NATURAL RADIOACTIVITY STUDIES OF PHOSPHATE FERTILIZERS APPLIED ON GREEK FARM SOILS USED FOR WHEAT CULTIVATION

NATURAL RADIOACTIVITY STUDIES OF PHOSPHATE FERTILIZERS APPLIED ON GREEK FARM SOILS USED FOR WHEAT... Abstract Naturally Occurring Radionuclide Materials (NORM) such as fertilizers, agricultural soils fertilized and wheat grain cultivated at those soils were studied regarding the radiation hazard to the public and workers. Activity of 238U, 226Ra, 232Th and 40K was measured hermitical sealing the marinelli beaker using the Stopaq FN 2100L material that prevent 222Rn escape from the baker. The phosphate fertilization of soil did not affect natural radioactivity in comparison with non-fertilized soils and also did not affect natural radioactivity of wheat grain cultivated since only 40K was measured. The high mean value of Dair 211 nGy h−1 for fertilizers compared to soils fertilized 53 nGy h−1 highlights the raising concern about radioprotection of workers occupied with tasks involving fertilizers. The maximum external dose rate 1.15 mSv y−1 calculated for truck drivers involved to transportation section is similar to annual external effective dose limit 1 mSv. Real concerns rise for workers in the fertilizers storage and supply department since the maximum dose rate estimated is 1.98 mSv y−1, while the mean value is 0.89 mSv y−1. INTRODUCTION Phosphate fertilizers, used on agricultural lands, are materials known as Naturally Occurring Radioactive Material (NORM), as they contain significant amounts of natural radionuclides. Their production is based on the manufacture of phosphoric acid, by the wet or thermal process of phosphate rock(1). The main phosphate rock deposits are part of the apatite group, and are both of igneous and sedimentary origin. The phosphates of sedimentary origin, accounting for 85% of the phosphate rock contain 100–200 ppm uranium and 2–20 ppm thorium and show a positive correlation between uranium and P2O5 content. Igneous phosphates, which are of volcanic origin, contain <10 ppm uranium but they have an appreciable amount of thorium, up to 20 ppm and rare earths(2). The most common method applied for the production of phosphoric acid is the wet process acid, which is based on the chemical treatment of the primary phosphate rocks by using sulfuric acid. This method produces also a by-product, called phosphogysum, an insoluble calcium sulfate salt. Phosphogysum is filtered off and pumped as slurry to nearby ponds, where it stays for a period sufficient to allow complete deposition. After this period, it is stockpiled and is considered waste, due to its elevated levels of impurities. In the phosphate rock, the natural uranium and thorium are in equilibrium. During the industrial process their equilibrium is disturbed and the radionuclides migrate to intermediate, final and by-products(3). Approximately 80–90% of the 226Ra concentrates in phosphogysum and ~80–85% of uranium and 70% of thorium concentrate in phosphoric acid(4, 5). The natural radionuclides concentrations in phosphate fertilizers are of critical importance, due to the fact that they will affect humans via a number of pathways. The use of phosphate fertilizers on agricultural lands may lead to an increase of natural radioactivity in farm soil and agricultural products and therefore it may enhance the absorbed dose of the farmers by external radiation and the public by internal radiation due to products intake. Wheat is one of the most important crops that man consumes and covers the largest portion of agricultural land. According to International Atomic Energy the main objectives of environmental radiation monitoring are to verify compliance with authorized discharge limits, to provide information and data for dose assessment purposes, to assess the exposure or potential exposure of critical groups and populations, to check the conditions of operation and to provide warning of unusual conditions(6). So, higher dose is expected for the workers in agriculture (farmers) due to elevate time spending outdoors. Moreover, the contamination of the area surrounding a Phosphate Industry installations is a further pathway that affecting the people living around(4). Another sensitive group of people affected is the workers in the Industry, especially them occupied to storage-supply and transportation of the fertilizers. Gamma spectrometry is common method to measure natural radioactivity concentrations of NORM samples(7). All three naturally decay series have a Radon isotope, which as noble gas escapes from solid matrix, where it was trapped before. Once this happens, the equilibrium between the post-radon nuclides, many of which have short half-lives and decay rapidly, will be lost. The half-lives of 219Rn in the actinium series and 220Rn in the thorium series are very short and even if radon escapes, equilibrium will be re-established within minutes. That is not the case of 222Rn, with a half life of 3.825 days. After loss of 222Rn, there is ample time for the decay of the daughter nuclides preceding 210Pb before re-growth of the 222Rn. As the post-radon nuclides are measured to estimate 226Ra activity, loss of radon will affect the whole activity measurement process. The solution is to encapsulate the sample and wait for ~10 half-lives of the 222Rn to allow equilibrium to be re-established(8, 9). Radon emanation rate for soil samples depends mostly on four parameters: soil type, grain size, temperature and water content(10, 11). When plastic beakers are used the real radioactive equilibrium is not reached due to the escape of gaseous radon through plastic(12). Subsequently, the hermitical sealing of sample’s containers for the prevention of 222Rn escape is essential to achieved, in order to be guaranteed the accuracy of gamma spectrometry results. The objectives of the current study is to screen natural isotopes 238U, 226Ra, 232Th and 40K in phosphate fertilizers, farm soils and wheat grains in Greece, to secure hermitical sealing for marinelli beakers and to estimate the radiation hazard through the effective dose rate received by the public and the workers in the agriculture land as well as to fertilizing industry. EXPERIMETNAL A total of 100 samples were collected. The 20 samples of phosphate fertilizers, 40 samples of surface soil and 40 samples of mature wheat grains. The sampling sites were randomly selected of the cultivated land of Greece. All sites were consistently fertilized with phosphate fertilizers. Wheat grains have been chosen as it is estimated that the largest portion of the global agricultural land is devoted to wheat(13). The measured fertilizers were commercial compound fertilizers, which are generally preferred to single-nutrient materials. Their preference is mainly for the farmer’s convenience. The compound fertilizers contain two or more nutrients and are formed either by mechanical mixing or via chemical processes. Either way, the raw materials are the same, thus the radioactive concentrations of NORM do not differ. Each fertilizer is described by a grade, namely the guaranteed nutrient content expressed in weight percentages of N, P2O5 and K2O in that order: N-P-K(14). The conversion between P (%) and P2O5 (%) and between K (%) and K2O (%) is: P2O5 (%) = 2.29 P (%) and K2O (%) = 1.2 K (%)(15). Soil and wheat samples were collected from the agricultural land of Greece. Soil sampling was performed at depths up to 25 cm using a soil corer. The samples were crushed, homogenized and sieved through a 200 μm mesh. Wheat grain samples were in full maturation at the time of selection. All samples were prepared according to the official methodology(16). Before the beginning of measurements, using a volumetric container, 500 ml of each sample were weighed and packed into marinelli beakers. Marinelli beakers have conventional and amenable geometry. These containers are ideally suited for the determination of low levels of gamma activity of environmental samples. Their shape ensures that the geometry results in a higher counting efficiency. The material of beakers and their lid is polypropylene. The chosen marinelli beakers were specified for liquid and solid samples. They have maximum height 10.7 cm, maximum diameter 11.7 cm, maximum well diameter 7.7 cm and height of well 6.8 cm. The beakers and their lids were made of polypropylene. Finally, all samples, after the above mentioned procedure, were marked and cataloged. Sealing of marinelli beakers Preliminary measurements involved the testing of four materials for sealing marinelli beakers, in order to prevent 222Ra to escape from the samples. More specific, Room Temperature Vulcanization (RTV) silicone glue, an epoxy plastic-glue, a gaffer tape and a product for sealing walls were evaluated. The preliminary tests have shown that only the last material with commercial name Stopaq FN 2100L was a successful candidate for efficient sealing of marinelli beakers. Therefore, the following experiments involved this material only. Overall, 10 samples of fertilizers and 20 samples of soil were measured. Each sample was measured for the activity concentration of 226Ra and then it was sealed hermitically using the Stopaq FN 2100L. The sealed containers were stored for 4 weeks, which is the required period to establish secular equilibrium between 226Ra, 232Th and their daughters, and then measured again. The insulating material Stopaq is a compound suited for sealing of house walls, pipe and hollow spaces, against gases, moisture, standing water and running ground water leaks. It is non toxic, single component, plastic–elastic and water and gas tight synthetic polyolefin compound. Its density is 1.2–1.5 g/cm3 and its moisture absorption is 10–20% (ASTM D 570). The results of experiments proved that Stopaq is a very efficient material for the sealing of the containers. Moreover it is very suitable one for this purpose, because it does not destroy marinelli beakers, thus, it allows their reuse. Gamma spectrometric analysis The samples were measured with a p-type coaxial lead shielded intrinsic HPGe detector of 61 mm diameter and 32 mm length. The gamma ray spectrometer has a relative efficiency of 15% for 1332 keV and resolution expressed by the full width at half maximum (FWHM) of 0.876 and 1.888 keV for the 122 and 1332 keV gamma-rays energies of 57Co and 60Co, respectively. The energy calibration of the detector was carried out with the use of a standard source of Εckert & Ziegler, containing mix sources of 241Am, 109Cd, 57Co, 139Ce, 203Hg, 113Sn, 85Sr, 137Cs, 88Y and 60Co. The source had the same geometry with the marinelli containers which housed the samples (ANSI/IEE, Std 325-1986). Spectra were analyzed using the software Genie 2000, 3.1 of Canberra. The counting time was preset at 72 000 s. The radionuclides in the uranium and the thorium decay chains cannot be assumed to be in radioactive equilibrium in NORM samples. The radionuclide 226Ra in the uranium chain may have slightly different concentration than 238U because of possible separation due to their differences in water solubility and mobility(17, 18). For this reason, it is not safe enough to assume that each daughter nuclide will be equal to the 238U activity, as the theory of NORM decay series predicted. Table 1 shows the natural isotopes that were measured in this study and the energy peaks that were used for these measurements. The 232Th and 226Ra contents were calculated as the weighted mean value of 228Ac, 208Tl, 212Pb and 214Pb, 214Bi concentrations, respectively. Table 1. Measured isotopes with the corresponding gamma-energies, detection limits (LD) and minimum detectable activities (MDA) for 95% confidence interval. Parent isotope  Daughter isotope  Energya (keV)  LD (counts)  MDA (Bq)  238U  234mPa  1001  156  38  226Ra  214Pb  352  291  0.6  295  703  1.3  214Bi  609  417  1.1  1120  235  3.5  1764  196  3.9  232Th  212Pb  338  231  1.3  208Tl  583  307  1.2  228Ac  911  257  1.5  40K    1461  156  4.9  Parent isotope  Daughter isotope  Energya (keV)  LD (counts)  MDA (Bq)  238U  234mPa  1001  156  38  226Ra  214Pb  352  291  0.6  295  703  1.3  214Bi  609  417  1.1  1120  235  3.5  1764  196  3.9  232Th  212Pb  338  231  1.3  208Tl  583  307  1.2  228Ac  911  257  1.5  40K    1461  156  4.9  aData from Brookhaven National Laboratory NNDC library (http://nndc.bnl.gov/nudat2/). View Large Table 1. Measured isotopes with the corresponding gamma-energies, detection limits (LD) and minimum detectable activities (MDA) for 95% confidence interval. Parent isotope  Daughter isotope  Energya (keV)  LD (counts)  MDA (Bq)  238U  234mPa  1001  156  38  226Ra  214Pb  352  291  0.6  295  703  1.3  214Bi  609  417  1.1  1120  235  3.5  1764  196  3.9  232Th  212Pb  338  231  1.3  208Tl  583  307  1.2  228Ac  911  257  1.5  40K    1461  156  4.9  Parent isotope  Daughter isotope  Energya (keV)  LD (counts)  MDA (Bq)  238U  234mPa  1001  156  38  226Ra  214Pb  352  291  0.6  295  703  1.3  214Bi  609  417  1.1  1120  235  3.5  1764  196  3.9  232Th  212Pb  338  231  1.3  208Tl  583  307  1.2  228Ac  911  257  1.5  40K    1461  156  4.9  aData from Brookhaven National Laboratory NNDC library (http://nndc.bnl.gov/nudat2/). View Large The concentration of each radionuclide (R) was estimated according to following formula:   R=NM⋅ε⋅y⋅twhere N is the net peak area in the sample spectrum, ε is the efficiency at photopeak energy, t is the live time of the sample spectrum collection in seconds, M is the mass in kg of the measured sample and y is the emission probability of the gamma peak(8, 19). Corrections due to nuclide decay during the storing period and the measuring time of samples were not applied as the measured radioisotopes are long life radionuclides. Moreover, correction due to random summing was not applied either as the counting rate was too low. Finally, as the efficiency calibration was performed using a suitable standard source, as it was described above; corrections due to self attenuation and to coincidence summing were omitted. Table 1 also gives the detection limit (LD) and the minimum detectable activity (MDA), as they have been calculated, of a typical background, by using the following equation for 95% confidence level:   MDA=LDε⋅t⋅ywhere detection limit given as follows:   LD=2.71+3.29⋅[B(1+n2m)]1/2where n is the number of channels within the peak region and m is the number of channels of upper and lower background region, before and after the peak region(8). The efficiency calibration of the gamma spectrometry system was performed with the radionuclide specific efficiency method in order to avoid any uncertainty in gamma ray intensities, as well as the influences of coincidence summation and self-absorption effect of the emitting gamma photons. The reference calibration materials (IAEA, RG-set) were certified to be of high quality, and they were enclosed in Marinelli beakers used for the measuring of the samples. The processes of energy and efficiency calibrations were repeated every week in order to be assured the quality and the accuracy of gamma spectrometry measurements(8, 20). The background due to the environment radiation, around the spectrometer, was recorded for 72 000 s. A sealed Marinelli beaker filled with inorganic, no radioactive material, with density 1 g/cm3, similar to the samples, was placed over the crystal, in order to simulate a sample’s measurement. The spectrum of the background was deducted from each sample spectrum, in order to make a peaked background correction possible. RESULTS AND DISCUSSION The activity concentrations of 226Ra in fertilizer and soil samples before and after sealing with Stopaq and storing for 4 weeks are given in Table 2. Difference is presented both among soil and fertilizer samples before and after sealing and storage. The selected material Stopaq FN 2100L prevents 222Rn to escape, and moreover it does not destroy marinelli beakers. However, the spectroscopy system applied does not allow differences lower than 20% (MDA) to be estimated with uncertainty lower than 50%; so few of the data obtained could be used. These differences express the 222Rn emanation factor that has mean value 30% for soils and 19% for fertilizer samples that are in agreement with previous published data in literature(9, 20, 21). Table 2. 226Ra (Bq kg−1) Activity concentration in soil and fertilizer samples before and after sealing and the corresponding increase—emanation factor (%).   226Ra ± σ  Radon emanation  Closed    Sealed    Increase  ±σ  S  15  1  19  2  MDA  S  25  2  33  2  24%  40%  S  17  2  20  2  MDA  S  49  3  69  5  29%  29%  S  54  4  70  4  23%  35%  S  28  3  40  3  30%  34%  S  19  2  23  2  MDA  S  23  2  33  3  30%  40%  S  30  3  38  3  MDA  S  37  3  40  3  MDA  S  10  2  11  2  MDA  S  13  2  27  3  52%  26%  S  16  2  18  2  MDA  S  14  2  19  2  MDA  S  11  2  21  3  48%  36%  S  17  2  18  2  MDA  S  19  2  22  2  MDA  S  18  2  20  2  MDA  S  27  3  36  3  25%  45%  S  35  3  40  3  MDA  F  126  7  135  9  MDA  F  243  16  334  22  27%  30%  F  266  18  336  22  21%  41%  F  103  7  123  9  MDA  F  30  3  34  3  MDA  F  227  19  276  19  MDA  F  94  6  135  9  30%  26%  F  114  10  135  9  MDA  F  182  12  228  8  20%  31%  F  207  14  242  16  MDA    226Ra ± σ  Radon emanation  Closed    Sealed    Increase  ±σ  S  15  1  19  2  MDA  S  25  2  33  2  24%  40%  S  17  2  20  2  MDA  S  49  3  69  5  29%  29%  S  54  4  70  4  23%  35%  S  28  3  40  3  30%  34%  S  19  2  23  2  MDA  S  23  2  33  3  30%  40%  S  30  3  38  3  MDA  S  37  3  40  3  MDA  S  10  2  11  2  MDA  S  13  2  27  3  52%  26%  S  16  2  18  2  MDA  S  14  2  19  2  MDA  S  11  2  21  3  48%  36%  S  17  2  18  2  MDA  S  19  2  22  2  MDA  S  18  2  20  2  MDA  S  27  3  36  3  25%  45%  S  35  3  40  3  MDA  F  126  7  135  9  MDA  F  243  16  334  22  27%  30%  F  266  18  336  22  21%  41%  F  103  7  123  9  MDA  F  30  3  34  3  MDA  F  227  19  276  19  MDA  F  94  6  135  9  30%  26%  F  114  10  135  9  MDA  F  182  12  228  8  20%  31%  F  207  14  242  16  MDA  MDA referred to values lower than 20%. View Large Table 2. 226Ra (Bq kg−1) Activity concentration in soil and fertilizer samples before and after sealing and the corresponding increase—emanation factor (%).   226Ra ± σ  Radon emanation  Closed    Sealed    Increase  ±σ  S  15  1  19  2  MDA  S  25  2  33  2  24%  40%  S  17  2  20  2  MDA  S  49  3  69  5  29%  29%  S  54  4  70  4  23%  35%  S  28  3  40  3  30%  34%  S  19  2  23  2  MDA  S  23  2  33  3  30%  40%  S  30  3  38  3  MDA  S  37  3  40  3  MDA  S  10  2  11  2  MDA  S  13  2  27  3  52%  26%  S  16  2  18  2  MDA  S  14  2  19  2  MDA  S  11  2  21  3  48%  36%  S  17  2  18  2  MDA  S  19  2  22  2  MDA  S  18  2  20  2  MDA  S  27  3  36  3  25%  45%  S  35  3  40  3  MDA  F  126  7  135  9  MDA  F  243  16  334  22  27%  30%  F  266  18  336  22  21%  41%  F  103  7  123  9  MDA  F  30  3  34  3  MDA  F  227  19  276  19  MDA  F  94  6  135  9  30%  26%  F  114  10  135  9  MDA  F  182  12  228  8  20%  31%  F  207  14  242  16  MDA    226Ra ± σ  Radon emanation  Closed    Sealed    Increase  ±σ  S  15  1  19  2  MDA  S  25  2  33  2  24%  40%  S  17  2  20  2  MDA  S  49  3  69  5  29%  29%  S  54  4  70  4  23%  35%  S  28  3  40  3  30%  34%  S  19  2  23  2  MDA  S  23  2  33  3  30%  40%  S  30  3  38  3  MDA  S  37  3  40  3  MDA  S  10  2  11  2  MDA  S  13  2  27  3  52%  26%  S  16  2  18  2  MDA  S  14  2  19  2  MDA  S  11  2  21  3  48%  36%  S  17  2  18  2  MDA  S  19  2  22  2  MDA  S  18  2  20  2  MDA  S  27  3  36  3  25%  45%  S  35  3  40  3  MDA  F  126  7  135  9  MDA  F  243  16  334  22  27%  30%  F  266  18  336  22  21%  41%  F  103  7  123  9  MDA  F  30  3  34  3  MDA  F  227  19  276  19  MDA  F  94  6  135  9  30%  26%  F  114  10  135  9  MDA  F  182  12  228  8  20%  31%  F  207  14  242  16  MDA  MDA referred to values lower than 20%. View Large The fertilizers’ measurements are presented on Table 3. The mean concentration values of 238U, 226Ra, 232Th and 40K were 377, 191, 22 and 2622 Bq kg−1, respectively. These values are in agreement with the concentrations reported in past studies(3, 4, 22–25) that are summarized in Table 4. Moreover, measured concentrations are lower than the permissible international radioactivity levels, given by IAEA: 1000 Bq kg−1 for 238U and 232Th series radionuclides and 10 000 Bq kg−1 for 40K(24). In fertilizers, 238U concentrations ranged from lower than the 38–703 Bq kg−1, with mean value of 377 Bq kg−1, 226Ra concentrations ranged from lower than the 1–529 Bq kg−1, with mean value of 191 Bq kg−1, 232Th concentrations ranged from lower than the 1–95 Bq kg−1 with mean value of 22 Bq kg−1 and 40K concentrations ranged from 37 to 4483 Bq/kg, with mean value of 2622 Bq kg−1. Measured concentrations of 238U, 226Ra in fertilizers, as shown in Figure 1, are checked whether any correlation exists between them and the % P2O5 concentrations. The value of correlation coefficient between 238U and P2O5 is higher than the one of 226Ra. This result is explained by the fact that during the fertilizers production, the greater part of, concentrates in phosphogysum while the greater part of uranium is transferred to phosphoric acid and thus in fertilizers. The correlation coefficient value between 232Th and P2O5 is similar to the 226Ra one. However, strong positive correlation between the concentrations of 40K and % K2O is proved, as it is shown in Figure 2. The fact that the line does not intercept zero, reveals that 40K is also present in phosphate rocks. Fertilizers with no K contain has one order of magnitude lower 40K than the others originated from the phosphate rocks Table 3. Fertilizers activity concentrations (Bq kg−1) as well as the related dose air, Dair (nGy h−1) and corresponding uncertainties. Fertilizer grade (N-P-K)  238U ± σ  226Ra ± σ  232Th ± σ  40K ± σ  Dair ± σ  0-0-50  <38  <1  <1  8325 ± 113  347 ± 22  16-5-8  41 ± 19  7 ± 1  <1  1707 ± 25  74 ± 2  20-5-10  88 ± 32  146 ± 1  36 ± 1  2603 ± 37  198 ± 4  20-8-6  230 ± 36  47 ± 1  <1  1532 ± 9  86 ± 1  24-8-7  250 ± 47  214 ± 2  54 ± 1  1694 ± 25  202 ± 4  20-10-0  575 ± 78  332 ± 2  5 ± 1  144 ± 4  162 ± 3  20-10-0  546 ± 78  222 ± 2  <1  428 ± 9  120 ± 2  12-11-18  140 ± 44  242 ± 2  56 ± 1  4483 ± 62  332 ± 11  12-11-18  274 ± 58  187 ± 1  <1  4098 ± 57  257 ± 7  12-12-17  655 ± 97  158 ± 1  <1  4164 ± 57  247 ± 7  12-12-17  495 ± 79  407 ± 3  53 ± 1  4311 ± 60  400 ± 11  25-15-0  553 ± 77  397 ± 3  77 ± 2  156 ± 5  236 ± 6  11-15-15  256 ± 48  52 ± 1  7 ± 1  3326 ± 47  167 ± 5  15-15-15  205 ± 52  228 ± 2  14 ± 1  3202 ± 45  247 ± 6  15-10-15  703 ± 104  299 ± 2  <1  3714 ± 52  293 ± 7  11-15-15  388 ± 67  31 ± 1  <1  3716 ± 52  169 ± 5  15-15-15  602 ± 94  529 ± 5  95 ± 2  3960 ± 55  467 ± 19  21-17-0  221 ± 35  159 ± 1  23 ± 1  680 ± 11  116 ± 2  16-20-0  663 ± 87  160 ± 1  4 ± 0  37 ± 2  78 ± 1  16-20-0  654 ± 85  11 ± 1  <1  160 ± 4  12 ± 1  Mean value ± σ  377  191  22  262  211  (Range)  (<38–703)  (<1–529)  (<1–95)  (238–4483)  (12–467)  Fertilizer grade (N-P-K)  238U ± σ  226Ra ± σ  232Th ± σ  40K ± σ  Dair ± σ  0-0-50  <38  <1  <1  8325 ± 113  347 ± 22  16-5-8  41 ± 19  7 ± 1  <1  1707 ± 25  74 ± 2  20-5-10  88 ± 32  146 ± 1  36 ± 1  2603 ± 37  198 ± 4  20-8-6  230 ± 36  47 ± 1  <1  1532 ± 9  86 ± 1  24-8-7  250 ± 47  214 ± 2  54 ± 1  1694 ± 25  202 ± 4  20-10-0  575 ± 78  332 ± 2  5 ± 1  144 ± 4  162 ± 3  20-10-0  546 ± 78  222 ± 2  <1  428 ± 9  120 ± 2  12-11-18  140 ± 44  242 ± 2  56 ± 1  4483 ± 62  332 ± 11  12-11-18  274 ± 58  187 ± 1  <1  4098 ± 57  257 ± 7  12-12-17  655 ± 97  158 ± 1  <1  4164 ± 57  247 ± 7  12-12-17  495 ± 79  407 ± 3  53 ± 1  4311 ± 60  400 ± 11  25-15-0  553 ± 77  397 ± 3  77 ± 2  156 ± 5  236 ± 6  11-15-15  256 ± 48  52 ± 1  7 ± 1  3326 ± 47  167 ± 5  15-15-15  205 ± 52  228 ± 2  14 ± 1  3202 ± 45  247 ± 6  15-10-15  703 ± 104  299 ± 2  <1  3714 ± 52  293 ± 7  11-15-15  388 ± 67  31 ± 1  <1  3716 ± 52  169 ± 5  15-15-15  602 ± 94  529 ± 5  95 ± 2  3960 ± 55  467 ± 19  21-17-0  221 ± 35  159 ± 1  23 ± 1  680 ± 11  116 ± 2  16-20-0  663 ± 87  160 ± 1  4 ± 0  37 ± 2  78 ± 1  16-20-0  654 ± 85  11 ± 1  <1  160 ± 4  12 ± 1  Mean value ± σ  377  191  22  262  211  (Range)  (<38–703)  (<1–529)  (<1–95)  (238–4483)  (12–467)  View Large Table 3. Fertilizers activity concentrations (Bq kg−1) as well as the related dose air, Dair (nGy h−1) and corresponding uncertainties. Fertilizer grade (N-P-K)  238U ± σ  226Ra ± σ  232Th ± σ  40K ± σ  Dair ± σ  0-0-50  <38  <1  <1  8325 ± 113  347 ± 22  16-5-8  41 ± 19  7 ± 1  <1  1707 ± 25  74 ± 2  20-5-10  88 ± 32  146 ± 1  36 ± 1  2603 ± 37  198 ± 4  20-8-6  230 ± 36  47 ± 1  <1  1532 ± 9  86 ± 1  24-8-7  250 ± 47  214 ± 2  54 ± 1  1694 ± 25  202 ± 4  20-10-0  575 ± 78  332 ± 2  5 ± 1  144 ± 4  162 ± 3  20-10-0  546 ± 78  222 ± 2  <1  428 ± 9  120 ± 2  12-11-18  140 ± 44  242 ± 2  56 ± 1  4483 ± 62  332 ± 11  12-11-18  274 ± 58  187 ± 1  <1  4098 ± 57  257 ± 7  12-12-17  655 ± 97  158 ± 1  <1  4164 ± 57  247 ± 7  12-12-17  495 ± 79  407 ± 3  53 ± 1  4311 ± 60  400 ± 11  25-15-0  553 ± 77  397 ± 3  77 ± 2  156 ± 5  236 ± 6  11-15-15  256 ± 48  52 ± 1  7 ± 1  3326 ± 47  167 ± 5  15-15-15  205 ± 52  228 ± 2  14 ± 1  3202 ± 45  247 ± 6  15-10-15  703 ± 104  299 ± 2  <1  3714 ± 52  293 ± 7  11-15-15  388 ± 67  31 ± 1  <1  3716 ± 52  169 ± 5  15-15-15  602 ± 94  529 ± 5  95 ± 2  3960 ± 55  467 ± 19  21-17-0  221 ± 35  159 ± 1  23 ± 1  680 ± 11  116 ± 2  16-20-0  663 ± 87  160 ± 1  4 ± 0  37 ± 2  78 ± 1  16-20-0  654 ± 85  11 ± 1  <1  160 ± 4  12 ± 1  Mean value ± σ  377  191  22  262  211  (Range)  (<38–703)  (<1–529)  (<1–95)  (238–4483)  (12–467)  Fertilizer grade (N-P-K)  238U ± σ  226Ra ± σ  232Th ± σ  40K ± σ  Dair ± σ  0-0-50  <38  <1  <1  8325 ± 113  347 ± 22  16-5-8  41 ± 19  7 ± 1  <1  1707 ± 25  74 ± 2  20-5-10  88 ± 32  146 ± 1  36 ± 1  2603 ± 37  198 ± 4  20-8-6  230 ± 36  47 ± 1  <1  1532 ± 9  86 ± 1  24-8-7  250 ± 47  214 ± 2  54 ± 1  1694 ± 25  202 ± 4  20-10-0  575 ± 78  332 ± 2  5 ± 1  144 ± 4  162 ± 3  20-10-0  546 ± 78  222 ± 2  <1  428 ± 9  120 ± 2  12-11-18  140 ± 44  242 ± 2  56 ± 1  4483 ± 62  332 ± 11  12-11-18  274 ± 58  187 ± 1  <1  4098 ± 57  257 ± 7  12-12-17  655 ± 97  158 ± 1  <1  4164 ± 57  247 ± 7  12-12-17  495 ± 79  407 ± 3  53 ± 1  4311 ± 60  400 ± 11  25-15-0  553 ± 77  397 ± 3  77 ± 2  156 ± 5  236 ± 6  11-15-15  256 ± 48  52 ± 1  7 ± 1  3326 ± 47  167 ± 5  15-15-15  205 ± 52  228 ± 2  14 ± 1  3202 ± 45  247 ± 6  15-10-15  703 ± 104  299 ± 2  <1  3714 ± 52  293 ± 7  11-15-15  388 ± 67  31 ± 1  <1  3716 ± 52  169 ± 5  15-15-15  602 ± 94  529 ± 5  95 ± 2  3960 ± 55  467 ± 19  21-17-0  221 ± 35  159 ± 1  23 ± 1  680 ± 11  116 ± 2  16-20-0  663 ± 87  160 ± 1  4 ± 0  37 ± 2  78 ± 1  16-20-0  654 ± 85  11 ± 1  <1  160 ± 4  12 ± 1  Mean value ± σ  377  191  22  262  211  (Range)  (<38–703)  (<1–529)  (<1–95)  (238–4483)  (12–467)  View Large Table 4. Activity concentrations (Bq kg−1) of fertilizers in bibliography. Country  238U  226Ra  232Th  40K  References  Egypt    1–950  1–162  10–23 845  (23)  Brazil  182–1158  <1.3–879  81–546  —  (3)  Malaysia    0.4–112  0.8–48  13–279  (24)  Italy  190–650  6–230  —  220–5200  (23)  Greece    16–4584    4–5254  (22)  Greece  312–936  19–1129  <1–12  53–16 700  (4)  Greece  <38–703  <1–529  <1–95  38–8325  This study  Global  23–2100  9–850  10–63  41–5900  (25)  Country  238U  226Ra  232Th  40K  References  Egypt    1–950  1–162  10–23 845  (23)  Brazil  182–1158  <1.3–879  81–546  —  (3)  Malaysia    0.4–112  0.8–48  13–279  (24)  Italy  190–650  6–230  —  220–5200  (23)  Greece    16–4584    4–5254  (22)  Greece  312–936  19–1129  <1–12  53–16 700  (4)  Greece  <38–703  <1–529  <1–95  38–8325  This study  Global  23–2100  9–850  10–63  41–5900  (25)  View Large Table 4. Activity concentrations (Bq kg−1) of fertilizers in bibliography. Country  238U  226Ra  232Th  40K  References  Egypt    1–950  1–162  10–23 845  (23)  Brazil  182–1158  <1.3–879  81–546  —  (3)  Malaysia    0.4–112  0.8–48  13–279  (24)  Italy  190–650  6–230  —  220–5200  (23)  Greece    16–4584    4–5254  (22)  Greece  312–936  19–1129  <1–12  53–16 700  (4)  Greece  <38–703  <1–529  <1–95  38–8325  This study  Global  23–2100  9–850  10–63  41–5900  (25)  Country  238U  226Ra  232Th  40K  References  Egypt    1–950  1–162  10–23 845  (23)  Brazil  182–1158  <1.3–879  81–546  —  (3)  Malaysia    0.4–112  0.8–48  13–279  (24)  Italy  190–650  6–230  —  220–5200  (23)  Greece    16–4584    4–5254  (22)  Greece  312–936  19–1129  <1–12  53–16 700  (4)  Greece  <38–703  <1–529  <1–95  38–8325  This study  Global  23–2100  9–850  10–63  41–5900  (25)  View Large Figure 1. View largeDownload slide Activity concentrations of 238U and 226Ra (Bq kg−1) correlated to P2O5 (%) content of fertilizer. Figure 1. View largeDownload slide Activity concentrations of 238U and 226Ra (Bq kg−1) correlated to P2O5 (%) content of fertilizer. Figure 2. View largeDownload slide Activity concentrations of 40K (Bq kg−1) correlated to K2O (%) content of fertilizer. Figure 2. View largeDownload slide Activity concentrations of 40K (Bq kg−1) correlated to K2O (%) content of fertilizer. The farm soil samples concentrations are shown on Figure 3 ranging from 8 to 68 Bq kg−1 for 226Ra and from 8 to 78 Bq kg−1 for 232Th and from 185 to 868 Bq kg−140K, respectively. These values are comparable to world average concentrations, which are equal to 35, 30 and 400 Bq/kg for 226Ra, 232Th and 40K, respectively(18) as well as to values reported in previous studies regarding Greek soils(26–31) that are summarized in Table 5. Both in Table 5 as well as in Figure 4, the data of Chernobyl 137Cs are also presented since the specific radioisotope could be considered now on as ‘natural occurring’ after all these years that it will be remain in any ecosystem of the environment. The specific data ranged between 4 and 33 Bq kg−1 comparable with 226Ra and 232Th concentration in some soils samples but the dose absorbed by the public is similar and even lower than the uncertainty of the dose received by NORM radioisotopes(28). Therefore, no more discussion on Chernobyl 137Cs influence on effective gamma dose rate is presented in the manuscript. The natural radiatiactivity results indicate that phosphate fertilization did not change the 226Ra, 232Th and 40K concentrations in comparison with non-fertilized soils. These results are in agreement with similar studies in other countries(3, 31). Figure 3. View largeDownload slide Mapping results of 226Ra 232Th and 40K activity concentrations (Bq kg−1) in Greek fertilized farm soils (the number of samples measured is presented inside the bracket). Figure 3. View largeDownload slide Mapping results of 226Ra 232Th and 40K activity concentrations (Bq kg−1) in Greek fertilized farm soils (the number of samples measured is presented inside the bracket). Table 5. Activity concentrations (Bq kg−1) and absorbed dose (nGy h−1) of Greek soils in bibliography. Regions  226Ra  232Th  40K  Dair [NORM]  Dair [137Cs]  References  Greece  9–54  3–58  160–750      (26)  Greece  4–44  5–37  155–575      (27)  Greece        5–220  0.3–1.1  (28)  Greek Islands  7–310  2–269  230–1796      (29)  S-W Greece  6–92  3–49  120–564      (30)  N-E Greece  8–68  8–78  185–868      This study  Global  35  30  400  57    (25)  Regions  226Ra  232Th  40K  Dair [NORM]  Dair [137Cs]  References  Greece  9–54  3–58  160–750      (26)  Greece  4–44  5–37  155–575      (27)  Greece        5–220  0.3–1.1  (28)  Greek Islands  7–310  2–269  230–1796      (29)  S-W Greece  6–92  3–49  120–564      (30)  N-E Greece  8–68  8–78  185–868      This study  Global  35  30  400  57    (25)  View Large Table 5. Activity concentrations (Bq kg−1) and absorbed dose (nGy h−1) of Greek soils in bibliography. Regions  226Ra  232Th  40K  Dair [NORM]  Dair [137Cs]  References  Greece  9–54  3–58  160–750      (26)  Greece  4–44  5–37  155–575      (27)  Greece        5–220  0.3–1.1  (28)  Greek Islands  7–310  2–269  230–1796      (29)  S-W Greece  6–92  3–49  120–564      (30)  N-E Greece  8–68  8–78  185–868      This study  Global  35  30  400  57    (25)  Regions  226Ra  232Th  40K  Dair [NORM]  Dair [137Cs]  References  Greece  9–54  3–58  160–750      (26)  Greece  4–44  5–37  155–575      (27)  Greece        5–220  0.3–1.1  (28)  Greek Islands  7–310  2–269  230–1796      (29)  S-W Greece  6–92  3–49  120–564      (30)  N-E Greece  8–68  8–78  185–868      This study  Global  35  30  400  57    (25)  View Large Figure 4. View largeDownload slide Mapping results of 137Cs activity concentrations (Bq kg−1) in Greek soils. Figure 4. View largeDownload slide Mapping results of 137Cs activity concentrations (Bq kg−1) in Greek soils. In wheat grains, specific radioactivity values of 226Ra and 232Th were below MDA of the detector, so only concentration of 40K is measured ranging from 109 to 200 Bq kg−1, with a mean value of 139 Bq kg−1. The respective range in soil samples was from 185 to 868 Bq kg−1, with mean value of 456 Bq kg−1. Figure 5 shows the stability of activity concentration of 40K in wheat grains irrespectively of its wide variation in soils. This stability confirms the potassium’s homeostasis of cereals that is known to occur(32, 33). This is because when K reaches the necessary level required by the organism of the reference person, it remains stable. Grasses of graminae family absorbs stable and only the amounts of potassium that can be metabolized. According to the measured activity concentrations in wheat grain, the use of fertilizers does not affect natural radioactivity of wheat grain. Figure 5. View largeDownload slide Activity concentrations (Bq kg−1) of 40K in fertilized soils correlated to 40K in wheat grains cultivated to these soils. Figure 5. View largeDownload slide Activity concentrations (Bq kg−1) of 40K in fertilized soils correlated to 40K in wheat grains cultivated to these soils. Dose assessments The gamma dose rate received by the public 1 m above the ground (Dair) in units of n Gy h−1 can be calculated by the following equation, assuming uniform distribution of naturally occurring radioactive nuclides: Dair (nGy h−1) = 0.462 CRa + 0.604 CTh + 0.0417 CK, where, CRa, CTh and CK are the specific activity in Bq/kg of 226Ra, 232Th and 40K, respectively. 238U radionuclides are ignored, as its gamma radiation has low energies and rate(18, 34). This formula is applicable to workers on agriculture land (farmers) working 50% of their time outdoors. In case of workers in Phosphate Industry it is applicable to workers in transportations (track drivers) as an upper limit but for workers in the fertilizers storage and supply department working indoors should be different since they work inside high piles of fertilizes packs. A model that describes the above situation could be the consideration of a cavity-spherical shell with 1 m in diameter surrounding by fertilizer and likewise the upper limit of dose air calculated using the formula: (nGy h−1) = 0.954 CRa + 1.128 CTh + 0.0830CK(18). The annual effective dose received by the population Eext, (mSv y−1) was estimated as follows considering the proper conversion coefficient (F) from absorbed dose in air to effective dose (0.7 Sv Gy−1) and the occupancy factor T (h y−1): Eext = 10−6 ×Dair × T × F(18). The occupancy factor, which implies that 50% of time is spent outdoors, is equal to 4400 h for the farmers and 3600 h for track drivers while 2920 h y−1 for workers at storage and supply department in the Phosphate Industry. Especially in case of the workers at storage and supply department a supplement study has been performed on internal alpha radiation due to ‘radon problem’ since high value of radium content in fertilizer in association to elevate radon emanation factor triggers high radon concentrations indoors. The range of public absorbed dose rate from soil due to NORM radioisotopes was calculated from 19 to 107 with mean value 53 nGy h−1. This value is very close to the average global 57 nGy h−1 as well as Greek data (Table 5), therefore, no problem is raised for due to soil radioactivity. The annual effective dose received by the public ranges from 0.03 to 0.15 mSv y−1 similar to worldwide mean value (Table 6). In case of workers in the field although double effective dose rate is calculated reaching values 0.3 mSv y−1 still no problem is raised for due to soil radioactivity since is lower than external effective dose limit 1 mSv y−1(18). Table 6. Annual effective dose (mSv y−1) received due to phosphate fertilizers application in the environment.     Eext  Eint  Range  Mean  Range  Mean  Outdoors  Public  0.03–0.15  0.07      Farmer  0.05–0.30  0.15      Truck drivers  0.03–1.15  0.52      Indoors  Storage-supply  0.05–1.98  0.98  0.01–0.27  0.27      Eext  Eint  Range  Mean  Range  Mean  Outdoors  Public  0.03–0.15  0.07      Farmer  0.05–0.30  0.15      Truck drivers  0.03–1.15  0.52      Indoors  Storage-supply  0.05–1.98  0.98  0.01–0.27  0.27  View Large Table 6. Annual effective dose (mSv y−1) received due to phosphate fertilizers application in the environment.     Eext  Eint  Range  Mean  Range  Mean  Outdoors  Public  0.03–0.15  0.07      Farmer  0.05–0.30  0.15      Truck drivers  0.03–1.15  0.52      Indoors  Storage-supply  0.05–1.98  0.98  0.01–0.27  0.27      Eext  Eint  Range  Mean  Range  Mean  Outdoors  Public  0.03–0.15  0.07      Farmer  0.05–0.30  0.15      Truck drivers  0.03–1.15  0.52      Indoors  Storage-supply  0.05–1.98  0.98  0.01–0.27  0.27  View Large The corresponding mean dose rate 211 nGy h−1 due to external radiation at 1 m from the source-fertilizers (Table 3) was calculated that is four times higher than average global. Calculated Dair ranged from 12 to 467 nGy h−1 and only four samples gave dose rates around the global mean while most fertilizers gave dose rate up to 5–10 times higher. The high value of Dair for fertilizers samples highlights the necessity of raising concern about the radioprotection of workers occupied with operations involving fertilizers. In case of the workers at transportations section (track drivers) the mean annual effective dose received 0.5 mSv y−1 is higher than maximum dose received by the farmers. The maximum dose rate 1.15 mSv y−1 calculated for truck drives is similar to annual external effective dose limit 1 mSv y−1. Real concerns rise for workers in the fertilizers storage and supply department since the maximum dose rate estimated is 1.98 mSv y−1, while the mean value is 0.89 mSv y−1. Especially these workers are exposed to supplement internal radiation due to radon inhalation leaving in high indoors radon environment since high value of radium content in fertilizer (Table 3) in association to elevate radon emanation factor (Table 2) triggers high radon concentrations indoors Based on model calculations of effective dose due to internal radiation (Eint) indoors proposed in Ref. (20) a mean dose value 0.07 mSv and maximum 0.27 mSv y−1 is estimated, lower than the internal effective dose limit 1.5 mSv y−1(18). However, the total maximum increment in effective dose received by industry worker could reach 2.25 mSv y−1 values similar to the annual limit of external plus internal exposure received by the public. CONCLUSIONS The objectives of the current study is to screen natural isotopes 238U, 226Ra, 232Th and 40K in phosphate fertilizers, farm soils and wheat grains in Greece hermitical sealing the marinelli beakers and to estimate any radiation hazard to the public. A sealing material, Stopaq FN 2100 L, was proved suitable in order to prevent 222Rn escape from sample matrix through the baker. The mean value and range of radioactive concentrations of fertilizers used in Greece were found to be lower than the permitted levels. The phosphate fertilization of soil did not change 226Ra, 232Th and 40K concentrations in comparison with non-fertilized soils. The use of fertilizers did not affect natural radioactivity of wheat grain since in wheat samples, only 40K was detectable with apparent stability confirming potassium’s homeostasis of cereals. The high mean value of Dair 211 nGy h−1 for fertilizers compared to soils fertilized 53 nGy h−1 highlights the raising concern about radioprotection of workers occupied with tasks involving fertilizers. The maximum external dose rate 1.15 mSv y−1 calculated for truck drivers involved to transportation section is similar to annual external effective dose limit 1 mSv. 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Google Scholar CrossRef Search ADS PubMed  24 Ibrahim, N. Determination of natural radioactivity in fertilizers by gamma ray spectroscopy. Radiat. Phys. Chem.  51( 4–6), 621 ( 1998). Google Scholar CrossRef Search ADS   25 United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). Ionizing radiation: sources and biological effects. United Nation Scientific Committee on the Effects of Atomic Radiation, Report to the General Assembly ( 1982). 26 Probonas, M. and Kritidis, P. The exposure of the Greek population to natural gamma radiation of terrestrial origin. Radiat. Prot. Dosim.  46, 123– 126 ( 1993). Google Scholar CrossRef Search ADS   27 Anagnostakis, M. J., Hinis, E. P., Simopoulos, S. E. and Angelopoulos, M. G. Natural radioactivity mapping of Greek surface soils. Environ. Int.  22, S3– S8 ( 1996). Google Scholar CrossRef Search ADS   28 Clouvas, A., Xanthos, S. and Antonopoulos-Domis, M. Radiological maps of outdoor and indoor gamma dose rates in Greek urban areas obtained by in situ gamma spectrometry. Radiat. Prot. Dosim.  112, 267– 275 ( 2004). Google Scholar CrossRef Search ADS   29 Florou, H., Trabidou, G. and Nicolaou, G. An assessment of the external radiological impact in areas of Greece with elevated natural radioactivity. J. Environ. Radioact.  93, 74– 83 ( 2007). Google Scholar CrossRef Search ADS PubMed  30 Papaeftymiou, H. V., Manousakas, M., Fouskas, A. and Siavalas, G. Spatial and vertical distribution and risk assessment of natural radionuclides in soils surrounding the lignite-fired power plants in Megalopolis basin, Greece. Radiat. Prot. Dosim.  156, 49– 58 ( 2013). Google Scholar CrossRef Search ADS   31 Fawzia, A. Impact of fertilizers on background radioactivity level in two newly developed desert areas. Radiat. Eff. Defects Solid  162, 31– 42 ( 2007). Google Scholar CrossRef Search ADS   32 European Union (E.U.). Practical use of the concepts of clearance and exception, Application of the concepts of exemption and clearance to natural radiation sources. Radiation Protection 122—Part II ( 2002). 33 Tracey, A. Plant Electrophysiology  ( Berlin-Heidelberg: Spinger) ( 2006) ISBN 978-3-540-32717-2. 34 Beck, H. L., Decompo, J. and Gologak, J. In situ Ge(Li) and NaI(Tl) gamma ray spectrometry. Health and Safety Laboratory AEC, New York, Report HASL 258 ( 1972). © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

NATURAL RADIOACTIVITY STUDIES OF PHOSPHATE FERTILIZERS APPLIED ON GREEK FARM SOILS USED FOR WHEAT CULTIVATION

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Abstract

Abstract Naturally Occurring Radionuclide Materials (NORM) such as fertilizers, agricultural soils fertilized and wheat grain cultivated at those soils were studied regarding the radiation hazard to the public and workers. Activity of 238U, 226Ra, 232Th and 40K was measured hermitical sealing the marinelli beaker using the Stopaq FN 2100L material that prevent 222Rn escape from the baker. The phosphate fertilization of soil did not affect natural radioactivity in comparison with non-fertilized soils and also did not affect natural radioactivity of wheat grain cultivated since only 40K was measured. The high mean value of Dair 211 nGy h−1 for fertilizers compared to soils fertilized 53 nGy h−1 highlights the raising concern about radioprotection of workers occupied with tasks involving fertilizers. The maximum external dose rate 1.15 mSv y−1 calculated for truck drivers involved to transportation section is similar to annual external effective dose limit 1 mSv. Real concerns rise for workers in the fertilizers storage and supply department since the maximum dose rate estimated is 1.98 mSv y−1, while the mean value is 0.89 mSv y−1. INTRODUCTION Phosphate fertilizers, used on agricultural lands, are materials known as Naturally Occurring Radioactive Material (NORM), as they contain significant amounts of natural radionuclides. Their production is based on the manufacture of phosphoric acid, by the wet or thermal process of phosphate rock(1). The main phosphate rock deposits are part of the apatite group, and are both of igneous and sedimentary origin. The phosphates of sedimentary origin, accounting for 85% of the phosphate rock contain 100–200 ppm uranium and 2–20 ppm thorium and show a positive correlation between uranium and P2O5 content. Igneous phosphates, which are of volcanic origin, contain <10 ppm uranium but they have an appreciable amount of thorium, up to 20 ppm and rare earths(2). The most common method applied for the production of phosphoric acid is the wet process acid, which is based on the chemical treatment of the primary phosphate rocks by using sulfuric acid. This method produces also a by-product, called phosphogysum, an insoluble calcium sulfate salt. Phosphogysum is filtered off and pumped as slurry to nearby ponds, where it stays for a period sufficient to allow complete deposition. After this period, it is stockpiled and is considered waste, due to its elevated levels of impurities. In the phosphate rock, the natural uranium and thorium are in equilibrium. During the industrial process their equilibrium is disturbed and the radionuclides migrate to intermediate, final and by-products(3). Approximately 80–90% of the 226Ra concentrates in phosphogysum and ~80–85% of uranium and 70% of thorium concentrate in phosphoric acid(4, 5). The natural radionuclides concentrations in phosphate fertilizers are of critical importance, due to the fact that they will affect humans via a number of pathways. The use of phosphate fertilizers on agricultural lands may lead to an increase of natural radioactivity in farm soil and agricultural products and therefore it may enhance the absorbed dose of the farmers by external radiation and the public by internal radiation due to products intake. Wheat is one of the most important crops that man consumes and covers the largest portion of agricultural land. According to International Atomic Energy the main objectives of environmental radiation monitoring are to verify compliance with authorized discharge limits, to provide information and data for dose assessment purposes, to assess the exposure or potential exposure of critical groups and populations, to check the conditions of operation and to provide warning of unusual conditions(6). So, higher dose is expected for the workers in agriculture (farmers) due to elevate time spending outdoors. Moreover, the contamination of the area surrounding a Phosphate Industry installations is a further pathway that affecting the people living around(4). Another sensitive group of people affected is the workers in the Industry, especially them occupied to storage-supply and transportation of the fertilizers. Gamma spectrometry is common method to measure natural radioactivity concentrations of NORM samples(7). All three naturally decay series have a Radon isotope, which as noble gas escapes from solid matrix, where it was trapped before. Once this happens, the equilibrium between the post-radon nuclides, many of which have short half-lives and decay rapidly, will be lost. The half-lives of 219Rn in the actinium series and 220Rn in the thorium series are very short and even if radon escapes, equilibrium will be re-established within minutes. That is not the case of 222Rn, with a half life of 3.825 days. After loss of 222Rn, there is ample time for the decay of the daughter nuclides preceding 210Pb before re-growth of the 222Rn. As the post-radon nuclides are measured to estimate 226Ra activity, loss of radon will affect the whole activity measurement process. The solution is to encapsulate the sample and wait for ~10 half-lives of the 222Rn to allow equilibrium to be re-established(8, 9). Radon emanation rate for soil samples depends mostly on four parameters: soil type, grain size, temperature and water content(10, 11). When plastic beakers are used the real radioactive equilibrium is not reached due to the escape of gaseous radon through plastic(12). Subsequently, the hermitical sealing of sample’s containers for the prevention of 222Rn escape is essential to achieved, in order to be guaranteed the accuracy of gamma spectrometry results. The objectives of the current study is to screen natural isotopes 238U, 226Ra, 232Th and 40K in phosphate fertilizers, farm soils and wheat grains in Greece, to secure hermitical sealing for marinelli beakers and to estimate the radiation hazard through the effective dose rate received by the public and the workers in the agriculture land as well as to fertilizing industry. EXPERIMETNAL A total of 100 samples were collected. The 20 samples of phosphate fertilizers, 40 samples of surface soil and 40 samples of mature wheat grains. The sampling sites were randomly selected of the cultivated land of Greece. All sites were consistently fertilized with phosphate fertilizers. Wheat grains have been chosen as it is estimated that the largest portion of the global agricultural land is devoted to wheat(13). The measured fertilizers were commercial compound fertilizers, which are generally preferred to single-nutrient materials. Their preference is mainly for the farmer’s convenience. The compound fertilizers contain two or more nutrients and are formed either by mechanical mixing or via chemical processes. Either way, the raw materials are the same, thus the radioactive concentrations of NORM do not differ. Each fertilizer is described by a grade, namely the guaranteed nutrient content expressed in weight percentages of N, P2O5 and K2O in that order: N-P-K(14). The conversion between P (%) and P2O5 (%) and between K (%) and K2O (%) is: P2O5 (%) = 2.29 P (%) and K2O (%) = 1.2 K (%)(15). Soil and wheat samples were collected from the agricultural land of Greece. Soil sampling was performed at depths up to 25 cm using a soil corer. The samples were crushed, homogenized and sieved through a 200 μm mesh. Wheat grain samples were in full maturation at the time of selection. All samples were prepared according to the official methodology(16). Before the beginning of measurements, using a volumetric container, 500 ml of each sample were weighed and packed into marinelli beakers. Marinelli beakers have conventional and amenable geometry. These containers are ideally suited for the determination of low levels of gamma activity of environmental samples. Their shape ensures that the geometry results in a higher counting efficiency. The material of beakers and their lid is polypropylene. The chosen marinelli beakers were specified for liquid and solid samples. They have maximum height 10.7 cm, maximum diameter 11.7 cm, maximum well diameter 7.7 cm and height of well 6.8 cm. The beakers and their lids were made of polypropylene. Finally, all samples, after the above mentioned procedure, were marked and cataloged. Sealing of marinelli beakers Preliminary measurements involved the testing of four materials for sealing marinelli beakers, in order to prevent 222Ra to escape from the samples. More specific, Room Temperature Vulcanization (RTV) silicone glue, an epoxy plastic-glue, a gaffer tape and a product for sealing walls were evaluated. The preliminary tests have shown that only the last material with commercial name Stopaq FN 2100L was a successful candidate for efficient sealing of marinelli beakers. Therefore, the following experiments involved this material only. Overall, 10 samples of fertilizers and 20 samples of soil were measured. Each sample was measured for the activity concentration of 226Ra and then it was sealed hermitically using the Stopaq FN 2100L. The sealed containers were stored for 4 weeks, which is the required period to establish secular equilibrium between 226Ra, 232Th and their daughters, and then measured again. The insulating material Stopaq is a compound suited for sealing of house walls, pipe and hollow spaces, against gases, moisture, standing water and running ground water leaks. It is non toxic, single component, plastic–elastic and water and gas tight synthetic polyolefin compound. Its density is 1.2–1.5 g/cm3 and its moisture absorption is 10–20% (ASTM D 570). The results of experiments proved that Stopaq is a very efficient material for the sealing of the containers. Moreover it is very suitable one for this purpose, because it does not destroy marinelli beakers, thus, it allows their reuse. Gamma spectrometric analysis The samples were measured with a p-type coaxial lead shielded intrinsic HPGe detector of 61 mm diameter and 32 mm length. The gamma ray spectrometer has a relative efficiency of 15% for 1332 keV and resolution expressed by the full width at half maximum (FWHM) of 0.876 and 1.888 keV for the 122 and 1332 keV gamma-rays energies of 57Co and 60Co, respectively. The energy calibration of the detector was carried out with the use of a standard source of Εckert & Ziegler, containing mix sources of 241Am, 109Cd, 57Co, 139Ce, 203Hg, 113Sn, 85Sr, 137Cs, 88Y and 60Co. The source had the same geometry with the marinelli containers which housed the samples (ANSI/IEE, Std 325-1986). Spectra were analyzed using the software Genie 2000, 3.1 of Canberra. The counting time was preset at 72 000 s. The radionuclides in the uranium and the thorium decay chains cannot be assumed to be in radioactive equilibrium in NORM samples. The radionuclide 226Ra in the uranium chain may have slightly different concentration than 238U because of possible separation due to their differences in water solubility and mobility(17, 18). For this reason, it is not safe enough to assume that each daughter nuclide will be equal to the 238U activity, as the theory of NORM decay series predicted. Table 1 shows the natural isotopes that were measured in this study and the energy peaks that were used for these measurements. The 232Th and 226Ra contents were calculated as the weighted mean value of 228Ac, 208Tl, 212Pb and 214Pb, 214Bi concentrations, respectively. Table 1. Measured isotopes with the corresponding gamma-energies, detection limits (LD) and minimum detectable activities (MDA) for 95% confidence interval. Parent isotope  Daughter isotope  Energya (keV)  LD (counts)  MDA (Bq)  238U  234mPa  1001  156  38  226Ra  214Pb  352  291  0.6  295  703  1.3  214Bi  609  417  1.1  1120  235  3.5  1764  196  3.9  232Th  212Pb  338  231  1.3  208Tl  583  307  1.2  228Ac  911  257  1.5  40K    1461  156  4.9  Parent isotope  Daughter isotope  Energya (keV)  LD (counts)  MDA (Bq)  238U  234mPa  1001  156  38  226Ra  214Pb  352  291  0.6  295  703  1.3  214Bi  609  417  1.1  1120  235  3.5  1764  196  3.9  232Th  212Pb  338  231  1.3  208Tl  583  307  1.2  228Ac  911  257  1.5  40K    1461  156  4.9  aData from Brookhaven National Laboratory NNDC library (http://nndc.bnl.gov/nudat2/). View Large Table 1. Measured isotopes with the corresponding gamma-energies, detection limits (LD) and minimum detectable activities (MDA) for 95% confidence interval. Parent isotope  Daughter isotope  Energya (keV)  LD (counts)  MDA (Bq)  238U  234mPa  1001  156  38  226Ra  214Pb  352  291  0.6  295  703  1.3  214Bi  609  417  1.1  1120  235  3.5  1764  196  3.9  232Th  212Pb  338  231  1.3  208Tl  583  307  1.2  228Ac  911  257  1.5  40K    1461  156  4.9  Parent isotope  Daughter isotope  Energya (keV)  LD (counts)  MDA (Bq)  238U  234mPa  1001  156  38  226Ra  214Pb  352  291  0.6  295  703  1.3  214Bi  609  417  1.1  1120  235  3.5  1764  196  3.9  232Th  212Pb  338  231  1.3  208Tl  583  307  1.2  228Ac  911  257  1.5  40K    1461  156  4.9  aData from Brookhaven National Laboratory NNDC library (http://nndc.bnl.gov/nudat2/). View Large The concentration of each radionuclide (R) was estimated according to following formula:   R=NM⋅ε⋅y⋅twhere N is the net peak area in the sample spectrum, ε is the efficiency at photopeak energy, t is the live time of the sample spectrum collection in seconds, M is the mass in kg of the measured sample and y is the emission probability of the gamma peak(8, 19). Corrections due to nuclide decay during the storing period and the measuring time of samples were not applied as the measured radioisotopes are long life radionuclides. Moreover, correction due to random summing was not applied either as the counting rate was too low. Finally, as the efficiency calibration was performed using a suitable standard source, as it was described above; corrections due to self attenuation and to coincidence summing were omitted. Table 1 also gives the detection limit (LD) and the minimum detectable activity (MDA), as they have been calculated, of a typical background, by using the following equation for 95% confidence level:   MDA=LDε⋅t⋅ywhere detection limit given as follows:   LD=2.71+3.29⋅[B(1+n2m)]1/2where n is the number of channels within the peak region and m is the number of channels of upper and lower background region, before and after the peak region(8). The efficiency calibration of the gamma spectrometry system was performed with the radionuclide specific efficiency method in order to avoid any uncertainty in gamma ray intensities, as well as the influences of coincidence summation and self-absorption effect of the emitting gamma photons. The reference calibration materials (IAEA, RG-set) were certified to be of high quality, and they were enclosed in Marinelli beakers used for the measuring of the samples. The processes of energy and efficiency calibrations were repeated every week in order to be assured the quality and the accuracy of gamma spectrometry measurements(8, 20). The background due to the environment radiation, around the spectrometer, was recorded for 72 000 s. A sealed Marinelli beaker filled with inorganic, no radioactive material, with density 1 g/cm3, similar to the samples, was placed over the crystal, in order to simulate a sample’s measurement. The spectrum of the background was deducted from each sample spectrum, in order to make a peaked background correction possible. RESULTS AND DISCUSSION The activity concentrations of 226Ra in fertilizer and soil samples before and after sealing with Stopaq and storing for 4 weeks are given in Table 2. Difference is presented both among soil and fertilizer samples before and after sealing and storage. The selected material Stopaq FN 2100L prevents 222Rn to escape, and moreover it does not destroy marinelli beakers. However, the spectroscopy system applied does not allow differences lower than 20% (MDA) to be estimated with uncertainty lower than 50%; so few of the data obtained could be used. These differences express the 222Rn emanation factor that has mean value 30% for soils and 19% for fertilizer samples that are in agreement with previous published data in literature(9, 20, 21). Table 2. 226Ra (Bq kg−1) Activity concentration in soil and fertilizer samples before and after sealing and the corresponding increase—emanation factor (%).   226Ra ± σ  Radon emanation  Closed    Sealed    Increase  ±σ  S  15  1  19  2  MDA  S  25  2  33  2  24%  40%  S  17  2  20  2  MDA  S  49  3  69  5  29%  29%  S  54  4  70  4  23%  35%  S  28  3  40  3  30%  34%  S  19  2  23  2  MDA  S  23  2  33  3  30%  40%  S  30  3  38  3  MDA  S  37  3  40  3  MDA  S  10  2  11  2  MDA  S  13  2  27  3  52%  26%  S  16  2  18  2  MDA  S  14  2  19  2  MDA  S  11  2  21  3  48%  36%  S  17  2  18  2  MDA  S  19  2  22  2  MDA  S  18  2  20  2  MDA  S  27  3  36  3  25%  45%  S  35  3  40  3  MDA  F  126  7  135  9  MDA  F  243  16  334  22  27%  30%  F  266  18  336  22  21%  41%  F  103  7  123  9  MDA  F  30  3  34  3  MDA  F  227  19  276  19  MDA  F  94  6  135  9  30%  26%  F  114  10  135  9  MDA  F  182  12  228  8  20%  31%  F  207  14  242  16  MDA    226Ra ± σ  Radon emanation  Closed    Sealed    Increase  ±σ  S  15  1  19  2  MDA  S  25  2  33  2  24%  40%  S  17  2  20  2  MDA  S  49  3  69  5  29%  29%  S  54  4  70  4  23%  35%  S  28  3  40  3  30%  34%  S  19  2  23  2  MDA  S  23  2  33  3  30%  40%  S  30  3  38  3  MDA  S  37  3  40  3  MDA  S  10  2  11  2  MDA  S  13  2  27  3  52%  26%  S  16  2  18  2  MDA  S  14  2  19  2  MDA  S  11  2  21  3  48%  36%  S  17  2  18  2  MDA  S  19  2  22  2  MDA  S  18  2  20  2  MDA  S  27  3  36  3  25%  45%  S  35  3  40  3  MDA  F  126  7  135  9  MDA  F  243  16  334  22  27%  30%  F  266  18  336  22  21%  41%  F  103  7  123  9  MDA  F  30  3  34  3  MDA  F  227  19  276  19  MDA  F  94  6  135  9  30%  26%  F  114  10  135  9  MDA  F  182  12  228  8  20%  31%  F  207  14  242  16  MDA  MDA referred to values lower than 20%. View Large Table 2. 226Ra (Bq kg−1) Activity concentration in soil and fertilizer samples before and after sealing and the corresponding increase—emanation factor (%).   226Ra ± σ  Radon emanation  Closed    Sealed    Increase  ±σ  S  15  1  19  2  MDA  S  25  2  33  2  24%  40%  S  17  2  20  2  MDA  S  49  3  69  5  29%  29%  S  54  4  70  4  23%  35%  S  28  3  40  3  30%  34%  S  19  2  23  2  MDA  S  23  2  33  3  30%  40%  S  30  3  38  3  MDA  S  37  3  40  3  MDA  S  10  2  11  2  MDA  S  13  2  27  3  52%  26%  S  16  2  18  2  MDA  S  14  2  19  2  MDA  S  11  2  21  3  48%  36%  S  17  2  18  2  MDA  S  19  2  22  2  MDA  S  18  2  20  2  MDA  S  27  3  36  3  25%  45%  S  35  3  40  3  MDA  F  126  7  135  9  MDA  F  243  16  334  22  27%  30%  F  266  18  336  22  21%  41%  F  103  7  123  9  MDA  F  30  3  34  3  MDA  F  227  19  276  19  MDA  F  94  6  135  9  30%  26%  F  114  10  135  9  MDA  F  182  12  228  8  20%  31%  F  207  14  242  16  MDA    226Ra ± σ  Radon emanation  Closed    Sealed    Increase  ±σ  S  15  1  19  2  MDA  S  25  2  33  2  24%  40%  S  17  2  20  2  MDA  S  49  3  69  5  29%  29%  S  54  4  70  4  23%  35%  S  28  3  40  3  30%  34%  S  19  2  23  2  MDA  S  23  2  33  3  30%  40%  S  30  3  38  3  MDA  S  37  3  40  3  MDA  S  10  2  11  2  MDA  S  13  2  27  3  52%  26%  S  16  2  18  2  MDA  S  14  2  19  2  MDA  S  11  2  21  3  48%  36%  S  17  2  18  2  MDA  S  19  2  22  2  MDA  S  18  2  20  2  MDA  S  27  3  36  3  25%  45%  S  35  3  40  3  MDA  F  126  7  135  9  MDA  F  243  16  334  22  27%  30%  F  266  18  336  22  21%  41%  F  103  7  123  9  MDA  F  30  3  34  3  MDA  F  227  19  276  19  MDA  F  94  6  135  9  30%  26%  F  114  10  135  9  MDA  F  182  12  228  8  20%  31%  F  207  14  242  16  MDA  MDA referred to values lower than 20%. View Large The fertilizers’ measurements are presented on Table 3. The mean concentration values of 238U, 226Ra, 232Th and 40K were 377, 191, 22 and 2622 Bq kg−1, respectively. These values are in agreement with the concentrations reported in past studies(3, 4, 22–25) that are summarized in Table 4. Moreover, measured concentrations are lower than the permissible international radioactivity levels, given by IAEA: 1000 Bq kg−1 for 238U and 232Th series radionuclides and 10 000 Bq kg−1 for 40K(24). In fertilizers, 238U concentrations ranged from lower than the 38–703 Bq kg−1, with mean value of 377 Bq kg−1, 226Ra concentrations ranged from lower than the 1–529 Bq kg−1, with mean value of 191 Bq kg−1, 232Th concentrations ranged from lower than the 1–95 Bq kg−1 with mean value of 22 Bq kg−1 and 40K concentrations ranged from 37 to 4483 Bq/kg, with mean value of 2622 Bq kg−1. Measured concentrations of 238U, 226Ra in fertilizers, as shown in Figure 1, are checked whether any correlation exists between them and the % P2O5 concentrations. The value of correlation coefficient between 238U and P2O5 is higher than the one of 226Ra. This result is explained by the fact that during the fertilizers production, the greater part of, concentrates in phosphogysum while the greater part of uranium is transferred to phosphoric acid and thus in fertilizers. The correlation coefficient value between 232Th and P2O5 is similar to the 226Ra one. However, strong positive correlation between the concentrations of 40K and % K2O is proved, as it is shown in Figure 2. The fact that the line does not intercept zero, reveals that 40K is also present in phosphate rocks. Fertilizers with no K contain has one order of magnitude lower 40K than the others originated from the phosphate rocks Table 3. Fertilizers activity concentrations (Bq kg−1) as well as the related dose air, Dair (nGy h−1) and corresponding uncertainties. Fertilizer grade (N-P-K)  238U ± σ  226Ra ± σ  232Th ± σ  40K ± σ  Dair ± σ  0-0-50  <38  <1  <1  8325 ± 113  347 ± 22  16-5-8  41 ± 19  7 ± 1  <1  1707 ± 25  74 ± 2  20-5-10  88 ± 32  146 ± 1  36 ± 1  2603 ± 37  198 ± 4  20-8-6  230 ± 36  47 ± 1  <1  1532 ± 9  86 ± 1  24-8-7  250 ± 47  214 ± 2  54 ± 1  1694 ± 25  202 ± 4  20-10-0  575 ± 78  332 ± 2  5 ± 1  144 ± 4  162 ± 3  20-10-0  546 ± 78  222 ± 2  <1  428 ± 9  120 ± 2  12-11-18  140 ± 44  242 ± 2  56 ± 1  4483 ± 62  332 ± 11  12-11-18  274 ± 58  187 ± 1  <1  4098 ± 57  257 ± 7  12-12-17  655 ± 97  158 ± 1  <1  4164 ± 57  247 ± 7  12-12-17  495 ± 79  407 ± 3  53 ± 1  4311 ± 60  400 ± 11  25-15-0  553 ± 77  397 ± 3  77 ± 2  156 ± 5  236 ± 6  11-15-15  256 ± 48  52 ± 1  7 ± 1  3326 ± 47  167 ± 5  15-15-15  205 ± 52  228 ± 2  14 ± 1  3202 ± 45  247 ± 6  15-10-15  703 ± 104  299 ± 2  <1  3714 ± 52  293 ± 7  11-15-15  388 ± 67  31 ± 1  <1  3716 ± 52  169 ± 5  15-15-15  602 ± 94  529 ± 5  95 ± 2  3960 ± 55  467 ± 19  21-17-0  221 ± 35  159 ± 1  23 ± 1  680 ± 11  116 ± 2  16-20-0  663 ± 87  160 ± 1  4 ± 0  37 ± 2  78 ± 1  16-20-0  654 ± 85  11 ± 1  <1  160 ± 4  12 ± 1  Mean value ± σ  377  191  22  262  211  (Range)  (<38–703)  (<1–529)  (<1–95)  (238–4483)  (12–467)  Fertilizer grade (N-P-K)  238U ± σ  226Ra ± σ  232Th ± σ  40K ± σ  Dair ± σ  0-0-50  <38  <1  <1  8325 ± 113  347 ± 22  16-5-8  41 ± 19  7 ± 1  <1  1707 ± 25  74 ± 2  20-5-10  88 ± 32  146 ± 1  36 ± 1  2603 ± 37  198 ± 4  20-8-6  230 ± 36  47 ± 1  <1  1532 ± 9  86 ± 1  24-8-7  250 ± 47  214 ± 2  54 ± 1  1694 ± 25  202 ± 4  20-10-0  575 ± 78  332 ± 2  5 ± 1  144 ± 4  162 ± 3  20-10-0  546 ± 78  222 ± 2  <1  428 ± 9  120 ± 2  12-11-18  140 ± 44  242 ± 2  56 ± 1  4483 ± 62  332 ± 11  12-11-18  274 ± 58  187 ± 1  <1  4098 ± 57  257 ± 7  12-12-17  655 ± 97  158 ± 1  <1  4164 ± 57  247 ± 7  12-12-17  495 ± 79  407 ± 3  53 ± 1  4311 ± 60  400 ± 11  25-15-0  553 ± 77  397 ± 3  77 ± 2  156 ± 5  236 ± 6  11-15-15  256 ± 48  52 ± 1  7 ± 1  3326 ± 47  167 ± 5  15-15-15  205 ± 52  228 ± 2  14 ± 1  3202 ± 45  247 ± 6  15-10-15  703 ± 104  299 ± 2  <1  3714 ± 52  293 ± 7  11-15-15  388 ± 67  31 ± 1  <1  3716 ± 52  169 ± 5  15-15-15  602 ± 94  529 ± 5  95 ± 2  3960 ± 55  467 ± 19  21-17-0  221 ± 35  159 ± 1  23 ± 1  680 ± 11  116 ± 2  16-20-0  663 ± 87  160 ± 1  4 ± 0  37 ± 2  78 ± 1  16-20-0  654 ± 85  11 ± 1  <1  160 ± 4  12 ± 1  Mean value ± σ  377  191  22  262  211  (Range)  (<38–703)  (<1–529)  (<1–95)  (238–4483)  (12–467)  View Large Table 3. Fertilizers activity concentrations (Bq kg−1) as well as the related dose air, Dair (nGy h−1) and corresponding uncertainties. Fertilizer grade (N-P-K)  238U ± σ  226Ra ± σ  232Th ± σ  40K ± σ  Dair ± σ  0-0-50  <38  <1  <1  8325 ± 113  347 ± 22  16-5-8  41 ± 19  7 ± 1  <1  1707 ± 25  74 ± 2  20-5-10  88 ± 32  146 ± 1  36 ± 1  2603 ± 37  198 ± 4  20-8-6  230 ± 36  47 ± 1  <1  1532 ± 9  86 ± 1  24-8-7  250 ± 47  214 ± 2  54 ± 1  1694 ± 25  202 ± 4  20-10-0  575 ± 78  332 ± 2  5 ± 1  144 ± 4  162 ± 3  20-10-0  546 ± 78  222 ± 2  <1  428 ± 9  120 ± 2  12-11-18  140 ± 44  242 ± 2  56 ± 1  4483 ± 62  332 ± 11  12-11-18  274 ± 58  187 ± 1  <1  4098 ± 57  257 ± 7  12-12-17  655 ± 97  158 ± 1  <1  4164 ± 57  247 ± 7  12-12-17  495 ± 79  407 ± 3  53 ± 1  4311 ± 60  400 ± 11  25-15-0  553 ± 77  397 ± 3  77 ± 2  156 ± 5  236 ± 6  11-15-15  256 ± 48  52 ± 1  7 ± 1  3326 ± 47  167 ± 5  15-15-15  205 ± 52  228 ± 2  14 ± 1  3202 ± 45  247 ± 6  15-10-15  703 ± 104  299 ± 2  <1  3714 ± 52  293 ± 7  11-15-15  388 ± 67  31 ± 1  <1  3716 ± 52  169 ± 5  15-15-15  602 ± 94  529 ± 5  95 ± 2  3960 ± 55  467 ± 19  21-17-0  221 ± 35  159 ± 1  23 ± 1  680 ± 11  116 ± 2  16-20-0  663 ± 87  160 ± 1  4 ± 0  37 ± 2  78 ± 1  16-20-0  654 ± 85  11 ± 1  <1  160 ± 4  12 ± 1  Mean value ± σ  377  191  22  262  211  (Range)  (<38–703)  (<1–529)  (<1–95)  (238–4483)  (12–467)  Fertilizer grade (N-P-K)  238U ± σ  226Ra ± σ  232Th ± σ  40K ± σ  Dair ± σ  0-0-50  <38  <1  <1  8325 ± 113  347 ± 22  16-5-8  41 ± 19  7 ± 1  <1  1707 ± 25  74 ± 2  20-5-10  88 ± 32  146 ± 1  36 ± 1  2603 ± 37  198 ± 4  20-8-6  230 ± 36  47 ± 1  <1  1532 ± 9  86 ± 1  24-8-7  250 ± 47  214 ± 2  54 ± 1  1694 ± 25  202 ± 4  20-10-0  575 ± 78  332 ± 2  5 ± 1  144 ± 4  162 ± 3  20-10-0  546 ± 78  222 ± 2  <1  428 ± 9  120 ± 2  12-11-18  140 ± 44  242 ± 2  56 ± 1  4483 ± 62  332 ± 11  12-11-18  274 ± 58  187 ± 1  <1  4098 ± 57  257 ± 7  12-12-17  655 ± 97  158 ± 1  <1  4164 ± 57  247 ± 7  12-12-17  495 ± 79  407 ± 3  53 ± 1  4311 ± 60  400 ± 11  25-15-0  553 ± 77  397 ± 3  77 ± 2  156 ± 5  236 ± 6  11-15-15  256 ± 48  52 ± 1  7 ± 1  3326 ± 47  167 ± 5  15-15-15  205 ± 52  228 ± 2  14 ± 1  3202 ± 45  247 ± 6  15-10-15  703 ± 104  299 ± 2  <1  3714 ± 52  293 ± 7  11-15-15  388 ± 67  31 ± 1  <1  3716 ± 52  169 ± 5  15-15-15  602 ± 94  529 ± 5  95 ± 2  3960 ± 55  467 ± 19  21-17-0  221 ± 35  159 ± 1  23 ± 1  680 ± 11  116 ± 2  16-20-0  663 ± 87  160 ± 1  4 ± 0  37 ± 2  78 ± 1  16-20-0  654 ± 85  11 ± 1  <1  160 ± 4  12 ± 1  Mean value ± σ  377  191  22  262  211  (Range)  (<38–703)  (<1–529)  (<1–95)  (238–4483)  (12–467)  View Large Table 4. Activity concentrations (Bq kg−1) of fertilizers in bibliography. Country  238U  226Ra  232Th  40K  References  Egypt    1–950  1–162  10–23 845  (23)  Brazil  182–1158  <1.3–879  81–546  —  (3)  Malaysia    0.4–112  0.8–48  13–279  (24)  Italy  190–650  6–230  —  220–5200  (23)  Greece    16–4584    4–5254  (22)  Greece  312–936  19–1129  <1–12  53–16 700  (4)  Greece  <38–703  <1–529  <1–95  38–8325  This study  Global  23–2100  9–850  10–63  41–5900  (25)  Country  238U  226Ra  232Th  40K  References  Egypt    1–950  1–162  10–23 845  (23)  Brazil  182–1158  <1.3–879  81–546  —  (3)  Malaysia    0.4–112  0.8–48  13–279  (24)  Italy  190–650  6–230  —  220–5200  (23)  Greece    16–4584    4–5254  (22)  Greece  312–936  19–1129  <1–12  53–16 700  (4)  Greece  <38–703  <1–529  <1–95  38–8325  This study  Global  23–2100  9–850  10–63  41–5900  (25)  View Large Table 4. Activity concentrations (Bq kg−1) of fertilizers in bibliography. Country  238U  226Ra  232Th  40K  References  Egypt    1–950  1–162  10–23 845  (23)  Brazil  182–1158  <1.3–879  81–546  —  (3)  Malaysia    0.4–112  0.8–48  13–279  (24)  Italy  190–650  6–230  —  220–5200  (23)  Greece    16–4584    4–5254  (22)  Greece  312–936  19–1129  <1–12  53–16 700  (4)  Greece  <38–703  <1–529  <1–95  38–8325  This study  Global  23–2100  9–850  10–63  41–5900  (25)  Country  238U  226Ra  232Th  40K  References  Egypt    1–950  1–162  10–23 845  (23)  Brazil  182–1158  <1.3–879  81–546  —  (3)  Malaysia    0.4–112  0.8–48  13–279  (24)  Italy  190–650  6–230  —  220–5200  (23)  Greece    16–4584    4–5254  (22)  Greece  312–936  19–1129  <1–12  53–16 700  (4)  Greece  <38–703  <1–529  <1–95  38–8325  This study  Global  23–2100  9–850  10–63  41–5900  (25)  View Large Figure 1. View largeDownload slide Activity concentrations of 238U and 226Ra (Bq kg−1) correlated to P2O5 (%) content of fertilizer. Figure 1. View largeDownload slide Activity concentrations of 238U and 226Ra (Bq kg−1) correlated to P2O5 (%) content of fertilizer. Figure 2. View largeDownload slide Activity concentrations of 40K (Bq kg−1) correlated to K2O (%) content of fertilizer. Figure 2. View largeDownload slide Activity concentrations of 40K (Bq kg−1) correlated to K2O (%) content of fertilizer. The farm soil samples concentrations are shown on Figure 3 ranging from 8 to 68 Bq kg−1 for 226Ra and from 8 to 78 Bq kg−1 for 232Th and from 185 to 868 Bq kg−140K, respectively. These values are comparable to world average concentrations, which are equal to 35, 30 and 400 Bq/kg for 226Ra, 232Th and 40K, respectively(18) as well as to values reported in previous studies regarding Greek soils(26–31) that are summarized in Table 5. Both in Table 5 as well as in Figure 4, the data of Chernobyl 137Cs are also presented since the specific radioisotope could be considered now on as ‘natural occurring’ after all these years that it will be remain in any ecosystem of the environment. The specific data ranged between 4 and 33 Bq kg−1 comparable with 226Ra and 232Th concentration in some soils samples but the dose absorbed by the public is similar and even lower than the uncertainty of the dose received by NORM radioisotopes(28). Therefore, no more discussion on Chernobyl 137Cs influence on effective gamma dose rate is presented in the manuscript. The natural radiatiactivity results indicate that phosphate fertilization did not change the 226Ra, 232Th and 40K concentrations in comparison with non-fertilized soils. These results are in agreement with similar studies in other countries(3, 31). Figure 3. View largeDownload slide Mapping results of 226Ra 232Th and 40K activity concentrations (Bq kg−1) in Greek fertilized farm soils (the number of samples measured is presented inside the bracket). Figure 3. View largeDownload slide Mapping results of 226Ra 232Th and 40K activity concentrations (Bq kg−1) in Greek fertilized farm soils (the number of samples measured is presented inside the bracket). Table 5. Activity concentrations (Bq kg−1) and absorbed dose (nGy h−1) of Greek soils in bibliography. Regions  226Ra  232Th  40K  Dair [NORM]  Dair [137Cs]  References  Greece  9–54  3–58  160–750      (26)  Greece  4–44  5–37  155–575      (27)  Greece        5–220  0.3–1.1  (28)  Greek Islands  7–310  2–269  230–1796      (29)  S-W Greece  6–92  3–49  120–564      (30)  N-E Greece  8–68  8–78  185–868      This study  Global  35  30  400  57    (25)  Regions  226Ra  232Th  40K  Dair [NORM]  Dair [137Cs]  References  Greece  9–54  3–58  160–750      (26)  Greece  4–44  5–37  155–575      (27)  Greece        5–220  0.3–1.1  (28)  Greek Islands  7–310  2–269  230–1796      (29)  S-W Greece  6–92  3–49  120–564      (30)  N-E Greece  8–68  8–78  185–868      This study  Global  35  30  400  57    (25)  View Large Table 5. Activity concentrations (Bq kg−1) and absorbed dose (nGy h−1) of Greek soils in bibliography. Regions  226Ra  232Th  40K  Dair [NORM]  Dair [137Cs]  References  Greece  9–54  3–58  160–750      (26)  Greece  4–44  5–37  155–575      (27)  Greece        5–220  0.3–1.1  (28)  Greek Islands  7–310  2–269  230–1796      (29)  S-W Greece  6–92  3–49  120–564      (30)  N-E Greece  8–68  8–78  185–868      This study  Global  35  30  400  57    (25)  Regions  226Ra  232Th  40K  Dair [NORM]  Dair [137Cs]  References  Greece  9–54  3–58  160–750      (26)  Greece  4–44  5–37  155–575      (27)  Greece        5–220  0.3–1.1  (28)  Greek Islands  7–310  2–269  230–1796      (29)  S-W Greece  6–92  3–49  120–564      (30)  N-E Greece  8–68  8–78  185–868      This study  Global  35  30  400  57    (25)  View Large Figure 4. View largeDownload slide Mapping results of 137Cs activity concentrations (Bq kg−1) in Greek soils. Figure 4. View largeDownload slide Mapping results of 137Cs activity concentrations (Bq kg−1) in Greek soils. In wheat grains, specific radioactivity values of 226Ra and 232Th were below MDA of the detector, so only concentration of 40K is measured ranging from 109 to 200 Bq kg−1, with a mean value of 139 Bq kg−1. The respective range in soil samples was from 185 to 868 Bq kg−1, with mean value of 456 Bq kg−1. Figure 5 shows the stability of activity concentration of 40K in wheat grains irrespectively of its wide variation in soils. This stability confirms the potassium’s homeostasis of cereals that is known to occur(32, 33). This is because when K reaches the necessary level required by the organism of the reference person, it remains stable. Grasses of graminae family absorbs stable and only the amounts of potassium that can be metabolized. According to the measured activity concentrations in wheat grain, the use of fertilizers does not affect natural radioactivity of wheat grain. Figure 5. View largeDownload slide Activity concentrations (Bq kg−1) of 40K in fertilized soils correlated to 40K in wheat grains cultivated to these soils. Figure 5. View largeDownload slide Activity concentrations (Bq kg−1) of 40K in fertilized soils correlated to 40K in wheat grains cultivated to these soils. Dose assessments The gamma dose rate received by the public 1 m above the ground (Dair) in units of n Gy h−1 can be calculated by the following equation, assuming uniform distribution of naturally occurring radioactive nuclides: Dair (nGy h−1) = 0.462 CRa + 0.604 CTh + 0.0417 CK, where, CRa, CTh and CK are the specific activity in Bq/kg of 226Ra, 232Th and 40K, respectively. 238U radionuclides are ignored, as its gamma radiation has low energies and rate(18, 34). This formula is applicable to workers on agriculture land (farmers) working 50% of their time outdoors. In case of workers in Phosphate Industry it is applicable to workers in transportations (track drivers) as an upper limit but for workers in the fertilizers storage and supply department working indoors should be different since they work inside high piles of fertilizes packs. A model that describes the above situation could be the consideration of a cavity-spherical shell with 1 m in diameter surrounding by fertilizer and likewise the upper limit of dose air calculated using the formula: (nGy h−1) = 0.954 CRa + 1.128 CTh + 0.0830CK(18). The annual effective dose received by the population Eext, (mSv y−1) was estimated as follows considering the proper conversion coefficient (F) from absorbed dose in air to effective dose (0.7 Sv Gy−1) and the occupancy factor T (h y−1): Eext = 10−6 ×Dair × T × F(18). The occupancy factor, which implies that 50% of time is spent outdoors, is equal to 4400 h for the farmers and 3600 h for track drivers while 2920 h y−1 for workers at storage and supply department in the Phosphate Industry. Especially in case of the workers at storage and supply department a supplement study has been performed on internal alpha radiation due to ‘radon problem’ since high value of radium content in fertilizer in association to elevate radon emanation factor triggers high radon concentrations indoors. The range of public absorbed dose rate from soil due to NORM radioisotopes was calculated from 19 to 107 with mean value 53 nGy h−1. This value is very close to the average global 57 nGy h−1 as well as Greek data (Table 5), therefore, no problem is raised for due to soil radioactivity. The annual effective dose received by the public ranges from 0.03 to 0.15 mSv y−1 similar to worldwide mean value (Table 6). In case of workers in the field although double effective dose rate is calculated reaching values 0.3 mSv y−1 still no problem is raised for due to soil radioactivity since is lower than external effective dose limit 1 mSv y−1(18). Table 6. Annual effective dose (mSv y−1) received due to phosphate fertilizers application in the environment.     Eext  Eint  Range  Mean  Range  Mean  Outdoors  Public  0.03–0.15  0.07      Farmer  0.05–0.30  0.15      Truck drivers  0.03–1.15  0.52      Indoors  Storage-supply  0.05–1.98  0.98  0.01–0.27  0.27      Eext  Eint  Range  Mean  Range  Mean  Outdoors  Public  0.03–0.15  0.07      Farmer  0.05–0.30  0.15      Truck drivers  0.03–1.15  0.52      Indoors  Storage-supply  0.05–1.98  0.98  0.01–0.27  0.27  View Large Table 6. Annual effective dose (mSv y−1) received due to phosphate fertilizers application in the environment.     Eext  Eint  Range  Mean  Range  Mean  Outdoors  Public  0.03–0.15  0.07      Farmer  0.05–0.30  0.15      Truck drivers  0.03–1.15  0.52      Indoors  Storage-supply  0.05–1.98  0.98  0.01–0.27  0.27      Eext  Eint  Range  Mean  Range  Mean  Outdoors  Public  0.03–0.15  0.07      Farmer  0.05–0.30  0.15      Truck drivers  0.03–1.15  0.52      Indoors  Storage-supply  0.05–1.98  0.98  0.01–0.27  0.27  View Large The corresponding mean dose rate 211 nGy h−1 due to external radiation at 1 m from the source-fertilizers (Table 3) was calculated that is four times higher than average global. Calculated Dair ranged from 12 to 467 nGy h−1 and only four samples gave dose rates around the global mean while most fertilizers gave dose rate up to 5–10 times higher. The high value of Dair for fertilizers samples highlights the necessity of raising concern about the radioprotection of workers occupied with operations involving fertilizers. In case of the workers at transportations section (track drivers) the mean annual effective dose received 0.5 mSv y−1 is higher than maximum dose received by the farmers. The maximum dose rate 1.15 mSv y−1 calculated for truck drives is similar to annual external effective dose limit 1 mSv y−1. Real concerns rise for workers in the fertilizers storage and supply department since the maximum dose rate estimated is 1.98 mSv y−1, while the mean value is 0.89 mSv y−1. Especially these workers are exposed to supplement internal radiation due to radon inhalation leaving in high indoors radon environment since high value of radium content in fertilizer (Table 3) in association to elevate radon emanation factor (Table 2) triggers high radon concentrations indoors Based on model calculations of effective dose due to internal radiation (Eint) indoors proposed in Ref. (20) a mean dose value 0.07 mSv and maximum 0.27 mSv y−1 is estimated, lower than the internal effective dose limit 1.5 mSv y−1(18). However, the total maximum increment in effective dose received by industry worker could reach 2.25 mSv y−1 values similar to the annual limit of external plus internal exposure received by the public. CONCLUSIONS The objectives of the current study is to screen natural isotopes 238U, 226Ra, 232Th and 40K in phosphate fertilizers, farm soils and wheat grains in Greece hermitical sealing the marinelli beakers and to estimate any radiation hazard to the public. A sealing material, Stopaq FN 2100 L, was proved suitable in order to prevent 222Rn escape from sample matrix through the baker. The mean value and range of radioactive concentrations of fertilizers used in Greece were found to be lower than the permitted levels. The phosphate fertilization of soil did not change 226Ra, 232Th and 40K concentrations in comparison with non-fertilized soils. The use of fertilizers did not affect natural radioactivity of wheat grain since in wheat samples, only 40K was detectable with apparent stability confirming potassium’s homeostasis of cereals. The high mean value of Dair 211 nGy h−1 for fertilizers compared to soils fertilized 53 nGy h−1 highlights the raising concern about radioprotection of workers occupied with tasks involving fertilizers. The maximum external dose rate 1.15 mSv y−1 calculated for truck drivers involved to transportation section is similar to annual external effective dose limit 1 mSv. 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Radiation Protection DosimetryOxford University Press

Published: Feb 3, 2018

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