Pesticides used for intensive agricultural production threaten the water resources of the French West Indies. For example, the pesticide chlordecone was used until the nineties in banana fields. Operational and simple tools are needed to assess the potential risk of pollution by pesticides. Here, we propose a method to assess the spatial variability of pollution risk on a watershed scale. This method proceeds in four steps: (1) surveying practices; (2) determining the pesticide load for each field in the watershed; (3) establishing an indicator of pollution contribution for each field based on the load of pesticide per year weighted by the toxicity and solubility of each molecule; and (4) spatializing the indicator. We applied this method to an agricultural watershed in the north of Martinique, French West Indies. The results showed detailed information of pesticide dose spread and the distribution of the pollution risk, highlighting risk areas. Specifically designed for the volcanic and tropical areas of the French West Indies, our method could be adapted by integrating other data on soil-climate conditions and the topography.