GEOELECTRICAL INVESTIGATION FOR GROUND WATER A CASE STUDY OF TAMBUWAL LOCAL GOVERNMENT AREA
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1.1 GENERAL STATEMENT
Water remains a major source of life to all and sundry big or small, it comes in either small or large quantity. The quality of water required for domestic and industrial use also needs to be taken into consideration. Underground Water Exploration using Electrical Resistivity Method in a sedimentary environment was done in Sokoto State (Alile et al., 2008).
Groundwater makes up about 1% of the water on the Earth (most water is in oceans. Groundwater is found beneath the unsaturated zone where all the open spaces between sedimentary materials or in fractured rocks is filled with water and the water has a pressure greater than atmospheric pressure. To understand the ways in which groundwater occurs, it is needed to think about the groundwater bearing formation properties such as porosity and permeability. The water bearing formation is termed AQUIFER. Aquifer: which is a geologic formation sufficiently porous to store water and permeable enough to allow water to flow through them in economic quantities. A relatively inexpensive way to prospect for groundwater, both on a small and large scale is by using electrical resistivity method of geophysical prospecting; this method is fast, repeatable, relatively cheap and non-intrusive, thus making it a practical alternative to traditional approaches (Skinner and Heinson, 2004). The electrical resistivity of rocks depend on several factors, some of which include; the presence of conductive minerals such as base metal sulphides or oxides and graphites in the rock. Most rocks without these minerals are usually poor conductors and their resistivity is determined primarily by their porosity, degree of fracturing and the degree of saturation of the pore spaces (Cook et al., 2001). Electrical method uses direct current or low frequency alternating current for subsurface investigation (Brooke and Kearey, 1984). Probing involves the adoption of the current and the potential electrode spread is used to delineate the apparent resistivity and it follows that the deeper the probing, the farther away from the current source is the measurement of the potential difference regardless of the electrode array utilized (Zohdy et al., 1980).
Groundwater is essential but necessary substitute to surface water in every society. It’s no doubt a hidden, replenish able resource whose occurrence and distribution greatly varies according to the local as well as regional geology, hydro geologic setting and to an extent the nature of human activities on the land. Groundwater occurrence in a Precambrian Basement terrain is hosted within zones of weathering and fracturing which often are not continuous in vertical and lateral extent (Jeff, 2006). There is a steady rise in the demand for groundwater in most hard rock areas most of which cannot boast of any constant surface source of water supply (Adanu, 1994). Kano area is underlain by rocks of the Nigerian Basement Complex comprising migmatite-gneiss complex, Younger Metasediments, Older and Younger Granites. The aquifers of the Basement Complex rocks are the regolith and the fractures in the fresh bedrock which are known to be interconnected at depth (Mohammed, 1984; Alagbe, 1987; Adanu, 1989; Uma and Kehinde, 1994). In a recent hydrogeological study carried out in parts of Kano area, Bala (2008) has shown that regolith aquifer derived from schists and gneisses of sedimentary origin (orthogneisses) proved to be a difficult groundwater terrain contrary to the observations in the earlier works that not only indicated similarity in aquifer performance across the different bedrock types, but also that these aquifers compare with those in other parts of the African Shield. He also noted that wells located in areas underlain by schists and similar rocks were generally deep and the depth to the water table in them is larger than in those located in the other rock types. The failure rate in most groundwater project recorded in Basement Complex aquifers has informed the general acceptance of a geophysical survey as a compulsory prerequisite to any successful water well drilling project (Dan Hassan, 1999). The electrical resistivity method involving the vertical electrical sounding (VES) technique is extensively gaining application in environmental, groundwater and engineering geophysical investigations (Zohdy et al., 1980; Aina et al., 1996; Olorufemi et al., 1993 and 2004; Afolabi and Olorufemi 2004 and Abubakar and Danbatta 2012).
The importance of groundwater as a water supply source to the socioeconomic development of any nation is tremendous. However, the difficulties in exploration and exploitation usually encountered in the basement areas where aquifers are both isolated and compartmentalized, requires the use of multi-disciplinary approach involving, geological and hydro-geological mapping and geophysical investigations to ensure success. The localization of groundwater within these zones is controlled by a number of factors which include the parent rock type, the depth, extent and pattern of weathering, thickness of the weathered materials, the sand/clay ratio and the degree of fracturing, fissuring and jointing (Oyawoye, 1964). These hydro-geological characteristics needs to be properly assessed and interpreted for sound groundwater development, modeling, well design and construction especially for localities lacking such useful information.
Electrical resistivity method of geoelectrical techniques happens to be the most preferred method in groundwater potential.
Vertical Electrical Sounding (VES) is a geoelectrical common method to measure vertical alterations of electrical resistivity.
The method has been recognized to be more suitable for hydrogeological survey of sedimentary basin (Kelly and Stanislav,
1993).The electrical resistivity technique involves the measurement of the apparent resistivity of soils and rock as a function of depth or position. The most common electrical technique needed in hydrogeologic and environmental investigations is vertical electrical soundings (resistivity sounding). During resistivity surveys, current is injected into the Earth through a pair of current electrodes, and the potential difference is measured between a pair of potential electrodes. The current and potential electrodes are generally arranged in a linear array. Common arrays include dipole-dipole array, pole-pole array, Schlumberger array and the Wenner array. The bulk average resistivity of all soils and rock influencing the current. It is calculated by dividing the measured potential difference by the input current and multiplying by a geometric factor specific to the array being used and electrode spacing.
1.2 STATEMENT OF PROBLEM
The persistence problem of poor geoeletrical investigation for ground water remains very worrisome to stake holders in the state and Nigeria in general. Scholars have attributed the problem to poor handling of ground water sources by inhabitance of the local governments. Previous studies also have investigated on the relationship between the experts who regulate and treat ground water and inexperienced persons who also regulates this water body. Water resources of the study area are threatened by increasing population trend with a resulting increase in water demand and the stresses of water use for various activities. The progressive population growth has led to severe shortage of potable water for the area. This poses as a great challenge to both the citizens and the Government. Thus, the inadequate supply of potable water in the area has been a chronic problem because prospection for a steady and reliable water supply from subsurface or groundwater has not been carried out.
1.3 AIMS AND OBJECTIVES OF THE STUDY
This study intends to undertake the investigation of the geoelectrical characteristics in Tambuwal Local Government in Sokoto State Nigeria.
The primary objectives of this study are to investigate Geoelectrical for groundwater in the study area and determine the geoelectric parameters and overburden thickness in the area from the VES data.
1.4 PHYSIOGRAPHIC SETTING
1.4.1 LOCATION, ACCESSIBILITY AND DRAINAGE PATTERN
Sokoto is located at the extreme Northwest of Nigeria. Tambuwal is a Local Government Area in Sokoto State, Nigeria near the confluence of the Sokoto River and the Rima River. As of 2006 it has a population of 427,760. Its headquarters are in the town of Tambuwal (or Tambawal or Tambawel) on the A1 highway at 12°24′00″N 4°40′00″E. It has an area of 1,717 km² and a population of 224,931 at the 2006 census. Sokoto is the modern-day capital of Sokoto State (and its predecessor, the north-western State). The study area is accessible as a result of the availability of roads; the terrain is generally low-lying with elevation between 8 -17m above mean sea level and slopes unperceptively towards the Atlantic Ocean (International Journal of Science and Technology Volume 3 No. 2, February, 2014).
1.4.2 CLIMATE AND VEGETATION
Tambuwal Local Government, Sokoto is in the dry Sahel surrounded by sandy savannah and isolated hills. With an annual average temperature of 28.3 °C (82.9 °F), Tambuwal Local Government, Sokoto is one of the hottest cities in the world, however the maximum daytime temperatures are generally under 40 °C (104.0 °F) most of the year, and the dryness makes the heat bearable. The warmest months are February to April, where daytime temperatures can exceed 45 °C (113.0 °F). The highest recorded temperature is 47.2 °C (117.0 °F), which is also the highest recorded temperature in Nigeria. The rainy season is from June to October, during which showers are a daily occurrence. The showers rarely last long and are a far cry from the regular torrential showers known in many tropical regions. From late October to February, during the ‘cold season’, the climate is dominated by the harmattan wind blowing Sahara dust over the land. The dust dims the sunlight, thereby lowering temperatures significantly and also leading to the inconvenience of dust everywhere in the house.
The region’s lifeline for growing crops is the floodplains of the Sokoto-Rima river system, which are covered with rich alluvial soil. For the rest, the general dryness of the region allows for few crops, millet perhaps being the most abundant, complemented by maize, rice, other cereals, and beans. Apart from tomatoes, few vegetables grow in the region. The low variety of foodstuffs available has resulted in the relatively dull local cuisine. In terms of vegetation, Sokoto falls within the savannah zone. This is an open tse-tse fly-free grassland suitable for cultivation of grain crops and animal husbandry. Rain starts late and ends early with mean annual rainfall ranging between 500 mm and 1,300 mm. There are two major seasons in Sokoto, namely wet and dry. The dry season starts from October, and lasts up to April in some parts and may extend to May or June in other parts. The wet season on the other hand begins in most parts of the state in May and lasts up to September, or October. The harmattan, a dry, cold and fairly dusty wind is experienced in the state between November and February
1.5 REVIEW OF PREVIOUS WORKS
The electrical resistivity method has been used by many researchers to detect bedrock fractures, overburden thicknesses, geoelectric layers, groundwater potential and so on. Ozegin et al., (2008) used combined electromagnetics method and vertical electrical sounding to establish groundwater viability in Oke-Agbe high school field located in Akoko North-west local government area of Ondo state based on a clear-cut relationship between electromagnetics method and vertical electrical sounding. Both methods were jointly used for investigation to determine the overburden thickness, geo-electric parameters and groundwater potential.
Selemo et al (1995) worked on an appraisal of the usefulness of Vertical Electrical Sounding (VES) in groundwater exploration in Nigeria and concluded that the results of VES with the schlumberger array in many parts of Nigeria have being very useful in identifying viable locations for water boreholes. Adiat et al., (2009) used integrated geophysical techniques involving the VLF-EM and the electrical resistivity sounding methods to map Oda town, Southwestern Nigeria to determine the groundwater potential of the town. The qualitative interpretation of the VLF-EM results identified areas of hydro-geologic importance and formed basis for vertical electrical sounding (VES) investigation. On the basis of the geo-electric parameters, the area was zoned into good, intermediate and poor groundwater potential zones. Olayanju G.M (2011) carried out a geophysical mapping involving VLF electromagnetic profiling along seven profiles, ten offset wenner and two azimuthal soundings in the study of perennial spring sites at Iloyin community in Akure metropolis, Southwest Nigeria. Odoh and Onwuemesi (2009) use Azimuthal Resistivity Survey (ARS) to determine and characterize the anisotropic properties of fractures in Presco campus of Ebonyi state University Nigeria, for evaluation of groundwater development and flow within the area. The azimuthal resistivity survey results show that there is significant anisotropy between depth and fractures.
Variation of the coefficient of anisotropy has been shown to have the same functional form as permeability anisotropy. Thus, a higher co-efficient of anisotropy implies higher permeability anisotropy. The results also indicate better permeability and porosity.
Isifile and Obasi (2012) used radial vertical electrical sounding (RVES) around Ifon, south western Nigeria, to determine electrical anisotropy and map trend of concealed structure. The results show that the concealed bedrock is anisotropic with causative features such as foliation, joints and fault which could favour groundwater storage.
Ajibade et al, (2012) used Azimuthal Resistivity Survey (ARS) to investigate the groundwater potentials and anisotropic properties of fractures for sustainable groundwater development within Ibadan metropolis. Result of groundwater head contouring showed that groundwater flow is dominantly in directions which are associated to fracture-controlled flow.
Olasehinde and Bayewu (2011) used evaluation of electrical resistivity anisotropy to report the potency of combination of anisotropy polygon and isoresistivity map in reducing ambiguity inherent in a single geophysical parameter in OdoAra, west central Nigeria.
Onabanjo (2001) carried out geophysical survey using resistivity method in Ago-iwoye southwestern Nigeria and discovered two types of subsurface zones which are associated with groundwater exploration namely the weathered basement and fractured basement in which both are separated by barriers of unaltered rocks tending to reduce the possibility of groundwater accumulation.
Oyedele (2001) carried out a geoelectric investigation of groundwater resources at Onibode area, near Abeokuta South-West of Nigeria. Reinhard (1994) carried out a combined geo-electrical and drill-hole investigation for detecting fresh-water aquifers in Northern Western Missouri. Application of surface geophysics to groundwater investigation has been carried out (Zohdy et al., 1974). Zohdy (1974) did a work on automatic interpretation of Schlumberger sounding curves. Alile (2008) carried out a geophysical investigation for underground water in the southern and central senatorial districts of Edo State, Nigeria, using electrical resistivity survey and seismic refraction survey methods.
1.6 SIGNIFICANCE OF STUDY
The reason why groundwater has become more popular as a source of potable water in Nigeria is due to its quality when compared to other water sources. It is known to be free most times from pollutants and hence requires little or no decontamination before use. It is noted that groundwater is most generally free from odour, colour and has very low dissolved solid. It is also not usually affected by natural factors such as drought. Geophysics surveys have been most widely used because of the basic advantage of providing more accurate results than other methods. The electrical resistivity method has been the most commonly used geophysical tool for groundwater investigation because of its advantage which include simplicity in field technique and data handling procedure. Electrical resistivity methods are effectively used for groundwater exploration in areas where good electrical resistivity contrast exists between the water bearing formation and the underlying rocks. The method enables the determination of subsurface resistivity by sending an electric current into the ground and measuring the electrical potential produced by the current. This study adds to the literature of researches on the Geoelectric investigation for ground water in Tambuwal Local Government area. This work first of all is timely in the light of recent concern about using resistivity technique to successfully investigate groundwater potential in different geological settings. Secondly, the result of the study will help the experts identify their strength and weakness and develop a desire to improve upon their weakness. Thirdly, this study will help policy makers and researches by exposing them to the need to adequate monitoring of ground water for the benefit of the masses. Finally, the findings from this study will serve as a reference material to undergraduate students from geophysics, physics, electrical and even marine engineering department in their future research.
1.7 SCOPE OF THE STUDY
The scope of the study is to carry out a Geoelectric investigation for ground water in Tambuwal Local Government in Sokoto State, Nigeria by determining the groundwater potential and also determine the geo-electric parameters and overburden thickness in the area from the VES data, and construct earth model revealing geo-electric sequence /subsurface lithology in the area.
1.7 LIMITATION OF STUDY
Study Area: The Geoelectrical investigation for ground water in Tambuwal local Government was a major limitation for this study due to the topography and land scape of the environment.
Population: The dense population of the inhabitance also served as a limiting factor to the study. Research has it that population contributes to the scarcity and abundance of ground water especially in a rural environment where modern waters sources are limited.
Financial constraint: the finance at the disposal of the researcher during the course of the study wasn’t sufficient enough to run the expenses of the research work.
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