Natural Radioactivity Levels in the Area around the Uranium Deposit of the Bahi District in Dodoma Region, Tanzania

In this study samples of soil and water from different villages in the Bahi wetlands were analyzed for radioactivity levels of 226 Ra, 232 Th and 40 K using gamma ray spectrometry. The mean activity concentration of 226 Ra, 232 Th and 40 K in soil were found to be highest in the northern study area were found to be lower than the guidance levels of these radionuclides (1.0 Bql -1 for 226 Ra and 232 Th) in drinking water recommended by the WHO.


INTRODUCTION
Natural radioactivity is part of our natural surroundings, and human and non human beings have been exposed externally and internally due to the presence of these radionuclides. Gamma radiation emitted from naturally occurring radioisotopes, such as 40 K, 232 Th, and 238 U and their decay products, which exists at trace levels in all ground formations, represents the main external source of irradiation to human body [1]. However, in some areas in the world the radioactivity concentrations are high due to the presence of phosphate as well as uranium deposits [2,3]. Accordingly UNSCEAR report (1993) [4] reported that 226 Ra, 232 Th, and 40 K activities are elevated in the granites which are rich in uranium and thorium.
In Tanzania high background radiation areas have been reported to be that in Minjingu where there is phosphates deposit as well as in Mkuju where there are uranium deposits [2,5]. Viable uranium deposits have also been discovered in Bahi and Manyoni districts. The deposits are reported to be shallow as that of Mkuju; therefore concentration of radioactivity might also be high in the areas at the vicinity of the deposits [6,7]. The main geographic feature in the district is the Bahi wetlands which serve as a source for agricultural and other economic activities within the area. The presence of uranium deposits in Bahi has brought a serious concern as several reports in the literature indicate high activity concentrations in regions near uranium deposits. Tripathi, et al. [8] in India, Geraldo et al. [9] in Brazil as well as Enkhbat et al. [10] in Mongolia reported higher activity concentrations of 238 U and 232 Th compared to the world average as per UNSCEAR report [11]. Mohammed and Mazunga [12] reported high activity concentration of 238 U in Likuyu village; 54 km east of Mkuju uranium deposit. The concern goes further to uranium mining that may start soon as uranium mining if not well controlled might contaminate the environment.
This study has determined the radioactivity concentrations of 226 Ra, 232 Th and 40 K in soil samples from villages in the wetlands so as to assess the radioactivity levels prior to mining.
The data will also act as reference in the future when assessing contamination of the area from mining.

Study Area
Bahi wetlands are about 60 km north-west of the capital Dodoma, between latitudes 05º51′ and 06º20′ South and longitudes 34º59′ and 35º21′ east. The wetlands cover an area of about 2000 km 2 with the population of about 150,000 [13]. The wetlands offer a wide range of livelihood options to communities as compared to the surrounding dry lands [14]. Several studies have highlighted the importance of the wetlands to the local communities, which include agriculture, livestock keeping, salt production and fishing [15,14]. Apart from river Bubu, Bahi wetlands receive water from various seasonal rivers, mainly draining from northern side of the Bahi wetlands. A major part of the wetlands is composed of a swamp known as the Bahi swamp and the remaining part is small scale farms as well as settlements. The swamp has an area of about 1000 km 2 and is located in the south-west of the wetlands. Major activities at the swamp are salt production as well as fishing, and the areas around the swamp are nice grazing area for cattle [16]. The climatic condition of the area is semi-arid with short rainy season extending from December to March. Mean annual rainfall across the area is approximately 600 mm with slightly higher rainfall at the higher altitudes in the northern part of the area [17]. Geology of the area comprises of biotite granites, sedimentary rocks as well as scattered exposures of the basement rocks of volcanic [39].

Sampling Methodology and Sample Preparation
A total of 25 undisturbed soil samples of about 2 kg each were randomly collected from 8 identified points using the standard sampling procedures [18]. The area was divided into three zones; northern zone (NZ) central zone (CN) and southern zone (SN) as shown in Fig. 1. The selection of sampling locations within each zone was based on the accessibility of the station to the public, as well as their proximity to the exploration sites. The surface soils were collected at the depth level between 0-15 cm from each location [19]. The collected samples were then placed in labelled polyethylene bags and transferred to the TAEC laboratory for preparation and analysis. In the laboratory, soil samples were oven-dried at the temperature of 105º C for between 3 and 4 hours until the moisture was completely removed [20]. The samples were then ground into fine particles and thoroughly mixed and pass through a fine mesh sieve (~2 mm) to obtain composite representative samples [21,20]. Finally, the samples were packed into 500 ml marinelli beakers, which were well sealed using silicon and plastic tapes for air tight for about 30 days in order to allow secular equilibrium between 226 Ra and its short-lived decay products in the 238 Useries. Also a total of ten (10) water samples were randomly collected in the study area. Out of these; 4 were collected from the surface of the Bubu River, 3 from three domestic water wells in Bahi town and Bahi Sokoni, and 3 from Bahi Swamp near the salt production area. Samples were collected and filled into 2 litres acid precleaned polyethylene container to avoid wall absorption [IAEA, 1989]. The containers were labelled and transported to the TAEC laboratory for preparation and analysis. The water samples were then transferred into well sealed 500 ml marinelli beakers without any special treatment.

Gamma Ray Spectrometry
The study used a P-type coaxial high purity germanium detector (HPGe) with relative efficiency of 51.0% and resolution of 1.80 keV at 1332 keV energy of 60 Co. Detector chamber is shielded with three layers of copper, cadmium and lead of 30 mm, 3 mm and 100 mm thick, respectively. Energy and efficiency calibration were performed using the multi-nuclide standard packed in a 500 ml marinelli beaker. The standard (MBSS 2) contained 10 radionuclides ( 241 Am, 109  Th and 40 K in the samples were calculated using the following analytical expression as shown in equation (1) [22].
where, A sp is the specific activity concentration of radionuclide in the sample, N sam is the net counts of the radionuclide in the sample, P E is the gamma-ray emission probability, ε(E) is the absolute counting efficiency of the detector system, T c is the sample counting time, M is the mass of the sample in kg or volume (l),

Radioactivity Concentrations in Soil
The activity concentrations of 226 Ra, 232 Th and 40 K in the soil samples collected from different locations in the three zones of the Bahi district and the average values of the zones are given in Tables 1.1, 1.2 and 1.3. The Northern zone (NZ) has the highest average activity concentrations of all radionuclides than Central zone (CZ) and Southern zone (SZ). The NZ includes the Bahi Township which is about 10 km from the exploration area at Kisalalo uranium deposits. The average activity concentrations of 226 Ra in the CZ and SZ are similar. This value is 37% lower than that of NZ.
The average values of the three radionuclides from the three zones of the Bahi wetlands are compared with those from other places in Tanzania and the world as shown in the Th and 40 K in NZ were found to be slightly higher than the values reported in Windhoek, Namibia about 400 km from a major uranium mine [3]. However these values were found to be 4, 3 and 2 times respectively, lower than the values reported from a high radiation background area of Lolodorf, southern part of Cameroon [23]. The mean value of 226 Ra in NZ is also about two times lower than the value reported in Ireland [24]. The mean concentrations of 226 Ra in samples from NZ was found to be higher than 15%, lower than 25% and similar to 60% of the concentrations reported in countries compiled by UNSCEAR, 2000. The mean concentration of 232 Th obtained from NZ is higher than 75% of the countries reported in the UNSCEAR, (2000) but comparable to the world average value and lies within the range of the reported countries [25]. The concentration levels of the three radionuclides were similar to the values reported in other places in the world.

Radioactivity Concentrations in Water
The activity concentrations of     Ra were found to be below the detection limit (MDL), the average value for 232 Th is 0.26±0.007 Bql -1 in a range of 0-0.68 Bql K in a sample collected from BSS were found to be higher than those of the other two locations. Water from Bahi Swamp is not used for any domestic or irrigation purposes due to its salt contents. However the water from this swamp is used for salt production as well as for fishing activities. Th. The highest activity concentrations of both 232 Th and 40 K were found in sample from BST located close to the rice farming fields. There is a possibility that the activity in this ground water is influenced as well by the radionuclides in phosphate fertilizer used in farming field.

Domestic water from ground water wells
El Arabi et al. [29] and Ajayi and Achuka [30] have reported elevated radioactivity concentrations of 226 Ra and 232 Th in ground water from Elba in Egypt and Ogun state, Nigeria. The values of 226 Ra in Elba and Ogun state were found to be 100 and 40 times, respectively more than the MDL of the system used in this study. The average value of 232 Th in this study was found to be about the same as the value reported in ground water in Elba and Ogun State. Fig. 2 compares the mean activities of 226Ra, 232Th and 40K obtained in samples of water analysed in this study. The mean values of 226Ra of all water bodies were found to be below the detection limit of the system used in this study. The ground water wells had the highest concentration of 232Th (0.68 Bql-1) whilst Bubu River and Bahi swamp had similar values. The level in ground water is 3 times higher than the value obtained in samples from Bubu River and Bahi swamp. Th of the domestic water in Bahi to that from other regions in the world. The values of 226 Ra in this study were below the MDL (0.11 Bql -1 ) of the system. This value was found to be about 2 times higher than the values reported in the drinking water in USA and Poland [32,33]. The value was found to be about the same as the values reported in drinking water in Mexico, China and France [34][35][36]. However, this value was observed to be 100 times lower than the value reported from the ground water in Elba (Egypt) and 40 times lower than that of ground water in Ogun state in Nigeria [29,30]. The value was also found to be more than 100 time lower than the value obtained in the drinking water in Switzerland [37].
The average value of 232 Th in this study was found to be about the same as the value reported in ground water in Elba (Egypt) and Ogun State (Nigeria) [29,30]. This value is higher than the values reported in Switzerland, USA, China Poland, and France [32,34,33,37,35]. However, the value was found to be lower than the value reported by Bituh et al. 2009 in Croatia [38,39].

CONCLUSION
In this study γ-ray spectrometry was used to determine the radioactivity levels of  [12]. The results in this study have shown also that the ground water wells had the highest concentrations of 232 Th than the water collected from Bubu River and Bahi Swamp. This might be because ground water is from deeper aquifers [30].