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 Groupand 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.
1.3 LOCATION AND ACCESSIBILITY
The Ishiagu area is located between latitude 5.54o N and 6.0o N and longitude 7o 30’ E to 7o35’E; and is in Ivo local Government Area of Ebonyi State, South Eastern Nigeria (Fig.1).
The area is easily accessible through road networks that link it to neighboring communities and cities (Fig. 1). The area is generally gently undulating with occasional hills traceable to intrusive bodies.
1.4 DRAINAGE, CLIMATE AND VEGETATION
The Ishiagu area is drained by several rivers such as the Ivo, Ngada and Iyi Odu rivers (Fig.1). The climate is typical of the tropics. The average annual temperature range is between 25o C and 31oC while an average annual rainfall of about 1500mm to 2400mm is prevalent (Aroh et al 2006, Akintola, 1986,). The area is characterized by two distinct climatic seasons, namely, the rainy season which usually starts and ends around April and October respectively and the dry season that prevails between November and March. A period of extreme coldness and dryness called the harmathan dots the dry season from around the months of December to January (Iloeje, 1981).
In terms of vegetation, the study area falls within the vegetation zone of Eastern Nigeria, which type is forest/derived savannah, dominantly made up of grasses, shrubs and trees.
1.5LITERATURE REVIEW
Since the late 1960s, several studies have been carried out to investigate the interconnection between aquifer geoelectric properties and their hydraulic parameters such as hydraulic conductivity and transmissivity as well as aquifer protective capacity. Such studies include those of Archie (1942), Zohdy (1976), Kelly (1977),Niwas and Singhal (1985), Salem (1999), Purvance, and Andricevic (2000). Although many of these studies were area specific, the approaches can be applied in different geological conditions. Three types of relationships have been derived between geoelectric and hydraulic properties:
i. Relation between Formation factor (F) and hydraulic properties
ii. Relation between Formation resistivity (ρ) and hydraulic conductivity
iii. Relation between transverse resistance (R) and transmissivity
A relation between the aquifer intrinsic permeability and Formation factor to estimate transmissivity from borehole resistivity measurements was developed by Croft (1971). Kosinski and Kelly (1981) did a work on geoelectric soundings for predicting aquifer properties. Niwas and Singhal (1981) established an analytical relationship between aquifer transmissivity and transverse resistance on one hand and between transmissivity and longitudinal conductance on the other hand. Niwas and Singhal (1985) noted that it is better to use a “modified transverse resistivity” instead of transverse resistance where there is remarkable variation in the groundwater quality. Frohlich, and Kelly, (1985) studied the relation between hydraulic transmissivity and transverse resistance in a complicated aquifer of glacial outwash deposits.Mazac et al. (1985) worked on the correlation between aquifer and geoelectrical parameters in both the saturated and unsaturated zones of the aquifers.Ako and Osondu (1986) carried out groundwater investigations at Darazo on the Kerri-Kerri Formation; they concluded that Dar-Zarouk parameters are related to borehole characteristics and pointed out that the highest traverse resistance (T) corresponds to the zone with the highest borehole yield. Mbonu, et al (1991) determined aquifer characteristics in parts of the Umuahia area of Nigeria using geoelectric sounding. McDonald et al (1999) used geoelectric method to estimate the transmissivity of aquifers in the Thames gravels in the basement complex area of Jos, Plateau State. De Lima and Niwas, (2000) estimated hydraulic parameters of shaly sandstone aquifers from geoelectrical measurements. Amadi (2006) carried out a hydrogeophysical survey of Ebonyi South area of Southeastern Nigeria, including Ishiagu and Ohaozara areas. Casmir (2006) used geoelectric investigation involving twenty-six vertical electrical soundings, to investigate the aquifer potential of the underlying bedrock as well as the transmissivity of aquifers in the Jos Plateau area.Yehia, (2004) used geoelectric resistivities sounding for ground water evaluation in the New Qena city and El-Khatatba area of Egypt. Ekwe, et al. (2006) used the concept of Dar-Zarrouk parameters (transverse unit resistance, R and longitudinal conductance, S) in porous media to determine aquifer hydraulic characteristics within the middle Imo river basin Nigeria. Most of the previous studies where the concept of Da- Zarouk parameters was applied were carried out in sandstone aquifers. However, Ekwe et al (2010) used thesame method to estimate aquifer hydraulic characteristics of low permeability (shaly) formation in Oduma town, Enugu State where they determined the aquifer hydraulic conductivity to fall between 0.624m day-1 and 5.5091m day-1 and transmissivity to range from 14.17 m2 day-1 to 174.89m2day-1. The geology and hydrogeology of the Oduma area can be said to be similar to thoseof the present study area, where shale isthe major lithology underlying the area.
Oladapo and Akintorinwa (2007) and Abiola et al (2009) related directly the longitudinal unit conductance, Sof overburden units above an aquifer to the protective capacity (insulation from pollution) of the aquifer.
Utom et al, (2012), used geoelectric methods to delineate groundwater parameters in parts of Enugu, South eastern Nigeria and successfully demarcated areas with good groundwater potential.
Amos-Uhegbu, (2013), established a graphical linear relationship between a set of pumping test data and vertical electrical sounding data and subsequently applied the relationship in determining aquifer parameters in areas where pumping test data were not available in some parts of Umuahia, South Eastern Nigeria.
Batayneh, (2013) applied the Da-zarouk Parameters in exploration of aquifer systems in the Gulf of Aqaba and demonstrated the behavior of the Dar-Zarrouk parameters and its patterns in space over large areas with respect to the occurrence of water aquifer systems in deltaic coastal area.
