EVALUATION OF GROUNDWATER POTENTIAL USING AQUIFER CHARACTERISTICS ON PARTS OF BOKI AREA, SOUTH – EASTERN NIGERIA

Vertical Electrical Sounding (VES) and pumping test (constant discharge and recovery test) was carried out in parts of Boki Local Government Area to evaluate the groundwater potential, using aquifer characteristics of the study area. Sixteen (16) VES point was employed for this study using the Schlumberger array, with a maximum spread of 400600m. The stimulated result from the field data shows 3-5 layers resistivity model with the following curve types A, AK, KH, KHA, QH, QHA and QHK. The geoelectric properties include resistivity of the various layers ranging from 33.58 2.29 x 105Ώm, thickness from 0.2 50.2m, depth to basement varies from 2.99 74.60m across the study area. The litho logs show a top layer comprising of laterite, gravel/gravelly sand and intercalations of siltstones, the layers underneath are made-up of clay, weathered basement containing migmatites and gneisses, fractured and unfractured basement are made up of granitic and metamorphic rocks. The weathered and fractured layers constitute the aquiferous layers in the study area. Hydraulic parameters show transmissivity (T) range of 4.1x 10-5 1.92 x 10-1 m 2 /day, specific capacity (SC) ranges from 2.09-21.42m2 /day, hydraulic conductivity (K) varies from 2.6 x 10-5 3.0 x 10-3m/day and mean static water level (SWL) of 7.39m. Iso resistivity map of saprolite, fractured basement map, isopach map and the transmissivity map show that the studied area falls within the low-moderate groundwater potential zone.


INTRODUCTION
In the basement complex of the Obudu Plateau, ground water constitutes the most reliable and major source of water for agricultural and domestic uses. It is found within the fractured / or weathered basement and are tapped by the use of hand-dug wells and boreholes. The discontinuous nature of the basement aquifers system makes detailed knowledge of the subsurface geology, it depth of weathering and structural disposition inevitable (Adiat et.al, 2009). Therefore, there is a need for the application of geophysical and hydrological techniques such as electrical resistivity, seismic refraction, magnetic, electromagnetic, ground probing radar, pumping test, and down-hole logging in water project (Anomohanran, 2013). The electrical resistivity method (vertical electrical sounding) is used in determining the resistivity distribution of the subsurface, it is simple to apply in the assessment of groundwater occurrence (Okongbo et al. 2011). A combination of Geophysical methods, hydrogeologic parameter analyses and geological surveys will further provide the needed information. The development of ground water can be viewed as a sequential process with three major phases. There exploration phase, evaluation stage and the exploitation phase (Freeze and Cherry, 1979). The exploration phase involves the use of surface and subsurface geological and geophysical techniques in search for suitable aquifers while the evaluation phase involve encompasses the measurement of hydro geologic parameters, the design and analyses of wells and the calculation of aquifer yield. Finally, the exploitation or management phase considers the impact between groundwater exploitation and hydro geologic system. The aim of this work is to analyze the groundwater potential and aquifer characteristics of the study area, using electrical resistivity and pumping test to delineate groundwater aquifers for the citing of boreholes and the computation of the specific yield of the various boreholes.

LOCATION OF STUDY AREA
he stu y e is oc te etween ongitu es 6 0 0 6 26 30 n titu e 8 30 20 9 27 0 . he oki area is an extension of the basement complex of the Obudu plateau, South -Eastern Nigeria, with the following dominant rock types Phylites, Hornblende schist, Gneisses, Migmatites, Igneous intrusive such as Granitoid, Dolerites. These rocks display Porphyroblastic, Granoblastic, Xenoblastic textures in metamorphic rocks while porphyritic, xenomorphic, and mortar features are present in igneous rocks. Ukaegbu and Oti (2005) had observed that polydeformation has resulted in multi -directional orientation of planar and linear structure in the basement rock of southern Obudu Plateau (Fig1).

MATERIAL AND METHOD
The acquisition of data was carried out in three phases, namely: Geological or surface survey Geophysical survey (VES) Hydrogeology survey (pumping test) In geophysical survey, Vertical Electrical Survey (VES) was carried out in 16 pre-determined communities (three VES points per sampled location) within parts of Boki Area in South-Eastern Nigeria. The Schlumberger array, with a maximum spacing of about 400-600m between the current electrodes ( fig 2) was employed. The process of carrying out resistivity involves the passage of current from the Geotron G41 terrameter into the ground through steel electrodes (current electrodes). The electrode where progressively expanded about a fixed point along the transverse line while maintaining the ratio of 1/5 of the potential electrode spacing to the current spacing. Data generated were collected along the course of traversing the survey line. The field data collected was recorded for each traverse line and was multiplied by the geometrical factor for the Schlumberger array (G=ƛ/4[L 2 -a 2 ]/a) to find the apparent resistivity. The data was analyzed and interpreted using a computer designed software called Interprex ID. In hydrogeology survey, 16 wells (single wells were used due to the absences of Observation Wells) within the pre-selected communities, was pumped at a constant rate until a near steady/steady state was attained using 1.0Hp (horse power) submersible Pump, dip meter and a stopwatch was used to measure the corresponding changes (drawdown) in the water level and the respective time. Prior to the pumping, the static water level (SWL) was obtained using the dip meter, coordinates and elevation was equally obtained using the Global position system (GPS) whiles a calibrated (20) twenty liters container was used in estimating the pumping rate. For the recovery test, the submersible pump was switched off, the aquifer allowed to recharge and the corresponding time and water level measurement was noted. The data generated was subjected to topographical and analytical analysis in estimating transmissivity of the aquifer (T), hydraulic conductivity (K), specific capacity (SC), thickness (B) and drawdown(s). Transmissivity of the aquifer was computed as follows The value of k varies from 0.0000025m/d to 0.0034m/d as shown in table 2.

RESULT AND DISCUSSION
From the geo-electric curves simulated, the curve type varies from K, A, H, KH, HKA, QHA, QHK and HAA type, showing that the sub surface formation varies in composition (complex). The resistivity curve shows a 3, 4, 5 geoelectric layers within the study area. Fig 4 and

GEOELECTRIC SECTIONS
The VES results were used in the preparation of 2-D geoelectric sections, which shows the resistivity of the various subsurface layers and their respective thickness (Fig 6-8). Each section was prepared in accordance to the VES points in each transverse. The geoelectric section is made up of 3-5 subsurface geoelectric units, inferred as laterite, gravel and gravelly sand; clay weathered and fractured basement, and fresh basement. The study area is made up of two aquifer type. Type1 are aquifer found on the overburden/weathered layers and are mostly exploited using hand dug well as a result of their near -to-surface depth while Type 2 aquifers are found on the fractured rocks of the basement complex. From the correlation of the geoelectric section along transverse section A-A 1 (fig, 6) with the following sampled points (VES 13,12,6,14 respectively), the resistivity curves show a 3-5 layer model with the following curve types KH, HAA, HK, AK along the transverse line. These layers are laterite, gravelly sand, clay, weathered basement and the un-weathered basement. The aquiferous layer for this transverse area is the weathered basement with esistivity v ues nging f om 60Ώm to310Ώm n thickness values ranging from 12m at VES 12 to approximately 19m at VES 6. The area will support hand-dug wells as a result of the depth of the aquifer to the surface. The resistivity of the sampled area shows that the weathered basement has high clay content therefore; the aquifer yield will be low despite the size of the thickness of the overburden.  ,7) with the following (Ves16, 8, 15 and 11 respectively). The geoelectric sections shows 3-5 layers model with a curve type of KH, k, KHA and QHA.VES 15 showing a confined aquifer. The probable aquiferous layer for this transverse section would be the weathered layered and the fractured basement. The sampled section will support both motorized and hang dug well. The thickness of the aquiferous unit ranges from 4m at VES 15 to 32m at VES 16 with the fractured basement resistivity ranging from 890Ώm to 4153Ώm. he geoe ect ic ye s inc u e laterite, gravelly sand clay, weathered basement, fractured basement and the fresh basement. The transverse C-C 1 (fig,8) shows three locations within the study area namely (VES 9, 5, and 7) with a resistivity nge of 89.59 to 229000Ώm. he geoe ect ic section shows show 3-5 layers model with the following curve type H, KQA, QHK. The geoelectric layers are interpreted as laterite, clay, weathered basement, fractured and un-weathered basement, with VES 7 showing a multi aquifer system at a depth of 20.5m-27.5m and 30m-33m. The possible aquiferous layer for VES 9 and 7 is the unconfined weathered basement while water can be tapped from a combination of the weathered and fractured basement in VES5.

ISOPACH AND ISO RESISTIVITY MAP OF THE AQUIFEROUS LAYERS
The overburdens are laterite, gravel /gravelly sands, clays, weathered basement. The overburden thicknesses (depth to basement) range from 2.99m at (lowest thickness) to 74.06m at (highest thickness) with an average thickness of 27.26m within the study area. The overburden thickness at each sounding location was gridded and contoured to produce an Isopach map ( Fig.9) of the overburden. An isopach map shows the spatial distribution of thicknesses within a locality. The map was broken into four zones based on ground water potentials. They zones are Zone A (high:65 and above), B (moderate:45-65), C (low:25-45), and finally D (minimal;0-25). The zones weighted as follows Zone A 6.3%, B 37.5%, C 31% and D 25%. Based on the zoning the study area could be classified into low to moderate ground water zone. According to Olurunfemi and Okhue (1992), Oyedele and Olayinka (2012), a minimum overburden thickness of 25m is required for viable ground water abstraction in the basement terrain. In the study area, the overburden thickness varies from 2.99 to 74.06m; an overburden thickness of 25 to 72m occurred in 74% of the study area, showing a very thick overburden and thus should support productive groundwater abstraction. However, according to Lenkey et.al (2005) a thick weathered layer (containing less percentage of clay) above the basement rock will constitute a water bearing layer. An Iso-resistivity map of the weathered basement (saprolite) was produced to show the spread of resistivity at a common depth within the study area and presented as Fig.10.The resistivity values of the saprolite (weathered basement) ranges from 33.58-918.63Ώm, n in ic tion of the p esent of te ites/c y, clayey/clay and sandy clay. The Iso-resistivity map show that the resistivity of the weathered basement is broken into the following zones in terms of groundwater potential: high g oun w te potenti (>650Ώm), moderate (400-650Ώm), ow (150-450), minimal (33.58-150). Wright (1992), Mallam(2004) andAdelusiet.al,(2004) had developed a scheme for ranking of ground water potential as a function of saprolite (weathered basement) resistivity as presented in the Table 2. From the table 2, the study area has 21.43% in the optimum weathered and ground water potential zone, 7.14% in the medium aquifer conditions and groundwater potential while 50% of the area within the study area is in the negligible zone with no appeal for groundwater potential. Finally, the limited weathering and poor groundwater potential zone constitute 21.43% of the study area. In the basement terrain, the aquiferous layers are usually found in the thick, porous weathered and the fractured part of the bedrock. The fractures are mostly associated with high ground water yield when compared with the overburden as a result of its relatively high permeability. A ranking scheme for aquifer potential, as a function of the fractured bedrock which was modified by Oyedele et al.,(2012) is shown in the table 3 below.

HYDROLOGICAL ANALYSIS
From the pumping test exercise carried out, the results obtained from the various pumped location is presented as table 3. Using Cooper -Jacob straight line equation, the data was handle graphically and analytically resulting in the plotting of time-drawdown curve on a semi-log graph for the various locations and the computation and analyses of aquifer parameters, here presented as appendix 2 and Table 3 respectively.

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UGBAJA, ANTHONY N., WILLIAM, GOD'SWILL A. AND UGBAJA, UDUAK A  Table 3 show the computed transmissivity value for the study area, which ranges from 0.00013 to 0.975 m 2 /day with an average transmissivity of 0.032m 2 /day within the pumped locations. Fig.4 is a transmissivity map showing the spatial distribution of transmissivity within the pumped locations in the study area. Offodite (2002) suggested a transmissivity range of 5to 50m 2 /day to represent high groundwater potential in crystalline rock while 0.0155m 2 /s (10000gal/day/ft.) represent good aquifer for well exploitation (Cherry and Freeze 1979). However, the transmissivity range of the pumped wells within the study area was characterized by negligible to low ground water potential.
The table 4 shows the saturated hydraulic conductivities (k) values found in nature.

EVALUATION OF GROUNDWATER POTENTIAL USING AQUIFER
From the computed hydraulic conductivity on the table 2 for the study area, the sampled location falls within the semi pervious zone with poor ground water potential.

THICKNESS OF THE AQUIFER
The aquifer thickness ranges from 2.2-50, 22m, the aquifers which is found within the weathered materials and fracture zones of the overburden has high clay content, which is very typical of the Obudu Basement Complex, as it is still undergoing weathering. Although the study area is characterized by zones of high over burden thicknesses, its high clay content places it within the low groundwater potential zones.

CONCLUSION
The use of geophysical and hydrogeological methods in the evaluation of the ground water potential, and aquifer characteristic in the study area have shown that the result from the field data shows 3-5 layers resistivity model with the following curve types A, AK, KH, KHA, QH, QHA and QHK. The geoelectric properties include resistivity of the various layers ranging from 33.58 -2.29 x 105Ώm, thickness f om 0.2 -50.2m, depth to basement varies from 2.99 -74.60m across the study area. The litho logs show a top layer comprising of laterite, gravel/gravelly sand and siltstone intercalations and the layers underneath are made-up of clay, weathered basement containing migmatites and gneisses, fractured and unfractured basement are made up of granitic and metamorphic rocks. The weathered and fractured layers constitute the aquiferous layers in the study area. Hydraulic parameters show transmissivity (T) range of 4.1x 10-5 -1.92 x 10-1 m 2 /day, specific capacity (SC) ranges from 2.09-21.42m2 /day, hydraulic conductivity (K) varies from 2.6 x 10-5 -3.0 x 10-3m/day and mean static water level (SWL) of 7.39m . Iso resistivity map of saprolite, fractured basement map, isopach map and the transmissivity map show that the studied area falls within the low-moderate groundwater potential zone.

RECOMMENDATION
As a result of the low-moderate aquifer potential of the study area, mechanized farming should be discouraged; however, two bore holes can be cited 50meters apart to improve the water supply within the farm. The need for an observation wells in future study, this will help in the computation of storativity values.