Evaluation of Groundwater Potential from Pumping Test Analysis and Vertical Electrical Sounding Results: Case Study of Okigwe District of Imo State Nigeria. Leonard I. Nwosu, M.Sc.*; Anthony S. Ekine, Ph.D.; and Cyril N. Nwankwo, Ph.D. Department of Physics, University of Port Harcourt, Port Harcourt, Nigeria. E-mail: [email protected]* [email protected] ABSTRACT 1978). Adequate water supply is essential to public health and wellbeing as well as agricultural Groundwater potential evaluation of Okigwe and industrial activities. Majority of the people in District of Imo State of Nigeria was carried out the study area do not have access to potable based on 120 vertical electrical soundings (VES) water. This is compounded by the gradual in the area. The Schlumberger electrode array increase in the rate of industrial and commercial was adopted with a maximum electrode spread of activities in the district. 900m. Twelve of the VES stations were sited near existing boreholes to enhance interpretation. The Certain aquifer characteristics are useful in results revealed a multi- layered earth. The assessing the water resources potential of a resistivity of the aquiferous zones ranged from place. These parameters include the hydraulic 33.1Ωm obtained at Umuedi (VES 7) in the conductivity (K), transmissivity (T), and storativity northern part to 32600Ωm obtained at Otoko (S). The determination of these aquifer (VES 93) in the southern part of the area. The characteristics is best done on the basis of data highest aquifer thickness of 104.4m was recorded obtained from pumping wells, (Mbonu et al., at Amonze (VES 95). The hydraulic conductivity 1990; Ekine and Iheonunekwu, 2007). These values obtained from the VES results varied from parameters are needed to execute proper water 9.8854 to 115.9646 m/day. The transmisivity planning and management and also in values ranged from 992.04 to 10263.65m3/day. determining the natural flow of water through an The storativity values determined for the area aquifer and its response to fluid abstraction ranged from 1.59x10-4 to 7.80x10-3 while the (Ekwe et al., 2006; Oseji et al., 2005; Odoh et al., specific capacity is fairly uniform with magnitude 2009). This study is therefore aimed at utilizing of about 877. Groundwater potential evaluation of the aquifer characteristics determined from the district based on borehole hydrogeological pumping test data and those calculated from information revealed that the Southern part of the surface geoelectric soundings to establish the district recorded the highest values of ground water potentials of the study area. groundwater yield of 8292m3/day at Isinweke, 1746 m3/day at Ugiri and 5237 m3/day at Umuelemai. This clearly indicates that the GEOLOGY AND HYDROGEOLOGY OF THE southern zone is the most prolific in terms of STUDY AREA groundwater exploitation and thus the most promising for siting productive boreholes. Okigwe District is in Imo State of Nigeria. The District is made up of six Local government (Keywords: transmissivity, storativity, groundwater Areas; Isiala Mbano, Ihitte Uboma, Ehime Mbano, yield, specific capacity, VES) Onuimo, Obowo and Okigwe. The area lies between latitude 5o03’N to 5o57’N and longitude 7o04’E to 7o26’E (Figure 1), covering a land area INTRODUCTION of about 1,824 km2. The study area is situated in a complex geological setting. The stratigraphic Water is an important element of the natural units observed include: the Benin Formation, The environment but is becoming increasingly scarce Ogwashi - Asaba Formation, the Bende - Ameki owing to increasing demand and deteriorating Formation, Imo Shale Formation, Nsukka quality due to pollution (Ayoade and Oyebande Formation and Ajali Formation. (Akaolisa and The Pacific Journal of Science and Technology –536– http://www.akamaiuniversity.us/PJST.htm Volume 14. Number 1. May 2013 (Spring) 7o 26’E 7o 26’E o o 5 57’N 5 57’N C 2 5 3 4 120 6 119 8 7 117 118 115 11 114 9 10 12 116 13 113 112 14 16 15 108 111 109 110 17 107 106 19 21 18 104 20 24 105 103 23 23 101 K 25 26 27 28 102 L 29 65 64 63 62 73 66 72 61 74 75 67 44 68 7 1 45 59 77 46 76 70 100 78 Abekonto 80 99 79 98 Amainyi- Ukwu D 81 97 95 94 e 83 93 83 84 91 85 92 88 89 86 87 5o 30’N 5o 30’N 7o 04’E Sounding Station 7o 04’E Sounding Station near Borehole Fig. 1: Map of study area showing sounding stations and interpretative Geoelectric Cross-Section (IGCS) traverse. Figure 1: Map of Study Area Showing Sounding Stations and Interpretative Geoelectric Cross-Section (IGCS) Traverse. The Pacific Journal of Science and Technology –537– http://www.akamaiuniversity.us/PJST.htm Volume 14. Number 1. May 2013 (Spring) Selemo, 2009). The Benin Formation is overlain the VES stations were sited near existing by lateritic overburden and underlain by the boreholes to enhance interpretation. Ogwashi - Asaba Formation which is in turn underlain by the Ameki Formation of Eocene to Oligocene age (Mbonu et al., 1990). The Benin Analysis of Pumping Test Data Formation consists of coarse - grained gravelly sandstones with minor intercalations of shales The pumping test data obtained for the study area and clay. The sand units which are mostly coarse is displayed in Table 1. grained, pebbly and poorly sorted, contain lenses of fine grained sands (Onyeaguocha, 1980; Short The Hydraulic Conductivity was determined using and Stauble, 1967). The Southern part of the the Logan (1964) approximation of Thiem’s study area covering Obowo, Southern part of equation given by Equation 1, for hydraulic Ehime Mbano and Isiala Mbano fall within this conductivity. formation. 1.18Q The Ogwashi - Asaba Formation is made up of K .......... .......... .......... ..............(1)1 variable succession of clays, sands and grits with hSmw seems of lignite. It also forms part of the study area while the Ameki Formation consists of and the transmissivity obtained from, greenish - grey clayey sandstones, shales and mudstones with interbedded limestones. This Tr = Kh (2) Formation in turn overlies the impervious lmo Shale group characterized by lateral and vertical where variations in lithology. It is underlain in succession Q = well discharge by Nsukka Formation, Ajali Sandstones and h = screen length Nkporo Shales. Smw = draw down. Values of K and Tr were computed for the METHODOLOGY boreholes using the pumping test data in Table 1. A total of 120 vertical electrical soundings (VES) Fetter, (2007) gave the following equations for were carried out in the study area (Figure 1) using determination of storativity of wells: the Schlumberger electrode configuration and a maximum electrode spacing of 900m. The ABEM h –h = (3) terrameter (SAS) 300B was used to acquire the o data. Four stainless non polarizable electrodes were used, two current electrodes and two and potential electrodes. A freshly charged 12V dc Trtu supply was used to supply current. S 2 .......... .......... .................... ..4(4) 360r The current electrode spacing was increased symmetrically about the station point, keeping the u = argument in the solution of the potential electrode constant until it became differential equation of the necessary to increase the potential electrode as confined flow of water in the the recorded signal diminished. The apparent aquifer. resistivity values computed were plotted against half of the current electrode spacing (L/2) on a ho – h = Ssw = draw down log-log graph scale. The sounding curves obtained were subjected to conventional partial r = radial distance of the Pumping curve matching using the Rijks Waterstaat (1988) Well master curves to obtain the initial model parameters (resistivities and thickness) for W(u) = Well function computer aided interpretation. The software package used is the Schlumberger automatic S = Storativity analysis version 0.92 (Hemker, 1985). Twelve of t = time of pumping The Pacific Journal of Science and Technology –538– http://www.akamaiuniversity.us/PJST.htm Volume 14. Number 1. May 2013 (Spring) Table 1: Existing Borehole/Pumping Test Data – Converted Units (Source: Imo State Cooperation, Owerri, Imo State). (m) (m) (m) (m) (m) (m) (m) Using Equation 3 the value of the well function The values obtained for the bore holes ranged W(u) was obtained and the equivalent value u, from 5.0 x 10-6 to 2.1x10-4 with an average obtained from the table and this enabled the values of 4.93 x10-5 . computation of S to be achieved using Equation 4. The Specific Capacity of the wells (Sc) was determined using Chatter Jee (2005) formula: The storativity values for the seven boreholes with draw down values are displayed in Table 2. The storativity values are fairly uniform and varied Sc = 2.73kh (7) from 7.3x10-5 to 2.558 x10-3 which agrees with the standard value, S < 0.005 (Fetter, 2007). Equation 6 can be modified by correct substitutions to give: The specific storage S is obtained from the s equation: S = 0.85kh c K = hydraulic conductivity S = S h (6) s h = screen length where h = screen length The Pacific Journal of Science and Technology –539– http://www.akamaiuniversity.us/PJST.htm Volume 14. Number 1. May 2013 (Spring) Table 2: Aquifer Characteristics Calculated for some Boreholes Located in the Study Area. The values calculated for the boreholes are Transmissivity of Aquifer Layer (Tr) shown in Table 2. Tr = KσR (9) Aquifer Characteristics Determined from VES R = transverse resistance Results σ = conductivity The aquifer characteristics for the VES stations were determined using the following equations: DISCUSSION OF RESULTS Hydraulic Conductivity (K) The results show that the study area is composed of multi geoelectric layers. There is marked K = (8) variation in resistivity with depth across the entire study area.