Investigation of the Radiological Risk of Farmlands and the Transfer Factor from Soil to Crops in Jalingo and Wukari L.G.A of Taraba State, Nigeria

The activity concentrations of radionuclides, absorbed dose rate, excess lifetime cancer risk, and soil-to-plant transfer factor have been evaluated in soil and crop samples from Jalingo and Wukari Local Government Area of Taraba State, Nigeria. The activity concentrations were determined with the aid of High Purity Germanium detector. The absorbed dose and excess lifetime cancer risk were evaluated and forecasted for 60 years using the ResRad off-site model. The average activity concentration of 40 K, 232 Th, and 238 U in the soil samples were 633.13, 141.15, and 71.20 Bq∙kg −1 respectively, for the Jalingo study area, and while that of the Wukari study area was; 199.21, 87.23, and 25.37 Bq∙kg −1 respectively. The average soil-to-plant transfer factors for 40 K, 232 Th, and 238 U were 0.51, 0.10, and 0.27 respectively for the Jalingo study area while that of Wukari are 0.40, 0.57, and 0.74 respectively. The mean annual effective dose equivalent for the study area is higher than the world average of 0.07 mS∙vy −1 . The excess lifetime cancer risk for the study areas has values that are higher than the safety limit. The ResRed model showed that direct radiation from the crops is the major contributor to excess cancer risk among other pathways. The radiological hazard indices reveal health risks to farmers, especially in the Jalingo area.


Introduction
To assess the internal doses to humans from ingestion of radionuclides present in agricultural products, it is important to know the main processes which determine the uptake of radionuclides by plants in the environment. Farmlands are being contaminated with both natural and artificial radionuclides that continuously disintegrate to release nuclear particles. Natural radionuclides like 238 U and 232 Th including their daughter nuclides and non-series 40 K are distributed by the geological and geochemical processes in the soils due to earth formation [1]. Internal exposure of humans is majorly attributed to food consumption which is due to radionuclides in the soil being transferred to food crops via uptake by crops [2].
Contamination of food by radionuclides may occur either by direct deposition of these radionuclides on the leaves of crops, stems, tubers, fruits, and roots or by absorption of radionuclides by plants (crop) from the soil. Transfer of these radionuclides from the contaminated region (soil, water, and air) is of great concern due to its long-term effect on humans and the environment. Nigeria as a developing nation is one among few countries of the world, that is faced with the challenge of indiscriminate dumping of refuge, the use of phosphate fertilizer to improve yield in farmlands, environmental pollution due to the use of explosives by Boko Haram and other terrorists on farmlands and communities, especially in the Northeast region where this research was carried out. Apart from the aforementioned, high background radiations are being noticed in farmland and dumpsites which could become treat human health through food chain [2].
Scientists across the globe are working hard to measure and analyze the radioactivity content of soils and their health implications to humans. Monitoring radioactivity concentration in farmland and its environment is of basic importance for human and environmental protection, but a faster and accurate method for the assessment of radioactivity is of great value [3]. This study aims to investigate the activity concentrations of natural radionuclides in locally grown food crops (yam, cassava, beans, and maize) in Jalingo and Wukari Local Government areas of Taraba state. These communities are the major agrarian communities of the state. The data got can be useful for the estimation of the internal radiation dose originating from the farmlands and the environment.

Study Area
In Taraba

Sample Collection and Preparation
The crops cultivated within the region and soil samples were collected from farmland at harvest. Only the edible portions of the crops were collected for analysis. Due to high cost of analysis and time constrain few commercial farm lands were chosen within the study area. Ten samples of food crops were collected into a nylon bag and labeled from all farmlands selected within the study area. The yam and cassava tubers were washed under running water to get rid of the dust and soil particles. From the samples collected, the edible parts were taken separately and their net wet weight of 1000 g was collected. These samples were chopped and spread on a clean table for drying at room temperature. Samples were then dried, pounded, sieved, and homogenized to get a composite mixture. Drying was conducted for 2 weeks at room temperature and then in an oven at 110˚C to get rid of the humidity and acquire a constant dry weight.
Samples were homogenized by grinding and sieving through a 2 mm mesh. The ashes of the samples were then transferred to wash sample containers of specific sizes and were hermetically sealed. The samples were shelved for a minimum of one month to confirm the secular equilibrium between the isotopes, before gamma spectroscopy analyses.

Radioactivity Measurement
A high-resolution measurement system, consisting of a high purity germanium detector linked to a PC, was used for radioactivity measurement of the collected The sample was placed directly on the hpge detector and counted for 10 hours. The counting system exhibited high detection efficiency since the amount of pulses under a photograph peak is proportional to the intensity of the radiation reaching the detector volume. The count under the corresponding peaks within the energy spectrum was computed by subtracting the counts' thanks from Compton scattering of the upper peaks and other background sources from the peaks and the web area. The background counts within the detector assembly were determined using an empty container sealed under identical measurement conditions and having identical geometry because the container was used for the sample measurement. This procedure is essential due to the existence of natural radionuclides in building materials, cosmic rays entering the atmosphere, and the contribution from other radioactive sources which could be present within the laboratory.

Transfer Factor
Radionuclide uptake by plants from contaminated soil represents a key step of radionuclide input into the human food chain; this phenomenon is described by the soil-plant transfer factor, which is explained as the ratio between crop-specific activity and soil-specific activity. Plants are the primary recipients of radioactive contamination to the soil following atmospheric release of a radionuclide. In the study of routine or accidental release of radionuclides, the transfer factor (TF) can be valuable to evaluate the impact on the environment of many important agricultural products. For other areas and particularly developing countries, the knowledge of Transfer Factors (TFs) particularly in most farm land is still lacking [2] [3]. The soil-to-plant transfer factor is employed in concert with the foremost important parameters in the environmental assessment needed for nuclear facilities. This parameter is essential for environmental transfer models which are useful within the prediction of radionuclide concentrations in agriculture crops for estimating dose intake by man [4].
The soil-to-plant Tfs of the plant to the soil were evaluated using the expression [5].
where CRC-activity concentration of radionuclides in crops in Bq•kg −1 dry and CRS-activity concentration of radionuclides in the soil in Bq•kg −1 dry.

Absorbed Dose Rate (AD)
The amount of absorbed radiation dose for the collected samples was evaluated using the expression.

Annual Effective Dose Rate (AEDR)
The annual effective dose rate of the considered population was evaluated using the conversion coefficient from the absorbed dose in the air to the effective dose given as (0.7 Sv•Gy −1 ) and taking into consideration the outdoor occupancy factor (0.2), and the indoor occupancy factor (0.8) (12). The outdoor AEDR is obtained from the expression (11): The total annual effective dose rate (AED) is the sum of the annual effective dose rate of natural radionuclides and the annual effective dose rate of radionuclides in (μSv/y).
The farmers spend more time on their farms; they spend about 5 to 7 hours of their day on the farm. This gave a factor of 5/24 = 0.2 for the outdoor occupancy factor [6].

Radium Equivalent Activity
The radium equivalent activity is used to evaluate the radiation doses accruing from 226 Ra, 232 Th, and 40 K. It is one of the most used hazard indices in radiation protection assessment [

Results and Discussion
The evaluation of radioactivity was carried out in ten farmlands of the study area to estimate the activity concentration in soil and crops, soil to plant transfer factor, radiological indices, and cancer risk probability due to continuous exposure to ionizing radiation as a result of ingestion and inhalation of radionuclides from farmlands, ambient radiation, use of inorganic fertilizer and agrochemicals. Bq/kg. Virtually the soils from the farmlands in Jalingo contained relatively high levels of 232 Th (Figure 2) compared to the world average value of 30 Bq/kg [11] [14]. The activity concentration of 40 K ranged from 332 to 1252 Bq/kg, with an average value of 633.13 Bq/kg, which is relatively much higher than the world average of 400 Bq/kg [14].
Radionuclide activity concentration of 238 U, 232 Th, and 40 K was also measured in harvested crops from each farmland corresponding to the soils collected from Jalingo L.G.A as shown in Table 2   crops. Again, the activity concentrations in tubers were higher than those of cereal crops in most of the farms evaluated.
The results available from the two farms shows that the level of radionuclide present in the soil does not automatically transfer to food crops, several factors Journal of Environmental Protection enhance the absorption or uptake of radionuclides into crops as already highlighted above. Some areas with high activity concentrations in soil have low activity concentrations in crops, particularly for 238 U and 232 Th. Though 40 K differs in this assertion in some locations and that is because 40 K plays a very important role in terms of plant nutrients and 40 K also helps in the regulation of water in the plant. Hence, its high uptake by plants.

Transfer Factor (TF) of 238 U, 232 Th, and 40 K from Soil to Crops
In Jalingo farms, the value of transfer factor for 40 K leads other radionuclides in the area with the following average values 0.270, 0.1037, and 0.5090 for 238 U, 232 Th, and 40 K respectively ( Table 3). The high value of 40 K could be a result of the high usage of inorganic fertilizer (NPK) and agrochemical in the area; this is because most of these farmers apply the inorganic fertilizers directly at the root region of crops and in high quantity. The high uptake of 40 K by crops may also be attributed to the essential nutrient properties of potassium to plants and the nature of the soil. The high TF values, recorded for 232 Th and 238 U, could be due to the long deposit of radionuclides over the years, type of soil, pH level, and the high ambient radiation of the zone ( Table 4). The activities of bandits and terrorists in the zone could have accounted for the high value too. This is because Taraba state in the northeast could have also had a fair share of the use of explosives by terrorists to attack and destroy houses and farmlands in the area. The use of these explosives over the years by these terrorists in the area could have deposited radionuclides to farmlands and crops as well and in turn, contributed to the level of TF in the area.
On the other hand, the case of the Transfer factor in Wukari differs greatly from Jalingo, the average transfer factor for Wukari is 0.7405, 0.5656, and 0.3946 (Table 3) for 238 U, 232 Th, and 40 K respectively. The TF of 238 U leads in the area followed by 232 Th and 40 K. The high value of TF for 238 U might be connected to the nature of the soil, the pH level of the area, the high ambient radiation of the area, and high organic matter content of the area (  [13]. The excess lifetime cancer risk was also evaluated and the result is found in Table 6. The values obtained for both Jalingo and Wukari are above the world value of 0.229 × 10 −3 [14]. Jalingo has an average value of 0.7287 × 10 −3 (Table  6), which is two times the world average value.

RESRAD Model
The total radionuclide Absorbed dose in 60 years as forecasted by the RESRAD computer model for Jalingo and Wukari farms was 2.5188 mSv/y (Figure 4(a)) and 3.4296 mSv/y (Figure 4(b)), and there the corresponding maximum total cancer risk for all the pathways was 5.465 × 10 −3 (Figure 4(c)) at year 32 years   (2). Analyses of the model reveal that the water-independent pathway is the most significant in calculating the excess cancer risk. In this pathway, the cancer risk is largely caused by direct radiation from the plant (direct and airborne), followed by soil (direct and airborne) and then inhalation followed by soil ingestion.

Radiological Indices
The absorbed dose rate, annual effective dose rate (AEDR), radium equivalent activity, internal and external hazard index of Jalingo and Wukari farms were evaluated and the result is found in Table 6. The absorbed dose rate and AEDR of the two locations were far above the safety limit of 59 nGy/h and 0.07 mSv/y, respectively, as approved by [14]. The results obtained for both the H int and H ext were all above unity, which is of great concern both for the farmers and the community at large.

Conclusion
The average activity concentrations of 238 U, 232 Th, and 40 K at various farm sites evaluated in Jalingo were generally above the safety limits. In Wukari farm sites, the average activity concentrations of 232 Th were the only ones above the safety limit as approved by UNSCEAR (2000). The activity concentration of crops in both Jalingo and Wukari increases in the order 40 K > 232 Th > 238 U. However, the radionuclides hazard indices and cancer risk showed exposure risk for Jalingo.
Hence, this may pose a radiological and cancer risk to farmers and residents in There is a need to monitor the seasonal variation of radionuclide levels in the various sampling points in the studied areas and extend it to other communities around the area to make conclusive predictions on exposure levels. However, food crops from these locations must be well prepared before consumption to safeguard the health of the residents concerning thorium and uranium in the affected sites. It is also recommended that further studies on radioactivity should be carried out to cover the entire Taraba state to get baseline data for future studies.

Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.