Modeling preindustrial ANC and pH during the spring flood in northern Sweden

The natural preindustrial ANC and pH decline associated with 22 melt events from 11 streams during spring flood of 1997 and 1998 in Northern Sweden ha ve been assessed using the Boreal Dilution Model (BDM). The results show th at the spring flood pH decline of 0.5 to 2.5 pH units in the streams was la rgely caused by natural processes. The most important driving factors influ encing pH were TOC increase in combination with ANC dilution. The study als o demonstrates that pH in some streams can decline to pH values close to 4. 5 as a result of natural processes alone. In general the anthropogenic comp onent of the pH decline was between 0.1 and 0.3 pH units superimposed on th e natural pH decline. Furthermore, the anthropogenic contribution to ANC an d pH decline generally followed the gradient of anthropogenic S deposition in the region. The sites with the greatest inferred anthropogenic influence are also those for which the assumptions used in the BDM were most sensiti ve. Nevertheless, the results from this study suggest that the regional imp act of anthropogenic acid deposition on the ANC and pH in northern Sweden i s currently limited.