Metals are introduced into the aquatic systems as a result of the weathering of soils and rocks; from volcanic eruptions; and from a variety of human activities involving mining, processing, or the use of metals and/or substances. The most common metal pollution in freshwater comes from mining companies. Usually, mining companies use an acid mine drainage system to release heavy metals from ores, because metals are very soluble in an acid solution. After the drainage process, they disperse the acid solutions containing high levels of metals into the groundwater. When the pH in the water decreases, the metal solubility increases and the metal particles become more mobile. That is why metals are more toxic in soft waters.