Radionuclides (210Po and 210Pb) and Some Heavy Metals in Fish and Sediments in Lake Bafa, Turkey, and the Contribution of 210Po to the Radiation Dose

The pollution level of Lake Bafa was investigated by collecting fish samples {Dicentrarchus labrax (sea bass), Liza ramada (mullet) and Anguilla anguilla (eel)}, surface sediment, and core samples. In all these samples, 210Po and 210Pb concentrations were estimated, and total annual dose rates were obtained for each species. Some heavy metal (Cr, Ni, Pb, Cd, Mn, Fe, and Zn) concentration levels were obtained for the fish and a core sample. The sediment mass accumulation rate was found to be 3.27 g·m−2·day−1 (0.119 g·cm−2·y−1) from a core sample. The heavy metal concentrations in the vertical profile of samples from the core were also observed. The measured concentration of Zn, Pb, Cd, and Cr were between the ERL (effects range low) and ERM (effects range median) limits, while Ni concentrations were higher than the ERM limit. The observed concentrations of Cd, Pb, and Zn in fish samples did not exceed the limits in accordance with Turkish Food Regulations. Further, the maximum effective dose equivalent of 210Po in the area was found to be 1.169 µSv·y−1.


Introduction
Wetlands are an important source of global biodiversity and have been threatened by natural and human activities [1]. Extensive literature shows that lakes are suffering from contamination of metals and radionuclides. Lake ecosystems have been contaminated from anthropogenic sources. They may contain heavy metals from various sources such as urban garbage, urban fugitive, and automobile exhaust [2]. Several metals such as Zn, Co, and Cu are necessary elements for life; on the other hand, Pb and Cd are toxic to a great variety of habitats. Moreover, the excess usage of all metals can have a toxic effect to an organism [3]. 210 Po and 210 Pb are well-known natural radionuclides and are absorbed by plankton in aquatic environments. These radionuclides are accumulated by marine organisms and transferred to people with ingested food [4]. The activities of these two elements may be increased by industrial waste and human activities.
The main aim of this study was to determine the concentrations of certain metals and radionuclidies in Lake Bafa by collecting biota and sediments. Biota samples {Dicentrarchus labrax (sea bass), Liza ramada (mullet), and Anguilla anguilla (eel)} and surface sediments were gathered monthly and seasonally, respectively. In addition, a core sample was obtained from the lake to get information about the activities of radionuclides and metal contents in recent years.
The pollution of the lake is very important for people and the habitat of the lake. Natural pollution is assisted by human activities. The Menderes River, the water source of Lake Bafa, has many industrial factories down the river. In addition, the lake is covered by vulnerable agricultural soil. All of these increase the level of pollution of the lake because of unsustainable source use, illegal fishing, factory wastes and excessive use of water of the river for agriculture. Singare et al. [5] stated that among the different water bodies, lakes have a complex and fragile ecosystem, as they do not have self-cleaning ability and therefore readily accumulate pollutants.

Study Area
Lake Bafa is located in the southwestern part of Turkey, at the boundaries of Mugla and Aydın, at the coordinates 37 • 30 N and 27 • 25 E, as shown in Figure 1. The lake is mostly shallow, with a maximum depth of 21 m and a surface area of approximately 7500 ha. It is a delta lake formed by the Büyük Menderes River. The sediments of the river filled the marine, the Latimen Gulf, over the past six or so millennia; thus, it is separated from the sea [6]. Because of the ancient location area and the habitat of the lake, it has been legally protected as a natural park since 1994. In the villages around the lake, agriculture and fishing still occupy an important part of their economies. that among the different water bodies, lakes have a complex and fragile ecosystem, as they do not have self-cleaning ability and therefore readily accumulate pollutants.

Study Area
Lake Bafa is located in the southwestern part of Turkey, at the boundaries of Muğla and Aydın, at the coordinates 37°30′ N and 27°25′ E, as shown in Figure 1. The lake is mostly shallow, with a maximum depth of 21 m and a surface area of approximately 7500 ha. It is a delta lake formed by the Büyük Menderes River. The sediments of the river filled the marine, the Latimen Gulf, over the past six or so millennia; thus, it is separated from the sea [6]. Because of the ancient location area and the habitat of the lake, it has been legally protected as a natural park since 1994. In the villages around the lake, agriculture and fishing still occupy an important part of their economies.

Sampling
To estimate the pollution level of the lake, certain fish samples, surface sediments, and core sample were collected. The most consumed fish samples were Dicentrarchus labrax (sea bass), Liza ramada (mullet), and Anguilla anguilla (eel), and surface sediments were collected monthly and seasonally, respectively. Surface sediments were collected by Ekmann Grab, and the core sample was obtained by a gravity core-sampler. Ekmann Grab collected the upper sediment layer, with a thickness varying from 4 to 20 cm, depending on the bottom hardness. By using all these samples, 210 Po and 210 Pb concentrations were estimated, and total annual dose rates were obtained for each species. Certain heavy metal (Cr, Ni, Pb, Cd, Mn, Fe, and Zn) concentration levels were obtained for fish and core samples.
The fish species were bought from fishermen of Lake Bafa in the Serçin Village in 2012. In the summer months, collecting fish in the lake is prohibited; therefore, there are no samples for the summer. The surface sediments were collected from about 8 cm deep in the middle of each season. The points where the surface sediment samples were collected are shown in Figure 2. The core samples were collected from the middle of the lake.
The muscles of the biotas were separated with a plastic knife. The muscle tissues were dried in an oven at 80 °C until they reach a constant weight. A core sample was cut into 6 mm long pieces for the first 10 cm and then into 12 mm long pieces until 33 cm. Core and surface samples were dried at 70 °C until a constant weight level was maintained. All samples were passed through a 250 µm mesh sieve for homogenization.

Sampling
To estimate the pollution level of the lake, certain fish samples, surface sediments, and core sample were collected. The most consumed fish samples were Dicentrarchus labrax (sea bass), Liza ramada (mullet), and Anguilla anguilla (eel), and surface sediments were collected monthly and seasonally, respectively. Surface sediments were collected by Ekmann Grab, and the core sample was obtained by a gravity core-sampler. Ekmann Grab collected the upper sediment layer, with a thickness varying from 4 to 20 cm, depending on the bottom hardness. By using all these samples, 210 Po and 210 Pb concentrations were estimated, and total annual dose rates were obtained for each species. Certain heavy metal (Cr, Ni, Pb, Cd, Mn, Fe, and Zn) concentration levels were obtained for fish and core samples.
The fish species were bought from fishermen of Lake Bafa in the Serçin Village in 2012. In the summer months, collecting fish in the lake is prohibited; therefore, there are no samples for the summer. The surface sediments were collected from about 8 cm deep in the middle of each season. The points where the surface sediment samples were collected are shown in Figure 2. The core samples were collected from the middle of the lake.
The muscles of the biotas were separated with a plastic knife. The muscle tissues were dried in an oven at 80 • C until they reach a constant weight. A core sample was cut into 6 mm long pieces for the first 10 cm and then into 12 mm long pieces until 33 cm. Core and surface samples were dried at 70 • C until a constant weight level was maintained. All samples were passed through a 250 µm mesh sieve for homogenization.

Alpha Spectrometric Analysis for 210 Pb
Measurements of 210 Po were realized through its 5.30 MeV alpha particle emission, using 209 Po (4.88 MeV, t1/2 = 109 y) as the internal tracer. After a standard addition of polonium tracer, each sample was dissolved using three portions of concentrated 20 mL HNO3 and then 2 mL H2O2 evaporated to near dryness on a hot plate. To the remainder, three portions of 20 mL HCl were added, and the solution was evaporated to dryness. For sediment samples, HF was also used in the dissolving process. Finally, polonium was plated on a copper disc from a dilute HCl medium in the presence of ascorbic acid to reduce Fe 3+ to Fe 2+ . Ortec Octete Plus with a 450 mm 2 ULTRA-AS detector was used to measure the alpha activities. Concentrations of 210 Po in all samples were well above the detection limit (0.0003 Bq). Sample count times were 86,400 s, with counting errors in the order of ±10% or less.
After the first deposition of 210 Po, the residual 0.5 M HCl was kept for one year to allow 210 Po in-growth from the 210 Pb contained in the solution to search supported 210 Pb in the samples. The samples were re-plated, and the 210 Po activities were determined. Well-known Bateman equations were used to determine 210 Pb activity from measured 210 Po activity [9]. The second deposition provided information on the 210 Pb content of the samples and hence on the extent to which the initial 210 Po was supported by its grandparent.

Dose Calculations
Due to the dense ionization in tissues caused by high-energy alpha particles, 210 Po has long been recognized as an important contributor to the radiation dose received by organisms [10]. For the calculation of dose due to 210 Po, the following formula was used [11].
where DPo is the collective committed effective dose of 210 Po via consumption of fish from intake during the sampling period. The unit of collective committed effective dose is (man Sv). The coefficients Cb and Fc denote activity concentration of 210 Po in the edible part of the fish samples (Bq·kg −1 ) and the catch calculated from Turkish Statistical Institute (TURKSTAT) statistics (kg·y −1 ), respectively. Fh is the fraction of the catch that is used for regional human consumption (assumed to

Alpha Spectrometric Analysis for 210 Pb
Measurements of 210 Po were realized through its 5.30 MeV alpha particle emission, using 209 Po (4.88 MeV, t 1/2 = 109 y) as the internal tracer. After a standard addition of polonium tracer, each sample was dissolved using three portions of concentrated 20 mL HNO 3 and then 2 mL H 2 O 2 evaporated to near dryness on a hot plate. To the remainder, three portions of 20 mL HCl were added, and the solution was evaporated to dryness. For sediment samples, HF was also used in the dissolving process. Finally, polonium was plated on a copper disc from a dilute HCl medium in the presence of ascorbic acid to reduce Fe 3+ to Fe 2+ . Ortec Octete Plus with a 450 mm 2 ULTRA-AS detector was used to measure the alpha activities. Concentrations of 210 Po in all samples were well above the detection limit (0.0003 Bq). Sample count times were 86,400 s, with counting errors in the order of ±10% or less.
After the first deposition of 210 Po, the residual 0.5 M HCl was kept for one year to allow 210 Po in-growth from the 210 Pb contained in the solution to search supported 210 Pb in the samples.
The samples were re-plated, and the 210 Po activities were determined. Well-known Bateman equations were used to determine 210 Pb activity from measured 210 Po activity [9]. The second deposition provided information on the 210 Pb content of the samples and hence on the extent to which the initial 210 Po was supported by its grandparent.

Dose Calculations
Due to the dense ionization in tissues caused by high-energy alpha particles, 210 Po has long been recognized as an important contributor to the radiation dose received by organisms [10]. For the calculation of dose due to 210 Po, the following formula was used [11].
where D Po is the collective committed effective dose of 210 Po via consumption of fish from intake during the sampling period. The unit of collective committed effective dose is (man Sv). The coefficients C b and F c denote activity concentration of 210 Po in the edible part of the fish samples (Bq·kg −1 ) and the catch calculated from Turkish Statistical Institute (TURKSTAT) statistics (kg·y −1 ), respectively. F h is the fraction of the catch that is used for regional human consumption (assumed to be 0.7), F e is the fraction that is actually eaten (assumed to be 0.8), D f is the delay factor between the catch and consumption time, and 4.3 × 10 −7 is the factor used for adult members of the public, the recommended dose conversion coefficient [12].

Heavy Metal Analysis
The microwave method was applied for the digestion procedure of samples. Dried biota samples of 0.5 g were placed in a Teflon vessel with 8 mL of 65% HNO 3 and 2 mL of 30% H 2 O 2 . Then, 0.5 g sediment samples were added to the vessels with 2.5 mL of 65% HNO 3 and 7.5 mL of HCI [13]. The samples in the vessels were then digested using an optimized microwave method. Then, they were cooled at room temperature. The residues were then dissolved and diluted to 15 mL. The chemicals used for the sample dissolution were of analytical grade. Ultra pure water was used throughout the study [14].
Cr, Ni, Pb, Cd, Mn, Fe, and Zn levels in the fish samples measured and are given in Table 1 with data from around the world. According to Turkish Food Regulations, the maximum Cd, Pb, and Zn concentrations should not exceed 0.05, 0.3, and 50 mg·kg −1 , respectively. The observed values in this study do not exceed the given limits. The high concentrations can be linked to the industrial facilities on the Büyük Menderes River, which is the largest water source for Lake Bafa.
The dose values are presented in Table 2. The calculated annual effective ingestion dose for humans due to 210 Po from the consumption of the lake fishes ranges from 0.011 to 1.169 µSv·y −1 . It is clear that the highest dose value (1.169 µSv·y −1 ) was observed of mullet. According to the UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) [18] report, the effective dose equivalent of 210 Po in areas of normal background radiation is approximately (130 µSv·y −1 ). This dose value is higher than the doses that were found in this study.
The average 210 Po and 210 Pb values are given in Figure 3 seasonally.
The average 210 Po and 210 Pb values are given in Figure 3 seasonally. There are significant seasonal differences in 210 Pb concentrations. The highest 210 Pb concentrations were observed in the winter. The sampling station is under the influence of Mediterranean climate with cool wet winters and warm to hot, dry summers. Most of the precipitation falls during the winter months. It is believed that the elevated concentrations of 210 Pb in sediments in winter could be linked to high atmospheric deposition of 210 Pb during this season [25].
The 210 Po: 210 Pb activity ratios in surface sediments samples varies between 0.85 and 4.48 as shown in Table 3, with a mean value of 2.14. There are significant differences in 210 Po and 210 Pb concentrations with respect to different seasons. These differences could be due to the local pollution sources as well as variations in the riverine and rainfall regimes.  There are significant seasonal differences in 210 Pb concentrations. The highest 210 Pb concentrations were observed in the winter. The sampling station is under the influence of Mediterranean climate with cool wet winters and warm to hot, dry summers. Most of the precipitation falls during the winter months. It is believed that the elevated concentrations of 210 Pb in sediments in winter could be linked to high atmospheric deposition of 210 Pb during this season [25].
The 210 Po: 210 Pb activity ratios in surface sediments samples varies between 0.85 and 4.48 as shown in Table 3, with a mean value of 2.14. There are significant differences in 210 Po and 210 Pb concentrations with respect to different seasons. These differences could be due to the local pollution sources as well as variations in the riverine and rainfall regimes.

Core Sample
The minimum, the maximum, and the average 210 Po concentrations in core samples were found to be 32 ± 1 Bq·kg −1 (at 33 cm), 362 ± 20 Bq·kg −1 (at the surface), 116 ± 5 and Bq·kg −1 , respectively. The minimum, the maximum, and the average 210 Pb concentrations in core samples were found to be 14 ± 2 Bq·kg −1 (at 34.2 cm), 201 ± 18 Bq·kg −1 (at 1.2 cm), and 71 ± 8 and Bq·kg −1 , respectively. Figure 4 shows 210 Pb ex activity concentrations on a logarithmic scale versus cumulative mass depth. Activity concentration of excess 210 Pb varies exponentially along the sediment profile. The respective maximum concentration (159 ± 18 Bq·kg −1 ) was measured in the 1.2 cm sediment layer. The equilibrium point is at 12.6 cm. The 210 Pb concentration in the core sample decreases with depth, so the 210 Pb flux of the sediment is constant. The age of the sediment layers was calculated with the CRS model. The results are given in Table 4. The deepest sediment layers, at depths between 31.8 and 33 cm, had been deposited around the beginning of the 1900s.

Core Sample
The minimum, the maximum, and the average 210 Po concentrations in core samples were found to be 32 ± 1 Bq·kg −1 (at 33 cm), 362 ± 20 Bq·kg −1 (at the surface), 116 ± 5 and Bq·kg −1 , respectively. The minimum, the maximum, and the average 210 Pb concentrations in core samples were found to be 14 ± 2 Bq·kg −1 (at 34.2 cm), 201 ± 18 Bq·kg −1 (at 1.2 cm), and 71 ± 8 and Bq·kg −1 , respectively. Figure 4 shows 210 Pbex activity concentrations on a logarithmic scale versus cumulative mass depth. Activity concentration of excess 210 Pb varies exponentially along the sediment profile. The respective maximum concentration (159 ± 18 Bq·kg −1 ) was measured in the 1.2 cm sediment layer. The equilibrium point is at 12.6 cm. The 210 Pb concentration in the core sample decreases with depth, so the 210 Pb flux of the sediment is constant. The age of the sediment layers was calculated with the CRS model. The results are given in Table 4. The deepest sediment layers, at depths between 31.8 and 33 cm, had been deposited around the beginning of the 1900s.    In the study, the sediment mass accumulation rate was found to be 3.27 g·m −2 ·day −1 (0.119 g·cm −2 ·y −1 ). The sedimentation mass accumulation rate in Lake Bafa is relatively lower than Lake Hachirogata and Lake Erie. Jin et al. [26] found the sediment mass accumulation rate in Lake Hachirogata 0.43 g·cm −2 ·y −1 and 0.45 g·cm −2 ·y −1 in two core samples, and Klump et al. [27] determined the accumulation rate in an average of 0.23 g·cm −2 ·y −1 for Lake Erie.
The heavy metal concentrations in vertical profile of samples from the core are given in Table 5. For all the metals investigated, there is a peak value at 16 cm and below this point the concentrations are decreasing with depth. Except Fe and Zn, the highest and lowest concentrations are at 16 cm and 4.5 cm, respectively. This coincides with the beginning of industrialization in the region. The measured concentration of Fe and Zn are in the range 3.11%-4.13%, 68-90 ppm, respectively. The Zn concentrations in the core sample are lower than the ERL (effects range low) and ERM (effects range median) limits [28] (Table 6). High Fe and Zn concentrations were observed at 3 and 6 cm.

Conclusions
In this study, the distribution of natural radionuclides and certain heavy metals were determined in Lake Bafa via sediment and biota samples. Surface sediment samples were taken seasonally. The highest 210 Pb concentrations were observed in winter because of the Mediterranean climate and high rainfall regime. Most of the precipitation falls during the winter months. This indicates that the main source of 210 Pb is from atmospheric deposition through precipitation. In addition, the sediment mass accumulation rate was found to be 3.27 g·m −2 ·day −1 (0.119 g·cm −2 ·y −1 ) from the core sample. The lake is the delta progradation of the Büyük Menderes River, so it could be considered a sediment trap area. The heavy metal concentrations in the vertical profile of samples from the core were also observed. The measured concentration of Zn, Pb, Cd, and Cr are between the ERL and ERM limits, while Ni concentrations were higher than the ERM limit. Although nickel does not accumulate in fish, plants, or animals, it does accumulate in soils and sediments and may ultimately have an adverse effect on water quality [29].
The 210 Po and 210 Pb concentrations were obtained in the range of other studies around the world. The differences in the radionuclide concentration can be explained by the feeding habitat.
Mullet is a pelagic species. They feed on epiphytic algae, detritus, and small benthic or planktonic organisms. Sea basses feed on small fish, polychaetes, cephalopods, and crustaceans. Eels are predators. In winter and spring, the maximum 210 Po concentrations were observed in eel fish samples. In addition, the highest dose value (1.169 µSv·y −1 ) was observed for sea bass and the annual effective ingestion dose for humans since 210 Po from the consumption of the lake fishes were calculated between 0.011 and 1.169 µSv·y −1 . 210 Po concentrations were highest in sea bass due to feeding habits. Aarkrog et al. [30] calculated that the individual doses ranged from 5.1 to 9.1 µSv·y −1 . Pollard et al. [31] and Nielsen et al. [32] estimated doses of 19 µSv·y −1 and 700 µSv·y −1 , respectively. Jia et al. [33] calculated exposure values between 50 and 200 µSv·y −1 . The maximum observed Cd, Pb, and Zn concentrations do not exceed 0.05, 0.3, and 50 mg·kg −1 , respectively, according to Turkish Food Regulations. Although contaminants measured in fish and sediments in this study do not pose a significant risk to human health, it is important to monitor the lakes from an ecological point of view.