ABSTRACT

This article discusses a passive methodology of introducing alkalinity to acid drainages in order to buffer pH decreases in constructed wetlands due to Fe hydrolysis. In 1988, the Tennessee Division of Water Pollution Control constructed and evaluated prototype passive anoxic limestone drains (ALDs) at two mine sites in Tennessee. Over 50 ALDs have now been constructed in the eastern US The ALD, which consists of a shallow, limestone-filled trench excavated into the spoil and sealed from the atmosphere, passively introduces buffering capacity, as alkalinity, into the acid drainage. The ALD consists of an open, unlined trench or excavation back-filled with gravel-sized, crushed, high-CaCO3 limestone. The purpose of an oxidation basin is aimed at high Fe influents. Alternatively, a modified marsh-pond cell with a major portion devoted to deep water might be more applicable to acid drainage with low to moderate Fe concentrations. Design, utility, and longevity of an ALD require further examination to develop guidelines and expectations.