Application of the Action Programme and impact evaluation at a catchment scale (Arquennes, Belgium). Two water catchments exploited by a water supply company (SWDE) in the rural village of Arquennes (Hainaut Province, Belgium) are contaminated by nitrate (concentrations higher than 50 mg NO3 -.l-1). Catched water is no longer used for distribution. The catchment basins (31 ha and 47 ha) are located exclusively in a cultivated area, mainly on loamy soils. Since October 2004, these basins are used as pilot areas for environmental evaluation of good agricultural practices in respect of use of nitrogen fertilizers. Farmers are supervised concerning nitrogen fertilisation with the aim to improve water quality, keeping optimum yield in the same time. This supervision fits into the general pattern of the Sustainable Nitrogen Management Program (PGDA in French), transposition of the European Nitrates Directive in Walloon legislation. Another objective of the working scheme is to study and to set up a decision-making tool concerning prevention of water contamination by agricultural nitrate around water catchments. Water and nitrate flux modelisation in both saturated and non-saturated zones is undertaken. Eight piezometers were drilled on the basins in order to describe soil, subsoil and aquifer and also to follow water table and water quality. Tracer tests in saturated and non-saturated zone were also achieved in order to determine transfer time of contaminants between soil surface and water catchments. The study shows that the impact of a modification of practices in surface can be detected in catched water quality within four or five years at the earliest. A gradual improvement in water quality is being seen in water catchments since the beginning of 2009.