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Where do metals come from?

Heavy metals in surface water systems can be from natural or anthropogenic sources. Currently, anthropogenic inputs of metals exceed natural inputs.

Natural sources:

  • Chemical and physical weathering of igneous and metamorphic rocks and soils often release heavy metals into the sediment and into the air.
  • Precipitation or atmospheric deposition of airborne particles from volcanic activity, wind erosion, forest fire smoke, and oceanic spray (Kennish, 1992).

Anthropogenic sources.

  • Surface runoff from mining operations usually has a low pH and contains high levels of metals such as iron, manganese, zinc, copper, nickel and cobalt.
  • Combustion of fossil fuels pollutes the atmosphere with metal particulates that eventually settle to the land surface.
  • Urban stormwater runoff often contains metals from roadways and parking lots, as well as atmospheric fallout
  • Domestic wastewater effluent contains metals from metabolic wastes, corrosion of water pipes, and consumer products.
  • Industrial effluents and waste sludges may substantially contribute to metal loading.

Transport in air:

Metals introduced into the atmosphere may be carried to the land surface by precipitation and dry fallout. Additionally, because metals readily sorb to many sediment types, wind-borne sediment is a significant source of metals to surface waters.

Transport in water:

The primary route for metal transport is stormwater runoff. Water transports metals that are bound to sediment and dissolved metals. Some groundwater transport of dissolved metals is possible though most dissolved metals are readily bound to soil particles.