The Federal Energy Technology Center of the U.S. Department of Energy is monitoring changes in water quality at three inactive mining sites where a coal combustion residue (fly ash) was injected into the spoil to control the production of acid mine drainage. Since most combustion residues are alkaline, their addition to the subsurface environment raises the pH, limiting the propagation of pyrite oxidizing bacteria and reducing the rate of acid generation. Grouts of fly ash, lime and cement, when injected underground, decrease permeability and porosity, diverting water away from the pyritic material. Both mechanisms, alkaline addition and water diversion, are expected to reduce the amount of acid produced. Changes in water quality parameters (pH, acidity, anions and cations) in water samples from wells and seeps are monitored to assess the effect of CCR injection. The concentration of heavy metals in the water flowing through the sites, before and after CCR injection, is also determined. The use of underground mine sites for disposal of coal combustion residues provides a stable, low maintenance alternative to landfills, benefitting the mining and electric power industries.
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