Evaluation of Acid-base Accounting to Predict the Quality of Drainage

The effectiveness of Acid-Base Accounting (ABA) for predicting surface coal mine drainage quality in Pennsylvania was evaluated. Comparisons between ABA and mine drainage alkalinity, acidity, and sulfate were made for 38 mines in the bituminous coalfield. Neutralization Potential (NP), Maximum Potential Acidity (MPA), and Net Neutralization Potential (NNP) were evaluated with and without "thresholds." Calculations using "thresholds" counted only those values for NP greater than 30 mt CaC03/l,OOO mt and percent sulfur greater than o. 5%. "Without thresholds" computations included all values. Stoichiometric equivalence factors of 31.25 and 62.5 were used to compute MPA. NP and NNP are the best predictors of postmining drainage quality. Alkaline or acid drainage quality is controlled by as little as 1% to 3% carbonate in the overburden. NNP less than 1% generally results in acidic drainage, and NNP greater than 3% yields alkaline drainage. An empirical relation exists between alkalinity and NP. Postmining alkalinity can be estimated as 4 to 6 times NP (without thresholds) . MPA is not a reliable predictor of drainage quality, except in the absence of calcareous strata, where a positive relation exists between acidity and MPA. Additional