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Acid rock drainage: More than just a mining project concern?

Acid rock drainage: More than just a mining project concern?

Acid rock drainage (ARD) is a well-known challenge in mining. What’s less understood is the prevalence of this concern in other industries. ARD can potentially impact any project that requires blasting or excavation of rocks containing iron-sulfide minerals. Because any project site—a railroad, a highway, a pipeline corridor, or a mine—involving rock disturbance may be at risk for acid runoff, planning is crucial.

What is ARD?

ARD forms when iron sulfide minerals are exposed to oxygen and water, creating acidic runoff. While sulfide-bearing minerals are regularly associated with many types of mineral deposits, excavation activities that expose these minerals are common at mining sites. This is why ARD is often associated with mining and sometimes called acid mine drainage.

Why does ARD matter?

ARD is an environmental concern because it can disrupt an area’s pH balance and negatively impact plants and animals. The runoff may contain high concentrations of metals from dissolving sulfide and other minerals. On larger-scale projects, ARD can pose lasting consequences to the local watershed and lead to long-term water treatment costs. If left untreated, ARD can persist for decades or even centuries, as observed in historic mining regions around the world—including throughout the western U.S.

Careful planning and mitigation at the beginning of a project can help minimize the risks and costs of long-term ARD.

Careful planning and mitigation at the beginning of a project can help minimize the risks and costs of long-term ARD. On sites with already problematic ARD, project owners can manage the environmental risks by designing ARD remediation, reclamation, and treatment systems.

Besides mining, which industries can be impacted by ARD?

Construction projects in areas with potentially acid-generating rocks can present ARD concerns if they involve exposure of those rocks. For example, Barr has provided ARD assessment services for several existing clients seeking to build wind turbines in reclaimed mining regions and other areas with ARD potential. Following assessments that indicated ARD potential, Barr prepared mitigation plans to be used when potentially acid-generating rock is encountered during construction.

Transportation projects also can pose ARD risks. On a recent highway realignment project in northern Minnesota, Barr assisted with ARD evaluation services. As a result of this project, the Minnesota Department of Transportation hired Barr to develop an ARD guidance manual with the University of Minnesota’s Natural Resources Research Institute. The manual describes how to screen, characterize, evaluate, monitor, and mitigate potentially acid-generating material. Minnesota’s guidance manual is the third of its kind in the U.S.; only Pennsylvania and Tennessee previously had such documents. Although focused on highway construction, the background and methods included in the manual can inform other mining, industrial, and construction projects in northern Minnesota and similar environments.

How do you prevent or treat ARD issues?

Preparing mitigation plans and designing remediation, reclamation, and treatment systems are vital steps in helping you prevent or treat ARD issues at your project sites. To support clients in this area, Barr brings together geochemists, geologists, geological engineers, environmental engineers, and scientists. We recently expanded our capabilities in ARD assessment and related projects by hiring two senior geochemists with extensive experience in mining and other industries. Bob Berry and Kathy Johnson joined Barr in 2021, each with more than 30 years of geochemical consulting experience. Bob and Kathy provide Barr’s existing geochemical team with additional capability and experience in modeling and interpreting the fate and transport of contaminants in the environment.

Contact Barr to learn more about our recent work or to assess ARD risk on your project.

About the authors

Denise Levitan is a senior environmental scientist and geochemist who has evaluated ARD potential for mining, power, and public-sector clients. She has a decade of experience with environmental analysis and assessment, solving problems related to all phases of industrial-project lifecycles.

Kevin Eisen is a senior geological engineer who has over 22 years of experience assisting clients with emerging contaminants, legacy contamination, and ARD evaluation and mitigation. He managed the overall development of the MnDOT PAG guidance manual and has experience in state- and federally led ARD-related projects.

 

Image gallery (below):

  1. One of Barr’s clients planned to use this type of quarry rock for a transportation project, but the sulfur content of the aggregate exceeded a threshold value and the project came to a halt. After a historic water-quality pH analysis indicated the rock was not acid-generating, the client was able to move forward with the project.

  2. Barr assisted with ARD evaluation services on a highway realignment project in northern Minnesota. The project involved excavating rock from the 2.5-billion-year-old Soudan iron formation. The folded and faulted stone contains sulfide minerals that, when exposed to air and water, can generate acidic runoff.

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