Natural acidity or anthropogenic acidification in the spring flood of northern Sweden?

Abstract

Episodic pH decline associated with spring flood runoff in a typical headwater stream in the boreal forest of northern Sweden is driven almost exclusively by natural processes. Despite a drastic decline in acid neutralisation capacity (ANC), a 75-fold increase in hydrogen ion concentration and an associated increase in inorganic monomeric aluminium fractions, anthropogenic deposition (the strong acids SO 4 2− and NO 3 − associated with anthropogenic deposition) made only a minor contribution (5–8%) to the ANC and pH decline during the spring flood of 1997. Instead the ANC decline from 60 μeq. l −1 during winter baseflow to −26 μeq. l −1 at peak flow, as well as the associated pH decline from approximately 6.4–4.6 was, according to an episodic acidification model, driven almost exclusively by organic acids originating from the soil and dilution by low ionic strength snowmelt water. The natural component of ANC and pH decline during spring flood, such as that reported here, has important implications for the aquatic ecology of the boreal zone in general, and for the Swedish liming program in particular, since government subsidies are used to keep pH above 6.0 throughout spring flood, in the belief that this is the natural pH level prior to the appearance of acid rain in Europe.