Contents lists available at ScienceDirect Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci The concentration, distribution and health risk from potentially toxic elements in the soil - plant - water system developed on black shales in SE Nigeria T.N. Nganje a,b , A. Edet a,∗ , S. Cuthbert b , C.I. Adamu a , A.S. Hursthouse b a Department of Geology, University of Calabar, P.M.B. 1115, Calabar, Nigeria b School of Science and Sport, University of the West of Scotland, Paisley, PA1 2BE, UK ARTICLEINFO Keywords: Potentially toxic elements Black shale Enrichment factor Geoaccumulation index Health risk assessment Nigeria ABSTRACT Potentially toxic elements (PTEs) are present in some natural geological materials and the black shales of southeastern Nigeria provides an example of this. The concentration and distribution of these PTEs such as Cd and Pb in shale rocks and soil-water-plant systems are spatially examined in this study. Geochemical surveys were undertaken in 6 shale formations. Some PTEs are highly enriched in the black shales (Mo, Se, Tl, and Zn) andsoils(As,Cd,Cr,CuandZn).Acknowledgedpollutionindicesrevealedvariouslevelsofsoil,waterandcrop plants contamination by PTEs. Distribution of PTEs may be attributed to diferences in geochemical nature of shale formations and degree of weathering. The concentrations may also be modifed by varying degree of pedogenetic processes that may lead to mobility and redistribution of PTEs in soil. Health risk assessment of the population, based on the Hazard quotients (HQ) and indices (HI), identifed minimal or no risk for drinking water consumption in the area. The cancer risk from As for individuals exposed through the soil and stream water pathways did not exceed the acceptable risk of 1 in 10,000 set for regulatory purposes. This study has provided data for the development, demobilization and implementation of a monitoring system and manage- ment of PTEs from shale bedrock. 1. Introduction Potentiallytoxicelements(PTEs)occurnaturallyinbedrockandare releasedintothesoilandwaterenvironmentthroughweathering(Peng etal.,2005).TheshalesofsoutheasternNigeriaprovideanexampleofa natural geologic source for PTEs. Most shale deposits of the world are enrichedwithavarietyofPTEs,especiallyMo,Zn,Ni,Cr,Cu,V,Co,Pb, UandAg(Vine and Tourtelot, 1970). In some cases, the concentrations of some PTEs are such that economic extraction is viable (Gustavson and Williams, 1981; Kelly and Taylor, 1997; Plumlee et al., 1999; Goodfellow, 2004). The weathering of shales can release PTEs into the soils where the PTEs can in turn be leached into surface water and groundwater and become available for uptake by plants. The dissolved and exchangeable forms of PTEs are usually absorbed and transferred by plants and this may accumulate to levels high enough to pose potential threat to human health from their consumption. The uptake by plants is de- terminedthroughmobilityandbioavailabilityofPTEsinthesoils(Zeng etal.,2011).Also,oxidationduringsurfaceexposureofsulfdeminerals and organic matter contained in shale can generate acidic solutions, which may be discharged into local streams and groundwater, thereby decreasingthepHofbothsurfacewaterandgroundwater(Kwongetal., 2009; Joeckel et al., 2005; Tuttle et al., 2009; Fang et al., 2001). This oxidation process releases associated metallic elements as a primary source of PTEs. The low pH in the shale system also enhances the re- lease of metal ions sorbed on mineral surfaces. Common metal oxy- hydroxides and organic matter, coupled with primary mineral dis- solution increases the total dissolved solid loading in associated surface and ground water. Due to their intrinsic toxicity and potential accu- mulation in the soil-water system, PTEs are considered an important contaminant for wider environment and human health risk assessment (Ngole-Jeme, 2016; Chiang et al., 2011). Efects of PTE in humans in- clude various forms of cancer, lung damage, kidney diseases, hy- perkeratosis, nervous disorder etc. (Chiang et al., 2011). Whilstmanystudiesreportonthemobilizationofmetalsfromshale rocksduetominingactivities(e.g. Tuttleetal.,2009),fewconsiderthe “natural” weathering process as a potential human exposure problem, particularly in developing countries, where high proportions of the https://doi.org/10.1016/j.jafrearsci.2020.103806 Received 27 February 2019; Received in revised form 22 February 2020; Accepted 24 February 2020 ∗ Corresponding author. E-mail address: aniekanedet25@gmail.com (A. Edet). Journal of African Earth Sciences 165 (2020) 103806 Available online 28 February 2020 1464-343X/ © 2020 Elsevier Ltd. All rights reserved. T