Some information comes from EPA Region 3 publication: A Citizen's Guide to Address Contaminated Coal Mine Drainage, EPA-903-K-97-003.
An aerobic wetland is a pond with a large surface, which collects AMD water and provides time for metals to and sediments to precipitate. Unlike anaerobic wetlands, aerobic wetlands do not add alkalinity and should not be used to treat very acidic water.
Aerobic wetlands contain cattails and other wetland vegetation to filter water and introduce oxygen, which causes iron to precipitate out of the water. They are only about one foot deep, but spread out over a large area to hold the same amount of water as the smaller, but deeper, components in a treatment system. This surface area also helps expose water to oxygen.
Because metals and sediment precipitate to the bottom of the pond, an aerobic wetland should not be used if iron levels are high, which would cause the pond to fill up quickly. Rather, aerobic wetlands are generally used after AMD has been treated in other ways, and they are often called "polishing ponds" for this reason.
Operations & Maintenance Considerations
As previously mentioned, metals that accumulate on the bottom of a settling pond need to be removed. They reduce the volume of the pond, decreasing the time that water is retained, thereby giving metals less time to settle out of it. Also, flows of water may create preferential paths through metal sludge over time. This results in short-circuiting, where water flows quickly in a current through the pond, without dispersing throughout or stalling long enough for metals to precipitate. One solution is to build berms, baffles, or other obstructions in the pond to deflect water from any preferential paths.
Of course, like most passive treatment systems, basic maintenance including mowing grass around a system and removing leaves that may clog conduits is necessary. Also, animals may damage a system, especially muskrats, whose tunnels must be blocked with dirt and rocks.