Heavy metals are metals of relatively high density (specific
gravity greater than about 5), e.g. lead (Pb), cadmium (Cd),
mercury (Hg), chromium (Cr).
Heavy metals are characterised by being elements, which
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cannot be transformed to other less harmful substances,
which is the case for many other priority soil and groundwater
heavy metals enter the soil environment through a number
of diffuse and point sources. Diffuse sources include atmospheric
deposition from dust, particle and smoke spreading from
industry and transport activities, use of nutrients and
pesticides in agriculture, sludge amendment, use of residual
products, etc. Point sources include various types of industrial
land use such as textile and dye production, pulp and paper,
electroplating, iron and steel works, etc.
In most cases, heavy metals are mainly found in the upper
layers of the soil and will not be considered a threat to groundwater
aquifers, since they are not very mobile in the soil environment.