report with conditions. The written conditions are found in the following two links to documents on the Site Report Tab:
Plant Site Characterization Report.
Appendix A (Groundwater Modeling) of the Plant Site Characterization Report.

What are the reasons for the length of this process?

The Colstrip site is a complex contamination site that has required thorough hydrological analysis. The coal ash disposal ponds are part of an evolving, dynamic industrial process.  Each of the sites has at least three or four active ponds of different construction and inactive filled ponds.  Consequently, the analysis of the site reports has been time consuming.  
The ponds are surrounded by a complicated network of hundreds of monitoring and capture wells. Contaminated water is pumped back from the capture wells into the ponds. Each active well is 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 groundwater flow caused by the complex area geology.  (Click here for a more detailed summary of the geology surround this site.)
As a result of the geological complexities, it is difficult to confirm predictions concerning how far and fast contamination will spread.
In addition, the site reports are involved documents describing the ash disposal ponds, the water chemistry of the ponds, estimates of pond leakage, 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. The AOC acknowledges that contaminated groundwater has continued beyond the capture well network in some areas.
The site reports also contain complex groundwater models. DEQ is evaluating the technical accuracy of every part of the site reports.

Questions about Public Involvement

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). We have valued 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 the public comment period:

Montana DEQ
Attn: Jake Kandelin
P.O. Box 200901
Helena, MT 59620-0901

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

DEQ is required to respond to substantive public comments in each written approval or disapproval of each report. DEQ’s decision on each report will be posted on this .  Once DEQ approves or disapproves PPL’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.


Questions about the Contamination

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*, termed “indicator parameters”, to define the amount, extent, and change in time of the contamination.

Additional chemicals may be identified as contaminants when the Cleanup Criteria and Risk Assessment Reports are submitted by Talen and reviewed by DEQ. Click here for a list all of the chemical parameters that are currently being tested at the Colstrip facility. 
*Specific conductance is the measure of the ability of water to conduct electricity.

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.

Detailed information concerning groundwater flow is presented in each Site Report.

How far has the contamination spread?

The extent of the contamination is represented in the maps from the groundwater model reports, the entirety of which can be found on this page.  While many different chemical parameters have been tested in the area, maps of the spread of contamination for boron are included below:  

DEQ will post more maps describing the spread of contamination to this page as the data becomes available.  

How fast is the contamination moving?

Even in areas that have much simpler geology than Colstrip does, the speed of groundwater is uncertain because highly complex underground geological factors govern its speed. The geology at Colstrip can vary horizontally quickly 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 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.

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.

Monitoring wells have detected leaking water from the ash disposal ponds. 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.

Other Frequently Asked Questions

Will the AOC process shut down the Talen (formerly PPL Montana) power plant?

No,Talen Enrgy is allowed to continue operating the Colstrip Power Plant while the groundwater contamination is addressed under the AOC.

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

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

What happens if Talen Energy closes the plant?

DEQ does not have bonding authority under the Major Facility Siting Act.  PPL Montana, however, voluntarily agreed to post bonds at certain phases of the AOC process.  PPL Montana’s submission of the bonds was made a requirement of the AOC.  PPL Montana submitted an irrevocable letter of credit for $7.5 million to ensure the continuing operation of current actions that are taking place to address the groundwater contamination.  Talen Energy will also be required to submit additional bonds when remedial actions are identified under the AOC and when final closure plans are approved 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, PPL has implemented several measures: 

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

A  paste disposal system, meant to reduce the free available water and leakage, has been implemented at the ponds 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.  

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.


Administrative Order on Consent

The Administrative Order on Consent sets forth the process DEQ and Talen must follow to address groundwater contamination at the Colstrip site.  

You can download the entire AOC at this link, and view related documents below: 


Site Reports

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, in addition to the two major pond complexes.  

Plant Site Reports and Related Documents:


Jake Kandelin

Environmental Science Specialist
P.O. Box 200901
Helena MT  59620-0901