A SCALED SORPTION MODEL VALIDATED AT THE COLUMN SCALE TO PREDICT CADMIUM CONTENTS IN A SPATIALLY-VARIABLE FIELD SOIL

Sorption of Cd is described with an extended Freundlich equation, that accounts explicitly for effects of pH and organic matter content. Parameters for this scaled sorption model were estimated using batch experiments with soil taken from an arable field. The sorption model explained differences in sorption between two soil samples from the same field that differed in pH and organic matter content. For validation of the sorption model predictions, a transport model was developed for cadmium in soil. Predictions of solute breakthrough were made that were in good agreement with measurements, using sorption parameter values that were derived from batch experiments that used the same background electrolyte concentration. With the validated sorption model total soil Cd contents along a transect in the field were predicted using measured values of pH, organic carbon content, and Cd concentration in soil solution. Although spatial variability of these parameters is distinct and not identical to the spatial variability of the total Cd content of the soil, model predictions showed good agreement with the observed spatial pattern. Spatial variability of pH and of organic matter content appeared to be an important factor which explained a large proportion of the heterogeneity of Cd contents in this soil. The scaled sorption model described Cd sorption well in both batch experiments, column scale experiments, and at the field scale, when all (scale dependent) relevant processes were taken into account.