Evaluation of methods to derive pesticide degradation parameters for regulatory modelling

Models to simulate the fate of pesticides in the environment are frequently used for risk assessments within the registration process. An adequate description of pesticide degradation in soil is important to provide input for these models. Often, DT50 values (time required for 50% dissipation of the initial concentration) are used as model input, but there is no widely agreed methodology to derive DT50 values from experimental data, DT50 values are often obtained by fitting first-order kinetics to observed degradation patterns. The result depends on the handling of pesticide data (e.g. logarithmic transformation) and initial concentrations (variable or fixed). Kinetics other than first-order may be more suitable to describe the decline of measured concentrations, but the derived DT50 values are then not appropriate as input for many simulation models. Field or laboratory DT50 values can be used for modelling and this has consequences for model parameterisation. Degradation parameters derived from static laboratory experiments may not be applicable to pesticide behaviour under flow conditions in the field. Several methods to simulate the fate of metabolites and to evaluate experimental data are available. The methodology used to derive model input parameters must be consistent with the approach used within the simulation model.