Acid-base accounting to predict post-mining drainage quality on surface mines.

Area of Science:

• Environmental Science
• Geochemistry
• Mining Engineering

Background:

• Acid-base accounting (ABA) is crucial for assessing pre-mining overburden rock potential.
• ABA data informs permitting decisions for surface mines to predict post-mining water quality.
• Previous methods relied on comparing acid-producing and acid-neutralizing rock quantities.

Purpose of the Study:

• To evaluate the accuracy of ABA in predicting post-mining water quality.
• To correlate overburden geochemical data with actual post-mining water quality at 56 West Virginian sites.
• To refine the interpretation of ABA values, specifically the NP to MPA ratio, for improved prediction.

Main Methods:

• Collected geologic and geographic data for 56 mined sites.
• Calculated maximum potential acidity (MPA), neutralization potential (NP), net neutralization potential (NNP), and NP to MPA ratios.
• Correlated ABA values with observed post-mining water quality from springs and seeps.

Main Results:

• Net neutralization potential (NNP) and the NP to MPA ratio showed the highest prediction accuracy for water quality.
• Established categories for NP to MPA ratios: <1 (acid), 1-2 (mixed), >2 (alkaline).
• Six sites (11%) deviated from predicted water quality, with four sites producing acid drainage despite high NP to MPA ratios due to external factors.

Conclusions:

• ABA, particularly NNP and the NP to MPA ratio, is a reliable tool for assessing overburden quality and predicting post-mining drainage.
• The established NP to MPA ratio categories provide a useful framework for predicting water quality.
• External factors beyond overburden geochemistry can influence post-mining water quality, necessitating careful site-specific evaluation.