Groundwater is that water found within the saturated voids beneath the ground. It is the major source of potable water supply in the Makurdi area. Groundwater is the subsurface water which fully saturates the pores and behaves in response to gravitational force (Straher, 1973). Water is an indispensable resource and the concern of many earth scientists and researchers have been on the acquisition of a reliable source of drinking water (Akinbinu, 2015). Surface and groundwater resources are abundant in Nigeria. The water resources master plan for Nigeria which was prepared by the Japan International Co-operation Agency (JICA) in 2006 indicates an estimated surface water resource of about .67  and groundwater storage of about  (Oteze, 2006). These figures greatly outweigh the country’s total water demand of about  (Oteze, 2006). Surface water is frequently found to be grossly degraded in quality because of its exposure to physical, biological or chemical contaminants (Edet, 2004). Groundwater on its own has less of a degree of contamination when compared with surface water and this has contributed to an increase in the number of boreholes drilled by the government, non-governmental organizations and individuals in Nigeria.

 

The availability and quality of groundwater in Makurdi, the capital town of Benue State, North Central Nigeria, are determined from a number of composite factors such as porosity and permeability. The Public Water Works which supplied water to all parts of the town has become dysfunctional and moribund. Most residents of the town have adjusted to providing their own domestic supplies by use of shallow hand dug wells (approximately 10 m deep) which are often poorly completed.  The Makurdi Sandstone which is the main aquifer that supplies water into wells for abstraction is frequently indurated to the extent that well failure is always recorded when it is encountered using manual digging as is the case with all shallow boreholes. The wells are neither cased nor are they properly capped after completion. The immediate surroundings of the wells are inadequately sequestered from unsanitary conditions. Since there was no profes-sional prospecting for the location of water bearing sediments, some of the boreholes have either failed entirely or partially because of uncoordinated drillings. When the rate at which a well is discharge is greater than the rate at which it is recharge, then the well may fail (Davison et al., 1997). The nature of the aquifer is a function of subsurface geological composition that play an important role in determining the circulation of water from the surface (infiltration) to subsurface water through recharge processes (Bashir et al., 2014).

 

It is believed that the thickness and the other properties of the aquifers where the water is tapped from are very important and can only be made known through geophysical surveys. According to Makinde (2002), groundwater is related to the nature of earth rocks. Information on these rocks can be provided by a number of physical techniques which includes: seismic reflection and refraction, electrical resistivity techniques, well-logging etc. The vertical electrical sounding (VES) method is a depth sounding galvanic method and has proved very useful in groundwater studies due to simplicity and reliability of the method. The electrical resistivity of rock is a property which depends on lithology and fluid contents.

 

The basement complex terrain has many challenges as regards to groundwater potential evaluation and it explains why yield in basement complex is lower than well yield in sedimentary terrain. Groundwater usually occurs in discontinuous aquifers in basement complex area. Using electrical resistivity method and borehole lithologic logs, Dan-Hasssan (2001) depicted that the aquifers of the basement complex rocks of north central Nigeria are predominantly weathered overburden aquifers. As the resistivity of sediments and rocks are controlled by the amount of water present and the salinity (electrolytic conduction), clay mineral, fine grained or increasing silt or clay content in poorly sorted rocks or sediments will reduce resistivity (Burger,1992). Resistivity imaging method has improved the chance of drilling successfully by identifying the fractured and weathered zones in hard and compacted terrain (Loke, 2001). The flow potential of groundwater is a measure of the transmissivity of the aquifer, which is the product of the aquifer thickness and hydraulic conductivity. The knowledge of aquifer characteristics is important in determining the natural flow of water through an aquifer, it response to withdrawal of fluid, the availability, quantity and quality of the groundwater. Hydraulic conductivity (k) is a measure of the ease with which a fluid will pass through a medium (Heigold et al., 1979). By definition, hydraulic conductivity depends not only upon the medium but also on the fluid (Heigold et al., 1979).

 

Geophysical site investigations for groundwater exploration are scanty and inadequate in the study area and the hydrogeology is not well developed. The present study is therefore aimed at evaluating the groundwater condition and the nature of the subsurface layers, which will constitute the baseline information about the hydrogeology of the area.

 

Location and geology of the study area

 

The study area is Makurdi, the Benue State Capital, North central, Nigeria (Figure 1). It lies between latitudes 7°40’N and 7°50’N of the Equator and between longitude 8°20’E and 8°40’E of the Greenwich Meridian, covering a total area of about 670 km². Makurdi lies within the Guinea savannah vegetation zone with a few patches of forests. The annual rainfall ranges between 1,500 to 2,000 mm with its peak rainfall in the month of July. Temperatures in March and April are about 38 and 48°C, respectively, while in December/January, the temperature is 27°C (Benue State Water Supply and Sanitation Agency, 2008). Makurdi belongs to the Makurdi Formation which overlies the Albian Shale. It consists of thick current bedded coarse grained deposits. The Makurdi sandstone has a thickness of about 900 m (Offodile, 1976). The southern part of the Benue valley is generally gently undulating and punctuated by a few low hills. But toward the northeast, the relief is exaggerated by hills like the Lammuder and Ligri hills, which rise up to 600 m above sea level. The drainage consists of rivers which meander into the River Benue from the north and south directions.

 

Geologically, the Benue valley consists of a linear stretch of sedimentary basin running from about the present confluence of the Niger and the Benue rivers to the north east, and is bounded roughly by the Basement Complex areas in the north and south of the River Benue. The elongated trough-like basin is continuous with the coastal basin, and in fact, has been correctly described as the longest arm of the Nigerian coastal basin (Offodile, 2002) (Figure 1).

 

Different types of geological, geotechnical and environmental problems have been studied using surface electrical resistivity methods. The resistivity survey in the study area was completed with thirty Schlumberger electrical sounding (VES)   (Figure 2). PZ-02 resistivity meter was used with maximum current electrodes spacing  of 100.0 m, of 15.0 m. The resistances of the subsurface were measured and recorded against the appropriate potential and current electrodes separation. The depth of penetration is proportional to the separation between the electrodes in homogeneous ground, and varying the electrodes separation provides information about the stratification of the ground (Dahlin, 2001). This method can be used in groundwater to determine depth, thickness and boundary of an aquifer (Zohdy, 1969). The Schlumberger electrode configuration was performed using the vertical electrical sounding field 

 

 

procedure to assess the electrical resistivity of the subsurface and the thickness of the aquifer. The apparent resistivity  was determined using

 

 

Where, AB is the distance between the two current electrodes, MN is the distance between the potential electrodes, and  is the apparent electrical resistance measured from the equipment. The equation can be simplified to

 

 

Where, K is the geometric factor given by 

 

Using the conventional partial curve matching technique with two-layer master curves in conjunction with auxiliary point diagrams (Orellana and Mooney, 1966), the initial estimates of VES data was achieved. From this, estimates of layer resistivities and thicknesses were obtained which served as starting points for computer-assisted interpretation. The conventional curves and auxiliary point diagrams (theoretical curves) used in the interpretation helped in obtaining a good fit between the observed field curves and the theoretical curves during total and partial matching. The computer software program WINRESIST was used and the data sets obtained from the manual interpretation stage were keyed as inputs into the computer modeling software (WINRESIST) to generate data for the estimated model (Figures 3 to 6). According to Heigold et al. (1979), hydraulic conductivity can be determined using:

 

 

 

 

 

 

 

 

 

 

The field results obtained within the survey area is presented in Table 1. The interpretation of the data identified aquifer layers  at various sounding points showing the variation of aquifer resistivity and thickness due to lithologic composition, from which the  longitudinal conductance, hydraulic conductivity and transmissivity were computed. The aquifer resistivity in the study area ranges from 10 to 702 Ωm with an average value of 190.4 Ωm. From the results obtained, aquifer thickness ranges from 5 to 65 m having an average value of 27.37 m. The high thickness values at some VES points makes it prolific and desirable. The VES with the greatest thickness of 65 m was observed at Nyiman layout while VES at Owner’s occupier, North Bank (INEC) and New G.R.A. have the thinnest of 5m. Table 1 also gives the result of hydraulic conductivity (K), transmissivity (T_r) and Longitudinal conductance (S). It gives the range of longitudinal conductance from 0.01 to 0.97 , the average value been 0.33 . Most of the VES points in the study area have values less than the good protective aquifer range of 0.7 .0, thus indicating that the aquifers are unprotected. The result gives the hydraulic conductivity ranging from 0.85 to 45.10 m/day with an average value of  7.05 m/day. The average theoretical value of hydraulic conductivity given by Niwas and Singhal (1981) is 8.64m/day for pure sand and gravel, hence our value falls below this value, indicating that 

 

                                 Vertical electrical sounding (VES).

Thirty (30) VES have been used to evaluate the subsurface hydrogeological conditions in Makurdi, Benue state, using  geoelectric parameters to characterise the groundwater conditions of the region. From the interpreted results, longitudinal conductance, hydraulic conductivity and transmissivity were calculated. These parameters were mapped to show lateral and longitudinal distributions of thicknesses, hydraulic conductivity and transmissivity  within the water bearing geomaterials. The groundwater flow potential is affected by aquifer thickness and hydraulic conductivity. The flow potential shows the study area to have 3.3% very low potential, 26.7% low potential, 66.7% moderate potential and 3.3% high potential. The transmissivity increases towards the southwestern part of the study area, thus, the southwestern part have high groundwater flow potential. The point with the lowest hydraulic conductivity (0.85 m/day) has the lowest transmissivity (4.25 m²/day) which gives the lowest well potential. Transmissivity values are suitable for sustainable groundwater development and zones with high yield have been determined for future development and for choosing the drilling sites.