Investigation of lithological control of iron enrichment in groundwater using geophysical techniques in Yenagoa, Southern Nigeria
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Hydrochemical analysis of water samples from Yenagoa in the Niger Delta shows widespread occurrence of iron (Fe) in the groundwater. The Fe concentration is more than 0.3 mg/L at many places, and the distribution is heterogeneous both vertically and horizontally. In order to identify the cause of the high heterogeneity, we carried out an integrated study consisting of hydrogeochemical, electrical resistivity sounding and induced polarization (IP) chargeability measurements at eleven sites and 2-D electrical resistivity profiling (at 2 sites). Data processing using inversion techniques resulted in 4-layered resistivity and chargeability—depth models. The results show that clean sand and gravel exhibit high resistivity but low chargeability and normalized chargeability values, whereas clay and sandy clay exhibit relatively low resistivity but high chargeability and normalized chargeability values. In sites where the aquifer is overlain by a thick clay layer, Fe concentration is high (Fe > 0.3 mg/L) in the groundwater and redox potential values range between 118 and 133 mV. We interpret that the low-permeability clay layer creates a relatively atmosphere-isolated state in the underlying aquifer, which is responsible for the reductive ambient subsurface groundwater environment. In sites where the aquifer is capped by silt, Fe concentration is low (< 0.3 mg/L) in the groundwater and redox potential values range between 115 and 164 mv indicating a mild oxidation environment. We interpret that the clay acts as a controlling factor to the Fe enrichment in the groundwater regime. Knowledge of the clay layer, which is identified in the present study, will be helpful in selecting suitable sites for boreholes.
KeywordsGroundwater Electrical resistivity Induced polarization Normalized chargeability Yenagoa
In the last decade, several investigations were carried out in parts of the Niger Delta, Southern Nigeria, to determine the regulating processes and spatial distribution of Fe in the shallow alluvial aquifer [1, 2]. The results of these studies show that the concentration of Fe in the groundwater abstracted from boreholes is high. Aside, the distribution of Fe in the groundwater was reported to be extremely heterogeneous, both vertically and laterally, within a scale of tens of metres. The issue of high iron concentration (> 0.3 mg/L) in groundwater is a common problem, since over 90% of residents in most cities in the Niger Delta depend on water abstracted from shallow private boreholes. Presently, we remain remarkably ignorant of the cause of the high Fe heterogeneity in the groundwater.
Considering the above, it was pertinent to investigate the probable reasons responsible for the high spatial variation of Fe in the groundwater and provide a sustainable solution for mitigation. Most studies in this direction only concentrated on the geochemical aspects of Fe contamination and suggested various purification techniques for iron removal . Designing various purification methods for iron removal is only temporary solution and suffers from many obvious problems such as waste disposal and maintenance and hence is not a sustainable solution. The strategic importance of groundwater in Yenagoa and the threats posed by excessive Fe concentration emphasize the significance of low-Fe groundwater sources and prompted the assessment of the hydrostratigraphy of the sedimentary sequences in the study area. In assessing the hydrostratigraphy of the sedimentary sequence, it was considered that investigating the alluvial aquifer over short distances ranging between tens to hundreds of metres and in different locations within the study area would perhaps help explain whether the variation in the groundwater Fe concentration was due to local variations in aquifer stratigraphy. Although borehole drilling could be one of the best ways to determine the lithological variation, this approach was considered tedious and would require drilling several boreholes of different depths. This will be time-consuming, laborious and cost-intensive. The use of non-invasive surface geophysical techniques is of great relevance in that reasonably factual subsurface information is obtained without any destruction to the environment within a relatively short time. Electrical resistivity and time domain induced polarization have shown a good complementarity in this regard. The geoelectrical method provides a wide range of variations in the subsurface electrical resistivity. The variations are often associated with water content and lithology; hence, it is one of the most powerful geophysical methods often used in providing solutions to hydrogeological problems [4, 5, 6]. Recently, induced polarization method which is based on the chargeability effect of the subsurface has proved to be of significant value in the investigation of lithological variability of unconsolidated sediments especially in the mapping of clay content. In this study, we explore the lithological control on Fe contamination using surface geophysical methods in Yenagoa and environs.
2 Description of the study area
2.1 Location, physiography and climate
2.2 Geology and hydrogeology of the study area
The study area lies within the fresh water swamps, backswamps, deltaic plain, alluvium and meander belt geomorphic unit of the Niger Delta ). The Niger Delta is basically an alluvial plain and consists of the modern and Holocene delta top deposits. Grain-size profiles of the Holocene alluvial deposits consist of a fining-up sequence of sand capped by fine silts and clay indicating a fluvial environment of deposition . The fine-grained silts and clay overlying the basal sandy sequence are often called the near surface aquitard. The near surface aquitard thickness varies between < 5 to about 12 m, and due to the varying clay, silt and fine sand content,  reported that the aquitard permeability is highly heterogeneous. The near surface aquitard becomes a confining unit if it is thick and impermeable, which prevents percolation of precipitation into the alluvial aquifer. Akpokodje  reported that groundwater flows from North to South in the region.
Three main subsurface lithostratigraphic units are reported in the Niger Delta . From top to bottom, they are Benin, Agbada and Akata Formations. The Benin Formation which is fluvial in origin is the main aquifer. Groundwater occurs mainly under unconfined conditions in the Benin Formation. Abam  observed that the sediments of the Benin Formation were deposited during the Late Tertiary–Early Quaternary period and are about 2100 m thick. The sediments are lenticular and unconsolidated and consist of coarse- to medium-fine-grained sands with localized intercalations of clay/shale. Gravel and pebbles are minor components. Mbonu et al.  reported that the sands are moderately sorted and poorly cemented. The presence of thin clay beds creates discontinuities in the vertical and lateral continuity of the aquifer, resulting in the presence of local perched aquifers . The aquifer is directly recharged through the infiltration of rain water. In the Niger Delta, the water table in many areas is close to the surface though subject to seasonal variations. The water table is about 3–4 m in the dry season , but rises considerably in the rainy season. Groundwater is the main source of drinking water for over 80% of the population in the study area.
3 Induced polarization (IP) method
4 Data acquisition and processing
4.1 Geoelectrical sounding and induced polarization
Summary of VES-IP model results and their corresponding thicknesses at low-Fe areas
Layer 1 (top soil)
Layer 2 (silty sand)
Layer 3 (sand)
Layer 4 (sandy clay)
4.2 Electrical resistivity imaging
We also acquired one each 2D electrical resistivity imaging profile in the low-Fe area and as well as in the high-Fe area using the Wenner array (Fig. 3). The 2D resistivity profile was acquired to supplement the vertical electrical sounding (VES) and IP sounding data. This is because the 2D resistivity imaging gives a clearer picture of the lateral and vertical variation of the subsurface geological sequences. The 2D resistivity imaging data were acquired manually using the Wenner configuration. Each 2D profile was 100 m in length. The electrode separation ranged between 5 and 30 m in an interval of 5 m, with a total of 21 electrode positions for each profile. Field measurements were taken using electrode spacing of 5.0 m at electrode positions 1, 2, 3 and 4 in each profile. Then, each electrode was moved a distance of 5.0 m (one unit electrode spacing), the active electrode positions being 2, 3, 4 and 5. This procedure was continued to the end of the profile with electrode positions for the last measurement being 18, 19, 20 and 21. The electrode spacing was then increased by 5.0 m, as mentioned above for measurements of next data level, such that the active positions were 1, 3, 5 and 7. The procedure was then repeated by moving each of the electrodes a distance 5.0 m (one unit electrode spacing) and maintaining the electrode spacing for the data level until the electrodes were at electrode positions 15, 17, 19 and 20. This procedure was continued until 6 data levels were observed, yielding a total of 63 data points in each of the profiles. RES2DINV computer code  was used in the inversion of the 2D data. The computer program takes advantage of the nonlinear optimization technique in which a 2D resistivity model of the subsurface is automatically determined for input apparent resistivity data [19, 20]. In this program, the subsurface is subdivided into a number of rectangular blocks based on the spread of the observed data. The 2D data were inverted using the least-squares inversion with standard least-squares constraint which minimizes the square difference between the observed and the calculated apparent resistivity values. The program displays the distribution of electrical properties in the form of 2D pseudo-section plot. 2D pseudo-section plot gives a simultaneous display of both horizontal and vertical variation of the subsurface resistivity and are useful for initial quality assessment . In constructing a 2D pseudo-section plot, each measured value is put at the intersection of two 45o lines through centres of the quadripole. Each horizontal line is then associated with a specific value of n (inter electrode spacing) and gives a pseudo-depth of investigation. It is pertinent to note that the larger the n-values, the greater the depths of investigation .
4.3 Hydrogeochemical analysis
Chemical composition of the groundwater at low-Fe areas
The drilled boreholes were lithologically logged and sampled at 3.0 m or more often when characteristics of the sediment changed based on their grain size and colour. Each sample was assigned to one of the three colours—grey, off-white and brown by virtual inspection of the sediments. The Fe concentration of about 3 g of the wet sediments was measured by AAS and after extraction with hydroxylamine hydrochloride (NH2OH.HCl) in 25% acetic acid, and filtering using a 0.45-µm cellulose acetic filter. The boreholes were screened at depth intervals of either oxidized brownish sand aquifers, off-white or greyish reduced sediments. The depth of these boreholes varied between 8 and 30 m. Fe concentration in the aquifer sediments is shown in Table 5.
5 Results and discussion
In this study, resistivity soundings and profiling were carried out in locations with contrasting dissolved iron concentrations, low and high in the groundwater (Fig. 3). IP soundings were also carried out simultaneously to support the resistivity interpretation for investigating the lithological control of Fe enrichment in the groundwater.
The chargeability (M) and normalized chargeability (MN) of the top soil range between 0.79 and 2.47 ms and 0.014–0.073 mS/m but range between 1.3 and 3.8 ms and 0.009–0.186 mS/m, respectively, in the silty sand layer. In the sandy layer, the chargeability and normalized chargeability values are between 0.71 and 11.25 ms and 0.001–0.031 mS/m but range between 1.72 and 126.3 ms and 0.007–0.424 mS/m in the sandy clay layer. The chargeability of a given medium indicates polarizability of the medium. Thus, chargeability is related to the permittivity and resistivity of the subsurface materials as well as the porosity and moisture content in the subsurface media. The normalized chargeability (i.e. the ratio between chargeability and resistivity) has also been reported to be a good parameter for discriminating lithotypes [17, 18]. These authors suggested that clean sands have low chargeability and low normalized chargeability, while clay and clayey sands have high chargeability and normalized chargeability values.
A careful analysis of the model results (Figs. 5 and 8) shows a strong correlation between the resistivity and chargeability anomalies with high resistivity values corresponding to relatively low chargeability and normalized chargeability values (Table 1). Deposits of clean sand and gravel are easily distinguished by their high resistivity values from their surrounding clay and silt [23, 24, 25]. Small measurable IP effects are associated with clean sand and gravel deposits . The model results show low chargeability and low normalized chargeability values in the second and third layers (i.e. silty sand and sand layers). This indicates that these layers are mainly composed of sandy materials and less disseminated clayey materials. In contrast, high chargeability and normalized chargeability values are observed in the sandy clay layer ranging between 1.72 and 126.3 ms and 0.007–0.424 mS/m relative to layers 2 and 3. This is consistent with the observation of Amaya et al.; Vonhala [26, 27] who suggested that strong IP effects are commonly observed in sediments containing clays disseminated on the surface of larger grains. Hence, the sandy clay layer displays large IP effects as a result of the presence of disseminated clay. The lithology suggested by the 2D resistivity interpretation (Fig. 6) correlated well with the observed subsurface materials obtained from ground-truthing in this location and also clearly delineate the basal sandy clay layer. Layer 3 serves as the aquifer in the area, and residents tap their water from this layer. Table 2 shows that the Fe concentrations obtained from the analysis of groundwater samples abstracted from boreholes in these areas are within WHO acceptable limits. Additionally, the measured redox potential values from the groundwater immediately after abstraction are relatively high, ranging between 115 and 164 mV, indicating a mild oxidation environment .
Summary of VES-IP model results and their corresponding thicknesses at high-Fe areas
Layer 1 (top soil)
Layer 2 (clay)
Layer 3 (sandy clay)
Layer 4 (sand)
Chemical composition of the groundwater at high-Fe areas
5.1 Source of Fe and role of near surface clay layer on groundwater Fe distribution
Fe concentration in the aquifer sediments
Fe conc. (mg/L)
Fe conc. (mg/L)
We infer therefore that the spatial distribution of groundwater Fe is as a result of the variation of redox conditions in the host aquifer. This implies that a reductive ambient subsurface environment is favourable to Fe ions transferring from the aquifer matrix into the groundwater. Although the decomposition of organic matter in groundwater and soil can consume dissolved oxygen and thus create a reductive hydrochemistry, in this case this effect is assumed to be significantly small. We opine that a relatively atmosphere-isolated state in the aquifer is responsible for the stronger reducibility of groundwater in the high-Fe locations. The investigations of the sediment stratigraphy and lithology across the study area show a near surface aquitard composed of argillaceous materials (clay) widely occurs in the upper most subsurface sediments in the high-Fe locations. This near surface aquitard heterogeneity and variation in thickness lead to variation in vertical recharge, localized dilution and confinement, resulting in varying redox conditions in the aquifer affecting Fe release.
An integrated hydrogeophysical investigation consisting of electrical resistivity and induced polarization techniques in parts of the Niger Delta delineated a widespread clay layer characterized by low resistivity (5–96 Ωm) and high chargeability and normalized chargeability values (0.67–13.9 ms and 0.134–2.242 mS/m) overlying the aquifer in locations that show elevated dissolved Fe concentrations. The thickness of the clay layer varies, pinching out at some places. In locations where the clay layer pinches out, the Fe concentration is within WHO acceptable limits (< 0.3 mg/L), implying that the lithological set-up plays a significant role in understanding Fe enrichment in groundwater in the Niger Delta. The low-permeability clay layer acts a confining layer and thus helps in creating atmosphere-isolated state in the underlying aquifer which is responsible for the reductive ambient subsurface groundwater environment favourable to Fe ions transferring from the aquifer matrix into the groundwater. The knowledge of the clay layer will be very helpful in selecting suitable sites for the installation of boreholes.
We are grateful to the Post-graduate Geophysics students in the Department of Physics who assisted with the field work and Mr. Udofia for producing the maps.
Compliance with ethical standards
Conflict of interest
We do not have conflict of interest in this paper.
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