Groundwater Evaluation Study Using Electrical Resistivity Measurements in Bunza Area of Kebbi State, Nigeria

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1 Article International Journal of Environment and Bioenergy, 2017, 12(2): International Journal of Environment and Bioenergy Journal homepage: ISSN: Florida, USA Groundwater Evaluation Study Using Electrical Resistivity Measurements in Bunza Area of Kebbi State, Nigeria *Umar A. B., Ladan B. and Gado A. A. Department of Physics, Kebbi State University of Science and Technology Aliero, Nigeria * Author to whom correspondence should be addressed; abdulbello2005@yahoo.com Article history: Received 19 April 2017, Revised 21 June 2017, Accepted 30 June 2017, Published 6 July Abstract: Agriculture is one of the key sectors the Nigerian government is focusing on in revatilising and diversification of the country s economy and Kebbi state is at the centre stage in rice and wheat cultivation. However, such efforts will always remain futile in the absence of sufficient water supply. Surface water is not much in the study area and the number of unsuccessful boreholes and hand dug wells is high. Geophysical methods (Schlumberger array) provides a simple and quick medium for determining the presence or otherwise of groundwater. Six VES points (VES 1-6) were analysed using the IP2Win software and VES 3 and 5 have the best aquifer and boreholes could be sited in these locations. A reliable interpretation was made by having a look at the lithological borehole descriptions and geophysical well logs within these areas of surveys. Keywords: Groundwater, resistivity and aquifers 1. Introduction In the past there has been a general assumption that groundwater is ever abundant and readily exploitable. However, recent studies have shown that special care and skill are needed for its exploration and exploitation. Even though a renewable resource, extra effort is necessary to determine the location(s) for development and the extent of the development in order to avoid over-exploitation which may lead to serious repercussions. It is therefore necessary to employ quick and efficient methods for groundwater exploration especially in these days of scarce resources. Geophysical studies provide useful information

2 101 on the possible sites for boreholes. Generally, a number of geophysical exploration techniques are available which enable an insight to be obtained rapidly into the nature of water bearing layers. These include: geo-electric, electromagnetic, seismic and geophysical borehole log techniques. The choice of a particular method is governed by the nature of the terrain and cost consideration. These methods have been used extensively in groundwater investigation, geologic mapping and engineering site investigations (Etu- Efeotor and Akpokodje, 1990; Obiakor and Chukwudebelu, 1992; Okwueze and Ezeanyim, 1985; Mbipom and Archibong, 1989). In geophysical investigations for ore prospecting, water exploration, depth to bedrock determinations, sand and gravel exploration, etc, the electrical resistivity method can be used to obtain, quickly and economically, details about the location, depth and resistivity of subsurface formations. The basis of the electrical resistivity method employs the measurement of electrical potential associated with subsurface electrical current flow generated by a Direct Current (DC) or slowly varying alternating current source (AC). Factors that affect the measured potential include the presence and quality of pore fluids and clays. The degree to which the potential at the surface is affected depends upon the size, location, shape and conductivity of the material within the ground. It is therefore possible to obtain information about the subsurface distribution of this material from the measurements of the electrical potentials made at the surface (Vingoe, 1972). Wiebeng (1955) noted that the ratio of the current applied, with ageometric factor K, which depend on the electrode separation gives the quantity termed apparent resistivity. This study describes a direct current geoelectric investigation for ground water potential undertaken. Groundwater is the largest available reservoir of fresh water. Water in rivers and lakes only account for less than 1% of the Worlds fresh water reserves. There must be space between the rock particles for groundwater to flow and the Earth s material becomes denser with more depth. Essentially, the weights of the rock above condense the rock below and squeeze out to open the pore spaces deeper in the Earth. That is why groundwater can only be found within a few kilometers of the Earth s surface. Observation shows that groundwater comes from rain, snow, sheet and hail that soak into the ground and become the ground water responsible for the spring, wells and bore holes (Oseji et al. 2005).Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of lithologic formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fracture sand voids in rock become completely saturated with water is called water table. Groundwater is often withdrawn for agricultural, municipal and industrial use by constructing and operating extraction wells. Groundwater is also widely used as a source for drinking supply and irrigation in food production (Zekster and Everett, 2004).Electrical resistivity method of geophysical techniques happens to be the most preferred method in ground water potential. Vertical Electrical Sounding (VES) is geoelectrical common method to measure vertical alterations of electrical resistivity. The method has been recognized to be more suitable for hydrogeological survey of

3 102 sedimentary basin (Kelly and Stanislav, 1993). The electrical resistivity technique involves the measurement of the apparent resistivity of soils and rock as a function of depth or position. The most common electrical technique needed in hydrogeologic and environmental investigations is vertical electrical soundings (resistivity sounding). During resistivity surveys, current is injected into the Earth through a pair of current electrodes, and the potential difference is measured between a pair of potential electrodes. The current and potential electrodes are generally arranged in a linear array. Common arrays include dipole-dipole array, pole-pole array, Schlumberger array and the Wenner array, but for the purpose of this work schlumberger array is used Electrical Resistivity The electrical resistivity of any material depends largely on its porosity and the salinity of the water in the pore spaces. Although the electrical resistivity of a material may not be diagnostic, certain materials have specific ranges of electrical resistivity. In all electrical resistivity surveying techniques, a known electrical current is passed through the ground between two (or more) electrodes. The potential (voltage) of the electrical field resulting from the application of the current is measured between two (or more) additional electrodes at various locations. Since the current is known, and the potential can be measured, an apparent resistivity can be calculated. The separation between the current electrodes depends on the type of surveying being performed and the required investigation depth. Electrical resistivity, also referred to as galvanic electrical methods, is occasionally useful for determining shallow and deep geologic and hydrogeologic conditions. By measuring the electrical resistance to a direct current applied at the surface, this geophysical method can be used to locate fracture zones, faults and other preferred groundwater/contaminant pathways; locate clay lenses, sand channels and locate perched water zones and depth to groundwater. A variety of electrode configurations or arrays e.g. Wenner, Schlumberger, dipole dipole can be used depending on the application and the resolution desired. Typically, an electrical current is applied to the ground through a pair of electrodes. A second pair of electrodes is then used to measure the resulting voltage. The greater the distance between electrodes, the deeper the investigation because various subsurface materials have different, and generally understood, resistivity values, measurements at the surface can be used to determine the vertical and lateral variation of underlying materials The Study Area Bunza local government Kebbi state northern Nigeria is situated within Latitudes "N to "N along the equator and Longitudes "E to3 59 8"E along the greenwhich meridian of the earth. However the raining season in the area is May to November and dry season is December to April. It is generally characterized by broadly slop topography with gentle undulations, and is largely drained

4 103 by rain. Mangrove and rain forests characterize the vegetation of the study area. The spill site lies within the coastal plain of Birnin Kebbi, which consists mainly of Cretaceous sediments deposited in a highenergy deltaic environment. The hydrogeological aspect of Bunza has not been studied. Its subsurface lithology consists of sandstone with shale intercalations, coarse grained, laterite, clay, clayey sand, white gravel, poorly sorted, sub angular to well rounded, contains lignite streak and wood fragment. These characteristics result in good porosity and effective permeability for effective retention and mobility of groundwater and other liquid substance within the subsurface (Obaje, 2009) Vertical Electrical Sounding (VES) Vertical electrical sounding technique also known as electrical drilling is very useful method adopted in groundwater investigation especially when dealing with a sedimentary terrain. The technique involves the measurement of electrical resistivity in a vertical plane section, which in turn yields the depth, and the corresponding resistivity of the geological formation. It is a technique in which the depths and types of different geologic materials are sampled from the surface by injecting current into the ground through current electrodes and receiving information from the probed sections of the subsurface through two potential electrodes in form of potentials. Current conductivity of different geologic materials depends on the grain size and degree of wetness. Thus by measuring the resistance of these materials using ABEM microprocessor known as Terrameter, the subsurface strata can be determined (Keller and Frischknecht, 1976). The aim of the vertical electrical sounding is to determine the apparent resistivity of various layers of the geological formation by increasing the current electrode distance (AB). The apparent resistivities obtained are plotted against AB/2 for Schlumberger configuration. From the sounding curve obtained one can draw conclusion about the true resistivities and the true thicknesses of the layers. Usually the curves from the field data are interpreted in terms of geologic layers, porosity and aquiferous potential by employing a curve-matching technique in any of the known methods often used for Schlumberger field data interpretations and deductions can therefore be made in terms of the thickness of layers, layer type, depth to the aquifer and layer characteristics and of course, the watertable (Kearey and Brooks, 1991). The assumptions on which the sounding techniques are based upon are: The surface of the geological formation is a horizontal plane; the boundaries of the layers are horizontal and plane parallel; the layers are horizontally of infinite extension; every layer is homogeneous and isotropic in itself. 2. Materials and Method 2.1. Materials

5 104 The following equipment was used during the VES survey: ABEM terrameter SAS 300, Measuring Tape, Reels of wires, 4 copper electrodes, and electrode field hammer, Dry Cell Battery which served as the source of main power supply and Global positioning system (GPS) device Methodology The study was carried out using the ABEM terrameter SAS 300 earth resistivity meter, measuring tapes, current and potential electrode, crocodile clips and hammers as listed above. For the purpose of this research, six (6) Vertical Electrical Sounding stations were obtained within the study area at different locations. The Schlumberger array experimental procedure was adopted throughout the survey. Current was passed into the ground through the pair of current electrode, then the corresponding potential difference, V was obtained through pair of potential electrodes, and then recorded on the resistivity recording sheets to determine the quotient V/I which represent the resistance using ohm s law relation. Basically, a station was chosen and an iron rod is driven into the ground, this marks the base station, which is served as a reference mid-point from where MN/2 (potential electrode) spacing are measured in both directions using the marked mid-point and measuring tape. The potential electrodes are driven in either side of the base stations at a specified distance. The current electrodes were driven in on either side and the spacing is given as AB/2, a straight line was maintained by the configuration of all the electrodes. The measurements were repeated and recorded with MN fixed at its initial distance (current electrode) AB/2 is symmetrically increased where the resistance measured becomes too small, MN/2 was increased symmetrically. The maximum spread of AB/2 is 550m, while the maximum spread of MN/2 is 30.0m.The change in distance between the current electrodes increases the depth range at which current penetrates, the apparent resistivity is then plotted against the corresponding half electrode spacing (AB/2) on a bi-log paper. The curve types were interpreted qualitatively and quantitatively. The quantitative interpretation involved the partial curve matching and the computer iteration using the IPI2 software, while the quantitative interpretation involved the inspection of the curve types Field Procedure Resistivity measurement using ABEM Terrameter SAS 300CVertical Electrical Sounding was carried out using ABEM SAS 300C Terrameter and Schlumberger electrode configuration was employed for reasons of logistics of manpower and its characteristic deep penetrations into the subsurface. The Schlumberger array requires that four steel electrodes are arranged and pinned collinearly into the earth with the current electrodes spacing much greater than the potential electrodes and ensuring that AB/2 5MN/2 where AB is current electrodes separation and MN is potential electrodes separation. This implies that for MN = a, AB = na+a from which their values were generated and approximated for convenience. The electrodes were connected appropriately to their respective

6 105 terminals on the terrameter through cables and hammered to make good contact with the earth. On sounding, the digital SAS 300Cterrameter sends down direct current into the earth subsurface through the pair of steel current electrodes, while the established subsurface potential difference across the subsurface under investigation is measured by the terrameter through the steel potential electrodes. For each sounding, the terrameter computes and displays a mean digital value of the apparent resistivity of the subsurface under investigation using the theory that the measured potential difference. I C1 P1 P2 C2 A M N B I L Figure 1. Schlumberger Electrode Configuration of Rasha Area The resistivity measurements are normally made by injecting current into the ground through two current electrodes (C1 and C2 in), and measuring the resulting voltage difference at two potential electrodes (P1 and P2). From the current (I) and voltage (V) values, an apparent resistivity (pa) value is calculated Basic theoretical consideration The fundamental equation for resistivity survey was derived from ohm s law (Telford et al., 1976, 1990), that is, ρ = RA L Where ρ is the resistivity, R is the resistance, L is the length of the homogenous conducting cylinder and A is the cross-sectional area. However for solid earth, whose material is predominantly made up of silicates and basically non-conductors, the presence of water in the pore spaces of the soil and in the rock features enhances the conductivity of the earth when an electrical current, I is passed through it, Thus making the rock a semi-conductor. Since the earth is not like a straight wire and its anisotropic, then equation (i) becomes (i)

7 106 ρ = V I (2πr) (ii) where V is the change in voltage and r is the radius of current electrode s small hemisphere. Since the earth is not homogeneous, equation (ii) is used to define an apparent resistivity ρ a which is the resistivity the earth would have if it were homogeneous (Telford et al., 1990). ρ a = V I (2πr) Where, 2πr is define as a geographic factor (G) fixed for a given electrode configuration. The Schlumberger configuration was used in the study. Thus the geographic factor, G of the earth is given by the equation below; 2 G = 2πr = π [ (AB 2 ) ( MN 2 2 ) 2 ( MN ] (iv) 2 ) Where AB and MN are the spacing between current and potential electrode respectively. However by substituting the value of 2πr from the above equation into equation (iii) and there by obtaining; (iii) ρ a = V I [ 2 π [ (AB 2 ) ( MN 2 2 ) 2 ( MN ] 2 ) Thus equation (v) can be customized to; ρ a=rg ] (v) (vi) Where R= V I and G= π [ (AB 2 )2 ( MN 2 )2 ] 2( MN 2 ) Therefore equation (ii) concluded for the fact that apparent resistance is the product of resistance of subsurface and geographic factor of the earth The Theory of the Experiment The theory and field methods used for resistivity surveys are based on the use of direct current, because it allows greater depth of investigation than alternating current due to the fact that complexities caused by the effects of ground inductance, capacitance and resulting frequency dependence of resistivity. However, in practice, actual direct current is infrequently used for two reasons: (1) direct current electrodes produce polarized ionization fields in the electrolytes around them, and these fields produce additional electromotive forces that cause the current and potentials in the ground to be different from those in the electrodes; and (2) natural Earth currents (telluric currents) and spontaneous potentials, which are essentially unidirectional or slowly time-varying, induce potentials in addition to those caused by the applied current. The effects of these phenomena, as well as any others that produce unidirectional

8 107 components of current or potential gradients are reduced by the use of alternating current, because the polarized ionization fields do not have sufficient time to develop in a half-cycle, and the alternating component of the response can be measured independently of any superimposed direct currents. The frequencies used are very low, typically below 20Hz, so that the measured resistivity is essentially the same as the direct current resistivity True Resistivity The true resistivity of earth material is dependent upon composition, grain-size, water content and other physical characteristics (Hasbrouck 2003). When the earth material is isotropic and homogeneous, the resistivity calculated from any electrode configuration should be constant and independent of both electrode spacing and surface location. This gives the true resistivity. Thus for homogeneous half space, the apparent resistivity is the true intrinsic resistivity, but resistivity differs due to inhomogeneity and anisotropy resulting in different layered conductivities in the earth. The measurements were made with great care and the following precautions taken for accuracy. 1). The instrument was properly checked and tested to ensure that it isin good condition for efficiency and effectiveness. 2). Care was taken to make sure that the profiles were taken in as straight line and that the cables do not touch or cross each other during measurement. 3). The electrodes were fixed deep into the ground to ensure good electrical contact. 4). Measures were taken to maintain the required AB/2 and MN/2 ratio of AB = 5MN and looping done at appropriate intervals. 5). Water was poured round the electrodes in places that are very dry for good current penetration and result. 6). Batteries were checked at intervals and recharged when necessary to avoid current drop. 7). Measurements were not taken close to electric power lines to avoid the effect of electro-magnetic effects on the readings. 8). Measurements were not taken during heavy thunderstorms to avoid inductive effects by lightning and also for safety Data Interpretation, Data Input and Format After the field survey, the resistance measurements are reduced to apparent resistivity values. Practically all commercial multi-electrode systems come with the computer software to carry out this conversion. The quantitative interpretation of the field data were carried out by the IPI2 application of a computer iteration package. This package is capable of converting the values apparent resistivity as function of electrode spacing acquired as the field data to values of true resistivity as function of depth

9 108 of individual layer for the actual condition in the ground to be interpreted. However, the qualitative interpretation of the VES curves assumes that the earth is composed of horizontal layers with different resistivity referred to as geoelectric formation. 3. Results and Discussion 3.1. Results The result of the experiment acquired from six vertical electrical sounding were presented as shown in the tables below. Table 1. The results of the resistivity measurements for different VES points S/N Current electrode separation AB/2(m) Potential electrode separation MN/2(m) Geographic factor, G VES1 ρ a (Ωm) VES2 ρ a (Ωm) VES3 ρ a (Ωm) VES4 ρ a (Ωm) VES5 ρ a (Ωm) VES6 ρ a (Ωm)

10 109 Figure 2: VES1 CURVE Figure 3: VES2 CURVE Figure 4: VES3 CURVE

11 110 Figure 5: VES4 CURVE Figure 6: VES5 CURVE Figure 7: VES6 CURVE

12 111 Table 2: Summery of computer iteration and geoelectric section of the VES locations VES Layers Resistivity ρ a (Ωm) Thickness (m) Depth (m) Top soil Clayey sand Clay Fine grained sand White gravel Top soil Clay Clayey sand White gravel Top soil Laterite Clay White sand Clayey sand Top soil Clayey sand Fine grained sand Clay White sand Top soil Laterite Sand White gravel Top soil Clayey sand White gravel Lithology Curve type RMS % Error P1>P2>P3<P4< QA P5 P1>P2<P3<P4 HA P1<P2>P3<P4> P5 KHK P1>P2<P3>P4< P5 HKH P1<P2<P3<P4 3.7 P1<P2<P3 A Discussion The results of the survey were presented as shown in tables 1 and table 2 with their respective curves, which were obtained by plotting the graph of vertical electrical sounding against current electrode spacing on a log by log graph as shown from figures 2 to 7 respectively. However the interpretation of these vertical electrical sounding curves reveals that, the nature of the curves obtained were, AH, KHA, A, HA, KHK, and HKH types. Thus, the iterated results analysis infers that the numbers of geoelectric layers generally vary from three to five with corresponding root mean square percentage error ranging from 2.9 to 17.0%. VES1, 3 and 4 constitute of 5 layers, which were made up of QA, KHK and HKH type curves respectively. The resistivity of these locations vary from 28.2Ωm to 699.9Ωm, 415.6Ωm to Ωm and 146.0Ωm to Ωm with thickness 2.2m to 10.0m, 0.7m to 54.8 and 0.8m to 3.7m respectively,

13 112 this therefore infer the fact from the lithology that, the sub-surface layers were mainly made up top soil, clay, clayey sand, white sand, fine grained sand and finally the white gravel. The aquifer for these locations has a total thickness of 10.0m at a depth of 17.9m in the fourth layer for VES1 location, 54.8m thickness at a depth of 59.3m in the fourth layer forves3 while VES4 location has thickness and depth at infinity. Moreover VES 2 and 5 comprises of 4 layers made up of HA and A curves type. The resistivity for these locations ranges from 5.3Ωm to Ωm with corresponding thickness varying from 4.8m to 6.0m at a depth of 4.8m to 16.2m for VES 2 location while VES 5 is of resistivity ranging from 368.3Ωm to Ωm with a resultant thickness varying from 0.8m to 18.8m at a depth of 0.8m to 34.5m.The lithology for this locations are mainly top soil, laterite, clay, sand and white gravel. The aquifer for VES 2 is at third layer with equivalent of thickness of 5.4m at a depth of 16.2m while in the other way round the aquifer for VES 5 has thickness of 18.8m at a depth of 34.5m. Finally VES 6 has three layers made up of A-type curve. The resistivity for the three layers are 175.1Ωm, 715.3Ωm and Ωm with corresponding thickness of about 7.7m and 98.0m at a depth of 7.7m and 105.7m respectively. However the lithologies of the subsurface layers are made up top soil, sand and white gravel. The aquifer in this location is at a depth of 105.7m. Conclusively, VES 3 and 5 have the best aquifer and boreholes could be sited in these locations. A reliable interpretation was made by having a look at the lithological borehole descriptions and geophysical well logs within these areas of surveys. This was purely in conformity with the vertical electrical survey results. 4. Conclusion After carrying out investigation of groundwater depth estimation using electrical resistivity in parts of Bunza Local Government Areas in Kebbi State to delineate the aquifers in the region and to ascertain the depth, the geoelectric units, topography as well as determining the direction of groundwater in the study area. Vertical Electrical Sounding (VES) technique using Schlumberger array has proved to be effective. The saturated aquiferous zones were Fine grained sand, Clayey sand, White sand, clay and Clayey sand as determined by the probes. From the geoelectric sections, one can infer that the aquifers encountered in the area were mainly confined. The study has shown that the community is underlain by 3 to 5 geoelectric layers within the depth penetrated. The research has enabled the determination of the depth to water table, aquifer thickness, and sub-surface geology of the study area, thus revealing its groundwater distribution as well as its potential as a substitute or compliment to the surface water resources. The results of the research have shown that locations of VES 3 and 5 have the best aquifer of thickness 54.8m and 18.8m at a depth of 59.3m and 34.5m respectively, for the maximum yield,

14 113 boreholes should be sited in VES 3 and 5 locations. However for VES 1, 2, 4 and 6 where some abortive boreholes are located and aquifers not well defined, there is need to increase the array spread for greater current penetration and deeper penetration beyond the depth of these locations. This is because there is tendency for aquifer bearing formations to be discovered beyond 10.2m, 59.3m, 8.1m and 105.7m for these four locations since there is an increasing value of apparent resistivity obtained from the curves. References Etu-Efeotor, J.O and Akpokoje, E. G. (1990), Aquifer systems of the Niger Delta, Nigerian Journal of Mining Geology, 26(2): Hasbrouck, J. R. (2003), Deep Groundwater Exploration Using Geophysics, N4/dept on the ground.pdf. Kelly, W.E. and M. Stanislav, (1993), Applied Geophysics in Hyrogeological and Engineering Practice. Elsevier, Amesterdam, pp: 292. Keller, G.V. and Frischknech, F. C. (1976), Electrical method in Geophysical prospecting: Pergamon Press, Inc. New York, USA. Pp Kearey, P. and Brooks, M. (1991), An Introduction to Geophysical Exploration. University Press, Cambridge. Pp Mbipom, E.M. and Archibong J.E (1989), Vertical electric Sounding of Coastal Acquifers near Qua- Iboe Estuany, Journal of Mining and Geology. 25: 51 Obaje, N. G. (2009), Geology and Mineral Resources of Nigeria. Springer-verlag, Berlin Heildelberg. ISBN; Obiakor, I.P. and Chukwudebelu, J.U. (1992), Geophysical Investigation for Groundwater in the Nanka Sands of the Anambra Sedimentary basin, Nigerian Journal of Physics. 43: Oseji, J.O., E.A. Atakpo and E.C. Okolie, (2005), Geoelectric investigation of the aquifer characteristics and groundwater potential in Kwale, Delta state, Nigeria, J. Applied Sci. Environ. Mgt., 9: Okwueze, E.E. and Ezeanyim, V.E. (1985), The Vertical Electrical Sounding (VES) Method in Laterite Regions and in Iron-rich Glaciated Areas. Nigerian Journal of Mining and Geology, 22(1, 2): Telford, W. M., Geldart, L.P., Sherrif, R.E., Keys, D.A. (1976), Applied Geophysics. Cambridge University press, New York, USA. 860 Telford, W.M., Geldart, L.P., and Sheriff, R.E. (1990), Applied Geophysics (2nd Edition), Cambridge University Press.

15 114 Vingoe, P. (1972), Electrical Resistivity Surveying Geophyics Memoranda, Abem Geophysics and Electronics Sweden. 5: 75 Wiebeng, W.A. (1955), Geophysical Investigation of Water Deposits. Western Australia Canberra, Bureau for Mineral Resources. Geology and Geophysics, Bulletin 30. Zekster IS, Everett LG (2004), Groundwater resources of the world and their use, IHP VI, Series on Groundwater No. 6. UNESCO (United Nations Educational, Scientific and Cultural organisation.