METAL POLLUTION

Contamination of soils, streams and groundwater with traces of metallic elements is widespread, but as these so-called heavy metals occur naturally in the rocks that are weathered to provide the parent materials for soils, all soils contain natural background levels. As a rule of thumb, any soil with metallic elements in excess of the background levels given in the Table may be considered as polluted.

Small quantities of manganese, boron and molybdenum are essential for normal plant growth, and ruminant animals require cobalt, the other metals are toxic particularly if the background concentrations are enhanced by anthropogenic sources. cadmium (Cd) concentrations in Silurian and Lower Lias black shale are abnormally high leading to teart pastures, and the use of phosphate fertilisers containing cadmium as a contaminant has led to enhanced concentrations in agricultural soils. lead (Pb) concentrations from mining and smelting, sewage sludge and vehicle exhausts are widespread, the latter particularly affecting roadside verges. All urban areas and particularly sites where metal-working has taken place have enhanced metallic concentrations. The effect of toxic metals in soils is long-lasting as the metal ions are held by the clay-humus complex and inhibit the beneficial activities of soil microfauna.

[See alsoarsenic (As), contaminated land, copper (Cu), iron (Fe), mercury (Hg), micronutrients, trace element, zinc (Zn)]