MEASURING CATION-EXCHANGE CAPACITY AND TOTAL EXCHANGEABLE BASES IN BATCH AND FLOW EXPERIMENTS

Cation exchange capacity (CEC) recoveries from a leaching procedure ut ilizing intact soil columns were compared to extracts from disturbed s oil samples to determine the influence of macropores and preferential flow on ion exchange. Eleven soils representing eight soil series with a variety of morphological and physicochemical characteristics were u sed in the study. Leachate was introduced into duplicate undisturbed s oil columns following the 1 M NH4OAc, pH 7.0 procedure at a weight to volume ratio equivalent to that used for the disturbed soil samples. E ffluents from disturbed and undisturbed samples were collected and ana lyzed for CEC and total extractable bases. Average CEC values for the intact columns were 49.1% lower than those measured by routine analysi s. Regressional analysis indicated a significant difference between th e two methods (p < 0.1). Particle size distribution was identified thr ough multiple linear regression analysis as the most influential physi cochemical property contributing to the difference between methods. So il columns with sandier textures displayed high recovery rates attribu table to uniform porosity and low CEC. As the silt fraction increased, the difference between methods increased due to formation of macropor es and associated preferential flow through the soil matrix. However, increased clay levels allowed more thorough hydration of the matrix wh ich apparently restricted flow, increased residence time and promoted exposure of more exchange sites, thus resulting in intermediate recove ry rates. These results suggest that routine CEC measurements based on batch extractions of disturbed soil samples may overestimate ion exch ange interactions, and therefore, overestimate true contaminant sorpti on capacities of soils.