Shabir Ahmad Dar and Aqat Javaid
Human activities such as industry, mining and sewage treatment discharges as well as electronic and agriculture wastes are some of the examples of manmade sources contributing to the higher levels of trace metals. Trace or heavy metals like cadmium, chromium, cobalt, copper, lead, iron, manganese, mercury, nickel, Uranium and are elements with an atomic weight greater than twenty. They fall under the category of harmful chemicals that are persistent, toxic and bio-accumulative, but some of them are also endocrine disrupting as well as carcinogenic.
In other words, heavy metals are those metallic chemical elements that have a relatively high density and is toxic or poisonous at low concentrations. The examples of these heavy metals are mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl), and lead (Pb). These metals are the natural components of the Earths crust. They cannot be degraded or destroyed. To a small extent they enter our bodies through food, drinking water and air. As trace elements, some heavy metals like copper, selenium, zinc etc. are essential to keep the metabolism of the human body. However, at higher concentrations they can cause poisoning. Heavy metal poisoning results in many problems like drinking-water contamination, high ambient air concentrations near emission sources, or intake via the food chain.
Heavy metals are dangerous because they are prone to bio- accumulate. Bio-accumulation means increasing the concentration of a chemical in a biological organism over time. It is the slight accumulation of substances such as pesticides or other chemicals in an organism. It occurs when organisms absorb the substances at a faster rate than the substance is lost by catabolism and excretion. Compounds accumulating in the living things are taken up at any time and stored faster than they are broken down (metabolized) or excreted.
Heavy metals can enter into a water supply by industrial and consumer waste or even from acidic rain, breaking down the soils and releasing heavy metals into streams, lakes, rivers, and ground water. Metal pollution can harm aquatic organisms through lethal and sub-lethal effects and can reduce or eliminate species from an ecosystem through increased susceptibility to fish disease, mortality and decreased fecundity. Aquatic flora and fauna such as algae, invertebrates (oysters, mussels) and fish can accumulate trace metals to several orders of magnitude (thousands to million times) above background levels in the aquatic environment.
Cultivation of crops for human or animal consumption on contaminated soil can potentially lead to uptake and accumulation of trace metals in edible plant parts with a possible risk to human and animal health. Irrigation water may transport dissolved metals to agricultural fields and thus accumulate in soils. The metal transfer from soils to plants is a key pathway of human exposure to metal contamination.
Organo- mercurial are readily absorbed into fish, therefore organic mercury in the form of methyl and ethyl mercury makes its way up the food chain through fish. The major toxic effects of methyl mercury are on the nervous system, the documented cases of mercury poisoning in Japan resulted in many deaths.
In summing up, any water contaminated with trace metals could be unsuitable for human drinking, livestock drinking, irrigation, aquatic ecosystems protection, recreation, and aquaculture Therefore, monitoring of trace metal pollutants in the aquatic environment is essential to safeguard the health of environmental water, biota, and human.
(The authors are Students of Department of Environmental Science and Technology, Central University of Punjab, Bathinda.)
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