U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12,1999--Volume 1 of 3--Contamination From Hard-Rock Mining, Water-Resources Investigation Report 99-4018A

Frequency Distribution of the pH of Coal-Mine Drainage in Pennsylvania

By Charles A. Cravotta III, Keith B.C. Brady, Arthur W. Rose, and Joseph B. Douds

This report is available in pdf format: Cravotta.pdf 446 KB

ABSTRACT

The pH of coal-mine drainage in Pennsylvania has a bimodal frequency distribution, with modes at pH 2.5 to 4 (acidic) and pH 6 to 7 (near neutral). Although iron-disulfide and calcareous minerals comprise only a few percent, or less, of the coal-bearing rock, these minerals are highly reactive and are mainly responsible for the bimodal pH distribution. Field and laboratory studies and computer simulations indicate that pH will be driven toward one mode or the other depending on the relative abundance and extent of weathering of pyrite (FeS₂; acid-forming) and calcite (CaCO₃; acid-neutralizing). The pH values in the near-neutral mode result from carbonate buffering (HCO₃^-/H₂CO₃ and HCO₃^-/CaCO₃ ) and imply the presence of calcareous minerals; acid produced by pyrite oxidation is neutralized. The pH values in the acidic mode result from pyrite oxidation and imply a deficiency of calcareous minerals and the absence of carbonate buffering. The oxidation of only a small quantity of pyrite can acidify pure water (0.012 g ^. L ^-1 FeS ₂ produces pH~4 and 20 mg ^. L ^-1 SO₄ ^2- ); however, because of the log scale for pH and ion complexation (SO₄ ^2- /HSO₄^- and Fe ³⁺ /FeOH²⁺), orders of magnitude greater oxidation is required to produce pH < 3. Laboratory leaching experiments showed that for a specific proportion of FeS ₂ :CaCO₃ , effluents produced under variably saturated hydrologic conditions, in which oxygen availability and pyrite oxidation were enhanced, had lower pH and greater dissolved solids concentrations than effluents produced under continuously saturated conditions, in which oxygen availability and pyrite oxidation were diminished.