Geophysical investigation of transmissibility and hydrogeological properties of aquifer system: A case study of Edem, Eastern Nigeria Open Access

Water is a requisite demand of life and can be a great challenge when its extraction/exploitation in an area is carried out without any geophysical knowledge on the inter-transmissibility magnitude of hydrogeologic units of the area. The residents of Edem in Nsukka, local government area of Enugu State, Nigeria, has been experiencing difficulties in accessing groundwater as no functional borehole was drilled in the study area (Omeje et al. 2021). The only groundwater source attempted was a manual hand dug well with no established scientific data attached to it. The major source of water available is the spring water located in Akpa and the surface water located in Ozzi and Edem-ani respectively, which are very far for most residents. The growing population of the community and the increasing demand of water for their domestic use and agricultural purposes has given rise to the urgent need to address the issue of aquifer transmissibility in Edem. For this reason, it would be very helpful to explore the geological formations of the aquifer units of Edem in order to identify potential zones with high transmissibility. Zones with high transmissibility can be explored using the electrical resistivity method. The electrical resistivity method involving vertical electrical sounding has proven to be useful in groundwater study (Omeje et al. 2021). Several researchers have employed several methods in studying the characteristic nature of hydrogeologic units of different geologic locations. Of all the research studies carried out on groundwater by different researchers on different locations in Nigeria, only one research study conducted by Omeje et al. (2021) has been carried out in Edem. Edem, which is the largest and most populated community in Nsukka, is faced with failures of water scheme projects leading to a high rate of water scarcity, and with a rapid increase in population and agricultural activities requires more research studies on its hydrogeologic units. This study aims at investigating the transmissibility and hydrogeological properties of aquifer units of Edem employing the electrical resistivity method. This aim cannot be successful without the knowledge of some important geologic units' properties. Omeje et al. (2021) employed some geohydraulic parameters which include; porosity, tortuosity, hydraulic conductivity and formation factor in characterizing the geologic units of Edem. In their study permeability of the aquifer layer was not analysed and for an aquifer to be productive it must be permeable. The present study tends to further introduce other hydrogeologic properties needed for clearer study and investigation of the aquifer unit. Hydrogeologic properties additionally considered in this study to ascertain the geological formations of the aquifer units include; permeability, hydraulic capillary radius, surface area per unit pore volume and transmissivity. These properties were considered because they conspicuously regulate the magnitude of fluid transmissibility in geologic units. Water-bearing characteristics of hydrogeological bodies can be vividly explained using permeability (kp) knowledge, in line with transmissivity. Thus, an idea on permeability and transmissivity distribution is essential in result analysis from hydrogeological studies (George et al. 2010; Ibuot et al. 2013). Permeability is highly correlated with hydraulic conductivity of water within and across aquifer repositories. Permeability magnitude essentially influenced by geologic formation nature permits the feasibility of delineating latent water-bearing units. The degree of distribution of permeability and hydraulic conductivity assist in classifying the aquifer unit of an area as a low or high fluid transmissible unit. In order words permeability and hydraulic conductivity of a geologic unit controls pore fluid transmissibility.

Transmissibility can be defined as the rate of flow of water through a vertical strip of the aquifer (George et al. 2018). Its magnitude determines the productivity level of an aquifer in an area. For high transmissibility of fluid through an aquifer, permeability and hydraulic conductivity of the aquifer must be high. This range of classification (low or high) is dependent on the geohydraulic characteristics of the geologic unit of the area. An increase in transmissivity may represent an increase in hydraulic conductivity, porosity and good interconnected pore spaces. This increase thus results in a high magnitude of transmissibility which approves high permeability presupposition (George et al. 2017). These hydrogeologic properties which aid in defining the transmissibility of an aquifer can be estimated employing the electrical resistivity method. Electrical resistivity method Being a geophysical method, the electrical resistivity method has the widest adoption in groundwater exploration among the various geophysical methods employed in groundwater investigation (Obiora & Ibuot 2020). This is as a result of its less fatigue in field operation; its relatively high diagnostic values, portability of equipment and greater depth of penetration. This method is economical and versatile in delineating locations of apparent thickness of the weathered areas and productive aquifer sites (Ezema et al. 2020). Advances in geophysical techniques with geologically available information enabled accurate estimation of porosity, permeability and other hydraulic parameters through ground-based geophysical measurements, which are affordable compared to laboratory measurements (George 2020).

In this study, estimation of the additional hydraulic properties employing the electrical resistivity method will serve as a tool to the investigation of transmissibility and hydrogeologic properties of aquifer systems in the study area. The magnitude estimated will be productive in groundwater development, exploration, protection, and flow modelling in locations with identical hydrogeologic controls (George et al. 2015). The effectiveness of these properties as it relates to transmissibility of pore fluid in geologic units will also assist in managing and monitoring contaminants in aquifer repositories.

The study area as presented is examined based on location and geology.

Edem is situated in the Nsukka area of Enugu, eastern Nigeria. It lies between latitudes 6 °50^′ N to 6 °54^′ N, and longitudes 7 °16^′ E to 7 °23^′ E (Figure 1), having an elevation variation above sea level ranging from 300 to 450 m. It covers an area of about 50.492 km² with an estimated total population of 309,633 (National Population Commission Census 2006). Edem is characterised with two distinct seasons (rainy and dry), and falls inside the Guinea savannah/tropical rain forest zone of Nigeria. Rainy season occurs between April to October while the dry season is between October and March (Iloeje 1995). It has a temperature range of 19–33 °C.

The study area, whose geologic formations are the upper Nsukka Formation and the underlying Ajali Sandstone (Figure 1), is within Anambra Sedimentary Basin with Upper Cretaceous rocks in age. The Nsukka formation has over 60% coverage of the entire surveyed area with its sediments serving as the geologic units. Edem is a community with elevated, flat and undulating topography. Dry valleys and residual hills characterise the study area as the prime landforms. These two major geomorphic structures are the resultant effect of weathering and differential erosion of clastic materials, which are remnants of Nsukka Formation (Ofomata 1967). The numerous conical hills and laterite capped are the outliers of Nsukka Formation. The laterite caps are aquiferous as a result of its porosity, permeability and anisotropic nature (Ugwuanyi et al. 2015). Water accumulation by plains in this region is from runoff of strands, the recharge of rainwater and from springs. The Ajali Formation which outcropped in some areas is a sandstone formation. The surveyed area is bounded on the east, west, south and north by Nsukka, Nrobo, Obimo,and Ibagwa-ani communities respectively.

A vertical electrical sounding (VES) survey employing Schlumberger electrode configuration was conducted in 21 locations of the study area with the aid of ‘SSR-MP-ATS’ Resistivity Meter. The 21 locations selected were based on nearness to community settlements, areas with difficulties in groundwater exploration, areas with level or close to level ground that can accommodate a full current electrode spread of 900 m and areas not close to high tension wires. Half potential electrode (⁠⁠) and half current electrode separation (⁠⁠) of 20 and 450 m respectively, were adopted to ensure deeper current penetration, with the resistivity meter placed in between the electrodes. AB values varied with locations and were placed on the earth at a distance ranging from 2 to 900 m while potential electrode (MN) ranged from 0.5 to 40 m (Akpan et al. 2013). Accessible roads were considered in locating areas for VES points so as to achieve a wide extreme current electrode (AB) spread of 900 m. For standard guarantee field measurements, the potential electrode separations were one-fifth of the current electrode separations (George et al. 2016).

A suitable geologic model of the field data was obtained employing both computer and manual modelling techniques (Zohdy et al. 1974; Akpan et al. 2009). A plot of apparent resistivity against half current electrode spacing on a bi-logarithmic graph was carried out to achieve the manual modelling. Lateral in-homogeneities were extracted from the produced curves through smoothening by deleting any outlier at the cross over points that did not conform to the dominant trend of the curve, or by averaging the two readings at the cross over points (Akpan et al. 2006; Chakravarthi et al. 2007). The computer modelling was carried out using a computer based VES modelling software programme called WINRESIST, which refined the bi-logarithmic graph. WINRESIST software programme uses the initial layer parameters to perform some calculations, generate geoelectric sounding curves in the process and classify the curves into layers with different resistivities, depths and thicknesses (Figure 2).

The aquifer layer resistivities having an average of 13,227.180 Ωm range from 34.8 to 67,561.2 Ωm. This range of values signifies the existence of medium to high resistive geomaterials in the aquifer layer. The aquifer thickness having an average value of 70.250 m range from 24.8 to 147.6 m. The hydraulic properties which provide an insight into the transmissibility and nature of permeability of the hydrogeologic units were estimated from the values of aquifer resistivity and thickness. Results of these properties were contoured to visualise its spatial variability. Values of these hydrogeologic properties were compared through a plot to show the dependency of permeability on these properties. Figure 3(a) is a contour map showing the variation of permeability in the study area. The map shows high values of permeability having a range K_p > 4.7 × 10⁻⁷ μm² in.the eastern and western parts of the study area. The lowest distribution having a range K_p was observed in the northwestern part. Areas having K_P > 4.7 E × 10⁻⁷ μm² reflects areas with good pore interconnectivity and high rates of groundwater transmissibility in the aquifer. This also shows that these areas of high permeability can be attributed to a coarse-grained soil with an increased void ratio.

Transmissivity (T_r) values with average of 118.1003 m²/day range between 0.8607 and 458.0727 m²/day. Transmissivity distribution as shown in Figure 6(a) shows high values in areas with values ranging from 120 m² per day to ≤200 m² per day and areas with values ≥200 m² per day. Transmissivity considered as a hydraulic property that measures the ability of an aquifer to transmit groundwater throughout the entire thickness, shows that areas with these ranges of values are dominated with moderate groundwater potential (Offodile 1983). The distribution reveals that the western part of the study area constitute areas with high transmissivity magnitude (⁠⁠). This conforms with the high permeability distributions observed along the western part of the study area, thus suggesting high indices of transmissibility magnitude and high groundwater potential compared to other parts. Values of hydraulic conductivity (K_h) range from 0.0121 to 14.0931 m/day. The map as shown in Figure 6(b) shows areas with K values ranging from⁠. These magnitudes reflect moderate to high hydraulic conductivity in major parts of the study area (Driscoll 1986; Todd 1980). Areas with values K_h < 1.2 m/day (northwestern part) reflect areas with low hydraulic conductivity, and this can be attributed to poor interconnectivity of the pores or low permeability (Aleke et al. 2018).

These variations in K_h values signify aquifer flow dynamic nature. It shows that water passes through the aquifer with ease in greater parts of the area and with much difficulty in the northwestern part. The aquifer fractional porosity as estimated from Equation (4) ranges from 0.256 to 0.326 with an average value of 0.290. The fractional porosity map as shown in Figure 7(a) ranged from 0.255 < Ø >0.315. This range may be due to the presence of sand and gravel for unconsolidated sediments and sandstone for consolidated sediments in the aquifer formation of the area (Roscoe 1990). The map reveals areas of high and low porosities with the highest porosity (Ø > 0.315) area discerned along the western part. Minimum porosity ranging from 0.255 to 0.285 is observed along the northwestern parts of the map. Comparing Figure 7(a) with 3a Figure 3(a), it is observed that the aquifer layers along the western part of the study area are both porous and permeable, suggesting a good fluid transmissibility of the aquifer. From this observation and also from the plot confirmation in Figure 7(b), it is right to say that water projects sited along the western area will be highly productive.

Figure 8(a) shows the variation of tortuosity (⁠⁠) values ranging from 1.500< >1.698. Tortuosity range (⁠ > 1.698) shows areas with the highest aquifer geometric complexity. The distribution shows that northwestern parts constitute areas of high tortuosity with least distribution discerned along the western part and a small portion of the eastern part. Comparing Figure 8(a) with Figure 3(a), it is observed that due to the high permeability distribution along the western part and small fraction along the eastern part, there is a great reduction in the tortuosity distribution along these parts. The map reveals an inverse variation of permeability with tortuosity which is confirmed in Figure 8(b). This relation may be attributed to high geometric complexity of the aquifer, resulting from net overburden stress on the aquifer layer. Permeability and tortuosity relation Figure 8(b) attests to the fact that permeability decreases with a high tortuous path and increases with low tortuosity areas.

The inter-transmissibility magnitude of aquifer units in Edem were evaluated from hydraulic properties estimated using aquifer resistivity and thickness. Estimation of hydraulic properties was achieved employing some empirical relations of petrophysics. Clear comprehensions of the distribution of hydraulic properties in Edem were provided with the aid of the contour maps generated. VES 1, 2, 3, 4, 16, 17, 19 and 20 were delineated as areas with high groundwater transmissibility magnitude compared to other VES stations in the study area, following the high permeability and hydraulic conductivity values observed in the aquifer layers. From the contour map, high distribution of permeability, hydraulic capillary radius, transmissivity, hydraulic conductivity and porosity with low distribution of surface area per unit volume and tortuosity was observed along the western and some eastern parts of the study area. The seemingly high values of these properties along the western parts and some at the eastern parts suggest high indices of groundwater transmissibility magnitude along these parts. VES stations found along northwestern areas are dominated with low permeability distribution and both high tortuosity and surface area per unit volume along the northwestern area. The low permeability distribution along the northwesten parts of the study area signifies a low magnitude in groundwater transmissibility. From the above, it is observed that the western and some eastern parts of the study area constitute areas with high transmissibility (high productive aquifer) and as such encourages establishment of water scheme in these areas. The results obtained from this study will serve as a guide to explorationists in areas with high productive aquifer thus limiting the rate of water scheme project failures in Edem. The established functional mathematical relations between permeability and other hydraulic properties can be utilised in characterizing the hydrogeologic units of the area for proper groundwater exploration, exploitation and management.

The authors declare that they have no competing interests

All relevant data are included in the paper or its Supplementary Information.