Assessment of Groundwater Potential in Ozoro , Delta State , Nigeria Using the Electrical Resistivity Method

Electrical resistivity survey involving six vertical electrical soundings was conducted in Ozoro to assess the groundwater potential of the area. Record of apparent resistivity and electrode spread were analysed using curve matching and computer iteration technique and the outcome compared with record of existing borehole. Result revealed the presence of four geoelectric strata which are topsoil, fine-medium grain sand, clay-clayey sand and coarse-medium coarse sand. The fourth layer with a resistivity range of 648-917 Ωm and depth range of 10.7-18.0 m is identified as the aquifer layer. Result also revealed that the aquifer layer is overlaid by a clayey aquitard of 4.2 m thick. It is recommended that the sourcing of water for domestic purpose by the people should be done in excess of 18.0 m.


Introduction
Exploration of groundwater has been the surest way to handle the ever increasing need for fresh and quality water used either for domestic, agricultural or industrial purpose.This is because of its relative abundance and quality with respect to many other sources.The development of this very important resource in Nigeria has not yielded a very impressive result that is commensurate with the financial resources expended on it (Edet & Okereke, 2014).This has therefore impeded the supply of quality water to a good percentage of the people of this country.Perdomo et al. (2014), emphasize the fact that limited fund has also hampered the provision of water and the availability of necessary information to help assess this resource in developing countries.
Assessing groundwater requires detailed geophysical exercise which includes geophysical well logging, seismic, magnetic, electrical method, inseam drilling and geographic information system.In a bid to effectively tap the groundwater resource, a method which is able to evaluate the aquifer layer, groundwater vulnerability, and quality is necessary.The electrical resistivity method is readily available to achieve all of these.This is because it can determine the resistivity variation of the various earth strata (Ibrahim, 2013).
The electrical resistivity method has been a very significant instrument employed in exploring the earth for groundwater (Lowrie, 2004).Electrical resistivity has been employed greatly to investigate or probe the subsurface layers of the earth.This is because it is able to distinguish between sandy and clayey formation, porous and impermeable rock, and fresh and saline water aquifers.It is used to obtain information concerning the subsurface layers and the location of groundwater (Ibrahim, 2013).The contrasts in the electrical resistivity of various lithological sequences in the subsurface are used to delineate the subsurface layers and assess the groundwater prospect of the area (Aanuoluwa & Ayobami, 2012).
Electrical resistivity surveys are carried out through an electrical resistivity sounding or horizontal profiling.The electrical resistivity sounding indicate the differences in apparent resistivity of the subsurface formation with respect to the depth while the horizontal profiling is employed to determine lateral variations in impedance (Anomohanran, 2014).In a situation where the space between the current electrodes and the potential electrodes are increased, it is observed that the current will move deeper through the ground and influence the apparent resistivity to greater depths (Todd, 2004).electrode spacing was increased in steps about a fixed point while maintaining the spacing between the potential electrodes until it became necessary to increase the potential electrode as the ground response becomes weak.The data obtained were interpreted first by partial curve matching using the master curves and the model parameters obtained were then employed as the input parameters in the computer based interpretation using the Resist software.

Results and Discussion
The data obtained from the field survey were interpreted both qualitatively and quantitatively.The resulting sounding curve obtained from the interpretation is presented as Figure 2.This figure shows that the resistivity curve types are mainly KH which are diagnostic of semi-confined or confined aquifers.The result of the quantitative interpretation is as shown in Figure 3.The result presented in Figure 3 was compared with record of existing borehole and the outcome shows close correlation between the field record and the log details. Figure 3 shows the resistivity values of the various formations observed in the area and also their depth from the earth's surface.The outcome of this study has indicated the existence of four formations (Figure 4).These are the topsoil, fine-medium grain sand, clay-clayey sand and coarse-medium coarse sand.
The first layer which is topsoil has a resistivity range of 160 to 376 Ωm and a thickness range of 0.8 to 1.6 m.
Underlying the topsoil is fine-medium grain sand formation having resistivity range of between 275 and 414 Ωm and a thickness range of between 6.2 and 12.0 m.This layer is an unconfined aquifer, which cannot be relied upon to provide adequate water for the people at all times.Besides the water content, this stratum will be very vulnerable to pollution from dumpsites and septic tanks.
The third layer is composed of sandy clay in VES 1, 2, 3 and 5 while VES 4 and 6 are clayey sand.The resistivity of this layer range between 83 and 160 Ωm while the thickness range between 3.1 and 5.1 m.This layer is fairly impermeable and hence could resist the influx of contaminants to the underlying layers.The fourth layer is made up of coarse sand in VES 1, 2 and 4 while it is medium coarse sand in VES 3, 5 and 6.The resistivity range of this layer falls between 648 and 917 Ωm while the depth range is between 10.7 and 18.0 m.This layer is the water bearing horizon in the area and is protected by the overlying clayey formation.Sourcing of groundwater for domestic and agricultural uses can therefore be made to the fourth layer aquifer.The contour map showing the depth to drill in order to strike the top of the aquifer is presented in Figure 5.This will assist the people of the area to know the depth to drill in order to obtain water from the fourth aquifer.The depth to the aquifer layer increases as you move northward meaning that the depth of boreholes sank in the area will deepen as you travel northward.The resistivity map of the aquifer layer is presented in Figure 6.This figure shows that the resistivity of the aquifer increases in the northeast direction.It therefore implies that the layer toward the northeast will be more prolific and will be of interest in the development of groundwater in the area.

Conclusion
The vertical electrical sounding has been conducted in this study to assess the groundwater potential of the area.
The data from the survey were interpreted qualitatively and quantitatively and the result showed the presence of four geoelectric layers comprising of topsoil, fine-medium grain sand, clay-clayey sand and coarse-medium coarse sand.The fourth formation with a depth range of 10.7 to 18.0 Ωm is identified as the aquifer zone.The survey has also demonstrated that the aquifer is covered by a fairly impermeable layer that is 4.2 m thick.Groundwater for domestic, industrial and agricultural use is suggested to be tapped from the fourth layer aquifer.

Figure 2 .
Figure 2. Sounding Curves Obtained from Study Area

Figure 5 .
Figure 5. Depth Contour Map of the Aquifer Layer Figure 6.Resistivity Contour Map of the Aquifer Layer