Geoelectrical investigation of aquifer problems in Gosa area of Abuja , North Central , Nigeria

1 Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310, Johor Bahru, Malaysia. 2 Centre for Research and Innovation Management, National Defence University, Malaysia. 3 Department of Geology, University of Nigeria Nsukka, Enugu State, Nigeria. 4 Centre for Energy Research and Development, University of Nigeria, Nsukka, Enugu State, Nigeria. 5 Department of Physics, School of Pure and Applied Sciences, Federal University of Technology, Yola, Nigeria.


INTRODUCTION
Basement complex rocks do not inherently make good aquifers.The hydrogeologic characteristics of basement rocks are only enhanced when the rocks are fractured and/or when they are weathered.The conditions are better enhanced when the rocks are overlain by thick overburden.Groundwater potentials of a basement complex area are often determined by geophysical means, which determines the thickness of the overburden and the network of fractures that may exist in the area.Geophysical surveys are also important for groundwater investigation in basement areas in view of the discontinuous (localized) nature of basement aquifers (Satpatty and Kanugo, 1976).The use of the vertical electrical sounding (VES) method is popular for groundwater investigation in both soft rock (sedimentary) and hard rock (igneous and metamorphic) terrains (Barongo and Palacky, 1989;De Beer and Blume, 1985;Mbonu et al., 1991;Shemang, 1993).In the Basement Complex of Nigeria, extensive application of geoelectrical method for groundwater investigation has been reported (Pulawski and Kurth, 1977;Acworth, 1987;Olorunfemi and Okankune, 1992;Olorunfemi and Fasuyi, 1993; *Corresponding author.E-mail: maxico3333@yahoo.co.uk.Edet and Okereke 1997; Nur and Ayuni, 2004).
The area of study includes most parts of Abuja main city, and western surburbs.It includes areas around Maitama, Asokoro, Wuse, Garki, Gwarimpa, Kado, Jabi, Kubwa, Dei-Dei, Lugbe and Gosa.It is bounded by Latitudes 8° 53`N to 9° 13`N and longitudes 7° 00`E to 7° 30`E.Gosa has become important because of the increasing population of the suburbs of Abuja.Many residents of the area embark on the development of private boreholes to augment public water supplies which are inadequate.But most boreholes here are abortive, and so counter the intentions of the owners, in addition to loss of capital.This work is a first-hand study aimed at establishing the groundwater conditions in the Gosa area of Abuja, Nigeria, which will help in adequate planning and drilling of boreholes in the area.

Geology and hydrogeology of the area
The area of study forms part of the Basement Complex of north central Nigeria; with lithologic units falling under three main categories, which include (1) Undifferentiated migmatite complex of Proterozoic to Archean origin, (2) Metavolcano-Sedimentary rocks of late Proterozoic age and (3) older granite complex of late Precambrian -Lower Paleozoic age, also known as Pan-African granites.All these rocks have been affected and deformed by the Pan-African thermotectonic event.Detailed reports of the lithological description, age, history, structure and geochemistry of the Basement Complex of Nigeria are given in Oyawoye (1972); Black et al. (1979); Ajibade et al. (1987); Rahaman (1988); Caby (1989) and Dada (2008).Figure 1 shows the Google earth map of the location.Figure 2 is the geologic map of Nigeria showing the study area with red arrow.
In the study area, all the three major rock categories mentioned above are well represented (Figure 3).The rocks are generally weathered into reddish micaceous sandy clay to clay materials, capped by laterite.The hydrogeology of basement areas is simple since there is an inherent limitation to the occurrence of groundwater.However, where the regolith is thick, and there is a dense network of fractures, the potentials for the accumulation of groundwater in basement complex rocks may increase.

Geophysical investigation
The search for groundwater in Gosa and it's environ was carried out through the use of electrical methods of geophysical survey, (Figure 4).The Schlumberger configuration in VES was used to obtain field data.Vertical electrical sounding probes the vertical variation in resistivity of the subsurface, thereby indicating the presence of fluid and ionic concentration in the subsurface materials.It is also applied to determine the depth to bedrock, delineate the various units that constitute the overburden (regolith), determine the degree of fracturing of the bedrock; all of which would help in making the choice for a feasible site for constructing a successful borehole.VES data for this work were obtained using the Allied Omega C2 Terrameter.The field data obtained was presented as curve of apparent resistivity values against half of the current electrode separation (AB/2) in metres on a log-log scale.

Drilling boreholes
Among the drilled boreholes, two monitoring boreholes for this work were drilled at Dei-Dei and Gosa for proper investigation of

Sample inventory
The identification of boundaries between layers with noticeably different particle sizes using visual manual logging method, record the thickness when the layer changes.Layer thickness change may range from less than one metre to tens of metres.After boundary of distinct layers have been clearly marked an indelible felt-tipped pen, using a single entry for each layer: The two sites were Dei-Dei (about 60 m), and Gosa (49 to 50 m), Figure 3 shows the drilling points.

Site one borehole (S1):
Details about the logging are given in Table 2.       3.

VES data analysis and interpretation
Twelve VES points were studied in this work; five from areas around Gosa and seven from other parts of Abuja, (Figure 4).The five points in Gosa area were close to abortive borehole sites.Table 1 shows a summary of the results of the interpretations.The table shows that at the location of VES 1, there is a thick lateritic topsoil of up to 3.4 m, underlain by a thick micaceous sandy clay layer of weathered basement rock that cannot sustain boreholes.This is directly underlain by unweathered, unfractured

Borehole log interpretation
It is clear from the above log that in Dei-Dei, 7.2 m thickness of the basalt is buried about 4 m of clay capped by a top laterite bed.The basalt lie directly a thin gravely and sandy layer marked by deep brownish laterite surface.The gravely layers, though highly permeable produces good yield, deep brown in colour, the brown colour resulting from high iron concentration derived from the intensive ferruginisation associated with the ground surface before the volcanic eruption.The fractured bottom layer of the basalt provides less contaminated water.
In Gosa, the coarse granites weather into water bearing sandy residue, the syenitic rock types, with the predominance of unstable minerals eg.Feldspars, decompose into plastic or soft clay and other argillites which behave only as aquitards or aquicludes.Generally, only small amount of water can be obtained or dry well in the freshly unweathered bedrock below the weathered layers.Even when fractured and prevented water from being transmitted into the borehole.

Conclusion
Results from the VES surveys and well log carried out in the study area indicated the presence of lateritic or sandy topsoil, weathered basement and fractured basement.The similarity in the geoelectric sections suggests a homogenous aquiferous setting in the area.However, this is not the case as boreholes drilled in the Gosa area were dry, even when drilled to depths of over 120 m.Based on the findings stated above, the following conclusions can be made.
(1) In Gosa, the area is inextensive fracturing in younger granite, and not often coincide with the broad depression of valley within the formation.In nature, it situated in an elongated depression simulating one of such situation but the hydrogeological information which are the deeply dissected by fractures within the rock formations are not interconnected.
(2) In Dei-Dei, the rock compositions are made up of coarse granitic material, gave relatively higher yields in the apparently fractured or faulted zones which could be the evidence of volcanic activity marked by the occurrence of flat toped lateritised basalt.
In this study, it suggests that most of the successful boreholes in the basement rocks should terminate at the depth of about 60 m in deeply weathered and fractured zones that are interconnected.

Figure 1 .
Figure 1.The Google earth map of the study area showing the study arean with blue dot.

Figure 2 .
Figure 2. Geological Map of Nigeria, showing the position of Abuja (red arrow) in the Basement Complex of North central Nigeria (modified from Obaje, 2009).

Figure 3 .
Figure 3. Geological map of the study area.

Figure 4 .
Figure 4. Accessibility map of the study area, showing positions of VES points.
(a) Record the date the sample is logged and the initials of the logger, (b) Record the designate geologically logged cut segment, (c) Determine and record the depth interval for each layer, (d) Photograph each layer using digital camera.

Table 1 .
Summary of results obtained from the computer output of the eight (12) VES in the area.