**3. Contamination of heavy metals**

Harmful trace metals are a significant threat to both aquatic and terrestrial ecosystems [31]. Upon release from both natural and anthropogenic sources, HMs contaminates natural aquatic bodies, sediments and soils. Ultimately, during volcanic eruptions and complex industrial emissions, heavy metals released into the atmosphere often return to the soil and cause water and soil contamination. They either collect in biota or leach down into ground water because heavy metals in the atmosphere are irreversible. There are significant public health effects of the contamination of biota and groundwater with potentially harmful heavy metals. In riverine settings, the degree of heavy metal pollution can be measured by observing the concentrations and distribution of these elements [32]. **Figure 4** provides a conceptual schematic of the contamination of the marine (riverine) environment with heavy metals. Various physicochemical and climatic effects affect the overall dynamics and biogeochemical cycling of heavy metals in the atmosphere.

#### **3.1 Water**

Water is regarded as the life-blood of the biosphere as well. It can dissolve distinct organic and inorganic chemicals and environmental contaminants, as water is a common solvent. They are vulnerable to pollution in both freshwater and marine aquatic environments. Heavy metals are important contaminants in the pollution of *Environmental Pollution with Heavy Metals: A Public Health Concern DOI: http://dx.doi.org/10.5772/intechopen.96805*

**Figure 4.** *A conceptual schematic of contamination of heavy metals in aquatic ecosystem.*

the marine environment. Water contamination by HMs is a critical environmental issue that adversely affects plants, animals and human health [33]. Also at very low levels, heavy metals are highly harmful to marine species [34]. These elements may cause significant physiological changes in the body and histopathological changes in the tissues of aquatic organisms, such as fish [35]. There are several sources of heavy metal pollution in water. Two main culprits of this industrialization and urbanization are the increased degree of concentration of heavy metal in water. Heavy metals from factories, municipalities and urban areas are carried by runoff [36]. This release of untreated industrial waste into marine bodies is a significant cause of surface and groundwater contamination [37]. Due to the environmental persistence, bioaccumulation, and biomagnification of food chains and the toxicity of these elements, contamination of water bodies with heavy metals is a global issue [38].

## **3.2 Sediments**

The contamination of sediments with HMs is a very important environmental problem with implications for marine life and human health. Sediments act as the main source of HMs in the aquatic environment. Their quality can indicate the contamination status of water [39]. Sediments act as a sink and heavy metal source, releasing them into the column of water [40]. Continued heavy metal accumulation in sediments can also contribute to groundwater pollution of these contaminants [41]. Many physicochemical variables such as temperature, hydrodynamic conditions, redox status, organic matter and microbe content, salinity, and particle size influence the adsorption, desorption, and subsequent concentrations of heavy metals in sediments [42]. The distribution of heavy metals in sediments is influenced by the sediment's chemical composition; grain size and total organic matter

content [43]. The pH is an important determinant of the bioavailability of metals in sediments. A decrease in pH increases the competition between metal ions and H+ for sediment binding locations and may contribute to the dissolution of metal complexes, thereby releasing free metal ions into the water column [44]. Higher toxic heavy metals concentrations in riverine sediments can pose an ecological risk to benthos (bottom-dwelling organisms) [45].

#### **3.3 Soils**

Heavy metals and metalloids are released into soils from activities and sources such as manufacturing activities, mine tailings, high metal waste disposal, leaded gasoline and paints, fertilizer land application, animal manures, sewage sludge, pesticides, irrigation of waste water, residues of coal combustion and petrochemical spillage, resulting in soil contamination by heavy metals [46]. Most HMs does not typically experience microbial or chemical degradation and thus, after being released to the atmosphere, their total concentrations last in the sail for a long time. The composition of the parent rock, the degree of weathering and physical, chemical and biological characteristics of soil and environment conditions are factors influencing the presence and distribution of heavy metals in soils [47]. Compared to virgin soils and soils with low inputs, substantial heavy metal enrichment has been recorded in soils receiving more fertilizer input and Cu fungicide [48]. Soils may be polluted with heavy metals from heavy vehicular activity on roads in urban areas. In urban areas, soil samples have elevated levels of Pb, of which 45–85% is bioaccessible [49]. The bioavailability of heavy metals in soils is of great importance for their environmental fate and for their plant uptake. Different HMs has different soil bioavailability and this bioavailability depends on the speciation of metals and the different soil physicochemical characteristics.

#### **3.4 Fish**

Aquatic biota is exposed to heavy metals by water, sediments and food on various routes [50]. Different toxic HMs released to freshwater bodies from various natural and anthropogenic sources are introduced to freshwater fish. Heavy metal pollution of fish has become a major global concern because it poses a danger to fish and poses health hazards to buyers of fish [51]. Assessment of the bioaccumulation of HMs in fish species from various aquatic ecosystems is very significant [52]. Assessing the amount of heavy metals in fish tissues is important for the conservation of marine environments and the human consumption of fish [53]. There are high levels of unsaturated fatty acids and low cholesterol levels in fish. They are a major protein source [54].

It is advantageous to use edible fish in human diets and is also recommended in healthy diets. Contamination of fish by toxic heavy metals is considered a risk to human health and has raised concerns about their consumption, especially among more vulnerable groups of people, such as women, children and people at risk of other diseases.

Heavy metal bioaccumulation in freshwater fish depends on different factors, including the characteristics of the fish and the external environmental factors. Fish-related factors include fish age, size (weight and length), feeding habits and physiology of the body, while external environmental factors include water column metal concentration and bioavailability, water physicochemical properties and other climatic factors. Depending on the structure and function of the tissues, the degree of accumulation of heavy metals in the various tissues of fish is usually different. Metabolically active tissues such as the gills, liver and kidneys typically

*Environmental Pollution with Heavy Metals: A Public Health Concern DOI: http://dx.doi.org/10.5772/intechopen.96805*

have higher heavy metal accumulations than other tissues such as the skin and muscles. The comparatively higher accumulation of heavy metals in metabolically active fish tissues is normally explained by the induction/occurrence in these tissues of metal-binding proteins called metallothioneins (MTs) upon exposure to heavy metals. Fish gills have been observed as the target tissue for heavy metals such as Ni to accumulate and eliminate [38]. While fish muscles are the tissue of poor heavy metal accumulation [55], from a human consumption point of view, they are essential. Trace metal bioaccumulation in fish muscles is typically species-specific [56].

The bioaccumulation of toxic HMs in freshwater fish has significant environmental, ecological and social consequences; it affects the carnivorous species and human by eating fish [57, 58]. Waterborne HMs are absorbed into fish and penetrate the human body through the food chain, thereby impacting human health [59]. In addition, poisonous HMs affects the health and well-being of fish as well.
