Organophosphorus pesticides in water and agricultural soils of Ngalda, Yobe State, Nigeria: Distributions and risks assessment

Contamination of soil and water by pesticides is a global concern due to their toxicity and persistence in both matrices. The Concentrations of four organophosphorus pesticides (OPPs) viz. dichlorvos, diazinon, chlorpyrifos, and fenitrothion were determined in water and soil samples from Ngalda Agricultural Area, Yobe State, Nigeria. Samples collection, preparations, eextractions and clean-up were carried out using standard procedures. The concentrations (mg/kg) of the OPPs were determined across three soil depth profiles (0-10, 10- 20 and 20-30 cm). Generally, the OPPs were found in 100 % detection frequency in all profiles with the top soil (0-10 cm) having the highest concentrations. The mean residual levels (mg/kg) of the OPPs in the soil samples decreased in the order dichlorvos (3.59E+00) > fenitrothion (3.35E+00) > diazinon (2.99E+00) > chlorpyrifos (2.04E+00). The results of the water analysis showed that the concentrations of the OPPs in the top river water samples ranged 9.53E-03 - 3.12E-02 mg/L, mid 4.29E-01 - 6.64E-01 mg/L and bottom 1.07E+00 - 1.72E+00 mg/L. The mean concentrations of the OPPs in the water samples follow the decreasing order: dichlorvos (2.38E+00 mg/L) > fenitrothion (2.18E+00 mg/L) > diazinon (1.84E+00 mg/L) > chlorpyrifos (1.51E+00 mg/L). Noteworthily, the mean pesticides residue levels obtained for all the water samples were higher than the maximum residue limits (MRLs) by FAO/WHO. Compared to soil, OPPs are less frequently detected in water sources. Comparatively, the OPPs residual concentrations (2.04E+00 - 3.59E+00 mg/kg) in the soil samples were remarkably higher than in the water samples (1.51E+00 - 2.38E+00 mg/L). The incremental lifetime cancer risk (ILCR) for the detected OPPs in soil through ingestion, dermal contact and inhalation by adults and children were calculated. The total cancer risk of OPPs in the studied soils ranks high (10−3 < value ≤ 10−1) with the calculated total cancer risk values ranged 7.39E-03 - 1.30E-02 and 1.12E-02 - 1.96E-02 for adults and children respectively. Moreover, for different exposure pathways, the decreasing trend in cancer risks for both age groups was ingestion > dermal contact > inhalation.