Department of Applied Geophysics,. Federal University of Technology, Akure, Nigeria. Corresponding Author: I.A. Adeyemo ...
Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3 (2): 368-373 © Scholarlink Research Institute Journals, 2012 (ISSN: 2141-7016) jeteas.scholarlinkresearch.org Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3(2):368-373 (ISSN: 2141-7016)
Hydrogeologic and Geoelectric Determination of Water Table at Aule Area, Akure, in the Hard Rock Terrain of Southwestern Nigeria I.A. Adeyemo and G.O. Omosuyi Department of Applied Geophysics, Federal University of Technology, Akure, Nigeria. Corresponding Author: I.A. Adeyemo ___________________________________________________________________________ Abstract Aule area is a fast growing residential area, near Akure, Nigeria. Although the eastern half of the study area is usually flooded in the raining season as result of ‘Ala’ stream that flows through the area, yet some parts of the study area have low groundwater potential. Static water level and total depth measurements of 55 wells and 21 electrical resistivity soundings was carried out in the area to estimate the depth to water table in order to evaluate the water column thickness across the area. The results of the static water level measurements and the geoelectric soundings were presented as maps, chart and geoelectric sections. The static water level varies from1.08 to 8.46m, while the aquifer depth varies from 0.5 to 5.5m respectively across the study area. The maps of static water table and aquifer depth shows that groundwater occurs at shallow depth at the northern and eastern part of the study area and occurs at greater depth at the western part and ground water flow direction is from west to east. The water column thickness map shows that water column is thicker at the northeastern and eastern parts of the study area and this correlates well with the static water level and aquifer depth maps. The geoelectric sounding results, reveal that resistivity values vary significantly across the study area, from 21 to 372ohm-m in the top soil, 4 to 1137ohm-m in the weathered layer and 99 to 6522ohm-m in the weathered/fractured/fresh bedrock. This study shows that there is a good correlation between hydrogeologic measurements and geoelectric sounding results across the study area. __________________________________________________________________________________________ Keywords: static water level, water column, geoelectric sounding and geoelectric section __________________________________________________________________________________________ INTRODUCTION 55 wells. The data enabled the preparation of a The current trends in the long-term sustainable potentiometric surface map for the area. Also, the development of groundwater aquifers resources are (21) geoelectric depth soundings equally conducted centred on integrative studies that combine geologic, in the area allowed the delineation of the vadoze and hydrogeologic and geophysical information phreatic zones and enabled the characterization of (Haverkamp et al., 1999). The approach has been their lateral and spatial variations across the area. The recognized to enhance the development of conceptual study is anticipated to be useful in the development hydrogeologic framework for the area under of conceptual model of the hydrogeologic system in investigation (MacDonald et al., 2005). the area. Hydrostratigraphically, the two major subdivisions of the subsurface are the vadoze zone and the phreatic Location and Geology zone separated by the imaginary water table (Fetter, Aule GRA is located along Akure-Aule village road 2011). The water table shows the level of saturation and it is also accessible through Alaba layout, off in an unconfined aquifer (Freeze and Cherry, 1979). Federal University of Technology, Akure road. The The hydrogeologic equivalent in a well is the static area is gently to moderately undulating with surface water level. In wells, static water levels can be elevation varying from 349-376m (Fig. 1). The area measured directly with appropriate instrument is drain by the popular ‘Ala’ stream which passes (Hiscock, 2005). through the north-east to south-east part of the area. The area fall within the tropical rain forest of The major difference between the vadoze and Southwestern Nigeria, where there exist two major phreatic zones is that the former is mostly filled with seasons; wet and dry. The rain season usually spans air while the latter is filled with groundwater (Freeze through the months of April to October, while the dry and Cherry, 1979 and Fetter, 2011) This difference in season is usually between November and March. The water content often enables subsurface geoelectric average annual rainfall is about 3000mm (Iloeje, stratification through geoelectric depth sounding 1980). The area falls within the Crystalline Basement (Patra and Nath, 1998). In this study conducted Complex of Southwestern Nigeria (Rahaman, 1989). around in Aule area, near Akure, Nigeria, The local rock units consist of Migmatite-Gneiss, hydrogeologic measurements were conducted across Older Granites and Charnockites (Fig.2). The Older 368
Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3(2):368-373 (ISSN: 2141-7016) Granites and Charnockites exist together as single body in all the outcrops visited, while the Migmatite-Gneiss occurs separately 805800
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Fig. 2: Simplified geological map of Akure (modified after Owoyemi, 1996). MATERIALS AND METHODS A combination of methods involving static water measurement and geoelectric sounding was adopted to determine depth to groundwater table at Aule Government area, near Akure, Ondo State, Nigeria and to bring to fore its relevance to groundwater potential. The static water measurement covers 55 available hand dug wells and 21 Vertical Electrical Sounding (VES) stations were occupied across the study area. Well whistle and tape rule were used to
measure static water level and well depth of all the 55 accessible wells. The well locations were painstakingly determined using 12 channels Garmin ETrex Legend Global Positioning System. These measurements were presented as static water level map and static water level variation bar chart. The vertical electrical sounding (VES) survey was carried out using PASI 16GL resistivity meter and its accessories. The current electrode separation AB was varied from a minimum of 2m to maximum of 200m. 368
Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3(2):368-373 (ISSN: 2141-7016) The field data were plotted on a bi-log graph sheet and subsequently interpreted using partial curve matching technique, involving a segment by segment curve matching of the field curves with the theoretical curves (Zohdy, 1965; Keller and Frischnect, 1966 and Koefoed (1979). The derived geoelectric parameters were further refined using a forward modelling computer software; Window Resist Version 1.0 (Vander Velpen, 1988). The VES results were presented as curve types, geoelectric sections and maps.
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RESULTS AND DISCUSSION The static water level map (Fig. 3) shows that depths to water table across the study area vary from 1.08 to 8.46m. In most of the wells static water level was intercepted at depths of 2 to 3m and 6 to 8m (Fig. 4). The static water level has good correlation with groundwater potential of an area (Haverkamp et al., 1999) and could serve as groundwater potential indicator. The static water level map reveal that groundwater occurs at greater depth at the western part of the study area, but occurs at shallow depth at the eastern part, perhaps this was caused by the area relative lower elevation and proximity to ‘Ala’ stream. The water column thickness map (Fig. 5) is a contour map of water column thicknesses estimated from each well. A comparison of the static water level map (Fig. 3) and the water column thickness map (Fig. 5) shows that areas with shallow static water level correspond to area with thicker water column. The geoelectric sounding delineated 2 to 5 geoelectric layers which corresponds to topsoil, weathered layer, weathered bedrock, fractured bedrock and fresh bedrock. Only VES20 gives 2 layer curve type; due to the fact that the bedrock is outcropping at this VES point (Figs.6a-e). The resistivity values vary respectively across the study area from 21-372ohm-m in the top soil, 4-1137ohmm in weathered layer and 99-6522ohm-m in the weathered/fractured/fresh bedrock.
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The geoelectric section along S-N direction (Fig. 7a) shows a thin topsoil with resistivity ranging from 46195ohm-m, while the weathered layer resistivity varies from 8-1137ohm-m. VES points 4, 8 and 9 have weathered layer resistivity typical of clay or highly water saturated geologic materials (8-39 ohmm), these correspond to zones were water table occurs at shallow depth. The geoelectric section along SWNE direction (Fig. 7b) reveals thinner topsoil with very low resistivity values (8-265ohm-m), while the Well Locations weathered layer resistivity ranges from 5-227ohm-m, Roads it is only beneath VES4 that we have higher resistivity value (227ohm-m), resistivity values at other VES points (9, 11, 14 and 13) varies from 5-
Fig. 3: Static water level map of the study area 370
Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3(2):368-373 (ISSN: 2141-7016) 39ohm-m, again this is indicative of clayey soil or highly water saturated geologic materials which are good for groundwater development (Omosuyi et al., 2003). Aquifer depth map (Fig. 8) was based on the geoelectric sounding results and was generated by contouring depths to the top of identified aquifer layers across the study area. The aquifer depth map (Fig. 8) shows that depth to aquifer layer varies from 0.5 to 5.5m across the study area; this shows a good
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correlation with static water level map (Fig. 3), which varies from 1.08 to 8.46m. In both maps, aquifers occur at shallow depth at most portions of the eastern and northern parts of the study area, while they occur at greater depth at the western portion. The little differences observed could be attributed to the fact that well data were not evenly distributed like the VES points.
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Fig. 6a-e: Typical curve types from the study area.
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Hiscock, K.M. (2005). Hydrology Principles and Practice. Blackwell Publishing, pp. 155-211.
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Iloeje, N.P. (1980). A New Geography of Nigeria (New Revised Edition). Longman Group: London, UK. pp. 32-45.
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Keller, G.V. and Frishchnecht, F.C. (1966). Electrical Methods in Geophysical Prospecting. Pergamon Press, New York, pp. 96.
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Koefoed, O. (1979). Geosounding Principles 1. Resistivity Measurements. Elsevier Scientific Publishing, Amsterdam, Netherlands, pp. 275.
Fig. 8: Aquifer Depth Map CONCLUSION The static water measurements and geoelectric sounding could be effectively used to determine depth to water table, which in turn could be used to determine groundwater potential of an area. The combination of maps of static water level, water column and aquifer depth revealed that aquifers could be intercepted at shallow depth at the eastern and northern eastern parts of the study area, while they occur at greater depth at the western parts of the study area, and consequently groundwater potential is higher at the northern and eastern parts.
MacDonalds, A, Davies, J. Calow, R. and Chilton, J. (2005). Developing Groundwater, A Guide for Rural Water Supply. ITDG Publishing, Warwickshire, UK, pp. 358. Omosuyi, G.O., Ojo, J.S. and Enikanselu, P.A. (2003). Geophysical Investigation for Groundwater around Obanla-Obakekere in Akure Area within the Basement Complex of South-Western Nigeria. Journal of Mining and Geology. 39(2):109-116.
ACKNOWLEDGEMENT The authors appreciate the assistance of the following undergraduate students of the Department of Applied Geophysics, Federal University of Technology, Akure who assisted during the data acquisition of this work; Oduwaye, A., Orekoya, A., Edika, O., Ajibade, A. and Jonibola, O.J.
Patra, H.P. and Nath, S.K. (1998). Schlumberger Geoelectric Sounding in Ground Water: Principles, Interpretation and Application. A.A. Bakema Publishers, Brookfield, pp. 183.
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