Heavy Metals
The term heavy metal refers to any metallic chemical element that has a relatively high density and is toxic or poisonous at low concentrations. Examples of heavy metals include mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl), and lead (Pb).
Heavy metals are natural components of the Earth's crust. They cannot be degraded or destroyed. To a small extent they enter our bodies via food, drinking water and air. As trace elements, some heavy metals (e.g. copper, selenium, zinc) are essential to maintain the metabolism of the human body. However, at higher concentrations they can lead to poisoning. Heavy metal poisoning could result, for instance, from drinking-water contamination (e.g. lead pipes), high ambient air concentrations near emission sources, or intake via the food chain.
Heavy metals are dangerous because they tend to bioaccumulate. Bioaccumulation means an increase in the concentration of a chemical in a biological organism over time, compared to the chemical's concentration in the environment. Compounds accumulate in living things any time they are taken up and stored faster than they are broken down (metabolized) or excreted.
Heavy metals can enter a water supply by industrial and consumer waste, or even from acidic rain breaking down soils and releasing heavy metals into streams, lakes, rivers, and groundwater.
Salinity increase: Elevated salt concentrations create increased competition
between cations and metals for binding sites. Often, metals will be driven off
into the overlying water. (Estuaries are prone to this phenomenon because of
fluctuating river flow inputs.)
Redox Potential decrease: A decreased redox potential, as is often seen under
oxygen deficient conditions, will change the composition of metal complexes
and release the metal ions into the overlying water.
pH decrease: A lower pH increases the competition between metal and hydrogen
ions for binding sites. A decrease in pH may also dissolve metal-carbonate complexes,
releasing free metal ions into the water column.
Heavy Metals and Water Quality
Absorption of toxins (chemicals, heavy metals and contaminants) may occur through the skin, by inhalation or ingestion. There are varying levels by which toxins can be absorbed, Acute dose: refers to a high content over short period of time. Chronic dose: refers to a lower content over a longer period of time.
Heavy metals:
| Metal | Source | Toxicity |
| Cadmium | industrial activities (electroplating, battery manufacture) and in some phosphate fertilisers. | accumulates in kidneys in humans and in shellfish. |
| Chromium | timber and electroplating. | very toxic to humans and aquatic life. bio-accumulates in shellfish, and is able to change to a more toxic form when oxidised. |
| Copper | timber treatment, maritime industries, industry and road run off, fungicides and piping. | very toxic to aquatic life. |
| Lead | industry and road run off | toxic to humans and aquatic life. |
| Zinc | urban road run off (tyres and braking), industry, plastics and paint. | toxic to aquatic life. |