Heavy metals
In its pure form, heavy metals have a high specific weight, which is why they are referred to as heavy. Various types of heavy metals can be found in rivers and seawater: the most important ones are zinc, chromium, copper, nickel, arsenic, lead, cadmium, tin, titanium and mercury. Some of these are of essential importance als trace elements for plants and animals. Very low concentrations are found naturally in the environment. In high concentrations, heavy metal compounds are poisonous. Due to pollution, there was a period when the concentrations that were found were too high. However, the present concentrations in sea are so low that they are no longer a threat. Thanks to international agreements and changes in production processes, dumping heavy metals is presently kept relatively well within limits.
- Amounts of heavy metals in the North Sea
The large decrease in discharges of metals led to a strong decline in concentration in the waters of the North Sea as well as in the organisms during the period 1980-2000. The graph above shows that regulations to decrease the discharges of cadmium, zinc and mercury since 1975 have had success. A strong declining trend began in 1985 (coastal waters) which led to a sufficient decrease in the concentrations of these materials so that the target values were reached by 1996 (Westerschelde).
Dealing with the discharges of chromium, copper, lead and nickel did not go as smoothly. After an initial decline between 1975 and 1981, the concentrations of these materials stabilized at a level which was still too high. Only after 1996 did the average concentrations reach the target values.
