Monitoring of radioactive contamination in Polish surface waters in 2012–2013

The 90Sr and 137Cs contamination in Polish surface waters has been monitoring since 1994. Surface water samples from six lakes and the Vistula and Oder Rivers were collected in spring and autumn 2012 and 2013. The mean 90Sr and 137Cs concentrations were 3.92 ± 0.40 and 4.49 ± 2.00 mBq L−1, respectively. Correlations were identified between the radionuclide concentrations and meteorological conditions and the original fallout distribution from the Chernobyl disaster. The annual average radionuclide concentrations were not significantly different from the concentrations found between 1994 and 2011. The 137Cs and 90Sr concentrations have been decreasing only slowly.


Introduction
Artificial radioactive isotopes are present in the environment because of human activities, and these isotopes include (a) fission products generated in nuclear explosions in the atmosphere, which were performed by several countries until 1980, and (b) the products of the Chernobyl nuclear reactor accident in April 1986. The radionuclides 137 Cs and 90 Sr are present in the environment at higher concentrations than other artificial radionuclides. The systematic monitoring of radionuclides in the Polish environment has been performed by the Central Laboratory for Radiological Protection (CLOR) since the early 1960s. For two decades the monitoring study was focused on radionuclide contamination in surface waters (the Baltic Sea and Rivers), tap water, soil, air, agricultural products, and foodstuffs that was predominantly caused by nuclear explosions in the atmosphere. Short-and long-lived radionuclides were measured, and the main indicators of long-term pollution related to weapons were 90 Sr (t 1/2 = 28.8 year) and 137 Cs (t 1/2 = 30.17 year). The extent of the monitoring study was increased dramatically after the Chernobyl disaster.
The CLOR and the network of stations that form the service for the measurement of radioactive contamination has been required to perform high-frequency analyses of the total b-particle activities in surface water (rivers and lakes) and tap water samples [1]. The CLOR started a systematic study of radioactive contamination in water and bottom sediment samples from Polish rivers and lakes in 1994. At present, six lakes in different areas and the two main river systems in Poland, the Vistula River and two of its tributaries (the Bug and Narew Rivers) and the Oder River and its main eastern tributary (the Warta River), are regularly monitored. The activity concentrations of 137 Cs and 90 Sr are measured in the water samples, and the 137 Cs and 239,240 Pu activity concentrations are determined in the bottom sediment samples. Here, we present the results of the analyses of the water samples for 137 Cs and 90 Sr.

Description of sampling sites
The Vistula River is the longest river in Poland, with a total length of 1,022 km. The part of the Vistula River drainage basin that is in Poland measures 168,868 km 2 . The main tributaries of the Vistula River come from the east, and are the Bug River (590 km of which is within Poland) and the Narew River (443 km of which is within Poland). The sampling points on the Vistula River were in the upper, middle, and lower parts, in Kraków, Annopol, Warsaw, Płock, and Kiezmark. The sampling points on the Narew and Bug Rivers were in Pułtusk and Wyszków, respectively.
The Oder River is 840 km long, and 726 km of the river is within Poland and along the Polish-German border. The part of the Oder River drainage basin that is within Poland measures 106,043 km 2 . The main tributary of the Oder River comes from the east, and is the Warta River, which is 795 km long. The sampling points on the Oder River were at Chałupki, Wrocław, Głogów, and Krajnik, and the sampling point on the Warta River was at Poznań.
Six lakes, each located in a different part of Poland, were selected for the water monitoring program. The lakes were Drawsko Lake (in the West Pomeranian District), Niesłysz Lake (in the Lubuskie District), Wadąg and Partęczyny Lakes (in the Masurian District), Wigry Lake (in the Podlaskie District), and Rogóźno Lake (in the Lubelskie District). The largest of the lakes were Wigry Lake (which has an area of 21.2 km 2 and a depth of 73 m) and Drawsko Lake (which has an area of 17.8 km 2 and a depth of 79.7 m), and Rogóźno Lake (which has an area of 0.57 km 2 and a depth of 25.5 m) was the smallest lake. The locations of the river and lake sampling sites are shown in Fig. 1.

Sample preparation
Water samples were collected in the spring and autumn each year. The water samples were 20 L, and they were collected from the main stream of each river and from a platform extending from the shore into each lake. The water samples were put into polyethylene containers and acidified with HNO 3 . In the laboratory, each water sample was evaporated to 450 mL and filtered through a hard filter paper. The filter was then ashed in an oven at 450°C, and the ash was dissolved in nitric acid and combined with the filtrate. Finally, each water sample was evaporated to 250 mL and the 137 Cs and 90 Sr concentrations in the sample were determined.

Analytical method
The 137 Cs in a sample was separated from the solution by filtering the sample through a 137 Cs-selective bed of ammonium molybdophosphate in a radiochemical funnel [2]. The b-particle activity in the ammonium molybdophosphate bed was then measured using a low-level beta GM multicounter system (DTU Nutech, Roskilde, Denmark).
After the cesium was removed, the 90 Sr concentration in the filtrate was determined using the Volchok method [3] with some modifications. The 90 Sr concentration was determined from the b-radiation emitted by 90 Y once equilibrium was reached in the 90 Sr-90 Y system. The same conditions were used for the 90 Sr measurements as were used for the 137 Cs measurements. The accuracy of each of the analytical methods was verified in national and international comparison exercises.

Results and discussion
The 137 Cs and 90 Sr concentrations in the water samples from the Vistula and Oder Rivers and their tributaries and from the lakes are shown in Table 1. The samples were collected in the same periods (spring and autumn) in 2012 and 2013, although the atmospheric and hydrologic conditions were different in the different years. There was a drought in 2012, and the spring samples were collected when there were low or very low water levels. This may have affected the overall results.
The highest 137 Cs concentration (12.33 mBq L -1 ) in the water samples from the Vistula River was found in the sample that was collected in spring 2012 in Annopol. Relatively high 137 Cs concentrations were also found in the samples collected from Warsaw (9.71 mBq L -1 ) and Kraków (7.05 mBq L -1 ) in spring 2012. The concentrations in the samples from Kraków and Annopol collected in autumn 2012 (3.40 mBq L -1 in Kraków and 3.67 mBq L -1 in Annopol) were lower by a factor of two to three than the concentrations in the samples collected in the spring. The concentrations in the samples from Warsaw were lower by a factor of eight (at 1.25 mBq L -1 ) in the autumn than in the  spring. The differences between the concentrations found in the river water samples collected in the spring and autumn in 2013 were fairly small. Relatively high 137 Cs concentrations were only found in the river water samples from Kraków in 2013 (5.65 mBq L -1 in the spring and 4.18 mBq L -1 in the autumn). The water samples collected from the lower Vistula River in 2012 and 2013 were characterized by low 137 Cs concentrations and only small variations in the concentrations, and this agrees with data for previous years [4]. The 137 Cs concentrations in the samples from the lakes varied strongly between sample sites and between sampling periods. The highest 137 Cs concentration was found in water from the small Rogóźno Lake, and significantly lower concentrations were found in the samples from the larger and deeper lakes (Wadąg and Wigry Lakes) in northeastern Poland. The mean 137 Cs concentrations found in the samples collected in 2012 and 2013 were 6.41 ± 1.79 mBq L -1 for Rogóźno Lake, 1.88 ± 0.49 mBq L -1 for Wadąg Lake, and 2.53 ± 0.46 mBq L -1 for Wigry Lake. Similar differences between 137 Cs concentrations in water samples from the lakes were found in previous studies [4,5].
The 90 Sr concentration in the river water samples varied less between sample sites and temporally than did the 137 Cs concentrations. The 90 Sr concentration ranged from 1.59 to 9.61 mBq L -1 over the study period, and the means (over 2012 and 2013) were 4.32 ± 1.20 mBq L -1 for the samples from the Vistula River, 4.02 ± 1.17 mBq L -1 for the samples from the tributaries of the Vistula River, 3.54 ± 0.64 mBq L -1 for the samples from the Oder River, and 4.39 ± 1.00 mBq L -1 for the samples from the tributaries of the Oder River. The highest 90 Sr concentration in the river water samples (9.61 mBq L -1 ) was found in a sample from the Vistula River collected in Warsaw in spring 2012, but a relatively high concentration (7.24 mBq L -1 ) was found in a sample from the Vistula River collected in Kraków in autumn 2012. In 2012, the 90 Sr concentration in the samples from the Oder (collected at Głogów) ranged from 2.15 mBq L -1 in the spring to 4.9 mBq L -l in the autumn, and these samples were collected when the water levels were low. The 90 Sr concentration in the sample collected in Chałupki in spring 2013 was 4.88 mBq L -1 , and this decreased to 3.01 mBq L -1 in the sample collected in autumn 2013. We concluded that the mean 90 Sr concentrations in the samples from both rivers were very similar. However, the 90 Sr concentrations in the samples from the Warta River were somewhat more variable, and the mean 90 Sr concentration in 2013 (6.11 mBq L -1 ) was higher than the mean 90 Sr concentration in 2012 (3.79 mBq L -1 ).
As was the case for 137 Cs, the 90 Sr concentrations were higher in the samples from the small Rogóźno Lake (in southeastern Poland), ranging from 6.19 mBq L -1 in spring 2012 to 9.2 mBq L -1 in autumn 2012, than the 90 Sr concentrations in the samples from the other lakes. The 90 Sr concentrations in the samples from Drawsko Lake were also relatively high, ranging from 3.67 mBq L -1 in spring 2013 to 5.36 mBq L -1 in spring 2012. Lower 90 Sr concentrations were found in Niesłysz Lake (1.35 and 1.60 mBq L -1 in the autumn samples and 2.49 mBq L -1 in the spring samples). The mean 90 Sr concentrations in the samples from the lakes were 3.84 ± 2.14 mBq L -1 in 2012 and 3.40 ± 2.33 mBq L -1 in 2013.
As is stated in the introduction, the radioactive contamination in water was monitored using the total b-particle activity before 1986. The mean b-particle activity found in the Polish water samples that were analyzed in 1986 was 1.5 Bq L -1 , and the highest total b-particle activity reached 417 Bq L -1 in May 1986, when the Chernobyl disaster occurred. The total b-particle activities found in water samples in 2001 were 24-386 mBq L -1 . Since 1986, the 137 Cs in Polish water samples has predominantly originated from the fallout from the Chernobyl accident, which caused severe contamination in some regions, especially in eastern and southwestern Poland. The 137 Cs concentrations in soil show that the amount of 137 Cs contamination in the Polish environment has decreased by a factor of 2.5 since 1988, but the geographical variations in the contamination have not changed [6]. There is still a clear relationship between the geographical location and the 137 Cs concentrations in soil and water samples. The main source of 137 Cs to rivers is it being washed out of the soil by water. This partly explains why there are higher 137 Cs concentrations in the upper parts of the Vistula and Oder Rivers, which are in southern Poland, than in the rivers near to their Baltic Sea estuaries. There was a correlation between the locations of the lakes and the radioactive fallout distribution. A particular case is Rogóźno Lake (in eastern Poland) which has no inflowing or outflowing rivers, so no water exchange can occur, and the only water supplies to the lake are springs and precipitation. All of the other lakes that were studied belong to water systems that include rivers.
The impact of contamination caused by the Chernobyl disaster and meteorological conditions on the 137 Cs concentrations in the river water samples can be identified by inspecting the long-term trends in the 137 Cs concentrations. The plots shown in Fig. 2 (see Table 1 for the detailed results) represent the mean annual 137 Cs concentrations over 20 y (1994-2013) at selected sampling points in the upper and lower parts of the Vistula River (Kraków and Kiezmark) and the Oder River (Chałupki and Krajnik) [4,5,7,8].
The levels of radioactive contamination were higher in the samples collected in the upper parts of the rivers, at Kraków on the Vistula River and at Chałupki on the Oder River, than in the samples collected in the lower parts of the rivers. This agrees with the fact that southern Poland was contaminated more than northern Poland with radioactive fallout from the Chernobyl accident.
The plots in Fig. 3 show the 137 Cs concentrations found in the lakes that were studied between 1994 and 2005 and between 2007 and 2013 [4,7,8]. The 137 Cs concentration remained more constant over the years in the lakes than it did in the main rivers. Again, the exception was Rogóźno Lake, in which the 137 Cs concentration decreased significantly over the study period.
It should be stressed that the first measurements of 90 Sr contamination of water in Poland were performed in 1961. A number of nuclear explosions, especially the intense period of nuclear tests in 1951-1958 and 1961-1962, caused the 90 Sr concentrations in the water samples collected in 1961 to have a very broad range, from 4 to 104 mBq L -1 . The 90 Sr concentrations in 1962 were 17-60 mBq L -1 . The mean 90 Sr concentration in water in the Vistula estuary on the Baltic Sea was 10.2 ± 1.5 mBq L -1 in 1983 and the 137 Cs concentration was only 1.0 ± 0.1 mBq L -1 . Similar concentrations have been found in lakes in northern Poland, e.g., the 90 Sr and 137 Cs concentrations in water samples from _ Zarnowieckie Lake in the late 1970s were 9.58 ± 1.39 and 2.68 ± 0.21 mBq L -1 , respectively [9]. Fallout connected with nuclear bomb explosions around the world between 1945 and 1980 is still the main source of 90 Sr in the environment. Comparable amounts of 90 Sr and 137 Cs were present in the bomb-related fallout. Smaller amounts of 90 Sr than 137 Cs were released by the Chernobyl accident because Sr is not very volatile, and this means that the environment was affected by much less Sr contamination than Cs contamination. Therefore, the presence of 90 Sr in the environment is mainly related to nuclear weapons testing in the atmosphere, and the reasons for the 90 Sr variations that are observed in water bodies are rather complex [10]. The 90 Sr concentrations found in the lake water samples are presented in Fig. 5. As stated earlier, the highest concentrations were always found in the Rogóźno Lake samples. The concentrations in the other lakes were quite constant between 2007 and 2013. The 90 Sr and 137 Cs concentrations in the lake water samples were comparable.
The radionuclide concentrations in the Polish water bodies were fairly similar to the concentrations that have been found in rivers and lakes in other regions of Europe. For instance, the 137 Cs concentrations found in Lugano Lake and the rivers that feed into the lake were around 1 mBq L -1 in 2003-2004 [12].
In 2012, the 137 Cs concentrations found in four rivers in Finland were in the range 1.5 to 22 mBq L -1 , and the 90 Sr concentrations were in the range 2.5 to 6.4 mBq L -1 [13]. 90 Sr concentrations of \1 to 6 mBq L -1 were found in the Netherlands in 2011 [14]. 90 Sr concentrations of 1.2-4.3 mBq L -1 and 137 Cs concentration of \1.5 mBq L -1 were found in three rivers in Portugal in 2011 [15].

Conclusions
The 137 Cs and 90 Sr concentrations found in surface water samples from Poland in 2012 and 2013 were compared with the concentrations found in previous years, with the aim of identifying temporal changes. The 137 Cs and 90 Sr concentrations have clearly but rather slowly decreased over the years. The 137 Cs mainly originated from the Chernobyl disaster, while the 90 Sr is to a large extent the residue of nuclear weapons explosions in the atmosphere that were performed in the 1950s and 1960s. Despite the differences in the times that the 137 Cs and 90 Sr were released into the atmosphere, the 137 Cs and 90 Sr concentrations in the surface water samples were quite similar.