Levels of the pesticide terbutryn in German rivers have not fallen, despite having been banned in 2003, say environmental researchers.

Terbutryn belongs to the group of triazine herbicides, which work by inhibiting photosynthesis, and were once widely used for agricultural weed control. However, due to their persistence in aquifers and consequent threat to drinking water, many triazine herbicides were banned in the 1990s. Terbutryn itself, which is slightly toxic to humans, was banned from agricultural use in Germany in 2003, but levels of the pesticide in rivers have not fallen since then, say environmental scientists.

 Kristin Quednow and Wilhelm Püttmann from J W Goethe University in Frankfurt, Germany, took water samples from four tributaries of the river Rhine in southern Germany from 2003 to 2006, and analysed them for the presence of terbutryn. Although concentrations varied significantly between sites and were generally higher in the summer, they exceeded threshold levels in more than half of the samples, said Quednow.

Quednow and Püttmann conclude that the ban has had 'no discernable influence' on terbutryn levels in rivers in the region, and suggest several reasons for this. Illegal agricultural application and run-off from farm equipment could be one factor. Another is that terbutryn is still permitted for use in garden ponds, aquaria, and in antifouling paints, said Quednow, which could explain its presence in river water in winter, when farm application typically ceases. She added, 'other sources, such as the leaching of terbutryn from river sediments and groundwater, will be the subject of further investigations.'

Kate Heppell, a hydrochemist from Queen Mary, University of London, UK, highlights the importance of this field of research. 'Identifying the provenance of different pesticides in watercourses is vital to ensure that the most appropriate management decisions are made when trying to reduce pesticide concentrations,' she said.

David Barden