1.0 Introduction
1.1 Background Of The Study
Hidden beneath the varied landscapes of the Niger Delta is a treasured and important natural resource. It is neither petroleum nor natural gas, which are the natural resources that are certainly important and have brought wealth to many people. This hidden treasure is water, and to be more specific, groundwater. Groundwater occurs everywhere but sometimes its availability in economic quantity depends solely on the distribution of the subsurface geologic units that are referred to as the aquifers.
This implies that where groundwater is not potentially endowed enough, there may be either complete lack or inadequacy due to increasing industrial and domestic needs (Akpan, 2006 and George, 2010). Although groundwater is a renewable resource, fear is being nursed about its imagined danger in case of inadequacy or lack. The universality of its utility heightens the degree of fear as no other fluid can replace the uncountable roles played by water in our communities. However, when many people think of a water source, they think of lakes, rivers and streams; in other words, surface water. Of all the usable freshwater in the world, approximately 97 percent of it is groundwater. According to the United Nations, 10 million cubic kilometres of water are stored underground. The United States Geological Survey states that there is about 4.2 million cubic kilometres of water within 0.8 kilometre of the earth's surface.
Groundwater is the water that lies beneath the ground surface, filling the pore space between grains in bodies of sediment and elastic sedimentary rocks and filling cracks and cavity in all types of rocks (Plummer et al, 1999). Observations have shown that a good deal of surplus rainfall runs-off over the surface of the ground while the other part of it infiltrates underground and becomes the groundwater responsible for the springs, lakes and wells (Oseji et al., 2006). Groundwater is often withdrawn for agricultural, municipal and industrial use by constructing and operating extraction wells.
Groundwater is also widely used as a source for drinking supply and irrigation (UNESCO, 2004). Although groundwater cannot be seen above the earth, a scope of techniques can be used in determining its availability in the subsurface. Surface investigation allows us in deciding the information about type, porosity, water content and density of subsurface condition. This is usually done with the help of electrical and seismic methods and without any drilling on the ground. The data supplied by these techniques are partly reliable and it is less expensive. It gives only indirect signs of groundwater so that the underground hydrologic record must be inferred from the subsurface investigations. Of all the surface geophysical methods, electrical resistivity has been employed most for groundwater exploration (Egbai, 2011). This is because the equipment is portable, simple, field logistics are easy and straightforward and the analysis of data is economical and less tedious than other methods (Zhody et al, 1993; Egbai, 2011).
As time goes on the demand for water for various purposes will be increasing day by day due to increasing population within Federal University of Petroleum Resources Effurun.
This work is aimed at delineating the depth to groundwater using Vertical Electrical Sounding (VES) with the Schlumberger electrode configuration in the Sports Complex of FUPRE to be able to infer suitable locations where prolific boreholes can be sited.
1.2 Statement Of Problem
Here this study focuses mainly on the delineating of good groundwater aquifer in the Sports Complex of FUPRE and how it can be evaluated using resistivity method. But first I will like to discuss briefly about the basic terms in groundwater study.
1.2.1 Basic Terms In Groundwater Study.
Aquifer:
An aquifer is a ground-water reservoir composed of geologic units that are saturated with water and sufficiently permeable to yield water in a usable quantity to wells and springs. Sand and gravel deposits, sandstone, limestone, and fractured crystalline rocks are examples of geological units that form aquifers. Aquifers provide two important functions:
- They transmit ground water from areas of recharge to areas of discharge.
- They provide a storage medium for useable quantities of groundwater.
The amount of water a material can hold depends upon its porosity. The size and degree of interconnection of pore spaces (permeability) determine the materials’ ability to transmit fluid.
Unconfined Aquifers:
An unconfined aquifer is one in which a water table varies in undulating form and in slope, depending on areas of recharge and discharge, pump rate from wells, and permeability. Rises and falls in the water table correspond to changes in the volume of water in storage within an aquifer.
Figure 1.1 shows an idealized section through an unconfined aquifer; the upper aquifer is also unconfined. Contour maps and profiles of the water table can be prepared from elevations of water in wells that tap the aquifer to determine the quantities of water available and their distribution and movement. A special case of an unconfined aquifer involves perched water bodies (Fig.1.1).
This occurs wherever a groundwater body is separated from the main groundwater by a relatively impermeable stratum of small areal extent and by the zone of aeration above the main body of groundwater. Clay lenses in sedimentary deposits often have shallow perched water bodies overlying them. Wells tapping these sources yield only temporary or small quantities of water.
Confined Aquifers:
Confined aquifers also known as artesian or pressure aquifers. Occur where groundwater is confined under pressure greater than atmospheric by overlying relatively impermeable strata. In a well penetrating such an aquifer, the water level will rise above the bottom of the confining bed, as shown by the artesian and flowing wells. Water enters a confined aquifer in an area where the confining bed rises to the surface; where the confining bed ends underground, the aquifer becomes unconfined. A region supplying water to a confined area is known as a recharge area; water may also enter by leakage through a confining bed. Rises and falls of water in wells penetrating confined aquifers result primarily from changes in pressure rather than changes in storage volumes. Hence, confined aquifers display only small changes in storage and serve primarily as conduits for conveying water from recharge areas to locations of natural or artificial discharge.
Figure 1.1: Schematic Cross-sections of Aquifer Types (Modified after Hartan et al, 1989) — The diagram is shown in the complete material which you will get before the end of this article
Leaky Aquifer:
Aquifers that are completely confined or unconfined occur less frequently than do leaky, or semi-confined, aquifers. These are a common feature in alluvial valleys, plains, or former lake basins where a permeable stratum is overlain or underlain by a semi-pervious aquitard or semi-confining layer. Pumping from a well in a leaky aquifer removes water in two ways: by horizontal flow within the aquifer and by vertical flow through the aquitard into the aquifer.
Aquitard:
An aquitard is a partly permeable geologic formation. It transmits water at such a slow rate that the yield is insufficient. Pumping by wells is not possible. For example, sand lenses in a clay formation will form an aquitard.
Aquiclude:
An aquiclude is composed of rock or sediment that acts as a barrier to groundwater flow. Aquiclude are made up of low porosity and low permeability rock/sediment such as shale or clay. Aquiclude have normally good storage capacity but low transmitting capacity.
Aquifuge:
These are a geologic unit that does not have interconnecting pores, it is neither porous nor permeable thus can’t store or give out water e.g. igneous rocks.
1.3 Aim And Objectives Of Study
This research is aimed at delineating the depth to groundwater using vertical electrical sounding (VES) with the Schlumberger electrode configuration in the Sports Complex, Federal University of Petroleum Resources Effurun, Delta state.
The Objectives of Study are:
- To detect subsurface layering and its resistivity
- To investigate the hydrological conditions of the area with the view of delineating the potential area for groundwater development.
1.4 Significance Of Study
The Significance of the Study is important in following ways:
- It will provide useful information on the ground water potential of the subsurface in the area of study, to aid the site for productive boreholes.
- Geo-electric details of the subsurface gotten from this study will give sound knowledge of the subsurface geology such as the subsurface layering and its resistivity, hydrological conditions and the structural geology.
1.5 Scope Of Study
The scope of study was directed towards data acquisition through Vertical Electrical Sounding (VES) carried out in the study area alongside data interpretation to investigate the subsurface geologic characteristics for delineating a good aquifer.
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