radiometric assessment of natural radioactivity levels ...

Radiation Protection Dosimetry (2013), Vol. 156, No. 1, pp. 59 –67 Advance Access publication 17 March 2013

doi:10.1093/rpd/nct039

RADIOMETRIC ASSESSMENT OF NATURAL RADIOACTIVITY LEVELS OF AGRICULTURAL SOIL SAMPLES COLLECTED IN DAKAHLIA, EGYPT Shams A. M. Issa* Physics Department, Faculty of Sciences, Al-Azhar University (Assiut branch), Assiut 71524, Egypt *Corresponding author: [email protected] Received July 16 2012, revised January 15 2013, accepted February 24 2013

INTRODUCTION In addition to being the main source of continuous radiation exposure to human, soil acts as a medium of migration for the transfer of radionuclides to the biological systems and hence, it is the basic indicator of radiological contamination in the environment. Moreover, the soil radioactivity is usually important for the purposes of establishing baseline data for future radiation impact assessment, radiation protection and exploration(1). The dose rates vary from one place to another depending upon the concentration of natural radionuclides such as 238U, 226Ra, 232 Th and 40K present in soil. These radionuclides pose exposure risks externally due to their gammaray emissions and internally due to radon and its progeny that emit alpha particles(2). The assessment of the activity concentrations of natural radionuclides is of particular importance because it embodies an important contribution to the external dose of the population. In this range, the United Nations Scientific Committee on the Effects of the Atomic Radiation (UNSCEAR) provides a direct correlation between the activity of 238U, 232Th and 40K in soils and the external doses absorbed by the population(3). The radionuclides present in soils can pass on to the food chain and the air, contributing to the internal dose received by the population. The present study examines the concentrations and the distribution of natural radioactivity in order to evaluate the current state of the environmental quality in the soils collected from 10 locations in Dakahlia (El-Senbllawen, Meit Ghamr, Mansoura,

Aga, Tmayy Alimdid, Belkas, Talkha, Jamasah, Shirbin and Minyat Elnassr), Egypt. The study area has many industries such as chemical, paper, organic fertilisers and construction materials, and the soils of the study region are used as a construction material. The Dakahlia governorate is a large farm, where the cultivated area of 642.339 acres represent 8.3% of the cultivated area at the level of the republic, and produce 13% of the cotton crop; 19% of the rice crop; 10% of the wheat crop; 5% of the maize crop also achieved high rates of production of major crops in the above rates on the level of the Republic. MATERIALS AND METHODS Study area Dakahlia is a governorate in the east of the Delta of the Nile that covers 3.459 km2 and it has a population of 5 million. It locates in a very strategic location overlooking the Damietta branch of the Nile River and the Mediterranean Sea coast as shown in Figure 1. The main economic resources of the governorate are derived from agriculture, as well as food, logistic and furniture industries. The governorate includes a wide range of agricultural lands and is famous for producing some agricultural products such as rice, wheat and cotton and many other crops. Dakahleya is famous for its ginning cotton and textile factories, petrochemical, paper, organic fertilisers and construction material industries and many other industries that depend mainly on agricultural products of the governorate.

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Determination of the natural radioactivity has been carried out, by using a gamma-ray spectrometry [NaI (Tl) 300 3 300 ] system, in surface soil samples collected from various locations in Dakahlia governorate, Egypt. These locations form the agriculturally important regions of Egypt. The study area has many industries such as chemical, paper, organic fertilisers and construction materials, and the soils of the study region are used as a construction material. Therefore, it becomes necessary to study the natural radioactivity levels in soil to assess the dose for the population in order to know the health risks. The activity concentrations of 226Ra, 232Th and 40K in the soil ranged from 5.7 + 0.3 to 140 + 7, from 9.0 + 0.4 to 139 + 7 and from 22 + 1 to 319 + 16 Bq kg21, respectively. The absorbed dose rate, annual effective dose rate, radium equivalent (Req), excess lifetime cancer risk, hazard indices (Hex and Hin) and annual gonadal dose equivalent, which resulted from the natural radionuclides in the soil were calculated.

S. A. M. ISSA


Figure 1. Location map of the studied areas in Dakahlia.

60

SOIL, EXCESS LIFETIME CANCER RISK, ANNUAL GONADAL

In order to measure the natural radioactivity in soil, surface soil samples were collected from different locations of the investigated area: three samples from El-Senbllawen, Meit Ghamr, Mansoura, Aga, Belkas and Talkha, two samples from Shirbin, Tmayy Alimdid and Minyat Elnassr and one sample from Jamasah (see Table 1). Samples from each location were collected by digging a hole at a depth of 0.5 m below the ground surface. The samples were dried in an oven at 1108C. The samples were crushed, homogenised and sieved through a 200 mesh, which is the optimum size enriched in heavy minerals. The samples were weighed and transferred to polyethylene bottles of 350 cm3 volume. The beakers were completely sealed for 4 weeks to reach a secular equilibrium, where the rate of decay of the progeny becomes equal to that of the parent (radium and thorium)(4) within the volume and the progeny will also remain in the sample.

Instrumentation and calibration Activity measurements were performed by a gamma-ray spectrometer, employing a scintillation detector (33 inch). It is hermetically sealed

Table 1. Locations of the study areas. Location

El-Senbllawein Miet Ghamr Mansoura Shirbin Aga Talkha Belkas Tmayy Alimdid Minyat Elnassr Jamasah

Code no.

Latitude (North)

Longitude (East)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

308520 47.2300

318270 46.7100

308430 10.5200

318150 33.6900

31820 26.9300

318220 42.7300

318110 26.7500

318310 27.4400

308560 30.1800

318170 36.4900

31830 17.6900

318220 48.8700

318120 50.7500

318210 27.7600

308560 40.9300

318310 43.6700

318250 30.4000

318410 47.6300

318260 28.7600

318320 12.3000

RESULTS AND DISCUSSION Radioactivity concentration Activity concentrations of 226Ra ranged from 5.7 + 0.3 to 140 + 7 Bq kg21, from 9 + 0.4 to 139 + 7 Bq kg21 for 232Th and 22 + 1 to 319 + 16 Bq kg21 for 40K. Table 2 shows that the average concentrations values of 226Ra, 232Th and 40K in the 10 various sites investigated in this study were 43 + 2, 54 + 3 and 183 + 9 Bq kg21, respectively. The average concentrations values of 226Ra and 232Th are higher than that of world’s average(3). Table 2 shows that the highest values of 226Ra, 232Th and 40K were found in the Aga region may be attributed to organic fertiliser industries. The concentrations of the above-mentioned radionuclides are compared with that of other author’s for soil in Table 3. In the first assessment of representative concentrations of these radionuclides in soil, the committee(7) suggested the maximum allowable values of 370, 25 and 25 Bq kg21 for 40K, 226Ra and 232Th, respectively. Figure 2 (a, b and c) shows the distribution of 238U, 232 Th and 40K in the graphical form. As can be seen in Figure 3, the strong correlation between 226Ra 61


assembly, which includes a NaI (Tl) crystal, coupled to PC-MCA Canberra Accuspec. To reduce 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 lead. 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 activity or background was 43 200 s. The background spectra were used to correct the net peak area of gamma rays of measured isotopes. A dedicated software program, Genie 2000 from Canberra, was used to carry out the on-line analysis of each measured gamma-ray spectrum(5). The 226Ra radionuclide was estimated from the 351.9 keV (36.7%) gamma peak of 214Pb and 609.3 keV (46.1%), 1120.3 keV (15%), 1728.6 keV (3.05%) and 1764 keV (15.9%) gamma peaks of 214Bi. The 186-keV photon peak of 226Ra was not used because of the interfering peak of 235U with the energy of 185.7 keV. The 232Th radionuclide was estimated from the 911.2-keV (29%) gamma peak of 228Ac and 238.6-keV (43.6%) gamma peak of 212Pb. The 40K radionuclide was estimated using 1.461-keV (10.7%) gamma peak from 40K itself. The below detectable limits (BDL) were 25.2 Bq kg21 for 40K, 6.5 Bq kg21 for 226Ra and 5.7 Bq kg21 for 232Th. All procedures were described in previous publications(6).

Sampling preparation

S. A. M. ISSA Table 2. Activity concentrations (Bq kg21) of 226Ra, 232Th and 40K. Location

226

Code No.

El-Senbllawein Miet Ghamr Mansoura Shirbin Aga Talkha Belkas Tmayy Alimdid Minyat Elnassr Jamasah Average Range

Th

26 + 1 27 + 1 74 + 4 23 + 1 35 + 2 29 + 1 26 + 1 32 + 2 33 + 2 31 + 1 37 + 2 124 + 6 108 + 5 140 + 7 11.7 + 0.6 45 + 2 26 + 1 26 + 1 5.7 + 0.3 14 + 0.6 27 + 1 30.7 + 1 70.7 + 3 31 + 1 36 + 2 43 + 2 5.7– 140

50 + 10 + 53.6 + 36 + 53 + 46 + 50 + 48 + 60 + 53 + 60 + 101 + 88 + 139 + 9+ 47.6 + 52 + 17 + 11 + 50 + 55 + 50.6 + 115 + 48 + 56 + 54 + 9– 139

2 0.5 3 2 3 2 2 2 3 3 3 5 4 7 0.4 0.2 3 0.8 0.1 +2 3 2 6 2 3 2

40

K

165 + 8 204 + 10 200 + 10 203 + 10 220 + 11 139 + 7 128 + 6 234 + 12 148 + 7 218 + 11 246 + 12 213 + 11 166 + 8 319 + 16 22 + 1 136 + 7 208 + 10 279 + 14 33 + 2 231 + 11 259 + 13 137 + 7 172 + 9 64 + 3 223 + 11 183 + 9 22– 319

Table 3. Activity concentrations (Bq kg21) of 226Ra, 232Th and 40K measured worldwide. Country/region

226

Alexandria, Egypt France Zacatecas (Mexico) Cyprus Sudan Punjab, Pakistan South India S. Punjab, Pakistan Jessore (Bangladesh) Spain Kohistan, Pakistan South west Nigeria Kenya China Republic of Ireland Saudi Arabia Tayma, Saudi Arabia Louisiana (USA) World’s average

16.7 9– 62 23 7.1 28.31 35 35 21.7 48.32 46 42.11 16.2 28.7 42.7 60 14.5 30.77 43–95 35

Ra

232

Th

19.4 16– 55 19 5.0 20.12 41 29.8 31 53.34 49 43.27 24.4 73.3 46.3 26 11.2 27.59 50– 190 30

40

K

262 120– 1026 530 105 280.29 615 117.5 393.2 481.35 650 418.27 234.8 255.7 578 350 225 161.82 43.729 400

Reference (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (3)

Radium equivalent activity (Raeq)

and 232Th for soil samples indicates that the individual result for any one of the radionuclide concentrations is a good predictor of the individual values for the other.

The results were evaluated in terms of the radiation hazard by means of the Ra equivalent activity (Raeq). The radium equivalent activity is a widely 62


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

232

Ra



Figure 2. Activity concentrations of (a)

226

Ra, (b)

63

232

Th and (c)

40

K for investigated samples.

S. A. M. ISSA

respectively. Column 3 of Table 4 gives the results for the absorbed dose rate in air for samples. The authors note that Aga shows the highest values of 166 nGy h21. The frequency distribution (in per cent) of the total absorbed dose rates of the 25 measured samples is plotted in Figure 4. Sixteen samples exhibit dose rates that ranged from 40 to 70 nGy h21. Three samples ranged from 100 to 130 nGy h21 and four of the samples exhibit values under the limit of 40 nGy h21.

used hazard index and it is calculated through the relation given by Beretka and Mathew(8). It is assumed that 370 Bq kg21 of 226Ra, 259 Bq kg21 of 232 Th and 4810 Bq kg21 of 40K produce the same gamma-ray dose rate. Raeq ¼ ARa + 1:43ATh þ 0:077AK

ð1Þ

where ARa, ATh and AK are the specific activities of 226 Ra, 232Th and 40K, respectively, in Bq kg21. Raeq was estimated for the collected samples and are given in Table 4. The values of Raeq varied from 24 to 364 Bq kg21 and the average value of Raeq was found to be 135 Bq kg21. The estimated highest value of 364 Bq kg21of Raeq in the present work are lower than that of the recommended maximum value of 370 Bq kg21(8).

Hazard indices (Hex and Hin) A widely used hazard index (reflecting the external exposure) called the external hazard index Hex was defined as follows(3):

Absorbed dose rate (Do)

Hex ¼

The absorbed dose rates in outdoor (D, nGy h21) due to gamma radiations in air at 1 m above the ground surface for the uniform distribution of the naturally occurring radionuclides (226Ra, 232Th and 40 K) were calculated based on the guidelines provided by ref. (9). The conversion factors used to compute the absorbed gamma-dose rate (D) in air per unit activity concentration in Bq kg21 (dry-weight) corresponds to 0.427 nGy h21 for 226 Ra (of U series), 0.662 nGy h21 for 232Th and 0.043 nGy h21 for 40K. D= 0:427CRa þ 0:662CTh þ 0:043CK

CRa CTh CK þ + 370 259 4810

ð3Þ

In addition to the external hazard index, radon and its short-lived products are also hazardous to the respiratory organs. The internal exposure to radon and its daughter progenies was quantified by the internal hazard index Hin, which is given by the equation: Hin ¼

CRa CTh CK + + 185 259 4810

ð4Þ

where, CRa, CTh and CK are the mean activity concentrations of 226Ra, 232Th and 40K in Bq kg21, respectively. The value of Hin must be less than the unity to have negligible hazardous effects of radon and its short-lived progeny, the calculated values of these indices are given in Table 4.

ð2Þ

where CU, CTh and CK are the concentration in (Bq kg21) of radium, thorium and potassium, 64


Figure 3. Linear regression of the activity concentrations of 226Ra versus 232Th for investigated samples.


Table 4. Radium equivalent (Bq kg21), the dose rate (nGy h21), hazard indices (Hex and Hin) (nGy h21), annual effective dose rate (mSv y21), excess lifetime cancer risk (ELCR) and annual gonadal dose equivalent (AGDE) (mSv y21). Code no.

Raeq

D. rate

Hex

Hin

A. Eff.

ELCR 1024

AGDE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Average Range

110 57 166 91 128 105 108 120 131 125 142 285 246 364 26 124 117 72 24 103 126 114 249 105 134 135 24–364

51 27 76 43 60 49 50 56 60 58 66 129 111 166 12 57 55 35 11 49 59 53 114 48 62 62 51–166

0.3 0.2 0.4 0.2 0.3 0.3 0.3 0.3 0.4 0.3 0.4 0.8 0.7 1.0 0.1 0.3 0.3 0.2 0.1 0.3 0.3 0.3 0.7 0.3 0.4 0.4 0.1–1.0

0.4 0.2 0.6 0.3 0.4 0.4 0.4 0.4 0.4 0.4 0.5 1.1 1.0 1.4 0.1 0.5 0.4 0.3 0.1 0.3 0.4 0.4 0.9 0.4 0.5 0.5 0.1–1.4

62 33 92 52 72 59 61 68 73 71 80 157 135 201 15 69 66 42 14 59 72 64 138 58 76 76 14– 201

2 1 3 2 3 2 2 2 3 2 3 5 0.5 2 2 1 0.5 2 3 2 5 2 3 5 7 3 0.5–7

341 190 514 288 399 325 331 376 401 389 443 873 753 1114 81 383 364 241 74 324 395 349 755 316 418 417 74– 1114

Annual effective dose

calculated using a conversion factor of 0.7 SvGy21, which was used to convert the absorbed rate to the human effective dose equivalent with an outdoor

The annual estimated average effective dose equivalent received by a member of the public was 65


Figure 4. Frequency distribution of the total absorbed dose rates of the measured soil samples.

S. A. M. ISSA

occupancy of 20 and 80% for indoors(10). The annual effective dose was determined as follows: Annual effective dose rate = D T F

values of 226Ra activities as shown in Table 2. Thus, this information is an important alert for the local people to avoid the use of this soil in the construction of dwellings. The data obtained in this study will serve as a baseline for assessing the radiation exposure of the residents.

ð5Þ

where D is the calculated dose rate (in nGy h21), T is the outdoor occupancy time (0.224 h x365.25 d1753 h y21) and F is the conversion factor (0.71026 SvGy21). The experimental results of the annual effective dose rate are presented in Table 4, column 6. From the results it is clear that the average total annual dose (based on the samples of this study) is 76 mSvy21. The International Commission on Radiological Protection (ICRP) has recommended the annual effective dose equivalent limit of 1 mSvy21 for the individual members of the public and 20 mSvy21 for the radiation workers(11).

ACKNOWLEDGEMENTS This work was carried out using the nuclear analytical facilities at the Physics Department, Faculty of Sciences, Al-Azhar University, Assiut, Egypt.

This work was supported by Physics department, Faculty of science, Al-Azhar University, Assiut branch, Egypt.

Excess lifetime cancer risk Excess lifetime cancer risk (ELCR) was calculated using the following equation and presented in Table 4. ELCR = AEDE DL RF

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ð6Þ

where AEDE, DL and RF are the annual effective dose equivalent, duration of life (70 y) and risk factor (Sv21), fatal cancer risk per sievert. For stochastic effects, ICRP 60 uses values of 0.05 for the public(12). The calculated value of ELCR showed that the highest value was in Aga. This value of ELCR was less than the world average 0.291023(3) for the soil sample in Dakahlia, Egypt. Annual gonadal dose equivalent The annual gonadal dose equivalent (AGDE, mSv y21) due to the specific activities of 226Ra, 232Th and 40K was calculated using the following formula(13): AGDE = 3:09 CRa + 4:18 CTh þ 0:314 CK

ð7Þ

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