GENERATION OF ALKALINITY IN AN ANOXIC LIMESTONE DRAIN' Robert W. Nairn, Robert S. Hedin and George R. Watzl af2 Abstract. The rate of 1 imestone dissolution and a1 kal inity generation in an anoxic 1 imestone drain is determined by many factors, including the qua1 i ty of the limestone used and the partial pressure of carbon dioxide within the system. Carbon dioxide concentrations greater than 600 times atmospheric levels have been found within an anoxic 1 imestone drain located in northwestern Pennsylvania. This situation greatly increases the sol ubi 1 i ty of 1 imestone, making elevated alkalinity concentrations possible. Before construction of the drain, the mine drainage contained over 400 mg/L acidity as CaC03 equivalent but alkalinity concentrations of more than 300 mg/L have been found at the exit of the drain. A1 kal inity generation rates of approximately 117 grams of a1 kaljni ty (as CaCO, eq.) meter-3 of drain (bulk vol ume) day- have been determined for this system. The anoxic limestone drain discharges the mine water into a settling pond-constructed wetland system where metal oxidation, hydrolysis and precipitation occurs in a strongly buffered, a1 kal ine solution. The use of the anoxic limestone drain resulted in substantial cost savings compared to conventional chemical treatment of thi s drainage. Additional Key Words: cal ci um carbonate, carbon dioxide, solubility, dissolution. 'paper presented at the 1992 National Meeting of the American Society for Surface Mining and Reclamation, Dul uth, Minnesota, June 14-18, 1992. Publication in this proceedings does not preclude authors from publishing their manuscripts, whole or in part, in other pub1 ication outlets. '~obert W. Nairn and Robert S. Hedin are Research Biologists and George R. Watzl af is an Environmental Engineer, U. S. Department of the Interior, Bureau o f Mines, Pittsburgh, PA 15236.