Radiological impact on uncultivated soil and Dittrichia viscosa plants around a Lebanese coastal fertilizer industry

Dany Saba; Omar El Samad; Rana Baydoun; Rola Bou Khozam; Nastaran Manouchehri; Lina Nafeh Kassir; Amine Kassouf; Hanna Chebib; Philippe Cambier; Naim Ouaini

doi:10.4103/rpe.RPE_15_20

ORIGINAL RESEARCH

Year : 2020 | Volume : 43 | Issue : 2 | Page : 61-69

Radiological impact on uncultivated soil and Dittrichia viscosa plants around a Lebanese coastal fertilizer industry

Dany Saba¹, Omar El Samad², Rana Baydoun², Rola Bou Khozam², Nastaran Manouchehri³, Lina Nafeh Kassir⁴, Amine Kassouf⁵, Hanna Chebib⁵, Philippe Cambier⁶, Naim Ouaini⁷
¹ Department of Food and Bioproducts Sciences and Processes, UMR SayFood, AgroParisTech, INRA, University of Paris-Saclay, Paris, France; Department of Chemistry and Biochemistr, Faculty of Arts and Sciences, Holy Spirit University of Kaslik, Jounieh, Lebanon
² Department of Environmental Radiation Control, Lebanese Atomic Energy Commission, National Council for Scientific Research, Tripoli, Lebanon
³ Department of Food and Bioproducts Sciences and Processes, UMR SayFood, AgroParisTech, INRA, University of Paris-Saclay, Paris, France
⁴ Department of Life and Earth Science, Faculty of Sciences, Lebanese University, Tripoli, Lebanon
⁵ Department of Chemistry and Biochemistry, Faculty of Sciences II, Lebanese University, Fanar, Lebanon
⁶ Department of Environment and Agronomy, UMR ECOSYS, AgroParisTech, INRA, University of Paris-Saclay, Thiverval-Grignon, France
⁷ Department of Chemistry and Biochemistr, Faculty of Arts and Sciences, Holy Spirit University of Kaslik, Jounieh, Lebanon

Date of Submission	07-Apr-2020
Date of Decision	02-May-2020
Date of Acceptance	05-Jun-2020
Date of Web Publication	27-Aug-2020

Correspondence Address:
Dr. Dany Saba
Department of Food and Bioproducts Sciences and Processes, UMR SayFood, AgroParisTech, INRA, University of Paris-Saclay, Paris; Department of Chemistry and Biochemistry, Faculty of Arts and Sciences, Holy Spirit University of Kaslik, B.P. 446, Jounieh

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/rpe.RPE_15_20

Abstract

Chemical fertilizers, phosphate ore treatments, and phosphogypsum wastes contribute to enhanced levels of natural radionuclides in the environment. A total of 27 soil samples were collected from nine uncultivated sites around a Lebanese fertilizer plant in order to analyze the gamma emitter radionuclides (²³⁸U,²³²Th,²²⁶Ra,²¹⁰Pb,¹³⁷Cs, and⁴⁰K) and to assess the radiological impact on the surrounding environment, through the calculation of different radiological index parameters. In addition, a total of 27 Dittrichia viscosa plant samples were gathered including roots, leaves, and stems, and the radionuclide transfer factors were determined. Measurements were conducted using a gamma spectrometer with high-purity germanium detectors. The highest values measured in soil samples were 77 ± 9 Bq/kg, 102 ± 10 Bq/kg, and 143 ± 5 Bq/kg for²³⁸U,²²⁶Ra, and²¹⁰Pb, respectively.⁴⁰K levels were comparable to other Lebanese provinces and about 50% less than the worldwide average value. The results showed the absence of radionuclide transfer between soil and plants, except for⁴⁰K. The average values of the total absorbed dose rate and the annual effective dose were comparable to the worldwide average values. Therefore, the external exposure index and Radium equivalent were found to be below the international recommended values.

Keywords: Radiological index parameters, fertilizer plant, gamma emitters, gamma spectrometer, phosphogypsum, radionuclides

How to cite this article:
Saba D, El Samad O, Baydoun R, Khozam RB, Manouchehri N, Kassir LN, Kassouf A, Chebib H, Cambier P, Ouaini N. Radiological impact on uncultivated soil and Dittrichia viscosa plants around a Lebanese coastal fertilizer industry. Radiat Prot Environ 2020;43:61-9

How to cite this URL:
Saba D, El Samad O, Baydoun R, Khozam RB, Manouchehri N, Kassir LN, Kassouf A, Chebib H, Cambier P, Ouaini N. Radiological impact on uncultivated soil and Dittrichia viscosa plants around a Lebanese coastal fertilizer industry. Radiat Prot Environ [serial online] 2020 [cited 2023 Mar 28];43:61-9. Available from: https://www.rpe.org.in/text.asp?2020/43/2/61/293621

Introduction

The naturally occurring radioisotopes in the environment are the series of²³⁸ U,²³² Th, and⁴⁰ K.^[1] Several anthropogenic activities emit large quantities of radionuclides into the environment, causing a major potential risk to the ecosystem and damaging the soil and vegetation.^[2] Chemical fertilizer plants are widespread around the world and generally use sedimentary phosphate rocks, that contain large amounts of radionuclides, as raw material.^[3] The main fertilizers produced are single superphosphate (SSP) and triple superphosphate (TSP) under the action of sulfuric acid (H₂ SO₄) and phosphoric acid (H₃ PO₄), respectively.^[4] Phosphate fertilizers, as well as the by-product phosphogypsum (PG), are considered to be concentrated with natural radionuclides of the Uranium series and toxic trace metals.^[4] Several studies, carried out in different countries, showed that industrial (PG) discharges can increase and redistribute the levels of natural radionuclides in soil profiles, leading to their uptake by plants, and thus, their transfer to the human and animal food chains.^[3] Indeed, soil–plant transfer depends on the studied radionuclide and the plant species.^[5],[6],[7] On another side, radioisotopes and nutrients can reach the plants through two mechanisms; dry or wet deposition and/or root uptake. Because the process is not selective, plants can also take up toxic elements from the soil, such as radionuclides.^[8]

In Lebanon, a fertilizer plant located in the northern region uses about 840 tons per day of sedimentary phosphate rocks (Ca₁₀F₂(PO₄)₆) imported from Syria.^[9],[10] It produces about 230,000 tons of SSP, 31,000 tons of TSP, and 160,000 tons of phosphoric acid per year.^[11] Many studies carried out around this plant, concerning biota, coastal soil, seawater, and sediment samples,^[12],[13] indicated that the surrounding environment was affected by the (PG) wastes released, either dumped in the sea or stocked on land.^[14],[15] This could cause significant risk for the surrounding environment (fauna and flora), resulting in adverse consequences on human and animal health.^[3] The transport of raw materials, industrial processes, wind direction and velocity, and precipitation play a fundamental role in the dispersion and deposition of dust loaded with contaminants in the environment and even toward distant regions.^[16],[17]

In this work, the activity concentrations of different gamma emitters radionuclides in uncultivated soils collected from the surrounding of the fertilizer plant will be determined. This aims to the estimation of radiological impact through the calculation of radiological parameters such as absorbed dose rate, annual effective dose, external exposure index, and radium equivalent. In addition, soil–plant transfer will be studied based on the activity concentration of radionuclides in Dittrichia viscosa plant species commonly distributed in the studied area and consumed in the form of herbal tea by the habitats of the region. The shrub is considered to be a pollution indicator, knowing that it has sticky leaves capturing the atmospheric fallout and diffuse pollution.

Materials and Methods

Study area

The study area is located in the Northern Lebanon province which is 40 km from the capital Beirut and near the Mediterranean coast. It is characterized by Eutric Leptosols and Eutric Fluvisols soil^[18] and pedo-geochemical background. Samples were collected from uncultivated areas located at different distances from the chemical fertilizer plant and from the areas adjacent to the trajectory of trucks transporting raw material. Uncultivated sites, with no added fertilizers, were chosen in order to get only the diffuse aerial deposit of radionuclides, i.e., the fertilizer plant. The sampling sites were designated as A, B, C, D, E, F, G, H, and I, with a reference site A. The distance and the direction of the sampling sites according to the plant are presented in [Table 1].

Table 1: Activity concentrations of radionuclides in soil samples at top layer and comparison with other studies

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[Figure 1] shows the locations of the sites as well as the wind direction that is southwest most of the year, according to the Wind Rose for Beirut International Airport.^[16] These uncultivated sites are located near residential and agricultural areas. Site A is a reference site having same characteristics as the studied area, located far away from the adjacent roads, at 1.5 km southeast from the plant and could not be affected by its pollution.

Figure 1: Fertilizer plant and study area

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Sampling and sample preparation

Two sampling campaigns of soil and plant samples were carried out in 2016 and 2017. A total of 27 soil samples, around 5 kg each, and 27 plant samples, around 600 g each, were taken from nine uncultivated terrains, distributed at different distances around the chemical fertilizer plant. The selection of sites was based on prior background studies conducted in the same region.^[12],[17] Three depth intervals were chosen for the study (0–5, 6–15, and 16–25 cm), to address the most probably contaminated surface layer and the layers involved in the soil–plant transfer. These depth intervals were chosen following a preliminary study that compared two depth profiles of radionuclides in (Ps) site (0–5/6–20 cm and 0–10/11–20 cm) which showed different distributions of radionuclides in the soils. The sampling soil area was from a flat (25 cm × 30 cm) area. In order to study the soil–plant transfer, an annual plant “D. viscosa” from Asteraceae family was chosen, as it is widely spread in the studied area and its leaves are traditionally consumed by local inhabitants and also used for medicinal purposes as an anticoagulant and antifungal agent. These plants were randomly taken from the uncultivated sites where the soil samples were collected. Large leaves at the end of the plant's living period were gathered so as to assure maximum accumulation of radionuclides. The collected samples were kept in well-sealed, airless, labeled polyethylene bags. All stones and impurities were eliminated from the soil samples. All soil samples were then sieved and ground in an agate mortar and pestle down to 0.2 mm, and then dried in an oven at 100°C for 48 h until constant weight. Radiological impact of a fertilizer industry. As for the plants, three different parts were studied (leaves, stems, and roots) and compared in order to determine the origin of radionuclides, whether from deposition or soil transfer, and which portions accumulate radionuclides the most. The plant's roots extended to a depth of from 4 to 24 cm were collected in order to determine soil–plant transfer, and were cleaned of all sand around their roots. All the three different parts were dried in the oven at 50°C until constant weight, and then ground and homogenized. All samples were preserved in polyethylene containers of 500 mL and sealed with thick vinyl tapes for 21 days in order to reach a secular equilibrium between²²⁶ Ra and its daughter isotopes (²¹⁴ Bi and²¹⁴ Pb).^[14],[20]

Measurement technique

Sample measurement was conducted at the accredited Gamma Spectroscopy Laboratory of the Lebanese Atomic Energy Commission, according to ISO/IEC 17025 standards for calibration and testing laboratories, using a high-resolution gamma spectrometer with high-purity germanium detector extended range, with Beryllium window and relative efficiency of 50%. To assure the accuracy and the reliability of the results, certified standard reference multigamma sand from Eckert and Zeigler Company (code EG-ML-1195-11-1), and a leftover proficiency test grass sample, MAPEP-RdV-2013, from the Department of Energy-Mixed Analyte Performance Evaluation Program, USA, were counted and analyzed routinely as part of the overall quality control procedure and for the energy calibration. The efficiency calibration was carried out by counting mixed 500-mL multigamma solution from GE Healthcare Limited (Amersham, Buckinghamshire, England) (code R8/31/32) and then plotting the efficiency curve (efficiency versus energy). The efficiency curves were plotted for various compositions, densities, and geometries using EFFTRAN software (Belgian Nuclear Research Center, Mol, Belgium) taking into consideration attenuation and absorption factors.^[29],[30] Efficiencies for the corresponding energies used are given in [Table 2].

Table 2: Efficiencies for soil and plant samples of the corresponding energies used

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Performance test and linearity of the detectors were checked periodically using¹⁵² Eu point source. The performance test is conducted at low and high energies, 122 keV and 1408 keV, respectively. The²¹⁰ Pb was determined via its gamma emissions at 46.5 keV.²³⁸ U was determined from the gamma line of its daughter^234m Pa at 1001 keV, and²³² Th was determined from its daughter²²⁸ Ac at 911 keV,^[20] while²²⁶ Ra was determined directly from its gamma line at 186 keV after correction for²³⁵ U interference.^[12],[30] The net peak area used in the calculation of²²⁶ Ra activity concentration consists in subtracting the net peak area corresponding to²³⁵ U at the gamma line 185.7 keV from the total net peak area at 186 keV. The latter is calculated from the activity concentration of²³⁵ U at the gamma line 163.3 keV.

All results were expressed in Bq/kg dry weight. The minimum detectable activities (MDA) of²³⁸ U,²³⁵ U,²²⁸ Ac,²²⁶ Ra,²¹⁰ Pb,¹³⁷ Cs, and⁴⁰ K in soil samples were, respectively, 30 Bq/kg, 3 Bq/kg, 3 Bq/kg, 10 Bq/kg, 10 Bq/kg, 0.4 Bq/kg, and 4 Bq/kg. The MDA of²³⁵ U,²²⁸ Ac,²²⁶ Ra,²¹⁰ Pb,¹³⁷ Cs, and⁴⁰ K in plant samples were, respectively, 6 Bq/kg, 2.6 Bq/kg, 10 Bq/kg, 7 Bq/kg, 0.4 Bq/kg^, and 11 Bq/kg.

Statistical analysis

The data were statistically analyzed using IBM SPSS Version 22 (IBM Corp., New York City, New York State, USA). A one-way ANOVA test was performed on²¹⁰ Pb activity concentration, measured on soil samples, distributed according to the sampling depth in all sites: 0–5, 6–15, and 16–25 cm. Spearman's correlation test was conducted on the studied radionuclides, as the data do not fit Gaussian distribution, to determine the origin of contamination in the soil samples. A significant difference was recognized, as P < 0.05, for a confidence interval of 95%.

Results and Discussion

Radioactivity concentration in soil

All results of the gamma spectrometry analysis for soil samples, expressed in Bq/kg dry weight, the world average values, and the average radioisotope data reported by other studies are presented in [Table 1]. The activity concentration of radionuclides in the topsoil layer in the analyzed soil samples ranged from 34 ± 6 to 77 ± 9 Bq/kg, 17 ± 1 to 56 ± 2 Bq/kg, 47 ± 4 to 102 ± 10 Bq/kg, 39 ± 2 to 143 ± 5 Bq/kg, and 100 ± 3 to 316 ± 9 Bq/kg for²³⁸ U,²³² Th,²²⁶ Ra,²¹⁰ Pb, and⁴⁰ K, respectively. These data were comparable with the ranges of coastal soil samples in the same area^[12] for²³⁸ U, but higher for²³² Th and⁴⁰ K. The activity concentration values for²³⁵ U were found to be below the MDA (3 Bq/kg), except for sites (B), (C), and (H) where the values were slightly higher than the MDA; 5.2 ± 0.5 Bq/kg, 3.2 ± 0.4 Bq/kg, and 4.1 ± 0.4 Bq/kg, respectively. The highest activity concentration value of⁴⁰ K was 316 ± 9 Bq/kg in site (A), while high activity concentrations of²³⁸ U,²²⁶ Ra, and²¹⁰ Pb were found in site (B); 69 ± 7 Bq/kg, 70 ± 15 Bq/kg, and 109 ± 4 Bq/kg, respectively, and in site (D); 77 ± 9 Bq/kg, 102 ± 10 Bq/kg, and 143 ± 5 Bq/kg, respectively. As for²³² Th, higher activity concentrations were found in sites (C), (D), and (F) with 56 ± 2 Bq/kg, 53 ± 2 Bq/kg, and 51 ± 2 Bq/kg, respectively. The values of²³⁸ U activity concentrations were found to be comparable to those of UNSCEAR (2000)^[19] and values reported in North Lebanon and Mount Lebanon.^[20],[21] The values were compared to those of other countries. They were found comparable to Greece, Iran, and Palestine^{[15],[20],[21],[24],[25]} except for sites (B) and (D), where values were twice the world average and values reported in Palestine and other Lebanese provinces. These values were twice higher than the values found in Syria for all sites, except for sites (B) and (D), where²³⁸ U activity concentrations were found to be four times higher.^[23] Values of⁴⁰ K activity concentrations in all sites were half the value reported by UNSCEAR (2000),^[19] Iran,^[25] Turkey,^[27] India,^[28] and by studies conducted in Europe^[22],[24] and higher than those found in Syria^[23] and Jordan,^[26] but they are in accordance with the work conducted by El Samad et al. in 2013 and 2018^[20],[21] in other Lebanese provinces and Palestine.^[15]²²⁶ Ra activity concentration values in all the sites were found to be comparable to the average value found in India^[28] and higher than the average value reported by the UNSCEAR (2000),^[19] North Lebanon, Iran, and Greece, except for site (E) where activity concentration of²²⁶ Ra was found to be comparable with that of Mount Lebanon and Turkey.^{[20],[21],[25],[27]} As for²³² Th, results were found to be lower than the average value found in India,^[28] comparable to the average values for the UNSCEAR, Europe, Syria, Palestine, Jordan, and other Lebanese provinces except for sites (C), (D), and (F), where values were higher and comparable to those found in Turkey and Iran.²²⁶ Ra was strongly correlated with²³⁸ U (R = 0.80) and²¹⁰ Pb (R = 0.82), as shown in [Table 3], but was not correlated with²²⁸ Ac (R = 0.03). This indicates that these three radionuclides have the same origin and chemical behavior in soils.^[12],[17] PG and phosphate ores contain more uranium and daughter isotopes, such as²²⁶ Ra and²¹⁰ Pb than²³² Th and⁴⁰ K, which can be related to its low activity concentration compared to other radionuclides in our soil samples. This trend is consistent with other studies conducted on PG and phosphate rock.^[4] Site (A) in Koubba, at 1.5 km southeast of the plant, was selected as a reference as it is not affected by the plant. The gamma-emitting radioisotope activity concentrations of the the site (A) were comparable to the worldwide average value for²³⁸ U, lower for²³² Th, and higher for²²⁶ Ra. The site (E), chosen to the south of the plant, in the opposite wind direction, also shows activity concentrations comparable to the worldwide values. For sites (B) and (I), along the routes of the trucks transporting raw material, the results in [Table 1] show that the activity concentration of²²⁶ Ra and²³⁸ U was higher than the worldwide values and higher than the activity concentrations of their daughters²¹⁴ Pb and²¹⁴ Bi, 61 ± 1 Bq/kg and 54 ± 1 Bq/kg, respectively, for site (B), 32 ± 1 Bq/kg and 29 ± 1 Bq/kg, respectively, for site (I). A higher activity concentration value for²²⁶ Ra and²³⁸ U could be due to there being slight contamination from the dust coming from the transport trucks. Sites (C), (F), (G), and (H), located to the northeast of the plant, showed higher activity concentrations for²²⁶ Ra more than its parent²³⁸ U. These results could be the consequence of the dispersion of PG waste by the plant. The high concentrations of²³⁸ U,²²⁶ Ra, and²¹⁰ Pb were found in the site (D) located north of the plant, with an activity concentration of²²⁶ Ra and²¹⁰ Pb higher than their parent²³⁸ U. These results could be attributed to the release of the PG by the plant, and the high level of²²⁶ Ra can be explained by the transport trucks spreading PG dust, rich in²²⁶ Ra (≈5% of PG were added to cement), because this site is on the trucks' route. Soil samples collected to the north or northeast of the plant showed higher radionuclide activity concentrations than those located to the south. This can be explained by the southwesterly wind that brings dust and disperses the contaminants coming from the chemical fertilizer plant.^[16]

Table 3: Spearman's correlation coefficients (R)

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Vertical distribution

Natural activity concentrations of the studied radionuclides were homogeneously distributed vertically in site (A) between the different soil layers. This was consistent with other studies showing that normally, the activity concentrations of natural²³⁸ U and²³² Th radioisotopes series are distributed identically in the different soil layers if there were no exogenous contamination contribution.^[21] As shown in [Figure 2], a negative trend was found in all sites, with²¹⁰ Pb showing higher activity concentrations values in the surface soil layer, except in site (A), where a homogenous distribution of²¹⁰ Pb was observed. This vertical distribution showed that the studied sites were affected by the plant's PG releases, the effect of trucks, wind direction, and meteorological data influencing radionuclide fallout. A one-way ANOVA test was used to test if the variation of the activity concentration of²¹⁰ Pb as a function of depth is significant or not. The test showed a statistically significant difference in²¹⁰ Pb values at each depth, in all sites except site (A), with P < 0.05. Therefore, sites (B), (C), (D), (G), (H), and (I) show higher concentrations of radioactive²¹⁰ Pb on the surface in comparison with site (A), more basically in sites (B), (D), and (H) showed 109 ± 4, 143 ± 5, and 90 ± 4 Bq/kg, respectively. These data indicate that there is an exogenous human-made source of²¹⁰ Pb at the surface, which can be explained by the wet and dry deposition incriminated in its deposition within the topsoil layers because it is the daughter of the Uranium chain series. This²¹⁰ Pb accumulation on the soil surface is a consequence of human activity as the superficial layer of soils is a good indicator of lead deposition. These high levels in the topsoil layer could be attributed to the chemical fertilizer plant located near these fields, which releases PG by-product, rich in²²⁶ Ra and its daughters.^[12] As for sites (C) and (F), both approximately located in the same field, the²¹⁰ Pb activity concentration was found higher in the superficial soil layers in the site (C) than in site (F). This could be attributed to the wet deposition of the radionuclide, especially that sampling from site C was carried out after small amounts of rain. The plausible enrichment in²¹⁰ Pb in these sites, compared with reference site, could be attributed to the northeastern wind direction prevailing the most period of the year, that could transport and disperse the contaminants toward sites C and F even though they are more far than others' sites.^[21]

Figure 2: Vertical distribution of²¹⁰Pb

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The results showed the presence of¹³⁷ Cs in all the analyzed samples with an activity concentration ranging from 0.5 ± 0.1 to 26.1 ± 0.8 Bq/kg [Table 1].¹³⁷ Cs is an artificial radioactive fallout element in Lebanese soil resulting from the Chernobyl nuclear accident that occurred in 1986.^[31],[32] It has been studied in order to determine if there were disturbances at the different soil layers. As shown in sites (B, C, D, F, and H), the exponential decrease of¹³⁷ Cs is proof that no disturbances in soil layers have taken place. Higher activity concentration of¹³⁷ Cs was found in site (D) (26.1 ± 0.8 Bq/kg). This could be attributed to the fact that site (D) is a water-gathering point due to its geographic characteristic. Because¹³⁷ Cs is easily transported through water,^[33] all of these factors have resulted in its high activity concentration in this site. However, this value is still comparable to values found in other studies conducted in Lebanese provinces.^[20],[21] However, no exponential decrease was shown in sites (A, E, G, and I), which means that there was soil degradation.^[32],[34] This can prove that²¹⁰ Pb at top layers in all sites, except in (A) and (E), is due to plant discharges and air deposition. However, sites (G) and (I) did not present an exponential decrease in activity concentration of¹³⁷ Cs but showed a decrease in²¹⁰ Pb activity concentration. This can be attributed to the geochemical composition of the soil and environmental conditions in the studied area.^[35]

Radioactivity concentrations in plants

The gamma spectrometry measurements for D. viscosa dried plant samples, for the different parts (leaves, stems, and roots) and the corresponding transfer factors (TFs), are given in [Table 4]. The results showed that radionuclide activity concentrations, for both the Thorium and Uranium series, are less than the MDA in most of the sites.²³⁵ U values were below MDA (6 Bq/kg) in all plant samples. Results for²²⁶ Ra were below the MDA (10 Bq/kg) in all sites, except in sites (D) and (E) where values were, respectively, 27 ± 4 Bq/kg in leaves and 14 ± 2 Bq/kg in roots. The presence of²²⁶ Ra in root samples of site (E) can be explained by the possible presence of soil particles. Despite their proximity and their location on the truck's route,²²⁶ Ra was detected in leaf samples of site (D) but not in sites (B) and (H). This can be attributed to the re-suspension and adhesion of soil to leaves,^[36] considering the high activity concentration in²²⁶ Ra in this site in comparison with others. In addition, PG dusts, rich in²²⁶ Ra, dispersed by the southwesterly wind, can explain such high concentration. Values of¹³⁷ Cs were 0.5 ± 0.1 Bq/kg in leaves, 1.0 ± 0.1 Bq/kg in roots in site (B), 0.7 ± 0.1 Bq/kg, 1.1 ± 0.1 Bq/kg, 0.6 ± 0.1 Bq/kg and 0.5 ± 0.1 Bq/kg for sites (C), (D), (G) and (H) respectively, but below the MDA (0.4 Bq/kg) in the other sites. The activity concentration of²²⁸ Ac,²¹⁰ Pb, and⁴⁰ K ranged from 3.0 ± 0.5 Bq/kg to 10 ± 1 Bq/kg, 10 ± 2 Bq/kg to 58 ± 3 Bq/kg, and 96 ± 4 Bq/kg up to 447 ± 14 Bq/kg, respectively. As shown in [Table 4], the highest values for²²⁸ Ac,²¹⁰ Pb, and⁴⁰ K were all found in plant leaves, respectively, in the sites (D), (E), and (A); these were used to determine the TF.

Table 4: Activity concentrations of radionuclides in the different plant parts and corresponding transfer factors

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Soil–plant transfer

The soil–plant TF of each radionuclide [Table 4] was calculated according to the following equation^[5],[7] which shows the accumulation of radionuclide by plants or their transfer in leaves from the corresponding soils (16–25 cm) through roots:^[37]

TF = A_p/A_s

Where TF is the TF, A_p and A_s represent the activity concentration of radionuclides in leaves and in soil collected from the layer (16–25 cm), respectively, in dry weight (Bq/kg).^[6]

[Table 4] shows that TF ratios for all the studied radionuclides (²²⁸ Ac,²²⁶ Ra, and¹³⁷ Cs) are <1 in all plants samples except for⁴⁰ K and²¹⁰ Pb. These results showed that there was no transfer of radionuclides such as²²⁸ Ac,²²⁶ Ra, and¹³⁷ Cs from roots to leaves. TF was >1 for⁴⁰ K in all sites except for site G and TF was >1 for²¹⁰ Pb in sites B, D, E, and H. The high transfer of⁴⁰ K can be related to its high mobility in soil and its subsequent plant uptake.^[38] The relatively high²¹⁰ Pb activity concentration in plants in comparison to its concentration in soil could be due to atmospheric fallout and hence could be the reason of high TF.^[12]

Determination of radiation index parameters

The absorbed dose rate (D), the annual effective dose rate (E), the external exposure index (E_ex), and the radium equivalent (Ra_eq) were calculated, respectively, using the following equations:

D = 0.427 A_Ra+ 0.604 A_Th+ 0.0417 A_K+ 0.03 A_Cs
E = T. f. Q. D.ε
E_ex= A_Ra/370 + A_Th/259 + A_K/4810
Ra_eq= A_Ra+ 1.43 A_Th+ 0.077 A_K

Where A_Ra, A_Th, A_K, and A_Cs, represent the activity concentrations of²²⁶ Ra,²³² Th,⁴⁰ K, and¹³⁷ Cs, respectively, in soil samples, in Bq/kg. T is time in seconds (8760 h/year), f is the occupancy factor which corrects the average time spent outdoors in the sites (0.2), Q is the quotient of the effective dose rate and absorbed dose rate in the air (0.7 Sv/Gy), D is the absorbed dose rate in air in (nGy/h), and ɛ is the factor converting nano (10⁻⁹) into micro.^{[6],[7],[8],[9],[10]}

The results of the total absorbed dose rates D expressed in nGy/h, total annual effective dose E in mSv/year, external exposure index E_ex in mSv/year, and radium-equivalent Ra_eq in Bq/kg are shown in [Table 5].

Table 5: Radiological parameters for the soil samples and comparison with other studies

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Absorbed dose rate and annual effective dose

The absorbed dose rate (D) was estimated in the air at 1 m above terrestrial level due to artificial and natural gamma emitter radionuclides in topsoil samples in order to assess radiation risk.^[40] The total annual effective dose (E) was assessed on the exposed public in order to evaluate biological exposure.^[19],[41] About 99% of the total dose calculated comes from natural radionuclide radiation. [Table 5] shows that the total absorbed dose rate and the total annual effective dose ranged, respectively, from 41 to 84 nGy/h, with a mean value of 56 ± 13 nGy/h, and from 0.05 to 0.10 mSv/year, with a mean value of 0.07 ± 0.02 mSv/year. The average value of the total absorbed dose rate was in accordance with the world average criteria (57 nGy/h)^[19] and lower than studies conducted in Turkey and Iran.^[25],[27] The total annual effective dose average value was seven times lower than the worldwide average value for terrestrial radiation (0.5 mSv/year)^[19] and comparable to average values found in Turkey and Iran.^[25],[27] Both average doses were found comparable to other studies conducted in Northern Lebanon,^[20] but higher than the average values reported in Mount Lebanon^[21] and in other countries such as Jordan, Kuwait, and Palestine.^{[15],[26],[39]}

External exposure index and radium equivalent

The external exposure index that is an essential parameter used to limit the radiation exposure of the population to natural radioactivity to by unity and the radium equivalent that defines the exposure due to gamma radiation,^[26],[42] have been compute. [Table 5] shows that the external exposure index and the radium-equivalent values ranged from 0.3 to 0.5 with an average value of 0.3 ± 0.1 and from 92 to 192 Bq/kg with an average of 127 ± 30 Bq/kg, respectively. The average values of both parameters appeared to be lower than the accepted global levels respectively of unity and 370 Bq/kg.^[19] The results of the external exposure index were found to be comparable to the ones found in Mount Lebanon,^[21] Jordan, Iran, and Turkey,^{[25],[26],[27]} and slightly higher than the average value found in Kuwait and Palestine.^[15],[39] The average value of radium equivalent was higher than the average value reported in a recent study conducted in Mount Lebanon, Kuwait, Jordan, and Palestine^{[15],[21],[26],[39]} and in accordance with the findings in Turkey and Iran.^[25],[27]

Conclusions

In this work, the activity concentrations of natural and artificial gamma emitter radionuclides, in soil and D. viscosa plant samples, were measured around a chemical fertilizer industry in Selaata, Lebanon, using gamma spectrometry. The highest values measured in the soil samples were 77 ± 9 Bq/kg, 102 ± 10 Bq/kg, and 143 ± 5 Bq/kg for²³⁸ U,²²⁶ Ra, and²¹⁰ Pb, respectively. Because the level of natural radionuclide activity concentrations in the studied soil samples was comparable to those determined in other Lebanese regions and only slightly higher than the acceptable worldwide average values, we could deduce that there are no serious radiological risks for the inhabitants of the region. Even if low activity concentrations of these natural radionuclides were found in plant samples, a continuous intake through the food chain may pose some health risks for consumers in the long term. Results of this study indicate that the fertilizer industry increases the amounts of radionuclide and show that sites located in the direction of the wind (northeast), (B), (C), (D), (F), (G), and (H) are affected by the plant and present higher activity concentration of radionuclides. The sites (A) and (E), located southeast and south the factory, showed the lowest value of radionuclides, whereas sites (B) and (D), located in the wind direction, presented the highest values. The winds in the area have disperse draw material, transported by trucks to be deposited in zones surrounding the trajectory. As these elements are toxic to humans, further study of their mobility and bioaccessibility, simulating the ingestion of soil dust or plant consumption by the nearby population and identifying the radioactive isotopes accessible in the human body, should be carried out.

Acknowledgment

The authors of this article are grateful to the National Council for Scientific Research (CNRS-Lebanon) for its financial support.

Financial support and sponsorship

This study was financially supported by the National Council for Scientific Research (CNRS-Lebanon).

Conflicts of interest

There are no conflicts of interest.

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