The impacts of metalliferous drainage on aquatic communities in streams and rivers

Introduction Metals are naturally occurring elements of the Earth's crust that can be released into the aquatic environment through the processes of weathering and erosion, where they are present in trace amounts and do not normally constitute an environmental problem. However, the activity of humans has increased the release of many metals to the environment. It is difficult to assess what the natural background levels would have been in affected areas, particularly where the influence has been prolonged, but it has been reported that in mining areas concentrations of metals in waters and associated sediments can be 3–4 orders of magnitude higher following mineral ore extraction (Runnells et al . 1992; Helgen & Moore 1996). Although some of the metals released by human activities, such as Fe and Zn, are essential elements for the successful growth and functioning of biota, the presence of these substances in elevated concentrations or in other chemical forms can be potentially toxic. In addition, many metals, such as Pb and Cd, have no known role in biological functioning and can be toxic to organisms at very low concentrations. Therefore, the release of metals into the environment poses a significant threat to the fauna and flora of receiving water courses.