Water is regarded as contaminated or polluted if there are substances in it to such an extent that it cannot be utilized for a particular reason. Polluted water is not safe for cooking, drinking, and other human uses. Water pollution occurs as a result of anthropogenic activities such as littering, mining, deforestation, and use of pesticides. It also occurs as a result of industrial activities such as oil spillage, sewage leakage, and industrial waste dumping. Other parameters that can contribute to water pollution include salinity pollution (the content of dissolved solids in water) and pathogenic pollution (coliform bacteria) from fecal contamination (UNEP, 2016).

More than 50% of river stretches in Latin America, Asia, and Africa are adversely affected by organic and pathogen pollution, whereas 33% of them are affected by salinity pollution. This deterioration is more pronounced in subdivision of these river stretches. In 1990, water pollution has risen to a serious level and by 2010, it worsened (UNEP, 2016).

In developed countries like China, increasing water pollution has been associated with the increase in industrial and economic activities. An example is Yangtze River in China. Waste from industries, mining process, and sewage discharged into the river weighs about 25 billion tons yearly. About 80% of this waste is not properly treated and 60% of the river’s length is affected by pollution (Yi, Yang, & Zhang, 2011). This pollution eventually spreads out to the other parts of the river, which have high densities of population and industry. The sewage discharge around the river also releases heavy metals into the river, which eventually accumulate in the water sediments, leading to ecological damage.

Heavy metals are those metals with relatively high density. About forty metals are classified as heavy metals based on their chemical properties (Zahra, Kalim, Mahmood, & Naeem, 2017). These metals are mostly found in rock formations, but are also frequently found in soil, aquatic habitats, and in the atmosphere in form of vapors. Some of these metals are not easily degradable and can therefore accumulate in the environment. Heavy metals are also classified as essential such as iron, zinc, manganese, copper, and non-essential metals such as mercury, lead, nickel, and cadmium. Essential metals are needed as micronutrients in the body. For example, zinc and copper act in two ways: as activators for enzyme-catalyzed reactions or as prosthetic group in metalloproteins. In addition, they are needed in redox reactions, electron transfer, and are essential in metabolism of nucleic acids (Zahra et al., 2017).  Essential metals only become toxic in high quantities while non-essential ones are toxic even in low quantities.

Heavy metals are released into aquatic habitats as a result of anthropogenic sources, such as mining of metals, metropolitan and sewage wastes, industrial emissions, and agricultural runoff (Fig. 1). They can also be released into an aquatic ecosystem due to natural causes such as biological weathering of rocks and atmospheric deposition (Iqbal, Tabinda, Yasar, & Mahfooz, 2017).