Effects of acidification on the concentrations of heavy metals in running waters in Sweden

Heavy metals in running water are analysed within the Swedish Surface Water Monitoring Programme at about 80 stations. Data for selected rivers and brooks has been used to assess the effect of soil acidification on the concentrations of metals in waters. In southern Sweden, acidification shows a significant effect on Zn and Cd. In the upper parts of the drainage areas, there is an marked increased leakage of these metals from soils to waters indicated by elevated concentrations of Zn and Cd in brooks and also by high maxima during periods of low pH values. The increased leaching from forest soils also influences the concentrations in rivers in southern Sweden. During periods of high water flow, the pH values of the rivers decreases and the concentrations of Zn and Cd increases. At pH levels of 6.2–6.5 the concentrations of these metals are mostly 2–5 times higher as compared to pH levels of about 7.0. Since high concentrations of Zn and Cd coincides with high water flow, the transport of these metals has most certainly increased several times compared to the pre-industrial period. Hence, the by far most important human impact on the loading of Zn and Cd on the marine areas, surrounding southern Sweden is the increased leakage of these metals due to acidification. In northern Sweden the effects of acidification on Zn and Cd are less evident. In general, the concentrations of these metals are lower and the connection between pH and these metals are much less significant in the rivers. The links between acidification and the concentrations of Pb and Cu in the watercourses are comparatively much weaker. These two metals are more related to the content of organic matter in the waters and a possible effect of acidification is overshadowed by natural transport processes in soils and waters.