Acid base accounting--An improved method of interpreting overburden chemistry to predict the quality of coal-mine drainage

Acidic mine drainage (AMD), which results from the accelerated oxidation of pyrite (FeS₂ ) in mined coal and overburden, has contaminated thousands of miles of streams in the Appalachian region of the United States. Acid‐base accounting (ABA), which simplifies the complex hydrogeochemical system through use of a limited number of variables, commonly is used to predict the post‐mining occurrence of AMD. ABA involves the measurement of sulfur (S) and carbonate (CO₃) concentrations in coal‐bearing rocks and the computation of overburden net‐neutralization potential (NNP) in units of tons of calcium carbonate per thousand tons of rock (tons CaCO3/1,000 ton) (Sobek and others, 1978). ABA was developed on the assumption that the stoichiometry of the following overall reaction of FeS₂ and CaCO₃ can be used to convert acid (H⁺) into units of CaCO₃:

FeS₂ + CaCO₃ + 3.75 O₂ + 1.5 H₂O --> Fe(OH)₃ + 2 SO₄^-2 + 2 Ca⁺² + 2 CO₂(g), (1)

where the H + from 1 mol (mole) of FeS2 [64 g (gram) of S] is neutralized by 2 mol of CaCO3 (200 g). This method presumes that gaseous carbon dioxide (CO2 ) will exsolve. Thus 3.125 g CaCO3 will neutralize the acid from 1 g S; or 31.25 tons of CaCO₃ will neutralize the acid from 1,000 tons of rock that contains 1.0 percent pyritic S. The total S concentration, in percent, is multiplied by 31.25 and is assumed to be pyritic and acid‐producing in order to compute maximum potential acidity (MPA) for comparison with neutralization potential (NP), in units of tons CaCO₃ /1,000 ton (Sobek and others, 1978). NNP is computed by subtracting mass‐ weighted MPA from NP (Smith and Brady, 1990). if the value of NNP is less than zero, the acid‐producing potential of the rock exceeds its neutralization potential and if mined, therefore, would be expected to produce AMD.