Natural radioactivity, dose assessment and uranium uptake by agricultural crops at Khan Al-Zabeeb, Jordan

Available online at www.sciencedirect.com

Journal of Environmental Radioactivity 99 (2008) 1192e1199 www.elsevier.com/locate/jenvrad

Natural radioactivity, dose assessment and uranium uptake by agricultural crops at Khan Al-Zabeeb, Jordan Samer J. Al-Kharouf a, Ibrahim F. Al-Hamarneh b,*, Munir Dababneh b b

a Royal Scientific Society, Amman 11941, Jordan Prince Abdullah Bin Ghazi Faculty of Science and IT, Al-Balqa Applied University (BAU), Salt 19117, Jordan

Received 3 September 2007; received in revised form 29 January 2008; accepted 8 February 2008 Available online 24 March 2008

Abstract Khan Al-Zabeeb, an irrigated cultivated area lies above a superficial uranium deposits, is regularly used to produce vegetables and fruits consumed by the public. Both soil and plant samples collected from the study area were investigated for their natural radioactivity to determine the uranium uptake by crops and hence to estimate the effective dose equivalent to human consumption. Concentrations of 238U, 235U, 232Th, 226 Ra, 222Rn, 137Cs and 40K in nine soil profiles were measured by gamma-ray spectrometry whereas watermelon and zucchini crops were analyzed for their uranium content by means of alpha spectrometry after radiochemical separation. Correlations between measured radionuclides were made and their activity ratios were determined to evaluate their geochemical behavior in the soil profiles. Calculated soileplant transfer factors indicate that the green parts (leaves, stems and roots) of the studied crops tend to accumulate uranium about two orders of magnitude higher than the fruits. The maximum dose from ingestion of 1 kg of watermelon pulp was estimated to be 3.1 and 4.7 nSv y1 for 238U and 234U, respectively. Estimations of the annual effective dose equivalent due to external exposure showed extremely low values. Radium equivalent activity and external hazard index were seen to exceed the permissible limits of 370 Bq kg1 and 1, respectively. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Natural radioactivity; Gamma spectrometry; Alpha spectrometry; Transfer factor; Soil; Watermelon; Zucchini; Annual effective dose equivalent

1. Introduction Study of natural radioactivity is usually done in order to gain information about the present levels of harmful pollutants discharged to the environment itself or in the living creatures. It is also important to understand the behavior of natural radionuclides in the environment, because such information can be used as the associated parameter values for radiological assessments (Vera Tome et al., 2003). Public radiation exposure from natural radioactive decay series of 238U, 235U, and 232Th occurs mainly because they can get dissolved in water and migrate to surface water reservoirs, leading to the possibility of contaminating foodstuffs following soileplant transfer as

* Corresponding author. Tel.: þ962777489677; fax: þ96253557518. E-mail addresses: [email protected], [email protected] (I.F. Al-Hamarneh). 0265-931X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2008.02.001

well as getting ingested into the human body (Chen et al., 2005). They can also be suspended in air, and in case of gaseous radionuclides, can emanate into air, facilitating their incorporation into the body through inhalation (Amaral et al., 2005). Both types of exposure depend on the activity concentration of the radionuclide, making radiometric techniques key instrument in estimating the radiation dose received by humans. The Earth’s crust in many regions in Jordan is covered by phosphate rocks that make the terrestrial environment rich in 238 U and its daughters. Khan Al-Zabeeb, situated about 60 km south of Amman e Jordan, is an agricultural land lies above 39 km2 of superficial secondary uranium deposits with an average estimated concentration of 1186 ppm and a maximum concentration of 4000 ppm (Al-Kharouf, 2006). It is essential to establish both radiometric and radiological baseline data, which do not exist yet, to investigate the potential radioactive impact of the uranium ore. The aims of the present

S.J. Al-Kharouf et al. / Journal of Environmental Radioactivity 99 (2008) 1192e1199

a 900 800 700

CPM

study are to evaluate the fluxes of natural radionuclides and radioactive disequilibria involved in uranium secondary deposits; to determine the soil and plant radioactivity in such area as well as their transfer factor; to estimate their radiological impact in crop soils. However, the results obtained from this study may prove to be a small but valuable addition to the knowledge gathered on this issue. It deserves mentioning that soil samples were prepared and measured in the radiation measurements and calibration laboratory at the Royal Scientific Society (RSS), an internationally (UKAS) and nationally (JAS) accredited lab. The radiochemical separation and alpha spectrometry measurements were performed at the Jordanian Nuclear Energy Commission (JNEC) labs.

2.1.2. Vegetation Watermelon (from locations S4, S5, S7, S8, and S9) and zucchini (from locations S2 and S4) crops were taken to determine their uranium content. In order to determine the uranium uptake by the edible part, watermelon was treated as three separate compartments; peel, pulp and green part including the root system. Uniquely identified and marked, fresh watermelons from each sampling site were picked on the same day, washed to remove any possible surface contamination and weighed. Each compartment was sliced, dried at the nominal temperature of 105  C, weighed and then ashed in an oven at 450  C without ignition to oxidize carbon in the organic matter. Zucchini, on the other hand, was treated as two separate compartments; fruit and green part including the root system. On a weekly basis, zucchini fruits were picked up and uniquely marked throughout the plant’s productive period. When zucchini green parts reached the end of their productive life, they were uprooted and then treated in the same manner applied for watermelon.

2.2. Radioactivity determinations 2.2.1. Soils The soil samples were analyzed by gamma spectrometry system consists of a closed end co-axial p-type HPGe detector (Tennelec’s model CPVDS3020190) with 20% relative efficiency and a resolution of 1.88 keV at 1.33 MeV. The detecting system was shielded by 5 cm thick cylindrical lead

500

300

S6

200 S3 234950 235000 235050 235100 Eas 235150 t 235200 235250

2.1. Sampling and preparation

b

S1

S4 S2

S5 S7

3492200 3492100 S8 3492000 3491900 rth

S9

No

3491800 3491700

180

Gauss Fit: χ2 = 534.1 R2 = 0.83 Center = 553.9 ± 9.8 cpm

160 140 120

Frequency

2.1.1. Soils Nine soil profiles were collected; eight of them from locations of high level of substrate radioactivity. In each location, soil samples were collected, in layers of 0e2, 2e7, 7e12, 12e22, 22e32 cm, to verify a possible variation in the concentrations of the measured radionuclides. The locations, marked as S1 to S9, are given in UTM coordinates in Fig. 1a and the characteristics of the soil layers can be found in Table 1. A surface soil scraper of 625-cm2 area was used to take the first two layers whereas the rest of the profile was collected using a cylindrical coring device. The collected soils were sieved using a sieve of 1-mm in mesh size to remove small stones, grinded to insure the soil particle size to be less than 400 mm, homogenized using a mixer/shaker and then filled into 450 ml Marinelli beakers for gamma measurements. Ten grams of aliquots were later used for soil pH determination using a Metrohm model 744-pH meter.

600

400

2. Experimental details

Prior to sampling, a radiation survey was carried out to get aid in selecting representative locations for soil and vegetation samples following the radioactivity of the substrate. The survey was performed using a Canberra 200  200 NaI detector model 802-3, connected to a Bicron model labtech scaler/ratemeter analyzer and connected to a laptop computer to facilitate collecting data every 0.1 min using LABVIEW software. Synchronized with the laptop, a handheld GPS system of Garmin-12 model was used to determine the survey path.

1193

100 80 60 40 20 0 300

400

500

600

700

800

900

CPM Fig. 1. The radiation survey trajectory (a) and its histogram distribution (b). lined from the inside with cadmium, copper and Plexiglas. Extra 5-cm thick lead bricks were added around the existing shield for extra reduction of the ambient background. Energy, shape and efficiency calibrations were performed using a 10-radionuclide mixed gamma standard and a 210Pb, both in Table 1 Average values of soil layers characteristics and average physicochemical composition of collected soils Depth (cm)

0e2

2e7

7e12

7e12b

22e32

12e22

5.16 7.90 1.33

6.43 8.07 1.38

8.49 8.25 1.39

6.58 8.14 1.31

11.02 8.49 1.30

9.80 8.31 1.31

a

Characteristics Moisture (%) pH r (g/cm3)

Physicochemical composition Compound SiO2 Al2O3 Fe2O3 TiO2 P2O5 CaO MgO K2O Concentration (%) 22.8 5.6 2.5 0.4 0.5 29.4 2.4 0.6 LOIc (%) 35.0 a

Average value. The second measure of sealed samples was carried out six months after the first one. c Loss on ignition at 1000  C to include the loss of carbon in carbonates (like CaCO3), organic matter and moisture. b

1194

S.J. Al-Kharouf et al. / Journal of Environmental Radioactivity 99 (2008) 1192e1199

water-equivalent solid matrix that was filled in a 450 ml Marinelli beaker similar to that used for counting the samples. All soil samples were counted for live counting times 53,000e153,000 s. To avoid the effect of peak shifting, a spectrum of the mixed gamma standard is acquired right after the end of the sample counting. A number of ambient background spectra of counting time 240,000e420,000 s were acquired during the period of counting in such a way that an ambient background spectrum acquisition is within a maximum of one month of the date of acquisition of soil sample spectrum. True summing correction factors were calculated using the method described by Drya´k et al. (1989) and applied to the efficiency calibration curve. To account for self-absorption, the chemical composition (see Table 1) of selected soil samples was analyzed using wavelength dispersive X-ray fluorescence (WD XRF). These samples were taken from the part of the profile that contains most of the root system mass, namely the 7e22 cm layer; onethird of each sample came from the 7e12 cm layer and two-third from the 12e22 cm layer. The self-attenuation coefficients were then calculated plugging the mass attenuation coefficient (Debertin and Helmer, 1988) for the relevant energy and the soil density in the activity calculation. Detection limits were determined following the method of Currie (1968). The calculated activities were corrected for soil moisture content right after counting the sample by drying around 10 g of the counted sample to constant weight at 105  C. The concentrations of radionuclides of interest and their associated uncertainties were determined by evaluating their respective energy lines and the lines of any radionuclides with interfering energy lines. 238U was identified using the gamma energies emitted by its daughters 234Th (63.28, 92.37 and 92.79 keV) and 234mPa (766.38 and 1001.03 keV) assuming secular equilibrium. The validity of this assumption was checked by re-counting one sample from each soil profile six months or more after its collection date and the results were compared. In evaluating 235U, it is to be noted that 226Ra, when present, contributes considerably to the photopeak at 186 keV. Therefore, 235U was first determined from its gamma lines at 143.76, 163.36, 202.11 and 205.31 keV. Then the counts coming from the 235U line at 185.72 keV were calculated and subtracted from the total counts under the 186 keV peak to get the counts coming from 226Ra and, hence the activity of 226Ra was calculated. This approach in determining 235U is sometimes risky; due to the low energy lines of 235U lie over a high and varying Compton continuum making the area determination of each line very sensitive to the way the peak is defined and how the spectral background is determined. Moreover, the 143.76 keV peak suffers from summing-out effect and the 163.36 and 205.31 keV peaks suffer from summing-in effect (Papachristodoulou et al., 2003). Therefore, a second approach was used to calculate the activity of 235U from that of 238U using their natural activity ratio, 235U/238U ¼ 0.046. The same de-convolution method described above to calculate the activity of 226Ra was then followed. 222 Rn was determined from its daughter 214Pb by considering its clean peak at 295.22 keV. 232Th was calculated from its decay products: 208Tl, 212 Pb, 212Bi, and 228A. Finally, 137Cs and 40K were directly determined using their gamma lines at 661.66 and 1460.81 keV, respectively. 2.2.2. Vegetation The ash taken from each vegetation sample was measured to determine the concentrations of 234U, 235U, and 238U by alpha spectrometry after radiochemical separation procedure (Vajda et al., 2003). About 0.1 ml of 232U tracer was added to each sample, and then the samples were evaporated to almost complete dryness with mineral acids. Uranium was then extracted by chromatographic separation using dipentyl-pentyl phosphonate (UTEVA). Thin alpha sources were prepared on filter paper by micro-co-precipitation. The alpha spectrometry system is Tennelec’s model TC257 eight-detector system, each of which is a 300-mm2 Passivated Implanted Planar Silicon (PIPS) detector with FWHM of 20 keV at 241Am. Each sample and the reagent blank were counted for 60,000 s.

2.3. Transfer factor determinations Transfer factors (TFs) were calculated as the ratio of the radionuclide concentration in plant (Bq kg1 plant) to its concentration in soil (Bq kg1 soil). As per convention, TF for fruits is based on their fresh weights to get a realistic

transfer factor (Carini, 2001). Soil radioactivity in the rooting zone was calculated assuming one-third of the activity came from the 7e12 cm layer and the rest came from the 12e22 cm layer. This choice was made because the part of the soil profile that contains most of the root system was found to be 7e22 cm.

2.4. Dose estimations Assuming that naturally occurring radionuclides are uniformly distributed in the ground, the dose rate (D) at 1 m above the ground surface is calculated by the following formula (Kohshi et al., 2001; UNSCEAR, 1988):
D nGy h1 ¼ 0:662ATh þ 0:427AU þ 0:0432AK ð1Þ and the effective dose rate (E ) outdoors is calculated by the following formula:

E mSv y1 ¼ D nGy h1  24 h  365:25 d  0:2  0:7  103 Sv Gy1 ð2Þ where 0.2 is the occupancy factor and 0.7 is the conversion coefficient. Radium equivalent activity (Raeq) is a widely used hazard index (Beretka and Mathew, 1985). It is based on the estimation that 370 Bq kg1 of 226Ra, 259 Bq kg1 of 232Th and 4810 Bq kg1 of 40K produce the same gammaray dose rate and can be expressed as ð3Þ Raeq ¼ ARa þ 1:43ATh þ 0:077AK Another radiation hazard index called the representative level index, Igr, is defined as follows (NEA-OECD, 1979):   ARa ATh AK Igr ¼ þ þ ð4Þ 150 Bq kg1 100 Bq kg1 1500 Bq kg1 where ATh, AU, AK, and ARa, in Eqs. (1), (3), and (4), are the activity concentrations (in Bq kg1) for 232Th, 238U, 40K, and 226Ra, respectively, measured in topsoil (0e2) cm. Finally, Saueia and Mazzilli (2006) gave the committed effective radiation dose EING (Sv y1) due to ingestion of a terrestrial foodstuff: EING ¼ AUFCDING

ð5Þ 1

where A is the activity concentration (Bq kg ) of the investigated radionuclide in the edible part of plant, U is the ingestion rate (kg y1), and FCDING is the dose conversion factor (Sv Bq1) due to ingestion of that radionuclide.

3. Results and discussion 3.1. Soil radioactivity The registered count rates (in cpm) of the radiation survey versus latitude and longitude coordinates are shown in Fig. 1a, indicating the level of radioactivity close to the soil surface. However, the frequency distribution of the registered count rates looks normal with a mean value of 515.3  119.8 cpm (range 220e950 cpm) as elucidated in Fig. 1b. The mean activities of the measured radionuclides in the nine soil profiles are given in Fig. 2. The surface soils showed both spatial and vertical heterogeneous distributions of radioactivity over the entire sampling sites. It is worth to mention that an ANOVA test performed to compare the activities of 235 U and 226Ra with that of 235UCal and 226RaCal showed no significant difference. Another ANOVA test performed to compare the measured concentrations of the investigated radionuclides six months or more after the first measure also showed no significantly statistical difference. However, the concentrations of 238U and 235U in the topsoil layer, 0e2 cm, were found to be 69.58  7.21 (range 36.2e

S.J. Al-Kharouf et al. / Journal of Environmental Radioactivity 99 (2008) 1192e1199

238

235

235

226

226

222

232

40

U RaCal

U Rn

UCal Th

a

Ra

650

550

100

500

550

450

500

400

222

Ra (Bq.kg-1)

600

226

450 350 400

10

300

350

250

300 0

2

4

6

8

10 12 14 16 18 20 22 24 26 28 30

Depth (cm)

1 0-2

2-7

7-12

12-22

22-32

Depth (cm)

b

Fig. 2. Mean activity concentrations of natural radionuclides of interest in soil profiles. Error bars of 95% confidence level are also shown.

232

A ( Th) = (22.18 ± 0.38) + (-0.20 ± 0.02) * Depth 2 R = 0.96 40 A ( K) = (235.18 ± 2.48) + (-3.21 ± 0.16) * Depth 2 R = 0.99

280 260

22

20 220 200

18

232

114.0 Bq kg ) and 4.99  0.41 (range 2.2e6.4 Bq kg ), respectively. Hence, the 238U activity of Khan Al-Zabeeb surface soils is about 2.8 times higher than the world’s mean concentration, 25 Bq kg1, that was reported for areas of normal radioactivity (UNSCEAR, 2000). Moreover, the 238U and 235 U concentrations seem to be nearly constant down to a depth of 22 cm with mean values of 71.54  18.80 and 8.24  2.37 Bq kg1, respectively. This behavior could be attributed to the fact that the study area is an intensively cultivated land, which leads to mixing soils from different depths. An abrupt increase by about 60% in the uranium activities has been observed in the deep layer (22e32 cm) with mean concentrations of 116.75  43.17 and 8.24  2.37 Bq kg1 for 238 U and 235U, respectively. This is most probably due to the closeness of the uranium ore to the surface at this depth. It is known that soil overburden at these sampling sites range in 25e40 cm (Al-Kharouf, 2006). 226 Ra and its daughter 222Rn have elevated concentrations compared with other measured radionuclides. In topsoil, their mean concentrations were 338.87  54.30 and 265.10  42.39 Bq kg1, respectively. The 226Ra activity lies in a wide range (62.1 up to 660.9 Bq kg1) and hence, in some locations exceeds the permissible value of 370 Bq kg1, which is acceptable as a safe limit (OECD, 1979). Radon, on the other hand, is a noble gas that emanates from the soil, but unless forced out by heating or flushing by other gases, a considerable portion remains adsorbed to soil clay particles. In deep layers, the best-fit plot of their mean concentrations with depth, Fig. 3a, reveals significant exponential correlations (R2 ¼ 0.97 and 0.99, respectively). 232 Th and 40K, in topsoil layer, showed homogeneous distributions of concentration with mean values (range) of 21.40  0.79 (18.42e25.17 Bq kg1) and 227.88  12.83 (179.07e307.60 Bq kg1), respectively. This homogeneity supports the assumption of soil mixing due to agricultural activities. In addition, 232Th and 40K activities tend to decrease

40

1

K (Bq.kg-1)

240 1

Th (Bq.kg-1)

Mean Activity (Bq.kg-1)

600

A (226Ra) = 357.63 ± 13.99 * Exp [(0.023 ± 0.002) * Depth] 2 R = 0.97 222 A ( Rn) = (260.40 ± 6.24) * Exp [(0.021 ± 0.001) * Depth] R2= 0.99

700

K

Rn (Bq.kg-1)

1000

1195

180 16 160 140

14 0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30

Depth (cm) Fig. 3. Mean concentrations of profiles.

226

Ra,

222

Rn (a),

40

K and

232

Th (b) in the soil

linearly in deep layers as depicted in Fig. 3b. The decreasing behavior of 232Th and 40K with depth can be explained by the effect of irrigation water in dissolving thorium and potassium compounds, the solutions move under the effect of heating by the sun towards the surface and are deposited by evaporation. Abbady et al. (2005), reported concentrations of 232 Th and 40K in sedimentary phosphate rocks comparable to those normally observed in soil. The activity of 137Cs in topsoils has been observed to vary in the range 0.96e3.53 Bq kg1 with mean value of 2.11  0.23 Bq kg1, whilst the 137Cs mean activity, calculated for all depths, was 1.60  0.34 Bq kg1. These values are significantly lower than the range of activities, 7.5e 576 Bq kg1, that were registered for 137Cs in soil samples collected from the northern part of Jordan in 2000 (Al Hamarneh et al., 2003). However, it is known that Khan Al-Zabeeb area is free from any direct anthropogenic radioactive fallout. As far as secular equilibrium is required in evaluating the radioactivity of natural series in soil, the relationships between 238 U, 235U, 226RaCal, and 222Rn mean activities versus 226Ra were investigated. The general trends of these relations, Fig. 4a, were of linear shape with strong correlation

S.J. Al-Kharouf et al. / Journal of Environmental Radioactivity 99 (2008) 1192e1199

238

U = (9.98 ± 23.99) + (0.15 ± 0.04) * 226Ra; R2 = 0.94 U = (1.20 ± 1.60) + (0.010 ± 0.003) * 226Ra; R2 = 0.78 226 RaCal = (9.09 ± 32.85) + (1.04 ± 0.06) * 226Ra; R2 = 0.99 222 Rn = (27.25 ± 5.70) + (0.70 ± 0.01) * 226Ra; R2 = 0.99 232 Th = (27.87 ± 1.01) + (-0.017 ± 0.002) * 226Ra; R2 = 0.96 40 K = (314.31 ± 11.43) + (-0.25 ± 0.02) * 226Ra; R2 = 0.98

800 700

A (Bq.kg-1)

260

235

900

240 220 200

(Bq.kg-1)

a

180

40K

1196

600 500 400 300

160

200 140

100 0

120 350

b

9.5

400

450

500

550

600

226Ra

(Bq.kg-1)

650

700

750

235

U = (0.55 ± 0.30) + (0.066 ± 0.003) * 238U

2 R = 0.99

9.0

235

U (Bq.kg-1)

8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 70

80

90

100

110

120

130

140

U (Bq.kg-1)

238

Fig. 4. Mean concentrations of various radionuclides versus 235 U versus that of 238U (b) in soil profiles.

226

Ra (a), and

coefficients, indicating similar physical transport of these radionuclides in soil profiles despite their different chemical behavior. The slope of each line in Fig. 4a was used to determine the activity ratio. The mean activity ratios  standard error (range) of 238U/226Ra and 235U/226Ra were 0.15  0.04 (0.02e0.55) and 0.010  0.003 (0.008e0.3), respectively. It is obvious that the 238U/226Ra activity ratio was disturbed in favor of 226Ra. Many authors (Karangelos et al., 2004; Bikit et al., 2001) intensively discussed the 238U/226Ra activity ratio. Actually, they used the 226Rae238U disequilibria in favor of 226 Ra as an evidence of natural origin of uranium in soil. However, disturbances of the radioactive equilibrium between 238U and 226Ra in environmental samples usually result from different geochemical behavior of these elements in the near-surface layers of the lithosphere. In many environments, repeatedly occurring changes both in time and space in pH values, temperature, humidity, and many other physical and chemical factors, like the chemical composition of ground waters reacting with minerals, are the main possible processes that may affected the equilibrium (Karangelos et al., 2004). Ivanovich and Harmon (1992) ascribed the 226Rae238U disequilibrium

to the precipitation/dissolution reactions. Sheppard et al. (2005) have explained the variation of uranium concentrations in soil by the changes that can occur in the oxidation state of uranium from IV to VI. The first oxidation state causes the uranium to form compound insoluble in water that precipitates in soil, while the second oxidation state causes uranium to form uranyl ion (UO2þ 2 ) complexes with carbonate, phosphate or sulphate ions. Radium, on the other hand, has one oxidation state of II. Its compounds found in soil, though range from slightly soluble to soluble in cold water, don’t need oxidizing conditions or a change in oxidation states to dissolve in water, which makes them easier to transport within the soil profile. The mean 222Rn/226Ra activity ratio of 0.70  0.01 (0.62e 0.77), deduced from Fig. 4a, showed much milder variation in soil than that observed for 238U/226Ra. The obtained 222 Rn/226Ra values were not affected by the time lapse between sampling and counting dates. Fig. 4a also shows the linear relationship between 226Ra and 226RaCal concentrations with strong correlation coefficient (R2 ¼ 0.99) and 226Ra226 Ra mean activity ratio of 1.04  0.06 (0.96e1.12). FiCal/ nally, strong inverse correlations between the mean concentrations of 232Th and 40K with 226Ra were also shown in Fig. 4a, indicating that no significant fractionation during weathering or involvement in metasomatic activity of the radionuclides had occurred as suggested by Chiozzi et al. (2002). El Mamoney and Khater (2004) suggested that potassiumeradium relationship is affected by their relative solubility in soil. Shown in Fig. 4b the significant linear relationship between the mean activities of 235U and 238U. The slope of the line, with its strong correlation coefficient, provides a mean 235 U/238U activity ratio of 0.066  0.003 (0.039e0.11) which is comparable to the natural 235U/238U activity ratio of 0.046 (Ivanovich and Harmon, 1992). Deduced from that the natural origin of uranium in Khan Al-Zabeeb soils. The observed deviation from the accepted value is mainly due to two reasons namely; the self-absorption effect that may amount in a systematic error and the emanation of radon from the sealed samples that may also cause an underestimation of uranium concentrations especially for soils of highly uranium content. Fig. 5 displays the mean 235U/238U activity ratio against soil depth. The best-fit plot results in strong linear correlation (R2 ¼ 0.86) of slope 0.0014  0.0003, implying the slight increase of the uranium activity ratio towards deep depths where the uranium ore layer is located. Moreover, the intercept of the line (0.0708  0.0011) presents the 235U/238U activity ratio at the surface, which is close to the average value presented earlier. 3.2. Vegetation radioactivity The concentrations of radionuclides in fruits can be indicative of ingestion dose to man. Therefore, watermelon and zucchini crops were analyzed for compartmental content of 234U, 235 U, and 238U. While watermelon pulps do not contain measurable amounts of 235U (<0.01 Bq kg1), the average 234U and 238U concentrations were found to be 0.017 and

S.J. Al-Kharouf et al. / Journal of Environmental Radioactivity 99 (2008) 1192e1199 235

Mean 235U/238U Activity Ratio

0.080

U/238U = (0.0708 ± 0.0011) + (0.0014 ± 0.0003) * Depth R2 = 0.86

0.078

0.076

0.074

0.072

0.070 0-2

2-7

7-12

12-22

22-32

Depth (cm) Fig. 5. The 235U/238U mean activity ratio against soil depth with 95% confidence level.

0.010 Bq kg1, respectively. Watermelon green parts with roots, with the average 234U and 238U concentrations of 0.81 and 0.65 Bq kg1, respectively, are an order of magnitude or higher than those in pulp. Zucchini fruits, on the other hand, have concentrations below the detection limit of 1  104 Bq kg1 for 234U and 238U, and 1  102 Bq kg1 for 235U. The average 234U, 238U, and 235U concentrations of zucchini green parts with roots were 0.75  0.04, 0.72  0.03, and 0.050  0.002 Bq kg1, respectively. It is noticeable that 234U uptake by zucchini is slightly higher than that of 238U with mean 234U/238U activity ratio of 1.05  0.05. This ratio, however, is in good agreement with the natural 234U/238U ratio in soil ranges between 1.0 (expected for secular equilibrium between 234U and 238U) and 1.3 (El Mamoney and Khater, 2004). Plants take up radionuclides that have similar chemical behavior as the essential nutrient. Therefore, radionuclides are transported to specific tissues based on the function of the element in plant metabolism and it is reflected in its higher concentration in a particular part compared to others. According to Chen et al. (2005), there are considerable differences in the uptake and translocation of long-lived radionuclides among different plant species. In order to study the accumulation of uranium isotopes in watermelon crops the total content of each isotope in whole plant has been normalized for dry weight fraction of each plant part. The percent accumulation of 234U and 238U in watermelon pulps was in the range of 0.5e3% and 0.6e6.4% with medians 1.8% and 2%, respectively. It deserves noting that more than 90% of uranium is seen to accumulate by the roots. 3.3. Transfer factors The compartmental TFs of 234U and 238U and the associated standard deviations for zucchini and watermelon are listed in Table 2 whilst that of 235U was not possible to calculate, as its concentrations were not detectable for most plant

1197

compartments. It can be deduced from the table that the average value of the total TF of 238U for zucchini and watermelon plants was 1.05  0.08  102 and 0.45  0.19  102, respectively. When the TFs are corrected for plant water content by assuming average water content for fruits and green parts to be 75%, they become 4.21  102 and 1.82  102, respectively. Similarly, the total TFs of 234U for zucchini and watermelon calculated on dry weight basis would be 4.44  102 and 1.83  102, respectively. Vera Tome et al. (2003) reported transfer factors of 238U in the range of 102 to 101, which is compatible with data of the present work. On the other hand, the fresh weight average TF of 238U for watermelon fruits (peel þ pulp) found in this study (0.80  103) is less than (3.4  103) that given by Carini (2001). This might be ascribed to differences in the soils and climatic conditions. However, the present TF results agreed with that obtained by Fernandes et al. (2006), as they stated that a range from 103 to 101 conveniently compasses most of the TF values for soileplant systems (involving different cultures, different soils and natural radionuclides). Nevertheless, some variations in TF values depending on soil and/or plant characteristics are notably possible. For example, watermelon green parts concentrate more uranium than zucchini green parts. This phenomenon indicates that plant-specific effects seemed to be considerably important in mobilizing the radionuclides in the soil for plant uptake. Others (Bolca et al., 2007) reported that variability in TF for a given radionuclide is mainly ascribed to differences in soil properties, rather than differences between plant species. Pulhani et al. (2005) mentioned that the soileplant transfer of naturally occurring radionuclides is mainly affected by the soil characteristics. Regarding the present data, we noticed that both 234U and 238U TF values in watermelon green parts enhanced with calcium and potassium content in soil, whereas the high soil pH seems to reduce their availability to watermelon. These findings are in general agreement with that obtained by Pulhani et al. (2005). Fujiyoshi and Sawamura (2004) stated that the low binding of sandy soils has been shown to increase plantesoil concentration rates of the natural radionuclides. 3.4. Radiation dose estimation The external gamma absorbed dose rate, D (nGy h1), in air outdoors as well as the annual committed effective dose equivalent, E (mSv y1), due to external exposure of the 232Th and 238 U series and 40K at Khan Al-Zabeeb area is summarized in Fig. 6. The D-values were in the range: 45.40 nGy h1 (at sampling site S3) to 71.06 nGy h1 (at sampling site S1) with an arithmetic mean value of 53.73 nGy h1 and standard deviation of the mean of 8.32 nGy h1. The E-values, on the other hand, were in the range 55.72e87.20 mSv y1 with arithmetic mean value of 65.94 mSv y1 and standard deviation of the mean of 10.21 mSv y1. It is worth mentioning that both Dand E-values follow the trend of the naturally occurring radionuclides distribution in soil and in general agreement with early predictions made via radiation survey. It is obvious

S.J. Al-Kharouf et al. / Journal of Environmental Radioactivity 99 (2008) 1192e1199

1198

Table 2 Transfer factor (TF) of uranium isotopes in watermelon and zucchini compartments Sampling site

Crop compartment

Compartmental TF  sa 234

235

238

(1.11  0.05)  102 NMb

(1.59  0.06)  102 NM

(1.05  0.04)  102 NM

(1.11  0.07)  102 NM

(1.61  0.11)  102 NM

(1.05  0.07)  102 NM

Watermelon Green parts Peel Pulp

(1.02  0.06)  102 (9.90  0.73)  104 (3.18  0.21)  104

(5.51  0.37)  102 (3.75  0.34)  103 NM

(1.16  0.07)  102 (8.46  0.64)  104 (2.62  0.18)  104

S5

Green parts Peel Pulp

(1.90  0.12)  102 (1.11  0.08)  104 (9.62  0.73)  105

(2.06  0.14)  102 NM NM

(1.67  0.11)  102 (1.64  0.11)  104 (1.41  0.10)  104

S7

Green parts Peel Pulp

(5.09  0.34)  103 (2.96  0.20)  104 (9.18  0.64)  105

(5.53  0.38)  103 NM NM

(4.71  0.32)  103 (2.74  0.14)  104 (1.23  0.08)  104

S8

Green parts Peel Pulp

(1.17  0.07)  102 (6.05  0.40)  104 (3.81  0.25)  104

(1.77  0.11)  102 NM NM

(1.36  0.08)  102 (4.35  0.25)  104 (1.23  0.06)  104

S9

Green parts Peel Pulp

(1.86  0.11)  102 (7.03  0.62)  104 (4.07  0.26)  104

(1.45  0.10)  102 NM NM

(1.84  0.11)  102 (5.35  0.30)  104 (2.84  0.18)  104

U

S2

Zucchini Fruits

S4

Fruits

S4

a b

U

U

All uncertainties are expressed at 1s (68% confidence). Not measured.

that the average value (0.066 mSv y1) obtained from Khan Al-Zabeeb area is comparable to the worldwide average value for outdoor annual effective dose of 0.07 mSv y1, reported by UNSCEAR (2000). It is clear that most contribution to the total absorbed dose rate in the study area comes from 238U series, particularly 226 Ra radionuclide. Therefore, the radium equivalent activities (Raeq) were calculated using Eq. (3) and the results are presented in Fig. 6. The maximum value of Raeq must be <370 Bq kg1 in order to keep the external dose <1.5 mGy y1 (OECD, 1979). As shown in Fig. 6, the Raeq values, ranged from 121.82 to 702.55 Bq kg1 with a mean value of 387.03 Bq kg1, are exceeding the recommended limit set by OECD (1979). In addition, Fig. 6 presents the values of the external hazard index Igr. The value of Igr must be lower than unity in order to keep the radiation hazard insignificant (Huy and Luyen, 2006). The maximum value of unity for Igr corresponds to the limit of 370 Bq kg1 for Raeq. In this study, the external hazard index, as shown in Fig. 6, was within the range of 0.87e4.72 with a mean value of 2.62, which is much greater than the limit of 1. Ingestion of radionuclides is a source of radiation dose received by the public. The importance of this source increased if the ingested radionuclide is an alpha emitter, as the case with uranium isotopes. The bio-kinetic models used to estimate ingestion dose contain a parameter ( f1), which represents the fraction of the nuclide transferred to the blood from the gut (IAEA, 1996). The dependence of the committed effective

dose via ingestion can be very significant. Hence, the ingestion gut transfer factor ( f1) of value 0.02 as well as the committed effective dose conversion factors of 4.5  108 and 4.9  108 Sv Bq1 due to ingestion of uranium isotopes 238 U and 234U, respectively, was adopted for calculating the committed effective dose. As there are no data available for

D

Raeq

E

1000

Iγr

100

10

1 1

2

3

4

5

6

7

8

9

Sampling Site Fig. 6. External gamma absorbed dose, D (nGy h1); annual committed effective dose equivalent, E (mSv y1); Ra equivalent activities, Raeq (Bq kg1); and external hazard index, Igr calculated for all sampling locations.

S.J. Al-Kharouf et al. / Journal of Environmental Radioactivity 99 (2008) 1192e1199

the plants intake by the public from Khan Al-Zabeeb crops, the maximum annual effective dose per kilogram of watermelon pulp intake was estimated to be 3.1 and 4.7 nSv y1 for 238U and 234U, respectively. It can be seen that these doses are much below the annual dose limit of 1 mSv for the general public (IAEA, 1996). 4. Conclusions Cultivated soils of Khan Al-Zabeeb area present slightly high level of uranium contamination, whereas the 40K and 232Th concentrations are comparable to the worldwide average values. There are, in generally, strong correlations among measured radionuclides indicating that these radionuclides have similar physical transport in soil profiles despite their different chemical behavior. However, disturbances were observed in the radioactive equilibrium of uranium series resulting from different geochemical behavior of these elements. The radiological study performed to quantify the transfer factors of 234U, 235U, and 238U in zucchini and watermelon crops has indicated that plant-specific effects seemed to be considerably important in mobilizing radionuclides in the soil for plant uptake. For both crops, the edible parts showed much lower tendency to accumulate uranium. Therefore, the internal ingestion of uranium showed extremely low values although estimations of radium equivalent activity and external hazard index were seen to exceed the permissible limits. Finally, a study of the kinetics and soileplant transfer of 226 Ra for a number of edible crops at Khan Al-Zabeeb is highly recommended as it showed considerably high concentrations. Acknowledgements The authors sincerely thank Mr. Tahseen Al-Abed from the JNEC for his work in the radiochemical separation. The authors wish to acknowledge the financial and technical support received from BAU, RSS, and JNEC. References Abbady, A.G.E., Uosif, M.A.M., El-Taher, A., 2005. Natural radioactivity and dose assessment for phosphate rocks from Wadi El-Mashash and El-Mahamid Mines. Egypt J. Environ. Radioact. 84, 65e78. Al Hamarneh, I., Wreikat, A., Toukan, K., 2003. Radioactivity concentration of 40K, 134Cs, 137Cs, 90Sr, 241Am, 238Pu and 239þ240Pu radionuclides in Jordanian soil samples. J. Environ. Radioact. 67, 53e67. Al-Kharouf, S.J., 2006. M.Sc. thesis (unpublished), Al-Balqa Applied University, Salt, Jordan. Amaral, R.S., Vasconcelos, W.E., Borges, E., Silveira, S.V., Barbara, P.M., 2005. Intake of uranium and radium-226 due to food crops consumption in the phosphate region of Pernambuco e Brazil. J. Environ. Radioact. 82, 383e393. Beretka, I., Mathew, P.I., 1985. Natural radioactivity of Australian building materials, waste and by-products. Health Phys. 48, 87e95. Bikit, I.S., Slivka, J.M., Krmar, M., Veskovic, J., Conkic, L., Varga, E., Curcic, S., Mrda, D., 2001. Determination of depleted uranium at the Novi Sad low-level laboratory. Arch. Oncol. 94 (4), 241e243.

1199

Bolca, M., Sac, M.M., Cokuysal, B., Karah, T., Ekdal, E., 2007. Radioactivity in soils and various foodstuffs from the Gediz River Basin of Turkey. Radiat. Meas. 42, 263e270. Carini, F., 2001. Radionuclide transfer from soil to fruit. J. Environ. Radioact. 52, 237e279. Chen, S.B., Zhu, Y.G., Hu, Q.H., 2005. Soil to plant transfer of 238U, 226Ra and 232 Th on uranium mining-impacted soil from southeastern China. J. Environ. Radioact. 82, 223e236. Chiozzi, P., Pasquale, V., Verdoya, M., 2002. Naturally occurring radioactivity at the AlpseApennines transition. Radiat. Meas 35, 147e154. Currie, L., 1968. Limits for qualitative detection and quantitative determination: application to radiochemistry. Anal. Chem. 40, 586e591. Debertin, K., Helmer, R.G., 1988. Gamma- and X-ray Spectrometry with Semiconductor Detectors. North-Holland, Amsterdam. Drya´k, P., Kova´rˇ, P., Plachova´, L., Sˇura´n´, J., 1989. Corrections for the Marinelli geometry. Radioanal. Nucl. Chem., Lett. 4 (135), 281e292. El Mamoney, M.H., Khater, A.E.M., 2004. Environmental characterization and radio-ecological impact of non-nuclear industries on the Red Sea coast. J. Environ. Radioact. 73, 151e168. Fernandes, H.M., Filho, F.F.L., Perez, V., Franklin, M.R., Gomiero, L.A., 2006. Radioecological characterization of uranium mining site located in semi-arid region in Brazil. J. Environ. Radioact. 88, 140e157. Fujiyoshi, R., Sawamura, S., 2004. Mesoscale variability of vertical profiles of environmental radionuclides (40K, 226Ra, 210Pb, and 137Cs) in temperate forest soils in Germany. Sci. Total Environ. 320, 177e188. Huy, N.Q., Luyen, T.V., 2006. Study on external exposure doses from terrestrial radioactivity in southern Vietnam. Radiat. Prot. Dosim. 118 (3), 331e336. IAEA, 1996. International basic safety standards for protection against ionizing radiation and for the safety of radiation sources. Safety series No. 115, Vienna, Austria. Ivanovich, M., Harmon, R.S., 1992. Uranium Series Disequilibrium: Applications to Earth, Marine and Environmental Sciences. Clarendon Press, Oxford. Karangelos, D.J., Anagnostakis, M.J., Hinis, E.P., Simopoulos, S.E., Zunic, Z.S., 2004. Determination of depleted uranium in environmental samples by gamma-spectroscopic techniques. J. Environ. Radioact. 76, 295e310. Kohshi, C., Takao, I., Hideo, S., 2001. Terrestrial gamma radiation in Koshi prefecture. Jpn. J. Health Sci. 47 (4), 362e372. NEA-OECD, 1979. Nuclear energy agency. Exposure to Radiation from Natural Radioactivity in Building Materials. Report by NEA Group of Experts. OECD, Paris. Papachristodoulou, C.A., Asymakopoulos, P.A., Patronis, N.E., Ioannidis, K.G., 2003. Use of HPGe g-ray spectrometry to assess the isotopic composition of uranium in soils. J. Environ. Radioact. 64, 195e203. Pulhani, V.A., Dafauti, S., Hegde, A.G., Sharma, R.M., Mishrad, U.C., 2005. Uptake and distribution of natural radioactivity in wheat plants from soil. J. Environ. Radioact. 79, 331e346. Saueia, C.H.R., Mazzilli, B.P., 2006. Distribution of natural radionuclides in the production and use of phosphate fertilizers in Brazil. J. Environ. Radioact. 89, 229e239. Sheppard, S.C., Sheppard, M.I., Llin, M., Thompson, P., 2005. Soil-to-plant transfers of uranium series radionuclides in natural and contaminated settings. Radioprotection Suppl. 40 (1), 253e259. UNSCEAR, 1988. Sources, Effects and Risks of Ionizing Radiation, Report on the General Assembly. United Nations, New York. UNSCEAR, 2000. Ionizing Radiation: Sources and Effects on Ionizing Radiation. Report to the General Assembly, with Annexes. United Nations, New York. Vajda, N., Molna´r, Zs., Kabai, E., Osvath, Sz, 2003. Simultaneous determination of long-lived radionuclides in environmental samples. In: Birsen, N., Kadyrzhanov, K.K. (Eds.), Environmental Protection against Radioactive Pollution. 9th Int. Symposium on Environ. Radiochem. Anal., Maidstone, U.K., 18e20 Sept. 2002. Royal Chemical Society, pp. 133e146. Vera Tome, F., Blanco Rodrı´guez, M.P., Lozano, J.C., 2003. Soil-to-plant transfer factors for natural radionuclides and stable elements in a Mediterranean area. J. Environ. Radioact. 65, 161e175.