The term “heavy metals” is used to designate a group of metals with particular characteristics. For the Rhine water quality, arsenic, lead, cadmium, chromium, copper, nickel, mercury and zinc are of particular relevance.
Heavy metals are used in many fields, particularly for metal refining to obtain the desired material properties. They adsorb to and accumulate in suspended matter. In small quantities they may be of vital importance (essential) for organisms. Even in low concentrations, many heavy metals, and this includes the essential ones, may be harmful to the health of organisms or toxic. EU environmental quality standards and target values have been developed in order to be able to assess noxious concentrations for organisms.
Even though the heavy metal concentrations in the Rhine water and in suspended matter have fallen, the contents of certain heavy metals - in particular of those of diffuse origin - continues to be too high. The environmental quality standards according to the Water Framework Directive are being exceeded in individual cases. The target values for zinc in suspended matter are still being exceeded.
The most important sources for zinc are industry, mining, burning of coal and waste as well as steel production. Galvanized sheet metal is for example used on roofs, for downpipes, gutters, chimney coverings and skylights and even for covering entire roofs. The zinc contained in the sheet metal reaches the Rhine via sewer systems and subsequent input pathways.
The main input pathways for zinc into the Rhine are treated urban wastewater and storm water overflows, combined mixed water inflows and sewer conducts not connected to the sewerage network. Regionally, surface runoff from sealed surfaces may be of great importance.