Quantifying sources of acid neutralisation capacity depression during spring flood episodes in Northern Sweden

Quantitatively separating the anthropogenic and natural contributions to acid neutralisation capacity (ANC) depression during spring flood is complicated by the many factors which contribute to this depression. Besides anthropogenic sulphate and nitrate, there are natural influences which include dilution, organic acids, sea salts, as well as strong mineral acid anions from bedrock and/or sediment. This paper presents a model for separating and quantifying the effects of dilution, organic acids and strong mineral acids on the ANC for five streams in Nothern Sweden during the spring floods of 1994 and 1995. The basis of the model is that the baseflow prior to and following the event is essentially unaffected by acid deposition and hence can act as the chemical baseline for the episode. The relative contribution of different factors to the spring flood ANC depression are calculated with reference to the dilution of a conservative hydrological tracer. The most important contributor to the ANC decline was found to be dilution of baseflow and organic acids in this remote region of Europe with relatively low acid deposition levels.