Geoengineering and Environmental Risk Assessment of The Near-Surface Geology at An Abandoned Dumpsite: A Case Study from Oluku, Ovia North-East, Southern Nigeria

Abstract

This research presents an integrated environmental and geotechnical assessment of the abandoned Oluku dumpsite in Benin City, Southern Nigeria. The investigation examined soil physicochemical properties, heavy metal concentrations, plant (Musa paradisiaca) metal uptake, groundwater quality, and geotechnical behavior, supported by risk indices and statistical analyses. Soil parameters included pH, electrical conductivity (EC), total dissolved solids (TDS), organic matter (OM), organic carbon (OC), nitrate, nitrite, phosphorus, sulphur, and particle size distribution. Heavy metals including Pb, Cd, Cr, Ni, Cu, Zn, Mn and Fewere quantified in soils, plantain leaves, and groundwater. Ecological and human-health risks were assessed using contamination factor (CF), geo-accumulation index (Igeo), enrichment factor (EF), potential ecological risk index (PERI), and transfer factor (TF). Regression analyses and a comparative temporal analysis with 2020 data were also performed. Water samples from boreholes around the dumpsite’s vicinity were assessed for physicochemical, and microbial parameters. Geotechnical parameters included Plasticity, compaction behavior, and shear strength. All measured parameters were evaluated against relevant national and international standards. The soils were predominantly loamy sand with low organic matter (2.50–5.76%) but from an engineering perspective, they exhibited clayey behavior with near-neutral pH (6.0–7.84) i.e., soils had high plasticity with presence of organic matter, conditions that can favor metal mobility and also affect engineering performance. Pbconcentrations in soils exceeded NESREA permissible limits at several sampling points, with risk indices classifying Pb contamination as significant and strongly anthropogenic in origin. Temporal comparison of soil data between 2020 and 2024 showed decreases in Cd, Ni, Cu, Zn, Mn, and Fe, indicating partial natural attenuation. However, Pb and Cr remained consistently high, suggesting persistent contamination sources. In plantain (Musa paradisiaca) leaves, Pb, Cd, Ni, Cr, Mn, and Cu were aboveFAO/WHO limits, indicating potential food-chain exposure. While Zn and Fe were within safe ranges,their marginal concentrations suggested micronutrient deficiencies. Regression analysis further revealed that Pb, Cr, and Cd jointly inhibited Fe uptake in leaves, explaining the observed chlorosis. Groundwater was acidic, poorly buffered, and variably impacted by leachate, with Pb, Fe, and Cr exceeding safe thresholds. Microbial assessment showed only low counts of heterotrophic bacteria, and no coliforms, E. coli, Staphylococcus aureus, or fungi, indicating that microbial quality met WHO and NSDWQ standards. Engineering investigations indicated particle size are fine-grained with presence of organic content having high compaction potential, with maximum dry densities of 1.46-1.57 g/cm3 at optimum moisture contents of 16.8-25.3%, and triaxial compression tests showing cohesion values between 30 to 36KN/m3 and corresponding friction angles 13.2-14.6o Based on these findings, the Oluku dumpsite poses significant environmental and engineering risks, and will require targeted remediation strategies such as land-use restrictions, phytoremediation, soil amendments, engineered capping, and continuous monitoring prior to any agricultural or structural development.