Introduction

The water that we drink contains some level of natural radioactivity due to water– soil interaction in various geological formations such as the earth crust, rocks, and soils, with water. The earth soil lithology plays a very important role in the estimation and determination of the source of radionuclide in the subsurface water aquifers. The emission of radiation from naturally occurring radionuclides in rock formations and soil are sources of pollution in the environment if allow to build up naturally or by anthropogenic activities. Monitoring of radioactive materials is, therefore, of primary importance for human well-being and environmental protection (El-Bahi 2004).

The human and natural activities in the terrestrial environment contribute to and impact the soil radioactive content, thereby polluting the flora, surface, and sub surface water that can cause harmful effect to man and other biotic organisms in the environment. Percolation, surface water run-off, leaching of water through the geological formation, and the bed rock–aquifer interactions contribute to the overall radionuclide concentration in subsurface water. The lithology of any given area, therefore, is a major determining fact of the water quality that is obtained in the subsurface water of the area. Ika South geological formation and soil lithology make it difficult to predict with precision water aquifer table, good, and portable water that meet national and international standard of drinkable water. The exploration for good water aquifer and exploitation through drilling deep wells for the supply of portable water that meets World Health Organization (2011) standard must, therefore, be carried out. These relevant procedures and analysis should include geophysical evaluation, radionuclide concentration assessment in the soil depth lithology, and the physico-chemical parameters of the water aquifer. However, soil radionuclide activity concentration is one of the main determinants of the natural occurring radiation and the quality of the infiltrated groundwater, because, when rocks are disintegrated through natural process, radionuclides are carried to soil by rain infiltration process, and end up in the subsurface water (Taskin et al. 2009).

Nevas et al. (2002a) reported that radioactivity concentration variation with geological formation, soil type, and soil lithology profiles are relatively new in the field of radiation studies and Nigeria is not an exception. However, studies have shown that, in soil lithology, radioactivity concentrations are highly dependent on the soil type and mineral content of the environment (Nevas et al. 2002a). Nevas et al. (2002b) determined the distributions of natural gamma-emitting radionuclide in three soil profiles developed on tertiary sedimentary materials in mountain landscapes of the Central Spanish Pyrenees and reported a depletion of radioactivity concentration with depth. Nevas et al. (2002a) also reported that soil properties affected the mobilization of natural radionuclide and the association of some radiologic properties with soil layers suggests a relationship between soil processes and radionuclide distribution.

Vukasinovic et al. (2017) assessed the radionuclide variation from a series of soil cores between 13 and 40 cm depth collected in different lithology and reported that radionuclides show variations in the depth profile as well as in the different morphedaphic environments studied. They further reported that variability in some radionuclides seems to be related to mineralogy derived from parent materials as well as with cryogenic and soil processes affecting the depth distribution of the granulometric fractions and organic matter. Avwiri et al. (2010) measured the radionuclide concentration with depth of Port Harcourt lithology, and reported that radionuclide concentration reduced with depth from 0 to 40 m in a significant manner down the soil depth. Avwiri et al. (2012) evaluated the radiological effects of soil profile samples of Udi and Ezeagu areas of Enugu State, Nigeria, and reported a contrast behaviour of the radionuclide concentration down the soil lithology.

The need to obtain accurate and reliable data of aquifer characteristics in terms of precise transmissivity value of the aquifer in the environment and to delineate areas with good groundwater potential and the uncertainty surrounding variation of natural radionuclide concentration with depth of soil lithology lend credence to this research work. This study is, therefore, aimed at assessing the groundwater potential and soil profile of the study area; determines the concentration of the three natural radionuclides with depth lithology up to 66 m depth; assesses the radiological health risk parameters associated with the samples. This will help to estimate the radionuclide content in the water aquifer to determine its portability. The obtained result will constitute a baseline data of radionuclide status of the geological formation and soil profile lithology of the study area, and will add to the world database of radionuclide variation with depth profile. The findings of the study will be useful to the government and geologist exploiting solid minerals in the area.

Materials and methods

Study area

Agbor, the study location, is the administrative headquarters of Ika South LGA of Delta State. It covers a landscape that spans the dwarf hills and extends down to the deep valley of Orogodo in Abavo. It has an area of 89.1 km2 and an estimated population of 15,922 people. It lies within longitude 6˚16′N–6˚16′N and latitude 6˚09′E–6˚12′E of the equator, with a mean annual temperature range of 28–34 °C. The geology of the area consists mainly of coastal plain sands, deltaic sands, and meandered belts and alluvia sands, all ranging from Oligocene to Holocene. The area is underlain by superficial sediment of Quaternary Age which is loose sand to clayey sand of the Benin formation which overlies the entire onshore of southern region of Nigeria (Etu-Efeotor and Akpokodje 1990; Egbai 2013; Ohwona et al. 2013). Drilling of borehole for drinking water in the study area has been a hectic task because of the heterogeneous lithology and topography of the environment. It is, therefore, necessary to carry out geophysical survey to determine the right water aquifer. The area is also noted for the exploration of solid minerals, especially coal, kaolin, limestone, and other solid minerals that may be radioactive.

Geoelectric measure of water aquifer

The Schlumberger array arrangement is an effective and easy to use method for geophysical survey, and thus, it was adopted for this study. Measurements were taken at expanding current electrode distance (A and B), while the potential electrode (M and N) spacing was kept constant for successive measurements, but adjustments were made only when the voltage readings becomes too small for instrument sensitivity in accordance with the procedure reported by (Egbai 2013; Ohwona et al. 2013). The current electrodes were kept fixed and the potentials were expanded in either case (Zohdy 1989; Egbai 2010, 2013; Ohwona et al. 2013). For every measurement, the ratio of potential to current electrode was 1.5. Three VES locations were strategically located and selected to be occupied with a maximum current electrode spacing of 500 m. The terameter was first observed at off mode before it was switched on, and then, the potential difference at electrodes, geometric factors, and the resistance were recorded. The apparent resistivity (\(\rho\)) values were determined, taking the product of the resistance measured on the earth resistivity meter and the geometric factor that is independent of the array used (Egbai 2012, 2013).

The apparent resistivity (\(\rho\)) was determined using the relation (Ohwona et al. 2013):

$${\rho _{\text{a}}}=KV{\text{/}}I,$$
(1)

where \(V\) is the voltage of the potential electrode, \(I\) is the current of the electrode, and \(K\) is the geometric factor for the Schlumberger configuration, as shown in Fig. 1, and is expressed as follows:

$$K=\frac{{\pi {{\left[ {{\text{AB/}}2} \right]}^2} - {{\left[ {{\text{MN/}}2} \right]}^2}}}{{{\text{MN}}}}.$$
(2)
Fig. 1
figure 1

Location map of the study area

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The field data were interpreted using both qualitative and quantitative processes through plotting of the apparent resistivity against the current electrode spacing, through partial curve matching method and computer iteration techniques using Resist Version 1 to evaluate the curves type and to determine the numbers of layers (Vander 1988).

Sample collection and preparation for radioactivity

Twenty-one soil samples were collected during the drilling process of a public water bore after using the geoelectric survey method to identify the right location and the water aquifer depth. Soil samples were collected first at the surface and then every 3.0 m depth corresponding to the length of drilling pipe. The soil samples were collected using a steel-hand geological auger, which was cleaned with acid, detergent, and rinsed with clean water.

Soil samples of about 450 g wet weight of the soil collected were then put in new aluminum foil and labeled and placed in black polythene bags. The soil samples were spread in a room at ambient temperature for equilibration. The soil samples were then oven dried at temperature between 60 and 70 °C for about 24 h. The dried samples were grounded with mortar and pestle, and then allowed to pass through 100-mesh sieve. Sample of 250 g dry weight was packed in air tight cylindrical plastic container which is of the detector geometry, and stored for a period of 4 weeks for secular equilibrium to be attained before counting (Zarie and Al-Mugren 2010; Avwiri et al. 2012).

Sample analysis

The samples were analyzed using a sodium iodide [Na(TI)] detector that was connected to an ORTEC 456 amplifier and a computer program MAESTRO window which match gamma energies to the library of possible isotopes. The cylindrical plastic containers holding the samples were made to sit on the precise geometry 7.6 cm  × 7.6 cm NaI(TI) of the detector. Near absolute-level shielding against the environmental background radiation was achieved by counting in the Canberra 100-mm-thick lead castle. Spectra were accumulated for background for 8 h at 900 V to produce strong peaks at gamma emitting energies of 1461 keV for 40K; 609 keV of Bi; 911 keV of 228Ac, which were used to estimate the concentration of 226Ra and 232Th, respectively (El-Gamal et al. 2007; El-Taher and Madkour 2014). The energy resolution of the detector using Cs-137 from International Atomic Energy agency (IAEA 1996) is 8.3% at 662 keV Cs-137 line, while the activity of the standard at the time of calibration is 25.37 KBq. The background spectra measured under the same conditions for both the standard and sample measurements were used to correct the calculated sample activities concentration in accordance with the standard procedures (Nevas et al. 2002b; Arogunjo et al.2005; Avwiri et al. 2012). The activity concentration (C) in Bqkg− 1 of the radionuclides in the samples was calculated after decay correction was made using the expression:

$$~~{A_{\text{c}}}=\frac{{{N_{\text{p}}}}}{{{\varepsilon _{{\text{ff}}~}} \times {M_{\text{s}}}~ \times ~{T_{\text{c}}} \times ~{\gamma _{\text{p}}}}}~{\text{Bqk}}{{\text{g}}^{ - 1}},$$
(3)

where \({A_{\text{c}}}\) is the sample concentration, \({N_{\text{p}}}\) is the net peak area of a peak at energy, \({\varepsilon _{{\text{f}}~}}\) is the efficiency of the detector for a γ-energy of interest, \({\varepsilon _{{\text{ff}}~}}\) is the sample mass, \({T_{{\text{c}}~}}\) is the total counting time, and \({\gamma _{\text{p}}}\) is the emission probability of radionuclide of interest.

Quality control and quality assurance

The calibration of energy and efficiency of the spectrometer system were performed 2 weeks before the analysis of samples using the standard reference radionuclide sources and the reference soil. The samples of 10% were selected at random for replicate analysis (n = 3) and the relative standard deviations of results were found to be less than 1.2%, which shows that the detector system is stable, and the results are reliable (Gang et al. 2012).

Radiological risk parameters

To assess, quantify, and estimate the radiation hazards associated with the study samples and arrive at a reliable conclusion, six radiological risk parameters were defined and determined (Diab et al. 2008; Kabir et al. 2009; Zarie and Al-Mugren 2010; Senthilkumar et al. 2010; Ononugbo et al. 2017).

Radium equivalent activity (Raeq)

Radium equivalent activity (Raeq) is a radiological risk parameter used to compare the specific activities of materials containing 226Ra, 232Th, and 40K by a single quantity, which considers the radiation hazards associated with them (Avwiri et al. 2013). The radium equivalent activity represents a weighted sum of activities of the three natural radionuclides and is based on the estimation that 1 Bqkg− 1 of 226Ra, 0.7 Bqkg− 1 of 232Th, and 13 Bqkg− 1 of 40K produce the same radiation dose rates. The radium equivalent activity index was estimated using the relation (Avwiri et al. 2013; Ononugbo et al. 2017):

$$R{a_{{\text{eq}}}}={C_{{\text{Ra}}}}+1.43{C_{{\text{Th}}}}+0.077{C_{\text{k}}},$$
(4)

where \(~{C_{{\text{Ra}}}},\;{C_{{\text{Th}}}}\), and \(~{C_{\text{k}}}\) are the radioactivity concentration in Bqkg− 1 of 226Ra, 232Th, and 40K respectively.

The use of a material whose \({\text{R}}{{\text{a}}_{{\text{eq}}}}{\text{~}}\) concentration exceeds 370 Bqkg− 1 is to be discouraged to avoid radiation hazards (Sam and Abbas 2001).

Annual gonadal equivalent dose (AGED)

The bone marrow, the gonads, and the bone surface cells are considered as vital organs of interest by UNSCEAR (2000). An increase in AGED has been known to affect the bone marrow, causing destruction of the red blood cells that are then replaced by white blood cells. This situation may result in a fatal blood cancer called leukemia. The AGED for the general public is estimated using the equation (UNSCEAR 2000; Avwiri et al. 2013):

$${\text{AGED}}~\left( {{\text{Svy}}{{\text{r}}^{ - 1}}} \right)~=~3.09{C_{{\text{Ra}}}}~+~4.18{C_{{\text{Th}}}}~+~0.314{C_{\text{k}}},$$
(5)

where \({C_{{\text{Ra}}}},\)\({C_{{\text{Th}}}},\) and \({C_{\text{k}}}\) are the radioactivity concentration of 226Ra, 232Th, and 40K, respectively.

External (H ex) and internal hazard index (H in)

Radionuclides occur naturally in terrestrial soils which produces an external radiation fields to which all human beings are exposed. If these radiations from the fields are inhaled, due to exposure to radon gas can lead to respiratory diseases like asthma and cancer (internal exposure). The external hazard (\({H_{{\text{ex}}}}\)) and internal hazard (\({H_{{\text{in}}}}\)) indices were evaluated using the relations (Ramasamy et al. 2009; Ononugbo et al. 2017):

$${H_{{\text{ex}}}}={C_{{\text{Ra}}}}{\text{/}}370+~{C_{{\text{Th}}}}{\text{/}}259+{C_{\text{k}}}{\text{/}}4810,$$
(6)
$${H_{i{\text{n}}}}~={C_{{\text{Ra}}}}{\text{/}}185+{C_{{\text{Th}}}}{\text{/}}259+{C_{\text{k}}}{\text{/}}4810.$$
(7)

The value of this index must be less than unity for the radiation hazard to be negligible (Ramasamy et al. 2009).

Representative gamma index (I γr)

The representative gamma index is used to estimate the γ- radiation hazard associated with the natural radionuclide in specific investigated samples. It is a screening tool for identifying materials that might become of health concern when used for construction (Tufail et al. 2007). The values of \(~{I_\gamma } \leqslant 1\) correspond to an annual effective dose of less than or equal to \(1.0~\;{\text{mSv}},~\) while \({I_\gamma } \leqslant 0.5\) corresponds to annual effective dose less or equal to \(0.3\;{\text{~mSv}}\) (Turhan and Gunduz 2008).

The representative gamma index is expressed as follows (Ashraf et al. 2010):

$${I_\gamma }~=~{C_{{\text{Ra}}}}{\text{/}}150~+~{C_{{\text{Th}}}}{\text{/}}100~+~{C_{\text{K}}}{\text{/}}1500.$$
(8)

Absorbed dose rate (D)

The absorbed dose rates’ outdoor (D) is the gamma radiations in air which represents a uniform distribution of the naturally occurring radionuclides (226Ra, 232Th and 40K). It is calculated based on guidelines provided by (UNSCEAR 2000) and is given by the relation (Ashraf et al. 2010):

$$D~\left( {\eta {\text{Gy}}{{\text{h}}^{ - 1}}} \right)=~0.462{C_{{\text{Ra}}}}+~0.621{C_{{\text{Th}}}}+~0.0417{C_{\text{K}}}.$$
(9)

Annual effective dose equivalent (AEDE)

The annual effective dose equivalent received outdoor by the populace was estimated from the absorbed dose rate by applying dose conversion factor of \(0.7~\;{\text{SvG}}{{\text{y}}^{ - 1}}\), and the occupancy factor for outdoor as 0.2 (5/24) and indoor as 0.8 (19/24) (Veiga et al. 2006 and Ravisankar et al. 2014). The AEDEs were determined using the following expressions:

$${\text{AEDE}}~\left( {{\text{outdoor}}} \right)\left( {\mu {\text{Sv}}{{\text{y}}^{ - 1}}} \right)=~{\text{Absorbed}}~{\text{dose}}\left( {{\text{nGy}}{{\text{h}}^{ - 1}}} \right)~ \times 8760\;{\text{h}}~ \times ~0.7\;{\text{SvGy}}{{\text{y}}^{ - 1}} \times ~0.2,$$
(10)
$${\text{AEDE}}~\left( {{\text{indoor}}} \right)\left( {\mu {\text{Sv}}{{\text{y}}^{ - 1}}} \right)=~{\text{Absorbed~dose}}~\left( {{\text{nGy}}{{\text{h}}^{ - 1}}} \right) \times ~8760\;{\text{h}} \times ~0.7\;{\text{SvGy}}{{\text{y}}^{ - 1}} \times ~0.8.$$
(11)

Excess lifetime cancer risk (ELCR)

This radiological risk parameter is used to estimate the probability of developing cancer over a lifetime at a given exposure level.

Excess lifetime cancer risk (ELCR) is estimated using the relation (Taskin et al. 2009):

$${\text{ELCR}}~=~{\text{AEDE}}~ \times {\text{~DL}}~ \times ~{\text{RF,}}$$
(12)

where AEDE is the annual effective dose equivalent (outdoor), DL is the average life span of man in Africa (estimated to be 70 years), and RF is the risk factor (Sv− 1), i.e., fatal cancer risk per Sievert. For stochastic effects, ICRP uses RF value of 0.05 for the public (Taskin et al. 2009).

Results and discussion

Results’ presentation

Table 1 presents the results of the three geoelectric survey evaluation using the vertical electrical sounding method (VES). From the results obtained in the three VES locations, site 3 was observed to having the best water aquifer and was chosen for the drilling of the borehole.

Table 1 Model parameter summary of computer iteration and the geoelectric section of the study location
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The results of the gamma ray analysis of the soil samples are presented in Table 2.

Table 2 Radionuclide activity concentration of the soil profile samples
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The results of the estimated radiological risk parameters of the soil samples are presented in Table 3.

Table 3 Radium equivalent and summary of calculated radiation hazard indices
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Table 4 presents a comparative assessment of the three natural radionuclide activities in soil sample of this present work with the reported values in some parts of the World.

Table 4 Comparison of the radioactivity concentration of the study area with some countries and regions of the world (Agbalagba et al. 2012)
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Discussion of results

Discussion of geoelectric analysis

The studied locations where the samples were investigated and the arrangement of Schlumberger Array for the geophysical investigation for the groundwater are shown in Figs. 1 and 2, respectively. The summary of the results obtained for the geoelectric evaluation using the vertical electrical sounding (VES) is presented in Table 1. The interpretation of field curves of the three (3) VES stations indicates that the curves are of types HK, AH, and AA. The HK type implies decrease in resistivity and then a progradational increase in sand content with increasing resistivity and depth. The AH and AA curves show that the resistivity increases with depth, but starts falling at certain depth in the case of AH type.

Fig. 2
figure 2

Arrangement of Schlumberger Array Method

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The top soil (first layer) in the study location had a resistivity value ranged from 143.4 Ωm to 494.1 Ωm with a thickness range of 0.9–3.1 m. This layer composed of fine-grained-to-medium-grained sands with inoculation of clay as obtained in Abavo area of Ika South LGA (Ohwona et al. 2013; Egbai 2013). The second layer has a resistivity value that ranged from 226.5 Ωm to 1268 Ωm, a corresponding thickness (3.9–25.6 m), and depth range of (4.9–13.7 m). The lithology of the second layer consists of pinch-out of fine-grained-to-medium-grained sands. The soil are loose type; thus, it can be recharged easily through high precipitation, and therefore, wells drilled in this layer will be of low or none productive. The third layer had a resistivity value range of 308.8–2215.2 Ωm, and a thickness of (25.2–147.9 m) with a depth value range of (30.1–174.4 m). This third layer is composed majorly of gravelly sand and some fine-grained sands, which progradationally increase in grain sizes with increase depth, that is from fine to gravelly sands. The fourth layer had a resistivity value that ranged from 1026 to 9347.1 Ωm. The thickness and depth of this forth layer is infinite, and it is composed of medium-grained-to-gravelly sands, and, hence, well drilled in this region which will have a high productive capacity, transmissivity, and groundwater yield because of it high porosity and permeability. The results obtained, therefore, indicate that layers three (3) and (4) have the best aquifer thickness and depths. This necessitated the depth of drilling of the borehole that is understudied for radioactivity concentration variation.

Discussion of radioactivity analysis

Table 2 presents the results of the assessment of radionuclide activities in the soil profile down the lithology depth. From Table 2, 40K activity concentrations ranged from 102.2 ± 31.5 Bqkg− 1 at depth profile of 51 m (sandy shale) to 413.3 ± 98.7 Bq kg− 1 at depth profile of 12 m (lateritic silt) with a mean value of 254.0 ± 62.7 Bq kg− 1. The activity concentration of 226Ra ranged from 10.3 ± 3.7 Bqkg− 1 at depth profile of 60.1 m (sand stone or gravelly sand)–22.9 ± 7.3 Bqkg− 1 at depth profile of 12 m (lateritic silt) with a mean value of 17.0 ± 6.3 Bqkg− 1, while the radioactivity concentration of 232Th ranged from 07 ± 3.0 Bqkg− 1 at depth profile of 51 m (sandy shale)–20.5 ± 7.5 Bqkg− 1 at depth profile of 12 m of lateritic silt. Lateritic silt soil layer at the depth of 12 m was found to be most radioactive soil type with 40K at this depth slightly higher than the recommend limit of 400 Bqkg− 1. However, these radionuclides’ average activity concentration values for 40K, 226Ra, and 232Th obtained in the studied borehole soil are below the world average value of 400 Bq kg− 1 for 40K, 35 Bq kg− 1 for 226Ra, and 30 Bq kg− 1 for 232Th, respectively (UNSCEAR 2000), and as shown in Figs. 3, 4, and 5. The examination of the variation of radioactivity concentration in the soil with depth shows no definite pattern as the activity concentration undulate in a sinusoidal wave form. This is seen in Fig. 6 where the radium equivalent values (a specific activities which account for 226Ra, 232Th, and 40K by a single quantity) were plotted against depth soil profile. The variation in the radionuclides down the depth profile can be attributed to parent rock materials and mineral in the soil lithology.

Fig. 3
figure 3

Comparison of radioactivity concentration of 40K in soil sample with world recommend limit for the public

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Fig. 4
figure 4

Comparison of radioactivity concentration of 226Ra in soil sample with world recommend limit for the public

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Fig. 5
figure 5

Comparison of radioactivity concentration of 232Th in soil sample with world recommend limit for the public

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Fig. 6
figure 6

Variation of radium equivalent with lithology depth profile of the study site

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Table 3 presents the results of the six evaluated radiological risk factor parameters. The calculated values of radium equivalent (Raeq) due to 226Ra, 232Th, and 40K in soil samples ranged from 33.3 to 84.0 mSvy− 1 with an average value of 56.9 Bqkg− 1, which are below the recommended permissible limit of 370 Bqkg− 1. The obtained estimated values for the annual gonadal dose equivalent (AGED) ranged from 100.7 to 286.0 mSvy− 1 with an average value of 189.3 mSvy− 1; this is below the recommended world permissible value of 300 mSvy− 1 (UNSCEAR 2000; Ononugbo et al. 2017). This indicates that the gonadal values may pose little or no threat to the bone marrow and the bone surface cells of the drillers exploring water in the study area.

The average estimated values of Hex and Hin are both 0.2, which keeps the radiation hazard level insignificant. The values of the radioactivity level index (Iyr) ranged from 0.2 to 0.6 Bqkg− 1 with an average value of 0.4 Bqkg− 1. These show that external hazard index, internal hazard index, and representative gamma indices are all less than the world permissible value of unity (Orgun et al. 2007). This indicates that the obtained values will not lead to respiratory diseases such as asthma and cancer and external diseases such as erythema, skin cancer, and cataracts (Avwiri et al. 2012). The estimated absorbed dose rate (D) values ranged from 14.5 to 41.1 ηGyh− 1 with a mean value of 27.6 ηGyh− 1; this is below the recommended limit 59 ηGyh− 1. The computed average annual effective dose rate (AEDE) for indoor and outdoor are 135 µSvy− 1 and 40.2 µSvy− 1, respectively, which are lower than the worldwide annual effective dose value of (450 µSvy− 1 for indoor and 70 µSvy− 1 for outdoor) (Orgun et al. 2007). The estimated average excess lifetime cancer risk (ELCR) is \(~0.12 \times ~{10^{ - 3}},\) and this is well below the recommended permissible limit of \(0.29~ \times {10^{ - 3}}.\) The overall radiological risk parameters examined show that none of the parameters is significant to cause any radiological health hazard or pose any immediate health challenge to the populace that will use the soil as building material nor will it cause any immediate radiological health-related sickness to those drinking water from this borehole water source.

Table 4 presents the comparison of the average measured radioactivity values in this study with activity concentration of reported values in some regions of Nigeria and other parts of the world. It was observed that the measured activity concentration of the three naturally occurring radionuclides in the study was lower than some of the reported values for countries, such as Republic of Ireland, Spain, Bangladesh, Punjab, Pakistan, Kohistan Pakistan, Sudan, Louisiana USA, Kenya, South India, Mexico, and South–South Nigeria. These obtained values are within the range of values reported in countries/regions like Alexandria Egypt, France, Sudan, Tayma Saudi Arabia, and South West Nigeria. However, these values obtained are higher than values reported in Japan, Malaysia, and Cyprus.

To identify the extent of the dependence and relationship that exist among these three natural radioactive nuclides within the soil investigated, correlation analysis were performed between the radionuclides (40K, 226Ra), (40K, 232Th), and (232Th, 226Ra). Figure 7 shows an exponential correlation of 40K and 226Ra which is \(y~=~64.464{e^{0.0773x}}\) with a regression value of R2 = 0.4782. This shows a weak correlation between the activities of the two radionuclides which implies that a slight relationship of increase in one indicates decrease in the other radionuclide exist. Figure 8 shows the exponential correlation of 40K and 232Th which is \(y=107.88{e^{0.0564x}}\) with a regression value of\({R^2}=0.316,\) which indicates that no clear relationship exists between the two radionuclides. While Fig. 9 presents the linear correlation between 232Th and 226Ra with a correlation expression of \(y=0.9091x - 1.2676~\)and a regression value of \({R^2}=0.6649\). This implies that a strong correlation exists between the two radionuclides (232Th and 226Ra) correlated. The relationship that exists between these two radionuclides may be attributed to the link between two observed decay series chain headed by 238U and 232Th (Ononugbo et al. 2017).

Fig. 7
figure 7

Exponential correlation of K-40 with Ra-226 of the study soil samples

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Fig. 8
figure 8

Exponential correlation of K-40 with Th-232 activity concentration of the study soil samples

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Fig. 9
figure 9

Linear correlation of Th-232 with Ra-226 activity concentration of the study soil sample

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Conclusion

The exploration for groundwater aquifer potential using geophysical survey method of vertical electrical sounding (VES) and the assessment of the natural radionuclide concentration of the soil lithology using sodium iodide [Na(TI)] detector has been carried out in Agbor Delta State. This research work has addressed the duo challenges of identifying the right water aquifer for drilling for portable drinking water and radioactivity content in soil which is transferable and transmittable to water, to determine the subsurface water quality that is best for drinking. The findings on the geophysical survey show that the formation of the three studied sites has curve types KH, AA, and AH, with average water aquifer table of 30.1 m. The results of the assessment of the activity concentrations of the naturally occurring radionuclides 40K, 226Ra, and 232Th in the drilled soil/sediment carried out indicate that all soil lithology activity concentration levels were below world permissible limits, and they compared well with those reported in some part of the world. The values of the six radiological risk parameters used in examining the health hazard indices were all below their recommended limits. The variation of radioactivity concentration of the soil down the well-depth profile shows no significant trend, but laterite soil was observed to have the highest activity concentration, while sandy shale have the lowest activity concentration in the soil lithology. The radioactivity concentration levels obtained in this study soil are thus low; therefore, the soil-to-water transfer ratio will be low (< 0.002). The radioactivity level in this water aquifer will be low and the water from this aquifer will be radiologically safe for drinking.