Natural Radioactivity and Associated Radiological Hazard Indices in Carbonatite Rocks from Oldoinyo Lengai, Tanzania

Natural Radioactivity and Associated Radiological Hazard Indices in Carbonatite Rocks from Oldoinyo Lengai, Tanzania

Erick Josephat Kalori

P.O.Box 47, Bagamoyo, Tanzania, Dr. Innocent J. Lugendo, P.O Box 35063, Dar es Salaam, United Repubilc of Tanzania

DOI: https://doi.org/10.51584/IJRIAS.2023.81101

Received: 07 November 2023; Revised: 11 November 2023; Accepted: 16 November 2023; Published: 27 November 2023

ABSTRACT

The distributions of naturally occurring radionuclides ²³²Th, ²³⁸U and ⁴⁰K in carbonatite rocks from Oldoinyo Lengai region, Tanzania were determined. The Hyper-pure Germanium (HPGe) detector based on gamma spectroscopy located at Tanzania Atomic Energy Commission (TAEC) in Arusha was used in order to assess the radiological health hazards associated with the use of carbonatite rocks. The mean activity concentrations of ²³²Th, ²²⁶Ra and ⁴⁰K were found to be 51.75 Bq/kg, 46.38 Bq/kg and 966.56 Bq/kg respectively. The results of mean activity concentration together with radiological hazard indices obtained in this study were all higher than their worldwide maximum recommended limits. This indicates that there exist radiation risks within the vicinity of carbonatite deposits in Oldoinyo Lengai. The region is known to be subjected to environmental degradations due to volcanic activities. Therefore, findings in this study could also serve as an important radiometric baseline data upon which future epidemiological studies and environmental monitoring initiatives could be based.

Key words: Natural radionuclides, Radiological hazard indices, Carbonatite Rocks, Oldoinyo Lengai, Excess lifetime cancer risk.

INTRODUCTION

Naturally Occurring Radioactive Materials (NORMs) are the major source of ionizing radiation exposure to humans (El Samad et al. 2013). Approximately 87% of radiation doses to which humans are exposed are from the naturally occurring radioactive isotopes of ²³²Th, ²³⁸U and their daughters as well as ⁴⁰K (Shetty and Narayana 2010). Natural radioactivity exists in the earth’s environment through various geographical formations such as water, soil and rocks (Shetty and Narayana 2010). Carbonatite rocks are among the rocks that are essentially known to host significant amounts of natural radionuclides (Otwoma 2012).

Carbonatite rocks are carbonate-rich igneous rocks containing at least 50% of carbonate minerals and other various minerals such as silicate and phosphate in fewer amounts (Woolley and Kjarsgaard 2008). These rocks are economically valuable to human being as they are the host of Niobium (as pyrochlore) and rare earth element (REE) such as bastnaesite, monazite and vermiculite (Simandl 2014). Apart from hosting these elements, carbonatite rocks, like other rocks are also used in construction activities (Downes et al. 2012). Nonetheless, studies have shown that carbonatite rocks contain natural radioactive elements at different concentrations. In fact, some carbonatite deposits have been reported to have higher radioactivity than the recommended world average levels (Achola 2009).

In Tanzania, carbonatite rocks are found in Wingu Hill and Luhombero in Morogoro, Panda Hill and Senjeri Hill in Mbeya and Galapo in Mbulu (Boniface 2017). However largest deposits of carbonatite rocks are at Oldoinyo Lengai in Arusha region due to continually eruption and solidification of carbonatite lava (Boniface 2017). The region is highly important for the lives of people around especially the Maasai tribe.

The socio-economic activities around Oldoinyo Lengai include agriculture, pastoralism, house construction, minerals extraction and tourism (Haulle and Njewele 2017). The houses construction and agriculture activities depend on the soils resulted from weathered carbonatite rocks. The outcrops of carbonatite rocks are consumed by both humans and animals (Haulle 2014).

The above-mentioned socio-economic activities around Oldoinyo Lengai coupled with the possible presence of natural radionuclides in carbonatite rocks may lead to excessive exposure to natural radiation for communities within and in the vicinity of the deposits. People around Oldoinyo Lengai may be exposed to radiation directly from primordial radionuclides present in carbonatite rocks or indirectly through consuming carbonatite outcrops and inhalation of carbonatite dust. Literature show that prolonged exposure to radiation may lead to serious health effects including cancer (Qureshi et al. 2014, Spycher et al. 2015). Therefore, there is a need for assessing radioactivity level in carbonatite rocks as well as the associated radiological hazards that may be encountered by the people of Oldoinyo Lengai. Several studies conducted to analyze the carbonatite rocks at Oldoinyo Lengai, based on their geochemical properties and their origin but they are silent on their activity levels (Carmody 2012 and Fischer et al. 2009). Therefore, this study aligned itself towards investigating the level of natural radioactivity in the carbonatite rocks of Oldoinyo Lengai and assessing the associated radiation hazard indices.

MATERIALS AND METHODS

 2.1. Sample collections

Oldoinyo Lengai region is in the proximity of Engaresero village, Ngorongoro district in northern west Arusha, Tanzania. The region consist of Mt Oldoinyo Lengai which is the active volcano that erupts the natrocarbonatite lavas which cools and solidifies leading to the formation of carbonatite rocks in different parts within the region such as Gelai, Ketumbeine and some parts of the lake Natron as shown in Fig 2.1 (Carmody 2012). However the availability of water falls within the Gregory rift just below the foot of Oldoinyo Lengai Mountain makes more important for domestic use by population around the region (Lengai et al. 2015). Thirty (30) samples of carbonatite rocks were randomly collected from different locations of Oldoinyo Lengai that is north, south, east and west of sampling area. The number of samples differed from sampling area to another due to the amount of carbonatite deposit available in a given location. The survey meter was used to identify the carbonatite deposit with high radiation in each location. Samples collection points were separated by a distance of 1 km and about 1 kg of each carbonatite rock sample were packed and transported to the TAEC laboratory for preparation and analysis.

Figure 2.1: Geological location of Oldoinyo Lengai

2.2. Sample Preparations and measurement

The carbonatite rock samples were crushed and pulverized into smaller pieces so that the samples can be reduced into approximately 75 m size of fine powdered rock to enhance homogeneity of the element in the samples. Figure 2.2 shows the laboratory rock crusher.

Figure 2.2: The Laboratory rock crusher/pulveriser (Kerur et al. 2010)

In order to remove the moisture, the samples were dried in the oven at 110^o C for 24 hrs. Using the electronic beam balance with sensitivity of 0.01 mg, the weight of the dried sample was measured. By using a pressure sensitive tape for air tight, the 20 mm level canister was used to seal 150 g of each sample as well as reference materials in order to avoid any possibility for radon escape. Both samples and reference materials were left for twenty one days in a well-sealed canister in order to allow the secular radioactive equilibrium between ²²⁶Ra and its short lived decay products. When the equilibrium was reached, the samples were analyzed using the gamma – ray spectrometer with high purity germanium detector (HPGe)(Makundi et al. 2018). The activity of natural radionuclides present in the rock samples were measured using the n-type coaxial high purity germanium detector system (HPGe) model number GEM40-83-SMP and serial number: 57P51572A. The detector has relative efficiency of 49% and energy resolution of full width at half maximum (FWHM) of 1.8 Kev for 1332 KeV energy of ⁶⁰Co. It was connected to Digital Spectrum Analyzer (DSA) with operating system of Gamma-version 8 software for data acquisition and analysis. The detector shielding consists of three layers of copper (30 mm thick), cadmium (30 mm thick) and lead (100 mm thick). This arrangement of shielding helps to reduce background scattering. In order for peaks to form, each sample was measured for 24 hours. Under the same condition as the measurement of the samples, the background level in counting room was measured and subtracted from spectra recorded from the samples. The activity concentration of ²³⁸U, ²³²Th and their decay product as well as ⁴⁰K were calculated from the progeny photopeaks under the secular equilibrium assumption using equation (1).

                                                                                                    (1)

whereby A is the specific activity of radionuclides in Becquerel per kilogram    (Bqkg^-1), N is the net peak area under the most prominent photo peaks,  is detector efficiency of the specific gamma ray,  the absolute transition probability of gamma decay, t is the counting time in seconds  and m is the mass of the rock samples in kilogram (kg) (Akkurt and Günoǧlu 2014) Evaluation of accuracy of gamma spectrometer needs calibration so that the relation between parameters such as spectrum channel number and energy as well as spectrum counts and activity can be well defined. To ensure accuracy of measurements, evaluation of detector efficiency and energy calibration were done daily before commencing analysis of the rock samples. The MBSS 2 standard source (with ¹³⁷Cs, ⁶⁰Co, ²⁴¹Am radionuclides) among others was used in this study for evaluating the detection efficiency of the detector. Moreover for energy calibration, the canister containing the standard reference material (malt-nuclide) was placed on the top of the detector for 10 hours and the relationship between gamma photon energy with its corresponding channel number in the spectrum was observed. Calibrations was done using a computer program ISOCS (In-Situ Object Calibration Software) developed by Canberra Company. Figure 2.3 shows the energy spectrum for standard sources.

Figure 2.3: Energy spectrum for standard sources

The validation process of HPGe detector was done where by parameters such as linearity, detection limit as well as the degree of accuracy were investigated in order to avoid the wrong measurement of activity concentration of radionuclides of interest. The multi-nuclide standard with certificate No 9031-OL-022\13 type MBSS 2 from Czech Metrological Institute with reference date 8^th February 2013 was used to check the validation of HPGe detector. By placing the canister containing standard reference material on the top of the detector for 36000 seconds, its activities for different energies were estimated and compared with the certified values calculated on the day of the measurement. The activity at the date of counting was calculated after correction for decay of the radionuclides. The obtained data were used in uncertainty evaluation associated with the measurements.

RESULTS AND DISCUSSION

3.1 Activity Concentration of Radionuclides in the Carbonatite Rock Samples

Table 3.1: The activity concentration of ²³²Th, ²²⁶Ra and ⁴⁰K (Bqkg^-1) in carbonatite rock samples

Key:  SEM – Standard error of the mean

The activity concentrations of radionuclides in carbonatite rocks at Oldoinyo Lengai were observed to vary depending on the site where the samples were taken. As Table 3.1 shows, the activity concentration of carbonatite rock samples varied from 25.42 to 95.53 Bqkg^-1 with an average concentration of 46.383 ± 2.647 Bqkg^-1 for ²²⁶Ra. The activity ranged from 30.66 to 121.15 Bqkg^-1 with an average concentration of 51.754 ± 3.09 Bqkg^-1 for ²³²Th and the activity concentration of ⁴⁰K varied from 538.76 to 1464.6 Bqkg^-1 with an average concentration of 966.564 ± 16.968 Bqkg^-1. Data presented in Table 3.1 reveal that ⁴⁰K has the highest activity concentration among the radioisotopes identified in this study. This is due to the fact that  carbonatite rocks of Oldoinyo Lengai are naturally rich in potassium (Mitchell 2005). From Table 3.1, it is also observed that the activity concentration of ²³²Th in carbonatite rock samples is higher than activity concentration of ²²⁶Ra. The reason may be the large difference in the mobility of the two elements. Uranium, the parent of ²²⁶Ra is quite mobile while thorium is much less mobile compared to uranium. Therefore, thorium concentration is more likely to remain higher in the carbonatites than Uranium (Achola 2009). The distribution of ²²⁶Ra, ²³²Th, and ⁴⁰K at different sampling sites along with their respective Global Average Values (GAV) are displayed in Figure 3.1

Figure 3.1: The activity concentration of ²³²Th, ²²⁶Ra and ⁴⁰K for all samples.

The data revealed by Figure 3.1 show that in some of collected samples the activity concentration was found to be high compared to other samples. For instance the highest activity concentration for both ²²⁶Ra and ²³²Th were detected in sample GW3 which was collected from Gelai west whereas for sample NN1 which was collected from Natron north, the activity concentration of both ²²⁶Ra and ²³²Th was low. On other hand, the highest activity concentration for ⁴⁰K was detected in the sample KW1 from Ketumbeine west. The lowest concentration of ⁴⁰K was seen in sample NN3 collected from Natron north. Figure 3.1 reveals that the activity concentrations for ²²⁶Ra, ²³²Th and ⁴⁰K were almost uniform is some samples such as KE1, KE2 and KE3. In some samples such as GN1, KS2 and KW2 the activity concentrations were completely not uniform. These variations of activity concentration were due to variations of concentrations of radionuclides and their geographical formation at different sampling sites. The global average activity concentrations for  ²²⁶Ra, ²³²Th and ⁴⁰K are  35 Bq/kg, 30 Bq/kg and 400 Bq/kg respectively (UNSCEAR 2000 and Akkurt et al. 2010). However, it can be observed from Figure 3.1 that for all collected carbonatite rock samples in this study, the activity concentration of both ²³²Th and ⁴⁰K were above the recommended GAV. It is only in few samples the activity concentration of ²²⁶Ra was below the recommended GAV. This indicates that the activity concentration of ²²⁶Ra, ²³²Th and ⁴⁰K in carbonatite rocks of Oldoinyo Lengai are higher than the tolerable limit.

The variation of each radionuclide concentration in carbonatite rock sample was expressed by its standard deviation via its frequency distribution curve. It was observed from Figure 3.2 that the standard deviation of ⁴⁰K was high compared to that of ²³²Th and ²²⁶Ra. Thus ⁴⁰K demonstrated the wide bell curve. This indicates that difference in concentration for ⁴⁰K was high throughout all samples compared to the difference in concentration for ²³²Th and ²²⁶Ra in one sample to another. In addition to that, the wide bell curves and their asymmetry nature of radionuclide distribution exhibited by ²³²Th, ²²⁶Ra and ⁴⁰K indicate the inconsistence in concentrations and multi-modal feature of radionuclides in carbonatite rocks from Oldoinyo Lengai. This multi-modal feature indicates the complexity of minerals in carbonatite rocks.

Figure 3.2: Frequency distribution curves of ²³²Th, ²²⁶Ra and ⁴⁰K

3.2 Comparison of Radionuclide Concentrations in Carbonatite Rocks from different Areas.

Due to complexities in composition of carbonatite rocks, results of this study were compared to results reported from other areas associated with carbonatite rocks.

Table 3.2: Comparison of radionuclide concentrations (Bq/kg) in carbonatite rocks from   different areas.

The data revealed by Table 3.2 show that radionuclide concentrations were also high in carbonatite rocks of different areas. Carbonatite rock such as that of Lambwe East in Kenya and in Rodberg (carbonatite associated rocks) in Norway had extremely high concentration of ²³²Th compared to carbonatite rocks of Oldoinyo Lengai. This is due to the fact that they contain large amount of monazite minerals which acts as the chief source of thorium (René 2017). However, low concentration of ²³²Th in carbonatite rocks of Oldoinyo Lengai may be due to their less amounts of monazite minerals. A large difference in the concentration of ⁴⁰K in different carbonatite rocks was also observed. For instance, as shown by Table 3.2, the concentration of ⁴⁰K carbonatite rocks from Rauhaugite and Rodberg in Norway and Lambwe East Kenya were low compared to that found in Oldoinyo Lengai. Table 3.2 shows that, in the rocks of Isparta volcanic area, the concentration of radionuclide ²²⁶Ra, ²³²Th and ⁴⁰K  were high since  the origin of Isparta potassic volcanism was associated with a common and enriched mantle source, which interacted with the carbonatite melts (Çoban, 2019).

3.3 Comparison of Radionuclide Concentrations in Carbonatite Rocks of Oldoinyo Lengai with other type of Rocks.

As reported by various studies, the various radionuclide concentrations are not only found in carbonatite rocks but also in other different type of rocks. In other rocks, the concentration of ²²⁶Ra, ²³²Th and ⁴⁰K were even higher compared to carbonatite rocks of Oldoinyo Lengai and much higher when compared to the recommended average global value as indicated in Table 3.3.

Table 3.3: Concentrations of radionuclides (Bqkg^-1) in different rocks from different regions

Table 3.3 shows that, ⁴⁰K concentration was extremely high in other rocks such as granite rock of Iran, Yemen (Juban town), Egypt and China (Cuihua) as well as basalt rocks of Kottur area in India. The concentrations of ²²⁶Ra and ²³²Th were also high in other rocks such as granite rocks of India (Punjab) and Egypt. This indicated that like in carbonatite rocks of Oldoinyo Lengai, even other rocks may have radioactivity levels that are intolerable to human beings. Since the activity level play the significant role in determining radiation exposure, then from Table 3.3, one should note that radiation exposure in a given area depends on carbonatite rocks together with other types of rocks.

3.4 Evaluation of Hazards Associated with Radionuclides in Carbonatite Rocks at Oldoinyo Lengai.

Radiation health risks are health effects resulting from one’s exposure to high level of radiation. The source of radiation may be the naturally occurring radioactive materials present on the earth’s environment. High level of radionuclide concentrations in the environment, results to high level of radiation exposure to the surroundings. Although low level of radiation may not cause the immediate health problems, long-term exposure to such radiations may contribute to long-term health problems. That is, long term exposure to radiation, even at the level of natural background radiation results to long-term health effect such as cancer and cardiovascular diseases (Spycher et al. 2015). In this study, radiation health risk assessment was done via radiological hazard indices by considering the activity concentrations of radionuclides. These indices were given by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) to determine the tolerable level at which one can be exposed to radiation without any significant harm (Akkurt et al. 2010). Table 3.4 presents the radiological hazard indices values calculated from ²²⁶Ra, ²³²Th and ⁴⁰K concentrations in carbonatite rocks of Oldoinyo Lengai.

Table 3.4: Radiological hazard indices values

3.4.1 Outdoor Absorbed Dose Rate (D_o)

The absorbed outdoor terrestrial radiation dose rate in air at a height of 1 m above the ground surface was computed from Equation (2)

                                                 (2)

based on the guidelines provided by UNSCEAR 2000. The calculation formula was obtained from  Akkurt et al. 2010 and Mehra et al. 2009. As shown in Table 4.4, the outdoor absorbed dose rate ranged from 56.36 to 170.32 nGy⋅h⁻¹ with mean value of 93 ± 4.97 nGy⋅h⁻¹ which was above the global average value of 55 nGy⋅h⁻¹ (UNSCEAR 2000 and Akkurt et al. 2010). It was also observed that, D_o values vary from one site to another. This variation is displayed in Figure 3.3.

Figure 3.3: Variation of D_o at Oldoinyo Lengai.

3.4.2 Indoor Absorbed Dose Rate (D_in)

The imparted indoor gamma dose by the emission of gamma-ray from ²³²Th, ²²⁶Ra, and ⁴⁰K in carbonatite rock samples when used as the building materials was calculated based on Equation (3),

                                                             (3)

The equation was obtained from Isinkaye and Emelue 2015 and Qureshi et al. 2014 for a standard room of dimensions 4 m x 5 m x 2.8 m. Table 4.4 shows that, the indoor absorbed dose rate ranged from 107.18 to 322.86 nGy⋅h⁻¹ with mean value of 176.93 ± 9.45 nGy⋅h⁻¹ which was above the global average value of 84 nGy⋅h⁻¹ (Isinkaye and Emelue 2015). The variation of D_in was observed to vary from one site to another as shown in Figure 3.4.

Figure 3.4: Variation of D_in at Oldoinyo Lengai.

Moreover, from Figure 3.4, it was observed that the D_in values were higher than D_o values. This indicates that using carbonatite rocks of Oldoinyo Lengai as building materials leads to higher amount of indoor absorbed dose rate. The D_in and D_o values were also used to determine the annual outdoor and indoor effective doses.

3.4.3 The Annual Outdoor Effective dose rates (AED_o)

In order to test the health effect of the outdoor absorbed dose rates, the annual outdoor effective dose rates were obtained. The annual effective dose equivalent for adults from outdoor terrestrial gamma radiation was calculated from Equation (4)

                 (4)

which was given by Akkurt and Günoǧlu 2014 and Kljajevi et al. 2012. From Table 4.4, the annual outdoor effective dose rate values varied from 0.07 to 0.21 mSv⋅y⁻¹ with the mean value of 0.11±0.06 mSv⋅y⁻¹. The obtained outdoor effective dose rate was greater than the global average value of 0.07 mSv⋅y⁻¹ in areas with the normal background radiation (UNSCEAR 2000 and Kljajevi et al. 2012). The variation of AED_o in all sites is displayed in Figure 3.5. 

Figure 3.5: Variation of AED_o at Oldoinyo Lengai.

3.4.4 The Annual Indoor Effective dose rates (AED_in)

Annual indoor effective dose equivalent to the population due to radioactivity in carbonatite rock samples was computed from Equation (5)

               (5)

 The equation was given by UNSCEAR 2000 and Isinkaye and Emelue 2015. Table 4.4 shows that the overall mean for the annual indoor effective dose rate was 0.87±0.01 mSv.y^-1 with range values from 0.53 – 1.58 mSvy^-1. However, the AED_in was greater than the world’s average value of 0.41 mSv.y^-1 (UNSCEAR 2000 and David 2012). The variation of AED_in from one site to another is displayed in Figure 3.6.

Figure 3.6: Variation of AED_in at Oldoinyo Lengai.

Moreover, from Figure 3.6, it was observed that the annual indoor effective dose rates were higher than that of the outdoors. This signifies that more radiation health effect results from absorbed indoor dose rate.

3.4.5 Comparison of hazard indices due to radionuclides in carbonatite rocks at Oldoinyo Lengai with the indices reported elsewhere.

The rate of radiation health effect as determined via annual effective doses varies depending on annual absorbed doses of a given area (David 2012). In that context, results from this study were compared to results reported from other areas associated with carbonatite rocks. As Table 3.5 shows, in other areas associated with carbonatite rocks, the values of D_o, D_in and AED_o were higher than that of Oldoinyo Lengai. For instance, areas such as Lambwe East and Homa bay in Kenya had higher values of D_o and D_in than the global recommended limit hence resulting to high values of AED_o. This signifies high radiation exposure in those areas than in Oldoinyo Lengai. Furthermore, the data shown in Table 3.5 reveal that, although the population of Oldoinyo Lengai faces intolerable level of radiation exposure, the populations of Lambwe East and Homa bay in Kenya are more likely to encounter radiation health risks.

Table 3.5: Comparison of D_o, D_in AED_o and AED_in of carbonatite rocks in Oldoinyo Lengai region and other areas associated with carbonatite rocks.

Meanwhile, the elevated values of D_in, D_o, AED_o and AED_in were also observed in other type of rocks. As Table 3.6 shows, granite rocks of Egypt and Yemen (Juban) resulted to high radiation exposure due to their high values in D_o and AED_o.  It was also observed that AED_o due to granite and sand stones rocks in Punjab, India as well as basalt rocks of Gulbarga area was higher than that of Oldoinyo Lengai and the recommended global average value. These variations in hazard indices depends on the amount of radionuclide concentration present in a given type of the rock (Kljajevi et al. 2012). Therefore, the information revealed in Table 3.6 indicates that radiation exposure together with its health effects can be enhanced by varieties of rocks around a given area.

Table 3.6: Comparison of hazard indices due to radionuclide in carbonatite rocks at Oldoinyo Lengai with the indices reported in other rocks.

3.4.6 Radium equivalent activity (Ra_eq)

Due to the non-uniform distributions of radionuclides in carbonatite rocks, radium equivalent activity is defined as a single radiological parameter that compares the specific activity of materials containing varying concentrations of ²³²Th, ²²⁶Ra and ⁴⁰K (Isinkaye and Emelue 2015). The radium equivalent activity is a weighted sum of activities of the ²²⁶Ra, ²³²Th, and ⁴⁰K radionuclides based on the assumption that 370 Bq/kg of ²²⁶Ra, 259 Bq/kg of ²³²Th, and 4810 Bq/kg of ⁴⁰K produce the same gamma-ray dose rate (Makundi et al. 2018). Radium equivalent activity was calculated from Equation (6)

                                                                             (6)

as suggested by El-mageed et al. 2011and Uosif et al. 2015.

From Table 4.4, the observed radium equivalent activity in this study ranged from 492.75 Bqkg^-1 to 1456.13 Bqkg^-1 with an average value of 864.65 ± 41.84 Bqkg^-1. As recommended by UNSCEAR, this mean value was 2.3 times higher than the maximum permissible limit of 370 Bqkg^-1 for radium equivalent activity (El-mageed et al. 2011). Therefore from the radiological protection point of view, the carbonatite rocks can lead to a potential radiation risk when used as materials especially in activities such as building construction. The radium equivalent was observed to vary from one site to another as shown in Figure 3.7.

Figure 3.7: Variation of radium equivalent at Oldoinyo Lengai.

3.4.7 External and Internal Hazard Indices

The assessment of external exposure risks due to gamma rays was also done by considering the external and internal hazard indices (H_ex and H_in). The external or internal exposure to radiation is generally said to be insignificant if the H_ex and H_in are respectively less than unity. Both the H_ex and H_in are evaluated by using the value of radium equivalent (Ra_eq). For the radiation exposure, the Ra_eq must be less than 370 Bqkg^-1. In this work_, the H_ex for each sample was calculated according to Equation (7)

                                                                                (7)

(Makundi et al. 2018 and Akkurt and Günoǧlu 2014). Meanwhile, if the maximum concentration of radium is half that of the normal acceptable limit then H_in will be less than 1.0  (UNSCEAR 2000, David 2012). The internal hazard index (H_in) due to the emitted gamma rays for each sample was calculated according to Equation (8)

                                                                                    (8)

(Isinkaye and Emelue 2015, David 2012). Figure 3.8 displays both the H_ex and H_in.

Figure 3.8: The external and internal hazard index for all samples at Oldoinyo Lengai.

Table 3.4 shows that, external hazard index varied between 0.32 and 0.99 with a mean value of 0.53± 0.03 while the internal hazard index ranged from 0.39 and 1.25 with a mean value of 0.65±0.04. The mean values were lower than the recommended global unit. Although the mean of H_ex and H_in were less than a unit, Figure 3.8 reveals that H_ex and H_in in some of collected carbonatite rock samples used in this study are higher than unity. A high value of both H_ex and H_in in these samples is a good indicator of the significant radiation exposure due to the carbonatite rocks of Oldoinyo Lengai.

3.4.8 Representative level index (Iγτ )

In order to estimate the level of gamma radiation hazards associated with natural radionuclides in the carbonatite rock samples, the representative level index was determined. According to the ICRP-60 recommendation, this value should not exceed a unit (David 2012). In this work, the representative level index was calculated using Equation (9)

                                                                                (9)

(El-mageed et al. 2011). It was observed that the values of the representative level index ranged from 0.89 to 2.79 with a mean value of 1.5, which is above the recommended limit. Figure 3.9 displays the variation of representative level index among various sites at Oldoinyo Lengai.

Figure 3.9: Variation of representative level index at Oldoinyo Lengai.

3.4.9 Gamma Activity Index (Iγ )

In order to assess the excess external and indoor gamma radiation from carbonatite rocks, the gamma activity index was used. As proposed by the European Commission (EC), the gamma activity index should not exceed a unit. It was calculated using Equation (10)

                                                                                                (10)

(David 2012). It was found that, the values of the gamma activity index ranged from 0.45 to 1.35 with a mean value of 0.74, which is below the recommended limit. However, as shown in Figure 3.10, in some samples such as GW1, GW2, GW3, KS2, KW1 and NS1 collected from Gelai, Ketumbeine and along Lake Natron, the gamma activity index was above the recommended limit. This indicates the excessive gamma radiation in some sites of Oldoinyo Lengai.

Figure 3.10: Variation of gamma activity index at Oldoinyo Lengai.

3.4.10 Excess lifetime cancer risk (ELCR).

In order to assess the probability or extra risk of developing lung cancer due to the indoor exposure to gaseous radionuclides incurred over the lifetime of an individual, the excess life time cancer risk index was evaluated using Equation (11)

                                                                                     (11)

(Qureshi et al. 2014 and SureshGandhi et al. 2014). It was found that the ELCR due the indoor exposure ranged from 1.8 x 10^-3 to 5.5 x 10^-3 with an average value of 3 x 10^-3. The ELCR obtained for the carbonatite rock samples collected from Oldoinyo Lengai was about 2.6 times higher than the global average value of 1.16 x 10^-3 (Qureshi et al 2014). This indicates that people living at Oldoinyo Lengai for a long time have a higher risk of developing cancer problems. Figure 3.11 shows the excess lifetime cancer risk for different sites at Oldoinyo Lengai.

Figure 3.11: Excess lifetime cancer risk in different sites at Oldoinyo Lengai.

CONCLUSION AND RECOMMENDATIONS

4.1 Conclusion

In this study, radioactivity levels of thirty (30) carbonatite rock samples from Oldoinyo Lengai and the associated radiation risks were assessed. Such an assessment was important because excessive exposure to the low level natural background radiation may result to several health effects including cancer. The determination of activity concentrations of radionuclides of interest in carbonatite rocks was carried out using gamma ray spectrometry. All sample analyses were carried out at the TAEC laboratory in Arusha.

The analysis revealed that, carbonatite rocks from Oldoinyo Lengai contain ²²⁶Ra, ²³²Th and ⁴⁰K at different concentrations. The average activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in all collected carbonatite rocks were found to be 46.38 Bqkg^-1, 51.75 Bqkg^-1 and 966.56 Bqkg^-1 respectively. The concentration of ⁴⁰K was higher than that of ²²⁶Ra and ²³²Th perhaps due to the fact that carbonatite rocks of Oldoinyo Lengai are more potassic in nature (Mitchell 2005). Furthermore, the concentrations of all radionuclides in the carbonatite rocks from Oldoinyo Lengai were above the global average values and maximum limits given by the UNSCEAR. This means that the radioactivity levels at Oldoinyo Lengai due to carbonatite rocks are higher than the tolerable limits. Therefore, people at Oldoinyo Lengai seem to be exposed to high background radiation doses.

This study has also observed that, the elevated radioactivity levels in carbonatite rocks of Oldoinyo Lengai results into higher risks to radiation effects as indicated by the values of radiation hazard indices. Hazard indices such as D_o, D_in, AED,   and ELCR were found to exceed the recommended global average limits. This signifies radiological health hazards at Oldoinyo Lengai. The higher values of Ra_eq and AED indicated that carbonatite rocks of Oldoinyo Lengai might be unsafe for uses as building materials. Hence, the intolerable radioactivity levels of ²²⁶Ra, ²³²Th and ⁴⁰K in carbonatite rocks together with the elevated values of radiation hazard indices show that the population around Oldoinyo Lengai is at high risk of being affected by ionizing radiation from the carbonatite rocks.

4.2 Recommendations

This study has revealed high concentrations of radioactive elements in the carbonatite rocks at Oldoinyo Lengai, which hosts large deposits of carbonatite rocks. Following the weathering process, carbonatite rocks transform to soil, which may be rich in radioactive elements. Since soil is important for various activities including agriculture, the crops grown around Oldoinyo Lengai may also contain significant levels of radioactive elements leading to various problems to the consumers. Besides, when the soil is used as the building material, it may result to high indoor concentrations of radon gas, which may lead to lung cancer. The authors therefore recommend that a comprehensive study should be conducted to evaluate the radioactivity levels in soil and crops from Oldoinyo Lengai especially around the carbonatite deposits.

Meanwhile, volcanic eruptions caused by the active Mount Oldoinyo Lengai produces large quantities of volcanic carbonatite ashes, which may be inhaled by animals and humans living around the region. Thus, the authors of this work recommend that another study should be conducted at Oldoinyo Lengai to determine the amounts of carbonatite in the volcanic ashes and quantify the radioactivity concentrations in the ashes. This will help to understand the biological effects, which may arise from inhalation of volcanic ashes with high concentrations of radioactive elements.

The author of this work also recommend that, to mitigate the heightened radiation risks associated with the elevated levels of radioactive elements in the Oldoinyo Lengai carbonatite rocks, immediate measures should encompass strict protective protocols for workers and residents, continual monitoring, public education campaigns, environmental assessments, and research initiatives. Implementing regulations, setting local exposure limits, exploring alternative materials, and undertaking targeted remediation efforts can collectively minimize exposure and potential health hazards, ensuring the safety of the community and the environment in the vicinity of these rocks.

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