Researchers collected water samples at 14 points along the river. SOS Mata Atlântica Twenty-five types of pesticides were found at least once in water samples collected at 14 points on the Tietê River during an expedition carried out by the SOS Mata Atlântica Foundation in partnership with universities and the Itaúsa Institute. The analyzes identified herbicides, fungicides and insecticides widely used in predominant crops in the Tietê basin, such as sugar cane, soybeans and citrus (orange and lemon). ?? After application to crops, only part of the pesticides reach the pests. The rest can be carried away by rain or infiltrate the soil, reaching rivers, streams and other water sources. The study, conducted by the CENA/USP Ecotoxicology Laboratory, identified the presence of atrazine, a herbicide used in Brazil to control weeds, but banned in the European Union since 2004 because of the risks to the environment and human health. In 2024, the World Health Organization (WHO) classified atrazine as a substance with the potential to cause cancer. In some stretches of the Tietê, the concentration of atrazine exceeded the limit established by Conama Resolution No. 357/2005, which defines water quality standards in Brazilian rivers. Research points to contamination in all sections of the Tietê River. The herbicides tebutiuron and clomazone, in turn, were found in all collection points along the river. The highest concentrations of these substances were recorded in the stretch between Pirapora do Bom Jesus and Barra Bonita, a region marked by intense agricultural activity. Even in the spring region, in Salesópolis, considered relatively preserved, herbicides and insecticides were identified. See below the list of pesticides found and the frequency with which they were detected in water samples collected at 14 points on the Tietê River. Tebuthiuron: 100% Clomazone: 100% Diuron: 92.86% Cyproconazole: 85.71% Hexazinone: 85.71% Atrazine: 85.71% Terbuthylazine: 85.71% Acetamiprid: 85.71% Azoxystrobin: 78.57% Ametrine: 78.57% Metalaxil-M: 71.43% Tebuconazole: 71.43% Metribuzim: 71.43% Promethrin: 64.29% Fipronil: 64.29% Imidacloprid: 57.14% Malathion: 50% Bentazone: 42.86% Thiamethoxam: 42.86% Bromacil: 28.57% Propamocarb: 21.43% Trifloxystrobin: 14.29% Fludioxonil: 14.29% Indaziflam: 7.14% Mesotrione: 7.14% Environmental and health risks According to the study, fungicides and insecticides found in the river can harm fish and other aquatic organisms. Among the impacts are changes in the functioning of the organism of these animals, changes in behavior and imbalances in the food chain. The problem can be even greater when different pesticides are present at the same time, as the combination of these substances can enhance their effects. "Although natural dilution and self-purification processes occur along the river, especially in the Middle and Lower Tietê stretches, water from these regions is used for public supply, which raises additional concerns," says the report. "This is because conventional treatment systems are not always fully effective in removing various organic contaminants, including different classes of pesticides", he adds. Tietê Expedition The expedition covered more than 1,100 kilometers of the Tietê River between June 9 and 14, 2025, from its source, in Salesópolis, to its mouth on the Paraná River, in Itapura. In addition to pesticides, the analyzes identified microplastics in all collection points and 16 substances, including medicines and illicit drugs, including cocaine. The team used boats on several sections of the expedition to collect water at the midpoint between the banks. SOS Mata Atlântica