AMD usually contains dissolved metals, such as iron, aluminum, and manganese. These dissolved metals decrease the tolerance of aquatic life to non-neutral pH. Dissolved metals also corrode man-made structures, such as bridges and wells, and cows will not drink water if it is high in iron, distressing milk production.*
Because most dissolved metals in AMD are likely to precipitate eventually after exiting the mine, most passive treatment systems try to hasten the process before the mine drainage reaches a stream or river. This generally requires merely keeping the AMD calm in a settling pond for a while, but sometimes pH is increased to precipitate metals faster. Some systems also use sulfate-reducing bacteria to cause metals to remove metals.
* Dairy Reference Manual, 1995. Third Edition. College of Agricultural Sciences, Pennsylvania State University & Northeast Regional Agricultural Engineering Service, 293p.