APPLICATION OF PRIMARY AND SECONDARY RESISTIVITY PARAMETERS IN EVALUATING AQUIFER POTENTIAL AND VULNERABILITY WITHIN KABBA, NORTH CENTRAL NIGERIA

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INTRODUCTION
Kabba is one of the fast-growing towns in Kogi State owing to already existing University of Agriculture and the newly created State-owned University.This led to rural-urban migration which brings increment in the population thereby deteriorating the available surface water in the area.The major source of water supply in the study area are surface water, rain water and groundwater.The surface water is contaminated as mentioned above and the rain water is usually seasoner making groundwater the most reliable source of providing potable water to inhabitant of the area.However, low yield and abortive boreholes has been reported to be drilled in some part of the study area which is due to lack of detail geological and geophysical knowledge.This indicates the variation in distribution of groundwater in the area.This formed the basis on which this research is carried out.Water availability has become a natural phenomenon that is of concern globally, because it is one of the indispensable and fundamental resources that sustains the livelihood of humans and animals within an environment.Water exists as groundwater (such as wells, and boreholes) and surface water (such as springs, lakes, and rivers).However, groundwater has been reported to be the major source of providing potable, uncontaminated and healthy water supply used for domestic, agricultural and industrial purposes (Obasi et al., 2022;Kizito et al., 2023b;Obaje et al., 2023).Exploration of groundwater within areas underlain by the basement rock is often difficult to carry out especially when the potential aquifer areas for groundwater are associated with fissures and fractures.As a result, the reservoir capacity of fractured crystalline basement rocks is limited, and the conductivity and transmissivity of groundwater take place along cracks and planar breaks (Adeniji et al., 2022;Simon et al., 2022;Kizito et al. 2023a).The occurrence of groundwater in basement complex is usually in closely spaced cracks and other fracture patterns characterized by large openings generated by the effect of tectonism and other related geological events.The sizes and connectivity of these fractures and fissures determines the volume of water within the basement complex.Adeniji et al. (2020) stated that the study of groundwater is extremely important because it has a great impact on a healthy and favorably quality of life.Groundwater is naturally guarded against migrating contaminants by existing subsurface structures, but in an environment where there is thin overburden; groundwater could be easily endangered by leachate contamination (Qishlaqi et al. (2018).Ebokaiwe et al. (2018) and Obasi et al. (2022) indicate that there are contaminants in surface waters, which may as well infiltrate into the groundwater system if the overlying geological materials permit their smooth transmission.Daud et al. (2017) reported that leachate is generated from landfill sites and contains high levels of contaminants and is hazardous to our ecosystems and indirectly to groundwater.Geophysical techniques have been used to resolve various exploration problems due to its ability to penetrate subsurface to a greater depth (Falae 2014, Oladunjoye et al., 2019;Mahmud et al., 2023).These methods include: Electrical resistivity Imaging, Ground penetration radar, Seismic, Electromagnetic, and magnetic.Electrical resistivity Imaging is the most used geophysical method for groundwater investigations due to its high capacity to determine hydrogeological properties like porosity, permeability, and conductivity (Helaly, 2017;Falae et al., 2019;Kizito et al., 2023a;Akpah et al., 2023).Hydraulic and electrical conductivities are physical parameters and lithological attributes that control the electric current and conduction as well as the fluid flow, hence, they are dependent on each other (George et al., 2015;Ogundana and Falae, 2023).Based on these principle, electrical resistivity technique is useful in accessing the hydrological condition of the subsurface and its aquifer protective capacity (Adeeko et al., 2019;Ogundana and Falae, 2023).
The application of Dar Zarrouk Parameters from secondary electrical resistivity data for evaluation of aquifer protective capacity or aquifer vulnerability has been carried out by Various researchers (Raji and Abdulkadir, 2020;Obasi et al., 2022;Adeniji et al., 2022;Simon et al., 2022;Kizito et al. 2023a;Ogundana and Falae, 2023).Within the study area, focus has been majorly on the study of the geology of the area (Kolawole et al., 2017;Bassey et al., 2021) which may be attributed to presence of Obajana Dangote cement company and the newly created Mangal cement and gold mineralization within the neighboring community.However, the study of groundwater potential and aquifer protective capacity is lacking.Therefore, the aim of this study is to use electrical resistivity method to investigate the groundwater potential and the protective capacity, thereby providing a comprehensive information that will serve as a guide for sustainable groundwater resources management within the study area.

Study Area Location and Geologic Setting
Kabba is one of the fast-growing towns in the western part of Kogi State, North-central Nigeria.It is situated in the basement complex of south-western Nigeria.The study area is bounded by latitude 7º 45` 00``N to 7º 52` 00``N and longitude 6º 00` 00``E to 6º 07` 36.67``Ecovering a total area of 75km2 (Figure 1).The study area is accessible through the major highway connecting Kabba-Okene and Kabba-Lokoja, minor roads and foot path.These make the study area accessible for the research purpose.Geologically, the study area lies within the Nigerian basement complex which is one of the three major litho-petrological components that make up the geology of Nigeria.Gokii et al. (2010) noted that deformation of the basement appears to be in two phases, a ductile phase which is responsible for the formation of planar structures (foliations and lineations) and a brittle phase resulting in joints and fractures that have been filled with quartzo-feldspathic veins, pegmatite, aplite, and dolerite dykes.According to Kolawole et al. (2017), the area is underlain predominantly by migmatite-schist suite comprising migmatite gneiss, migmatite schist and a quartzmica schist-quartzite complex in which quartzite occurs as elongated ridges.Bassey et al. (2021) added that, there are four major lithologic units in the study area which include; Migmatites, Granite-gneiss, Porphyritic Granite and Garnetiferous Schists.Minor rocks type include: pure quartzite, pegmatite, aplite and quartz veins.From this study as shown in Figure 1, the major rocks include; migmatitegneiss, granite-gneiss, schist, and minor occurrence of charnockite.Structural features identified are foliations, lineations, folds, joints, fractures and faults.The structures observed on the rocks in are those formed due to compressional forces resulting in ductile structures and tensional forces resulting in brittle structures.

MATERIALS AND METHODS
The DDR-3 resistivity meter was applied in this study which is capable of measuring the subsurface resistivity variation at greater depth with high accuracy and precision.Other materials used are 2 pairs of current and potential electrodes, 2 pairs of reels cable, a Global Positioning System (GPS), a Direct Current Source (Dry Cell batteries), measuring tapes and survey data sheets.Vertical electrical sounding (VES) was carried out at forty (40) locations (see figure 1) so as to obtain detail information of subsurface resistivity variation across the different major rock types within the study area.Schlumberger electrode configuration was used.This array is a reliable method for delineating horizontal layers of rocks with adequate depth sensitivity.The depth that can be penetrated by resistivity survey is roughly 1/3 of the total current electrode distance (AB) (Maiti et al., 2011).The value of current electrode spacing (AB/2) ranges from 1m to 200m while the potential electrodes range from 0.5m to 15m.The field data were converted to apparent resistivity (ρa) in ohmmeter by multiplying the resistance value with Schlumberger geometric factor (k).The apparent resistivity (ρa) values versus AB/2 were plotted manually on a logarithmic sheet of paper to obtain the apparent field curves.The number of layers with their corresponding resistivity and depth obtained from the manual plotting through partial curve matching were incorporated into a computer program with the aid of a computer software WINRESIST version 1.0.The software helps in curve smoothening and enhancement and gives corresponding resistivity, thickness, and depth of various subsurface lithology called the geoelectric layers.Aquifer resistivity and aquifer thickness are significant parameters that help in identifying the aquifer properties which are important factors in groundwater potential and vulnerability assessment.These parameters were used in deriving the secondary parameters described as Dar Zarrouk parameters.These include longitudinal conductance, transverse unit resistance, transmissivity and hydraulic conductivity.Longitudinal conductance (Lc) was calculated using Equation 1 as used by Akpan et al. (2015), Obasi et al. (2022), Kizito et al. (2023aKizito et al. ( , 2023b)).Transverse unit resistance (Tr) was obtained from equation 2 as used by Simon et al. (2022), Adeniji et al. (2022), Kizito et al. (2023aKizito et al. ( , 2023b)).Hydraulic conductivity (Hc) was calculated using Equation 3 as used by Obiora et al. ( 2016), Raji and Abdulkadir (2020), Obasi et al. (2022), Kizito et al. (2023aKizito et al. ( , 2023b)).Transmissivity (Tm) was calculated using Equation 4as used by Raji and Abdulkadir (2020), Kizito et al. (2023aKizito et al. ( , 2023b)).Where ρq and h are the aquifer resistivity and thickness respectively.
All the value of parameters obtained were used to generated contour maps showing their spatial distribution using Surfer version 25.1.229.

RESULTS AND DISCUSSION
From the VES result as shown in Table 1, the study area is characterized by four (4) and five (5) geo-electric layers with majority having five layers.These layers consist of top soil having resistivity and thickness ranges from   The aquifer thickness ranges from 2.7 m to 37.2 m with an average value of 16.4 m (Table 2).These thickness values reveal that the study area has good groundwater potential for drilling motorized boreholes or hand-dug wells, which will serve both domestic and industrial purposes.The map showing the distribution of the aquifer thickness (Figure 3b) further reveal that areas with low resistivity has higher thickness which showed relationship between the aquifer thickness and resistivity (Adeniji et al., 2022;Ogundana and Falae, 2023).

Figure 3b: Aquifer Thickness Variation Map of the Study Area
The depth to aquifer layer varies from one location to other and ranges from 6.5 m to 122.2 m with an average value of 52.94 m (Table 2).The average depth value reveals that the study area has a shallow aquifer depth which is less than 55.00 m as indicated in Figure 3c.The average depth to groundwater in this study correlate with the work of Aizebeokhai et al. (2018), Raji and Abdulkadir (2020), Kizito et al. (2023aKizito et al. ( , 2023b) ) within the basement Complex of Nigeria.Longitudinal conductance value ranges from 0.01 siemens to 2.85 siemens with an average value of 0.16 siemens (Table 2 and Figure 3d).Longitudinal conductance is used to describe the aquifer protective capacity of an area.Areas that have poor and weak longitudinal conductance are more prone to contamination, areas that are moderate are less vulnerable and areas with good protective capacity are not vulnerable to contamination from leachate and infiltration.According to Henriet (1976) and Oladapo et al. (2004) classification (Table 3) as used by other authors, the aquifer protective capacity of the study area is classified into poor, weak, moderate and good.However, majority of the area has poor to weak protective capacity as seen in Figure 3d and the mean value of longitudinal conductance indicates that the study area has moderate protective capacity and this showed that the study area is more vulnerable to contamination from leachate and infiltration.The transverse unit resistance (Table 2 and Figure 3e) value ranges from 146.06 Ωm 2 to 34141.82Ωm 2 with an average value of 7133.11Ωm 2 .The highest borehole yields usually come from the zone with the highest transverse value (Opara et al., 2012;Simon et al., 2022).Highest values are found in the southwest and part of northeast of the study area, this indicate that these areas have higher aquifer thickness.However, the mean value revealed that the study area has moderate to good to groundwater yield as indicated earlier by other parameters.The value of hydraulic conductivity ranges from 0.36 m/day to 55.54 m/day with an average value of 3.88 m/day (Table 2 and Figure 3f).Hydraulic conductivities describe the vertical movement of water in the aquifer and can be used to express aquifers potential recharge (Adeniji et al., 2022;Obasi et al., 2022).Higher hydraulic conductivity greater than 10.00 m/day is found in the central and the extreme end of the northern part while lower to moderate hydraulic conductivity values were observed in the remaining part using the classification scheme of Singhal and Gupta (1999) as in Table 4.The value of transmissivity ranges from 3.05 m 2 /day to 1266.32 m 2 /day with an average value 76.72 m 2 /day (Table 2 and Figure 3g).Aquifer transmissivity has been used as an indirect indicator of borehole yield and it describes the lateral movement of groundwater in the aquifer (Graham et al., 2009 andMacDonald et al., 2012).Based on Krasny (1993) classification of transmissivity (Table 5)

CONCLUSION
Evaluation of aquifer parameters using the geology, primary and secondary resistivity parameters was carried out within the study area with the view to assess its potential and vulnerability.Four major rocks were identified and they include: migmatite-gneiss, granite-gneiss, schist, and minor occurrence of charnockite.The geoelectric layers are made up of topsoil, lateritic clay, confining basement, weathered/fracture basement aquifers, and fresh basement.
The primary parameters (aquifer resistivity, thickness and depth) from VES result revealed that the groundwater can be classified into good, moderate and low.This was further confirmed by secondary parameters like transverse resistance, hydraulic conductivity and transmissivity and correlate with the finding of Okogbue and Omonona (2013), Raji and Abdulkadir (2020), Kizito et al. (2023a).Aquifer potential zones compared with the geology of the area revealed that migmatite gneiss, and schist have very good potential for APPLICATION OF PRIMARY AND SECOND… Hudu et al.,

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groundwater except where fractures are not well pronounced.Granite gneiss and charnockite lack pronounced fractures, but where these fractures are identified in granite gneiss, it can produce water for domestic use at low or moderate yield.Average depth to groundwater is 52.94 m indicating shallow to deeper aquifer depth within the study area.Shallow aquifer depth is distributed within the different rock types while deeper aquifer depth is found mostly in the migmatite gneiss and granite gneiss.The longitudinal conductance showed that the aquifer protective capacity of the study area varies from poor to good with majority of the study area having weak and poor protective capacity.This indicate that the groundwater is susceptible to contamination.In conclusion, the study revealed that the area requires geological and geophysical investigation for groundwater exploration especially in the granite gneiss and charnockitic rocks which are dominant towards the Northeastern and few of western parts.

Figure 1 :
Figure 1: Geology Map of the study Area Showing VES Points () =   ℎ (ℎ. 2 ) (2)   () = 386.40(  ) −0.93283 (/) (3)  () =  × ℎ ( 2 /) (4) 79.1 Ωm to 989.6 Ωm and 0.5 m to 5.4 m, lateritic clay with resistivity and thickness ranges from 7.1 Ωm to 579.3 Ωm and 2.7 m to 13.0 m, partially weathered basement has resistivity and thickness ranges from 38.1 Ωm to 8065.3 Ωm and 508 m to 98.8 m, weathered/fractured basement aquifers has resistivity and thickness ranges from 8.0 Ωm to 1773.8 Ωm and 4.7 m to 37.2 m, while fresh basement has resistivity ranges from 174.5 Ωm to 21385.6 Ωm with infinite thickness.There are two types of curves as revealed from the result, these are HA and KQ types with majority having HA curves type.KQ curves types occur only in VES 21 and VES 28.Four sample of the curve as seen in Figure2a, 2b 2c and 2d showed the basement complex signature with both decrease and increase in resistivity values.The resistivity of the aquifer layer ranges from 8.0 Ωm 1773.8Ωm with an average value of 509.17 Ωm (Table2).Resistivity ranges were used to generate the aquifer resistivity map (Figure3a) to visualize the distribution of the aquifer types within the study area.It was observed that high (> 1000 Ωm) resistivity value are concentrated in some areas underlain by migmatite gneiss and granite gneiss while low (<600 Ωm) to moderate (600-1000 Ωm) resistivity are distributed within the various rock types.This resistivity range was use to classified the groundwater potential of the area.VES locations with aquifer resistivity less than 600.0 Ωm were classified as those with good groundwater potential (part of the central towards the north); those VES locations with resistivity less than 1000 Ωm were classified as having moderate groundwater potential (few parts of the south and northeast); while areas with resistivity greater than 1000 Ωm are classified as having poor groundwater potential (southwest and some portion in the northeast.However, majority of the VES locations within the study area were classified under moderate to good groundwater potential which corroborate with the findings ofOkogbue and Omonona (2013),Raji and Abdulkadir (2020),Kizito et al. (2023a) in the southwestern basement complex.

Figure 3c :
Figure 3c: Aquifer Depth Variation Map of the Study Area

Figure 3d :
Figure 3d: Longitudinal Conductance Variation Map of the Study Area

Figure 3f :FJSFUDMA
Figure 3f: Hydraulic Conductivity Variation Map of the Study Area have an intermediate transmissivity and the remaining part of the area have lower transmissivity.Therefore, the study area has good groundwater potential.Both transmissivity and hydraulic conductivity plots showed the same spatial distribution indicating that the eastern part of the study area have highest groundwater potential.The average value of hydraulic conductivity and transmissivity from this study using resistivity data correlate with the result obtained by Okogbue and Omonona (2013) Sule and Ayenigba (2017) and Kizito et al. (2023b) using pumping test data.

Figure 3g :
Figure 3g: Transmissivity Variation Map of the Study Area