Sources of cadmium, its distribution and turnover in the freshwater environment.

Cadmium enters the aquatic environment from numerous sources, e.g., via the atmosphere, from which the cadmium released by combustion, mainly of fossil fuels, is deposited. Since some fertilizers contain up to 40 mg/kg cadmium, wash-out from agricultural land is another source. Zinc ores contain up to several percent of cadmium, and there may be high concentrations in raw zinc, zinc alloys and zinc compounds, so that cadmium may enter the aquatic environment from mining residues, solid wastes and wastewater discharges. Once in the aquatic environment, cadmium is highly mobile. Its dissolved species are extremely labile, and are the first to be released, e.g., when salinity increases in estuaries. Sediments reflect the metal loading in the past, and their analysis can be the key to an understanding of the fate of anthropogenic discharges into the hydrosphere. The internal cadmium loading of aquatic systems can be estimated from the remobilization rates of cadmium from sediments.