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Acid-Base Accounting for Surface Mines

A methodology for geologic testing for land disturbance.

A dump truck unloads a load of fill from a mine site.

Acid mine drainage and acid sulfate soils are common consequences of disturbing earth materials containing pyrite and other sulfide-bearing minerals. In order to predict the acid producing potential of geologic layers in eastern US coal mining regions, Acid-Base Accounting (ABA) was developed by researchers at West Virginia University. The objective of this is to demonstrate the use of this method and its interpretation as a prediction tool, and to evaluate its accuracy from literature sources.

ABA is an analytical procedure that provides an assessment of the acid-producing and acid neutralizing potential of soils, sediments and rocks prior to coal mining, highway construction, and other large-scale earth-moving excavations. ABA includes techniques that measure the reactive sulfur content (which is converted to the acid-producing potential, Maximum Potential Acidity or MPA) and the reactive carbonate content (which is converted to acid-neutralizing potential, Neutralization Potential or NP). These two quantifiable properties in ABA are primarily used to predict the quality of drainage and soil quality by subtracting MPA from NP, resulting in a net NP value (either positive or negative). If the MPA value is higher for the sample (negative net NP), the rock sample is predicted to produce acidic drainage upon weathering and leaching. If the number for NP is higher (positive net NP), the rock is predicted to produce alkaline drainage. Other parameters such as rock type, color and paste pH help to refine the interpretation and prediction of net NP.

After passage of laws requiring an assessment of surface mining on water quality, ABA became the preferred method to predict post-mining water quality, and permit decisions for surface mines are largely based on the net NP values of ABA. With this information, mining plans are developed which may include mixing overburden materials during mining and reclamation or removing acid-producing materials from the site, selective handling of these materials and placing in specific areas within the backfill, and amending these acid materials with alkaline material. ABA has proven to be a good tool to predict overburden quality that allows the application of prevention procedures to alleviate post-mining water and soil quality problems. Studies comparing the post-mining water quality with predictions made by ABA have confirmed the utility of ABA for permit decisions, pre-mine planning and reclamation practices.

Read the full article at Science Direct