Acid–Base Accounting to Predict Post‐Mining Drainage Quality on Surface Mines

Abstract

Acid-base accounting (ABA) is an analytical procedure that provides values to help assess the acid-producing and acid-neutralizing potential of overburden rocks prior to coal mining and other large-scale excavations. This procedure was developed by West Virginia University scientists during the 1960s. After the passage of laws requiring an assessment of surface mining on water quality, ABA became a preferred method to predict post-mining water quality, and permitting decisions for surface mines are largely based on the values determined by ABA. To predict the post-mining water quality, the amount of acid-producing rock is compared with the amount of acid-neutralizing rock, and a prediction of the water quality at the site (whether acid or alkaline) is obtained. We gathered geologic and geographic data for 56 mined sites in West Virginia, which allowed us to estimate total overburden amounts, and values were determined for maximum potential acidity (MPA), neutralization potential (NP), net neutralization potential (NNP), and NP to MPA ratios for each site based on ABA. These values were correlated to post-mining water quality from springs or seeps on the mined property. Overburden mass was determined by three methods, with the method used by Pennsylvania researchers showing the most accurate results for overburden mass. A poor relationship existed between MPA and post-mining water quality, NP was intermediate, and NNP and the NP to MPA ratio showed the best prediction accuracy. In this study, NNP and the NP to MPA ratio gave identical water quality prediction results. Therefore, with NP to MPA ratios, values were separated into categories: <1 should produce acid drainage, between 1 and 2 can produce either acid or alkaline water conditions, and >2 should produce alkaline water. On our 56 surface mined sites, NP to MPA ratios varied from 0.1 to 31, and six sites (11%) did not fit the expected pattern using this category approach. Two sites with ratios <1 did not produce acid drainage as predicted (the drainage was neutral), and four sites with a ratio >2 produced acid drainage when they should not have. These latter four sites were either mined very slowly, had nonrepresentative ABA data, received water from an adjacent underground mine, or had a surface mining practice that degraded the water. In general, an NP to MPA ratio of <1 produced mostly acid drainage sites, between 1 and 2 produced mostly alkaline drainage sites, while NP to MPA ratios >2 produced alkaline drainage with a few exceptions. Using these values, ABA is a good tool to assess overburden quality before surface mining and to predict post-mining drainage quality after mining. The interpretation from ABA values was correct in 50 out of 52 cases (96%), excluding the four anomalous sites, which had acid water for reasons other than overburden quality.

Acid Drainage Technology Initiative: Continuing Progress in Coal Related Topics

“…The results of the ABA analyses were found to be correct in 82% of the cases using the NP/MPA parameter; this indicates that ABA is a good way to predict postmining and reclamation water quality at a mine site (Skousen et al, 2002). More work is planned or is underway to refine this useful analytical tool.…”

“…(Skousen et al, 2002) The NMLRC has been studying the effectiveness of acid base accounting for predicting postmining and reclamation water quality (Skousen et al, 2002). Overburden analyses, permit maps, and predictions of postmining and reclamation water quality data were collected from regulatory agency permit files from several states in the Appalachian coal region.…”

Prediction of acid mine drainage generation potential of various lithologies using static tests: Etili coal mine (NW Turkey) as a case study

“…The ABA test evaluates the balance between maximum acidgenerating processes (oxidation of sulfide minerals) and acid-neutralizing processes (dissolution of alkaline carbonates, displacement of exchangeable bases, and weathering of silicates) (Sobek et al 1978;Ritcey 1989;Skousen et al 2002;Yeheyis et al 2009;Banerjee 2014). The maximum acidproducing potential (MAP) of the sample is calculated on the basis of its total sulfur (S) content: for each 1 % S contained in the material, 31.25 metric tons (t) of calcium carbonate are required for neutralizing the sulfuric acid produced from 1000 t of the material (Sobek et al 1978;Skousen et al 2000).…”

“…If the sample has an NPR between 2 and 4, then it is potentially acid consuming. If the NPR is greater than 4, the sample is considered to be acid consuming (Price and Errington 1998;Skousen et al 2002). NPR values ranged between 0.001 and 27.15, and according to this classification, 14 samples were categorized as acid generating (Fig.…”

Acid drainage potential from coal mine wastes: environmental assessment through static and kinetic tests

“…The acid base accounting (ABA), a form of static test, predicts mine drainage quality by comparing the sample material's maximum potential acidity (MPA) with its neutralization potential (NP) (Skousen et al 2002). Sobek et al (1978) formally presented a detailed laboratory methodology for conducting the ABA on mine overburden material and is frequently cited as a pioneer protocol, whereas Noll et al (1988) provided an exhaustive account of considerations fitting to planning an overburden analysis and collection of samples.…”