Behaviour and fate of chemicals in aquatic systems

Behaviour and fate of an active ingredient of a herbicide

The main route of entry for pesticides (e.g. herbicides, insecticides, fungicides) into water bodies can be either surface runoff or aerial drift. Evaluation of the behaviour of pesticides in natural surface water, in particular after changed weather conditions, is difficult due to the diffuse routes of entry. In the FSA, studies of this kind are more controllable and the results are easier to interpret. In September 2002, the herbicide Metazachlor was dosed in a stream and a separate pond of the outdoor system. The half-life (i.e. the stability of the active ingredient) was more than 2 to 4 times higher under these more realistic conditions than in laboratory experiments.

The low degradation rate of Metazachlor here, in comparison to laboratory experiments at 20°C, was partly due to the low temperature prevailing in the outdoor system during the study, which took place in autumn (Fig.1). The conditions in the FSA are much closer to natural conditions, since winter rape for example is sown towards the end of August and the beginning of September and is commonly treated with Metazachlor. If Metazachlor is then discharged into surface waters by surface runoff or aerial drift, it can be assumed that the active agent will be degraded at a slow rate similar to that observed in the FSA study.