This study proposes to evaluate the presence of the herbicide paraquat in runoff water in a catchment in the municipality of Bom Jardim – RJ, using two steps: modelling and simulation with GIS (Geographical Information System). The step of modelling was developed specifically for five standarded Wischmeier scenarios, considering the respective crops, management, soil type and the following parameters: culture interception and soil of the sorption. The slope was considered only for the simulation steps of the GIS. The results were represented as rates paraquat concentrations in runoff and afterwards validated with real data measured in the study area and analysed by EMBRAPA SOLOS. After validation, the scenario results was georeferenced as raster images using the Arcgis Software 9.2, beginning the result simulation step for the watershed. The aim is also to evaluate the influence of some parameters: paraquat interception by the crop and slop of the catchmente. The results of paraquat concentrations in the runoff can, in the future , be compared with accepted standards for environmental safety, defining thereshobes limits of paraquat use in similar watersheds. We also expect the development and application of the same modeling approach used in this study for other types of pesticides and in different watersheds.