MODELING THE INFLUENCE OF SORPTION AND TRANSFORMATION ON PESTICIDE LEACHING AND PERSISTENCE

Pesticide leaching to ground water at 1 m depth and pesticide persistence in the plow layer were calculated with a mathematical model for a sandy soil continuously cropped with maize (Zea mays L.) and exposed to weather conditions in a temperate climate. The pesticide was applied in spring. In the model, water flow was described by Darcy's law and water uptake by the crop was included. Daily averages of meteorological conditions (rainfall, evapotran-spiration, soil temperature) were used as input. The model assumes first-order transformation, equilibrium sorption (Freundlich equation), and passive plant uptake. Pesticide leaching and persistence were calculated as a function of pesticide sorption (characterized by the organic-matter/water distribution coefficient, K(om) and of transformation rate. It was found that pesticide leaching is very sensitive to both K(om) and the transformation rate: changing K(om) or the transformation rate by a factor of 2 changes the fraction of the dose leached typically by about a factor of 10. Pesticide persistence in the plow layer was found to be sensitive to K(om) at low transformation rates and sensitive to the transformation rate at high K(om) values. Additional calculations showed that autumn application results in much higher leaching of nonsorbing pesticides with short half-lives than spring application (difference of two orders of magnitude).