Soil quality and extent of soil-plant transfer of trace metals in areas adjoining the Benin-Ethiope fluvial system in the vicinity of Sapele, Nigeria

Spillages of oil from processes of discharge of imported petroleum products and lubricating base oil and the process of production of lubricating oils can have adverse effect on water, sediments, soil, and biota of an environment. The study examined the effect of these spillages of oils on the soil by determining oil residues, physicochemical parameters of soils, and the concentrations of seven heavy metals in soil and plant of the area. Soil and plant samples were collected from areas around the Benin-Ethiope Fluvial system in the vicinity of Sapele Town and her satellite towns. Total petroleum hydrocarbons TPH and other physicochemical parameters of soil were determined by standard methods. Heavy metals in soil and plant were determined by atomic absorption spectrometry after appropriate wet acid digestion procedures. Average values of important parameters determined in soil were: pH, 6.5±1.0; TPH, 1930 mgkg; Cr, 4.4±2.1 mgkg; Cu, 7.0±4.3 mgkg; Zn, 99±63 mgkg; Pb, 35.5±9.0 mgkg; Cd, 3.3±0.9 mgkg; Ni, 7.6±5.5 mgkg and Mn, 167±170 mgkg. Results showed that the levels of pH and pollutant parameters (which include TPH and the heavy metals) were significantly higher in the study area than the control area. The average values of pH, Cd, Zn and TPH in most of the sampling stations exceeded soil guideline values (SGVs) (lower than guideline range in the case of pH). Transfer factors of most of metals indicated that they had been moderately transferred from soil to plant. Various indexes calculated in the study also showed that the soils in the study area were of low quality as a result of these pollutants.