The objective of this study was to assess the seasonal contamination of the Lobo River by pesticide residues from surrounding crops. Water samples were taken eight times, in the dry and wet seasons, at the exit of agricultural plots (upstream) and in the watercourse (downstream). These samples were analyzed using a SHIMAZU liquid chromatograph. The analyses revealed the presence of nine different pesticides in the samples. Mancozeb, 2, 4-D and deltamethrin were the most detected at respective mean concentrations of 0.767 µg/L; 9.437 µg/L; 10.953 µg/L upstream and 0.326 µg/L; 8.579 µg/L; 9.160 µg/L downstream. Furthermore, the measured concentrations generally exceeded the international standards recommended by WHO, both for the total pesticides and for most individual molecules. The results also showed that agricultural activities exerted significant pressure on the water quality of the Lobo River, especially during the rainy season. Pesticides used in crops were drained by runoff into the watercourse, contaminating the aquatic ecosystem.
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