Investigation of aquifer storage properties and groundwater contamination risk potential using an electrical resistivity technique: a sedimentary environment case study

a

pore geometrical factor

AB

current electrode distance

APC

aquifer protection capacity

d_w

water density

F

formation factor

g

acceleration due to gravity

GLSI

Geoelectric Layer Susceptibility Index

h

thickness

K_h

hydraulic conductivity

K_p

permeability

LC

longitudinal conductance

LGA

local government area

LGAs

local government areas

m

cementation exponent

MCDA

Multi-Criteria Decision Analysis

MN

potential electrode distance

n

number of layers

OBAKIKE

Obot Akara and Ikot Ekpene

R_A

apparent resistance

RMSE

root mean square error

S

saturation at specific retention

S_r

aquifer-specific retention

S_y

aquifer-specific yield

VES

vertical electrical sounding

λ

the aquifer-specific yield – specific retention ratio

ρ

resistivity

ρ_A

apparent resistivity

ρ_b

aquifer bulk resistivity

ρ_sat

resistivity of the saturated portion of the aquifer

ρ_unsat

resistivity of the unsaturated geological unit overlying the topmost aquifer

ρ_w

water resistivity

σ_w

conductivity

aquifer porosity

μ_d

viscosity of water

The necessity of water for life has sparked a global surge in groundwater studies. Groundwater is stored in underground rocks called aquifers. The rock must be both permeable and porous in order to function as a prolific aquifer. The ability of an aquifer to release water from storage in response to a decrease in hydraulic head or to increase heads in response to an accumulation of water is defined by its storage characteristics. Aquifer storage capacity is greatly influenced by aquifer effective porosity, which is the fractional volume of interconnected empty spaces in the aquifer material. The amount of water that an aquifer can store depends on its effective porosity. The capacity of the aquifer to store water increases with porosity and vice versa. Nevertheless, not all the stored water in the aquifer is discharged during pumping into wells or boreholes. For unconfined aquifers, the fraction of the volume of water that the aquifer discharges by gravity to the overall volume of the aquifer is referred to as aquifer-specific yield or storativity S_y (Lohman 1972; Fetter 1994; Schwartz & Zhang 2003). Aquifer-specific retention S_r, on the other hand, is the fraction of the volume of water reserved or retained by the aquifer to the overall aquifer volume (Healy & Cook 2002). Aquifer effective porosity is the sum of specific retention and specific yield (Frohlich & Kelly 1988; Tizro et al. 2012). Factors affecting specific yield include grain size, shape and distribution of pores in the rock material. Adequate understanding of aquifer storage properties especially specific yield and retention of unconfined aquifers is important in the evaluation of groundwater potential of a given area (Lin et al. 2023). In this context, groundwater potential pertains to the capacity of an aquifer to produce water in sufficient quantity given certain hydrogeological circumstances. Groundwater potential investigation entails the identification of regions with prolific aquifers for optimum location of water wells (Ekanem et al. 2022a; Ekanem & Udosen 2023a). When the specific yield is higher than the specific retention, the aquifer releases more water than it keeps back and therefore has a high potential to supply water to a well or borehole during pumping. The reverse is also true and corresponds to low groundwater potentiality.

Undoubtedly, groundwater potentiality is high in most sedimentary basins (Ekanem & Udosen 2023a, 2023b; George et al. 2024). However, groundwater contamination by anthropogenic and natural sources poses a serious risk to the quality of groundwater (Thomas et al. 2020; Ekanem 2022a; Ekanem et al. 2022b; Ikpe et al. 2022; Ekanem & Udosen 2023b). This could be the result of human-induced processes or natural processes, which include agricultural practices, erosion, salt intrusion, poor sewage system, poor waste disposal, mining activities, landfill leachates and poor channelization of surface run-off (Uddin et al. 2021; Ekanem et al. 2022a). Wash-off from leachates and other chemical substances emanating from the breakdown of these poorly managed waste materials can drip down into the subsurface to cause groundwater contamination. Depending on the nature of the geomaterials of the vadose zone, the underlying aquifers may or may not have some degree of protection from surface or near-surface contaminants (Ekanem 2020; Ekanem et al. 2021; George 2021; Ikpe et al. 2022). This is referred to as aquifer protectivity. Aquifer susceptibility potential is the likelihood of the aquifer being contaminated by surface or near-surface contaminants (Awawdeh & Jaradat 2010; Ekanem et al. 2022a; Ekanem & Udosen 2023b). Investigation of groundwater contamination risk potential involves identifying areas that are prone to contamination arising from either anthropogenic or natural processes. This investigation is indeed necessary for the sustainable exploitation and management of groundwater georesource to meet the growing demand for potable water.

As a result of significant human population growth and other business operations in Obot Akara and Ikot Ekpene local government areas (LGAs) of Akwa Ibom State, there are growing worries regarding sustainable and sufficient supplies of clean water in the area. Studies by Ekanem et al. (2021) on the assessment of aquifer protectivity and corrosivity of shallow layers in some communities in the Obot Akara LGA have shown that most of the aquifer units in the area lack adequate protection against surface or near-surface contaminants. In the same manner, studies by Ikpe et al. (2022) on the assessment of the protectivity of hydrogeological units in some communities in Ikot Ekpene LGA reveal that a greater proportion of the aquifers in the area are characterized by weak/poor protection. Their findings demonstrate that the aquifer overburden layers are made up of previous geomaterials, which enhance easy percolation of any contaminated fluid into the aquifer system to cause groundwater contamination. George (2021) evaluated the vulnerability potential of the hinterland aquifers in some communities in the Niger Delta region where this research was carried out and found that a greater proportion of the aquifers have moderate/high vulnerability potential. The results of Ekanem (2022a) and Ekanem et al. (2022a) still in parts of the Niger Delta region equally demonstrate that most of the aquifer units in the area are characterized by moderate/high vulnerability potential. These results, they attributed to the low slope terrain in the area which enhances rapid infiltration of any leachates and other chemical substances originating from the decomposition of poorly disposed solid waste materials littering some streets in the study area during rainfall. All these studies however fail to evaluate the aquifer storage properties (specific yield and retention), which may have a link with the groundwater potential for the sustainability of water wells. Equally, it is paramount to have a fuller understanding of the groundwater contamination risk potential in the area to guide the development, exploitation and utilization of groundwater in the area. To date, the area has no groundwater contamination potential map. Consequently, the main aim of this study is to investigate aquifer storage properties and groundwater contamination risk potential in Obot Akara and Ikot Ekpene (OBAKIKE) LGAs of Akwa Ibom State in the Niger Delta region of southern Nigeria via the deployment of the surface resistivity technique. Ikot Ekpene municipality, being a major commercial nerve centre in the state continues to experience population increase over the years. Public water supply facilities in OBAKIKE LGAs are far too insufficient to cater for the water requirements of the inhabitants. Thus, the local population massively depends on groundwater abstracted from boreholes or hand-dug wells in the area, most of which is accomplished through a wild-cat drilling approach (Ekanem et al. 2021; Ikpe et al. 2022; Ekanem & Udosen 2023a, 2023b). The productivity and sustainability of such boreholes and water wells depend to a large extent on the hydraulic and storage characteristics of the aquifer system in the area. Furthermore, of major concern is the status of the groundwater quality from such boreholes and wells, which depends on the susceptibility potential of the aquifer to contaminants infiltration. To this end, an investigation of the storage properties and contamination risk potential of the aquifer units in the OBAKIKE LGAs is essential to map out areas for optimum borehole sustainability and low susceptibility potential to contamination. Groundwater contamination risk potential can be effectively controlled and reduced by detecting and monitoring contaminated areas.

The resistivity method is a low-cost technique for quickly determining the resistivity distribution of the subsurface. In this method, a DC current or low-frequency AC current is injected into the earth via two electrodes on the surface and the resultant potential difference produced is detected by another pair of inner electrodes also planted on the Earth's surface (George et al. 2016, 2017, 2020, 2021; Ekanem et al. 2020). Ohm's law provides the basis for getting the apparent resistance of the earth layers penetrated by the current. This technique has been extensively used by many scholars for groundwater studies globally (e.g. Evans et al. 2015; George et al. 2018, 2022a, 2022b; Umoh et al. 2022; Inyang et al. 2023, 2024 etc.). The storage properties considered in this study are aquifer-specific yield and retention, which have a link with groundwater potential. Groundwater contamination risk potential was evaluated via the use of Longitudinal Conductance (LC) and Geoelectric Layer Susceptibility Index (GLSI) parameters. This project was designed especially to achieve the following objectives: evaluation of aquifer-specific yield and retention, mapping of locations with high groundwater capability to sustain water wells, assessing the susceptibility of the hydrological units to contaminations that might emanate from or near the surface, generation of the groundwater and groundwater contamination potential maps in the study area. The results of this study provide valuable guidelines to the local government authorities and other policymakers in formulating sustainable groundwater exploration and management strategies in the area.

Geologically, the overall chronology of the Niger Delta region comprises the Benin, the Agbada and the Akata Formations in that sequence (Obaje 2009; Udo & Udofia 2020; Udo et al. 2020; Udofia & Udo 2021). The Benin Formation is the youngest unit in the Niger Delta and is composed of sands of unequal grain sizes ranging from fine sands to coarse sands and gravelly sands with diverse thicknesses, intermingled with thin clay beddings, lenses, and lignite streaks in some places (Mbipom et al. 1996; Udo & Mode 2013a, 2013b; Udo et al. 2023a, 2023b, 2023c; Allen et al. 2024). The local population in OBAKIKE obtain their groundwater from the Benin Formation, which constitutes the main hydrostratigraphic unit in the area. Where they occur, the sand–clay interbeddings cause the establishment of multi-aquifer systems (Esu et al. 1999).

The electrical resistivity method involving vertical electrical sounding (VES) was employed in this study to examine the subsurface resistivity distribution. Data obtained were analyzed by the use of Microsoft Excel and interpreted by the use of WINRESIST computer software. Aquifer storage properties were estimated by the use of empirical relations between the aquifer geoelectric properties derived from VES data interpretation while aquifer contamination risk potential was evaluated by using LC of the aquifer overlying layers and the GLSI approach. The image maps were generated by the use of SURFER 10 software via the deployment of the kriging method. The kriging method is more versatile and efficient in creating attractive maps for the majority of data sets. The kriging model employed was the linear variogram model.

Equation (7) forms the basis of using electrical resistivity data to calculate aquifer-specific yield (Frohlich & Kelly 1988).

The findings of this study agree with previous results of George et al. (2021, 2024), Ekanem et al. (2021, 2022a, 2022b), Ikpe et al. (2022), George (2021) and Ekanem (2022a). Although the results demonstrate that the study area has high groundwater potential for the sustainability of water wells, the aquifer units in the area are mostly unconfined. Most aquifers by and large lack impermeable overburden layers, which can retard the percolation of contaminated fluids or filter contaminated fluids before reaching the aquifer system except at a few locations in the northeastern and southeastern parts of the study area as illustrated in Figure 10. Since rainfall is the major source of recharge in the area, by implication the aquifer system in these locations can easily be contaminated by surface run-off, carrying debris and other harmful dissolved substances infiltrating into the aquifer system. Consequently, town planners and other government agencies in the area need to ensure proper channelization of run-off in the area aside from making stringent policies to regulate indiscriminate dumping of wastes on the streets of the study area. Water from boreholes already drilled in the contamination-prone localities in the area needs to be treated before consumption and the locations of new ones need to be checked to forestall the outbreak of epidemics in the area.

This study involves the investigation of aquifer storage properties and contamination risk potential using an electrical resistivity technique in Ikot Ekpene and Obot Akara LGAs in Akwa Ibom State. The two LGAs are shown to comprise three to four lithological successions of sandy and gravelly layers with slight clay intercalations. Groundwater abstraction in the layer is shown to take place in the third layer at some locations and the fourth layer at other locations. The depth to the top of this hydrostratigraphic layer varies from 5.4 m at Utu Ikot Inyang to 121.4 m at Ikot Udom communities, respectively, while the resistivity lies between 234.9 and 1,506.3 Ωm. Also, the aquifer thickness varies from 37.6 to 82.4 m at VESs 13 and 17, respectively. Aquifer porosity ranges from 0.28 to 30 while hydraulic conductivity and permeability, respectively, vary between 0.68 and 2.61 m/day and 1,132.33 and 4,380.18 mD. The estimated aquifer-specific yield values obtained lie between 0.12 and 0.29 while the specific retention lies between 0.02 and 0.16. The aquifer-specific yield – specific retention ratio computed varies from 0.79 to 17.15. The results show that an inverse relation exists between the aquifer-specific yield and specific retention. At a value of 0.15 for specific yield and specific retention corresponding to a unit value of specific yield – specific retention ratio (λ), the aquifer releases the same amount of water as it retains during pumping. However, at values of specific yield greater than 0.15 and specific retention less than 0.15, more water is released by the aquifer during pumping and λ is greater than one. The reverse is the case when the specific yield is less than 0.15 and the specific retention is greater than 0.15. On this basis, about 94% of the study area is shown to have good groundwater potential for the sustainability of water boreholes in the area.

Groundwater contamination risk potential was assessed via the use of LC and GLSI. Accordingly, the values of LC vary from 0.01 to 0.26 S while that of GLSI lies between 1.00 and 2.75. 87.5% of the study area is shown to have weak/poor protection while 12.5% has moderate protection against infiltrating surface or near-surface contaminants based on the LC results. On the basis of the GLSI values, 21.9% of the study is shown to have a moderate susceptibility rating while the remaining 78.1% has low susceptibility potential rating. The identified areas with weak/poor protection and moderate susceptibility are adjudged to have high risk to groundwater contamination. Although the area is characterized by high groundwater potential adjudged from the computed aquifer storage properties, the majority of the aquifer units lack impervious confining layers and hence have poor protection against any surface or near-surface percolating contaminated fluid. Surface wash-off of leachates and other hazardous chemical substances from the breakdown of poorly disposed wastes in the area can thus infiltrate easily to contaminate the groundwater system. This therefore calls for a need to formulate stringent waste disposal schemes and proper channelization of run-off in the area by town planners and other local government agencies. There is also a need for water treatment for the boreholes already drilled in the contamination-prone localities while there should be effective regulations on the locations of new ones to forestall the outbreak of epidemics in the area. The findings of this study ultimately will aid in formulating efficient groundwater exploration and management strategies in the area such as demarcation of sites for drilling new water boreholes, proper monitoring of waste disposal in the area by residents to forestall leachate percolation to contaminate groundwater, construction of sufficient gutters for run-off on the major streets and removal of blockages in the existing ones where necessary.

The authors are grateful to the postgraduate students and members of the GRG of the Department of Physics, Akwa Ibom State University for their supports especially during the data acquisition stage of this study.

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

The authors declare there is no conflict.