Abstract: Background and Objective: Soil contributes significantly to the internal and external exposure to environmental radioactivity by gamma rays and beta radiation that increase the risk to human health, therefore, the present study dealt with measuring radiation levels and radionuclides distribution (226Ra, 232Th and 40K) for soil around non-nuclear industries by studying the effect of residues of Abu Karqas Sugar Factory on agricultural soil. Materials and Methods: Twenty five soil samples (N = 25) were collected from different locations around Abu Karqas Sugar Factory, Upper Egypt. Activity measurements have been performed by gamma-ray spectrometer, employing a high-resolution scintillation detector NaI (Tl) crystal 3×3 inch. Also multi-variate statistical analysis such as variance, skewness, kurtosis, Pearson correlation and cluster analysis was performed utilizing Statistics Software Bundle SPSS version 19.0 for Windows. In addition, the radiological hazards were calculated for the investigated samples. Results: The study indicated that, the average values of activity ranged from 8±0.7 to 33±2, 8±0.3 to 19±1 and from 111±4 to 209±12 Bq kg–1 for 226R, 232Th and 40K, respectively. Conclusion: According to the obtained results, all samples would not present significant radiological hazards.
INTRODUCTION
About 96% of the total radiation dose is from natural sources exists in various geological formations such as soils, rocks, sediments, vegetation, water and air, while 4% is of artificial origin1. Soil is the most important source of the terrestrial radionuclides whose activity concentrations depend primarily on the geological and geochemical conditions of each region in the world2. Terrestrial radionuclides contain the radioactive series of uranium-radium (238U-226Ra), thorium (232Th) and radioactive potassium (40K) in the earth’s crust3. Long-term exposure to uranium and radium through inhalation has several health effects such as chronic lung diseases, acute leucopenia, anemia and necrosis of the mouth. Radium causes bone, cranial and nasal tumors. Thorium exposure can cause lung, hepatic, bone and kidney cancers and leukemia4. Hence, humans should be aware of their natural environment with regard to the radiation effects due to the naturally occurring and induced radioactive elements5.
Soil contributes significantly to the internal and external exposure to environmental radioactivity by gamma rays and beta radiation that increase the risk to human health. The level of exposure depends on the climatic factors, fertilizing, local geology, drainage patterns which are different at each region in the world6. Therefore, the aim of the present research was to study the effect of residues of Abu Karqas Sugar Factory in Al-Ibrahimeh canal by measuring the concentrations of 226Ra, 232Th and 40K in the agricultural soil around the factory.
MATERIALS AND METHODS
Samples description: This study was done in the period between June-November, 2017. Twenty five samples of agricultural soil were collected from different regions around Abu Karqas Sugar Factory (Fig. 1). The samples coded by (S1-S25). Soil is a mixture of mineral and organic matter, the composition and proportion of these components greatly influence soil physical properties, including texture, structure and porosity, the fraction of pore space in a soil and it is mainly made up of oxygen (46.7%), silicon (27%), aluminum (8.1%) and iron (5.0%).
| Fig. 1: | Map showing the studied area |
Sample collection and preparation: The agricultural soil samples were collected by a coring tool to a depth of 5 cm or to the depth of the plough line7. The collected samples each were about 600 g in weight. All samples were dried in an oven at about 110°C for 24 h to ensure that moisture is completely removed. All samples were crushed, homogenized and sieved through a 200 μm, which was the optimum size enriched in heavy minerals. Samples were placed in polyethylene beaker, of 250 cm3 volume each and weighted. The beakers were completely sealed for 4 weeks to reach secular equilibrium radium and thorium and their progenies8,9.
Instrumentation and calibration: Radioactivity measurements were determined by using gamma ray spectrometer, employing a high-resolution scintillation detector NaI (Tl) crystal 3×3 inch. It had a hermetically sealed assembly, which included a NaI (Tl) crystal, coupled with a PC-MCA Canberra Accuspec. To decrease the gamma-ray background, a cylindrical lead shield (100 mm thick) with a fixed bottom and movable cover shielded the detector. The lead shield contained an inner concentric cylinder of copper (0.3 mm thick) in order to absorb X-rays generated in the lead10,11. In order to determine the background distribution in the environment around the detector, an empty sealed beaker was counted in the same manner and in the same geometry as the samples. The measurement time of the activity or background was 43,200 sec. The background spectra were used to correct the net peak area of the gamma rays of the measured isotopes. A dedicated software program was used Genie-2000. The detection array was energy calibrated using 60Co (1173.2 and 1332.5 keV), 133Ba (356.1 keV) and 137Cs (661.9 keV). The curve of efficiency calibration was made using different energy peaks covering the range up to ~2000 keV. The 226Ra radionuclide was predestined from the 351.9 keV γ-peak of 214Pb and 609.3, 1120.3, 1728.6 and 1764 keV γ-peak of 214Bi. The 232Th radionuclide was predestined from the 911.2 keV γ-peak of 228Ac and the 238.6 keV γ-peak of 212Pb. The 40K radionuclide was estimated using the 1461 keV γ-peak from 40K itself12-14. For quality control, the uncertainties of the measured values have been calculated from all parameters.
Multivariate statistical analysis
Basic statistics: Statistical behavior of the measured data (range, minimum, maximum, sum, arithmetic mean (AM), arithmetic standard deviation (SD), variance, skewness, kurtosis and the type of frequency distribution for the three radionuclides for all the soil samples were performed utilizing Statistics Software Bundle SPSS version 19.0 for Windows. Skewness characterized the degree of asymmetry of a distribution around its mean15,16. Kurtosis is a measure of the peakedness of the probability distribution of a real-valued random variable. It characterizes the relative flatness or peakedness of a distribution compared with the normal distribution. Positive kurtosis indicates a relatively peaked distribution.
Pearson’s correlation coefficient and cluster analysis: Cluster analysis and Pearson correlation were done keeping in mind the end goal to clear up the relationship among the factors, particularly the impact of dregs radiological parameters on the appropriation of common radionuclides. Principal components analysis (PCA) is the most common technique used to summarize patterns among variables in multivariate datasets. The PCA is a way of identifying patterns in variables and expressing data in such a way as to highlight their similarities and differences. The main advantage of PCA is that, once the patterns have been found, data can be compressed reducing the number of dimensions, without much loss of information17.
RESULTS AND DISCUSSION
The distribution of the detected radionuclides, 226Ra, 232Th and 40K and their corresponding total uncertainties for samples under investigation were listed in Table 1. While Fig. 2 shows a comparison between the activity concentrations in Bq kg–1 for all soil samples under investigation. From these results, the 40K activity concentration dominated over that of the 226Ra and 232Th elemental activities. The highest value of activity concentration for 226Ra was found in soil sample coded by (S15), while the lowest one was found in sample coded by (S17). For 232Th values, the highest value of activity concentrations in soil sample coded by (S25), while the lowest value in soil sample code by (S12). In case of 40K, the lowest value was found in soil sample code by (S25), while the highest one was in (S16) sample. The variation of radionuclides concentration in studied soil samples may be due to the geological and geographical conditions18 and/or the using of chemical fertilizers. The worldwide average concentrations of the radionuclides 226Ra, 232Th and 40K, reported by UNSCEAR19 are 35, 35 and 370 Bq kg–1, respectively. The results showed that the average activity concentrations of 226Ra, 232Th and 40K in soil samples were lower than the worldwide average concentrations.
| Fig. 2: | Comparison between values of 226Ra, 232Th and 40K activity concentration in Bq kg–1 for soil samples around Abu Karqas Sugar Factory |
| Table 1: | Activity concentrations (Bq kg–1) of 226Ra, 232Th and 40K in soil samples around Abu Korqas sugar factory |
| 40K: Potassium, 232Th: Thorium, 226Ra: Radium | |
Table 2 shows a comparison of the radioactivity concentrations in the soil with other areas of the world.
| Table 2: | Comparison of the activity concentrations of the soil with other countries |
| 40K: Potassium, 232Th: Thorium, 226Ra: Radium | |
Statistical behavior of the measured data (range, minimum, maximum, sum, arithmetic mean (AM), arithmetic standard deviation (SD), variance, skewness, kurtosis and the type of frequency distribution for the three radionuclides for all the soil samples) presented in Table 3. The basic statistics show that the AM of activity concentrations are different from each other. The precipitation affects the natural radioactivity of the soils, when rain water mixed with SO2 of the air, then rain become acidic. Acid rain causes accelerated mobilization of many materials in sediments, especially 238U 30. The highest value of AM was observed for 40K (158.2 Bq kg–1) and the lowest was for 232Th (12.2 Bq kg–1). The basic statistics showed that the AM of activity concentrations for all locations were different from each other.
The values of skewness and kurtosis for 226Ra, 40K and 232Th were near to 0 and negative, respectively therefore, this radionuclide follows normal distribution.
| Table 3: | Descriptive statistics of radiological parameters |
| SD: Standard division, 40K: Potassium, 232Th: Thorium, 226Ra: Radium | |
| Fig. 3: | Frequency distribution of 226Ra |
| Fig. 4: | Frequency distribution of 232Th |
While positive skewness indicates a distribution with an asymmetric tail extending towards values that were more positive as observed in 232Th but negative skewness indicated a distribution with an asymmetric tail extending towards values that were more negative as observed in 226Ra and 40K. Lower skewness value form generally normal distributions. Negative kurtosis indicated a relatively flat distribution (shown in this study case). Higher kurtosis means more of the variance was the result of infrequent extreme deviations.
The frequency distribution for a ll radioactive variables in sediment samples were analyzed, where the histograms given in Fig. 3, 4 and 5. The graph of 226Ra and 40K showed that these radionuclides demonstrate a normal (bell-shape) distribution.
| Fig. 5: | Frequency distribution of 40K |
But 232Th exhibited some degree of multi-modality. This multi-modal feature of the radio elements demonstrated the complexity of minerals in sediment samples.
The results for Pearson correlation coefficients between all studied radioactive variables for soil samples shown in Table 4. From these results, the high good positive correlation coefficient was absorbed between 232Th and 226Ra because radium and thorium decay series occurs together in nature31,21. The positive correlation coefficient was absorbed between 226Ra, 232Th and 40K with all the radiological parameters. This implied that there is very strong relationship between the radionuclides in soil and descriptive statistic.
Table 5 shows the results of data analyzed by graph pad prism 5 programs. As shown in Table 5, 232Th was high, significantly different (p<0.001) from 40K in the mean concentration activity, also 232Th was significantly different from 226Ra (p<0.05) in concentration activity. 226Ra was high significantly different (p<0.001) from 40K in the mean concentration activity.
Finally, cluster analysis was performed using average linkage method, to calculate the Euclidean distance between the variables. The derived dendrogram is shown in Fig. 6. In this dendrogram, all 6 parameters were grouped into five statistically significant clusters.
Radiological hazard indices
Radium equivalent activity (Raeq): The radium equivalent activity was used to obtain the sum of activities to compare the activity concentration of soil samples, which contain 226Ra, 232Th and 40K.
| Fig. 6: | Dendrogram shows the clustering of radionuclide sand their radiological parameters |
| Table 4: | Pearson correlation coefficients between radioactivity variables in soil samples |
| Hex: External hazard index, AED: Annual effective doses, Iγ: Gamma index, D: Absorbed gamma dose rate, 40K: Potassium, 232Th: Thorium, 226Ra: Radium | |
| Table 5: | Results of data analyzed by graph pad prism 5 programs |
| ***Very high significantly different at p<0.001, **High significantly different at p<0.01, *Significantly different at p<0.05, 40K: Potassium, 232Th: Thorium, 226Ra: Radium | |
The radium equivalent activities (Raeq) have been calculated on the estimation that 370 Bq kg–1 of 226Ra, 259 Bq kg–1 of 232Th and 4810 Bq kg–1 of 40K produces the same gamma ray dose rate. Therefore, the Raeq was given by Beretka and Mathew32:
| (1) |
where, ARa, ATh and AK were the activities of 226Ra, 232Th and 40K (Bq kg–1), respectively. The results of radium equivalent activities (Raeq) for soil were presented in Table 6. From Table 6, it was observed that, the values of radium equivalent fluctuate from 27.49 Bq kg–1 in soil sample coded by (S17) to 69.59 Bq kg–1 in soil sample coded by (S15). These values were lower than the allowed maximum value32 of 370 Bq kg–1. Figure 7 showed the relative contributions to Raeq owing to 226Ra, 232Th and 40K for soil samples. It was noticed that 226Ra and 232Th were the main contributor to Raeq in all samples.
Absorbed gamma dose rate (D): The measured activity concentrations of 226Ra, 232Th and 40K were converted into doses by applying the conversion factors 0.462, 0.604 and 0.0417 for uranium, thorium and potassium19, respectively. These factors were used to calculate the total dose rate (nGy h–1) using the following equation:
| Fig. 7: | Relative contribution (%) of 226Ra, 232Th and 40K to Raeq in soil samples Abu Karqas Sugar Factory |
| Table 6: | Raeq, D, AED and hazard indices (Hex, Hin, Iγ and ELCR) for investigated samples |
| Raeq: Radium equivalent activity, D: Absorbed gamma dose rate, AED: Annual effective doses, Hex: External hazard index, Hin: internal hazard index, Iγ: Gamma index, ELCR: Excess lifetime cancer risk | |
| Fig. 8: | Relative contribution (%) of 226Ra, 232Th and 40K to D and AED in soil samples Abu Karqas Sugar Factory |
| (2) |
where, ARa, ATh and AK has the same meaning as in Eq. 1. The calculated values of absorbed gamma dose rate for the samples were presented in column 2 of Table 6 and ranged from 13.66-32.67 nGy h–1, those were lower than the allowed maximum value19 of 59 nGy h–1. The contributions to dose rate (D) and annual effective doses (AED) owing to 226Ra and 232Th higher than the contributions owing to 40K, except in samples coded by (S19 and S21), the 40K is highest one as shown in Fig. 8.
Annual effective dose (AED): The annual effective dose rate outdoors in units of (μSv/year) is calculated by the following formula19:
| (3) |
The AED values for the soil samples vary from 16.59-39.68 μSv/year, these values were lower than the world average values33 at 70 mSv/year as observed in Table 6.
Hazard indices: Beretka and Mathew32 defined tow indices that represented external and internal radiation hazards. The external hazard index (Hex) was determined from the criterion formula as:
| (4) |
where, CRa, CTh and CK are activities of 226Ra, 232Th and 40K, respectively in Bq kg–1. On the other hand, the internal hazard index (Hin) given the internal exposure to carcinogenic radon and its short-lived progeny and it was given by the following formula32,33:
| (5) |
where, ARa, ATh and AK having the same meaning as in Eq. 1. Table 6 showed that the calculated average values of hazard indices for all samples were less than unity19, which did not cause any harm to the farmers and populations in region under investigation. Figure 9 shows the relative contributions to Hin owing to 226Ra, 232Th and 40K for soil samples. As shown in Fig. 9, 226Ra was main contributor to Hin in all soil samples.
Gamma index (Iγ): Another radiation hazard index called the representative level index, Iγ, was defined from the following formula34, where, ARa, ATh and AK having the same meaning as in Eq. 1:
| (6) |
The calculated Iγ values for the samples under investigation were given in Table 6. It was cleared that the soil samples lower than unity35. Figure 10 showed the relative contribution to Iγ owing to 226Ra, 232Th and 40k, from this figure 226Ra was the higher contribution to Iγ in all soil samples, except in samples coded by (S18 and S25), 232Th was the higher contribution.
| Fig. 9: | Relative contribution (%) of 226Ra, 232Th and 40K to Hin in soil samples Abu Karqas Sugar Factory |
| Fig. 10: | Relative contribution (%) of 226Ra, 232Th and 40K to Iγ in soil samples Abu Karqas Sugar Factory |
Excess lifetime cancer risk (ELCR): Excess lifetime cancer risk (ELCR) could be defined as the excess probability of developing cancer at a lifetime due to exposure level of human to radiation. Excess lifetime cancer risk (ELCR) was calculated by using the following Eq.36-41:
| (7) |
| EDR | = | Annual effective dose equivalent |
| DL | = | Duration of life (30-70 years) |
| RF | = | Risk factor (Sv–1) fatal cancer risk per Sievert. For stochastic effects, ICRP 60 uses values of (RF = 0.05) for public |
The values of excess lifetime cancer risk (ELCR) for soil samples listed in Table 6. As shown in Table 6, it could be seen that, the values of excess lifetime cancer risk were ranged from 5.81E-05-1.39E-04, these values were less than the worldwide recommended value16 of 29E-05.
The current study results showed that the average activity concentrations of 226Ra, 232Th and 40K in soil samples were lower than the worldwide average concentrations. The variation of radionuclides concentration in studied soil samples may be due to the geological and geographical conditions18 and/or the using of chemical fertilizers. It was recommended to reduce the dependence on chemical fertilizers to fertilize the soil, because these fertilizers contain high concentration of radioactive material, which may leads to health and environmental problems in the future.
CONCLUSION
As shown from the results, the activity concentration of naturally occurring radionuclides in soil samples around Abu Karqas Sugar Factory were within the world average ranges which are 35, 35 and 370 Bq kg–1 for 226Ra, 232Th and 40K, respectively. The radiological hazards in all soil samples were lower than the world average, so it is safe for farmers, population living and can be used as a building raw materials or other human activities without any radiological risk.
SIGNIFICANCE STATEMENT
This study discovers the natural radioactivity levels and associated radiation hazards in soil samples around non-nuclear industry. The novelty of the present study is evidence that there is no effect of non-nuclear industries on the concentrations of 226Ra, 232Th and 40K for some environmental samples (soil), by studying the effect of residues of Abu Karqas Sugar Factory on agricultural soil.
This study is the first investigated in this area, so this study can be used as a baseline data for future investigations in pollution assessment and natural radioactivity mapping and could serve as a reference data for monitoring pollution studies in future.