PREDICTING REMOVAL OF MAJOR SOIL CATIONS AND ANIONS DURING ACID INFILTRATION - MODEL EVALUATION

Models for the effects of acid deposition on soil acidification generally have not been adequately validated, especially under conditions of simultaneous water flow. The objective of this study was to assess the capability of a simple model for soil acidification and solute transport to describe changes in solution- and sorbed-phase compositions of acid soil subjected to inputs of H ion. Columns of Cecil series (clayey; kaolinitic, thermic Typic Kanhapludult) and Candler series (hyperthermic, uncoated Typic Quartzipsamment) soils were leached with pH 3.0, 3.7, and 4.7 solutions. Effluent was collected in fractions and analyzed for ionic composition. Evaluation of model performance was based on a comparison of soil column leachate compositions to model predictions. The model adequately described effluent pH. However, it tended to overpredict concentrations of basic cations removed by infiltration of pH 3.0 and 3.7 solutions. Conversely, the model underpredicted concentrations of Al in the effluent from columns leached with pH 4.7 acid. These discrepancies may reflect an inadequacy of simple equilibrium models for cation exchange and Al solubility. Extension of the model to include exchange kinetics improved predictions of effluent Ca and Mg concentrations. An empirical model for Al solubility better described effluent Al concentrations than did the equilibrium dissolution of gibbsite.