Definition and integration on regional scale of action scenarios to fight diffuse pollution in rural environnements

Abstract

A non point source pollution diagnosis method has been built and tested on Cétrais watershed (Western part of France). This method includes a description of farmers' practices for several years, the ability of soils to transfer nutrients and pesticides, and the main pathways between fields and the watershed outlet. As phosphorus was mainly provided by a little town, this diagnosis method has not been applied for phosphorus. For nitrogen, measurements in water indicate that the method seems relevant, because it describes with a relatively good precision the amounts of nitrogen flowing from fields to the water. Fertilisation reasoning appears to be the main cause for nitrogen losses, especially because of manuring methods and grasslands ploughing, which allows accumulation of delayed nitrogen release. The ability of soils to transfer nutrients will then determine the speed of leaching. For pesticides, the first investigations confirm that some ditches are able to decrease the amounts transferred from the fields to the river. The ability of a wetland located near the outlet to collect the water coming from the river has been evaluated.