SOIL CONTAMINATION AND RISK ASSESSMENT IN AREAS COVERED BY SHALE BEDROCK IN PARTS OF SOUTH EASTERN NIGERIA

This investigation aimed to evaluate the concentrations of metals in agricultural soils and vegetables grown on soils developed from shale bedrock areas in parts of Southeastern Nigeria and also to assess the possible health risk to humans through soil and food chain exposure pathways. Soils and vegetables samples were prepared and analyzed for their major and trace elements content by Inductively – Coupled Plasma Spectrometry (ICP-AES and ICP-MS). The results obtained showed that the mean concentrations of Ca, Mg, Na, K, Co, Cu, Fe, Pb and Zn for soils are above the ranges set for agricultural soils in Nigeria and world average value. The calculated enrichment factor of soil showed that the soils are highly enriched with Cu, Pb and Zn relative to average background values of soils in Nigeria and elsewhere, with 21.31% of the sites in the class of very high enrichment. The mean transfer factor for potential toxic elements such as Cu, Pb and Zn were less than 1 and are in the order of Zn>Cu>Pb indicating that Zn is more bioavailable to plants than other elements.

Based on Principal Component Analysis (PCA), PC1, PC3 (surface soil) and PC1, PC2 (subsurface soil) are said to be associated with lithogenic factors derived from parent soil material, while PC 2 (surface soil) and PC3 (subsurface soil) probably originates from anthropogenic sources such as the Pb-Zn mineralization hosted by the shale bedrock. The concentration of these elements in the soils and vegetables reflect the geochemical composition of the shale bedrock in the study area. The calculated potential ecological risk value indicated that only 2.45% of the sites are in the class of great to very great risk zone, whereas 4.92% and 92.63% are in the class of considerable and low risk zones respectively. In addition, calculated composite ecological risk indices also revealed that 95.91% of the sites are in the class of low risk zone, 2.50% and 1.63% are in the class of moderate to considerable risk and very great risk zones and this is mostly attributed to Pb contamination. However, risk assessment calculated via exposure by soil ingestion and dermal contact by these toxic elements were generally low and within safe limit.