Degradation of mecoprop and isoproturon in soil influence of initial concentration

Models used to describe rates of degradation are presented and exemplified, and data from mecoprop at 0.0005 to 5000 mg kg(-1) and isoproturon at 0.00 1 to 5000 mg kg(-1) were tested in the models. Degradation was described by evolution of (CO2)-C-14 from C-14 labelled pesticides incubated in soil sa mpled in plough layer and in subsurface. For mecoprop the degradation rate of 0.0005 mg kg(-1) followed first-order models in both plough layer and in subsoil. At 5 mg kg(-1) the degradation showed kinetics with exponential growth in both surface and subsoil. At 500 0 mg kg(-1) the degradation was very slow. The degradation of isoproturon at all concentrations and soil types followe d kinetics without growth of microorganisms. The model that gave the best f it for degradation of isoproturon was a three-half order model consisting o f one first-order process and one of zero-order. The rate of degradation for both pesticides and soil types was highest at t he low concentrations, whereas at 5000 mg kg(-1) the degradation was very l ow. Thus degradation appears even at concentrations near the drinking water Limit whereas the degradation at very high concentration e.g. near point s ources with pesticides may be very limited or absent.