ASSESSMENT OF NATURAL RADIONUCLIDES IN SOIL SAMPLES FROM MICHIKA, ADAMAWA, STATE NIGERIA

Naturally occurring radionuclides in the environment constitute about 85% of public exposure to terrestrial gamma radiation. These radionuclides can be found in sands,clays, soils and rocks, and many ores and minerals, commodities, recycled residues, and other appliances used by humans. There is therefore the need to assess the occurrence of natural radionuclides in Soil Samples from Michika, Adamawa State Nigeria. Forty-six (46) soil samples were collected across different location in Michika, Adamawa State, Nigeria. Gamma ray spectroscopy was used for the analysis. The results obtained show that the mean activity concentration for 226 Ra, 232 Th and 40 K are 81.27, 101.28 and 324.79 Bq/kg respectively. These values are higher than the control value of 35,30 and 35 Bq/kg for 226 Ra, 232 Th and 40 K set by UNSCEAR. The absorbed dose rate (D) ranges from 60.77 to 248.65 nGy/h, which is above the maximum UNSCEAR accepted value of 59nGy/h. The values of total annual effective dose lie between 0.08 and 0.31 mSv y -1 with mean value of 0.14mSv y -1 . Also, the mean values of Radium Equivalent Activity were 251.11 Bq/kg. Almost all the value of R aeq were below the maximum recommended value. The mean value of total cancer risk range from 3.28E-6 to 1.3E-5 with mean value of 5.93E-6. All the mean values were above the acceptable range. It can be concluded that the soil is radiologically unsafe for human and animal use. Continuous radiological monitoring of the soil is recommended to safeguard the health of the populace.


INTRODUCTION
Naturally occurring radioactive materials (NORM) due to presence of primordial 238 U , 232 Th and 40 K radionuclides constitute about 85% of public exposure to terrestrial gamma radiation in the environment (Bajoga et al., 2019;UNSCEAR, 2000). They can be found in clays, rocks, soil and sands, and many minerals and ores, recycled residues, commodities, , and other appliances used by humans. Despite the fact that their distribution depends and is not uniform on lithology and extent of mineralization of the rocks formation in a given area, shale and phosphate rocks have relatively high radiation levels. The natural radionuclides are normally found in air, water, plant, rocks and soil (Ibrahim et al., 1993;Malanca et al., 1996). The information about these natural radionuclide concentration levels and their environment effect is of paramount importance in several fields of engineering and science. Therefore, knowing the distribution source of rock materials with high levels of natural radionuclides in soil is very useful. (Aysha et al 2022). Radiation health effects from uranium in the northern part of Adamawa state, Nigeria has attracted a lot of attention. It has been reported and confirmed from hospitals in the host communities that, several mysterious deaths, still born babies, deformed babies (like single leg, smooth featureless face) has been witnessed in the area. This was corroborated by a Daily Trust Newspaper report of 3 rd August 2016 and Oak TV report of October 19, 2016, that uranium ore mineral radiation exists in communities of Michika LGA following activities of the defunct uranium mining company jointly owned by Nigeria andFrench Companies between 1980-1983. A research was carried out to asses radiometric geology survey at Ghumchi (Michika) by Saleem et al., (2017), and the result revealed that the radiometric survey shows a high anomaly, which is observed in the northward of the study area at a fault zone associated with Cataclasite and Mylonite which may be due to the concentration of radionuclide such as Potassium, Thorium and Uranium. Also another research was conducted by Zarma et al., to asses the level of heavy metals for drinking water in michika. And the result revealed that the water is not safe for drinking purpose. Thus, it is necessary to assess the safety and quality of different soils sources used in the area, especially for domestic purpose. This study attempts to assess the radiological status of different soil samples used for farming and building in Michika Local Government Area, Adamawa State, Nigeria.

MATERIALS AND METHODS Study Area
The study was carried out in Michika Local Government Area, Adamawa State, Nigeria ( Figure 1). It comprise sisxteen (16) wards and eight (8) districts. The districts are Michika, Baza, Nkala, Ghumchi, Mbororo, Futu, Himike, Sina and Garta town. with population of 179,460 (2011 NPC projection) and an area of 967km 2 . The area lies between the latitudes 10°32'N to 10°41'N and longitudes 13°19'E to 13°25'E, and it is bounded to the South by Mubi Local Government Area, to the East by Republic of Cameroon, , to the North by Madagali Local Government Area and to the West by Borno State, respectively. The area is relatively flat in the west and hilly in the eastern part. There are patches of outcrops of granitic rock except in the southeastern part where the elevations of the mountains attain over 2500 feet, There are many rivers originating from the mountains and generally flow towards west and northwest of the study area. The rivers include RafinWantse, Yedseram, and Rafin Nanda. The rocks aid in the formation of dendritic pattern of drainage network. The valleys that drained the rivers have alluvial flood plains comprising mainly of coarse quarzitic materials. However, granites ranging from fine course grained, pegmatite, granodiorites and biotite granite predominantly occupy the southern part of the area.

Sample Collection
Soil samples were collected within the defund Uranium exploration area, farmlands and residential villages. The soil samples were collected using a mechanical hand auger to a depth of 25-30 cm. A Global Positioning System (GPS) was used to measure sampling point locations. Soil samples were collected from seven villages in the vicinity of the defunct uranium exploration area and two other villages far off from the exploration area to serve as control. One kilogram (1 kg) of each sample was collected in very clean polythene bag, well labeled and taken to the laboratory for analysis.

Sample Preparation and Analysis
The collected soil samples were air dried for 72hrs under laboratory temperature (ambient temperature of 27 o C) and average relative humidity of 70% (IAEA, 1989). The samples were then ground and packed to fill already weighed cylindrical plastic containers of dimension 7.2cm in diameter and 6.0cm high. (Ibeanu, 1999). Before sealing, the mass of each sample was determined and recorded. In order to determine the activity concentrations of uranium, thorium, and potassium in the soil samples, NaI(Tl) detector gamma spectroscopy was used. The choice was based on its availability, speed of analysis, relative ease of sample preparations and the simplicity of the data reduction procedures.The characteristic photon energy was measured and used to quantify the different radionuclides. (Ibeanu, 1999). The Radium Equivalent Activity (Raeq) was calculated from radioactivity concentration of 226 Ra, 232 Th and 40 K, using Equation (1) (Jibiriet al., 2007;Belivermiset al, 2009 al, 2009 andJibiriet al., 2007). D(nGy/h) = 0.462ARa+ 0.604ATh + 0.0417AK (2) Equations (3) to (5) were used to calculate Annual Effective Dose due to External gamma radiation (AEDγ), due to ingestion ( ) and total annual effective dose (TAED) (UNSCEAR, 2000;ICRP, 2012). AEDγ = D (nGyh -1 ) x 8760h x 0.2 x 0.7Sv Gy x 10 -3 (3) ( Svy -1 ) = x x (4) TAED = AEDγ + (5) where 0.7 SvGy -1 = conversion coefficients, 0.2 = occupancy factor, AR = mean activity concentration of radionuclides in a sample (Bq/kg), IR = the water consumption rate per year (730Ly -1 ) (DEA, 2010). DCF = effective dose coefficient in SvBq -1 for the ingestion of natural radionuclides of 226 Ra, 232 Th and 40 K with values of 4.50E-08.

RESULT AND DISCUSSION
The results obtained for activity concentration and other related radiological parameters is presented in Table 1.  (2000). For 232 Th concentration, only B4 had a value within the recommended value of 30 Bq/kg. However, only eight out forty-six sample locations had the value above maximum concentration. The variation of the concentration of this radionuclide may be due to geological conditions like transport process and formation of rocks (Choubeyet al., 1999). Considering 40 K as the only natural isotopes of potassium the high concentration of may also be due to the use of fertilizer rich in potassium. The calculated absorbed dose rate ranges from 60.77 nGy/hr (GA2 sample) to 248.65 nGy/hr (MB2 sample) , with mean value of 112.26. These values were found to be higher than the recommended value of 59 nGy/hr (UNSCEAR, 2000).The total annual effective dose was found to be between 0.08 mSv to 0.31mSv with mean value of 0.14 mSv. All the obtained values are less than world average value of 0.5 mSv. From the result obtained it clearly shows that the annual effective dose for soil samples is within the world average value.  (1979). All the remaining locations have values lower than the accepted value. The values of Hex, Hin, and lies in the range of 0.37 to 1.46, 0.45 to 2.17, 0.48 to 2.50 and 0.10 to 1.31, with mean value of 0.68, 0.90, 1.00 and 0.41 respectively. Almost all the value of both external and internal hazard indices are less than unity except two samples for Hex and nine samples for Hin. The total excess life cancer risk ranged between 3.28E-6 to 1.3E-5, with mean value of 5.93E-6. All the values were found to be within the accepted range, as they are all less than unity (Dankawu et al., 2021).

CONCLUSION
Assessment of Natural Radionuclides ( 226 Ra, 232 Th and 40 K) in the Soil Samples collected from different locations of Michika Local Government Area, Adamawa State Nigeria, was carried out using gamma spectroscopy. The result shows that the mean activity concentration of 226 Ra and 232 Th were almost three times higher than the recommended value. However, the concentration of 40 K in all samples were above the accepted ranges, except for four samples namely: S5, S3, F2 and N6 with value below the accepted value of 35 Bq/kg. The value of the absorbed dose values was found to be higher than the recommended value of 59 nGy/hr. The result obtained clearly shows that the annual effective dose for soil is within the world average value. Radium Equivalent value for all samples location is within recommended value except for one sample location MB2 with value below 370 Bq/kg. Almost all the values of both external and internal hazard indices and excess life cancer risk are less than unity. Therefore, the soil for this study area is safe for farming and construction material.