Contribution of organic acids to the acidity of Finnish lakes

摘要

This study provides a comprehensive assessment of the contribution of organic carbon and organic acids to the acid-base status of Finnish lakes, summarizing empirical organic acidity measurements (from 16 lakes) combined with the Finnish Lake Survey data set (978 lakes sampled in autumn 1987). Due to the random selection of the lakes the high median total organic carbon (TOC = 12 mg l(sup -)l) and colour (100 mg l(sup -)1 Pt) can be considered representative for small lakes in Finland. Latitude and the proportion of the catchment covered by peatlands were the most important catchment variables determining TOC concentrations in the lakes. Organic carbon in Finnish lakes was dominated by organic acids, averaging about 84 % of DOC (dissolved organic carbon) with smaller amounts of neutral and basic components. In forested, peatland-dominated regions the mean annual leaching of organic acids was calculated to exceed the mean annual deposition of anthropogenic acids. The average total acidity of organic carbon (at pH values existing in natural waters) was 9.7 (mu)eq (mg DOC)(sup -)1. Each milligram of TOC per liter was estimated to lower Gran alkalinity by about 5 (mu)eq. The median acid-neutralizing capacity provided by organic anions during Gran alkalinity titrations was estimated to be 1.6 (mu)eq (mg TOC)(sup -)I. Considerable effort was devoted to the development of an empirical model for organic anion, the dissociation product of organic acids, based on fractionations of DOC and titrations of isolated organic acids. Using the Finnish Lake Survey data base the developed model well agreed with the two most widely used methods of estimating organic acidity: 1. the charge balance approach and 2. the model of Oliver et al. (1983)