2.1. Study area

Köprübaşı district is located 120 km northeast of Manisa in the Aegean region of Turkey. It is bordered by Gördes in the northwest, Demirci in the northeast, Gölmarmara in the west, Demirköprü Dam in the southeast, Salihli in the southwest, Kula and Alaşehir in the south, and Selendi in the east. Köprübaşı is located at 38°44’N latitude and 28°24’E longitude. It has a surface area of 447 km². The Dibek Mountains are in the southwest of the district, the Çanak Mountains in the northeast, and Kayran Mountains in the north [15]. The elevation of Köprübaşı is 250 m. It has a Mediterranean climate with hot and dry summers, and rainy and mild winters.

According to the General Directorate of Mineral Research and Exploration of Turkey (MTA) inventory of mining in the Aegean region, the Köprübaşı area contains marble, feldspar, phosphate, sulfur, titanium and zeolite deposits [16]. There are also uranium deposits in fluvial sedimentary rocks. The Köprübaşı uranium deposit was discovered by MTA in 1961 [17]. It is the largest known uranium deposit in Turkey [18]. Apatite, biotite, feldspar, ilmenite-magnetite, muscovite, quartz, rutile, tourmaline, and zircon minerals are found in Köprübaşı uranium deposits [19].

In this work, lichen and soil samples were collected from seven sampling locations in Köprübaşı in July 2013. The sampling locations are shown in Fig. 1.

Fig. 1.Full-screen View

(Color online) Map showing the sampling locations (1, 2, 3, 4, 5, 6 and 7) at Köprübaşı-Manisa: (1) Çarıklar village, (2) Kozakli village, (3) Kasar uranium deposit, (4) West of Borlu, (5) Northeast of Köprübaşı, (6) the Köprübaşı-Gordes road, and (7) the Salihli-Simav road.

The lichen samples were collected from the surfaces of trees and then placed into the paper bags after wrapping in paper towels. The sampling locations, location number, date, type of substrate, altitude, and coordinate information determined by Garmin brand GPS devices were noted. Soil samples were also obtained at each of the locations.

2.2. Determination of ²¹⁰Po and ²¹⁰Pb activity concentrations

The inessential materials were cleaned from each sample in the laboratory. Samples were dried at room temperature and then grinded and sieved. One gram of sample was weighed and concentrated acids (HNO₃ and H₂O₂) were added, before leaving the samples overnight. The following day, the solution was heated on a hot plate and evaporated to dryness. Mixed acids (HNO₃ and HCl) were added to the dried residue. The solution was then evaporated to dryness again by heating on a hot plate. This process was repeated a total of four times. After evaporation, 0.5 M HCl acid was added to the dry residue. The solution was filtered with filter paper (particle retention 10-15 µm, 125 mm diameter, 84 g.m^-2 weight). Ascorbic acid was added to the solution to reduce Fe³⁺. A copper disc of diameter 2.5 cm was prepared. ²¹⁰Po was spontaneously accumulated onto the copper disc at 70 °C for 5 h.

The ²¹⁰Po accumulated onto the copper disc was counted for 5 h with an alpha spectrometer equipped with a PIPS detector placed in a vacuum chamber connected to a 1024 multichannel analyzer (Canberra). The energy resolution was ≤ 20 keV with a detector source spacing equal to the detector diameter. The detector efficiency was ≥ 25% of the detector source spacing of ≤ 10 mm. The background was ≤ 1 count/hour above 3 MeV. The system calibration was performed with an ²⁴¹Am point source.

The measurement of ²¹⁰Po was performed using the alpha particle emission peak with 5.30 MeV energy, using ²⁰⁹Po as the internal tracer (National Institute of Standards & Technology, SRM 4326 consists of radioactive polonium-209 chloride and hydrochloric acid dissolved in 5ml of distilled water. The solution mass was 5.160 ± 0.003 g. It is enclosed in a flame sealed NIST borosilicate glass ampoule. The ²⁰⁹Po massic activity of the solution was 85.42 Bq.g^-1).

In this study, the chemical efficiency was calculated as 37.7% using the ²⁰⁹Po standard. The total efficiency was found to be 102%. The ²¹⁰Po activity concentration for each sample was corrected for recovery using the total efficiency.

The ²¹⁰Pb activity measurement was performed indirectly from its measured product ²¹⁰Po activity after reaching radioactive equilibrium. The ²¹⁰Pb and ²¹⁰Po isotopes reached secular equilibrium at least six months after the first ²¹⁰Po electro-deposition. At this time, ²¹⁰Po grew from the ²¹⁰Pb of the sample. The ²¹⁰Po deposition in the lichen samples for the ²¹⁰Pb activity was collected on the cupper discs at 70 °C for 5 h and counted for 5 h by the alpha spectrometer. The ²¹⁰Pb activity concentration from the measured ²¹⁰Po activity in the analyzed samples was calculated by the Bateman equation [20].

$A_0{(}^{210}\text{Pb})=\frac{A_2{(}^{210}\text{Po})}{1-e^{-\lambda (t_2-t_{1)}}}$,

where A_o (²¹⁰Pb) is the activity of a sample during the collection time, A₂ (²¹⁰Po) is the activity of ²¹⁰Po ingrown from ²¹⁰Pb after the second electro-deposition, t₁ is the time between the sample collection time and the first ²¹⁰Po activity measurement, t₂ is the time between the sample collection date and the second ²¹⁰Po activity measurement, and λ is the decay constant of ²¹⁰Po [20].

2.3. Determination of gamma radioactivity in soil samples

The soil samples collected from the seven sampling locations in Köprübaşı were milled and dried in an oven in the laboratory, before being sieved and weighed at 100 g in a polyethylene beaker. They were tightly closed and stored for as a minimum of four weeks to permit the ²³⁸U and ²³²Th to arrive at equilibrium with their decay products. The gamma measurements were performed by a gamma ray spectrometer using a 3x3 inch NaI(Tl) (ORTEC-905-4) detector. The system calibration was made with standard samples (52 % K, 625 ppm eU and 150 ppm eTh) under appropriate conditions. The best available resolution was <7.5% for the gamma peak of ¹³⁷Cs (662 keV). The activity concentrations for ⁴⁰K, ²³⁸U, and ²³²Th were evaluated from the radioactive potassium peak (1.46 MeV), the ²¹⁴Bi peak (1.76 MeV), and the ²⁰⁸Tl peak (2.61 MeV), respectively. All the samples were counted for 7200 s. The ⁴⁰K, ²³⁸U and ²³²Th activities were then calculated.

3.1. Results of ²¹⁰Po and ²¹⁰Pb activity concentrations in samples

The 17 lichen samples of six different species (Cladonia convoluta, Parmelina tiliacea, Physcia stellaris, Pleurosticta acetabulum, Xanthoparmelia conspersa and Xanthoria parietina) and seven soil samples were collected from seven sampling locations in Köprübaşı district. The lichen species are presented in Fig 2. The results of the ²¹⁰Po and ²¹⁰Pb activity measured in the samples, according to sampling location, are presented in Table 1.

Fig. 2.Full-screen View

(Color online)Lichen species collected from the Köprübaşı district: (a) Cladonia convoluta, (b)Parmelina tiliacea, (c)Physcia stellaris, (d)Pleurosticta acetabulum, (e)Xanthoparmelia conspersa and (f)Xanthoria parietina.

At location 1, according to Table 1, there were two different lichen species and the highest ²¹⁰Po and ²¹⁰Pb activities of 577 Bq.kg^-1 and 493 Bq.kg^-1, respectively, were obtained in Xanthoria parietina. These values were the highest activities among all lichen samples. In location 2, while the highest value for ²¹⁰Po was obtained as 290 Bq.kg^-1 in Xanthoparmelia conspersa, the highest value for ²¹⁰Pb was calculated as 270 Bq.kg^-1 in Physcia stellaris. There were six different lichen species in location 3 where the Kasar uranium deposit is located. The highest activities for ²¹⁰Po and ²¹⁰Pb of 504 Bq.kg^-1 and 565 Bq.kg^-1, respectively, were obtained for Parmelina tiliacea. In location 4, there was only one lichen species, namely Xanthoria parietina. In location 5, there were three different lichen types, with the highest obtained activities for ²¹⁰Po (171 Bq.kg^-1) and ²¹⁰Pb (144 Bq.kg^-1) in Xanthoria parietina. In location 6, the highest activities for ²¹⁰Po (176 Bq.kg^-1) and ²¹⁰Pb (190 Bq.kg^-1) were detected in Parmelina tiliacea. In location 7, there was only one lichen species. The activities of ²¹⁰Po and ²¹⁰Pb in Xanthoria parietina were 450 Bq.kg^-1 and 227 Bq.kg^-1, respectively. The polonium accumulation is also affected by such factors as the age, health, and type of the lichen species, including their positions on trees, surface structure [21], altitudes where the lichens grow, and meteorological conditions (wind and rain). The mean annual rainfall of Köprübaşı is 575.8 mm and average annual temperatures range from 4.1 to 27.3 °C in 2013. Xanthoria parietina is the most common lichen species and was found at all locations except for location 2. It was assumed to be the most suitable bioindicator for pollution in the study area due to its high capture efficiency, geographical and climatic suitability, and common occurrence.

While the ²¹⁰Po activity levels in soil samples collected from six locations, except for location 3, varied between 14 and 53 Bq.kg^-1, the ²¹⁰Pb activity levels ranged from 19 to 55 Bq.kg^-1. It can be seen that the activity concentration in the samples in location 3 (1268 Bq.kg^-1 for ²¹⁰Po and 1113 Bq.kg^-1 for ²¹⁰Pb) were higher than in the samples from the other locations. Uranium mining had previous occurred in this area (location 3). In the other locations, ²¹⁰Po and ²¹⁰Pb concentrations in the soil samples were lower than those for lichen. This implies that lichens do not feed from the soil as they do not have roots and are affected by atmospheric fallouts.

The ²¹⁰Po and ²¹⁰Pb activities in the lichen and soil samples in this study were compared with those in the literature in Table 2. In the study, the mean ²¹⁰Po and ²¹⁰Pb activity levels in the lichen samples were similar to those of similar studies in the literature. The ²¹⁰Po/²¹⁰Pb ratio in the present study was found to be unity, implying the equilibrium between the ²¹⁰Po and ²¹⁰Pb radioisotopes. The ²¹⁰Po measured in lichens originates from ²¹⁰Pb, which is its parent in the uranium chain.

The mean ²¹⁰Po and ²¹⁰Pb activities according to the lichen species are shown in Fig. 3. The mean activity ratios of ²¹⁰Po/²¹⁰Pb are presented in Table 3.

Fig. 3.Full-screen View

The mean ²¹⁰Po and ²¹⁰Pb activity in the lichen species at Köprübaşı.

The mean ²¹⁰Po and ²¹⁰Pb activities in lichen species ranged from 154 to 390 Bq.kg^-1 and from 153 to 378 Bq.kg^-1, respectively. While the highest mean activity for ²¹⁰Po was detected in Xanthoparmelia conspersa, the lowest mean activity was seen in Pleurosticta acetabulum. For ²¹⁰Pb, while the highest mean activity was seen in Parmelina tiliacea, the lowest activity was found in Cladonia convolute (Table 3).

The mean activity ratios of ²¹⁰Po/²¹⁰Pb in the lichen species ranged from 0.85 to 1.52. The activity ratio of ²¹⁰Po/²¹⁰Pb was unity in Xanthoparmelia conspersa. This demonstrates the equilibrium between the ²¹⁰Po and ²¹⁰Pb radioisotopes. In three lichen species (Parmelina tiliacea, Physcia stellaris and Pleurosticta acetabulum), the ²¹⁰Pb deposition was greater than the ²¹⁰Po deposition. This disequilibrium indicated the presence of ²¹⁰Pb due to the atmospheric deposition in the lichen species. Since the ground surface is the main source of airborne ²¹⁰Pb, the air concentrations of ²¹⁰Pb show the local, geological and global climatological background of the areas observed [24]. In the Cladonia convoluta and Xanthoria parietina lichen species, the ²¹⁰Po deposition was higher than the ²¹⁰Pb deposition (Table 3). The activity ratio of ²¹⁰Po/²¹⁰Pb in the Cladonia convolute lichen species is higher than the other results. This lichen species was collected at two locations (locations 1 and 3). When the activity ratios for this species were calculated according to the sampling locations and were 1.3 and 1.63 in locations 1 and 3 (Kasar uranium deposit), respectively. According to the results, the ²¹⁰Po activity concentrations were higher than the ²¹⁰Pb activity concentrations. This result shows that the polonium was a contribution not only from the predominant ²¹⁰Pb but also from the environment. The Kasar uranium deposit is located in location 3 and it can be concluded that the uranium mine affected the polonium activity. The polonium accumulation is affected by the deposits of marble, feldspar, phosphate, sulfur, titanium, and zeolite. This is because the level of ²¹⁰Po in the atmosphere of tungsten, molybdenum, iron, and phosphate rocks is higher [25-26]. The Köprübaşı uranium deposits are found in apatite, biotite, feldspar, ilmenite-magnetite, muscovite, quartz, rutile, tourmaline, and zircon minerals.

A correlation graphic of ²¹⁰Po activity versus ²¹⁰Pb activity in the lichen samples is given in Fig. 4. It can be seen that there was a positive correlation of 0.91 between the two radionuclides, with an R² value of 0.78.

Fig. 4.Full-screen View

Relationship between the ²¹⁰Po and ²¹⁰Pb activity in the lichen samples.

3.2. Results of gamma radioactivity in soil samples

The ⁴⁰K, ²³⁸U and ²³²Th activities were measured by gamma spectrometry and the obtained results are given in Table 4.

Standard error values for the ⁴⁰K, ²³⁸U, and ²³²Th activities were calculated separately and were 129, 161 and 8, respectively. These are shown in Fig. 5.

Fig. 5.Full-screen View

(Color online) ⁴⁰K, ²³⁸U, and ²³²Th activities in the soil samples.

In the soil samples, the ⁴⁰K activity ranged between 250.25 and 1064.60 Bq.kg^-1 with an average value of 506.19 Bq.kg^-1. The highest concentration of ⁴⁰K was measured in the sample collected from location 5 (Table 4 and Fig. 5). The world average for ⁴⁰K is accepted as 400 Bq.kg^-1 [27] and therefore 71.5% of the results in this study exceeded the world average.

The ²³⁸U activity in the soil samples ranged from 41.73 to 1267.8 Bq.kg^-1, with an average value of 49.5 Bq.kg^-1, except at location 3. The ²³⁸U activity concentration in all soil samples was higher than the world average for ²³⁸U which is accepted as 35 Bq.kg^-1 [27]. In particular, the ²³⁸U activity concentration in the soil sample taken from the Kasar uranium mine (location 3) is very high (approximately 36 times the world average).

The ²³²Th activity varied from 14.61 to 111.92 Bq.kg^-1 with a mean value of 39.92 Bq.kg^-1. The results of two samples (location 2 and location 3) were lower than the world average of 30 Bq.kg^-1 [27]. The mean of the ⁴⁰K, ²³⁸U, and ²³²Th activities in this study were exceeded the accepted values (400, 35, and 30 Bq.kg^-1, respectively).

In the soil samples, the correlation between the ²³⁸U activity concentration measured by gamma spectrometry and ²¹⁰Po activity concentration measured by alpha spectrometry is demonstrated in Fig. 6. It can be seen that the correlation between the ²³⁸U and ²¹⁰Po activity is a positive (0.75) with an R² value of 0.998.

Fig. 6.Full-screen View

The correlation between the 238U activity and 210Po activity concentrations in the soil samples.