Microbial Characterization and Public Health Risk Assessment of Heavy Metals in Agricultural Soils in Kwale, Delta State

Abstract

Introduction: the indiscriminate application of agrochemicals threatens soil biodiversity and poses significant public health risks through heavy metal accumulation. Aim: this study evaluated the impact of agrochemical inputs on soil physicochemical properties, microbial populations, and enzymatic activities, while simultaneously assessing the ecological and human health risks of heavy metals in agricultural soils in Kwale, Delta State. Duration and Place of Study: Department of Environmental Management and toxicology, Federal University of Petroleum Resources, Effurun/ 4 months. Methodology: soil samples were collected from agrochemical-polluted farm and control site. Physicochemical parameters were analyzed alongside bacteria, fungi, actinomycetes, and phosphate solubilizers’ counts. Soil health was further gauged via enzymatic assays for urease, phosphatase, and dehydrogenase activity. Ecological risks were quantified using Contamination Factors (CF), Geo-accumulation Index (Igeo), and Pollution Load Index (PLI), while human health risks were estimated using United States Environmental Protection Agency models for Non-Carcinogenic (Hazard Index, HI) and Carcinogenic Risks (CR). Results: results showed that the agrochemical-polluted soil exhibited significant acidification (pH 5.75 as against 7.00 in control) and reduced moisture content, despite higher total organic carbon (0.26%). Urease and phosphatase activities were suppressed in farm soils, indicating impaired nitrogen and phosphorus cycling; however, dehydrogenase activity was unexpectedly higher (6.11 μTPF/kg/h contrasted with 5.06 μTPF/kg/h), suggesting a stress response by resistant microbial communities. Ecological assessments identified severe heavy metal enrichment, specifically Cadmium (CF = 1266) and Zinc (CF = 15.51), classifying the site as having "very high contamination". Health risk assessments were alarming: the Hazard Index (HI) for both adults (1825.0) and children (3443.0) far exceeded the safety threshold of 1, driven primarily by Cadmium, Lead, and Nickel. Furthermore, Carcinogenic Risk (CR) for Chromium and Cadmium surpassed the acceptable limit (1x10-4), with children exhibiting significantly higher vulnerability (CR = 0.329) compared to adults. Conclusion: Probable health risk was observed as the hazard index of some metals exceeded 1. However, children were found to be more susceptible to heavy metal contamination than adults.