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

Abstract

Black shales have a great environmental geochemical significance and act as a source of metal enrichment which upon weathering could release trace elements into soil system where they are in turn mobilized and release into surface water and groundwater systems. An investigation of environmental geochemistry of rock, soil and water in parts of Abakaliki, southeastern 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 analyzed for major and trace elements using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). In situ parameters such as pH, in water was measured in the field. The results shows 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 concentration 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 concentration of Al, Fe, Mn and Pb in groundwater and surface water was above the regulatory standards indicating contamination of the water. The calculated enrichment factor, anthropogenic inputs, 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..

An integrated approach to the bioavailability, ecological, and health risk assessment of potentially toxic elements in soils within a barite mining area, SE Nigeria

“…In this study, an EF value (EF > 2) was used to estimate the anthropogenic input (AI) of trace elements; hence, the mean EF values depicted slightly to moderate (1 < AI < 50) inputs for Fe, Pb, Zn, and Cd, with Fe, and Cd showing higher enrichment. However, they appear to be negligible anthropogenic inputs for Ba and Mn, implying that these elements were mainly derived from the parent material (Nganje et al, 2021). Hence results from this study have shown that the enrichment of heavy metals in the soil is majorly influenced by anthropogenic activities and to a lesser extent by geogenic controls (from parent material).…”

“…The high enrichment of Fe in the soil is attributed to weathering of Fe-rich minerals that underlie the area. Moreover, the chemical weathering of shale is accompanied by the dissolution of clay minerals such as illite and chlorite, and Fe and Al are the common constituents of clay minerals (Reimann et al, 2003, Nganje et al, 2021. The concentration of Ba deflects from 21.34 mg/kg-1 to 65.34 mg/kg-1.…”

“…EF values ranging from 0.5 to 1.5 are an indication that the element is entire of crustal (natural) origin; however, if EF values exceed 1.5, the elements are more likely to be of anthropogenic origin (Simex & Helz, 1981). Conversely, if the concentration of heavy metals exceeds the background value (EF > 1), the enrichment may be attributed to both crustal and anthropogenic origin; hence, discriminating between the different sources may be difficult (Nganje et al, 2021). Moreover, the trace metal contents are most times influenced by the parent material with concentration sometimes recording values above the threshold.…”

“…Furthermore, the percentage mean extraction of metals using both EDTA and glycine decreased in the order Cd > Zn > Pb > Ba, with higher bioavailable/bioaccessible fractions obtained for Cd and Zn (Table 8). The high values obtained for Cd and Zn can be attributed to their high solubility and mobility potential in soil (Kabata-Pendias, 2002, 2004Nganje et al, 2020Nganje et al, , 2021, whereas Pb is known to be the least bioavailable and mobile element in most soil physicochemical conditions (Fernandez-Turiel et al, 2001).…”

“…Although some elements (e.g. Fe, Al, Cu, Mn, Zn) when found in low concentration in soil play a vital role in plants and animals metabolism, however, when found in high concentration can be considered toxic for humans, plants, and aquatic life (Adepoju-Bello et al, 2009;Yadav, 2018;Nganje et al, 2020Nganje et al, , 2021. Following their persistence in the geoenvironment and indestructible nature, most of them have the potential to pollute soils and groundwater (Pourret & Hursthouse, 2019;Pourret et al, 2021).…”