Characterization of Aquifers in Parts of Abia State, Southeastern Nigeria

: This study was carried out to map and characterize the water bearing formations (aquifers) in Abia State, southeastern Nigeria. Aquifer hydraulic properties were obtained from existing borehole logs and pumping tests data obtained from the field. More recent productive boreholes in the study area were accurately geo-referenced by taking GPS elevations and coordinates. A synergy of all data collected during field visits and those gleaned from the literature and the geologic information available were analysed and interpreted to meet the broad framework and specific objectives of this study. Vertical electrical resistivity depth sounding for inferring the resistivity and thicknesses of the vertical succession of different conducting zones in the subsurface was employed in characterization of the subsurface aquiferous units. The depth of the boreholes drilled varies from 21.02m - 250m, while the static water level (SWL) varies between 4.57m around Ukwa-West to about 35.97m around Aba South. The depth range of 9.15m to 53.05m to the aquiferous zone for Umuahia Local Government Area is in agreement with the value of 35.0m. Very productive aquifers in Abia State are limited to the alluvial deposits and the Coastal Plain Sands lithologies comprising: fine, medium and coarse-grained and often pebbly sands with some intercalations of clays. The alluvium occurs mainly in Ukwa West and East Local Government Areas. Drilling depths in this aquifer range from 30 to 140m. Prolific production is expected from this alluvium which has an approximate permeability of 35m /day. The High production rate 2 (Permeability = 35m /day) is expected in the Coastal Plain Sands sediments found in all Local Government Areas 2 stretching south of Umuahia and Bende. Drilling depths ranging from 40 to 250 m are recommended for boreholes in these LGAs. However, more precise drilling depths must be confirmed by hydro-geophysical site survey, because the geology of Abia State becomes more complicated north of Umuahia and Bende LGAs. This complication in geology affects all other LGAs north of Umuahia. in State


INTRODUCTION
developed to its full potential as a source of high Groundwater forms the most important source of of Abia State [1]. Rapid urban expansion has often water supply in the urban, sub-urban and rural areas of resulted in increased risk to groundwater quality in areas most Sub-Saharan African (SSA) countries. It therefore of recharge. In the past several decades, climate change drives a country's socio-economic development. Abia has resulted in unpredictable rainfall events. This State, a component part of Nigeria, a Sub-Saharan country generally affects groundwater recharge quality and can also derive its socio-economic activities from the quantity negatively and also controls available surface sustainable development of available groundwater (in the water. All these create enormous challenges and pose State). Profitable groundwater everywhere in the world threats to groundwater and consequently affect exists in the sub-surface geologic material known as the continued provision of adequate and safe potable water aquifer. Since it exists below the earth's surface and is not for present and future generations of people everywhere visible to anybody, its occurrence, movement, flow on the planet earth. Abia State is not excluded from such direction and other attributes are poorly understood by negative impacts. The mapping and characterization of most people. Consequently, groundwater has not been aquifers in Abia State at a time like this, has therefore economic activities and good water quality in many parts become necessary so that adequate planning to ensure about unprecedented socio-economic progress and continued economic activities through sustainability of continued supply of safe potable water now and in the groundwater. It is anticipated that this process will bring future. The study area, The topography in the southern part of the state is Abia State is located in the south-eastern part of Nigeria low-lying while the other parts of the state have ( Fig. 1). The State is known for its commercial activities moderately high plains with elevations ranging between centered at Aba, which was formerly a British Colonial 20 and 200 metres above sea level (Fig. 3). The vegetation Government outpost. The entire state lies approximately of the area is that of the rainforest comprising various between latitudes 4°48'N and 6 °02'N and Longitudes species of shrubs and high forest trees all over the area -7°09'E and 7°58'E of the Greenwish Meridian (Fig. 2). On both within the hilly and depressed areas. the north and the northeast, the state is bounded by Geologically, Abia State is located within the Enugu and Ebonyi states respectively. The eastern transition zone of the Benin and Ogwashi-Asaba boundary is occupied by the Cross River State, while the Formations of the coastal sedimentary rocks of the southeast border is shared by Akwa Ibom State. Rivers northern Niger Delta. Therefore, the major geologic State occupies the southern and southwest boundaries.
sequences encountered in the area include the Coastal The western and northwest borders are occupied by Imo Plain Sands otherwise known as Benin Formation, the and Anambra States respectively. The entire State is Ogwashi-Asaba and Bende-Ameki Formations (Fig. 4). divided into seventeen (17) administrative units called These lithologies are underlain by the Paleocene Imo Local government Authorities (LGAs) (Fig. 2).
shales which are conspicuous in the northern part of the The physiographic information on Abia State reveals study area. The Benin Formation (Late Tetiary-Early that the climate of the area falls within the equatorial Quaternary age) is the most predominant and it is climatic belt with alternating dry and wet seasons overlain by the Recent Alluvium and underlain by the (Igboekwe and Nwankwo, 2011). The wet season starts Ogwashi-Asaba Formation. The Benin Formation is about from March and ends around October, while the dry 200m thick (Ebilah-Salmon and Partners, 1993) and the season spans between November and February each year.
lithology is unconsolidated fine-medium-coarse-grained, The seasonal variation in the climate of the state is caused cross bedded sands occasionally pebbly with localized by the northeast trade wind that blows across the Sahara clays and shales (Igboekwe et al., 2006). Most of the Desert and the southerly humid marine air mass that aquifers in the study area tap from this formation. The blows across the Atlantic Ocean. The annual rainfall is yield is prolific. The Ogwashi-Asaba Formation is made between 2000 mm and 2250 mm in the south and between up of variable succession of clays, sands and grits with 1250 and 2000 mm in the northern part of Abia (Igboekwe seams of lignite. It is directly underlain by the Bendeand Nwankwo, 2011).

Methods of Study:
The materials used in the execution of boreholes were chosen on account of availability of this project include the location map ( Fig. 2) which was lithologic logs and pumping test results. Supplementary divided into grids to enable the proper planning and geological and hydrogeological information were obtained execution of the exercise. The approach for aquifer from available maps and records of hand dug wells. mapping in the area was carried out systematically, from The logs enabled the establishment of the subsurface one Local Government to the other. At every borehole stratification, casing and screen positions, static water site, the coordinates of the location were taken using a levels and aquifer textural properties while the aquifer Geographical Positioning System (GPS) and recorded in pumping test data for each borehole comprised 1 to 3 the field notebook. Detailed information such as the yield, stage step-drawdown tests, single stage and recovery age of the borehole as well as the name of the measurements. These were used to determine aquifer community/village where the water project is situated was hydraulic properties such as transmissivity (T), hydraulic equally obtained from individuals and from public sign conductivity (K) and specific capacity (Cs). Pumping test post describing the project and locations (Table 1). data analysis was based on the Cooper-Jacob's [7] Two methods of aquifer characterization the non-equilibrium graphical method. This method has been geolectrical method which is a well-known and found suitable where the abstraction well itself serves as established method of surface, non-invasive and the observation well. By this method, the transmissivity quantitative geophysical technique of evaluating and (T) is given by: locating subsurface aquiferous zones employing vertical electrical resistivity depth sounding called the (1) Schlumberger array adopted by Oborie et al. [5] and Abija et al. [6] in aquifer characterization in Bayelsa and Rivers states respectively and characterization of aquifer where, hydraulic characteristics based on Cooper and Jacob [7] Q = discharge or yield (m /sec). non-equilibrium graphical method of pumping test S = change in drawdown over one log cycle. analysis also implemented by Tse and Amadi, 2009 in a This formula is useful because some of the boreholes are Nigerian basin were adopted for this study based on confined while others are semi-confined. The average availability of equipment and interpretive software.
drawdown for the twelve boreholes is about 20.41m. The Lithologic logs and pumping test results for hydraulic conductivity (K) was calculated from: representative boreholes were obtained from the Anambra Imo River Basin Authority and supplemented with field pumping test results in few locations for study. The (2) Q Cs S = where T = as in (1) above m/sec -0.781 m/sec in the south and north respectively. B = aquifer thickness (equivalent to the total screen The specific capacity values range from 0.053m /sec/mlength).
0.26m /sec/m in the south and 1.03 x 10 m /sec/m -The specific capacity (Cs), a measure of well The aquifers may thus be classified on the basis of yield productivity, was computed from: as poor to moderate in the North of the State and Good to (3) scheme of Dike [9]. Table 3 is the range of some aquifer where Q = as in (1) above State while Table 4 shows the drilled boreholes S = maximum drawdown [8].
information in the various Local Government Areas of Water level measurements for boreholes within the the impact of the lithology which is characterized by clay study area were obtained from pumping test records of lenses and matrix that occur within the materials of the the boreholes. Water level data employed for this work two aquifer units in the Lower Benue Trough. Since and depths of boreholes drilled in different parts of the groundwater is the main source of supply in the area and state are presented in Table 2.
given that the boreholes available are incapable of

RESULTS AND DISCUSSION
State, it implies that more boreholes drilled to specific Aquifer Hydraulic Characteristics: Results of lithologic hand dug wells will be required to augment the shortfalls logs carefully studied to delineate the water-bearing since increasing population will mean more water demand. sections of the subsurface soil sequence show Surface water impoundment by earth dams will also hydro-stratigraphic units in the study area to be coastal increase the quantity of water available for supply. The plain sands, alluvial deposits, shaley sandstones and Southern part of the state however is rich in groundwater fracture sets. These constitute the distinct aquifer units of supplies and boreholes will always be productive all the area. The overburdens of the basement areas are season. In addition, an efficient water management policy characteristically clayey and pebbly, with the clay forming is imperative for sustainable water supply planning. the matrix. Table 1 shows the summary of aquifers and A detailed examination of the geologic map of Abia characteristics. Borehole properties in terms of depth, State (Fig. 4) clearly showed the distribution of aquifers in screen intervals (assumed aquifer thickness), static and the area under study. All Local Government Areas dynamic water levels, drawdown and yield together with underlain by formations such as the Recent Alluvium in the aquifer constants T, K and Cs obtained from the Ukwa East and Ukwa West; and all LGAs underlain by the Cooper-Jacob methods are presented in Table 2. In order Benin Formation, such as Ukwa East, Ukwa West, to derive aquifer constants, curves of drawdown Obingwa, Aba (North and South), Isiala Ngwa (North and against time were constructed using the pumping test South), Osisioma and Umuahia (North and South), have results. In general, results from the recovery method seem aquifers with fairly high permeability of about 35m /day. more reliable since the recovery data represent in-situ Local Government Areas such as Ikwano, Central parts of aquifer conditions and they are free from the effects of Umuahia (North and South) and eastern-most part of pumping and interference (mainly human errors). From Umuahia South, are underlain by the Ogwashi-Asaba Table 3, transmissivity values range from 2.05 m /sec to Formation with a moderate permeability of 5m /day. Niger Delta basin aquifer and Northern Lower Benue through the south-westernmost part of Isiukwato, run in Trough aquifers respectively with proportional variations a generally NW-SE orientation through central Umuahia with yields indicative of the permeability in the study area.
South and the southern part of Bende LGA and go The low T values also imply that it will take a through northernmost portion of Ikwano LGA, all have an considerable time for the aquifers recharge into wells average permeability value of 10m /day. Productivity from removed during pumping. Hydraulic conductivity values all such formations as the Bende-Ameki and Ogwashirange from 0.385 m/sec -1.71m/sec in the south and 0.045 Asaba will be moderate.     Moderate productivity due to permeability value of 50m /day is expected by some lenticular portions of the 5m /day is expected from the False-Bedded Sandstone friable Ajali Formation (Upper Coal Measures). This 2 (Nsukka Formation) which underlie a narrow N-S portion aquiferous formation occurs N-S in Umu-neochi, runs of Umu-neochi, NW Isiukwato and the E-W portions of NW-SE through Isiukwato and is bifurcated by Bende and Ohafia LGAs. A very good permeability of the impermeable Imo Shale Group and Eze-Aku Shales. The upper limb of this bifurcation goes through Abia State, where only very deep boreholes can Bende LGA and broadens N-S across Ohafia and meet low to moderate productive aquifers. This again Arochukwu LGAs (Fig. 4). All other formations such as proves the deterministic control of geology. Figure Jacob, 1946. A generalized production from the Ajali Formation with permeability of graphical method for evaluating formation constants 50m /day derives from artesian conditions and the friable, and summarizing well-field history. Trans America 2 poorly sorted lithology with the shales of the False Geophysical Union, 27: 526-534. Bedded sandstones on top and the impervious sediments 8. Tse, A.C. and P.A. Amadi, 2009. Hydraulic properties of the underlying Lower Coal Measures with extremely from pumping tests data of aquifers in Azare area, low permeability, providing useful sandwich for confining North Eastern Nigeria. Journal of Applied Sciences conditions. Drilling depth in the range of 250 to 300m to and Environmental Management, 12(4): 67-78. encounter Nsukka and Ajali Formations are sure to produce well. The range is caused by different rates of erosion. It will be necessary to depend on hydrogeophysical site study to limit this drilling depth range.