Simulation of pesticide leaching at Vredepeel and Brimstone farm using the macropore model PLM

The distributions of bromide, bentazone and ethoprophos in the light-textured and unstructured soil at Vredepeel was simulated using the Pesticide Leaching Model (PLM). Distributions of all compounds were satisfactorily simulated using the data provided and without calibration. Since bromide is used as a tracer of water movement, this agreement indicates that PLM was able to model the movement and dispersion of water in the soil profile. However traces of bromide that remained near the soil surface were not predicted. Although PLM was designed to simulate concentrations of pesticide that reach drainage waters by preferential flow in structured soils, the model was still able to predict the distribution of the large proportion of bentazone and ethoprophos that remained in the topsoil at Vredepeel. The soil at Brimstone farm is a very highly structured, heavy, cracking-clay soil. PLM was used to simulate concentrations of isoproturon measured in samples of drainage water taken from the site. Simulated concentrations similar to those measured were obtained but only after extensive calibration of the model.