The cation denudation rate model — Its continued validity

Abstract

The rationale of the cation denudation rate (CDR) model is reviewed, and the history of the application of the model is described. The CDR model was first applied to rivers in Nova Scotia, some of which are strongly colored and carry significant amounts of organic matter. Analytical problems encountered in strongly colored coastal rivers in the determinations of sulfate ions, and their effects upon the application of the model are discussed. The methyl thymol blue (MTB) method for sulfate incorporates part of the organic anion, here called soluble reactive organic anion, or SRA⁻. Charge balance is obtained between MTB ‘sulfate’ and major ionic species. The CDR model requires charge balance; therefore MTB ‘sulfate’ yields are used for the model. This is equivalent to using sulfate plus SRA⁻ yields for the model. Comparing the status of these rivers in the early 1970s and in the early 1980s, we find that the excess sulfate yields are lower in the latter period, and the mean pHs are generally higher. The model is used to predict the mean pH of these rivers to be expected in the absence of SRA⁻, or of excess sulfate, or of both, that is, for simple cation-bicarbonate waters.