CHARACTERISTICS OF ABANDONED MINE DRAINAGE IN PENNSYLVANIA

Discharges from 140 abandoned underground coal mines in the bituminous and anthracite coalfields of Pennsylvania were sampled by the U.S. Geological Survey in 1999. The pH ranged from 2.7 to 7.3; with two modes at pH 2.5 to 4 (acidic) and pH 6 to 7 (near neutral). The bimodal frequency distribution for pH was distinctive for fully oxidized samples; few samples had intermediate pH values. Upon aeration, the pH increased because of exsolution of carbon dioxide and hydrogen sulfide, or it decreased because of oxidation and hydrolysis of dissolved iron. Many samples that were initially near neutral ultimately had acidic pH as indicated by their “hot” acidity. Concentrations of dissolved sulfate, iron, and aluminum ranged from 36 to 2,000 mg/L, 0.046 to 512 mg/L, and 0.007 to 108 mg/L, respectively. Concentrations of sulfate and iron were not correlated with pH. However, discharges with pH <4.5 had greater median concentrations of aluminum, various transition metals, and rare earths than higher pH solutions. Although the median sulfate and iron concentrations for bituminous discharges exceeded those for anthracite discharges at successive pH class intervals, the median loads of sulfate and iron for anthracite discharges exceeded those for bituminous discharges at a given pH.