Buffering processes in a boreal dissolved organic carbon-rich stream during experimental acidification

The role of organic acids on surface water acidity as well as their bufferi ng during anthropogenic acidification and subsequent recovery was studied i n a field experiment on a total organic carbon (TOC)-rich stream draining t he Svartberget catchment in northern Sweden. H2SO4 was added to the stream to increase SO42- concentration by 90 mu eq l(-1) for 30 h. About 60% of th e added H+ was buffered by protonation of organic acids, another 20% was bu ffered by base cations released from the surface of the stream channel and only ca. 20% of the added acid remained unbuffered. TOC concentrations (27 mg l(-1)), and site density of carboxylic groups - 8.6 mu eq (mg TOC)(-1) - remained stable during the experiment. Two models of organic acid dissocia tion (a triprotic model and a monoprotic pH-dependent pKa model) were fitte d to the experimental results. These models explained the observed variatio ns in organic anions, but the model parameters were quite different from th ose reported by studies in Northern America and Central Europe. This experi ment had substantially more buffering effect of TOC between pH 4.4 and 5.3, which is an environmentally important pH range. (C) 1999 Elsevier Science Ltd. All rights reserved.