Mining has proven its significant role in the global economic growth. This starts from huge benefits derived by the operating companies, a source of revenue to the government and employment generation to a large number of people. However, the negative impact of mining on the environment at local, regional and global levels continues to be a general interest (Obiora, 2015). Mining is often referred to as a practice that has negative impact on environment of both extent and diversity. Tahseen (2016) listed some of these effects to include biodiversity loss, formation of sinkhole, erosion and groundwater contamination by impurities from the mining process especially in open-pit mining.
Water is natural gift given to us by nature, a basic and a non-negotiable ingredient that supports life just like air. Water is generally defined as a universal solvent because it is a valuable natural asset, an essential necessity for human, and a prime resource (National Water Policy, 1987). According to Oladejo et al., 2013; Akinrinade and Adesina, 2016; Adagunodo et al., 2018; Anomohanran et al., 2017; Emenike et al.,2017, Water is the most vital natural resource that sustains life and could be gotten from surface flow as rivers and streams, the troposphere as rain, and subsurface flow as groundwater.
Groundwater is water that occurs in saturated zone with the source mainly from atmospheric precipitation, which has percolated down into the subsurface (Obiora et al., 2015; Kwami et al., 2018). Porous and fractured rock formations (aquifer) in the subsurface abhor groundwater and are identified through scientific methods before locating and extracting groundwater (Todd, 1980; Adagunodo, 2017). Naturally, Groundwater is excellent in quality, often having nothing to do with colour, pathogens, turbidity, and can be consumed/taken directly without being treated (Jain et al., 1996). It has peculiar features which makes it appropriate for public supply and therefore an indispensable resource (Offodile, 1983).
In the Basement Complex environment in Africa, groundwater exploration is often conducted using Vertical Electrical Sounding (VES) (Olasehinde and Bayewu, 2011; Oloruntola and Adeyemi, 2014, Falade et al., 2019). The success rate achieved in the exploitation of groundwater within the basement complex terrain requires a comprehensive knowledge of the hydro-geological properties of the aquifer units with respect to how susceptible/vulnerable they are to environmental contamination/pollution.
Contamination of groundwater occurs when constituents or materials of different chemical composition and unhealthy benefit for human, comes in contact with groundwater and its pot-ability altered. The pot-ability of groundwater can be altered by leachate from waste sites, saline intrusion, mining activities, oil spillage, sewage (from underlined petroleum pipes, latrines and septic tanks) (Obiora et al., 2015).
Groundwater vulnerability by meaning is subjective and had been defined in diverse ways; according to US National Research Council (NRC, 1993); it is the propensity and likelihood for contaminants to infiltrate the groundwater system after it has been introduced at some point in the surface. Groundwater vulnerability study is based on the relationship of the subsurface characteristics and the ease of groundwater contamination through anthropogenic activities which may be damaging to the quality of the resource. Aquifer vulnerability represents the intrinsic (natural ability of geo-materials) properties of the aquifer which determine its tendency of being affected by contaminant load imposed on it. Intrinsic aquifer vulnerability describes the relative extent of natural safety of the groundwater from contamination due to the physical properties of the surface and subsurface (Jessica et al., 2011). The intrinsic vulnerability (the study focus) is recognized as the natural susceptibility to contamination based on physical parameters of the environment; in other words, the intrinsic vulnerability is the vulnerability of groundwater to contaminants/pollutants created by human activities, while considering the inherent hydrological, hydrogeological and geological properties of an area but being independent of the type of contaminants. Some of the intrinsic parameters that determine contaminants attenuation are: the thickness of the superficial deposits, the permeability and clay content of inter-granular bedrock aquifers and the depth to the water table in both superficial and inter-granular bedrock aquifers, the permeability and clay content of superficial deposits and the mode of groundwater flow in bedrock aquifers,. Vulnerability assessment is governed by the travel time of water from the surface to reach a producing aquifer and the tendency of the geo-materials (vadoze/unsaturated zone) to attenuate (filter, delay and possibly degrade by biological activity) contaminants as it is considered the first line of natural defence.
To understand the vulnerability of groundwater to contamination, the protective capacity of the aquifer must be evaluated. The measure of the protective capacity is the capability of an earth medium to impede and filter percolating fluid. However, the overburden protective capacity which is maintained by retarding and filtrating percolating pollutants is directly proportional to the thickness and inversely proportional to its hydraulic conductivity of the overburden/geo-material. Clayey material content has high protective capacity because of its characteristic low resistivity, low permeability, high longitudinal unit conductance values and low hydraulic conductivity. Hence, protective capacity is proportional to the longitudinal conductance (S). Therefore, higher longitudinal conductance of the overburden in an area leads to higher aquifer protective capacity of that area. This method thrives well when the overburden/geo-material thickness is high.
Geo-electric Layer Susceptibility Index (GLSI) (Oni et al., 2017) was used in assessing the protective capacity of an overburden. The method accounts for the effectiveness of geo material like laterite that has capacity like clay to filter and degrade contaminants due to its low permeability, but highly resistive compared to clay that exhibits low resistivity.
This study focuses on understanding how to protect groundwater resources using electrical methods rather than only detecting new groundwater resources. Electrical prospecting methods, over the years have become very effective tools for distinguishing shallow geological targets, environmental and engineering applications as fluid migration, contaminant detection, structural mapping of internal landfill, imagery of faults and engineered hydraulic barriers as well as landslide investigations. Several researchers have proved the sensitivity and efficiency of electrical resistivity method in groundwater prospection and environmental investigation (Omosuyi et al., 2008; Abiola et al., 2009; Ariyo and Adeyemi, 2009; Mbimbe, et al., 2010; Muraina et al., 2012; Ogungbemi et al., 2013; Obiora et al., 2015; Olatunji et al., 2015; Anomoharan et al., 2017; Falade et al., 2019; Akintorinwa et al., 2020).
Location and geology of study area
The research area is located at the North-Eastern part of Ijero-Ekiti, a mining site and its environment (Figures 1and 2). It is located between Latitudes 7º 49’36” N and 7º 49’53” N; Longitudes 5º 3’58” E and 5º4’19” E; with an elevation above sea level of 532m. It is accessible by road transport; a federal road network from Akure through Ado - Ekiti, the state capital.
Ijero-Ekiti lies in the Northwestern part of Ekiti State. The research area is largely characterized by the basement complex rocks of Southwestern part of Nigeria. It consists of the quartzite, migmatite gneiss, schist biotite gneiss, epidiorite, granite, calc-gneiss, biotite-schist, pegmatite and amphibole schist (Figure 3). Varied granitic rocks constitute about 25% of the whole area of Ijero-Ekiti, composed of fine-grained granite and medium to coarse-grained varieties with porphyritic texture while the low-lying charnockitic rocks are associated with coarse-grained porphyritic granites. Xenoliths are often formed in both charnockitic and granitic bodies. The quartzites occur as elongated bodies enclosed within the migmatitic, charnockitic and granitic rocks. Most exposed low-lying migmatites are characterized typically by foliation and intrusion of granitic and charnockitic rocks in some places.