The context of this study is the contamination of groundwater by pesticide, and specially by atrazine (At) and one of its degradate : deethyatrazine (DEA). Because of the frequency and the high concentration of in groundwater, the application of atrazine in France in field is forbidden since 2003. Consequently, the comportment of the substitutes of atrazine in the environment have to be known. At the scale of a watershed, the objectives of this study was i) to assess the groundwater contamination risk of acetochlor (ACT), the substitute of atrazine used, and 2 of its degradates (ESA and OA), ii) to analyse the impact of the ending of atrazine application in the field since april 1999 on the water quality, iii) and to make a conceptual representation of the watershed in order to model pesticide fate. To study pesticide pollution in the entire watershed, several scales have been chosen : the experimental plot, and the entire watershed.