Soil contamination by heavy metals is increasing day by day. In this study, pollution indices which include geo-accumulation index (I-geo), enrichment factor (EF), environmental risk factor (ERF), contamination factor (CF), degree of contamination (CD), contamination/pollution index (C/PI) and pollution load index (PLI) among others were used. The City was divided into four zones; Northwest (NW), Northeast (NE), Southwest (SW) and Southeast (SE). The average total levels of heavy metals in soil samples analyzed in SE, SW, NE, NW and control were 143.1, 118.6, 129.1, 143.3 and 0.85 mg/kg respectively. The levels of Cd in soils from all the zones were greater than the target values. The results of the I-geo ranged between highly polluted to very highly polluted. The CF showed very high level of contamination for all the heavy metals in all the zones. The C/PI values were less than 1 (contamination range) for all the heavy metals in all zones except for Cd which was greater than 1 (pollution range) in all zones. The negative values of ERF recorded for all the metals in all zones implied that the heavy metals analyzed may present potential environmental threat. The PLI values were 331.08, 313.12, 287.54 and 313.86 for NW, NE, SW and SE respectively. This study revealed that automobile workshop is one of the major sources of anthropogenic soil pollution in Benin City and its environs. Keywords : Automobile Workshop, Heavy Metals, Pollution Indices and Soil. DOI: 10.7176/CMR/12-4-04 Publication date: April 30 th 2020

Heavy metals are non-biodegradable. They accumulate in the environment and subsequently contaminate the food chain. It is therefore essential to monitor heavy metals content in the soil, so as to prevent too much accumulation in human beings and animals through food chain. Test soil samples were collected from Nigeria Ports Authority (NPA) Expressway open dump, and background (control) soil samples were also collected at about 2 km from the open dump and spatial test samples were also collected. The physicochemical properties of the soil were determined. Tessier’s sequential extraction protocols were used to assess the geochemical forms of Cr, Pb, Zn, and Mn in the soil of the open dump. The concentrations of the heavy metals in the extracts were determined in a pre-calibrated atomic absorption spectrophotometer and they are above the background sample values. The metal assessment index (Igeo) evaluated, indicated that the soil in the vicinity of the open dump was highly polluted. The results obtained showed the mobile metal pools which are available to plants roots.

The rapid growth in industrialization and urbanization in Nigeria and its attendance environmental pollution problem, call for the need for the regular assessment of pollution prone areas particularly in commercial cities. Benin is a transitory town and a fast growing City with small-scale enterprises/artisans (such as auto repair workshops) scattered all over the city particularly along major ways in order to cater for the ever growing population and its daily traffic. In this study, Soil samples from four auto repair workshops in Benin City, Nigeria were assessed for physico-chemical properties, heavy metals (HMs), polycyclic aromatic hydrocarbon (PAH) and total petroleum hydrocarbon (TPH) using standard methods. The concentration of the metals from the different zones and depths spanned between 450-1940 mg/kg for Fe, 10.20-28.60 mg/kg for Mn, 25.80-53.60 mg/kg for Zn, 7.90-15.20 mg/kg for Cu, 2.88-7.03 mg/kg for Cd, 2.10-10.10 mg/kg for Cr, 3.15-6.52 mg/kg for Ni, 2.93-5.92 mg/kg for V and 4.74-15.5 mg/kg for Pb respectively. The total levels of PAHs and TPHs from the different zones and depth ranged 1.9988.682 and 8.869-24.714 mg/kg respectively. The studies revealed elevated levels of these pollutants.

Majority of contaminated sites in the world contain complex mixtures of heavy metals and organic contaminants from diverse natural processes and anthropogenic activities. Mixed interactions of heavy metals and organic contaminants may affect their bioavailability and accumulation in soil and biota through synergistic or antagonistic processes. Evaluation of contaminant bioavailability is a necessary component of the overall site assessment process for establishing either bioavailability-based or risk-based, site-specific remedial options. However, contemporary approaches aimed at the effective characterisation of contaminated soils for risk assessment, remedial and regulatory purposes are frequently challenged by knowledge gaps in contaminant bioavailability, mixed contaminant effects and emerging contaminants. Understanding mixed contaminant interactions at the elemental and molecular levels is, therefore, imperative not only to explain the underlying mechanisms controlling the fate and transport of these contaminants in soils, but also predict their bioavailability, ecotoxicological effects on natural communities under realistic exposure conditions and remediation endpoints. In this paper, scattered literature is harnessed to review specific soil-contaminant interactions, inter-contaminant (metal-metal, organic-organic, metal-organic) interactions and their implications for bioavailability, risk assessment and soil remediation. Key words: Heavy metals, organic contaminants, mixed contaminant interactions, co-contaminated soil, bioavailability, risk assessment, soil remediation.