Effectiveness of passive treatment systems for acid mine drainage under different flow conditions

Sixteen treatment projects using 5 different types of passive treatment were constructed between 1995-2007 to reduce acid mine drainage (AMD) impacts in a 70-km² watershed in Western Pennsylvania.   The combined median discharge for the treatment systems was 5100 L/min, which represented about 10% of stream discharge at the bottom of the watershed.  On average, passive treatment raised effluent pH to 6.9, increased alkalinity by 78.6 mg/L and reduced acidity by 161.9 mg/L, resulting in a net alkaline discharge. Fe, Al and Mn were reduced by an average of 41.5,  9.2 and 19.6  mg/L, respectively.  Higher AMD discharges during storm events were associated with lower acidity and Fe concentrations from dilution effects, however, their loadings increased, presumably because high flows flooded previously dry mine workings.  Alkalinity generation by treatment systems was maintained at higher AMD loadings because the dissolution of calcite within the systems is relatively rapid. The effects of flow on AMD treatment are difficult to generalize because they are highly dependent on the hydrology of different AMD inputs (surface or deep) and kinetics within the systems, which vary depending on the chemical parameter and type of treatment system.