Introduction

The importance of monitoring radionuclides and trace metals is related to the impact of these elements on the marine environment. Several natural and artificial radionuclides have been used in environmental studies, especially in marine processes. 210Po and 210Pb are important natural radionuclides used in studies on the marine environment.

210Po (t 1/2 = 138 days), a high-energy α-particle emitter in the 238U decay chain, is a naturally occuring radionuclide formed by the beta decay of its grandparent of 210Pb (t 1/2 = 22.3 years) via 210Bi. The main source of 210Po and 210Pb is 222Rn emanation from the continents. In the aquatic environment, 210Po is largely produced from the decay of 210Pb deposited from the atmosphere (Stepnowski and Skwarzec 2000). The naturally occuring radionuclides 210Po and 210Pb are important because of their contributions to the natural radiation dose and technologically enhanced releases from sources of natural radioactivity (Vreček et al. 2004).

Coastal environments are subjected to metal contamination via inputs from main natural sources (rock weathering, soil erosion, dissolution of water-soluble salts), industrial and urban sources (municipal wastewater-treatment plants, manufacturing industries, and agricultural activities etc.) that are transported via river discharge and eolians processes (Güven and Akıncı 2008; Uluturhan et al. 2011).

Major indicators of pollution in aquatic environments are contaminated sediments. Sediments are the primary repository of radionuclides and chemicals entering the marine environment (Saçan et al. 2010). Thus, marine sediments are commonly used as environmental matrices in chemical and radioactive monitoring programs.

This study presents 210Po,210Pb and trace metal levels that were measured in marine sediments from two points in the Aegean Sea coast (Izmir Bay and Didim).

Materials and methods

Sample collection and preparation

Surface marine sediment samples were collected using van-veen grab for trace metals and radionuclides during 2006–2007 from two stations in the coast of Aegean Sea every month. The locations of sampling stations are given in Fig. 1.

Fig. 1
figure 1

The map of the study areas

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210Po and 210Pb measurements

The marine sediment samples were weighed and oven-dried to a constant weight at 80 °C and then thoroughly mixed. The samples were ground and passed through a 2 mm mesh followed by homogenization.

After the addition of a standardized amount of 209Po (4.88 MeV alpha emission, t 1/2 = 109 year) tracer, each sample was dissolved using three portions of concentrated 20 mL HNO3 and evaporated to near dryness on a hot plate at 55 °C. Then 2 mL H2O2 was added, and evaporated to near dryness. The sample residuals were treated with three portions of 20 mL HCl, and evaporated to near dryness. Finally, the samples were dissolved in 200 mL of 0.5 M HCl, and ascorbic acid of 4 mg was added to the plating solution to reduce iron. The temperature of the solution was kept constant at 70 °C while stirring, and a copper disk was introduced into the solution in such a way that only one side of the disk was available for plating. The polonium was spontaneously deposited onto the surface of the disk for 5 h at 70 °C. The copper disk was then removed, washed with distilled water and dried at room temperature (Flynn 1968).

210Po levels were measured via 5.30 MeV alpha particle emission rates using a high-resolution alpha spectrometer equipped with 450 mm2 Passivated Implanted Planar Silicon (PIPS) detector (Canberra Model 7401 Alpha PIPS dedector). The spectrometer was connected to a conventional personal computer (PC) using a network connection. The different parameter settings and the viewing of spectra were performed using the commercially available software (Genie-2000 Basic Spectroscopy Software). Contamination of detectors with polonium isotopes such as 210Po and 209Po probably occurs by some other process than alpha recoil. This is probably due to the inherent volatility of polonium at low pressure. Polonium activity is transferred from the sample sources to the detectors, a very serious problem with the long-lived Po-210 and even worse when working with Po-209 (t 1/2 = 102 y) as a yield tracer (Sill and Olsen 1970).

After the first deposition of 210Po, the residual 0.5 M HCl was kept for 1 year to allow 210Po in-growth from the 210Pb contained in the sample solution.

The samples were re-plated and the 210Po activities were determined. The second deposition provided information about the 210Pb content of the samples and thus indicated the extent to which the initial 210Po was supported by its grandparent species.

Well known Bateman equations were used to obtain 210Pb activity from measured 210Po activity.

Lower limit of detection (LLD) was calculated using the Currie definition (Currie, 1968). The concentration of 210Po in a small number of samples was below the detection limit, but most of the 210Po levels were within detection limits (0.0003 Bq). Counting period was adjusted to obtain relative standard error of approximately 5 %. Final activity calculations were attained to include the appropriate corrections for blanks and also for collection date. Recovery was obtained to vary between 70 and 81 % for samples.

Metal analysis

One gram of the sediment sample was dissolved in concentrated nitric acid in a Teflon beaker and small amount of hydrofluoric acid was added. 5 mL of concentrated H2SO4 was added on the sample and the beaker placed on the hot plate at 70–80 °C. After, a small amount of the concentrated HNO3 was added very slowly and continued heating at 120 °C. When the sample solution became liquid, hydrogen peroxide was added and heating continued at the same temperature for 30 min. The hydrogen peroxide was added until the sample became clear. After that, the sample was diluted to 100 mL with 2 % HNO3 in a volumetric flask (Topçuoğlu et al. 2002). Trace metal levels (Mn, Fe, Cr, Ni, Zn, Cu, Cd and Pb) were determined by a Perkin-Elmer inductively coupled plasma-optical emission spectrometry (ICP-OES).

Results and discussion

The activity concentrations of 210Po and 210Pb were determined within the range of 24 ± 5 to 126 ± 6 Bq kg−1 dry wt. with an average value of 67 ± 2 Bq kg−1 and 18 ± 3 to 59 ± 4 Bq kg−1 dry wt. with an average value of 37 ± 1 Bq kg−1, respectively. These values were comparable with those given in literature for sediments from different region in Aegean Sea as shown in Table 1. The activity of 210Po and 210Pb in marine sediment samples of Didim and Izmir Bay is also shown in Figs. 2 and 3. The activity of 210Po and 210Pb in sediment samples was found to be high in 210Po compared to 210Pb. The highest 210Po concentrations were measured in Didim (126 Bq kg−1) sediments. The possible source for enhanced 210Po, especially in Didim, is the discharge of Büyük Menderes river. The river flows into Didim station after carrying fertilized agricultural soil.

Table 1 210Po and 210Pb concentrations in sediment from different regions of the Aegean Sea
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Fig. 2
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Activity concentrations of 210Po (Bq kg−1 dry wt.) in sediments of the Izmir Bay and Didim

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Fig. 3
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Activity concentrations of 210Pb (Bq kg−1 dry wt.) in sediments of the Izmir Bay and Didim

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Likewise, Saçan et al. (2010) studied 210Po and 210Pb concentrations in sediment samples from the Izmir Bay (Aegean Sea), and determined that the highest 210Po activities were observed in winter and summer periods. They pointed out that an increase in 210Po concentrations has been observed in Izmir Bay, probably due to inputs from the Melez stream which is polluted by untreated wastewaters such as industrial, domestic and agricultural sources.

The 210Po/210Pb activity ratio is also investigated in the present study. The 210Po/210Pb ratio for Didim and Izmir Bay stations is found to be ranged from 0.83 to 4.03, and from 0.88 to 3.21, respectively. Also, the mean 210Po/210Pb ratio for Didim and Izmir Bay is determined to be 1.86 and 1.96, respectively. In general, the 210Po/210Pb activity ratios are higher than unity in the measured samples. In study, the highest 210Po/210Pb activity ratios were obtained in Didim for April.

The range of trace metal concentrations (μg g−1 dry wt.) in the Izmir Bay and Didim sediments (respectively) were: Cr 9–65 and 9–22, Cu 9–38 and 3–9, Fe 4709–18470 and 1271–11405, Mn 76–542 and 42–371, Ni 5–33 and 3–18, Pb 0–16 and BDL, Zn 17–85 and 3–30 μg g−1.

Minimum and maximum concentrations of trace metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) determined in the sediments are presented in Table 2. The highest concentrations of metals were found in the Izmir Bay which is intensely industrialized (mainly iron, paper and pulp factories, antifouling paints, chlorine-alkali plants, chemical industries, textile industries, metal processing, timber processing, cement factories, tanneries, oil, soap and a very busy harbor) compared to Didim.

Table 2 Minimum and maximum values of metals in sediments during 2006– 2007 from Izmir Bay and Didim (μg g−1 dry wt.)
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In the past, various studies were conducted to determine the trace metals in the sediments of Izmir Bay.

Metal concentrations (μg g−1 dry wt.) in sediments of Didim and Izmir Bay were compared to other studies in sediments from different regions of the world (Table 3). Comparison of data set revelated that observed trace metal levels in the Didim and Izmir Bay were generally lower than other regions. On the other hand, the levels of metals in this study were higher than Egypt. Different extraction procedures were used in the previous studies and this may have contributed to the differences (Güven and Akıncı 2008).

Table 3 The metal concentrations in sediments (μg g−1 dry wt.) from Izmir Bay and Didim and different regions of the world
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Conclusions

The following conclusions can be derived:

  1. 1.

    The activity concentrations of 210Po and 210Pb were determined in the surface marine sediments in Izmir Bay and Didim (Aegean Sea).

  2. 2.

    The activity concentrations of 210Po were determined within the range of 24 ± 5 to 126 ± 6 Bq kg−1 dry wt.

  3. 3.

    The activity concentrations of 210Pb were determined within the range of 18 ± 3 to 59 ± 4 Bq kg−1 dry wt.

  4. 4.

    The highest 210Po concentration was observed at the Didim station.

  5. 5.

    210Pb activity concentrations in most of the sediment samples were lower than 210Po, and usually 210Po/210Pb activity ratios are much higher for natural levels.

  6. 6.

    The overall order of the metal concentrations found in sediments in our study was: Fe > Mn > Zn > Cr > Cu > Ni > Pb > Cd. Our results show that the distribution of metals in the surface sediment is variable.