Acid Rain Buffering Potential in Oregon Cascade Lakes: Secondary Mineral Solubility Control of Solution Ionic Composition

摘要

Results from Nelson and Delwiche (1983) were extended to determine possible geochemical controls on the chemical composition of Oregon's Cascade lakes. Two types of chemical equilibrium relationships were developed: (1) predominance-area diagrams relating primary and secondary minerals to determine the phase most likely to be in thermodynamic equilibrium with lake water solutions and (2) chemical speciation calculations, emphasizing aluminum, for the controlling geochemical phase and lake susceptibility to acidification. Comparison of chemical compositions of eight lakes to the phase regions of predominance diagrams indicated that kaolinite was the secondary mineral phase controlling solution chemistry. Gibbsite controlled solution chemistry in one lake. Speciation calculations showed aluminum to be predominantly in hydroxide complexes with free aluminum ion concentration never exceeding 2% of monomeric aluminum. Fluoride and sulfate complexes were generally negligible. Stability indices from solution compositions gave agreement with predicted controlling secondary mineral phases from predominance-area diagrams. The Oregon Cascade lakes studied appear to be in equilibrium or derived from secondary mineral phases formed during the incongruent dissolution of parent feldspar minerals. These geochemical forms, known for their low dissolution kinetics and resistance to chemical weathering, explain the dilute solution chemistry and weak buffering capacity of the lakes and their susceptibility to acidification.