Assessment of Heavy Metal Concentration in Soil Impacted Mining-Overburden in Enyigba, Abakaliki, Ebonyi State, Nigeria

This study assessed the concentration of selected heavy metals (Lead and Zinc) from surrounding soil of Royal Salt mining company, Enyigba in Abakaliki LGA, Ebonyi State. Soil samples were collected from nine (9) different locations at depths of 0-15 and 15-30cm. Two (2) samples were collected from each of the nine locations, making it total of eighteen (18) samples. Soil analysis carried out were particle size distribution, pH, available Phosphorus, total Nitrogen, Organic Carbon, organic matter, Calcium, Magnesium, Potassium, Sodium, exchangeable acidity, Aluminium, effective cation exchange capacity, base saturation and two selected heavy metals, Lead (Pb) and Zinc (Zn). Results shows that heavy metals were highest in soil around the disposal site of mine waste. The highest value of Pb and Zn was recorded on tailing down (TD) 14.82 and 25.33mg/kg at 0-15cm depth, 7.22 and 18.93mg/kg at 15-30cm depth respectively. The accumulation of Pb and Zn at the mining site (TD) may be due to mining activities, civil, industrial/anthropogenic activities going on around the site. This study recommends regular monitoring of heavy metal concentration in soil within the mining site and their disposal site in order to conform to international standards set by the WHO. DOI: https://dx.doi.org/10.4314/jasem.v24i7.7 Copyright: Copyright © 2020 Uchendu et al. This is an open access article distributed under the Creative Commons Attribution License (CCL), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dates: Received: 16 May 2020; Revised: 29 June 2020; Accepted: 07 July 2020

Mining industries throughout history has seriously caused immense environmental pollution in industrialized and developing countries and are seen as significant sources of heavy metal contamination of soil (Obiora and Chukwu, 2016). Mining activities leaves several heaps of Lead-Zinc mine waste rocks and also brings to surface large quantities of minerals that are unstable in the weathering environment. The mine itself disrupts the landscape, and the increased surface area of the broken and crushed rocks from mining and extraction set the stage for erosion and leaching of metals to the environment (Ogbonna et al., 2013). The release and contamination of soils with heavy metal through industrial effluents especially mining is of major environmental concern because the accumulated metals may adversely affect soil ecology, agricultural production, product quality, animal and human health as well as ground water quality (Adriano et al., 2001). Indeed, unlike organic contaminants, most heavy metals are not biodegradable and therefore total concentrations and ecotoxicological effects persist for very long periods after their introduction to the soil, they accumulate in organisms and cause numerous diseases and disorders (Eze and Chukwu, 2011). Over recent decades, the annual worldwide release of heavy metals reached 783,000 metric tons of Lead and 1,350,000 metric tons of Zinc (Singh et al., 2003). Because of its environmental significance, studies to determine risk caused by metal levels in soil on human health and forest ecosystems have attracted attention in recent years (Arantzazu et al., 2000;Krzyztof et al., 2004). It has been known that Lead-Zinc mining Leads to considerable heavy metal pollution in the soil of the surrounding area and thus, investigation of heavy metal content in soils and evaluation of the potential risks and human health risks from heavy metals in the waste dumpsite of this mining area are of vital importance to better understanding. Studies so far on the impacts of mining in Abakaliki Lead-Zinc mining area have focused mainly on water and plants metal load (Edeogu et al., 2007: Eze andChukwu, 2011), their health and environmental impact. The impacts of Lead-Zinc mining on plant physiology, anatomy and biochemistry are also available in local and international literatures (Ogbonna et al., 2013). The health impacts of heavy metal contamination are well documented. However, there is scarcity of literature on the impacts of Lead-Zinc waste on the fertility indices of the soil, soil microbial activity and their ecological impacts. The concentration of heavy metals in soil solution plays a critical role in controlling the availability of ions to plant. The solubility and therefore the bioavailability of heavy metal ions vary widely because many factors affecting metal availability such as soil pH, clay content and organic matter content (Davies et al., 2005;Friedland et al., 1992). It has been found that soils contaminated by heavy metals tend to exhibit reduced porosity, poor hydraulic conductivity and high rate of compaction (Udom et al., 2004). Heavy metal has been shown to affect the soil actual content, plant and soil microbial function. Therefore, it is important to assess the degree of Lead-Zinc mining waste in this study area. This study is relevant to understand the effects of such heavy metals on the fertility indices of the soil and in comparing the degree of contamination between tailing down (TD) and refuse dump site (RDS). This aim of this research is to assess the concentration of selected heavy metals (Pb, Zn) on the fertility indices of the soil from Lead-Zinc mining at Royal Salt Limited, Enyigba in Abakaliki LGA, Ebonyi State.

MATERIALS AND METHOD
The Study Area: The Abakaliki mining area lies between 06° 08' N and 06° 24' E and falls within the lower region of the Benue Trough. It includes Abakaliki town and the highly mineralized rural communities (Amagu, Ameri, Enyigba and Ameka) which are about 14 kilometres south of the metropolis (Abakaliki) (Chukwuma, 1993). The area is underlain with poorly bedded shales of the Abakaliki shale age and deposits of galena (PbS) and sphalerite (ZnS) occur as veins and lodes in the oldest exposed sequence of the Abakaliki Basin within which the highly mineralized rural community are located. The mining areas are inhabited by mostly agrarian people with artisanal Lead-Zinc mining as secondary operation. The Benue trough is one of the known major areas with Pb-Zn deposit in West Africa. Abakaliki, especially the Enyigba area which is about 14km south of Abakaliki, is overlain with tropical rocks which constitute gneisses, granites, shales, sphalerite and crustal rocks (Berti and Cunningham, 1997). The prevailing climate is laden with high rainfall, high temperature, high atmospheric humidity and precipitation usually exceeding evapotranspiration for more than half the year. The Enyigba Lead and Zinc mine was intermittently mined for Lead from 1925. Mining operations ceased due to low economic returns as well as the 1967 -1970 Nigerian Civil War that badly affected the industry. This Abakaliki Lead-Zinc mining area, comprising of Ameka, Ameri and Enyigba provide the inhabitants with economic sustenance next to agriculture. With the discovery and exploration of solid minerals in Abakaliki division of old eastern Nigeria, in the 1940s, artisan mining activities have become significant occupation among the inhabitants, to the extent that all the households within the mining areas are involved in artisanal mining.
Sample Collection and Analysis: Soil samples were collected at depth 0-15cm and 15-30cm from a mine waste dumpsite known as tailing down (TD) and a refuse dumpsite (RDS) within the premises of Royal Salt Limited (Mining Division) and a non-mining area 100 metres away from the company which was used as the control site (Ctrl) using a soil auger. Two (2) soil samples, 5 meters apart from Nine (9) different collection points were taken making it a total of 18 samples. The samples were taken to the laboratory; air dried, sieved and kept in a clean and dry place for analysis and the determination of particle size distribution, texture, some chemical properties and selected heavy metals. Soil samples were analysed in the laboratory using standard laboratory procedures for some selected physical and chemical properties as well as heavy metal concentration of Lead and Zinc.
Particle Size Distribution was determined using the hydrometer method by Bouyoucus (1962), Soil pH was determined both in water and 1N KCl in 1:2.5 soil water ratio using the glass electrode meter as described by Thomas (1996), Available Phosphorus in soil was determined using Bray-2 method as described by Bray and Kurtz (1945), Total Nitrogen in the soil sample was determined by Kjedahl digestion method as described by Bremmer and Mulvaney (1988), TEB (Calcium, Magnesium, Potassium, Sodium) were determined using 1N NH4 AOC method, The exchangeable acidity was determined by 1N KCl Extraction by (Mclean 1982), Effective cation exchange capacity was determined or calculated by the summation of total exchangeable bases (TEB) (Ca, Mg, Na, K) and Exchangeable Acidity (EA), Soil organic carbon was determined by Walkey and Black Wet -Oxidation Method as modified by Nelson and Sommers (1998), Lead and Zinc were determined using the digestive method of A.O. A.C. (2002). Data analysis on the soil physico-chemical properties, heavy metal distribution in the soil samples were subjected to analysis of variance (ANOVA), while the mean values were separated using the Least Significant Difference (LSD) at P<0.05.

RESULTS AND DISCUSSION
The mean effect of the particle size analysis of soils at different depth and location are shown in Table 1 below. The result showed that sand particles were significantly (P≤0.05) high at the Lead and Zinc tailing down (TD) with 77.7% at 0-15cm and 74.17% at 15-30cm depth, followed by 67.90% and 64.70% at 0-15cm and 15-30cm respectively..  The lowest value of sand was obtained on the sample from RDS (refuse dumpsite) with 46.90% at 0-15cm and 38.43% at 15-30cm. Silt and clay particles of the sample soil followed a reverse trend. Thus the lowest values of silt and clay were recorded on the sample from TD, 10.57% silt at 0-15cm and 11.73% at 15-30cm depth, while the highest value on silt and clay were obtained on sample from RDS 27.7% silt at 0-15cm, 25.47% silt at 15-30cm and 32.80% clay at 0-15cm and 29.13% clay at 15-30cm. The increase in sand and decrease in silt and clay particles may be attributed to the mining activities, civil and industrial anthropogenic activities around the mining site which may lead to loss of biomass from the soil surface (Pehlivan et al., (2009). The result is in agreement with Abimbola et al., 2003, where they noticed an increase in sand particles as a result of mining activities and increase in heavy metals (Lead and Zinc) deposition of soil surface. However, Ogbonna et al, 2013 reported no change on sand, silt and clay particles on the sol tested from mining site. The mean effect of the chemical properties of soils at different depth and location are shown in Table 2 below. The result showed that pH of the sampled area was generally low, showing that the soil is acidic in nature. pH value of the studied locations ranges from 5.73 -4.70. The highest pH value was recorded on RDS 5.73 at 0-15 cm followed by 5.5 from the control site while the lowest value on pH was recorded from the sample collected at TD 5.47, at 0-15cm and 4 at 15-30cm. The result shows a significant difference (P≤0.05) on the pH across the tested areas. The lower pH recorded on the mining site may be due to the long-term toxic effect of the mining activities at the area as reported by Obiora and Chukwu, (2016).
Available phosphorus (Av. P) shows a significant increase as shown on table 2. The highest value on Av. P was recorded on samples collected from refuse dumpsite RDS (25.07mg/kg) at 0-15cm followed by 24.13mg/kg from TD at 0-15cm, the lowest value of Av. P was obtained from the control site (22.03mg/kg). There is a decrease in the Av. P down the depth. Total Nitrogen values ranges from 0-178 -0.84% at 0-15cm. The result shows a significant difference (P≤0.05) in Nitrogen. Highest value on Nitrogen was obtained on the samples collected from TDS, 0.178% at 0-15cm and 0.133% at 15-30cm depth, followed by 0.129% from control site (Ctrl) at 0-15cm and 0.108% at 15-30cm depth. The lowest value was recorded on sample collected from TD 0.084% at 0-15cm and 0.061%. The decrease in Nitrogen at TD may be due to the removal of biomass and the toxicity of the heavy metal around the mining site (Leyval and Joner, 2001). Organic Carbon (OC) and Organic Matter (OM) followed the same trend, there is a significant increase (P≤0.05) on the organic carbon and organic matter content of the soils. The highest value on OC and OM were recorded on RDS (1.99%) and (3.43%) followed by Ctrl 1.47% and 2.54% at 0-15cm while the lowest value of OC and OM was obtained from RDS with OC (1.07%) and OM (1.85%  (Pb) and Zinc (Zn) contamination has negative effect on the chemical properties of soil which will in turn Lead to low fertility status of the soil around the mining site (Aydinalp and Marinova, 2003). From table 3 above, the highest value of Pb and Zn was recorded on the sample 14.82 and 25.33mg/kg at 0-15cm and 7.22 and 18.93mg/kg at 15-30cm. Followed by the sample collected from refuse dump Pb (0.19mg/kg), Zn (4.69mg/kg) at 0-15cm and 0.12 and 2.29mg/kg at 15-30cm. The accumulation of Lead and Zinc at TD may be due to mining activities, civil, industrial/anthropogenic activities going on around the site. This result is in agreement with the findings of Abimbola et al., (2003), Clark, (1992, Pehlivan et al., (2009), where they reported an increase in the Pb, Zn concentration on the soil from the mining sites. The increase recorded on the heavy metals (Pb and Zn) at the mining site is a clear indication that the soil around the Enyigba mining site is highly polluted by Lead and Zinc as a result of mining the two selected heavy metals in that area (Edeogu et al., 2007) Conclusion: This study showed that there was a significant difference in the level of Lead and Zinc content of soil from tailing down indicating that the soil is polluted due to the mining activities and disposal of mine waste. The high level of Lead and Zinc in the tailing down also affects the organic matter content of the soil hence, affecting the structural stability and productivity of the soil. This study therefore recommended the need for phytoremediation and development of an optimum land use plan for maximizing agricultural production.