Heavy metals

All trace metals occur naturally in the environment, however concentrations are greatly increased where they are have been extracted and are used for industrial purposes.

Arsenic, cadmium, chromium, copper, nickel, lead and mercury are the most common heavy metal pollutants, and mercury and cadmium are of the greatest concern. The problems posed by heavy metals vary. Dissolved copper can affect lower trophic levels such as phytoplankton. Lead, cadmium and mercury become concentrated through the food-chain, posing a toxic risk to species higher in the food chain, and to humans.

Hot spots for heavy metals releases to the environment are operational and former mining sites (extractive industry q.v.), industrial production ? particular foundries and smelters, untreated sewage sludge and diffuse sources such as metal piping, traffic and combustion by-products from coal-burning power stations (other industry q.v.). Coming from such point-sources, there are normally localised hot-spots of heavy metal concentrations in the coastal zone, often near particular river-mouths. In other cases, however, heavy metals mercury are introduced into the atmosphere and thus the threat they pose occurs in locations quite remote from their point of origin. Globally, air pollution is as important as rivers in carrying copper and nickel, and it is more important for cadmium, mercury, lead and zinc. Most of the mercury found in high concentrations in the Everglades in Florida comes from thousands of miles away, travelling on trade winds from Europe and Africa. Although it appears that less mercury than previously thought is polluting Greenland, global transfers of mercury to the poles are still substantial, with base-levels three times what they were two centuries ago.