Transfer of 40 K, 226 Ra and 210 Pb from soil to plants in various locations of El-Jadida agricultural area (north-western Morocco)

. The present study examines the radionuclides ( 40 K, 226 Ra and 210 Pb) concentrations in soil and plants collected from El Jadida agricultural area. The transfer factors of radionuclides from the soil to different components of the plants (stems, roots, leaves, and fruits) was also determined and assessed. The plants include various vegetable crops, including pumpkin, zucchini, beans, fennel, potato, sunflower, broccoli, onion, carrot, cabbage, tomato, and mallow from three different locations. A wide-range HPGe gamma-ray spectrometer is used for radionuclide activity measurement. The radionuclides activities of soil samples display a wide range of variability from one location to another. The concentrations of 226 Ra vary between 18 ± 1 Bq.Kg -1 and 80 ± 4 Bq.Kg -1 , the concentrations of 210 Pb range from 49.7 ± 4.4 Bq.Kg -1 to 269 ± 16 Bq.Kg -1 , and the 40 K values fluctuate between 94,1 ± 6,5 and 286,1 ± 15,3 Bq.Kg -1 . The study showed that the transfer factor for K-40 is more significant than Pb-210 and Ra-226, mainly related to the use of fertilizers rich in potassium in the agricultural areas, leading to its concentrations increase.


Introduction
Radionuclides occur naturally in Earth's crust. This naturally occurring radioactive element is part of the biosphere and is present in the soil, sediment, air, water, food, biota, and human bodies [1]. Soil and air are known as pathways to the transfer of radionuclide to the plant. The Transfer factor (TF) soil-to-plant is widely utilized for measuring radiological human dose through the ingestion pathway [2] and as a convenient and useful approach allowing to define the number of radionuclides estimated to enter a plant from the soil [3][4]. Otherwise, TF is considered one of the important parameters utilized for the environmental safety assessment required for nuclear facilities [5].
Up to date, many studies have investigated the transfer of radionuclides from soil to plants [6,7,8,9,10,11,12,13]. In the El Jadida region of northwest Morocco, almost all soil research has focused on evaluating the concentration of heavy metals and soil quality [14][15]. In contrast, studies focusing on the analysis of radionuclide concentrations in the El Jadida area and surrounding areas use sediments as quantification of the concentration of radioactive elements (such as Pb-210; K-40; Cs-137, Ra-228) and for establishing The pollution history of these elements, heavy metals and rare earth elements [16,17,18,19,20,21,22,23]. Thus, the present study is the first assessment of radionuclides concentrations in El Jadida's agricultural areas aiming at identifying and quantifying the radionuclides concentration and their Tfs from soil to plant. Therefore, this study aims to analyze soil quality by determining the concentration of radionuclides in soil samples and determining the transfer factors of 40 K, 226 Ra, and 210 Pb from the soil to vegetables grown in the El Jadida agricultural area.

Study area, Sample collection and physical preparation
Soil and plant samples were collected at six locations at different distances from the phosphate complex of Jorf-Lasfar (at 17 km from El Jadida city) in Morocco. Two locations in the Jorf-Lasfer area, three in Sidi Moussa and one in Ouled Ganem ( Figure 1 and Table  1). The study area is characterized by a climate semiarid Mediterranean, with fresh and humid winters and hot and dry summers. The average annual temperature is between 14°C and 21°C, and the average annual rainfall is about 389 mm; November, December, and January are the rainiest months (from 68 to 75 mm). The less rainy months are June, July, August and September (El Jadida weather station). The soil in the study area is generally sandy clay [24]. In the laboratory, we used deionized water to wash vegetables in order to remove all observable soil particles. Before being ground into powder, all collected plant parts and soil samples are dried at 85°C for 72 hours to constant weight. The soil samples were passed through a 2 mm mesh sieve. Non-soil particles, stones, woodblocks, rocks, gravel, and/or organic debris have been removed. The chemical and physical properties of the soil (e.g organic matter, pH, conductivity) are also analyzed.

Determination of radionuclides concentrations
The samples (plants and soil) were analyzed using high-resolution gamma-ray spectrometry to determine the activity concentrations of 226 Ra, 40 K and 210 Pb. The detector used is a low background CANBERRA high purity coaxial germanium (50% efficiency), and the resolution was 2 keV for the 1332 keV 60 Co γ-peak, housed in a 10-cm-thick high-purity lead shield. The gamma spectra delivered by the detector are analyzed by Genie 2000 gamma analysis software. 226 Ra was determined by measuring their gamma emitter daughters 214 Pb (351.9 keV) and 214 Bi (609 keV). The photopeaks used for 40 K are 1460.82 keV and for 210 Pb are 46.5 keV.

Determination of transfer factors (TFs)
The transfer factor is calculated as the ratio of the radionuclide activity concentration measured in the plant (A plant ) divided by the activity concentration in the corresponding soil sample (A soil ), as shown in Equation (1). Table 2 shows the physicochemical parameters of soil samples collected from the Jorf-Lasfar phosphate complex at different distances. The pH value of the soil sample is between 8.16 and 8.63, with an average value of 8.48, signifying that it is alkaline. The soil EC (the standard measure of salinity) ranges from 0.12 to 0.32, with an average of 0.19 mS cm -1 . The organic matter in the soil ranges from 2.04% to 7.25%. Sites 1, 2, and 5 are characterized by high organic matter content. However, sites 3, 4, and 6 show low OM content.

Radionuclide concentrations in soil
The determined activity concentrations of 226

Radionuclide concentrations in plants
Results revealed that the concentration of 226 Ra varied between 2.25±0.6 and 7.15±0.52 Bq.kg −1 with an average value of 4.54±0.59 Bq.kg −1 . The maximum activity of 226 Ra was found in mallow root (site 1) (7.15 ± 0.52 Bq.kg −1 ), and the minimum activity was found in carrot fruit (site 3) (2.25±0.6 Bq kg −1 ). The activity of 210 Pb ranged from 3.44±0.55 to 37.76± 0.42 Bq kg −1 with an average value of 15.00±0.59 Bq kg −1 . The lowest activity for 210 Pb was found in cabbage fruit (site 4) (3.44±0.55 Bq kg −1 ), while the highest activity for 210 Pb was found in mallow root (site 1) (37.76±0.42 Bq kg −1 ). The mean concentrations of 40 K varied between 379.2±27.54 and 1094.3±57.75 Bq kg −1 with an average value of 891.67±50.33 Bq kg −1 , in all the vegetable samples with a highest recorded in courgette fruit (site 6), while the lowest activity for 40 K was registered in cabbage root (site 4). The activity concentrations of 40 K in all plant species were more significant than those of 210 Pb and 226 Ra. This is probably related to potassium being an essential component of crop growth [25].

Transfer factor of radionuclides from soil to plants
The TF values calculated for 210 Pb, 226 Ra and 40 K to various agricultural products are presented in Table 3. The transfer factor for 210 Pb ranges from 0.06 to 0.47, with an average of 0.23. The highest and lowest TF of 210 Pb was found in mallow root (site 1) and in cabbage fruit (site 4). The results of 226 Ra showed that the transfer factor for 226 Ra ranged from 0.09 to 0.34 with an average of 0.19. The sunflower root (site 6) displayed the highest value (0.34), while the lowest value (0.09) was found both in mallow root and (stem+leaves) (site 1). The bean root (site 2) indicated the lowest TF (2.07), whereas tomato (root+stem+leaves) presented the highest TF (10.96) ( Table 3). The Tfs values calculated for 40 K are considerably higher than those of 226 Ra and 210 Pb, suggesting higher uptake levels of 40 K. It is worthily noting that although some plants are grown in soils of similar Physico-chemical characteristics and similar concentrations of 210 Pb, 226 Ra and 40 K, the transfer factors are dissimilar for different species. The transfer factors of 210 Pb reported for fruit vegetables vary from 0.01-0.34 [26] and from BDL-0.80 for medicinal plants [11]. The transfer factors of 226 Ra ranged from 0.097-0.504 with an average of 0.17 for grass pasture [26]. Manigandan & Chandrashekar [27] reported that the transfer factor for 40 K varied from 0.128 to 0.954 for six different plants. The 210 Pb and 226 Ra transfer factors observed in the present study are comparable to the values of the studies mentioned above [11,26,27]. The transfer factors for 40 K are relatively higher than the values found in Manigandan & Chandrashekar [28]. This high TF of 40 K is probably linked to the excessive use of fertilizers rich in potassium in the studied sites.

Conclusions
The concentrations of naturally occurring radionuclides in various locations of El-Jadida agricultural area in soil, vegetables, and TFs for soilto-plants were determined.
The measured concentrations of radionuclides (Pb-210 & Ra-226) were similar to the soil guideline values reported by UNSCEAR (2000) and relatively low for plants samples as well. K-40 exhibited higher values in analyzed samples (soil and plants), which is probably linked to the application of fertilizers rich in potassium in the study area. The transfer factor of radionuclides from soil to plant is relatively low for Pb-210 and Ra-226 compared to K-40, which showed higher values.
Further investigations need to be developed to assess the distribution of radionuclides in El Jadida agricultural areas and accurately quantify their transfer factor to plants. This work contributes to establishing a database on the concentration of radioactive elements (Ra-226, Pb-210, and K-40) in soils and plants and their soil-plant transfer factor in the semiarid environment of Morocco.