Transposition of the Nitrate Directive (EC) in the Walloon Region (Belgium): establishment of the potentially leachable nitrogen references in the Walloon Region. October 10th, 2002, the Walloon Government transposed the Nitrate Directive (91/676 EC) in the Walloon right and initiated the Durable Management Programme of Nitrogen in agriculture (in french PGDA). PGDAspecifies agricultural practices ensuring the protection of water quality, develops a Quality Approach (in french DQ) and set up a framing structure ensuring the follow-up of the Directive and an assistance to the farmers. In the farms registered in DQ, five fields are sampled each year in order to establish the profile for nitric nitrogen concentration in the soil. These results are compared with Potentially Leachable Nitrogen values (in french APL) of reference (in terms of reasoned fertilization) established annually by GRENeRA and ÉCOP (UCL). These references are built on the basis of measurements carried out annually in a serie of selected points representative of the Walloon agriculture called Agricultural Surfaces Survey (in french SSA) set up by these two scientific partners of Nitrawal. Twenty five farms currently constitute this SSA in which two hundred plots are followed annually for nitric nitrogen concentration in the soil.