Radiological Impact Assessment of Farm Soils and Ofada rice (Oryza sativa japonica) from Three Areas in Nigeria

Oryza sativa japonica (ofada rice) is largely grown in Aramoko, Abakaliki and Ofada communities and consumed by both the poor and rich in Nigeria. A total of twenty ofada rice farmlands were identified in each study area and rice samples were randomly collected, thoroughly mixed to make a representative sample from each farmland. Soil samples were collected in each farm to a depth of 5-15cm from at least eight different points and thoroughly mixed together to form a representative sample. The samples were thereafter taken to the laboratory for preparation and spectroscopic analysis. A well-calibrated NaI(Tl) gamma-ray detector was used in spectrometric analysis of the samples and descriptive statistics was used to analyze the results. The respective mean 40 K, 238 U and 232 Th activity concentrations in the rice were 261.8±52.5Bq/kg, 9.6±1.2Bq/kg and 8.7±1.0Bq/kg (Ofada); 257.3±39.0Bq/kg, 9.3±1.1Bq/kg and 7.8±1.8Bq/kg (Abakaliki); and 248.2±54.8Bq/kg, 9.3±0.9Bq/kg and 7.6±1.5Bq/kg (Aramoko). The respective mean 40 K, 238 U and 232 Th activity concentrations in the soils were 333.9±62.8Bq/kg, 11.1±1.1Bq/kg and 11.0±1.4Bq/kg (Ofada); 306.8±36.0Bq/kg, 10.7±0.8Bq/kg and 9.4±1.8 Bq/kg (Abakaliki) and 321.8±42.4Bq/kg, 10.9±0.5Bq/kg and 9.5±0.6Bq/kg (Aramoko). The highest mean ingestion dose of 106.0±8.0μSv/y and outdoor effective dose of 32.4±3.40μSv/y were recorded in Ofada community. The highest mean ingestion cancer risk of (0.33±0.004) x10 -3 was recorded in Aramoko. The results indicated significantly lower radionuclide ingestion dose than the world limit value of 290μSv/y and higher cancer risks than the UNSCEAR recommended limit of 0.29x10 -3 , therefore consumption of ofada rice from the areas may not pose any serious health impact.


Introduction:
The formation of soil is due to the weathering of parent rocks in the earth crust that contains decay series 238 U and 232 Th and non-series 40 K radionuclides since the creation of the earth. Natural terrestrial radioactivity is linked to the activity concentrations of the series and non-series decay radionuclides in the earth crust and soils. Humans are exposed to the terrestrial radioactivity through direct and indirect pathways from radionuclides in the earth crust (1,2). It is obvious to note that farmers repeatedly used and exhausted few available farmlands thereby caused soil infertility and consequently reduced the annual farm yields. The soil infertility motivated farmers to use various form of fertilizers to supplement the nutrients in farms to increase the annual food crop yields. Fertilizers contain uranium, thorium and radium radionuclides and their application especially those with phosphate rocks enhances the activity concentrations of natural radionuclides in the soil (3,4). The radionuclides in the soil are transferred into food crops through plant-root uptake and the subsequent health impact when ingested by a human is continuously causing concern to scientists all over the world. These natural radionuclides in soil find their ways into the root system, in the same manner, the nonradioactive elements enter the plant roots and transferred to the edible parts of food crops (5) and the consumption or intake of radioactive contaminated foods is one of the sources of internal radiation exposure to human (6).
The United Nation's advocacy against the consumption of food contaminated with radionuclides has motivated various authors in the field of environmental radioactivity to continuously assess the health impact of radionuclides in various food matrices across the globe. In fact, the potential dangers envisaged in internal radiation exposure of human have encouraged many authors to study the radioactivity levels in various food items from different parts of the world (7)(8)(9)(10)(11)(12). The estimated amounts of each radionuclide that are absorbed into the human body organs can be determined if the concentration of different radionuclides in a food item is measured and the consumption rates of the foodstuff by individuals are known. In other words, the level of radiation dose to man depends on the quantity of food consumed and the concentrations of the radionuclides in such foodstuffs.
Food in liquid or solid form is the foremost necessity among the needs for survival of human. It provides nourishment, energy and vitality. The food contains nutrients like carbohydrate, fat and oil for energy and heat; protein for body-building; minerals and vitamins for body protection and proper functioning of the body. In Nigeria, carbohydrate such as cereals and tubers constitute the most important food basket (13) and rice (cereal food crop) is commonly consumed by both the rich and poor. Although rice is grown in almost all the parts of the country the most popular areas that produce ofada rice varieties are Abakaliki in Ebonyi State, Ofada in Ogun State and Aramoko in Ekiti State. The geology of each of the study area is shown in Fig. 1.

Figure1. Generalized geological map of Nigeria showing the study areas (14)
The three areas are characterized by different geological settings. The study is therefore aimed at measuring the natural radioactivity in the rice and farm soils from the three areas using gamma ray-spectrometer and determining the resulting health effect on human.
Omotayo et al. (15) reported that a farmer from each of the study areas produces as much as 136 metric tons of ofada rice per annum. Ofada rice is recently becoming famous and prominence among the species of rice because of its natural taste that inspired high demand for consumption (16). This is because the mode of preparation, serving and the emotional value obtained in eating ofada rice is traced to its originality that connects people to their cultural heritage and food habit (17). Osaretin et al. (18), reported that ofada rice contains higher protein, fat, and fiber than imported rice (aroso rice). However, there is a need to ensure compliance with the United Nation's advocacy for food free from contaminants by examining the radioactivity levels. The aim of the study, therefore, is to determine the radiological impact due to the soil and the ofada rice produced in the three communities, consumed by the people from the three communities and sell to other areas in Nigerian for consumption.

Rice Sampling
Rice samples were collected from three study areas including Aramoko, Abakaliki and Ofada, in Ekiti, Ebonyi and Ogun States respectively. The three areas are known for the large production of ofada rice (Oryza sativa japonica) in Nigeria. Twenty ofada rice farmlands in each study area were identified and sampling was carried out in the farmlands during the harvesting period to facilitate and ensure site preciseness. Sampling was carried out randomly at different points and mixed to obtain a representative sample in each farmland. Thus, a total of twenty rice samples were collected in each study area. The samples collected in each farm were carefully packed in well-labeled polythene bags before being transported to a radiometric laboratory for preparation and spectroscopic measurements.

Soil Sampling
Soil samples were collected from the same twenty ofada rice farmlands. Soil samples were collected at eight or ten points and the same spots where the rice samples were taken. Thereafter, the soil samples were thoroughly mixed to obtain a representative sample. This gave a total number of twenty soil samples each collected for Aramoko, Abakaliki and Ofada communities. The soil samples from farms were carefully packed in black polythene bags and properly labeled. Thereafter, all the soil samples were transported to a radiometric laboratory for preparation and spectroscopic measurement.

Food sample preparation
The study focuses on human ingestion of rice grown and consumed hence only the edible portions of the rice samples were prepared for spectrometric analysis. The rice shafts were removed and the edible parts were thereafter ovendried at 110 o C to attain constant mass. The rice samples were then milled and sieved with a 2.0mm mesh sieve to attain the same matrix as the standard food sample before homogenized. 200g each of the rice sample was packed into uncontaminated geometry sample containers of uniform sizes. The containers were then sealed for about 28 days (4 weeks) to allow for secular equilibrium between 226 Ra and 228 Ra and their respective progenies before gamma spectroscopy.

Soil sample preparation
The soil samples were crushed, grounded and sieved with a 2.0mm mesh sieve, after drying for about twenty days at room temperature. Thereafter the soil samples were oven-dried at 110 o C to attain constant mass. 200g of each sample was transferred into uncontaminated empty cylindrical plastic containers of uniform sizes and then sealed for about four weeks like the food samples. This was to allow secular equilibrium between 226 Ra and 228 Ra and their respective progenies before gamma spectroscopy.

Radioactivity determination
The sample analysis was carried out using a single crystal 5.0cm × 5.0cm NaI(Tl) detector, manufactured by Scintitech Instrument, USA, coupled with a Hamamatsa (R1306NSV3068) photomultiplier tube and a Multichannel Analyzer, MCA (2100R:01) manufactured by Price gamma Technology, USA that required no inbuilt PC memory for operation. The MCA with attached Quantum MCA software is capable of performing automatic adjustment of the detector bias and amplifier gain. All calibration functions were made through the software. The detector has adequate energy resolution (FWHM) of about 8.0% in 0.662MeV ( 137 Cs) to distinguish the gamma-ray energies of radionuclides in each sample. The 214 Bi with peak gamma-ray energy of 1.760MeV was selected to provide an estimate of 226 Ra ( 238 U) in the sample, the radionuclide 208 Tl with gamma-ray energy 2.615MeV was selected for estimating 232 Th. The 40 K radionuclide was determined by measuring the 1.460MeV gamma-ray emitted during its decay. Each sample was counted for 36000 seconds. The expression for activity concentration is given by where A represents activity concentration in Bq/Kg; N, is a number representing the count rate under the photo peak, ε in cps/Bq represents the efficiency of radionuclide at the specific γ-ray energy, I γ is a number representing the gamma yield, m s represents the mass of the sample in kg and t is representing time.

Effective dose due to ingestion of food crop
According to Jibiri et al. (20), the effective dose due to ingestion of food is given by: Where the coefficients U i and C i r denote the consumption rate (kg/yr) and activity concentration of the radionuclide (Bq/kg) respectively and g T r is the dose coefficient for ingestion of radionuclide r (Sv Bq -1 ). The values of g T r for 40 K, 226 Ra, 232 Th and 137 Cs are 5.9 x 10 -9 Sv Bq -1 , 4.8 x 10 -8 Sv Bq -1 , 2.3 x 10 -7 Sv Bq -1 and 1.3 x 10 -8 Sv Bq -1 respectively for members of the public (adult) (21). These conversion factors were used to determine the effective doses due to dietary intake of radionuclides contained in ofada rice from the study areas. The rice consumption rate (U i ) value of 26.35 kg/Person in Nigeria (22) was used in the study.

Absorbed and outdoor effective dose rates
The absorbed gamma dose rates D R in the air at 1m above the ground level were calculated using (4): where D R is the dose rate in nGy/h and C Ra , C Th and C K are the activity concentrations (Bq/kg) of radium ( 226 Ra), thorium ( 232 Th), and potassium ( 40 K), respectively. The determination of absorbed gamma dose exposure of human is not sufficient to quantify the radiological health effect. However, the better feature in quantifying the radiological health impact from the soil is by determining the effective dose rates. The effective dose rate due to soil samples from the farmlands in each site was calculated using (7): 4 Where D is the absorbed gamma dose rate in (nGy/h); 0.2 is the occupancy factor; 0.7 Sv/Gy is the conversion factor and 8760 h/y as recommended by UNSCEAR and reported in (4).

Lifetime cancer risks due to ingestion of ofada and background radiation exposure from farm
The irregular cells growth either due to the action of carcinogens or likely exposure to gammarays may end up in the creation of tumor and later develops to cancerous cells. The cancerous cells can attack the adjoining body system and later spread to different organs of the body. A lifetime cancer risk is defined as the probability of a member of a population dying from cancer at a lifetime due to internal or external radiation exposures (23). The lifetime cancer risks associated with intake of rice in the study was determined from the cancer risk coefficients for ingestion of radionuclides and percapita intake of the radionuclides given by (24): r i is the cancer risk coefficient for i th radionuclide, I i is the per-capital activity intake of the radionuclide, A i is the activity concentration of the i th radionuclide, C is the food consumption rate and T is the average lifetime expectancy. The average lifetime expectancy at birth in Nigeria is 45.5 years (24) and the values of risk coefficients, r for 226 Ra, 232 Th and 40 K are 9.56x10 -9 Bq -1 , 2.45x10 -9 Bq -1 and 5.89x10 -10 Bq -1 (25).
The cancer risks due to the external radiation exposure from farm soils were evaluated using the activity concentrations of 40 K, 226 Ra, and 232 Th radionuclides in the farm soils and carcinogenicity radionuclide slope factors for environmental exposure to radionuclides by the United States Environmental Protection Agency and the average life expectancy (25): Where A is the activity concentration in Bq/kg, r i is the cancer mortality risk coefficient for i th radionuclide and T is the average life expectancy. The value of r for 226 Ra, 232 Th and 40 K are 1.33 x 10 -17 kg/Bq-s, 1.97x10 -19 kg/Bq-s and 4.66 x 10 -16 kg/Bqs respectively. The total cancer risk, R T was evaluated by summation of the cancer risks due to the ingestion of ofada rice and the radiation exposure from farm soils: where R S is the mean cancer risks due to radioactivity in soils and R F is the mean cancer risks due to radioactivity in food ingestion.

Radiological hazard indices
Radiological hazard index isa concept used to assess the radiation hazard associated with natural radionuclides in environmental matrices. The hazard indices are useful tools in determining the radiological suitability of soil or other environmental substances for building construction. The radiological parameters including radium equivalent activity (Ra eq ), internal hazard index (H in ), external hazard index (H ex )and gamma index (I γ ) are determined to appraise the radiological health risks due to radiation exposure from soils.

Radium equivalent activity
Radium equivalent activity is used to assess the hazards associated with materials that contain 226 Ra, 232 Th and 40 K in Bq/kg in materials used in building and it is determined on the assumption that 10 BqKg -1 of 226 Ra, 7 BqKg -1 for 232 Th and 130 Bq/kg for 40 K produce the same gamma-ray dose rate (26). The radium equivalent activity was calculated using (7): where A Ra , A Th and A k are the activity concentrations of 226 Ra, 232 Th and 40 K respectively.

External hazard index
The external hazard index (H ex ) is a radiation criterion used to evaluate hazard level from natural gamma radiation due to exposure to radon and its decay progeny (27) and it was calculated using (7): where A Ra , A Th and A K are the activity concentrations of 226 Ra, 232 Th and 40 K respectively.

Internal hazard index
The internal hazard index is also a radiation criterion due to exposure to radon and its decay progeny (27) and it was using (7): where A Ra , A Th and A K are the activity concentrations of 226 Ra, 232 Th and 40 K respectively.

Gamma index
The gamma index (I γ ) is a hazard parameter used to correlate the annual dose rate due to external gamma radiation. It is used only as screening tool for identifying materials that might pose health risk when used as construction materials (28). The gamma index was calculated using the model from European Commission (29). where A Ra , A Th and A K are the activity concentrations of 226 Ra, 232 Th and 40 K (Bq/kg) respectively

Results and discussion: Activity concentrations in rice from the study areas
The activity concentrations of 40 K, 226 Ra and 232 Th were calculated using Eq. 1 and the results are presented in Table1. The results indicated that the natural radionuclides were present in the rice samples and no trace of artificial radionuclide was detected. It was observed that 40 K exhibited the highest activity concentration in each rice sample investigated in the study. Similar findings of 40 K exhibiting the highest concentration in radioactivity were reported (6). The radioactivity levels obtained in the ofada rice from Ofada community were slightly higher than any of the other two communities. However, the 226 Ra and 232 Th activity concentrations obtained in rice from Abakaliki and Aramoko were very similar. This is assumed to be due to the radionuclide absorption capability of the ofada rice irrespective of the geological formations of the different communities. The mean activity concentrations of 89.3±6.2Bq/kg for 40 K, 2.8±0.7Bq/kg for 226 Ra and 7.5±2.7Bq/kg for 232 Th reported for paddy rice from Kampung Permatang TokLabu in Malaysia (30) were lower than the values obtained for ofada rice from all the three study areas. The average activity concentration of 40 K reported as 109.9Bq/kg in rice (Siam species) (31) was about one-half less than the values obtained from each of the three study areas.
The respective values of 226 Ra and 232 Th concentrations reported for rice (White glutinous species) (31) were higher than the values in the present study. Mlwilo et al., (8), reported radioactivity levels in rice from Tanzania as24.7Bq/kg for 40 K, 5.02Bq/kg for 226 Ra and 3.8Bq/kg for 232 Th. These values were lower than the corresponding values in the present study.

Activity concentrations in farm soil from the study areas
The activity concentrations of 40 K, 226 Ra and 232 Th in the farm soils in the study areas shown in Table 2 varied from238.5Bq/kg (Abakaliki) to 450.5Bq/kg (Ofada); 8.7Bq/kg (Aramoko) to 12.9Bq/kg (Ofada) and 6.2Bq/kg (Abakaliki) to 13.0Bq/kg (Abakalilki) respectively. Generally, the radioactivity levels in soils reported in studies from other countries were higher than the values obtained in the present study. For instance, the activity concentrations in the soils around the proposed site of Lambapur Peddagattu and Serpally India were 807.08±255.87Bq/kg, 48.07±22. 30   The effective dose of 294.3±49.8μSv/y due to the ingestion of rice reported for Kampung Peratang Malaysia (30) was about three times higher in magnitude than the value got in ofada rice from Ofada Community. The respective effective doses of 153.4±332.2μSv/y and 236.1±26.8μSv/y were reported for rice from other two areas, Sungai Besar and Kampung Sakan in Malaysia (30). The effective dose from each of the two reported areas in Malaysia was higher than the values reported in the present study. The estimated effective dose obtained in the study was higher than 64.23μSv/y (12) reported for Bangladesh but lower than the world average value of 290μSv/y (7).
Although the effective dose of the rice from the Ofada community was higher than the values obtained in Abakaliki and Aramoko, the radiological impact may depend on the frequency and quantity of the rice consumed. Ofada rice constitutes the major food types of nutritive importance in Nigeria and is consumed mostly during ceremonies and festivals. Therefore expected radiological effects due to ingestion of rice from the study areas depend largely and directly on the reality of dietary habits and the food choices of individuals.

Absorbed and outdoor effective doses in farm soils
Eqs.3 and 4 were respectively used to determine the gamma absorbed dose and effective dose rates in the farm soils from the study areas. The mean absorbed dose rates (nGy/h) in the air at 1.0 m above the ground determined in farm soils at each study area are shown in Table2. The mean absorbed dose in the soil samples obtained in Ofada community was about 4.0 nGy/h greater than the value obtained in Aramoko and almost 2.0nGy/h higher than the value obtained in Abakaliki. However, the absorbed dose value in the soil samples in each of the study areas was far below the world average value of 59.0nGy/h (7). The effective dose rate was 0.032±0.003mSv/y at Ofada, 0.029±0.002mSv/y at Abakaliki and 0.030±0.002mSv/y at Aramoko. Each of these values was ten times lower in magnitude than the world average value of 0.30 mSv/y (38). The outdoor effective doses received from farm soils by the local population in each of the study areas were low and insignificant radiological health burden could only be expected.
The degree of the association between the outdoor effective dose due to the farm soils and effective dose due to ingestion of the rice from the study areas was determined using statistical analysis (Pearson's rank correlation). Fig. 2 shows the regression lines and the correlation coefficients of 0.61 for Ofada, 0.42 for Abakalilki and 0.22 for Aramoko. The statistical analysis indicated that the correlation between the outdoor effective dose and effective dose due to ingestion of ofada rice at Ofada community showed a significant coefficient while the two others indicated low coefficients.

Figure 2.Correlation between outdoor effective and ingestion doses in (a) Ofada (b) Abakaliki (c) Aramoko communities
Lifetime cancer risks due to ingestion of rice and exposure from farm soils Using eqs. 5 and6, the cancer risks due to ingestion of rice were evaluated and the results are presented in Table 1 for Ofada, Abakaliki and Aramoko communities as (0.32±0.03) x10 -3 , (0.31±0.03) x10 -3 and (0.30±0.03) x10 -3 respectively. The cancer risks resulting from the ingestion of ofada rice from the study areas were almost the same. However, the cancer risk due to ingestion of the ofada rice from each community was slightly higher than the value of 0.29 × 10 −3 reported by (12). Eq.7 was used to determine the cancer risks due to background radiation exposure of individuals from each of the study areas and the results are presented in Table 2 as (0.22±0.04) x10 -3 , (0.20±0.02) x10 -3 and (0.22±0.03) x10 -3 for Ofada, Abakaliki and Aramoko communities respectively. The cancer risk value obtained in each study area is lower than the UNSCEAR recommended value of 0.29 x 10 -3 reported by (7,39). Eq. 8 was used to determine the total cancer risks, R T due to ingestion of rice and gamma exposure from the farm soils in the three study areas and the results were 0.54 x 10 -3 , 0.51 x 10 -3 and 0.52 x 10 -3 for Ofada, Abakaliki and Aramoko communities respectively.
Table2. Activity concentrations of 40 K, 226 Ra and 232 Th radionuclides, gamma absorbed and outdoor effective doses in farm soil and cancer risks from the three communities

Radiological hazard indices due to radionuclides in the farm soils
Eqs. 9, 10, 11 and 12 were respectively used to determine the radium equivalent activity (Ra eq ), external hazard index (H ex ), internal hazard index (H in ) and gamma index (I γ ) in the farm soils from Ofada, Abakaliki and Aramoko and the results are presented in Table 3. From a radiological and safety perspective, the radium equivalent activity in any material for building construction must be less than or equal to 370Bq/kg i.e. Ra eq ≤ 370 Bq/kg (2).The mean radium equivalent activity of farm soils from each of the communities was lower than 370Bq/kg, thus the use of the farm soils for construction of human dwellings is not expected to pose any serious radiological effect.
From a radiological point of view, the external hazard index, internal hazard index and gamma index of any building material must respectively be less than one (2). The values of H ex , H in , and I γ in the farm soils from each study area were less than one, therefore no radiological health effect is expected when the soils are used for construction of human dwellings. The profile of Ra eq , H ex , H in , and I γ with reference to farm land number in Ofada, Abakaliki and Aramoko are respectively shown in Figs

Conclusion:
The activity concentrations of natural radionuclides 40 K, 226 Ra and 232 Th in rice and the farm soil samples from Ofada, Abakaliki and Aramoko Communities have been measured using gamma-ray spectrometry method. The effective dose rates are estimated from the activity concentrations of the three natural radionuclides in the ofada rice. The gamma absorbed dose and effective dose rates due to external exposure from farm soil radioactivity are calculated. From the results, the mean radioactivity levels in the rice appear very close and this is attributed to the absorption pattern of the ofada rice irrespective of the geological setting of the areas. The radiological impact in the study is insignificant as the cancer risks due to ingestion of the ofada rice and the farm soils in the study are low when compared to the United Nations Scientific Committee on the Effects Atomic Radiation (UNSCEAR) recommended value of 0.29 x10 -3 .
University of Technology Ogbomosho for allowing us to use the gamma-ray spectrometry system for the study.