One reason for their relative success is that they typically treat flows of less than 400 Litres per minute [L/min](ca. 100 gpm), although some wetland systems treat flows of more than 2,000 L/min (+600 gpm) (Doug Kepler, Damoriscotta, Clarion, PA, USA. Personal communication). Systems treating coal-generated AMD range in size from a few hectares to more than 300 hectares. Other types of mines usually have larger flows, requiring wetlands with larger surface areas (until recently considered to be prohibitively large).
While coal-generated AMD is not homogeneous in composition, it usually contains high concentrations of aluminum, iron, and manganese. It often has a low pH and high acidity, although treatment of this type of water has only been successful in recent years. In contrast, mine drainage from other sources tends to be more diverse, making it more difficult to correlate system designs with one another. Systems treating coal-generated AMD are almost always designed to neutralize water pH and to remove the above metals. This is typically accomplished by using a combination of anoxic limestone drains (ALD's), ponds and wetlands (Skousen et al., 1994).
Early enthusiasm for Sphagnum-based systems waned because of their limited lifespan (as described by Wieder, 1993). Nowadays, virtually every wetland is vegetated with cattails (Typha latifolia). These plants are abundant in that region, they can withstand extremes in water pH and metal concentrations, and they are easily grown and transplanted.
The evolution of system designs is checkered, yet failures were just as informative as successes. At first, only near-neutral water could be treated, or water with sufficient alkalinity to buffer against hydrolysis of dissolved metals. New design concepts, such as the development and incorporation of ALD's, anaerobic wetlands and "successive alkalinity-producing systems" (SAPS), have allowed for the treatment of acidic drainages. Robust design criteria have been developed over time, which relate surface area requirements to mine water of specific composition. The US Bureau of Mines has published an excellent information circular detailing the derivation of such design criteria (see this abstract elsewhere on Enviromine).
A word of caution: design criteria developed in Appalachia may not apply in other parts of the world, as others in Montana have discovered (Hiel and Kerins, Jr., 1988; J. Koerth, Montana Dept of Environmental Quality, Helena, MT, USA. Personal communication). Only when we develop a better understanding of biogeochemical processes acting within treatment wetlands will such transferability will be possible.
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