Impact of atmospheric long range transport of lead, mercury and cadmium on the Swedish forest environment

Abstract

Emissions of Hg, Pb, and Cd to air are transported over wide areas in Europe and deposited far away from their sources. About 80% of the atmospheric deposition of these metals in south Sweden originate from emissions in other countries. As a result of the increased anthropogenic deposition the concentrations of Hg, Pb, and Cd in the mor layer of forest soils have increased considerably, mainly during the 20th century. Although the atmospheric deposition of these elements has declined during the most recent decades, the reduction of the input of Hg and Pb is not sufficient to prevent a further accumulation. The concentrations of Hg and Pb are still increasing by ca. 0.5 and ca. 0.2% annually in the surface layer of forest soils. In contrast, the Cd concentration is currently decreasing in a large part of Sweden as a result of both deposition decreases and enhanced leaching induced by soil acidification. The accumulation factors of Hg and Pb, especially in the forest topsoils of south Sweden, are already above those at which adverse effects on soil biological processes and organisms have been demonstrated in studies of gradients from local emission sources and laboratory assessment. There are also indications of such effects at the current regional concentrations of Hg and Pb in mor layers from south Sweden, judging from observations in field and laboratory studies. There is an apparent risk of Pb induced reduction in microbial activity over parts of south Sweden. This might cause increased accumulation of organic matter and a reduced availability of soil nutrients. At current concentrations of Hg in Swedish forest soils, effects similar to those of Pb are likely. Increased concentrations of these elements in organs of mammals and birds have also been measured, though decreases have been demonstrated in recent years, related to changes in atmospheric deposition rates. As a result of current and past deposition in south Sweden, concentrations of Hg in fish have increased about fivefold during the 20th century. This implies risks for human health, when fish from inland waters are used for food. Although the concentration of Hg in fish has decreased ca. 20% during the last decade, probably as a result of the reduced deposition, the level still exceeds the general limit (0.5 mg kg-1) in about half (ca. 40 000) of the Swedish lakes. In order to reduce concentrations in fish to the level recommended, and avoid further accumulation of Hg in soils, the atmospheric deposition has to be reduced to ca. 20% of the current deposition rate. This can only be achieved by international co-operation.