Piles of rock, sand and other neutral materials at a construction site in northern Minnesota where crews and large trucks work to lay the bed for a new road.

Guidance for Working With Potentially Acid-Generating Materials

A new resource is available to help agencies greatly reduce the risk of disturbing potentially acid-generating (PAG) rock in places like northern Minnesota when conducting road projects. When exposed to air and water, PAG minerals can generate drainage that is hazardous to the environment. A MnDOT-sponsored research team developed a best practices manual that provides comprehensive steps to identify, mitigate and monitor PAG material during highway construction.  

What Was the Need?

Across Minnesota and particularly in the northern half of the state, the bedrock and glacial overburden contain minerals that are potentially acid-generating. When sulfur-rich rock and soil are disturbed and exposed to air and water, they can generate acidic drainage. This drainage can contaminate surface and groundwater, and harm vegetation and aquatic life.  

Proposals to mine copper and nickel in northern Minnesota have increased local community awareness and concerns about PAG minerals. Further, highway construction projects often utilize and expose local bedrock and soil, which creates potential environmental risks. Acid generation in highway settings has also been responsible for metal deterioration, pavement spalling and unstable slopes and fills.

“This new manual will provide MnDOT’s highway construction decision-makers with effective tools and protocols to evaluate the presence of acid-generating minerals, as well as methods to manage their potential risks,” said Jason Richter, chief geologist, MnDOT Materials and Road Research Laboratory.

Before the recent Trunk Highway 169 (TH 169) Eagles Nest Lake Area reconstruction project in northern Minnesota, MnDOT had no guidelines for managing PAG materials. Consequently, the Eagles Nest project required considerable time, effort and expense to develop a plan to characterize, mitigate and monitor substantial quantities of PAG materials that were present along the construction route.  

MnDOT sought to develop a guidance manual that would establish methods of identifying and mitigating the effects of PAG materials present at future highway construction sites.

What Was Our Goal?

The objective of this project was to develop specifically for MnDOT a guidance manual that could be used to properly identify and characterize PAG materials prior to maintenance and construction projects, properly handle and mitigate these materials during projects, and, if needed, monitor them after projects are completed.

What Did We Do?

Experts developed the PAG guidance manual through several steps. First, they reviewed and gathered existing methods of PAG material screening, characterization, mitigation and monitoring used in Minnesota. They also evaluated practices used by other government agencies, particularly the Minnesota Department of Natural Resources (MnDNR) and the Pennsylvania and Tennessee Departments of Transportation; international agencies; and the hard rock and coal mining industries.

An investigator collects groundwater samples near the construction site.
Groundwater samples are collected and monitored after construction.

Next, they developed a spatial database using available geologic information to locate and assign risk rankings to PAG materials within MnDOT’s right of way. With this information, they developed a geographic information system (GIS) based tool that would enable MnDOT to make PAG material risk evaluations for highway projects. A geostatistical analysis of data acquired during the TH 169 Eagles Nest project was also performed, which provided parameters for the PAG characterization program. 

Investigators then drafted the guidance manual, which begins by explaining the acidic drainage process and how it relates to Minnesota’s geology. The manual then takes the user through a step-by-step project evaluation process, from desktop review through post-construction monitoring.

Before design for a project begins, a desktop study is conducted that screens the project areas using new GIS resources that show top of bedrock and PAG risk. This step may necessitate further field reconnaissance to verify the findings of the desktop study or it may conclude that no PAG is present, in which case no further investigation would be required.

“Northern Minnesota’s highway projects will benefit as design and construction decisions will now be guided by foreknowledge of the presence of PAG minerals and solid strategies to deal with them,” said Ward Swanson, senior environmental scientist, Barr Engineering.

If a PAG risk is identified during the field reconnaissance, the quantity of material to be excavated or imported will determine if either de minimis measures or a characterization program should be undertaken. The characterization program will provide the methodology and rationale for acquiring and testing soil and rock samples to more accurately determine the risk level of the material. Material may be determined to be non-PAG, uncertain or positively tested as PAG. The risk level will determine whether or not further characterization or mitigation is necessary. Chemical data about surface and groundwater will also be acquired to use as baselines for future monitoring.

If mitigation is deemed necessary, then a plan that includes treating, isolating or avoiding the PAG materials will be developed. Mitigation may include the addition of neutralizing agents to PAG materials, covering PAG materials with membranes or disposing of PAG materials off-site, to name a few strategies.

Finally, the manual includes development of a baseline and post-construction monitoring plans, as well as reporting to ensure that PAG mitigation efforts have been effective.

What Was the Result?

This project drew upon decades of research and professional experience from the public and private sectors to create a first-of-its-kind manual for MnDOT that addresses transportation-related PAG material concerns, particularly for northern Minnesota. MnDOT decision-makers now have a readily available resource that will help them greatly reduce a variety of risks associated with managing PAG materials in future highway projects. 

What’s Next?

The MnDOT Guidance Manual for Potentially Acid-Generating Materials in Northern Minnesota is available on MnDOT’s website.

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