Abstract
The appearance of pesticides in ground water necessitates the accurate prediction of pesticide transport in ground water. Adsorption and desorption are major mechanisms affecting the movement of pesticides in ground water. Equilibrium and nonequilibrium (chemical and physical) adsorption expressions are reviewed with the merits and disadvantages of each enumerated. The occurrence of desorption and hysteresis of desorption are discussed. It appears that physical nonequilibrium (mass transfer and diffusion limitations) models most accurately describe and predict the pesticide transport experimentally observed. However, the governing equations are the most complex and costly to solve in terms of computer time and memory. Equilibrium adsorption expressions are more easily solved with analytical solutions available for simple conditions. The modeler must thus decide what level of sophistication of the adsorption/desorption processes is merited relative to the level of uncertainty of other soil parameters.
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