Geoelectric Sounding to Delineate Shallow Aquifers in the Coastal Plain Sands of Okitipupa Area, Southwestern Nigeria
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Geoelectric Sounding to Delineate Shallow Aquifers in the Coastal Plain Sands of Okitipupa Area, Southwestern Nigeria. G.O. Omosuyi, J.S. Ojo, and M.O. Olorunfemi* Department of Applied Geophysics, Federal University of Technology, PMB 704, Akure, Ondo State, Nigeria. *Department of Geology, Obafemi Awolowo University, Ile-Ife, Nigeria. ABSTRACT INTRODUCTION In Okitipupa area, south western Nigeria, the The Okitipupa area is underlain by the aquifers of the Coastal Plain sands are largely sedimentary rocks of south western Nigeria. The Quaternary porous sediments, but without records zone constitutes the easternmost segment of the of their subsurface disposition. In this study, fifty extensive Dahomey basin. Studies have shown Schlumberger soundings were conducted over the that sedimentary basins generally contain study area, aimed at delineating the boundaries, enormous quantities of water (Freeze and depth range of the shallow aquifer units, and Cherry, 1979; Fetter, 1980; Kehinde and assessing their vulnerability to near-surface Loehnert, 1989; Edet and Okereke, 2002). To contaminants. realise this fact, several countries worldwide have engaged in extensive studies of basins within The study delineated two major aquifer units their frontiers, using geological/hydrogeological, within the area: (i) the upper/surficial aquifer geophysical, or chemical data, or a combination system, which occur at depths ranging from 5.8m of the above, for proper identification and (around Agbabu) to 61.5m (around Ikoya), and delineation of the water bearing units. Such with materials of higher average resistivity (504.7 studies (Leit and Barker, 1978; van Overmeeren Ωm), suggestive of gravelly/coarse to medium- 1989; Mbonu et al., 1991; El-Waheidi et al., 1992; grained sand and (ii) the intermediate aquifer Kalinski et al., 1993; Frohlich et al., 1994; system, characterised by depth range of 32.1- Ebraheem et al., 1997; Choudhury et al., 2001; 127.5m, average resistivity of 296.8Ωm, typical of Edet and Okeke, 2002; and Lashkaripour, 2003), medium-grained sand saturated with water. often afford the optimisation and proper management of the basins in order to enhance The highly resistive, impermeable materials safe discharge and appropriately safeguarding overlying the aquifer units around Ajagba, the quality status of the water. However, any Aiyesan, Agbetu, Ilutitun, Igbotako, and Erinj rational development groundwater requires suggests that the aquifer units are less vulnerable geophysical data input, among others. to near-surface contaminants than in Agbabu, Igbisin, Ugbo, and Aboto where aquifers are Few studies so far conducted in the area (Ako, overlain by less resistive materials. This study 1982; Omosuyi et al., 1999; Omosuyi, 2001) enabled the delineation of shallow aquifers, their were based on borehole lithologic correlation and subsurface disposition, and identified promising hydro-geophysical characterisation of the areas for elaborate groundwater development in subsurface sequence for suitable location of the area. An integration of such geophysical study groundwater abstraction points. with lithologic logs/drilling data would enhance accurate delineation of aquifers and vulnerability In this study, geoelectric resistivity sounding data quantification in the area. have been used to delineate the boundaries, subsurface disposition and assess the (Keywords: geoelectric sounding, aquifer units, coastal vulnerability status of the shallow water bearing plain sands, aquifer vulnerability) aquifers in the Coastal Plain around Okitipupa. The Pacific Journal of Science and Technology –562– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall) 0 00’N and 6 40’N. The terrain is regionally gently The study is envisaged to upgrade our knowledge undulating southward; topographic elevations of the basin and enhance the success rate of vary from about 84m above sea level in the groundwater abstraction boreholes in the area, in northern part, with gradual slope to a near sea addition to serving as a useful guide to an level swamp flat in the coastal area to the south. elaborate groundwater development programme The area is drained by many perennial streams proposed for the area. and rivers among which are Ominla, Oluwa, Akeun, Ufara, and Oni, while the southern part is particularly characterised by lagoons, coastal PHYSIOGRAPHY, GEOLOGY AND creeks, canals, and several tributaries to the HYDROGEOLOGY extensive River Oluwa. 0The annual temperature ranges from 24 to 27 C and the mean annual The study area (Figure 1) is bounded by 0 0 0 rainfall is over 2500mm (Iloeje, 1981). Longitudes 4 22 ‘E and 5 05 ‘E, and latitudes 6 4 30' 5 00' Odigbo ODIGBO Mkt Otu Ofosu ORE Oja Fagbo Olorunsogo Onisere oke alafia Soba Camp Omotoso Kajola Olowo Orisunmibare Oniparaga Ilebe Lamudifa Mulekangbo Otugbembo Lokubo Foriku Poroye Camp Abusoro Igbolouye Ilubirin Loda Origbe Ayadi Ilado Agbetu Erekiti Lejuwa Aroromi Obu Agbabu Irowo Ladenkehin Legbagbo Iju Oke Ode Aye Iwada Igbotako Settl Ladan Camp Ijuodo ement Igbaje Lamira OKITIPUPA Laworo IRELE Arijan Ilutitun Oluagbo Ayesan Ikoya IRELE 6 30' OKITIPUPA Omi Mahintedo Oloto Ode - Igbodigbo Igodan Araromi - Ayeka Igboegunrin Old Ayeka Shabomi LEGEND Ajoda Italuwa Igbotu Salawa Madagbayo Ebute - Erinje Ojuala Nagbore's Camp Atijere Ago Ukgbene Ipoka Gbelemotin IGBEKEBO Major roads Shegbemi Yopele Aboto Gbeleju Ijaw Camp ILAJE IGBOKODA Minor roads Uba - Arogbo Okorogbon Fiafio Okomu VES Location BI Ebute Ipare Amapere GH Arogbo Saforogbon T Kurawe OF Local Councils Capital B ESE - ODO Olowoso EN IN Fishing Camp Ayande Rivers Mahin Ashisha Biagbene Ugbo Ugbonla Gbu Footpath Aiyetoro Idi Ogba Ajapa Upe Oroto Okoron ado 0 10 Km Ilepete Akimo Obe Adu Upe Obenla 6 00' Sekelewu Ojumole Figure 1: Location Map showing Okitipupa Area. –572– The Pacific Journal of Science and Technology e 9. Number 2. November 2008 (Fall) Volum http://www.akamaiuniversity.us/PJST.htm The area is underlain by the Coastal Plain Sands sands, as well as the underlying impervious or the Benin Formation (Figure 2). The sediments clay/shale member of the Akinbo Formation, of the Coastal Plain, deposited during the Late constitute protective configuration for the aquifer Tertiary – Early Quaternary period (Jones and units. Also, the high annual rainfall and other Hockey, 1964), consist of unconsolidated, coarse favourable climatic and geologic factors to medium- fine grained sands and clayey shale in guarantee adequate groundwater recharge in the places (Okosun, 1998). The sands are generally area. moderately sorted and poorly cemented. The Benin Formation is overlain by lateritic overburden or recent alluvial deposits and underlain by the DATA ACQUISITION AND INTERPRETATION Paleocene Akinbo Formation. This formation is predominantly shally. Outcrops of shales were Fifty (50) Schlumberger vertical electrical mapped around a spring at Ode Aye (Omosuyi, soundings (VES) were conducted across the 2001). The Akinbo shale is underlain by the study area using a maximum current electrode continental Cretaceous sediments of the separation (AB) of 650m. Figure 2 shows the Abeokuta Group (Omatsola and Adegoke, 1981). VES locations. Resistivity measurements were The Coastal Plain sands constitute the major made with a digital resistivity meter (PASI – E2 shallow hydro-geologic units in the area. Aquifers DIGIT) which allows for readout of current (I) and are characteristically continental sands, gravels, voltage (V). or marine sands. The lateritic earth overlying the Figure 2: Geological Map of the Study Area (PTF, 1997), Showing the Sample Locations. The Pacific Journal of Science and Technology –564– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall) The field curves were interpreted through partial occurring at varying depths with variable curve matching (Koefoed, 1979) with the help of thicknesses. The sand and gravel layers master curves (Orellana and Mooney, 1966) and constitute the aquifer units. The geoelectric auxiliary point charts (Zohdy, 1965; Keller and parameters of the aquifer units were determined Frischnecht, 1966). From the preliminary from the interpretation of the sounding curves, interpretation, initial estimates of the resistivity assisted by the distinctive resistivity contrasts and thickness of the various geoelectric layers at between the discrete geoelectric layers. The each VES location were obtained. These upper and lower aquifer horizons work are geoelectric parameters were later used as starting referred to as the surficial (upper) and model for a fast computer-assisted interpretation intermediate (lower) aquifers respectively. (Vander Velpen, 1988). The program takes the manually derived parameter as a starting The results of the interpretation were used to geoelectric model, successively improved on it construct three interpretive sections AB, CD, and until the error is minimised to an acceptable level. EF, taken in the north-south, west-east and northwest-southeast directions (Figure 4). RESULTS AND DISCUSSION The sections (Figures 4a and 4b) reveal that the surficial and intermediate aquifers are overlain in Geoelectric and Lithological Characteristics places by materials with variable thickness and resistivity parameters. In section AB (Figure 4a), Figure 3 shows typical geoelectric curves the resistivity of materials overlying the upper corresponding to VES data from the study area. aquifer ranges from 2 Ωm to 5622Ωm. Curve types identified ranges from simple AK,