Seasonal Variations in Physico-Chemical Properties and Heavy Metal Risk Assessment in Vegetable Farm Soils of Lagos State, Nigeria

Background and Objective: Agrochemicals are essential for farming and are a significant source of environmental heavy metals, which could be influenced by the soil’s physico-chemical parameters. Hence, seasonal variations of the physico-chemical parameters and concentrations of some heavy metals in vegetable farm sites from the Far Eastern, Eastern, and Western agricultural zones of Lagos State were evaluated. Materials and Methods: Three blocks were selected from each zone where vegetables are planted. Six sampling locations and 3 sampling points were established for each block, coupled with a control (non-farm) site per zone. Soil samples were collected from these points during the wet and dry seasons at soil depth 0-30 cm and analyzed for physico-chemical parameters using standard methods, and an Atomic Absorption Spectrophotometer was used to quantify the heavy metals. The physico-chemical parameters and heavy metals data were analyzed for mean and subjected to Analysis of Variance (ANOVA) at a p<0.05 significant level using SPSS version 26. Results: The physico-chemical properties varied significantly across the three zones in both seasons. The highest pH of 6.05 was detected in the Western zone during the dry season, and the lowest of 4.61 in the Eastern zone during the wet season. The soil texture revealed that sand is the dominant component, with a mix of silt and clay. Heavy metal concentrations showed the highest presence of Fe (6.10 mg/kg) during the dry season in the Eastern zone, while Cr (0.0008 mg/kg) had the lowest in the Western zone during the wet season. Dry season heavy metal concentrations varied significantly (p<0.05) when compared with the wet season across the zones. The non-dietary risk assessment of heavy metals showed that the Hazard Quotient (HQ) was <1 for non-carcinogenic and ≤10⁻⁶ for carcinogenic risks in children and adults across the zones. Conclusion: However, continuous monitoring of agricultural sites is still required to maintain a healthy environment that will support biota.