Hydro-Geophysical Study of River Niger Floodplain at Jebba-North, Nigeria

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1 Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3 (1): Scholarlink Research Institute Journals, 212 (ISSN: ) jeteas.scholarlinkresearch.org Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3(1): (ISSN: ) Hydro-Geophysical Study of River Niger Floodplain at Jebba-North, Nigeria Omotoso, Oladele A., Ojo, Olusola J., orakinyo, Ebun P., and Alao, David A Department of Geology and ineral Sciences, University of Ilorin, Nigeria Corresponding Author: Ojo Olusola J Abstract The Hydro-geophysical study of part of River Niger floodplain at Jebba-North, north central Nigeria was carried out using Vertical al Sounding method to assess potential groundwater aquifers and delineate lithologies beneath the floodplain. Schlumberger electrode array system was used for the survey. Twelve Vertical al Sounding (VES) stations were probed and the sounding data were processed and interpreted using partial curve matching method and computer iteration techniques. Field data shows that the study area is situated across the Precambrian basement complex of Nigeria on one part and the sedimentary basin (Bida Basin). The basement rocks comprises of migmatites gneiss, quartzite complex, granitoids and minor acid dykes, while, the sedimentary terrain consists of essentially of conglomerates, sandstones and claystones of Campanian to aastrichtian age. The VES data revealed three to four geo-electric layers which include the top soil (resistivity range from , with average of and average thickness of 1.5m:), the weathered horizon (resistivity range from , average and average thickness of 9.6m) which probably comprise of sandstone and sandy claystone, and the basement rock (resistivity range from , average ). The weathered zone constitutes the main aquifer units. The results show that the study area could be classified as prolific zones for groundwater development and therefore, has potential to sustain the FADAA (Federal Government of Nigeria Agriculture intervention) project Keywords: floodplain, aquifer, fadama, sandstone, jebba, resistivity INTRODUCTION One of the most essential factors sufficiently required for sustainable development of any region in the world today is water. Nevertheless, in most developing countries like Nigeria, inadequate supply of surface water in terms of quality and quantity has become a big challenge with respect to rapid increase in demand for water due to urbanization and agricultural activities. In the study area, water for domestic and agricultural uses are sourced from rivers, streams, ponds, and such other surface water systems which are usually located at great distances from the areas they serve. At times, the limited water resources are shared among human beings, crops and animals and are therefore often highly insufficient and polluted. With increased awareness and understanding of personal and domestic hygiene, the only alternative source of perennial water supply for the ever growing populations is groundwater. On the other hand, farmers should not only depend on surface water for irrigation, there should be access to groundwater, as this would supplement surface water all through the year. According to Singh (1984), reliable groundwater potential data is very vital and fundamental for large scale development of groundwater. This is possible by a systematic exploration program using modern scientific tools. The use of geophysical methods provides valuable 152 information with respect to distribution, thickness, and depth of groundwater bearing formations. Various surface geophysical techniques are available but the most commonly used in Nigeria for rural/urban water supply is the al Resistivity ethod because of its low cost and relatively high diagnostic value. The investigated area is located within longitudes 4 o E, 4 o 51 6 E and latitudes 9 o N, 9 o 6 55 N. The climate of the study area usually alternates between dry and rainy season. The area lies within the middle belt of Nigeria with a total annual rainfall between 127 mm and 1524 mm, spread over the month of April to October (ccurry, 1973). The highest amount of rainfall is observed in the month of August. onthly temperature is highest in arch at about 3 C and lowest in August at about 25 C (Ajibade, 1982). The vegetation of the area is of the guinea savannah, comprising of various species of shrubs and high forest plants along the streams and depressions in the area (Ajibade and Woakes, 1976). This study is aimed at evaluating the groundwater potential of the River Niger floodplain around Jebba North (adjacent to complex) by applying electrical resistivity techniques and also to determine the subsurface lithological characteristics in the area. The occurrences of groundwater in recoverable

2 quantity as well as its circulation are controlled by geological factors (Olorunfemi and Fasuyi, 1993; Amadi and Olasehinde, 21). The study is significant because it is expected to provide data base necessary for the Fadama project in the area which require availability of water and it is the pioneer work in this regard in the study area. ETHODOLOGY al resistivity method (Schlumberger electrode array) was used to investigate subsurface conditions by passing electric current into the ground through a pair of current electrodes and measuring the resulting voltage difference between a pair of potential electrodes. Twelve Vertical al Soundings were carried out at Jebba-North (Fig. 2). Since the values measured in the field correspond to resistances or potentials, the first step when processing the data was to calculate the apparent resistivity. This was computed using the formula relevant to the electrode configuration. The calculated apparent resistivity values were plotted against half the current electrode spacing on a log-log graph. The curves obtained were interpreted both qualitatively by inspection and quantitatively by matching small segments of the field curves using two-layer model and their corresponding auxiliary curves. The resistivity and thickness obtained from the partial curve matching were improved upon by employing an iterative computer program to obtain the layers parameter (resistivity, thickness and depth). The numerous layers that were generated by the computer were grouped into relevant geologic depth intervals called geo-electric sections. The type of curves (Selemo et al, 1995), the resistivity of the sediments (Oyedele, 21) and the knowledge of the local geology were used as guides in the interpretation and analysis of the geo-electric parameters in terms of probable, potable and sustainable water supply. Curve matching and computer iteration methods were used for the data interpretation. RESULTS AND DISCUSSION The statistical summary of the VES data are presented in tables 1-12 and some of the resistivity curves are also presented in figures 3 and 4. The results from each VES are presented below; Vertical al Sounding 1 (VES 1) Using curve matching techniques, the type of curve that was determined in this cell is H-type curve. The curve falls to minimum then increases again due to an intermediate layer that is a better conductor than the top and bottom layer. Furthermore, three geo-electric layers were interpreted for the VES data. Table 1 shows the depth, thickness and corresponding resistivity value of each geo-electric layer. From the curve, there is an indication that the resistivity (377.4) of the first geo-electric layer (topsoillaterite) is greater than that of the second layer (47.8) for wet horizon and further increased in the fresh basement (978). The third geo-electric layer represents the basement rock with infinite thickness. Vertical al Sounding 2 (VES 2) Three geo-electric layers were represented for the VES data in this location with an H-type curve. The first layer is the top soil (laterite), with resistivity value of 146.1, and a thickness of about 1.7m at a depth of 1.7m (Table 2). The second geo-electric layer is claystone with an average resistivity value of 82.7, thickness of 14m and at the depth of about 16.7m. The third layer is a fresh basement with an average resistivity value of at an undetermined thickness and depth. Vertical al Sounding 3 (VES 3) Four geo-electric layers were delineated for the VES data in this location with an H-type curve. The first layer is regarded as the topsoil (laterite) and it contains some proportion of sand, clay and gravel. Average resistivity value is 444.3, and is 1.9m thick at the depth of about 1.9m. The second layer is the weathered bedrock with an average resistivity value of 254.1, and thickness of 3.8m at depth of about 5.8m. The third geo-electric layer is the claystone horizon with resistivity value of 26, and a thickness of 1.2m at a depth of 16.m. The fourth layer is the basement complex of resistivity average value, with an infinite thickness at depth (Table 3). Vertical al Sounding 4 (VES 4) Four geo-electric layers were delineated for the VES data in this location (Table 4). The type of curve is AKH- type. The first layer is regarded as the topsoil (laterite) containing some proportion of sand, clay and gravel and having an average resistivity value of 254.9, and thickness of 1.5m at the depth of about 1.5m. The second layer is the weathered bedrock with an average resistivity value of 572.3, and thickness of 4.2m at depth of about 5.7m. The third geo-electric layer is the clay horizon with resistivity value of 25.9, with a thickness of 1.6m at a depth of 16.3m. The fourth layer is the basement complex of resistivity average value, with an infinite thickness at depth. Vertical al Sounding 6 (VES 6) Three geo-electric layers were represented for the VES data in this location with an H-type curve. The first layer is the top soil (laterite), with resistivity value of 192.5, and a thickness of about 1.m at a depth of 1.m. The second geo-electric layer is a weathered zone (probably claystone or sandstone) with an average resistivity value of 44.4, thickness of 4.4m and at the depth of about 5.4m. The third layer is the basement rock with an average resistivity value of at an undetermined thickness and depth (Table 5). 153

3 Vertical al Sounding 7 (VES 7) The table 6 shows the interpreted VES data for this location with the various thicknesses, resistivity values and depth of each horizon. From the VES curve and the pseudo sections, it is observed that the location presents four geo-electric layers. The shape of the curve observed is of type HK. The top soil (laterites) is of thickness of about 1.2m and average resistivity of (Table 6). The second layer is a fractured horizon with the thickness of about 2.6m, depth of about 3.8m. The third layer is the weathered layer with an average resistivity value of 21.2 and having a thickness of about 13m at a depth of 16.8m. The fourth layer is the basement rock with resistivity having indefinite depth and thickness. Vertical al Sounding 9 (VES 9) Four geo-electric layers were observed with H-type curve. The comprises of the topsoil (laterite) of thickness of 2.6m and resistivity of The second layer is probably a weathered bedrock zone with average resistivity value of and a thickness of 1.5m at a depth of 4.1m. The third layer is assumed to be a claystone horizon with a very low resistivity of 1 and a thickness of about 25.9m at a depth of about 29.9m. The fourth layer is the fresh basement with an average resistivity value of 646.5, having an infinite thickness at unknown depth (Table 7). Vertical al Sounding 11 (VES 11) Four geo-electric layers were delineated for the VES data and the shape of the curve observed is H-typecurve. This is an indication that the average resistivity values of the first layer were greater than the second and third layers and that of the fourth layer greater than the previous layers. The values are presented in Table 8. From the resistivity values of each layer the respective could be delineated. The first layer is the topsoil (laterite), the second and third layers are clay deposit and the fourth layer is the fractured basement complex due to its varied resistivity values. Vertical al Sounding 13 (VES 13) The shape of the geophysical curve obtained here is of type-h with three geo-electric layers. The first layer which is the top soil (laterite) has an average resistivity value of , and thickness of about 1.6m (Table 9). The second layer could be a weathered bed rock with an average resistivity value of and a thickness of about 8.7m at a depth of about 1.3m. The third layer is a fractured basement with varying resistivity values but with an average resistivity value of about having infinite thickness. Vertical al Sounding 2 (VES 2) Three geo-electric layers were defined from the VES data in this location with an H-type curve. The first layer is the top soil (laterite), with resistivity value of 59.9, and a thickness of about 1.4m at a depth of 1.4m. The second geo-electric layer is clay with an average resistivity value of 4.7, thickness of 8.1m and at the depth of about 9.5m (Table 1). The third layer is the basement rock with an average resistivity value of at an undetermined thickness and depth. Vertical al Sounding 21 (VES 21) Three geo-electric layers were represented for the VES data in this location with an H-type curve. The first layer is the top soil (laterite), with resistivity value of 377.4, with a thickness of about 1.3m at a depth of 1.3m. The second geo-electric layer is claystone with an average resistivity value of 91.4, thickness of 18m and at the depth of about 19.3m. The third layer is the basement with an average resistivity value of at an undetermined thickness and depth (Table 11). Vertical al Sounding 22 (VES 22) Four geo-electric layers were interpreted for the VES data and table 12 shows the interpretation. The shape of the curve observed is of type-h-curve. The first layer, top-soil (sand stone) has an average resistivity of with a thickness of about 1.2m (Table 12). The second layer could also be sandstone with an average resistivity of with a thickness of 1.4m at a depth of 2.6m. The third layer is assumed to be a weathered zone, probably clay deposit of 53.7 resistivity with a thickness of 15.4m and at the depth of 18m. The fourth layer is also assumed to be a basement rock which is fractured, judging from varying resistivity values with an infinite depth. The Parameters of the Sequence Three to four geo-electric layers were delineated (fig 5) and discussed below. Three s Six VES stations in the study area have three geoelectric layers. The first layer which is the top soil has resistivity values ranging from 59.9 to with an average of and thickness ranging from 1. to 1.7m with an average of 1.4m. The second layer which could be regarded as weathered zone (claystone, sandstone, clayey sandstone or sandy claystone) has resistivity values ranging from 4.7 to with an average of 71. and thickness ranging from 4.4 to 18.m with an average of 12m. The third layer is the depth to basement with varying resistivity values ranging from to with an average of having an infinitive depth. Depth to basement ranges from 5.4 to 19.3m deep. In addition, this layer may be fractured owing to different resistivity values. This 154

4 zone could be regarded as water saturated zone and it could be developed for ground water. Four s Six Vertical al Sounding stations have four geo-electric layers in this study area viz: the top soil, the weathered layers (conglomerate, sandstone, clay or the combinations) and basement rock. The top soil has resistivity values ranging from to with an average of 54.8 and thickness ranging between 1.2m and 2.6m with an average of 1.7m. The top soil which is the first layer could be categorised as Laterite because of the varying resistivity. The second layer is the weathered layer having resistivity values in the range of 73 to with an average of and thickness in the range of 1.4 to 4.2m with an average of 2.5m. The third layer is also a weathered layer with resistivity values of 1 as minimum and 53.7 as maximum with an average of 26.4 and thickness of between 1.2m to 25.9m (an average of 14.3m). Depth to basement ranges from 14 to 29.9m deep. The resistivity values in the presumed geo-electric bedrock vary from to It has been discovered that the range constitutes the highest frequencies in this geo-electric layer. Where it is fractured or sheared and saturated with fresh water, the resistivity often reduces below 1 (Olayinka et. al., 1992). This zone is water saturated. Assessment of Groundwater Prospect and Aquifer Delineation The interpretation of the 12 VES conducted in the area of study reveals the presence of three to four geo-electric layers. For the four geo-electric layer delineated, the weathered layer lies within the second and the third which could be sandstone, sandy claystone, clayey sandstone or claystone. The second layer serves as the weathered zone for the 3 layered geo-electric sequences. Where the basement rock is fractured, it could serve as an aquifer and this could complement the weathered zone. The weathered and the fractured zones are favourable for construction of shallow boreholes within a depth of 14m and 29.9m for the 4 layered geo-electric zones. Sitting borehole within 5.4m and 19.3m would be appropriate for the 3 layered geo-electric zones. CONCLUSION VES data and the interpretation indicate that the study area is underlain by three types of (geo-electric layers) namely; the top soil (mainly laterites), the weathered zone/ layer and the basement rock. The top soil which constitute laterites of resistivity values ranging from (average=524.6 ) with a thickness range of 1 to 2.6m (average=1.5m). The weathered zone which could be sandstone, sand, clay or mixture has resistivity values range from (average= 195.4) with a thickness range from m (average=9.6m). The third geo-electric layer is the basement rock which was fractured in some areas and massive in others. It has a resistivity range of (average= ). The weathered zones offer the best potential for groundwater accumulation and the results show that the study area is prolific with respect to groundwater potential and development. ACKNOWLEDGEENTS We appreciate the technical assistance of r A.D. Adedoyin and some students of the Department of Geology and ineral sciences, University of Ilorin, Nigeria, during the field work. REFERENCES Ajibade, A.C The origin of the Older Granites of Nigeria: some evidence from the Zungeru region. Nigerian Journal of ining and Geology, 19(1), Ajibade, A.C. and Woakes, Proterozoic crustal development in the Pan-African Regime of Nigeria. In: C.A. Kogbe (Editor) Geology of Nigeria. Published by Rock, View (Nigeria.) Ltd., Amadi, A.N., Olasehinde, P.I., Okunlola, I.A., Okoye, N.O., Waziri, S. 21. A multidisciplinary approach to subsurface characterization in Northwest of inna, Niger State, Nigeria. Bayero Journal of Physical and athematical Science, 3, c Curry, P A general review of the Geology of the Precambrian to lower Paleozoic rocks of Northern Nigeria- A review; In Kogbe, C.A. (ed). Geology of Nigeria- Elizabeth Publishing Co. Ibadan, Nigeria. pp Obaje, N.G., Wehner, G., Scheeder, G., Abubakar,.B. and Jauro, A., 24. Hydrocarbon Prospectivity of Nigeria s Inland Basins: From the Viewpoint of Organic Geochemistry and Organic Petrology: AAPG Bulletin, 8, Olayinka, A.I Geophysical sitting of boreholes in crystalline basement areas of Africa. Journal of African Earth Sciences, 14, Olorunfemi,.O., Fasuyi, S.A Aquifer types, geo-electric and hydrogeologic characteristics of part of the Central Terrain of Nigeria (Niger State). J. Afr. Earth Sci., 16, Oyedele, K. F. 21. electric investigation of Groundwater resources at Onibode area, near Abeokuta, South-west Nigeria. Geophysical Prospecting, 2,

5 Selemo, A. O. I, Okeke, P. O. and Nwankwor, G. I An Appraisal of the usefulness of VES in Groundwater Exploration in Nigeria. Water Resources, 6, No Singh, C.L., 1984, Role of Surface Geophysical ethods for Groundwater Exploration in Hard Rock Areas. Proceedings of International Workshop on Rural Hydrology and Hydraulics in Fissured Zones APPENDIX a VES 2 Figure 1: Geologic ap of Nigeria showing the study area (Obaje et. al, 24) b Fig 3: Resistivity curves for (a) VES 1and (b) VES 2 Figure 2: Topographical ap of Jebba-North showing the study area a 156

6 CELL 5 VES 3 VES 4 VES b Fig 4: Resistivity curves for (a) VES 3 and (b) VES 4 CELL a b VES1 VES 6 VES 7 VES 13 VES 14 VES 21 VES Top Soil Weathered zone Fractured Fractured Top soil m CELL VES 2 VES 5 VES 11 VES 2 2m m Weathered Bedrock 123 Resistivity (ohms-m) m wearthered zone 3m Weak Zone resistivity in ohms-meter Top Soil Weathered Bedrock weathered zone Fresh 4m 5m resistivity in ohms-meter c Figure 5: electric section for (a)ves 1, 6, 7, 13, 14, 21, 22; (b) VES 2, 5, 11, 2; (c) VES 3, 4, 9 Table 1: Summary of the VES 1 with the interpreted Thickness, Top Soil Weathered Zone Rock Table 2: Summary of the VES 2 with the interpreted Top Soil Weathered Horizon Fresh Table 3: Summary of the VES 3 with the interpreted Top Soil (Laterite Weathered Bedrock Weathered Horizon

7 Table 4: Summary of the VES 4 with the interpreted Top Soil (Laterite Weathered Bedrock Weathered Zone Table 5: Summary of the VES 6 with the interpreted Top Soil Weathered Zone(Clay/Sand) Fresh Table 6: Summary of the VES 7 with the interpreted Top Soil Fractured Weathered Zone Rock Table 7: Summary of the VES 9 with the interpreted Topsoil Weathered Bedrock Weathered Zone Rock Table 9: Summary of the VES 13 with the interpreted Top Soil Weathered Fractured Table 1: Summary of the VES 2 with the interpreted Top Soil Clay Table 11: Summary of the VES 21 with the interpreted Top Soil Weathered Zone Table 12: Summary of the VES 22 with the interpreted Topsoil (Sandstone) Weathered zone(sandstone) Weathered zone Weathered basement Table 8: Summary of the VES 11 with the interpreted GEO- ELECTR IC LAYER RESISTIVI TY, THICKNE SS, m DEPT H, m INTERPRE TED LITHOLOG Y Top Soil Clay Clay Fractured 158