The role of sulfate and ionic strength on the shift from acid to alkaline mine drainage in southwest Pennsylvania

Four Mile Run, Latrobe, PA, receives discharges from abandoned deep mines. In 1971, the effluent was characterized as having low pH, high acidity, and high concentrations of iron, manganese, and sulfate. After 22 yr of neglect, the water flow rate, pH, and alkalinity all increased, while the acidity, iron, manganese, and sulfate concentrations decreased. The overall loading of iron and sulfate to the stream has not changed significantly. This change from acid to alkaline drainage is fairly typical for mine effluent in the surrounding area. However, the mechanism for change is not well understood. Laboratory studies have shown that the neutralization rate of marble chips (CaCO3) is depressed by high ionic strength or elevated levels of sulfate. The decrease in sulfate levels may be an important factor contributing to the change in water quality discharged into Four Mile Run. A similar kinetic phenomenon may occur with mine spoils and anoxic drains