A man-made marsh that is designed to be slow-draining so that specific species will flourish, primarily to replace natural wetlands that have been drained and filled prior to development. In mining, some constructed wetlands are designed to fix metals and other contaminants, primarily by the reduction of metal sulfates to sulfides, or the formation of oxides or carbonates. When the term is also applied to systems constructed without plants, they consist of buried substrates through which the contaminated water is passed under low oxygen or reducing conditions.