Rapid industrial enterprise is the hallmark of the recent on-going economic liberalization within the world (Albanese and Cicchella, 2012; Filippelli, et al., 2012). Within the technology-driven urbanized world, unethical practices of environmental deposition culminate in varied varieties of pollutants that often cause harm to environmental systems (Massas, et al., 2013; Rapant, et al., 2011). In some areas, the environmental deposition of pollutants has reached a level that is harmful to human and different living organisms (ATSDR, 1999). This may cause serious trouble if it remains uncurbed, particularly in cities ill-equipped and having no efficient waste disposal systems or effluent treatment plants.
There are two varieties of anthropogenic contaminants; the contaminants discharged from the point and non-point sources. The point sources are those that release contaminants from a distinct geographic location, which includes the discharge of effluents from industries and domestic waste matter into rivers and their surroundings, together with several different commercial activities (Vega, et al., 2009). The non-point sources of contamination are widespread in nature. It is, therefore, difficult to trace contaminants of nonpoint sources back to their point of origin. Agricultural activities, i.e., application of fertilizers, urban run-offs and atmospheric deposition, are potential non-point contamination sources (Vega, et al., 2009).
Although PHHMs will have an effect on the health of all living organisms, a number of them are essential in trace amounts (Figueira, et al., 2002; Jeffrey, et al., 1992). There’s a broadband of safe and adequate levels of intake for trace elements. However, definitely in long-run intake and accumulation of bioavailable toxic metals (e.g., Hg, Cd, As, Pb) in organisms will cause ailments and death (Sante et al., 2009). On the opposite hand, lack of a vital micronutrient element (e.g., Se, Zn, and Cu) in an organism’s food supply may also cause ill health and death (Sante et al., 2009). The controls on the metal input to an ecological system and output through it, whether or not balanced, are inherent functions of active geo-environmental processes mediated by physical, chemical and biological factors (Gasparatos, 2013).
Water constitutes a part of important environmental, ecological and agricultural resource (ATSDR, 1999). Environmental deposition of anthropogenic chemicals majorly contributes to PHHMs water pollution (Hong and Barlett, 2008; UNEP, 2009). Piling of PHHMs will considerably make the water toxic and therefore unusable for drinking, industrial, agricultural, recreational or other purposes (Carpenter et al., 1998; Vega, et al., 2009). Toxicity effects are divided into two categories: acute and chronic effects. Acute effects appear instantly or shortly after exposure, whereas chronic effects manifests many years later and their etiological origins are usually difficult to trace (Farcas et al., 2013). Therefore, it is necessary to prevent water from degradation to ensure an adequate supply of safe water for the increasing population of the world.
Sediments are ecologically vital components of the aquatic habitat and are a reservoir of contaminants that play a major role in maintaining the trophic status of any water body (Golterman, 1967; Singh et al., 1997). Depending on the limnological conditions, the sediment can act both as source as well as a sink for the nutrients and other elements (Golterman, 1967). Singh et al (2002) reported that highly contaminated sediments are adversely affecting the ecological functioning of rivers because of heavy metal mobilization from urban areas into the biosphere. Therefore, measurements of pollutants in the water solely don\'t seem to be conclusive because of water discharge fluctuations and low resident time. Assessment of sediment for PHHMs as contaminants of surface water is vital as they have an extended residence time.
2.2 Potentially hazardous heavy metals and their effects
The term ‘‘heavy metal’’ refers to any metal and metalloid element that has a relatively high density ranging from 3.5 to 7 gcm-3 and is toxic or poisonous at low concentrations, and they include arsenic (As), cadmium (Cd), chromium (Cr), lead (Pb), mercury (Hg), copper (Cu), zinc (Zn) and others. Although ‘‘heavy metals’’ is a general term defined in the literature, it is widely documented and frequently applied to the widespread pollutants of soils and water bodies (Duffus, 2002).
PHHMs are natural constituents of the earth’s crust and are present in varying concentrations in all ecosystems (Figueira, et al., 2002). Heavy metals originate from natural processes, such as atmospheric deposition and geological weathering, and from anthropogenic sources, such as untreated domestic and industrial wastewater discharges, accidental chemical spills, direct soil waste dumping, and residues from some agricultural inputs and are present in air, sediment and water (Liu and Li, 2011; Varol, 2011). They are stable and persistent environmental contaminants (Sante, et al., 2009) since they cannot be degraded or destroyed. Therefore, they have an inclination to accumulate within the soils, seawater, freshwater, and sediments. Excessive levels of metals within the aquatic surroundings will have an effect on marine biology and create a health risk to human shoppers of marine food moreover, adverse effects on the surroundings (Muhammad et al., 2010).
PHHMs entering the aquatic systems ultimately find their way into the sediments through the processes of precipitation and sedimentation (Dekov et al., 1997; Rodríguez and Avila-Pérez, 1997). Direct discharge of wet and dry depositions of contaminants increase the concentrations of PHHMs of aquatic systems, thus resulting in their accumulation in sediments (Baruah et al., 1996). Sediments, therefore, are important sources for the assessment of contamination in rivers as they have a long residence time.
Studies in the Kenyan waters have shown the presence of heavy metals in the water bodies (Mwamburi, 2000; Ochieng et al., 2007; Ogayi, et al., 2011; Ndeda and Manohar, 2014). In particular, River Nzoia has shown the presence of heavy metals such as Cd, Cu, Zn and Pb (Ogayi et al., 2011). Major rivers that are feeding Lake Victoria, for example, Nzoia, Nyando, Kibos and Kasat have recorded high levels of Pb and Cd (Mwamburi, 2000). Metal status of Nairobi river waters and their Bioaccumulation in Labeo cylindricus studied by Budambula and Mwachiro (2005) revealed that lead, chromium, iron and manganese were above the critical limit of World Health Organization and Kenya Bureau of Standards. Ndeda and Manohar (2014) carried a study to determine levels of heavy metals in Nairobi dam. It revealed that the levels of four metals Pb, Cd, Cu, Ni are higher than WHO and KEBS maximum permissible levels for drinking water. Njuguna et al (2017) assessed macrophyte, heavy metal and nutrient concentrations in the water of the Nairobi River, Kenya and noted Cr, Pb, Fe, and Mn had concentrations exceeding the WHO permissible limit for drinking water. High levels of PHHMs in water systems pose health risks to human beings and other animals especially when the water is for drinking.
2.2.1 Arsenic
Arsenic is a group 15 (VA) elements in the periodic table that occur in large quantities in the earth\'s crust and in trace quantities in rocks, soil, water and air (WHO, 2014). Environmental arsenic mainly exists as sulphide complexes for example realgar (As2S2), or orpiment (As2S3) and iron pyrites (FeAsS) (Gorby 1988). Industrial effluents also contribute arsenic to water in some areas (WHO, 2014). Commercially, arsenic is used in alloying agents and wood preservatives. Inorganic arsenic can occur in the environment in several forms but in water, it is mostly found as trivalent arsenite (As(III)) or pentavalent arsenate (As(V)) (Bhattacharya et al. 2006).
Poisoning can arise from the ingestion of as little as 100 mg arsenic dioxide; chronic effects may result from the accumulation of arsenic compounds in the body at low intake levels. More than 130 million people from Asian countries, as well as people in parts of Argentina, Mexico, Mongolia, and Taiwan, have been exposed to arsenic contamination, at concentrations of 100 to over 2000 μg/L (WHO 2011; Bhattacharya et al. 2006).
Symptoms of exposure to high levels of arsenic may include stomach pain, vomiting, diarrhoea and impaired nerve function that may result in “pins and needles” sensation in hands and feet. Arsenic can also produce a pattern of changes in your skin, which includes darkening of wart-like growths – most frequently found on the palms or soles (Bhattacharya et al. 2006). Long-term exposure to even relatively low concentrations of arsenic in drinking water can increase one\'s risk of developing certain cancers including skin, lung and kidney and bladder cancer (Elkins and Pagnotto, 1980). Cases of skin lesions on palm and soles have been reported (Mandal ., 1998; Smith ., 2000). WHO (2011) guideline value of As is 10 μg/L.
2.2.2 Cadmium
Cadmium is an element of Group 12 (IIB) of the periodic table that co-exists as greenockite (CdS) with zinc sulphide in ores (Morrow, 2001; Warfvinge, 1998; Satarug et al., 2001). Small amounts of cadmium are found in zinc, copper, and lead ores, which is generally produced as a by-product from the smelting of these metals, particularly zinc (Satarug et al., 2001; Schutte et al., 2008). It can also be found in superphosphate fertilizers (Baum, 2002).
Cadmium is used in electroplating of other metals or alloyed with other metals to form easily fusible compounds, which are coatings for other materials, in welding and in soldering process (Satarug, 2001; Schutte et al., 2008). Some of its compounds are used in printing, textiles, television phosphors, photography, lasers, semiconductors, solar cells, dental amalgams and as pesticides (Nawrot et al., 2010; Walakira, 2011).
Cadmium exposure in the population occurs mainly through the ingestion of contaminated food, drinking water and air (Nawrot et al., 2006). Cadmium replaces zinc in the body, which is required for many enzymes including, the enzyme for the immune system, digestion and cardiovascular health (Ruviko et al., 1997). Cadmium is associated with hardening of arteries, hardening and destruction of the kidney and all the organs (Johannes et al., 2006).
Its toxicity is associated with low sperm county, impotence in men, reduced fertility in women, and increased interest in sex by replacing some of the deep feminine mineral like manganese, selenium and others (Ruviko et al., 1997; Johannes et al., 2006). At steady state, the kidney and liver have the highest concentrations of cadmium and they contain about 20 to 30 % of the body burden of the metal respectively (Schutte et al., 2008; Walakira, 2011). Heavily polluted areas such as parts of Japan, cadmium intake from food and water have been reported to be considerably greater (Al-saleh et al., 2010; Walakira, 2011). Cadmium limits Permissible in drinking water by WHO is 0.03 mg/l.
2.2.3 Chromium
Chromium is a transition element in group 6 (VIB) of the periodic table, which can display oxidation numbers from 0 to VI (Rêczajska et al., 2005). Only two of them, trivalent and hexavalent Cr, are, however, stable enough to occur in the environment (Rêczajska et al., 2005). Cr (VI) is considered the most toxic form of Cr, which usually occurs associated with oxygen as chromate (CrO42−) or dichromate (Cr2O72−) oxyanions. Cr (VI) exerts toxic effects on biological systems (Rêczajska et al., 2005). Inhalation and retention of Cr (VI) containing materials can cause perforation of the nasal septum, asthma, bronchitis, pneumonitis, and inflammation of the larynx and liver and increased incidence of bronchogenic carcinoma (Gad, 1989). Skin contact of Cr (VI) compounds can induce skin allergies, dermatitis, dermal necrosis and dermal corrosion (Lee et al., 1989).
Cr (III) is less mobile, less toxic and is mainly found bound to organic matter in soil and aquatic environments (Becquer et al., 2003). Cr (III) is considered a trace element essential for the proper functioning of living organisms. Contamination of water and sediment due to the use of Cr in various anthropomorphic activities has become a serious source of concern over the past decade (Prasad, 2008).
2.2.4 Copper
Copper is a transition element in Group 11 (IB) of the periodic table that displays four oxidation states: Cu (O), Cu (I), Cu (II), and Cu (III) (ATSDR, 2004). Copper occurs naturally in plants and animals, in rock, soil, water and sediment (Inam et al., 2012). The principal ores are the oxides, sulphides and halides. Oxides are cuprous and cupric. Among the numerous copper sulphides, the important examples include Cu2S and CuS.
Cu is an essential element for all known living organisms including humans and animals at required levels of intake but not microorganisms. The human body contains copper at the level of about 1.4 to 2.1 mg/kg of the body mass (Duruibe et al., 2007; Ravesteyn, 2009). It is a major component of the oxygen-carrying part of blood cells (ATSDR, 2004; Inam et al., 2012). Copper, along with vitamin C, is important for keeping blood vessels and skin elastic and flexible. It also helps the body to produce chemicals that regulate blood pressure, pulse and healing (Inam et al., 2012).
General symptoms of not getting enough copper in the diet include arthritis (painful swelling of the joints), anaemia (a condition in which the blood cannot supply enough oxygen to the body), and many other medical problems. High intake can cause irritation of the nose, mouth and eyes, headaches, stomach aches, dizziness, vomiting and diarrhoea. At excessive levels, it causes anaemia, liver and kidney dysfunctions and a decline in intelligence in young adolescents (ATSDR, 2004; Emelina, 2011). The WHO (2011) limits of copper in drinking water 0.05 mg/l.
3.1 Iron (Fe)
Iron is a group 9 (VIIIB) chemical element of the periodic table that is abundant as it forms most of the earth’s outer and inner core (Rana et al. 2012). Excessive iron intake enhances the incidence of carcinogen-induced mammary tumours in rats and estrogen-induced kidney tumours in Syrian hamsters. Estrogen administration increases iron accumulation in hamsters and facilitates iron uptake by cells in culture. In humans, increased body stores of iron have been shown to increase the risk of several estrogen-induced cancers (Jarup, 2003; Liehr and Jones, 2001).
Iron acts as a catalytic centre for a broad spectrum of metabolic functions. Iron is additionally an element of assorted tissue enzymes, like the cytochromes, that are vital for energy production, and enzymes necessary for system functioning. The very fact that serum copper has been found to be low in some cases of iron deficiency anaemia suggests that iron standing has an impression on copper metabolism (Michael et al., 2009).
Iron deficiency includes symptoms such as reduced resistance to infection, reduced work productivity, reduced physical fitness, weakness, fatigue, impaired cognitive function, and reduced learning ability, increased distractibility, impaired reactivity and coordination, itching, inability to regulate body temperature and eating pica (Beard, 200; Jarup, 2003).