Occurrence and environmental risk assessment of pesticides reveal important threats to aquatic organisms in precordilleran rivers of north-central Chile

While pesticides are essential for food production, their widespread use poses environmental risks beyond lowland areas. Recent evidence indicates that mountain ecosystems are also vulnerable due to both local agriculture and long-range atmospheric transport. This study assesses pesticide contamination and ecological risks in five mountainous agricultural watersheds of north-central Chile, where pesticides support intensive crop production. Using primarily polar organic chemical integrative samplers (POCIS), complemented by sediment samples, we found pesticides at 26 of 30 sampled sites. Detection varied by location and method. Desethylatrazine, an atrazine metabolite, was most frequently found in POCIS samples, detected at 20 sites across all watersheds. While other pesticides only occurred at few sites, their presence across multiple, geographically dispersed locations contributed to extensive ecological risk. Northern watersheds (Limarí, Choapa, Aconcagua) showed the highest ecological risks, despite lower pesticide loads, due to the presence of highly toxic insecticides. Key factors influencing pesticide distribution included water conductivity, agricultural land use, and latitude. Ecotoxicological risk assessments revealed eight pesticides exceeding high-risk thresholds for aquatic organisms—mainly insecticides and fungicides. Pyrethroids such as deltamethrin, cyfluthrin, and lambda-cyhalothrin posed severe threats to fish and invertebrates. High-risk levels were also detected in sediments, particularly in the northernmost Limarí watershed. These findings underscore the urgent need for targeted monitoring and stricter pesticide regulation in mountain freshwater ecosystems of Chile, which are vital water sources and harbor unique biodiversity. This study provides one of the first comprehensive evaluations of pesticide risks in mountainous rivers, highlighting the ecological threats from agricultural contaminants.