Agricultural intensification and pesticide pollution on water: cross-scale insights from a subtropical watershed

Agricultural intensification has increased pesticide use worldwide, raising concerns about their effects on aquatic ecosystems. This study assessed pesticide contamination on water quality in the San Salvador River watershed (Uruguay), a region mainly reliant on rainfed agriculture. A total of 52 surface water samples were collected from 13 sites across four seasons (2022-2023) and analyzed for 138 compounds using complementary analytical techniques. Thirty-eight pesticides (14 herbicides, 13 insecticides, and 11 fungicides) were detected, with all samples containing at least 9 of the evaluated contaminants and up to 24. Herbicides, especially glyphosate, its metabolite AMPA, metolachlor, and atrazine degradation products, were most frequently found and reached the highest concentrations and loads. Seasonal variations were observed with higher pesticide diversity in autumn and greater loads during high-flow periods. Land use analysis, based on remote sensing and evaluated across multiple spatial scales, showed that agriculture covered up to 40% of the watershed area, followed by pastures and afforestation. Statistical models revealed that pesticide concentrations and loads were positively related to agricultural land and forest plantations, and negatively related to natural forests and seeded pastures, with stronger relationships at scales beyond 200 m from the watercourse. These results demonstrate the widespread and ongoing presence of pesticides in surface waters of one of Uruguay's most farm-intensive regions, emphasizing the influence of land use at multiple spatial scales and providing essential information for pesticide management and water quality protection in South American agricultural landscapes.