Relationships between Precipitation Chemistry, Hydrology, and Runoff Acidity.

Abstract

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).