As evidenced by catastrophic cadmium and mercury poisonings in Japan, heavy metals belong to the most toxic environmental pollutants. Through the investigation of sediments, the extent, distribution, and provenance of heavy-metal contamination in rivers and lakes can be determined and traced.
Eight heavy metals (Cd, Hg, Pb, Zn, Cu, Cr, Ni, Co) in the clay fraction of sediments from major rivers within the Federal Republic of Germany (Rhine, Danube, Ems, Weser, Elbe) were determined by means of atomic adsorption spectrometry. The addition of the eight metals results in average values higher than 1,000 ppm for each river; in the Elberand Weser over 2,000 ppm are found. The zinc concentration in each river is higher than the other seven heavy metals together.
Heavy metals known for their high toxicity are most enriched—mercury, lead, and zinc by a factor of 10; cadmium by a factor of 50—compared with the natural background of these elements.
A geochemical reconnaissance survey on extreme cadmium concentrations in the sediments of the Neckar river (tributary of the Rhine) led to (a) the detection of extreme cadmium concentrations in the river water, (b) the detection of fish highly polluted with cadmium, and (c) the discovery of the source of the cadmium: a factory producing cadmium pigments.
The potential danger of the heavy-metal accumulation in river sediments lies in the possibility that, under certain circumstances (changes in eH-pH within the sediment), a dissolution or desorption might lead to a release of metals into the river water.
The mobilization of heavy metals from the suspended load and from the sediments—as is observed in rivers approaching the marine environment—could endanger marine organisms, thus negatively influencing the aquatic food chain.