Changes in base cation concentrations due to freshwater acidification

Summary

Empirical models predict the response of acidified or acidifying lakes to changes in acidic atmospheric inputs. However, it is necessary to know how the concentration of non-marine Ca plus Mg (CM*) changes in lakewater as acid inputs are changed. Two approaches have been used, based on chemical data from 719 lakes in southern Norway with similar bedrock lithology, across a gradient in acid loading, as reflected by the non-marine sulfate concentration (SO₄*) in the lake. 1, Increases in CM* from increased weathering caused by increased SO₄* should be nearly equal in lakes and drainage basins, grouped by similar elevation. Lakes with the lowest CM* at elevated SO_* in each group can have increased CM* by a maximum amount corresponding to the present concentration. A maximum increase factor of about 0.4 μeq CM*/μeq SO₄* is derived. 2, A similar value is obtained from 486 lakes with pH < 5.0, assuming that H⁺ and Al concentrations are negligible if HCO₃⁻ is present, and that an increase in SO₄* is compensated by H⁺, aluminum, and possibly increased CM*. This neglects SO₄* used to compensate for lost HCO₃⁻. Both estimates of increased CM* caused by increased SO₄* give maximum values. Effects of changed SO₄* on pH may be calculated for a maximum or minimum pH change.