Fate of the herbicide linuron in outdoor experimental ditches

The fate of the urea-herbicide linuron was studied in experimental ditches. Eight ditches were sprayed three times at monthly intervals, while two served as controls. As a simulation of spray drift, four doses (nominal: 0.5; 5; 15 and 50 μg.dm-3) were applied in duplicate. After each herbicide application, the ditches were kept stagnant for a week, followed by a flow period of three weeks with fresh water. No clear stratification of linuron could be found in the water compartment. During the stagnant period, half-lives for the disappearance of linuron in the water compartment ranged from 7.2 to 11.8 days. The rate of disappearance was slower in the ditches treated with the highest dose and in colder treatment periods. A maximum of 6% of the linuron dose was found in the sediment and 1% in the macrophyte compartment. Approximately 20% of the fraction in the sediment compartment was present in the pore water.