Annual effective dose and radiological risk assessment from selected salt pans from the lagoon of Erongo region, Namibia.

Radioactivity levels in salt pans from the Erongo region of Namibia have been investigated. Ten composite salt samples, collected from salt pans of the Walvis Bay lagoon were analyzed for activity concentrations of 2226 Ra, 232 Th and 40 K. This was done using a high-resolution gamma-ray spectrometer. The average activity concentrations in (Bq.kg -1 ) of 226 Ra, 232 Th and 40 K were found to be 2.17 ± 0.19, 0.20 ± 0.02 and 2.28 ± 0.39, respectively. These activity concentrations were used to calculate the annual effective dose and radiological health risk from the ingestion of salt for the different age groups. The average annual effective dose in (μSv/yr) for the age ranges (2-7 years), (7-12 years), (12-17 years) and ≥ 17 years were found to be 2.67 ± 0.22, 3.33 ± 0.28, 6.08 ± 0.53 and 1.22 ± 0.10, respectively. All these were lower than the worldwide average of 0.29 mSv/yr as reported by UNSCEAR in 2000. The total average radiological risk (unitless x 10 -8 ) for the age ranges (2-7 years), (7-12 years), (12-17 years) and ≥ 17 years were found to be 30.15 ± 2.48, 60.30 ± 4.96, 89.8 ± 7.44 and 422.09 ± 34.75, respectively. All these were lower than the recommended limit of between 1x10 -6 to 1x10 -4 as reported by USEPA in 1991. Therefore, the results from this study indicated that the salt samples do not pose a radiological risk to members of the public.


INTRODUCTION
Humans are continuously exposed to naturally occurring radioactive materials (NORM) that originate from either natural or man-made radiation sources (Lin et al., 2015;Vives I Batlle et al., 2018). In recent years, radiological effects of naturally occurring radiation has received considerable attention in many countries around the world (UNSCEAR, 2000, Zivuku et al., 2018, Xinming and Wuhui, 2018, Harikrishnan et al., 2018. This has been due to their acute or chronic health effects (Reda et al., 2018, Sahin Bal, 2018Faisal et al., 2015). Beyond certain limits, radiation can cause longterm health effects such as cancer and cardiovascular diseases (William, et al., 2000;Busby, 2010, Innocent et al., 2013, Onjefu et al., 2020. Irrespective of NORM origin, internal exposure to radiation is mainly through ingestion and inhalation of radionuclides (UNSCEAR, 2008;Uwatse et al., 2015).
It is therefore imperative to monitor the levels of radiation that humans are exposed to through the different pathways. One of the exposure pathways is through the ingestion of salt. Salt plays an important role in our diet for preserving and flavouring food. It is found naturally in seas or underground as rock salt deposits (Kansaana, et al., 2012).
There has been great interest expressed worldwide for the study of radiological impact due to the ingestion of NORM from salt. In 2012, Kansaana and others (Kansaana et al., 2012) investigated radioactivity levels in salt from Panbros Salt Industry Limited in Accra, Ghana and discovered that the activity concentrations of 226 Ra, 232 Th and 40 K in salt samples varied from 1. 12-1.91, 2.11-3.47 and 30.25-47.65 mBq/l, respectively. The annual effective dose calculated for the salt samples varied from 0.00202 to 0.00305 mSv/yr with a mean value of 0.0025 ± 0.00053 mSv/yr. In studies carried out in Pakistan at the Khewera Salt Mines, the mean activity concentrations of 226 Ra, 232 Th and 40 K in salt samples were found to be 790 ± 262, 640 ± 162 and 23 000 ± 6000 mBq/kg, respectively (Tahir and Alaamer, 2008). The mean annual effective dose due to the intake of these natural radionuclides from the rock salt were estimated to be 0.0638 ± 0.015 mSv, a value lower than the worldwide average of 0.29 mSv/yr as reported by UNSCEAR in 2000 (Tahir and Alaamer, 2008). In a related study on radioactivity measurements of different types of salt using SSNTD in Eygpt, it was found that the annual effective dose calculated from the corresponding radon concentration ranged between 10.47-13.69 mSv which was higher than the recommended value (Shabaan, D.H., 2018). Salt samples were also collected from different areas in the western desert of Egypt and analysed for natural radionuclides. An absorbed dose rate of gamma radiation from 226 Ra, 232 Th and 40 K was estimated to be 1. 46-16.13 nGy/h (El-Bahi, 2003).
In Namibia, several studies have looked at the measurement of natural and artificial radioactivity in soils to evaluate the potential health hazards on the local population (Oyedele et al., 2010;Onjefu et al., 2017;Zivuku et al., 2018). However, no data has been reported concerning concentrations of natural radioactivity in salt mined and salt consumed in Namibia. It was therefore, the aim of the study to determine the activity concentrations of Naturally Occurring Radioactive Materials (NORMS) in salt from Walvis Bay lagoon and to calculate the radiological risk associated with the salt consumed by the population. It was also the aim of the research to evaluate the annual effective dose and the risk to the population according to the different age groups. The outcome of the study would form part of baseline values for the Regulatory Authorities to enable them develop national safety guidelines as a result of consumption of salt in Namibia.

Description of the Study Area
The study was undertaken along the coastline of the Erongo region of Namibia and lies between Latitude 22°57'27"S and Longitude: 14°30'19"E. It is bounded by the Walvis Bay lagoon on the south and on the north and east by the Atlantic Ocean (Fig. 1). The study area is made up of vast areas of flat land surrounded by low dykes and covers a total area of 4000 ha which has been developed into pans (Fig. 2). The region has a rainfall pattern, which ranges from 200 mm to 350 mm per year, with average maximum temperatures of between 28 0 C to 32 0 C and average minimum temperatures of 2 0 C to 8 0 C.

Salt Production Process
In Namibia, sea water from Walvis Bay lagoon provides the source of salt. Fig. 3 shows the processes involved in salt production.

Pre-evaporation
During this stage, seawater is pumped from a natural lagoon at a rate of 240 m 3 per minute into a series of preevaporation ponds, changing the salinity of seawater from 3.5 % to 15 %.

Crystallisation
During this stage, the concentrated brine with salinity greater than 25 % is pumped into cystallisation ponds. Sodium Chloride crystallizes to form a layer of crystals on the various crystallizer pavements in readiness for harvesting. Unwanted chemical impurities are removed through evaporation.

Salt Washing (Processing)
During this stage, harvested crude salt is then transported to the processing plant. The crude salt is washed and dried in order to remove some more chemical impurities. Calcium sulphate and magnesium that adheres to the salt are removed during the washing process. The final product, Sodium Chloride at this stage will be more than 99 % pure.

Distribution and Marketing
After washing and drying, the salt is then stockpiled in readiness for marketing and distribution.

Sampling and Sample Preparation
A hand auger was used to collect five samples from each of the 10 salt pans. The samples were then transferred in polyethylene zipper bags and labelled accordingly. All 5 samples from each pan were mixed together thoroughly, to obtain 10 composite salt samples representing each pan. All samples were then transported to the laboratory where the samples were kept to dry in the oven at 100 o C. The dry salt samples were then transferred into 500 ml Marinelli beakers and firmly sealed and stored for 31 days for secular equilibrium to be reached (Onjefu et al, 2017).

Experimental Radiometric Analysis
The counting of radionuclides present in salt samples were analysed with a high-resolution gamma-ray spectrometer using a coaxial (62.80 X 64.80 mm) Canberra high purity germanium (HPGe) detector Model No. GC4520 SN 10882 with 45% relative efficiency and resolution of 2.00 keV full width at half maximum (FWHM) at 1.33 MeV peak of 60 Co and 1.200 keV (FWHM) at 122 keV (Uwatse, 2015). The detector was shielded with 15 cm lead encasement to reduce the background radiation and cooled using liquid nitrogen. A computer-based Multichannel Analyser (MCA) Genie 2000 software from Canberra was used for data acquisition and analysis of gamma spectra. Each marble samples were counted for 53200 s in a reproducible sample detector geometry, and the same configuration and geometry was used throughout the analysis. The gamma spectrometry system was energy and efficiency calibrated using a range of gamma-ray energies ranging from 0.060 MeV to 2 MeV mixed radionuclides standard in a 500 ml Marinelli beaker. This energy range was analysed for the absolute photo-peak efficiency and energy calibration of the HPGe detector using a multinuclide calibration standard with an initial activity of 40 kBq homogeneously distributed in silicone matrix, which was supplied by Eckert & Ziegler Nuclitec GmbH, Germany, SN. AM 5599. The 295.22 keV, 351.93 keV for 214 Pb and 609.32 keV, 1120.29 keV and 1764.49 keV for 214 Bi gamma lines were used in the assessment of activity concentration of 226 Ra, while 911.21 keV for 228 Ac and 968.97 keV and 238.63 keV for 212 Pb were used for 232 Th. The 40 K activity was obtained from the measurement of the single gamma line at 1460.8 keV. The background activity counting due to naturally occurring radionuclides in the room housing the detector was subtracted from obtained peak of each samples. Equation (1) (Caridi, 2016) where C is the counting rate for a specific gamma line given in count per second corrected for background, ε(E) represent absolute photopeak efficiency, I is the intensity of gamma-ray line, t is the time for data collection in seconds and m is the mass of each samples in kg.
The Annual Effective Dose owing to ingestion of 226 Ra, 232 Th and 40 K in salt was calculated using Equation (2) (Poltabtim, 2019).

AED = A × I ×
(2) where is the activity concentration of radionuclides (Bq/kg), is the annual intake of salt (5 g per day, which is equivalent to 1.825 kg/yr) (WHO, 2012), F is the dose conversion factor (SvBq -1 ). Table 1 shows the dose conversion factors that were used in calculating Annual Effective Dose.
where AR is the activity concentrations in Bq/kg and I is the ingestion rate (kg/yr) taken as 5 g per day (WHO, 2012).

RESULTS AND DISCUSSION
The activity concentrations of 226 Ra, 232 Th and 40 K in salt samples are presented in Table 2. The table also presents average daily intake values due to ingestion of salt. The average activity concentrations in (Bq.kg -1 ) of 226 Ra, 232 Th and 40 K were found to be 2.17 ± 0.19, 0.20 ± 0.02 and 2.28 ± 0.39, respectively. The result showed a range of variation in the activities of the radionuclides (Fig. 4). This variation may be attributed largely on the geographical and geological setting of the location and also to the extent of anthropogenic activity in the study area (Turekian, 1970;NCRP, 1975;Ravisankar et al., 2007;Folkner and Williams, 2008;Paschoa and Steinhausler, 2010;El-Taher, 2010). 40 K had the highest average value followed by 226 Ra while 232 Th had the least value (Fig. 4). This was the same trend with average daily intake values. The high concentrations of 40 K is because of its dominance in ocean owing to its natural relative abundance (NCRP, 1987). According to UNSCEAR (2000) these average activity concentrations of radionuclides were much lower than the worldwide average of 32, 45 and 420 for 226 Ra, 232 Th and 40 K, respectively. The activity concentrations of the radionuclides were then used to calculate the annual effective dose ( μSv/yr ) due to ingestion of salt for the different age groups. Table 3 presents the annual effective dose (μSv/yr) due to ingestion of salt for the different age groups. The average annual effective dose for the age ranges (2-7 years), (7-12 years), (12-17 years) and ≥ 17 years were found to be 2.67 ± 0.22, 3.33 ± 0.28, 6.08 ± 0.53 and 1.22 ± 0.10, respectively. The age range (12-17 years) had the highest annual effective dose followed by the 7-12 years range. The ≥ 17(Adult) year age group had the minimum average annual effective dose. All these values were however much lower than the average worldwide exposure of 0.29 mSv due to ingestion (UNSCEAR, 2000). The average daily intake values presented in Table 2 were used to calculate the radiological health risk. Table 4 shows results of radiological health risk calculations made due to ingestion of natural radionuclides 226 Ra, 232 Th and 40 K from salt for members of the public of different age groups. The total average radiological risk (x 10 -8 ) for the age ranges (2-7 years), (7-12 years), (12-17 years) and ≥ 17 years were found to be 30.15 ± 2.48, 60.30 ± 4.96, 89.8 ± 7.44 and 422.09 ± 34.75, respectively. The risk was mostly pronounced in the ≥ 17 years age group (adults). Although the risk is higher in this age group, it is still much lower than the recommended limit between 1x10 -6 to 1x10 -4 as reported by USEPA in 1991. This allowable limit means that 1 person in 1 million to 1 person in 10,000 is acceptable according to USEPA (1991). These results therefore indicate that there is no radiological risk that may be posed through the ingestion of salt by members of the public.
Comparing the measured values of activity concentrations of 226 Ra, 232 Th, and 40 K from this study with some studies around the world showed that the average concentration of 226 Ra from this study is greater than the activity of 226 Ra from Romania, Ghana and Pakistan but less than the activity of 226 Ra obtained from the study in India (Table 5). The average concentration of 232 Th in this present study is less than those obtained from Romania, Ghana and India. Similarly, the average activity concentration of 40 K in this present study was found to be higher than the value obtained for 40 K from Romania but lower than the value for 40 K from Ghana, India and Pakistan, respectively (Table 5). The performed correlation between the specific activities of 226 Ra, 232 Th, and 40 K as presented in Figures 4, 5 and 6, showed weak positive correlations with coefficient (R 2 = 0.05) for 40 K and 226 Ra, weak negative correlation (R 2 = 0.08) for 232 Th and 226 Ra, and a strong negative correlation (R 2 = 0.09) for 40 K and 232 Th, respectively.

CONCLUSION
In this study, ten composite salt samples were collected from salt pans of the Walvis Bay Lagoon in Namibia. These salt samples were analyzed for activity concentrations of 2226 Ra, 232 Th and 40 K with the aim to estimate the radiological risk associated with a number of different age categories. The analysis was done using HPGe gamma spectrometry. The average activity concentrations (Bq.kg -1 ) of 226 Ra, 232 Th and 40 K in salt due to ingestion were found to be 2.17 ± 0.19, 0.20 ± 0.02 and 2.28 ± 0.39, respectively. These activity concentrations were used to calculate the annual effective dose and radiological health risk as a result of ingestion of salt for the different age groups. The average annual effective dose in (μSv/yr) for the age ranges 2-7 years, 7-12 years, 12-17 years and ≥ 17 years were found to be 2.67 ± 0.22, 3.33 ± 0.28, 6.08 ± 0.53 and 1.22 ± 0.10, respectively. All these were lower than the world wide average of 0.29 mSv/yr as reported by UNSCEAR in 2000. The total average radiological risk (unit less x 10 -8 ) for the age ranges 2-7 years, 7-12 years, 12-17 years and ≥ 17 years were found to be 30.15 ± 2.48, 60.30 ± 4.96, 89.8 ± 7.44 and 422.09 ± 34.75, respectively. All these were lower than the recommended limit of between 1x10 -4 to 1x10 -6 as reported by USEPA in 1991. Therefore, these results do not pose any radiological risk to members of the public.

ACKNOWLEDGMENT
The authors would like to acknowledge with thanks the assistance received from the management of Walvis Bay Salt Industry Limited during the period of sampling. We also appreciate the support by Namibia University of Science and Technology with sample analysis.