High precipitation amounts of strongly varying ionic composition make the western parts of southern Norway well-suited for studies of certain relationships between precipitation chemistry, hydrology, and runoff acidity. Ions from sea-spray are the predominant ions in precipitation in these areas. The remaining ions are almost exclusively acidifying compounds (H+, NH4+, SO42−, NO3), negatively correlated to the sea-spray ions.

Hydrochemical observations during 5 years confirm that Cl and, to a considerable extent, also SO42− move rather easily through the catchments, while NO3 is strongly adsorbed. The acid top soil tends to lead to acid surface runoff, while water penetrating deeper mineral soils becomes enriched in Ca2+ and depleted in H+, resulting in a high correlation between runoff acidity and runoff rate. The short-term variations in runoff acidity are thus governed largely by runoff rate and by atmospheric deposits of Cl and SO42−. In this particular precipitation climate this yields in fact a negative short-term correlation between precipitation acidity and runoff acidity.

A long-term acidification, indicated by declining fish populations, does probably arise from a SO42− -increase in runoff (caused by increased atmopheric deposition), and from a change in soil properties (caused by acid precipitation, and possibly also by changed vegetation).

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