The International Cooperative Programme on Assessment and Monitoring of Acidification of Rivers and Lakes (ICP-Waters) is designed to assess the degree and geographical extent of acidification of surface waters. During the last ten years international emission reduction measures in Europe and North America have resulted in a decrease in atmospheric sulphur depositon of up to 50%. Nitrogen deposition has stayed almost constant. Results from The Nine Year Report (Lükewille et al. 1997) show that trends in water chemistry indicate that sulphate concentrations are decreasing at almost all ICP-Waters sites, and in almost all cases the decreases in the 1990s are larger than in the 1980s. This is partly also reflected in a recovery of the invertebrate fauna at many sites. Decreasing sulphate concentrations emphasise the importance of nitrate as the second important acidifying anion.

The Norwegian Pollution Control Authority