Environmental geochemical signature of shale in pollution assessment of trace metals in soil and water in parts of southern Benue Trough, southeastern Nigeria

An investigation of environmental geochemistry of shale rock, soil and water in parts of Abakaliki, southern Benue Trough, Nigeria was carried out in order to evaluate the impact of shale bedrock in the soil and water environment. Shale rock, soil and water samples were collected from ten (10) locations within the study area and were analysed for major and trace elements using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). In situ parameters such as pH, in water were measured in the field. The results show that Pb is highly enriched while As. Cr, Co, Fe, Ni, Sr, V and Zn are slightly enriched in the shales relative to the global shale average. Similar geochemical trend was observed for surface and subsurface soils. Also, the mean concentrations of As, Co. Cu. Mn, Pb and Zn in soils at both depths are higher than the mean concentrations in the shale bedrock in the area of study, implying relative enrichments of these metals in the soils of the study area. In addition, the mean values of Al, Fe, Ti, Cr, Ni, Sr and V in both soil depths are lower than the values recorded in the shale bedrock. Relationship between pH and metal load suggested that the waters are dominantly near neutral to alkaline range suggesting that acidic contaminants do not affect the water quality. The mean concentrations of Al, Fe, Mn and Pb in groundwater and surface water were above the regulatory standards indicating contamination of the water. The calculated enrichment factor, anthropogenic input, pollution index for soil indicated that the soil are highly contaminated through numerous geogenic and anthropogenic sources with Pb being the most toxic metal with the shale bedrock and the mineral load contained in them as the major contributor of these contaminants.