Effect of organic anions on acid neutralizing capacity in surface waters

In order to assess the contribution of organic anions to acid neutralizing capacity, 40 Finnish lakes and 23 streams were sampled during 1991 and 1992. Based on total organic carbon (TOC), dissolved inorganic carbon (DIC), and acid neutralizing capacity (ANC) measurements, the contribution of organic anions to the measured alkalinity was estimated to be on averege 1.9 and 1.8 μeq per mg C in lake and stream water samples, respectively. The organic anion contribution to the Gran titrations varied from 0 to 71 μeq/L. The effect of organic anions on measured alkalinity was highest in samples with high TOC concentrations and relatively high pH. In most samples, calculated ANC (the difference between nonhydrogen cation and strong acid anion concentrations) was greater than the measured Gran ANC. The discrepancy ranged from 5 to 211 μeq/L and was greatest for samples with high TOC concentration.