Natural and anthropogenic acidity sources for Finnish Lakes

The headwater lakes in peat-rich areas in Finland commonly have high organic matter concentrations and are surrounded by soils and bedrock poor in bases. As a result the organic acid load on the watercourses is generally high. The significance of the organic vs anthropogenic acidity in 78 moderately acid lakes (pH range 4.3 to 7.1, TOC range 0.6 to 33.9 mg L^?1) was evaluated in areas receiving different levels of acid deposition. The lakes were sampled during autumn overturn, and their water quality (mean TOC = 10.9 mg L^?1, mean pH = 5.9) represents rather well the average water quality in small lakes in Finland. According to the correlation and regression analyses, TOC is a better predictor of pH than non-marine sulfate. Base cation concentrations are important in determining the sensitivity of the lakes to organic and anthropogenic acidity. The highest minerogenic acid contribution is found in high-deposition areas, while the percent organic acid contribution is greater in low deposition areas although TOC levels are comparable in both areas. It would appear that the pH of humic lakes is determined to a greater extent by individual watershed characteristics than by their geographical location at the present atmospheric loadings received in Finland.