In-situ assessment of natural terrestrial-radioactivity from Uranium-238 (238U), Thorium-232 (232Th) and Potassium-40 (40K) in coastal urban-environment and its possible health implications

The risk of natural terrestrial radioactivity on human health is often underestimated, and environmental safety awareness is necessary. Hence, this study aims to assess natural sources of gamma radiation emitter in coastal urban-environment using the radiometric technique. The dosage of gamma radiation from a parent radionuclide such as Uranium-238 (238U), Thorium-232 (232Th) and Potassium-40 (40K) and were measured using portable gamma spectroscopy. The result showed that the measured value of 238U activity was between 10.81 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document}± 0.69 and 46.31 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document}± 1.43 Bqkg−1. The mean value was estimated to be 35.44 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document}± 0.97 Bqkg−1 which is slightly higher than the world average. Meanwhile, 232Th activity ranges from 28.42 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document}± 1.12 to 69.43 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document}± 1.76 Bqkg−1 with the calculated mean value of 92.57 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document}± 1.17 Bqkg−1 while 40K activity ranged between 31.30 ± 1.32 and 328.65 ± 2.32 Bqkg−1 with the estimated mean 137.59 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document}± 2.42 Bqkg−1. Radiological parameters such as radium equivalent (Req), internal hazard (Hint) and external hazard (Hext) assessment were in the range of 66.00 Bqkg−1 to 141.76 Bqkg−1, 0.232 to 0.452 and 0.178 to 0.383, respectively. The measured values of gamma dose-rates ranged between 54.283 ± 0.78 and 117.531 ± 1.14 nGyh−1 with the calculated mean value of 84.770 ± 0.97 nGyh−1.


Methodology
Geological description of the sampling area. The study region is generally a gently sloping low-lying area. It falls within the eastern Dahomey (or Benin) Basin of southwestern Nigeria, stretching along the Gulf of Guinea's continental margin. The study area's local geology lies within the sedimentary rock sequence of Dahomey Basin, which extends from the eastern part of Ghana through Togo and Benin Republic to the western margin of the Niger Delta 21,22 (Fig. 1). The local geology's sequence arrangement that underlain the study area is as follows: Recent Alluvium (Quaternary age), which trends towards the south-east and central part of the study area and formed a boundary with Coastal Plain Sands in the west. This formation is followed by Coastal Plain Sands (Tertiary age-Pliocene), which is located in the west, southwest, and eastern part of the study area and also formed the boundary with the Ilaro Formation in the northwest. Ilaro Formation (Tertiary age-Eocene) overlaid both Coastal Plain Sands and Recent Alluvium and created the border with Ewekoro Formation/Osho-sunFormation/Akinbo Formation. This formation cuts across the northwest to the northeast of the study area. This geological formation is followed by Ewekoro Formation/Oshosun Formation/Akinbo Formation (Cretaceous-Paleocene). It cuts across north-north to northeast trend. The last geological formation that underlies Ewekoro Formation is Abeokuta Formation, Cretaceous age (Senonia). This formation formed a boundary with the Basement Complex in the North Ewekoro Formation in the northeast 23 .
The Dahomey basin comprises the Ogun River and Owena basin. The basin's tectonic structure is simple, forming a monocline against the basement outcrop to the North, with only a little evidence of faulting 24 . The area is characterised by two major climatic seasons: dry season spanning from November to March and rainy (or wet) season between April and October. Occasional rainfalls are usually witnessed within the dry season, particularly along the region adjoining the coast. Mean annual rainfall is more significant than 2000 mm and forms the primary groundwater source in the area.
Instrumentation. The instrument used to measure gamma-dose rates and the emitted radionuclides used in this study is the portable handheld radiation detector (Super-SPEC RS 125). This instrument works based on the principle of radioactivity, which is the spontaneous disintegration of radioactive elements with the emission of gamma radiation and other particles such as alpha and beta. The equipment has a high degree of accuracy with probable measurement errors of about 5% of measured radionuclides concentration. The portable equipment came with an integrated design and a large detector, direct assay read-out, storage data point, full of weather protection, easily used, and highly sensitive. The count display of RS-125 Super-SPEC on the front side of the panel in cps at 1/sec update rate. The variable of the SCAN mode of RS-125 Super SPEC usually stores data in the device's memory through Bluetooth connection to external storage of the handheld device. The data's location is gotten through the External Global Positioning System (GPS) connection to the data stream via Bluetooth connection to the device. RS-125 Super SPEC provides the analysis of sample concentration and directly displayed the radionuclides, namely Potassium-40 K (%), Uranium-238 U (ppm), and Thorium-232 Th (ppm). It also has a user-selectable sample time for optimum analysis. The RS-125 Super SPEC comes with utility software used for downloads of the stored data in memory. All the data in the memory of the handheld Super SPEC (RS-125) device can be transferred to the personal computer through Bluetooth or USB. Its operation does not require sources of radioactive content, and it was manufactured by an independent private company called Radiation-Solutions Incorporation situated at 386, Watline Avenue, Mississauga in Ontario, Canada) 24  www.nature.com/scientificreports/ bration of the handheld radiation detector was done according to the guidelines of IAEA-Tecdoc before use. This procedure starts the arrangement between the measured radionuclides' counts (Thorium, Potassium, and Uranium). This approach allows the spectro-meter to decide on subjective ascertainment of Uranium, Thorium, and Potassium make-up of soils, environmental-wastes, and surface-rocks. The calibration of both ground and airborne radiation instruments is done based on the global quality invented by Canada's Geological Survey (GSC) traceable 25 . The portable instrument's calibration was done to ensure consistency and accuracy in estimating Potassium, Uranium, and Thorium. The impact of this radiation in the atmosphere could be demonstrated due to the rate of exposure rate or dose rate absorbed by the use of conversion factors emanating through radio-element concentrating in the samples to be measured. The measured data can be converted using the following conversion factor: For Uranium-238, 1 ppm = 12. where C s and C ref are the activity concentrations in Bq/kg of the measured data. P s , P ref and P b are the photopeak areas, standard reference materials and the background photopeak gamma lines, respectively, which is dimensionless. Also, D s , D ref and D b are the counting duration/time in seconds, standard reference materials, and background. The radiation detector (Fig. 2) was held 1 m above the ground at every measurement point; readings were taken four times at every station, and their average was calculated to ensure accuracy. In addition, the GPS coordinates were noted at each station, and readings were taken at an interval of 90 s at each measuring location. The instrument's reading was in parts per million (ppm); the mean results were obtained and converted to Becquerel per kilogram (Bqkg −1 ).
(1)  www.nature.com/scientificreports/ Estimation of radiological parameters. The radiological parameters indices were determined from the measured data. These radiological parameters used in this present study include radium equivalent, internalradiation hazard index, and external-radiation hazard index. These parameters have been established based on equations reported by 1 and have been used by various researchers 8,[28][29][30][31][32][33] , which have proved the reliability of these equations.
Radium equivalent radiological factor, (R eq ). The radium equivalent radiological factor refers to the frequent denominator used to compare radionuclides present in any material, which has been applied in this study to compare the radionuclides concentrations measured from the subsurface. Radium equivalent activities were evaluated based on Uranium-238, Thorium-232, and Potassium-40 at typical values of 370, 259, and 4810 Bqkg −1 . Equation (2) 1,8 was used in the estimation of the radium-equivalent activity.
where C U , C Th, and C K are activity concentrations of 238 U, 232 Th, and 40 K, respectively, in Bqkg −1 .

Internal hazard assessment (H int ).
The hazard which is defined by H in and can be determined using Eq. (2) 4,8 : External hazard assessment (H ext ). The estimation of external risk assessment (H ex ) associated with gamma dose rays emanating from the subsurface was done by applying Eq. (4) as used by 1 and 8 .
Where C U , C Th , and C K are the concentrations of activities in Bkg −1 .

Results and discussions
Measured naturally-occurring radiation (MNOR). The measured radionuclides and the distribution patterns for Uranium-238 ( 238 U), Thorium-232 ( 232 Th), and Potassium-40 ( 40 K) from 120 sampling points in the study area are shown in Table 1 The standard deviation was also calculated to be 6.57 ± 0.59 Bqkg −1 . The observation showed that the estimated mean value of uranium was high when compared with the world average. This occurrence might have been due to anthropogenic activities such as construction activity, which involves constructing materials such as cement, sand, and other imported decorative materials buried in the subsurface after usage. The distribution pattern of uranium concentration in the study area is shown in Fig. 3, with a hotspot indicated in the eastern part of the study area selected for this investigation. In addition, in observing Fig. 3, it was observed that the hotspot region for the Uranium-238 concentration was noted in the central, northeastern and southeastern of the study area. Table 1 showed the measured concentration of Thorium ( 232 Th). It ranges from 28.42 ± 1.12 to 69.43 ±1.76 Bqkg −1 . The observed lowest value was noted at location 47, while the highest noticed at location 112 with the calculated mean value of 92.57 ± 1.17 Bqkg −1 . Furthermore, the estimated mean value was compared with the world average of 45 Bq/kg. Therefore, it was noticed the estimated mean value was higher. The pattern distribution of 232 Th concentration is shown in Fig. 4. Also, Thorium-232 ( 232 Th) concentration increased from the western part of the study area to the eastern part, as shown in Fig. 4. Therefore, the hotspot region for the 232 Th   Fig. 5 showed the pattern of Potassium-40 ( 40 K) distribution in the study area. The distribution of Potassium-40 ( 40 K) radionuclide observed increased from western toward the northern part study area while decreasing the trend as approaching east of the study area. The hotspot region for Potassium-40 ( 40 K) was observed at the central part of the study area, which flagged toward the North. However, comparing the average value with the world average of 420 Bqkg −1 , the values were far below the recommended limit. Also, the three primordial radionuclides were compared; the results showed that Uranium-238 ( 238 U) and Thorium-232 ( 232 Th) are less in comparison with the Potassium-40 ( 40 K). This is because Potassium-40 ( 40 K) may be associated with the coarse organic-rich rocks, which are radioactive. Furthermore, the measured radionuclides were compared with a similar study by [34][35][36] .

Internal hazard index, (H int ).
The estimation of the internal hazard index was done using Eq. (2), which is associated with the gamma dose rate measured in the study area, shown in Table 2. For the study area to be suitable for siting building structure for safety reasons, the estimated internal hazard index must be less than unity as recommended by 37,38 and 1 . This present study observed that H in ranged between 0.232 and 0.452, with a calculated mean of 0.671. The lowest and highest values of the internal hazard index were noted at measuring points 23 and 112. Table 2 showed the estimated external hazard index. The estimated value of this risk ranged from 0.178 to 0.383. The highest value of 0.383 was noted at measuring point 112, while the lowest was at 23. Furthermore, the estimated values were less than unity or one, as suggested by UNSCEAR 1 . However, compared with the international reference value's internal hazard index, it was observed that the estimated values were minor in comparison, suggesting that the impact of the radionuclides emanating from the study area will be more indoor compared to outdoor.

External hazard index, (H ext ).
Measured gamma dose-rates, (D rate ). The gamma dose rates measured are shown in Table 2. The measured values ranged between 54.283 ± 0.78 and 117.531 ± 1.14 nGyh −1 . The lowest is noted at measuring point 21 and the highest value at measuring location 112, with the estimated mean value of 84.770 ± 0.97 nGyh −1 . However, high values of measured gamma dose rates above the recommended limit value were observed across the measuring The result showed that the study area might be rich in coarsegrained inorganic rocks, sandstones, and non-detrital siliceous sediments due to three minerals K-feldspar, K-mica and glauconite 39,40 . This shows that there is the possibility of a high concentration of radionuclides in the study area, especially Potassium-40 ( 40 K). Radiological parameters estimated were correlated with each other. The increasing values of the radiological parameter measured were in the following order: radium equivalent (Req) ˃ gamma dose-rates (D rate ) ˃ internal hazard (H int ) ˃ external hazard (H ext ). As a result of this variation in both H int and H ext , the study suggests that the impact of radioactivity measured will be more inside than outside.
Generally, it was observed from Table 3 that the measured natural radioactivity was in increasing order ( 238 U < 232 Th < 40 K), with the highest concentration being 40 K. Furthermore, in Figs. 3, 4 and 5, it was noticed that the hotspot region for 238U, 232Th and 40K was observed to be situated in the eastern part of the study area. In terms of radiological parameters, radium equivalent (Req) is within the range of reference value of 370 Bqkg −1 . In addition, estimated internal hazard (H int ) has higher values than external hazard (H ext ), as shown in their calculated mean value (Table 3). Also, the measured gamma dose rate was above the reference value of 59 nGyh −1 , and This was compared with a similar study carried out by 27 on soil samples of the study area. The observation showed that it correlates, suggesting that the possible sources of radionuclides in the study area may be associated with the natural deposit of kaolin and gypsum.

Conclusions
In-situ assessment of naturally occurring radiation level in the coastal environments has been carried out using the ground radiometric technique. Radionuclides such as 238 U, 232 Th, and 40 K and gamma dose rates were measured. The result showed that the measured value of 238 U ranged between 10.81 ± 0.69 Bqkg −1 and 46.31 ± 1.43 Bqkg −1 . The mean value was estimated to be 35.44 ± 0.97 Bqkg −1 compared with the world average of 33 Bqkg −1 . The observation showed that the estimated mean weight of uranium was high when compared with the world average. The measured 232 Th ranges from 28.42 ± 1.12 to 69.43 ± 1.76 Bqkg −1 with the calculated mean value of 92.57 ± 1.17 Bqkg −1 .
Furthermore, the estimated mean value was compared with the world average of 45 Bqkg −1 . It was noticed that the estimated mean value was higher. The measured value of Potassium-40 ( 40 K) ranged between 31.30 ± 1.32 and 328.65 ± 2.32 Bqkg −1 with the estimated mean and standard deviation values of 137.59 ± 2.42 and 70.10 ± 6.40 Bqkg −1 respectively. In terms of radiological parameters, radium equivalent (Req) is within the range of reference value of 370 Bqkg −1 . In addition, estimated internal hazard (H int ) has higher values than external hazard   (Table 3). Also, the measured gamma dose rate was noted to be above the reference value of 59 nGyh −1 . Radiological parameters estimated were also correlated with each other. The results are in the following order: radium equivalent (Req) ˃ gamma dose-rates (D rate ) ˃ internal hazard (H int ) ˃ external hazard (H ext ). In addition, the results were compared with the geology of the area. The result showed that the location might be rich in coarse-grained inorganic rocks, sandstones, and non-detrital siliceous sediments, radioactive. Therefore, the site may not be safe for residents due to the deposition of anthropogenic radioactive materials and the natural deposit of kaolin and gypsum in the study area. This suggests that gamma radiation monitoring should always be carried out in the study area before embarking on a new building project, whether for office, industry and other uses such as residential, educational institution etc. This is to ensure the area's safety and radioactive content in building materials used. Also, natural ventilation should be used where there are existing buildings to avoid the long accumulation of impact of radionuclides on the residents, which may cause lung cancer. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.