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Hydrogeophysical Evaluation and Vulnerability Assessment of Shallow Aquifers of the Upper Imo River Basin, Southeastern Nigeria

Eke D.R., Opara A.I., Inyang G.E., Emberga T.T.*, Echetama H.N., Ugwuegbu C.A., Onwe R.M., Onyema J.C., Chinaka J.C

Department of Physics and Industrial Physics, Federal Polytechnic Nekede, Owerri

Abstract: Detailed hydrogeophysical study of the aquifers of the Upper Imo River Basin, Southeastern Nigeria was carried out to delineate the aquifers, evaluate their geometric characteristics and to assess their vulnerability of pollution from surface contaminants. Forty (40) Schlumberger Vertical Electrical Soundings (VES) were carried out in various parts of the study area with a maximum electrode separation of 1000 metres. Four parametric soundings were carried out at the exact positions of existing boreholes with available pumping test and electric log data for the purpose of comparison. The VES data were processed using a combination of curve matching techniques and computer iterative modeling. Layer parameters interpreted from the VES data together with the available well data were used to assess the vulnerability of the shallow aquifers using the DRASTIC model. Results of the study revealed the presence of 4-7 geoelectric layers with a multi-aquifer system. The aquifers are variable in thickness with Umuekwule having the thickest aquifer of 108m while Ukomo Ibeku has the least aquifer thickness of 1.7m. Similarly, depth to water table varies from 0.6m at Ajata ibeku to 91.8m at Umukabia. Information from iso-resistivity model and diagnostic factor revealed a distinct hydrogeological divide in line with the geology of the study area. The aquifer vulnerability index assessment revealed that about 55% of the study area falls within the moderate vulnerability zones with DRASTIC index values ranging from 102 to 140. About 30% of the study area have high vulnerability index while the remaining 15% of the study area have low vulnerability index with DRASTIC index values of between 85 and 99.

Keywords: hydrogeophysical, aquifer vulnerability, drastic, Vertical Electrical Sounding, Imo River Basin

Received May 15, 2015; Revised June 04, 2015; Accepted June 14, 2015

Citation: Eke D.R., Opara A.I., Inyang G.E., Emberga T.T., Echetama H.N., Ugwuegbu C.A., Onwe R.M., Onyema J.C., Chinaka J.C, Hydrogeophysical Evaluation and Vulnerability Assessment of Shallow Aquifers of the Upper Imo River Basin, Southeastern Nigeria. American Journal of Environmental Protection, 3 (4). 125-136. January 2015. doi:10.12691/env-3-4-3

1. Introduction

Access to potable water is a major challenge facing most developing countries of the world today. With the collapse of regional rural water schemes in the study area, more than half of the populace therefore relies on rain water, surface water and most often the poorly drilled private boreholes for their daily water needs.

The sedimentary sequences of Southeastern Nigeria including those of the Imo River basin are known to have several aquiferous units [23]. The groundwater recharge in this area is fairly good due to high yearly average rainfall of about 2270mm. However, several boreholes in the study area are unproductive due to improper planning and lack of technical geo-scientific surveys to unravel the complex geology of the area. This problem is made worse by the poor knowledge of the aquifers (the geometry and nature of their hydraulic boundaries) been exploited. Although numerous boreholes have been drilled in the Imo River Basin, systematic and comprehensive studies have not been carried out to establish the nature and distribution of the aquifers within the basin [23]. In addition to the need for quantitative description of the geometric and hydraulic characteristics of these aquifer, the assessment of the vulnerability of these aquifers to contamination from surficial sources is also very essential. This is important because the groundwater quality in the study area has deteriorated over the years due to population growth, urbanization and the resulting problems of improper waste management practices.

Groundwater pollution can be defined as the artificially induced degradation of the natural ground water quality. In contrast to surface water pollution, subsurface pollution is difficult to detect, more difficult to control and may persist for years, decades or even centuries unnoticed [20]. Prevention of contamination is therefore critical for effective groundwater utilization and management. Spatial variability and data constraints are the major constraints to effective monitoring of the groundwater system and make remediation activities expensive and ineffective in developing countries and most especially Nigeria. Aquifer vulnerability assessment has been recognized therefore for its ability to map areas susceptible to contamination as a result of anthropogenic activities on the earth’s surface.

Groundwater vulnerability is a function of the geologic and geographical setting of an area, as this largely controls the residence of the infiltrating and percolating rain water through the soil profile. Parameters such as depth to water table, aquifer recharge rate, nature of the aquifer media, nature of the soil material at the surface, the topography of the area, impact of the vadose zone as well as the hydraulic conductivity are factors that influence the infiltration of materials to the aquifer. An assessment of these parameters will give the vulnerability level of the aquifers in a particular region. Once vulnerable areas are identified, they can be protected by proper land use planning and intensive monitoring. An accurate groundwater resource assessment and a quantitative description of aquifers have therefore become imperative in other to address several hydrological and hydrogeological problems associated with groundwater management in the study area. The integration of aquifer hydraulic parameters, well information from existing boreholes and geo-electric parameters extracted from surface resistivity measurements can be highly effective in delineating the hydrological and hydro geophysical properties of these aquifers. Several authors have previously delineated shallow aquifers and their vulnerability using this approach [4,10,21,22].

The objective of the present study is to appraise the hydrogeophysical and aquifer vulnerability of the groundwater system of the Middle Imo River Basin, Southeastern Nigeria.

Figure 1. Location/topographic map of the study area (GSAN, 2011)

Figure 2. Geological map of the study area (GSAN, 2011)

1.1. Location, Physiography and Geology of the Study Area

The study area lies between longitudes 7o 17lE to 7o 34lE and latitude 5o 26lN to 5o 38lN. The area consists of parts of Obowo, Umuahia and Ihitte Uboma Local Government areas of Southeastern Nigeria. The study area lies within the sub - equatorial climatic belt characterized by two major seasons: the wet and dry seasons. Rainfall is high with an annual average of about 2,270mm. Relative humidity is also high and generally over 70%. Mean annual temperature is about 27°C while the mean evaporation rate is 3.0mm/day. The area has low-lying to moderately high plain topography. The elevation stands at about 152m above sea level within Umuahia and environs [13]. However, recent updated maps published by the Geological Survey Agency of Nigeria (GSAN) clearly showed that the elevation above sea level in the study area ranges from over 200m to about 400m (GSAN, 2011), with a general slope of about 0.0014 southward [23]. The area is drained by the Imo River and its tributaries which flows in a southern direction and empties into the River Niger and finally the Atlantic Ocean (Figure 1).

The area is underlain by the Benin Formation (Miocene- Recent) and the Eocene – Oligocene Ameki Formation as shown in Figure 2. The sediments of the Benin Formation consists of lenticular, unconsolidated coarse to fine grained sands and clayey shales [19]. The sands are generally moderately sorted, poorly cemented and angular to sub-angular in shape [13]. The Benin Formation overlies the Ameki Formation and dips southwest ward [23]. The Ameki Formation (Eocene-Oligocene) consists of medium to coarse grained white sandstones, bluish calcareous silt with mottled clay, thin limestone beds and abundant calcareous shale.

2. Data Acquisition and Processing

Direct current resistivity data was acquired using the Vertical Electrical Sounding (VES). The Schlumberger electrode configuration was adopted with a maximum current electrode separation of 1000m. In the Schlumberger array, the current and potential electrode pairs have a common midpoint but the distances between adjacent electrodes differ significantly. Theoretically, the resistivity (ρ) of a material is directly proportionally to the resistance (R) of the material to current flow. Thus;

[picture] | (1)

where K is the Geometric factor and is obtained using the formular in equation (2) below:

[picture] | (2)

a and b being half current electrode spacing and potential electrode spacing respectively.

A total of forty (40) vertical electrical sounding data with a maximum current electrode separation (AB) of 1000m were acquired using the ABEM Terrameter SAS 4000. The vertical electrical sounding(VES) points and the interpretative cross sections are shown in Figure 3 below. The profiles were taken for the purpose of regional correlation across the study area. The field data were converted to apparent resistivity values by multiplying with the Schlumberger geometric factor K as shown in equation (3) below:

[picture] | (3)

[11]

Figure 3. Location map of the study area showing VES points and interpretative profiles

Computer iterative modeling of the VES data was done using the Schlumberger software to generate the layer parameters. The apparent resistivity field values were then plotted against the half current electrode separation to generate geo-electric curves.

2.1. Aquifer Vulnerability Assessment

Groundwater vulnerability is a function of the intrinsic properties of the aquifer system and their sensibility to human and natural activities. Many approaches have been designed to evaluate aquifer vulnerability, however, the most commonly used are the index methods which combine factors controlling the movement of pollutants from the ground surface into the saturated zone resulting in vulnerability indices at different locations. The DRASTIC model has been the most effective and widely used aquifer sensitivity assessment method [24]. The model was developed by the United States Environmental Protection Agency (EPA) to evaluate groundwater pollution potentials [2]. DRASTIC is an index model designed to produce vulnerability scores for different locations by combining different thematic layers into a single predictive model. The model is based on the concept of the hydrogeological setting that is defined as a composite description of all the major geologic and hydrogeologic factors that affect and control groundwater movement into, through and out of an area [2,3,7,8,12,14].

The acronym DRASTIC corresponds to the initials of the seven parameters used in the model which are: Depth to water; Net Recharge; Aquifer media; Soil media; Topography; Impact of the vadose zone and Hydraulic Conductivity. The significant media type of each of these parameters are assigned a subjective rating varying from 1 to 10, based on their relative effect on the aquifer vulnerability. Every parameter in the model has a fixed weight multiplier indicating the relative influence of the parameter to contaminant transport [1,2]. The final DRASTIC index (Di) is the weighted sum overlay of the seven parameters using the following equation:

[picture] | (4)

Where D, R, A, S, T, I, C are the seven parameters, and the subscripts r and w are the corresponding ratings and weights respectively. The parameter D represents the depth to the water table. The depth to water level used in this work was estimated from information from pumping test data, well logs and VES data. The Net Recharge(R) represents the amount of water per unit area that penetrates the ground surface and percolates down to the water table. The Net Recharge is as a rule of thumb taken as 12% of the average annual rainfall [6,14]. The parameter A represents the Aquifer media while S connotes the Soil media. The topography (T) refers to the slope or steepness of the land surface. It dictates whether the runoff will remain on the surface to allow contaminant percolation to the saturated zone or not. Flat areas are therefore assigned higher rates because the run off tends to be less. The study area was found to be relatively flat with the slope ranging from 0-2%. The impact of the vadose zone(I) refers to the part of the earth surface found between the aquifer and the soil cover in which pores and joints are unsaturated or partially saturated. The influence of the Vadose zone on aquifer vulnerability depends on its permeability and on the attenuation characteristics of the media. Finally, the hydraulic conductivity is the ability of the aquifer to transmit water. An aquifer with high hydraulic conductivity will be more vulnerable to contamination as a contaminant plume will easily pass through the aquifer. The basis of the classification of aquifer vulnerability using the DRASTIC index is presented in Table 1 below.

Table 1. Aquifer Vulnerability classification based on the final DRASTIC index [14]

3. Result Presentation, Interpretation and Discussion

Interpretation of the VES data was carried out using a combination of curve matching and computer iterative modeling techniques. The field data were used to generate layer parameters and geo-electric curves. Typical geo-electric curves generated from the study area include HKH, HK, HH and KHK as shown in Figure 4 below. The shape of the curve for each sounding point gave an insight into the character of the beds or layers between the surface and the maximum depth of penetration. This is because the shape of a VES curve depends on the number of layers in the subsurface, the thickness of each layer, and the ratio of the resistivity of the layers [18]. The general sequence of the curves suggests alternate sequence of resistive-conductive layers. Table 2 is the summary of the layer parameters interpreted from the VES data. Geo-electric layers interpreted ranges from 4 to 7 layers.

Figure 4. Typical geo-electric curves generated from the resistivity data of the study area

3.1. Aquifer Resistivity, Depth and Thickness of the Study Area

Some of the aquifer properties (including resistivity, depth, thickness, etc) interpreted from this work are presented in Table 3 below. Aquifer resistivity in the study area varies from 136Ωm at VES 19 (Umulu Ikwuano) to 4640 Ωm at VES 1(Umulogho Obowo) with a mean value of 1193.8250 Ωm. The depth to the aquifer across the study area ranges from 0.6m at VES 13 (Ajata Ibeku) to 94.5m at VES 37 (Abueke Ihitte Uboma) with a mean value of 40.76m. The spatial variation of aquifer depth in the study area is shown in Figure 5 below. Shallow aquifer depth values dominated the northeastern section of the study area with Ibeku and environs having the least value. Deeper aquifers are distributed within the central part of the study area. Similarly, the aquifer thickness in the study area varies from 1.7m at VES 27(Ukome Ibeku) to 126.2m at VES 24 (Umuchoko Nkata) with a mean value of 41.49m. The isopach map is presented in Figure 6 below. The central area and the northeastern part of the study area appear to have the thickest aquifers.

Table 2. Summary of interpreted layer parameters from the resistivity sounding

Table 3. Summary of aquifer properties interpreted from resistivity data

Figure 5. Contour map of aquifer depth across study area

3.2. Iso- Resistivity Modeling

The iso-resistivity maps derived at depth intervals AB/2 = 10m to AB/2 = 500m, revealed vertical variation of resistivity with depth. Figure 7 shows some iso- resistivity maps generated from the resistivity data. The iso-resistivity maps revealed a progressive decrease in resistivity with depth suggesting a highly resistive overburden to less resistive layer at the base. However, at the depth intervals AB/2 =200m to AB/2 = 500m, high resistivity values demarcated by intermittent low resistivity values were intercepted. The iso-resistivity maps also revealed two distinct and distinguishable zones based on resistivity contrast. This contrasting resistivity zones are in line with the geology of the area underlain by the Ameki and Benin Formations respectively. The western axis of the study area is homogeneous in terms of the geo-electric properties and is therefore distinct from the eastern part of the study area. This divide is very visible on the maps presented in Figure 7 below. This divide is probably indicative of a regional groundwater divide in the study area. The iso-resistivity maps also revealed a multiple aquifer system in the study area separated hydraulically by thick bands of clay, sandy clay and shales. Uma [23] delineated a multi-aquifer system in the Imo River Basin viz: a shallow unconfined aquifer, a middle semi-confined aquifer and a deep confined aquifer.

Figure 6. Contour map of aquifer thickness (isopach) across study area

Figure 7. Contour map of the iso-resistivity values across study area: (a) AB/2 = 20 (b) AB/2 = 50m (c) AB/2 =150m (d) AB/2 =200m (e) AB/2 =300m (f) AB/2 =500m

3.3. Interpretative Profiles

For the purpose of correlation, four profiles were taken across the study area as shown in Figure 3 below. The profiles include profiles A-AI , B-BI , C-CI and D-DI . Profile A-AI which is 13.5km in length was taken along the northwest- southeast (NW-SE) direction within the southwestern section of the study area and traverses several towns including Umulogho, Amuzi and Avutu. The profile revealed six distinct geo-electric units as shown in the generated earth model in Figure 8 below. Around Umulogho, a minor intercalation of clay was noticed between the unsaturated sand unit and the saturated sand with the clay intercalation increasing in thickness as it dips southwestward. At the base is a conductive shale unit of resistivity ranging from 384Ω to 527Ω. The extent of the shale unit is however not resolved in this study due to limitations resulting from depth of probe.

Figure 8. Geo-electric layer along profile A-AI

Profile B-BI is 8.0 km long and is located at the northwestern section of the study area and transversed the following towns: Etiti, Lowa, Abueke and Umuezegwu. The profile showed six distinct geo-electric units as shown in Figure 9 below. The topsoil is underlain by a lateritic sand layer. Underlying the lateritic sand is an unsaturated sand unit which in turn is underlain by a shale unit. Underlying the shale unit is the saturated sand unit. The shale unit and the saturated sand unit appear to dip southwestward with increasing layer thickness around Umuezegwu. At the base is a conductive shale unit.

Figure 9. Geo-electric layer along profile B-BI

The C-CI profile is located at the eastern part of the study area. The profile traverses Ajata Ibeku, Ukomo Ibeku and Okwoyi Ibeku towns with a length of about 9.0km. This profile revealed only three geo-electric units with a thick shale unit at the base (Figure 10). The clayey sand unit pinched out at Ukomo Ibeku.

Figure 10. Geo-electric layer along profile C-CI

Similarly, profile D-Dl which measures about 11.3km long and traverses Itaja Olokoro, Umuobia Olokoro, Amachara and Atta Ibeku areas trends in the northeast-southwest (NE-SW) direction. This profile revealed seven distinct geo-electric layers (Figure 11) with a conductive shale layer at the base. Overlying the shale base is the saturated sand unit.

Figure 11. Geo-electric layer along profile D- DI

Similarly, for the purpose correlation and comparison the generated geo-electric sections were correlated with strata- log sections. This done at VES points were some parametric soundings were carried and strata- log information exist. Presented in Figure 12 below are the strata-log correlation at Olokoro and VES 16 and the strata-log section at Avutu and VES 32. These correlations revealed a fairly good correlation indicating that the VES data are good representation of the subsurface materials.

Figure 12. Correlation of lithologs from boreholes with VES Data: (a) Amangwu Olokoro (b) Avutu

Figure 13. Contour map of of the diagnostic constant (kσ)

3.4. Diagnostic Constant (Kσ Value)

In areas of similar geologic setting the diagnostic constant, Kσ (the product of hydraulic conductivity (K) and electrical conductivity (σ)) may be fairly constant [5,15,16]. The contour map of the diagnostic constant of the study area revealed two distinct hydrogeological zones (Figure 13). This hydrogeological divide is in line with the geology of the area. The western axis covering areas such as Avutu, Umulogho, Olokoro, Amachara is hydrologically homogeneous with an average Kσ of 0.008 which correlates with the hydrogeological properties of the Benin Formation. The other zone has lower Kσ values and transverses Ajata Ibeku, Alaocha and Etiti areas. It is hydrologically homogeneous with an average Kσ value of 0.0026 and has the attributes of the Ameki Formation.

3.5. Aquifer Vulnerability Assessment Using the DRASTIC Model

Information extracted from strata-logs, electric logs, VES data and hydraulic conductivity estimated from Dar-zarrock parameters were used to predict the DRASTIC Index of the study area (Table 4). The DRASTIC Index map of the study area showed that about 55% of the study area falls within the moderate vulnerability zone with DRASTIC index values of between 102 to 140 (Figure 14). The high vulnerability zones covered about 30% of the study area. The areas within Ajata, Umuire and other adjoining area falls within this zone with a DRASTIC index ranging from 143 to 179. The remaining 15% of the study area have low vulnerability rating with DRASTIC index values ranging from 85 to 99. High vulnerability rate in these areas may be attributed to shallowness of their aquifer and the fact that most of the aquifers in the areas may be unconfined. The low vulnerability index in these areas may be attributed to deep water table.

Figure 14. Contour map of DRASTIC index of the study area showing aquifer vulnerability

4. Summary, Conclusion and Recommendation

Result of this study has revealed that the aquifer resistivity in the study area varies from 136Ωm at VES 19 (Umulu Ikwuano) to 4640 Ωm at VES 1(Umulogho Obowo) with a mean value of 1193.8250 Ωm. The depth to the aquifer across the study area varies from 0.6m at VES 13 (Ajata Ibeku) to 94.5m at VES 37 (Abueke Ihitte Uboma) with a mean value of 40.76m. Similarly, the aquifer thickness in the study area varies from 1.7m at VES 27 (Ukome Ibeku) to 126.2m at VES 24 (Umuchoko Nkata) with a mean value of 41.49m. The geo-electric cross sections and litho-logs revealed an average of six geo-electric layers with the lithostratigraphy within the Benin Formation comprising of medium to coarse grained sands and gravel with minor clay intercalations. The lithostratigraphy of the Ameki Formation is characterized by sandy clay at very shallow depth with a thick layer of shale/clay extending over 350m [21,22].

Table 4. Summary of aquifer DRASTIC Index in the study area

The study has helped to delineate the aquiferous horizons within the study area. Data from the Iso-resistivity models revealed a multi- aquifer system. This finding is in close agreement with previous publications done in the study area [13,23]. The study area is therefore divided into two distinct hydrogeological zones on the basis of information from diagnostic parameter and iso-resistivity models. Earlier scholars within Imo River Basin and adjoining areas have also used the same parameters to demarcate hydrogeological zones [5,17,21,22]. The difference between these two zones is related with the subsurface geology in the area. The western axis covering areas such as Avutu, Umulogho, Olokoro, and Amachara is hydrologically homogeneous with an average Kσ of 0.008 which indicates the Benin Formation. The other zone traversing Ajata Ibeku, Alaocha and Etiti is hydrologically homogeneous with an average Kσ value of 0.0026 and indicate the Ameki Formation.

Result of the aquifer vulnerability index modeling revealed that about 55% of the study area falls within the moderate vulnerability zones with DRASTIC index values of between 102 to 140. The high vulnerability zones covers about 30% of the study area with DRASTIC index values ranging from 143 to 179. The remaining 15% of the study area have low vulnerability rate with DRASTIC index values ranging from 85 to 99. Generally, the result of the aquifer vulnerability assessment using DRASTIC model clearly revealed that the area is moderately vulnerable to groundwater contamination.

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