Introduction

In recent years the importance of organic farming methods has increased due to growing consumer interest in certified organic products. Today the most widely used commercial farming systems are organic and conventional farming. Phosphate fertilisers are used in conventional systems, while in organic agriculture organic fertilisers are used, such as bovine manure. As a result of the consumption of products that are grown in the areas where phosphate fertilisers are used, human exposure to radiation due to the ingestion of food may increase on fertilised farmlands. Organic farming is a method of agricultural production where each phase from production to consumption is controlled in order to certify that the products are free of synthetic pesticides, hormones and chemical fertilisers. In organic farming, the soil is considered alive therefore the continuity of its health is considered in each phase. In this practice, certain substances (natural herbicides, insecticides, and rodenticides) are applied to plants in various forms in order to protect them against diseases and pests. Organic fertilisers used in organic farming linger in the soil for a longer period of time and provide a better environment for microorganisms. Since they have a better capacity to hold moisture, organic fertilisers inhibit the loss of soil minerals through washing thus preventing the salt ratio from increasing in soil and desertification in the long-term.

It is very well known that about 1300 different radionuclides exist that originate from both natural sources, and anthropogenical production [1]. Natural radionuclides can be generated by the activation of stable isotopes using cosmic radiation, or originate during the creation of the universe. The latter are known as primordial radionuclides and include 40K (t 1/2 = 1.28 × 109 year) and isotopes of uranium and thorium which give rise to various daughter nuclides, including 226Ra (t 1/2 = 1602 year), 210Pb (t 1/2 = 22.3 year), 228Ra (t 1/2 = 5.75 year), and 210Po (t 1/2 = 138 days). These radionuclides are known to be present in foodstuffs as a result of their uptake by plants from soil and their concentrations vary from region to region. In Turkey, organic agriculture originated in İzmir in the form of dried fruit production in 1985. Since then, the region has become the biggest producer and exporter of organic products from Turkey. In the literature, there are some studies that investigate the differences in quality between products from conventional and organic farming or foodstuffs produced with the aid of different fertilisation systems, but there are not many studies about the radioactivity levels on conventional and organic farmlands [2]. Very few studies have been conducted on the natural radioactivity levels in the organic and conventional farmlands of Turkey. Therefore, the objective of this study was to determine the activity concentrations of 210Po, 210Pb, 226Ra, 228Ra and 40K in wheat, grape leaf, garlic, onion and soil samples of conventional and organic farming management systems and to estimate the soil-to-plant transfer factor.

Experimental

Soil and plant samples were collected from commercial organic and conventional farms in the İzmir region. Plant samples were collected from six different conventional farms and seven organic farms in three different regions (Ahmetli-Manisa, Tahtalı dam basin, and Menemen Soil and Water Research Institute) as shown in Fig. 1. The sampling sites were selected as areas where both conventional and organic farming are intensely carried out. Plants and their corresponding soils were collected randomly from several cultivation plots. The soil was collected from the surface to a depth of 20 cm.

Fig. 1
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The sampling areas

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The soil samples were homogenised and oven-dried at 75 °C, ground in an agate mortar, and sieved using a “30 mesh”. Each dried sample was sealed in a 1000 ml Marinelli beaker prior to analysis. Plant samples were carefully cleaned with a plastic brush to eliminate extraneous soil particles and then weighted (wt). The plant samples were dried until a constant weight was achieved. Dried samples were ground in a small mill and the ground material was transferred to plastic sample containers (45 mm in diameter). They were stored for 1 month to attain radioactive equilibrium between 226Ra and 222Rn before being subjected to γ-spectrometric analysis.

210Po and 210Pb determinations in plant and soil samples

Measurements of 210Po were realised through its 5.30 MeV alpha particle emission line, using 209Po as the internal tracer, each sample was completely dissolved in HCl and HNO3. For soil samples HF was also used in the dissolving process. Polonium was spontaneously plated onto silver discs in 0.5 M HCl in the presence of ascorbic acid to reduce Fe3+–Fe2+. In order to find the optimum conditions for plating, the standard technique given by Flynn was modified [3]. Alpha activities were measured by PIPS detectors, Ortec 450 mm2 with 20 µm of depletion depth. The counting period was adjusted to obtain relative standard errors of 5 %. Following the initial plating of 210Po, a silver disc was suspended in the plating solution which was stirred overnight to remove the remaining traces of 210Po and 209Po. The solution was then re-spiked with a known activity of 209Po and left for at least 6 months to allow the ingrowth of 210Po from 210Pb. The sample was re-plated and the 210Po activity was determined. Well-known Bateman equations were used to obtain the 210Pb activity from the measured 210Po activity [4]. In this way, the second period of plating provided information on the 210Pb content of the samples.

Gamma spectrometric analysis of 226Ra, 228Ra and 40K in plant and soil samples

The 226Ra, 228Ra and 40K activity concentrations of plant samples were determined using a Canberra 76 × 76 mm, high sensitivity NaI(Tl) detector connected to a Nucleus PCA-8000 multichannel analyser. The detector was well-shielded with 75 mm thick lead bricks to reduce the background counting rate. The 226Ra, 228Ra and 40K contents were detected using 1.76 MeV 214Bi, 2.62 MeV 208Tl and 1.46 MeV 40K lines, respectively. The detection limits of the spectrometer system for 238U, 228Ra, and 40K are 5.92, 1.84, and 1.27 Bq kg−1, respectively.

The soil samples were analysed for 226Ra, 228Ra and 40K by direct gamma assay, using a 184 cc p-type coaxial HPGe detector with a relative efficiency of 25 % and a resolution of 1.85 keV at 1.332 MeV (with associated electronics provided by EG&G Ortec). The detector was sealed by 100 mm thick lead bricks internally lined with 1.5 mm thick copper foil. The spectrum was acquired and analysed using a PC-based 8 K multichannel analyser and its associated software. 40K was detected via its gamma emission lines at 1.46 MeV, 232Th was detected by the 2.62 MeV gamma rays from 208Tl and 226Ra using the 295 and 352 keV γ-rays emitted by its daughter isotope 214Pb. Counting times varied from 10 to 24 h, depending on the activity in the samples, providing a precision of better than ±10 % and a 90 % level of confidence [5]. Corrections were made for the effect of self-absorption of low energy γ-rays within the sample [6].

Background counts were measured at regular intervals to ensure that low background characteristics were maintained. In order to confirm the results, three sub-samples were taken from each plant and the same chemical and radiometric procedure was repeated on them.

The soil-to-plant radionuclide transfer factor (TF) is defined using the following equation [7]:

$$ {\text{Transfer}}\,{\text{factor}}\, ( {\text{Fv)}}\,=\, \frac{{{\text{Activity}}\,{\text{of}}\,{\text{radionuclide}}\,{\text{in}}\,{\text{plant}}\,{\text{material}}\, ( {\text{Bq} \,{\text {kg}^{-1},}}\;{\text{dry}}\,{\text{weight)}}}}{{{\text{Activity}}\,{\text{of}}\,{\text{radionuclide}}\,{\text{in}}\,{\text{soil}}\, ( {\text{Bq}\,{\text{kg}^{-1},}}\,{\text{dry}}\;{\text{weight)}}}} $$

Results and discussion

The mean activity concentrations of 210Po, 210Pb, 226Ra, 228Ra and 40K radionuclides in plant and soil samples from three different areas (Ahmetli-Manisa, Tahtalı dam basin, and Menemen Soil and Water Research Institute) are presented in Figs. 2, 3, 4, 5 and 6. In plants collected from organic fields, the relative concentration ranges of 210Po, 210Pb, 226Ra, 228Ra and 40K were between 0.21 ± 0.04 and 1.40 ± 0.36 Bq kg−1, 0.67 ± 0.10 and 4.81 ± 0.67 Bq kg−1, ND and ND, ND and ND, and ND and 7.34 ± 1.50 Bq kg−1, respectively. In the soils of organic agricultural areas the activity concentrations of 210Po, 210Pb, 226Ra, 228Ra and 40K were between 19 ± 1 and 48 ± 1 Bq kg−1, 24 ± 2 and 47 ± 3 Bq kg−1, 20 ± 1 and 52 ± 3 Bq kg−1, 23 ± 1 and 40 ± 2 Bq kg−1, and 505 ± 26 and 602 ± 31 Bq kg−1, respectively. On conventional farms, the activity concentrations of soils were found to vary between 49 ± 1 and 78 ± 2 Bq kg−1 for 210Po, 45 ± 3 and 96 ± 6 Bq kg−1 for 210Pb, 33 ± 2 and 48 ± 2 Bq kg−1 for 226Ra, 35 ± 2 and 42 ± 3 Bq kg−1 for 228Ra and 630 ± 32 and 694 ± 36 Bq kg−1 for 40K. In plants collected from conventional fields, the relative concentrations of 210Po, 210Pb, 226Ra, 228Ra and 40K were found to be between 1.09 ± 0.30 and 2.18 ± 0.40 Bq kg−1, 6.56 ± 0.80 and 17.00 ± 1.25 Bq kg−1, ND and 18.5 ± 2.0 Bq kg−1, ND and 1.92 ± 0.30 Bq kg−1, and ND and 174 ± 9 Bq kg−1, respectively.

Fig. 2
figure 2

Comparison of 210Po activity concentrations (Bq kg−1) in plant and soil samples collected from organic and conventional farming areas

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Fig. 3
figure 3

Comparison of 210Pb activity concentrations (Bq kg−1) in plant and soil samples collected from organic and conventional farming areas

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Fig. 4
figure 4

Comparison of 226Ra activity concentrations (Bq kg−1) in plant and soil samples collected from organic and conventional farming areas

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Fig. 5
figure 5

Comparison of 228Ra activity concentrations (Bq kg−1) in plant and soil samples collected from organic and conventional farming areas

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Fig. 6
figure 6

Comparison of 40K activity concentrations (Bq kg−1) in plant and soil samples collected from organic and conventional farming areas

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In the organic farming areas, grape leaf-4 samples have the maximum 210Po and 210Pb concentrations as 1.40 ± 0.36 and 4.81 ± 0.67 Bq kg−1, respectively. 40K showed the lowest activity concentrations in these farming areas. 210Po:210Pb ratios in plant samples were also investigated in the present study to examine the unsupported 210Po enhancement. Except for grape leaf-2 (1.45), the 210Po:210Pb activity ratios were derived as between 0.03 and 0.75. The organic farming area, in which grape leaf-4 samples were collected, was a conventional farm up till 2 years ago and the excessive use of fertilisers during conventional farming may be a reason for the higher activity concentration of 210Po in these samples. In addition, it is considered that the existence of conventional nearby farming areas nudged up the activity concentrations in plants and soil samples in that region. The two 238U daughters, 210Po and 226Ra, showed a very strong relationship with an almost perfect correlation. The overall 210Po:226Ra activity ratio is almost 1 indicating a secular equilibrium in the soil collected from organic farming areas (Table 1). The 210Po:210Pb activity ratio was found to be slightly higher in soil where grape leaves were collected. The maximum 210Pb:226Ra activity ratio was observed in soil where grape leaf-3 was collected.

Table 1 The activity ratios in soil samples collected from organic farming areas
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In the conventional farming areas, grape leaf-1 samples have the maximum 210Po, 210Pb, 226Ra, 228Ra and 40K concentrations as 2.18 ± 0.40, 17.00 ± 1.25, 18.50 ± 2.00, 1.92 ± 0.30 and 174 ± 9 Bq kg−1, respectively. The 210Po:210Pb activity ratios in plant samples from conventional farms were estimated and the minimum and maximum values of the ratio were 0.13 and 0.30. 210Po:226Ra and 210Pb:226Ra activity ratios were found to vary between 1.40 and 1.64 and 1.32 and 2.00 respectively, indicating that the main source of 210Po and 210Pb in soils is not merely the radioactive decay of 226Ra but also the intensive use of fertilisers (Table 2). The 210Po:210Pb activity ratio was almost 1. The 210Po concentrations in garlic and grape leaf-1 from the conventional farming areas, were higher than the others and the highest 210Po concentrations were measured in soil samples where grape leaf-1 samples were grown. A conversation was held with the owner of the farmland in order to understand the reason why high levels were observed in the farmland where grape leaf-1 samples originated from. It turned out that in that farmland artificial fertilisers were used intensively.

Table 2 The activity ratios in soil samples collected from conventional farming areas
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The relationship between the organic and conventional farming areas was investigated by a t test statistical analysis. Some significant differences (0.003 < 0.05) were observed between the two types of farming in terms of 210Po, 210Pb, 226Ra and 228Ra radionuclides. The results of the analysis indicated that the activity concentrations of 210Po, 210Pb, 226Ra, 228Ra and 40K of the plant and soil samples that were collected from the conventional farming areas were higher than the ones collected from the organic farming areas.

Lindahl et al. indicated that no significant difference (p > 0.05) in activity concentrations of the four radionuclides determined in wheat grains (i.e. 40K, 226Ra, 228Ra and 228Th) was observed between organic and conventional agricultural systems in Belgium [8]. For organic and conventional areas the concentration ratios (CR) were almost 1 for stems and grains. However for roots, 40K and 228Th exhibited tendencies of higher uptake in the conventional fields even though there is a trend suggesting a lower uptake of 226Ra in the same fields. Similarly, no difference was observed for Ra and U in vegetables grown in organic and conventional farming systems in Brazil (Lauria et al. [9]).

Soil-to-plant transfer factors (TF) for the natural radionuclides 210Po, 210Pb, 226Ra, 232Th and 40K were calculated and are shown in Tables 3 and 4. The values were generally lower for 210Po than for 226Ra and 210Pb. In organic farming systems, the lowest TFs were obtained for 40K and 210Po. Statistically (Paired t-test) significant differences p > 0.05 were found between the data sets of TF values for Po, Pb and K isotopes from conventional and organic agriculture managements. In general, the TF values fall within the range given in the literature. Vandenhove et al. indicated that TF-Pb was highest for pastures/grasses (1.4 × 10−1), followed by leafy vegetables (8.0 × 10−2) and fodder (2.5 × 10−2) and was lowest for tubers (1.5 × 10−3)6. Highest TFs-Po (1.2 × 10−2) were found for pastures and grasses and the observed difference is significant [10].

Table 3 The radionuclide transfer factors for organic farming
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Table 4 The natural radionuclide transfer factors for conventional farming
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Karunakara et al. have investigated soil-to-rice transfer factors for 226Ra, 228Ra, 210Pb, 40K and 137Cs in Kaiga where a nuclear power station has been in operation since 1999 [7]. The mean soil-to-rice plant transfer factors for 40K and 210Pb were 1.5 and 1.4 × 10−1, respectively.

Strok and Smodis calculated soil-to-plant transfer factors for natural radionuclides in grass in the vicinity of a former uranium mine in Slovenia [11]. The researchers found that the transfer factors varied between 3.46 × 10−2 and 4.65 × 10−1 for 226Ra, and 9.83 × 10−2 and 1.52 for 210Pb.

Fernandes et al. have calculated soil-to-plant transfer factors in a semi-arid region in Brazil [12]. The researchers stated that a range from 10−3 to 10−1 may conveniently encompass most of the transfer factor values for soil/plant systems, i.e. involving different cultures, different soils and natural radionuclides.

Asaduzzaman et al. have investigated soil-to-plant transfer factors for tapioca and sweet potato in west Malaysia [13]. The researchers found that the uptake of radionuclides during the middle or late growth stages produced higher TFs than those during the early growth stage.

Al-Kharouf et al. have calculated soil-to-plant transfer factors in Jordan [14]. The researchers found that the average value of the total TFs of 238U for courgette and watermelon plants were 1.05 ± 0.08 × 10−2 and 0.45 ± 0.19 × 10−2, respectively.

James et al. have investigated soil-to-leaf transfer factors for the radionuclides 226Ra and 40K in the region of Kaiga in India [15]. 226Ra and 40K activities in leaves of herbaceous plants are higher than those of tree leaves. The soil-to-leaf transfer factors for 226Ra and 40K were found to be in the range of 0.03–0.65 and 0.32–8.04, respectively.

Pulhani et al. have calculated soil-to-wheat grain transfer factors in Maharashtra, India [16]. The soil-to-wheat grain transfer factors were calculated and observed to be in the range of 4.0 × 10−4 to 2.1 × 10−3 for 238U, 6.0 × 10−3 to 2.4 × 10−2 for 232Th, 9.0 × 10−3 to 1.6 × 10−2 for 226Ra and 0.14 to 3.1 for 40K.

Conclusions

In this study, 12 plant and 11 soil samples were investigated. The samples were taken from the farmlands where conventional and organic farming are performed. The results of the analysis indicated that the activity concentrations of 210Po, 210Pb, 226Ra, 228Ra and 40K of the plant and soil samples that were collected from the conventional farming areas were higher than the ones collected from the organic farming areas. The determined soil-to-plant transfer factors are comparable with the results given in the literature and by IAEA [17]. Further studies are needed to provide comparative data on natural radioactivity levels in different types of plant samples in the Aegean region in order to confirm the preliminary observations reported in this study. In addition, as Al-Masri stated it is important to investigate the differences between the open and sheltered systems [18].