Organic vs. minerogenic acidity in headwater streams in Finland

This study provides an assessment of the transport of organic carbon and organic acids from typical Finnish forested catchments since the 1960's. The 21 study catchments (0.69–56 km²) are located over Finland excluding the northernmost regions. The mean annual total organic carbon (TOC) load from the catchments ranges from 2 600 to 8 800 kg/km²/a. Using a total acidity of 9.7 eq/mg DOC this would represent organic acid load of about 25–85 keq/km²/a which is higher than the mean annual deposition of strong acids in the highest deposition areas in southern Finland. The study demonstrates that organic acids dominate stream water acidity over large regions in Finland. In southern Finland minerogenic acidity is, however, more important. Organic acids exceeded mineral acids not only in autumn but also during spring snowmelt in 18 catchments. Moreover, the lowest pH values (25th percentile) were dominated by organic acids in 20 catchments. In most catchments TOC better explained pH than non-marine sulphate. In stepwise multiple regression base cations and TOC explained 67–83 % of the variation in pH for catchments divided into six groups according to season, deposition and peatland percentage; non-marine sulphate had minor contribution.