Influence of pH-dependent sorption and transformation on simulated pesticide leaching

The leaching of a substance is influenced by its physico-chemical characteristics as well as environmental conditions. In spatially distributed modelling the influence of soil properties on the half-life and the sorption constant of the substance might become important and can be taken into account. The GeoPEARL model includes options to account for sorption and transformation being dependent on soil characteristics. Using some of these options in calculations for a herbicide with both sorption and transformation dependent on the pH of the soil, the calculated leaching from an application in spring appeared to be higher than anticipated from calculations according to the so-called paired parameter approach, in which the leaching is assessed for pairs of sorption and transformation parameters at regular pH intervals. The reason for the higher leaching was that the most critical leaching conditions were not covered by the selected pH values. A 'paired approach' might however be useful as a first tier assessment of the leaching potential. The maximum leaching is expected with the highest DegT(50)/K(om) ratio, which might be obtained from plotting this ratio against the characteristic soil property. The leaching potential of the parent was more important for the leaching of the metabolite than the leaching potential of the metabolite itself. This should be accounted for in the evaluation procedure.