Projects IGS Helps State Improve Waters Impacted by Acidic Mine Drainage by Ron Smith

Untreated AMD at the former Friar Tuck abandoned mine land site, Sullivan County, Indiana Photo by John Comer

According to the U.S. Geological Survey, more than 250,000 miles of streams in the United States are adversely affected by acidic mine drainage (AMD). In Indiana, the Department of Natural Resources, Division of Reclamation (IDOR) has been working to mitigate this problem by reclaiming mine lands and constructing AMD treatment facilities. The Indiana Geological Survey (IGS) assists in these efforts to protect the state’s water resources by helping to evaluate the effectiveness of treatment methods.

Research conducted by the IGS and others shows that mine water not only carries high levels of acidity and sulfate, but potentially toxic levels of certain metals as well. These metals originate in minerals that dissolve when acidic water flows through mine refuse on disturbed land. Most, but not all, of the dissolved metals precipitate from solution when AMD is neutralized.

Constructed wetlands have proved to be the most cost-effective way to neutralize AMD in Indiana. The wetlands are designed so that natural processes promote the removal of acidity and metals, while reducing the volume of runoff discharged from disturbed lands and providing storage for precipitates. A recent study performed by the IGS and funded by a Surface Mining and Reclamation Technology (SMART) Grant from IDOR indicated that properly constructed wetlands can improve the quality of severely contaminated mine water, and that treated water meets most of the primary water quality standards of the Safe Drinking Water Act.

The design of treatment wetlands can be complex, and a variety of schemes are used to produce the alkalinity required for effective treatment. Organic substrates are used to enhance biological processes that fix metals and generate alkalinity, and the growth of emergent plants extends the water’s residence time in the wetland so that neutralization and precipitation reactions can proceed. Although biological activity is low during winter, constructed wetlands continue to produce high-quality effluent because of the high surface area and sorptive capacity of the materials present.

Wetland sediments serve as a sink for metals; however, they can also be a source of metals if hydrologic conditions and water chemistry change. High concentrations of potentially toxic metals have been found in sediments throughout the treatment wetlands, including the final treatment cells where water leaves the wetland. As wetland cells fill with precipitated sediment, the massive deposits of precipitated metals must be removed to restore the original function of the treatment structures.

Since the periodic excavation of metal-rich sediment is necessary, it would be advantageous if some economic value could be recovered from the operation. In late 2003, the IGS was awarded another SMART Grant to evaluate methods of controlling metals deposition in AMD treatment wetlands so that the metals can be recovered in forms that have market value. This will provide a way to offset the cost of reclamation projects, saving money for the state of Indiana, while protecting the environment and providing more efficient reclamation techniques. The use of recovered metals also decreases the quantity of metals that the U.S. must otherwise mine and process to meet the needs of industry and commerce.

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