Cause of pH decline in stream water during spring melt runoff in northern Sweden

This study has sought to distinguish the anthropogenic and natural factors that drive episodic pH decline in northern Sweden. Approximately 600 stream water chemistry samples from 12 streams during the spring melt runoff of 1 997 and 1998 were collected. Although the acid deposition levels of the reg ion are relatively low (2-4 kg SO42-S.ha(-1).year(-1)), the pH decline in a ll of the almost two dozen spring melt events ranged from nearly 1 to 3 pH units. By using the sum of base cation concentration as a dilution index an d an organic acid pH model, the sources contributing to the pH decrease wer e quantified. For a majority of the spring melt events, organic acids contr ibuted over 75% of the acidity at peak runoff (minimum pH). In only three o f the monitored events was the anthropogenic SO42- contribution as high as that from natural sources. NO3- did not contribute to the pH decline during spring melt in this study. An interannual variation was observed that was probably due to a larger anthropogenic deposition load during the winter of 1997-1998 and a more rapid snowmelt during the spring of 1998.