Screening of Pesticide Residues in a Small Agricultural Watershed in Ogbomoso, Southwest Nigeria.

Pesticide residues in soil and water along the Oba River watershed in Ogbomoso southwest, Nigeria, were investigated to assess environmental contamination from agricultural practices. Surface water and soil samples (36 each) were collected during the dry (December 2024) and wet (April 2024) seasons from six locations, including five agricultural sites (Ladokun, Iluju Maize Farms, Iluju Irrigation Scheme, Ikose Potatoes Farms, Vegetable Farms Iluju) and control site (Yaku Ogbomoso). Samples were analyzed using Gas Chromatography-Mass Spectrometry (GC-MS) following the QuEChERS method. Semi-structured interviews with 55 farmers provided insights into pesticide use patterns. Results revealed significant contamination, with pesticide concentrations frequently exceeding the European Union Maximum Residue Limits (EU-MRL: 0.001- 0.01 mg/kg for soil, 0.1 mg/kg for water). In soil, Ladokun had the highest residues during the dry season, including Dichlorvos (1.52 ± 0.31 mg/kg), α-Lindane (1.35 ± 0.30 mg/kg), and Atrazine (1.27 ± 0.03 mg/kg). During the wet season, Ikose Potatoes Farms showed high levels of Chlorpyrifos (0.40 ± 0.08 mg/kg) and Dichlorvos (0.43 ± 0.12 mg/kg). In water, Heptachlor peaked at Ladokun (2.13 ± 0.42 mg/kg) and Carbofuran at Ikose Potatoes Farms (2.13 ± 0.32 mg/kg) during the wet season. Organochlorines (Heptachlor, Dieldrin) and organophosphates (Chlorpyrifos, Dichlorvos) dominated, reflecting intensive pesticide use and persistence. The control site showed minimal residues, confirming agricultural activities as the primary contamination source. Farmers’ limited knowledge of pesticide application, revealed through interviews, exacerbates environmental risks. These findings emphasize the need for sustainable agricultural practices, biopesticide adoption, and enhanced farmer education to mitigate ecological and health risks in the region.