Trace metals in the environment: why they matter

Did you know that your food may contain toxic levels of trace metals or your drinking water may contaminate with toxic levels of trace metals? How can we ensure that trace metal levels stay within safe limits in our foods, in our water?

What are these trace metals? and how they enter into our environment?

Trace metals are elements such as iron (Fe), copper (Cu), zinc (Zn), chromium (Cr), cobalt (Co), nickel (Ni), Manganese (Mn), vanadium (V), and molybdenum (Mo) that are normally occur at very low concentrations in the environment. Sources of trace metals in the environment can be natural (geogenic) or anthropogenic. Natural processes include breakdown of rocks, volcanic activities, and spreading of mid-ocean ridges. Human activities including mineral resource extraction, fossil fuel combustion, industrial manufacturing, and wastewater disposal release trace metals in to the environment. Most of the usual things we use in our day to day life like oil and lubricants, cosmetics and personal care products, medicines, and fertilizers and pesticides contain trace metals and we release them into the environment as waste.

Natural Copper Ore

Some trace metals are inorganic micronutrients that are required by living things, in very low concentrations, but ingestion of, or exposure to, excessive quantities can be toxic. For example, Fe is an essential element in human blood which transports oxygen around the body. However, if too much of Fe is consumed, it can be toxic to human body. The toxicity of trace metals depends not only on the concentration in the environment, but also where it is found in the environment (water, soil, air), chemical forms of the metals (species), acidity of the environment, source of the metal (natural or anthropogenic) etc.

Trace metal contamination in aquatic ecosystems

Human activities have resulted in widespread contamination of aquatic ecosystems by a wide range of inorganic substances, including numerous trace metals (including metalloids and non-metals with metal-like properties). Trace metal contamination in aquatic environments is a ubiquitous, persistent and complex problem, often with significant consequences for ecological and human health. Anthropogenic inputs from activities including mineral resource extraction, fossil fuel combustion, industrial manufacturing and other non-point sources are overwhelmingly the primary cause of most metal contamination in aquatic ecosystems and have made metals much more bioavailable. Unlike organic contaminants, inorganic trace metal contaminants do not decompose over time into less toxic substances, but accumulate and persist in a range of chemical forms (species) with varying biological availabilities and toxicities. While some trace metals have no positive biological role (e.g. Hg, Cd, Pb, As, U), many are essential micronutrients (e.g. Cu, Zn, Fe, Ni and Cr) with a significant role in metabolic pathways, are co-factors for many enzymes and have structural functions when associated with carbohydrates, nucleic acids and proteins. Aquatic organisms have therefore evolved to take up trace metals at typical background concentrations and many have developed complex biochemical pathways. However, at elevated metal concentrations or with high proportions of bioavailable species, many organisms find it difficult to adequately regulate their tissue levels and trace metals become toxic.

Therefore, how these metals dissolved and move around in water and what other chemicals they attach to are important. Metals in aquatic systems have the tendency to bind to organic matter, or suspended particles, and thus accumulate within fine-grained sediments. However, with changing oxygen concentrations, redox conditions or pH, metals can be remobilized into pore waters and the overlying water column, and/or accumulate within benthic organisms and the associated food webs. Therefore, sediments act as long-term sinks, but also as potential sources of metal contaminants in aquatic systems.

More sources of metal poisoning:

Mercury (Hg) poisoning; Minamata disease in Japan in 1956

Cadmium (Cd) poisoning; itai-itai disease in Japan

Arsenic (As) contamination in drinking water in Bangladesh

If you like to read more about the sources, sinks, and internal cycling of trace elements in ocean please visit international GEOTRACES website

Credit: Nadeeka Rathnayake

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