Determination of Radionuclides in Soil and Yam Tubers from Selected Towns across Ogun and Oyo States

Introduction

Human exposure to radiation from both natural (background) and artificial sources has been a major concern to nuclear scientists all over the world. The experiences of the Chernobyl nuclear accident and the Hiroshima nuclear weapon test, in which victims suffered a wide range of casualties, including tumors, cancer, and even death, are a clear example of the deleterious effects of radiation [1]. The level of terrestrial radiation varies from one geographical location to another depending on the variation in radionuclide concentration in soil, which largely depends on the local geology [2,3].

The greatest contribution to the average public radiation exposure comes from radioactive elements in the earth’s crust and from cosmic radiation originating from space. Natural sources contribute, on average, more than 98% of the human radiation dose, excluding medical exposure [4]. The global average background nuclear radiation, as estimated by UNSCEAR (1982) [5], is 2.4 mSv/y.

The presence of primordial radionuclides such as natural potassium-40 (⁴⁰K), Thorium-232 (²³²Th), Uranium-238 series (²³⁸U), and Caesium-137 (¹³⁷Cs) from artificial sources (flowmeters, nuclear weapon tests) in the environment has been established in some locations within the southwestern part of Nigeria [6-8]. Olomo (1990) [9] found that the levels of radioactivity for ²³⁸U and ²³²Th in food products are very low in Nigeria, with the single largest contributor being ⁴⁰K. Arogunjo (2003) [10] measured natural radionuclide contents present in cowpea, guinea corn, rice, and soya beans in Akure, southwestern Nigeria, and later established the presence of radioactivity levels of these natural radionuclides.

Natural radioactivity is present in soil and concentrations may be changed by anthropogenic activities (like the use of fertilizers) and transported from one place to another into the ground by rain or wind [11]. Yam is one of the most important tubers for human consumption in different parts of the world, especially in Africa (West Africa in particular). It is consumed and processed in different forms (roasted, boiled, fried).

Potassium-40 was found to contribute the highest activity in all food products, especially in yam tubers [12]. Little work has been done on soil and yam samples in these study areas – Isara, Ilisan-Remo, Ifo, and Ilaro in Ogun State; Iseyin, Saki, Eruwa, and Lalante in Oyo State. Thus, it becomes necessary to investigate the presence of radionuclides and establish whether there is a high concentration of radionuclides in soil and yam tubers across the study area. The data obtained would become a baseline for radioactivity levels in soil and yam tubers in Oyo and Ogun States. Therefore, the radiological implications of the activity level in soil and yams would be estimated for the population in the study area.

Materials and Methods Sampling Procedure and Preparation

Sixteen (16) samples were collected in all – eight (8) samples from four towns in Ogun State and another eight samples from four towns in Oyo State. The locations were: Isara, Ilisan-Remo, Ifo, and Ilaro in Ogun State; Iseyin, Saki, Eruwa, and Lalante in Oyo State. A tuber of local yam (Dioscorea spp.) was purchased directly from the farmers in each of the towns, with their soil samples collected after the topsoil containing decayed organic matter and roots had been removed.

The soil and the sliced pieces of yam samples were first sun-dried to constant weight. They were ground using a mole grinding machine, then sieved with a 2 mm wire mesh to obtain a fine texture of the sample. The sieved yam tuber and soil samples were weighed to about 80 g using a top-pan balance, then sealed in different cellophane bags. The cellophane bags were properly labeled to indicate the location where each sample was collected.

Sample Analysis

The samples were taken to a laboratory of the Ghana Atomic Energy Commission (GAEC), Accra, Ghana, for analysis using a gamma-ray spectroscopy machine. The analysis was done with a coaxial-type High-Purity Germanium (HPGe) detector. The relative efficiency of the detector was 28.5% with an energy resolution of 1.8 keV at a gamma-ray energy of 1332 keV of ⁶⁰Co. Calibration of the measuring system was done using certified reference standards for various radionuclides. The standard samples were provided by Deutscher Kalibrierdienst (DKD-3) QSAGlobal GmbH, Germany. The spectral analysis was performed with the Ortec Multichannel Analyzer (MCA) and Meastro-32 evaluation software for acquisition and processing.

The specific activity concentration (Asp) of ²³⁸U, ²³²Th, and ⁴⁰K was determined in Bq/kg for soil and yam samples, respectively. The activity concentrations of the radionuclides were determined using the equation below [13-15].

Where A_sp – the specific activity of the radionuclide in the sample

N_sam – net counts of the radionuclide in the sample

P_E – gamma ray emission probability (gamma yield)

E – total counting efficiency of the detector system

Tc – sample counting time

M – mass or weight of the sample

Note: Geographical Location of the selected areas in Ogun and Oyo States, Southwestern Nigeria

Figure 1: Geographical Location of the Two States

Results and Discussion Activity Concentration

(Table 1 and Table 2) present the activity concentrations of radionuclides in soil and yam samples from selected locations across Ogun and Oyo States, respectively.

Table 1: Specific Activity Concentration of Radionuclides in Samples from Ogun State

Table 2: Specific Activity Concentration of Radionuclides in Samples from Oyo State

(Figure 2) and (Figure 3) show the activity concentration of radionuclides in soil and yam samples from selected towns in Ogun State, while (Figure 3) and (Figure 4) show the activity concentration of radionuclides in soil and yam samples from four selected towns in Oyo State. The activity concentration of ⁴⁰K ranged from (12.74 ± 1.91) Bq/kg in Isara to (46.81 ± 1.45) Bq/ kg in Ifo, with a mean activity concentration of (37.11 ± 1.01) Bq/kg. The activity concentration of ²³²Th ranged from (1.23 ± 0.19) Bq/kg in Isara to (3.34 ± 0.08) Bq/kg in Ilisan-Remo, with a mean activity concentration of (2.73 ± 0.12) Bq/kg. The activity concentration of ²³⁸U ranged from (1.56 ± 0.23) Bq/kg in Isara to (5.24 ± 0.13) Bq/kg in Ilaro, with a mean activity concentration of (3.87 ± 0.16) Bq/kg for soil samples from Ogun State.

The activity concentration of ⁴⁰K ranged from (3.78 ± 0.32) Bq/ kg in Saki to (51.34 ± 0.21) Bq/kg in Iseyin, with a mean activity concentration of (37.40 ± 0.18) Bq/kg. The activity concentration of ²³²Th ranged from (0.32 ± 0.08) Bq/kg in Saki to (3.81 ± 0.05) Bq/kg in Iseyin, with a mean activity concentration of (2.64 ±

0.05) Bq/kg. The activity concentration of ²³⁸U ranged from (0.41 ± 0.13) Bq/kg in Saki to (4.87 ± 0.09) Bq/kg in Iseyin, with a mean activity concentration of (3.59 ± 0.26) Bq/kg for soil samples from Oyo State.

The activity concentration of these radionuclides in the food samples from Ogun State ranged from (3.44 ± 0.52) Bq/kg in Ilaro to (18.00 ± 0.09) Bq/kg in Ilisan-Remo, with a mean activity concentration of (8.79 ± 0.91) Bq/kg, (0.03 ± 0.02) Bq/kg in Ilaro to (3.16 ± 0.02) Bq/kg in Ilisan-Remo, with mean activity of (1.11 ± 0.02) Bq/kg, and (0.38 ± 0.04) Bq/kg in Isara to (4.22 ± 0.04) Bq/ kg in Ilisan-Remo, with a mean activity of (1.64 ± 0.04) Bq/kg for ⁴⁰K, ²³²Th, and ²³⁸U, respectively.

The activity concentration of these radionuclides in the food samples from Oyo State ranged from (3.41 ± 0.08) Bq/kg in Saki to (46.27 ± 0.08) Bq/kg in Iseyin, with a mean activity concentration of (24.92 ± 0.26) Bq/kg, (0.02 ± 0.01) Bq/kg in Lalante to (3.24 ± 0.03) Bq/kg in Iseyin, with mean activity of (1.76 ± 0.03) Bq/ kg, and (0.29 ± 0.04) Bq/kg in Lalante to (4.59 ± 0.03) Bq/kg in Iseyin, with a mean activity of (2.54 ± 0.06) Bq/kg for ⁴⁰K, ²³²Th, and ²³⁸U, respectively.

Note: Activity concentration of radionuclides in soil from selected locations in Ogun State.

Figure 2: Activity Concentration of Radionuclides in Soil from Selected Locations in Ogun State

Note: Activity concentration of radionuclides in yam from selected locations in Ogun State

Figure 3: Activity Concentration of Radionuclides in Yam from Selected Locations in Ogun State

Note: Activity concentration of radionuclides in soil from selected locations in Oyo State

Figure 4: Activity Concentration of Radionuclides in Soil from Selected Locations in Oyo State

Note: Activity concentration of radionuclides in yam from selected locations in Oyo State

Figure 5: Activity Concentration of Radionuclides in Yam from Selected Locations in Oyo State

From the soil samples in the selected towns in Ogun State, ⁴⁰K, ²³²Th, and ²³⁸U contributed about 84.9%, 6.3%, and 8.9%, respectively, while in the soil samples from the selected towns in Oyo State, ^[1]K, ²³²Th, and ²³⁸U contributed about 85.7%, 6.1%, and 8.2%, respectively. In the yam samples from the selected locations in Ogun State, ⁴⁰K, ²³²Th, and ²³⁸U contributed about 76.2%, 9.6%, and 14.2%, respectively, while in the yam samples from the selected locations in Oyo State, ⁴⁰K, ²³²Th, and ²³⁸U contributed about 85.3%, 6.0%, and 8.7%, respectively.

40K is found to contribute the highest percentage in both soil and yam samples due to its natural abundance in the Earth’s crust, plants, and animals, which, when decomposed, contributes to the highest radiation dose [16]. The influence pf human activities such as fertilizer applications also contributes to the high levels of 40K in these samples. Soil-plant transfer factor (TF) also influences the uptake of radionuclides in the selected areas [17].

(Table 3) and (Table 4) show the mean absorbed dose rate for these selected locations in Ogun and Oyo States, which were (5.03 ± 0.19) nGy/h and (4.87 ± 0.15) nGy/h, respectively. These values are less than the world mean values of the absorbed dose rate, which range from 18 to 93 nGy/h, with an average value of 70 nGy/h for areas of normal background radiation [11]. (Table 5) and (Table 6) show the mean annual outdoor effective dose rate for the selected locations in Ogun and Oyo States, which were 0.010 mSv/y and 0.006 mSv/y, respectively. These values are lower than the average value (2.4 mSv/y) recommended by the International Commission on Radiological Protection [18,11] (ICRP, 1999; UNSCEAR, 2000) for normal background and lower than the Nigerian average of 98 µSv/y [2].

Table 3: Mean Absorbed Dose Rate in Air from Selected Locations in Ogun State

Table 4: Mean Absorbed Dose Rate in Air from Selected Locations in Oyo State

Table 5: Mean Annual Effective Dose Rate from Selected Locations in Ogun State

Table 6: Mean Annual Effective Dose Rate from Selected Locations in Oyo State

(Table 7) and (Table 8) show the ingestion dose for Ogun and Oyo State. It ranged from 1.5 µSv/y in Isara to 13.0 µSv/y in Ilaro for ⁴⁰K, from 0.1 µSv/y in Ilaro to 10.9 µSv/y in Ilisan-Remo for ²³²Th, and from 6.5 µSv/y in Isara to 72.2 µSv/y in Ilisan-Remo for ²³⁸U in Ogun State. In Oyo State, it ranged from 1.3 µSv/y in Saki to 17.4 µSv/y in Iseyin for ⁴⁰K, from 0.1 µSv/y in Lalante to 11.2 µSv/y in Iseyin for ²³²Th, and from 5.0 µSv/y in Lalante to 79.2 µSv/y in Iseyin for ²³⁸U.

Table 7: Effective Dose of Radionuclides in Yam Samples from Selected Locations in Ogun State

Table 8: Effective Dose of Radionuclides in Yam Samples from Selected Locations in Oyo State