ABSTRACT
An investigation was carried out on the groundwater potentials and aquifer protective capacity of Ishiagu area, southeastern Nigeria. The study area lies within the lower Benue trough of Nigeria and is underlain by the Albian Asu River Group and the Turonian Ezeaku Shales.The methodologies employed in the study include measurements of static water levels of 15 hand dug wells, pumping tests carried out on 4 boreholes and vertical electrical sounding (VES) of 20 stations. Groundwater potentials of the area was thoroughly characterizedusing aquifer parameters of hydraulic conductivity and transmissivity within unit cells.These parameters were supplemented with those determined from empirical relationships.The hydraulic conductivity and transmissivity determined from the pumping test data range from 0.6m/day to 3.04m/day and 4.86m2/day to 34.93m2/day respectively while those from empirical relationships range from 0.04m/day to 4.34m/day and 0.07m2/day to 61.69m2/day respectively.Three groundwater potential ratings were defined based on the aquifers’ hydraulic conductivity and transmissivity data; poor (45%), fairly good (35%) and good (20%). The hydraulic head map reveals two divergence and two convergence zones. The computed hydraulic heads range from 58.2m to 84.5m.Vertical electrical sounding reveals that the area is characterized by 4-, 5- and 6- subsurface geo-electric layers with the 6-layer type being the dominant type. Longitudinal unit conductance of the 20 VES stations was estimated from the layers’ resistivity and thickness data.Longitudinal unit conductance of the overburden units ranged from 0.07mhos to 2.22mhos. Based on the estimated longitudinal unit conductance, three aquifer protective capacity types were defined namely, weak (25%), moderate (40%) and good (35%). It was observed that areas of good groundwater potentials also have good aquifer protective capacity. Groundwater development should therefore be concentrated more in areas of good groundwater potential for continuous/steady supply of potable water.
CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND INFORMATION
The Ishiagu area lies within the lower Benue trough of Nigeria and is underlain by the Albian Asu River Group and the Turonian Ezeaku Shales. The area is also intruded in many places by Santonian intrusive rocks and mineralized in some parts by hydrothermal Pb –Zn ores. (Kogbe, 1976)
The geology of the area is not favourable for groundwater availability. With population explosion in Ishiagu town occasioned by the presence of several mining and quarrying companies such as Crushed Rock Industries Ltd., Setraco Nig. Ltd. etc, a greater need for potable water has become obvious. More so, the need to assess the protective capacity of the aquifer in the area cannot be overemphasized as there is the danger of its contamination from surface industrial effluents and pollutants emanating from mining and quarrying activities in the area.
As exploitable quantity of groundwater is expected to be available only in fractured or weathered zones of the area, surface geoelectrical method becomes very necessary to locate such areas of reasonable groundwater potential. Furthermore, knowledge of values of aquifer parameters such as hydraulic conductivity and transmissivity in the area is useful in evaluating the groundwater potential of the area.
Although the conventional way of determining the mentioned aquifer parameters has been the use of pumping test method, it has proved to be expensive, tedious and time consuming. Also various formulas available for calculating the aquifer characteristics from pumping test data analysis are valid only if various assumptions about aquifer continuity, thickness, homogeneity, isotropy, well storage and nature of fluid flow are valid under field conditions (Freeze and Cherry 1979),hence the need for a reasonable alternative.Surface geoelectric methods have proved to be a handy alternative to pumping test method in the determination of aquifer parameters (Guérin, 2005). The method is cost and time effective and is used to predict the parameters for even those areas where there are no existing wells (Ekwe et al 2006).
1.2 OBJECTIVESAND SCOPE OF THE STUDY.
The objectives of this research work are to:
(1) Determine the lithological succession and the prospective water bearing horizons in terms of resistivity, thickness and depth across the study area using vertical electrical sounding method
(2) Evaluate hydraulic conductivity and transmissivity of aquifers in the study area using Da-Zarouk parameters from vertical electrical sounding
(3) Compare the value of aquifer hydraulic conductivity obtained from this study with that obtained by pumping test method using statistical correlation tool.
(4) Assess the overall groundwater potential across the study area
(5) Assess the level of protection of aquifers in the Ishiagu area from surface pollutants(aquifer protective capacity) using Da-Zarouk parameters from vertical electrical sounding
(6) Correlate geoelectric sections in the study area with existing borehole logs.
(7) Determine groundwater flow direction using existing well data and elevation values.