IW3 Harmful substances

>> Background information

Development

Loading of heavy metals from industrial sources to inland waters has decreased significantly during the last twenty years. This has been a result of improvements in both treatment of waste waters and industrial processes. Cadmium, mercury and lead are potentially the most harmful metals. Their development has followed the general trend with a major decrease in the late 1980s and early 1990s. Majority of the cadmium, mercury and lead loads have come from chemical and metal industries. The average loads seem to have been somewhat greater again in recent years, but the increase is mostly due to new industrial plants added to the register.

The actual concentrations of heavy metals in inland waters have remained approximately at the same level or decreased during the last decade. However, the concentrations of cadmium have increased in the beginning of the 21^st century and decreased thereafter. Cadmium as well as for example zinc and nickel have a tendency to leach into rivers from acid sulphate soils. These so called AS soils have been formed by natural processes, and are found in many parts of the western coast. Leaching of metals from these soils increases when the ground water level lowers. This may happen for example as a result of ditching for agricultural purposes. In addition, water bodies receive heavy metals from atmospheric deposition.

Impact on biodiversity

Heavy metals occur naturally in bedrock, soil and organisms in several different forms. They are released into waters as a result of both human activity and natural processes. Many heavy metals are essential trace elements for plants and animals but when a high concentration is reached they become toxic. Heavy metals typically cause deformations for zoobenthos and fish, and high concentrations may even be lethal.

Heavy metals tend to accumulate in organisms, and their concentration increases on every level of the food chain. Thus the top predators are often affected by them the most. For example seals have suffered from high heavy metal concentration of fish. It lowered the reproductive success of seals, especially in the 1960s and 1970s when the metal loads were very high. Recommendations have been given even for the people, regarding certain fish species and the amount of them which can be safely consumed.