There are more than 50 continental and marine salt lakes located in the Crimea(Balushkina et al., 2009). The lakes contain an almost inexhaustible supply of sodium, magnesium, bromine and other elements(Ponizovskii, 1965), being a potentially valuable source of raw materials for large-scale chemical industry. They represent a unique water ecosystem with a peculiar hydrochemical regime(Bulyon et al., 1989; Balushkina et al., 2009; Belmonte et al., 2012). The high salinity of the water in these lakes is mainly caused by intense evaporation, especially in summer. This may result in the net accumulation of many chemical elements, including radioactive isotopes(Bulyon et al., 1989). Natural radioactivity of environmental waters is primarily caused by the presence of ²²² Rn, ²²⁰ Rn(thoron), ²²⁶ Ra, ²²⁸ Ra, ²²⁴ Ra, ²³⁴ U, ²³⁸ U, ⁴⁰ K, ²¹⁰ Po, and ²¹⁰ Pb; artificial radioactivity is mostly due to ⁹⁰ Sr and ¹³⁷ Cs(Bulatov, 1996).

Artificial radionuclides in the environment originate from nuclear weapons testing, as well as a result of accidents arising from nuclear industries involved in energy production. On the 26 ^th of April 1986, the Chernobyl nuclear power plant(NPP)accident occurred, which was the largest nuclear disaster of the 20th century(Izrael, 1998; Appleby et al., 1999). Within ten days, while there were emissions, 1.9 EBq of radioactive material, represented by the fi ssion products and transuranic products of activation, equivalent to 3%–4% of the activity contained in the reactor zone, entered into the environment.

Apart from the inert gases, 20% of the iodine in the active zone(670 PBq of ¹³¹ I), 10% of the total amount of cesium(19 PBq of ¹³⁴ Cs; 37 PBq of ¹³⁷ Cs), 8 PBq of ⁹⁰ Sr, and 0.1 PBq isotopes of plutonium entered the atmosphere(Izrael et al., 1987; Polikarpov et al., 2008a). Presently, the contamination is restricted to ¹³⁷ Cs and ⁹⁰ Sr, as well as long-lived isotopes of plutonium and americium. The significance of the ¹³⁷ Cs and ⁹⁰ Sr entry into the environment as the result of the Chernobyl NPP accident(89 and 7.4 PBq, respectively)can be compared with a release of these radionuclides caused by nuclear weapons testing in open environments: 1 300–1 500 and 650–1 300 PBq, respectively, as well as with entry of these radionuclides due to other nuclear incidents(Polikarpov, 1966; Joseph et al., 1971; Gudiksen et al., 1989). Radioactive contamination of aquatic ecosystems, located both near and far away from the site of the accident depends on the emission into the atmosphere and aerial transportation of the radioactive products and aerosol particles. In the fi rst months after the accident, the Black Sea water basin was exposed to strong radioactive contamination. In May 1986, 1.7–2.4 PBq of ¹³⁷ Cs and 0.3 PBq of ⁹⁰ Sr were deposited on the surface of the Black Sea(Polikarpov et al., 2008a). In the years following the accident, the radioecological situation in the Crimea region was determined by secondary chronic radionuclide contamination, primarily ⁹⁰ Sr, with fl ows of the rivers, mainly the Dnieper, mainly due to the water feeding from the North-Crimean Canal(NCC)(Mirzoyeva et al., 2008a). An important feature of the Chernobyl accident was that the radioactive contamination of the environment occurred on a time scale much shorter than the characteristic time course of biogeochemical processes. Therefore post-accident ⁹⁰ Sr and ¹³⁷ Cs as radiotracers can characterize the intensity of hydrological and biogeochemical processes in aquatic ecosystems.

Mercury is one of the most dangerous chemical pollutants in marine waters due to the stability of its compounds in the saline environments, as well as by its accumulation in bottom sediments and in aquatic organisms(Kostova et al., 2001). The persistence of this element also provides a further opportunity to study possible sources of intake of the pollutants(in addition to that of ⁹⁰ Sr)into the aquatic ecosystems of the salt lakes.

Many of the salt lakes of the Crimea are used for recreational and economic purposes(Pervolf, 1953; Ponizovskii, 1965). The medicinal properties of the water and bottom sediments are often associated with increased levels of radon(Pervolf, 1953), one of the daughter products of the decay of uranium, the concentration of which in natural water reservoirs is often directly proportional to their salinity(Buesseler and Benitez, 1994).

Despite the growth of human infl uence and climate change in the region, environmental studies of the Crimean salt lakes were conducted only occasionally in the last 23 years(Sivash region, 2007; Gulina and Gulin, 2011; Dyakov et al., 2013). The presented radiochemoecological studies allow to estimate the infl uence of radiation and chemical pollution on water quality and on aquatic plants, and to determine the role of living and inert matter in the transfer, migration and elimination of toxicants of different nature.

The purpose of this investigation was the evaluation of the ecological status of salt lakes of the Crimea regarding the content in them the radioactive substances and chemical contamination for period 2009 and 2012–2014, the consideration of the ability of use of radiotracer methods for assessment of the intensity and dynamic of the biogeochemical processes, occurring in the ecosystem of the salt lake of the Crimea.

In accordance with the overall purpose of the investigations, the following tasks were performed: We determined the concentration and redistribution concentrations of ⁹⁰ Sr and of mercury in the components of aquatic ecosystems of different salt lakes(Lakes Kiyatskoe, Kirleutskoe, Kizil-Yar, Bakalskoe, Donuzlav);

We conducted a comparison of the water concentration levels observed for ⁹⁰ Sr and mercury of the Black Sea ecosystems, located in areas close to the location of the salt lakes and the content of these pollutants in the lake water, with the goal of identifying possible sources of intake of ⁹⁰ Sr and mercury into the aquatic ecosystems of the salt lakes;

We determine the contents of the vertical distribution of natural(²³⁸ U, ²³² Th, ²²⁶ Ra, ²¹⁰ Pb, ⁴⁰ K) and anthropogenic(¹³⁷ Cs)radionuclides in sediments salt Lake Koyashskoe;

We used Lake Koyashskoe as a model to explore the use of radiotracer methods to estimate the intensity of long-term dynamics and biogeochemical processes with special emphasis on sedimentation; We calculated the radiation doses received by aquatic plants from the ionizing radiation of ⁹⁰ Sr and its daughter product ⁹⁰ Y in the period after the Chernobyl NPP accident.

The study of the salt lakes was conducted during the period 2009, 2012–2014. Material for the study(water, aquatic plants, bottom sediments)was collected by the staff of IBSS, the Department of Radiation and Chemical Biology(DRChB), during expeditions to the Tarkhankut(Donuzlav, Bakalskoe), Evpatoria(Kizil-Yar), Perekopskaya(Kirleutskoe, Kiyatskoe), and Kerch(Koyashskoe)groups of lakes, which differ in origin, water balance, concentration and chemical composition(Ponizovskii, 1965)(Fig. 1). The sampling sites are given in Table 1, together with associated information on sample details.

Fig. 1 Location of the sampling stations at the salt lakes of the Crimea

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Table 1 Sites and details of sampling

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The Kiyatskoe and Kirleutskoe lakes(Fig. 1a)are part of the Perekopskaya group of lakes with a continental origin(Ponizovskii, 1965). Lake Kiyatskoe has the following characteristics: length, 10 km; maximum width, 2.5 km; average depth, 2 m; maximum depth, 4 m; area, 12.5 km ² ; catchment area, 68.4 km ² ; altitude of 4.0 m below sea level; and a salinity of 21.6‰. Kiyatskoe lake water, according to its chemical composition, is characterized by an increased content of NaCl(83.8%) and MgCl ₂(13%) and a low content of CaSO ₄(1.6%) and Ca(HCO ₃)₂(0.07%). Gypsum is the only sulfate salt present(Ponizovskii, 1965). The lake has no drainage and the average annual precipitation is ＜400 mm. Water is supplied to the lake from the groundwaters of the Black Sea artesian basin, waste and drainage water. Lake Kiyatskoe is connected to a drainage system and the North-Crimean Canal(Oliferov and Timchenko, 2005). In the 1970s this lake was considered unique, due to the high quality of brine. Since then the quality of the Lake Kiyatskoe’s ecosystem was destroyed by long-term discharge of wastewaters from the Crimean soda plant(the North-Crimean industrial complex)(Oliferov and Timchenko, 2005).

Lake Kirleutskoe(Fig. 1a)is the third largest(20.8 km ²)lake of the Perekopskaya groups; it has a catchment area, 101 km ² ; length, 13 km; maximum width, 3 km; maximum depth, 3 m; and it is located at an altitude of 3.9 m below sea level. The lake is not used for economic activities. The chemical composition of the lake water is brine, containing mainly sodium chloride, potassium, magnesium, calcium, and magnesium sulfate. The average annual precipitation is ＜400 mm. The lake is fed from groundwater from the artesian basin of the Black Sea, waste and drainage water. Also, an unnamed river fl ows into this lake(Ponizovskii, 1965; Oliferov and Timchenko, 2005).

Lake Bakalskoe(Fig. 1b)is the third largest lake in the Tarkhankut group of Crimean salt lakes(Shadrin and Anufriieva, 2013). It has a surface area, 7.1 km ² ; catchment area, 257 km ² ; length, 4 km; average width, 1.7 km(maximum 3.5 km); average depth, 0.4 m(maximum 0.85 m); and it is located at an altitude of 0.4 m below sea level. Like all lakes of the Tarkhankut group, Lake Bakalskoe has a similar chemical composition of Lake Kirleutskoe. Lake Bakalskoe has no drainage and the average annual precipitation is 350–400 mm(Ponizovskii, 1965). Primary water sources are marine waters(Shadrin and Anufriieva, 2013) and surface and underground waters(artesian basin of the Black Sea), and fl ow from two rivers(the Romanovka and the Dzhugenskaya-Ahtanskaya). The lake is used for medical treatment and recreation. Lake Bakalskoe is one of two lakes of the Crimea(other one is Lake Koyashskoe)that belongs to the protected natural objects of this region(Ponizovskii, 1965; Oliferov and Timchenko, 2005).

Lake Donuzlav(Fig. 1c)is the deepest and the largest lake in the Crimea and the Black Sea region. It belongs to the group of Tarkhankut lakes. It has a water surface area, 48.2 km ² ; lake coastline, 104 km; maximum depth, 27 m; catchment area, 1 288 km ² ; and a salinity of 7.06×10^-3. The northern part of the lake contains fresh water. The origin of the lake is tectonic, and the Old Donuzlav, the Donuzlav, the Burnuk, and the Chernushka rivers drain into it. Presently Lake Donuzlav can be considered a “manmade bay”: as a result of the construction of a naval base in 1961, the isthmus that separated the inl and lake from the Black Sea water was broken and the lake became connected with the Black Sea(Ponizovskii, 1965; Oliferov and Timchenko, 2005). Lake Kizil-Yar(Fig. 1d), like other salt lakes located in the neighborhood, was formed from a sea bay which cuts deeply into the dry l and . It belongs to the Evpatorian salt lakes group, and is separated from the sea by a narrow spit. It has a surface area, 8 km ², greatest depth, 3.7 m; catchment area, 328 km ² ; and is fed by the River Tobe-Chokrak. The lake is used for medical treatment and recreation(Ponizovskii, 1965; Oliferov and Timchenko, 2005) and is now a freshwater lake(Shadrin et al., 2012).

Lake Koyashskoe(Fig. 1e)has a marine origin, it belongs to the Kerch group of salt lakes(Ponizovskii, 1965; Oliferov and Timchenko, 2005). It has a length, 3.84 km, maximum width 2.81 km; and catchment area, 23 km ² . The chemical composition of the brine is that of a chloride-sulfate type lake. Lake Koyashskoe is one of the most saline of the Crimean salt lakes(salinity, 184– 340 g/L)(Abatzopoulos et al., 2009; Balushkina et al., 2009; Gulina and Gulin, 2011), and it has no outlet. Average annual precipitation is less than 400–450 mm. As stated above, the lake is part of a nature reserve. Water enters the lake from the sea, from surface water and from the groundwater artesian basin(Abatzopoulos et al., 2009). The lake is used for recreation(Ponizovskii, 1965; Oliferov and Timchenko, 2005).

Samples of sea water from the Black Sea areas, located close to the studied salt lakes were collected to compare the water concentration levels observed for ⁹⁰ Sr and mercury of the Black Sea ecosystems and to identify possible sources of entry of ⁹⁰ Sr and mercury in the aquatic ecosystems of the salt lakes. In addition to the water samples, bottom sediments and aquatic plants(Cladophora sp., Cystoseira sp., Potamogeton crispus L.)were analysed(Table 1).

For the period of research the following amount of samples were collected and analysed: water: 42, bottom sediments: 33, water plants(3 species): 10.

Each sample, which was collected for the radiochemical analysis of ⁹⁰ Sr, has such weight or volume characteristics: 20 liters of water, 50 g of dried bottom sediments, 50 g of ash of aquatic plants. ⁹⁰ Sr determination was based on the following radiochemical method: after acid leaching and /or preconcentration of strontium with carbonate(for water)or oxalate(for aquatic plants and bottom sediments), it was purifi ed from interfering elements by hydroxide precipitation. After equilibrium between ⁹⁰ Sr and the daughter product ⁹⁰ Y was established(after at least 14 days), ⁹⁰ Y was separated from ⁹⁰ Sr solution and measured by its Cerenkov radiation in a low background LKB “Quantulus 1220” liquidscintillation counter. The limit of detection was 0.01– 0.04 Bq/kg for aquatic plants and bottom sediments and 0.01–0.04 Bq/m ³ for water samples. Radiochemical recovery of ⁹⁰ Sr was calculated from recovery of added stable strontium carrier. Stable strontium was determined by fl ame photometry and then gravimetrically from the yttrium oxalate for ⁹⁰ Y(Harvey et al., 1989; Mirzoyeva and Kulebakina, 2008). Each result is reported as the mean of the activity values of parallel replicate samples, which were measured separately. The total relative error of each result did not exceed 20%.

The quality of the analytical methods and the reliability of the results were supported by regular participation in international inter-calibrations during the period 1990–2004, under the aegis of the IAEA(Vienna, Austria). Results of the IBSS participation in the inter-calibration were included in the intercalibration report materials(as examples IAEA, 1998, 2004) and each were certifi ed as reliable data.

The radiological dose(Gy/year)for the aquatic plants was calculated using the mean ⁹⁰ Sr concentrations in each group of aquatic plants, ⁹⁰ Sr concentrations in water and bottom sediments from the area of their habitation, and using individual coefficient dose-rate conversion factors(Amiro, 1997; US Department of Energy, 2001). Values of dose conversion factors for calculation of internal and external doses of ⁹⁰ Sr for aquatic organisms were derived from worksheets of the computer program of the RAD-BCG Calculator(US Department of Energy, 2001). The dose estimates were compared with the dose limits for aquatic organisms from the DOE St and ard(2001) and with the scale of zones of chronic dose rates and their effects in the biosphere proposed by Polikarpov(1998)(Mirzoyeva et al., 2008b, 2013).

To determine the concentration of mercury in water of the salt lakes of the Crimea, 26 samples were collected from the surface of reservoirs in one-liter fl asks. Immediately after sampling, all water samples were conserved by addition of concentrated nitric acid(10 mL acid to 1 L of water). H and ling and preparation of samples for the measurements were performed as described(Uniform Methods ..., 1986). In the laboratory, the sample water was fi ltered through 0.45 μm pore size Nucleopore fi lters. Dissolved forms of mercury were determined in the fi ltrate, and solid forms on the fi lters. The suspended matter on the fi lters was subjected to chemical treatment. The total amount of mercury consists of the sum of the quantities of the dissolved and suspended mercury in water. The mercury concentration was measured using fl ameless atomic absorption spectrophotometry(Igoshin and Bogusevich, 1969; Kostova et al., 2001). Measurements were performed on a “Julia-2” mercury analyser with a sensitivity of 0.001 μg. The relative error of measurement for mercury in water and suspended matter was 6.4% and 13.6%, respectively.

Sediment cores with undisturbed stratification were sampled in the coastal south-eastern part of Lake Koyashskoe, approximately 15 m from the shore(Table 1, Fig. 1e). The depth of the bottom at the sampling point was 15 cm. An acrylic tube with an internal diameter of 58 mm and a sharpened lower edge was immersed in the sediment to a depth of about 20 cm. Then the lower edge of the tube was closed with a rubber stopper-piston and the top by a plastic cap cover. In the laboratory of the DRChB of IBSS, horizontal layers of the core were sliced to a thickness of 1 cm using ~100 μm thick aluminium foil and a piston extruder as described by Papucci(1997). To reduce wall effect, the 1.5 mm edge of each layer of the core was cut off using rings of a smaller diameter(55 mm). Thereafter, sliced samples were weighed, and dried at a temperature of 40–50°C to a constant weight to determine the amount of evaporated water. To evaluate the porosity of sediments, the content of salts dissolved in the water capillaries was calculated(Buesseler and Benitez, 1994). Overall number of sediment samples taken in Lake Koyashskoe was 23 from which 12 samples were treated with gamma spectrometry. Their average dry weight was 20.7±2.8 g. The activity of the radionuclides of ²³⁸ U, ²³² Th, ²²⁶ Ra, ²¹⁰ Pb, ⁴⁰ K, and ¹³⁷ Cs in the dried samples was determined using the highpurity germanium(HPGe)detector EG&G ORTEC GMX10P4(geometry: coaxial, diameter 47.8 mm, length 45.6 mm; resolution: 1.75 keV/1.332 MeV; efficiency: 16.2%). The average counting time in gamma detector=2.7±0.8 day. The detector was calibrated using st and ard samples of bottom sediments IAEA-306 and IAEA-315, supplied by the IAEA(IAEA, 1998), having a shape and size similar to our bottom sediment samples. A detailed description of the method of measuring the activity of radionuclides in the Crimean lake samples was given by Gulina and Gulin(2011). In all cases, the relative error did not exceed 10%. The relative content of organic matter in sediments was determined by the change in weight of the sample after ashing in a muffl e furnace at 450°C.

Table 2 indicates that the highest concentration of ⁹⁰ Sr in water was observed in the lakes of the Perekopskaya group(Kiyatskoe and Kirleutskoe lakes)(see also Figs.2, 3). The increased concentration of the post-accident ⁹⁰ Sr in water of Lake Kiyatskoe resulted, above all, from the significant anthropogenic pressure on the studied ecosystem, as well as the peculiarities of the hydrological and hydrochemical characteristics of the investigated reservoir. It is known(Ponizovskii, 1965; Oliferov and Timchenko, 2005)that before 2014, the Dnieper water(which has regularly been entering through the waste ditches and branch duct system of the North-Crimean Canal(NCC)), provided a secondary chronic source of dissolved strontium from the region of the Chernobyl NPP accident(Gulin et al., 2013). Long-term wastewater discharge by the Crimean soda plant(where before 2014, the waters of the NCC and water of the Siwash Bay had been used in its operating cycle)was an additional source of ⁹⁰ Sr in the period after the Chernobyl NPP accident. The concentration of ⁹⁰ Sr in water of Lake Kiyatskoe, which was restored on 1986, was 730.2 Bq/m ³ . This value corresponds to the largest concentration of this radionuclide(728.8 Bq/m ³)that arrived in the dissolved form in the waters of the Dnieper River into the Kakhovskoe reservoir and then into the NCC in November 1986(Mirzoyeva et al., 2008a, 2013). In May 2014, the concentration of ⁹⁰ Sr in Lake Kiyatskoe water was 3.6 times lower than in February 2013(Table 2). This is probably due to the lack of entry of the Dnieper water from the NCC into the Crimea region in 2014(Ukraine builds dam on North Crimean Canal..., 2014), the redistribution of ⁹⁰ Sr between the ecosystem components in the lake and radionuclide deposition(primarily in the bottom sediment).

Table 2 Concentration of ⁹⁰ Sr in water of the salt lakes of the Crimea and in control samples

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Fig. 2 Concentration of 90 Sr in water of the salt lakes of the Crimea depending on the salinity and pH (+) of the environment 2: Lake Kizil-Yar; 3: Lake Kiyatskoe; 4: Lake Kirleutskoe; and control water samples (1: tap water in the DRChB laboratory),sampling: May, 2014.

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Fig. 3 Concentration of 90 Sr in water of the salt lakes of the Crimea and control samples 1: Lake Donuzlav; 2: sea near Lake Bakalskoe; 3: sea near Evpatoriya city; 4: sea near Tarkhankut Cape; 5: Lake Bakalskoe;6: Lake Kiyatskoe; sampling: February, 2013.

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Human activity in Lake Kirleutskoe is not active(Oliferov and Timchenko, 2005). However, its close proximity to the NCC and the peculiarities of the water supply(groundwater artesian basin of the Black Sea, waste and drainage water)were the main cause of dissolved post-accident ⁹⁰ Sr entering with Dnieper waters into the ecosystem(Table 2, Fig. 2).

To prevent the increase of quantity and level of the groundwater caused by construction of the NCC in the northern Crimea, drainage systems were built using the Perekopskaya group of salt lakes as reservoirs for water storage(Ponizovskii, 1965). At the control sampling stations(2–4), (Fig. 3), ⁹⁰ Sr concentrations in the Black Sea water along the northwestern part of the Crimean peninsula did not exceed the levels of this radionuclide before the Chernobyl NPP accident(Mirzoyeva et al., 2013). This is because the biogeochemical and hydrological processes in the Black Sea ecosystem reduce the presence time of ⁹⁰ Sr in the environment to 106–127 years(Mirzoyeva et al., 2013). In sea water, near the Tarkhankut Cape, the ⁹⁰ Sr concentration in waters of the salt lakes Kirleutskoe and Kiyatskoe was higher(by 1.7 and 23.4 times, respectively)than the content of this radionuclide determined in the waters of the Crimea before the Chernobyl NPP accident(Table 2, Figs.2, 3)(Polikarpov et al., 2008a).

A slight excess of ⁹⁰ Sr in sea water near the Tarkhankut Cape was expected(Mirzoyeva et al., 2013). It is caused by secondary chronic radionuclide contamination of ⁹⁰ Sr entering with Dnieper waters through the branches of the North-Crimean Canal(Gulin et al., 2013; Mirzoyeva et al., 2013), as well as hydrological and biogeochemical processes occurring in the marine ecosystem. The solubility of ⁹⁰ Sr is largely dependent on the level of salinity of these lakes and is not essentially dependent on pH(Table 2, Figs.2, 3). This is because the alkali metal chlorides and other salts dramatically increase the solubility of strontium salts. Thus, when the content of NaCl in water is >15%, the solubility of strontium sulfate increases 13 times(Ponizovskii, 1965). In the Black Sea and the salt lakes of the Crimea the NaCl content range from 72.8% to 83.8% of the total amount of salts, regardless of the level of salinity(with a minimum value in Lake Kirleutskoe and a maximum in Lake Kiyatskoe)(Ponizovskii, 1965; Zaitsev, 1998). Therefore possibly, ⁹⁰ Sr is mainly in the ionic form in aquatic ecosystems of the Black Sea and the Crimean salt lakes.

Redistribution of ⁹⁰ Sr between aquatic plants of the studied salt lakes is insignificant(Fig. 4). As opposed to marine hydrophytes in the region of the Tarkhankut Cape(Mirzoyeva et al., 2013)(Fig. 4), no speciesspecificity of ⁹⁰ Sr accumulation by algae and higher aquatic plants was observed from the salt lakes of the Crimea. The ⁹⁰ Sr concentration in aquatic plants of the genus Potamogeton and Cladophora from Lakes Kiyatskoe and Bakalskoe, with different levels of salinity, was within the limit of the confi dence interval and amounted to 0.30±0.04 to 0.439±0.04 Bq/kg wet weight(W.W.). The ⁹⁰ Sr concentration in Potamogeton crispus was at the detection limit(0.07±0.01 Bq/kg W.W.)(Fig. 4, Table 3). The accumulation factor(AF)of ⁹⁰ Sr in aquatic plants of Lake Bakalskoe, which is marine in origin, and the Black Sea(Table 3)corresponded to the range of changes of the AF in Cystoseira sampled from the Black Sea(16.7–60.1)(Polikarpov et al., 2008b).

Fig. 4 Concentration of 90 Sr in aquatic plants, selected from the salt lakes of the Crimea and the Black Sea coastal area (the area of the Tarkhankut Cape), sampling 2013 – 2014

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Table 3 Average concentrations and Accumulation Factors (AF) of ⁹⁰ Sr in the aquatic plants from the Crimean salt lakes and from the control sampling stations

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Previously, it was determined that in fresh water reservoirs of the Ukraine fi lamentous green algae of the genus Cladophora, and higher aquatic plants of the genus Potamogeton, are both indicator species for the accumulation of ⁹⁰ Sr in freshwater hydrophytes, having significant accumulation factors(12–1 025)(Mirzoyeva et al., 2008a, 2013). The low ⁹⁰ Sr AF for aquatic plants from Lake Kiyatskoe(Cladophora sp.) and Lake Donuzlav(P . crispus), (0.97 and 6.20, respectively)can be explained by the presence of salts in the investigated matrices, which may interfere with the mechanism of absorption of strontium radionuclides from the water by these freshwater plant species.

Despite the significant content of radionuclides in the water, the ⁹⁰ Sr concentration in the bottom sediments in Lakes Kiyatskoe and Kirleutskoe(0.45±0.22 and 0.62±0.23 Bq/kg dry weight(D.W.)), respectively)was almost 6 times lower than the average concentration in bottom sediments of different Sevastopol bays(3.1±0.1 Bq/kg(D.W.)). In 2013–2014 the ⁹⁰ Sr concentration in the water of all investigated salt lakes of the Crimea and control sampling stations did not exceed the maximum permissible concentration(MPC)for ⁹⁰ Sr in drinking water(NRS-99/2009).

The total external and internal exposure doses from ionizing radiation of ⁹⁰ Sr for the aquatic plants from Lake Kiyatskoe were determined to be 3.2×10 ^-6 Gy per year and persisted for the whole period of investigation(2013–2014)within the “Well-being Radiation Zone” according to the scale of the “Zone of chronic exposure to ionizing radiation”, which was proposed by Polikarpov(1998).

According to the levels of ⁹⁰ Sr concentration in water, bottom sediments, and hydrophytes sampled from other investigated salt lakes of the Crimea and the Black Sea coastal area(Table 2, Fig. 3), the radiation doses received by hydrophytes from these aquatic ecosystems were also within the “Well-being Radiation Zone”. Consequently, the absorbed doses, which had formed in the hydrophytes from ionizing radiation of ⁹⁰ Sr and its daughter product ⁹⁰ Y, had no noticeable impact on aquatic plants in the period after the Chernobyl NPP accident.

The highest concentration of total mercury was determined in water of Lake Kiyatskoe and the Black Sea area located near Lake Kizil-Yar in August 2012(363.2 and 368.8 ng/L, respectively)(Table 4, Fig. 5). These concentrations were 3.5 times higher than the MPC(Guidance, 1993). The high mercury content of these waters is due to anthropogenic impacts on these aquatic ecosystems. Lake Kiyatskoe, for example, is used in the operating cycle of the Crimean soda plant for wastewater discharge(Oliferov and Timchenko, 2005). The highest concentrations of mercury were observed in summer(August, 2012) and the lowest in winter(February, 2013). Many chemical elements, including mercury, can become concentrated in the summer time due to intensive evaporation of water in the lakes(Bulyon et al., 1989). In the summer of 2012 the main contribution to the total concentration of mercury in investigated reservoir waters, with the exception of Lake Kirleutskoe, was in dissolved form(Table 4, Fig. 6). In February 2013, only insignificant concentrations of mercury were measured in water of Lakes Bakalskoe and Donuzlav and in seawater near the Tarkhankut Cape. The pollutant was only found in the suspended form. The mercury content in the water of the Crimean salt lakes and the studied areas of the Black Sea did not depend on the salinity(Fig. 6). Since the intake of mercury in the environment is largely determined by human activity(Prokofiev, 1981), we believe that the reduction in mercury concentration that occurred in the waters studied in February 2013, was due to a lower economic activity, so that infl ow of pollutants into the environment was reduced.

Table 4 Concentration of mercury in the sampled waters

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Fig. 5 Concentration of total mercury in water of salt lakes of the Crimea and adjacent regions of the Black Sea (control sampling stations)

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Fig. 6 Levels of salinity, concentrations of dissolved and suspended form of mercury in water of the salt lakes of the Crimea

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Table 5 presents the vertical distribution of natural and artificial radionuclides in the bottom sediments, sampled in the coastal zone of Koyashskoe salt lake(Gulina and Gulin, 2011). The combined natural radionuclides ²³⁸ U, ²³² Th, ²²⁶ Ra, and especially ²¹⁰ Pb and ⁴⁰ K, contribute the largest contribution to the total radioactivity in the bottom sediments(generally, accounts for ~97% of the total activity), (Table 5). In contrast, the average level of radioactivity in sediments from anthropogenic ¹³⁷ Cs(which was released into the environment after the atmospheric nuclear weapons tests in 1950–1960 and as a result of the Chernobyl NPP accident in 1986)was 2–3 times lower than, for example, in the coastal zone and in the bays of Sevastopol(Gulin et al., 2002; Gulin, 2008). Noteworthy is the high content in the bottom sediments of the natural ²¹⁰ Pb(Table 5), produced by the radioactive decay of ²²² Rn. Typically, the amount of this radionuclide in the surface layer of the Black Sea coastal sediment is 5–10 times less than the values specifi ed in Table 5(Gulin, 2008). At the same time, the content of ²²⁶ Ra, which is formed from ²²² Rn, in the surface layer of the bottom sediments of Lake Koyashskoe was not significantly different from the values found earlier in the shallow waters of the Black Sea(Gulin et al., 2002; Gulin, 2008).

Table 5 Vertical distribution of naturally occurring and man-made radionuclides in coastal sediments of Lake Koyashskoe

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The data may indicate an active fl ow of gaseous ²²² Rn, having only a short half-life of 3.8 days, from the lower layers of the bottom sediments of Lake Koyashskoe. It should be noted that the concentration of ²³⁸ U is higher in the bottom sediments of Lake Koyashskoe compared to the other study areas. This is because the concentration of ²³⁸ U in water bodies is directly proportional to salinity(Buesseler and Benitez, 1994). Data on the vertical distribution of radionuclides in Lake Koyashskoe sediments(Table 5, Figs.7, 8)allow us to determine a range of biogeochemical parameters, especially the rate of sedimentation, for which ²¹⁰ Pb and ¹³⁷ Cs may be used(Buesseler and Benitez, 1994; Appleby, 1998). The profile of the vertical distribution of ²¹⁰ Pb in the bottom sediments of the coastal zone of the lake can be satisfactorily described by the exponential function: ²¹⁰ Pb(Bq/kg)=513.44 × e ^-0.266 cm(R ₂ =0.85)(Fig. 7)(Gulina and Gulin, 2011). According to the equation used to calculate the rate of sedimentation of bottom sediments(Buesseler and Benitez, 1994), the rate of this exponent(λ / S =-0.266)corresponds to a sedimentation rate of 0.117 cm per year.

Fig. 7 Vertical distribution of excess 210 Pb (●) and its mother radionuclide 226 Ra (○) in coastal sediments of Lake Koyashskoe SR: average sedimentation rate.

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Fig. 8 Vertical distribution of 137 Cs in the coastal sediments of Lake Koyashskoe SR: average sedimentation rate.

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Additional verification of the sedimentation velocity was performed using ¹³⁷ Cs(Fig. 8). The vertical distribution of this long-lived anthropogenic radionuclide had a subsurface maximum at the 2.5 cm horizon. This distribution characteristic of ¹³⁷ Cs in the Black Sea region is explained by its intense fallout after the Chernobyl NPP accident(Polikarpov et al., 2008a). As the maximum precipitation of the postaccident ¹³⁷ Cs over the Black Sea was observed in early May 1986(Izrael et al., 1987), the observed depth of maximum activity of the radionuclide in the investigated bottom sediment column corresponded to a sedimentation rate of 0.109 cm per year. This value is almost identical to measurements obtained in our study using ²¹⁰ Pb.

The highest concentrations of the artificial radionuclide ⁹⁰ Sr during the period 2013–2014 were determined in water of Lakes Kiyatskoe(350.5 and 98.0 Bq/m ³) and Kirleutskoe(121.3 Bq/m ³). This was primarily due to discharge of the Dnieper waters from the North-Crimean Canal(before 2014)into the Perekopskaya group of salt lakes. The results of these studies have shown, that the concentration of dissolved ⁹⁰ Sr in the aquatic ecosystems of the salt lakes of the Crimea depends on the availability of a secondary source of pollution of this post-accident radionuclide. No species-specificity of ⁹⁰ Sr accumulation by algae and higher aquatic plants was observed. The received absorbed doses in the hydrophytes from ionizing radiation of ⁹⁰ Sr and its daughter product ⁹⁰ Y had no noticeable impact on aquatic plants in the period after the Chernobyl NPP accident. Redistribution of ⁹⁰ Sr between the aquatic plants and bottom sediments was insignificant. The main content of ⁹⁰ Sr is present in the water. The solubility of ⁹⁰ Sr is largely dependent on the level of salinity of these lakes. This is due to the high(>70% of total salts)content of NaCl, which, like other chlorides of alkali metals, promotes the solubility of strontium salts. The ⁹⁰ Sr concentration in water of all investigated salt lakes and reference sampling locations for the period 2013–2014 did not exceed the maximum permissible concentration for ⁹⁰ Sr in drinking water.

The highest concentrations of mercury were found in Lake Kiyatskoe(363.2 ng/L) and in seawater near Lake Kizil-Yar(368.8 ng/L), values on average were 3.5 times higher than the MAC. Anthropogenic pressure is the determining factor for the input and changes in the mercury concentrations in the studied ecosystems. The mercury content in the water was not dependent on the level of salinity. The dissolved form of mercury, in general, contributes to the majority of the total amount of mercury in the investigated waters.

The suitability of radiotracer methods to estimate the intensity and the long-term dynamics of the biogeochemical processes in the salt lakes of the Crimea was proven for Lake Koyashskoe. The average sedimentation rates in Lake Koyashskoe, determined using ²¹⁰ Pb and ¹³⁷ Cs data, were 0.117 and 0.109 cm per year, respectively. The obtained results are the basis for the use of radiotracer methods for the geochronological reconstruction of the multi-year(50–100 years)dynamics of the sedimentation and other biogeochemical and ecological factors in the bottom sediments of the Crimean salt lakes.

The authors express their sincere appreciation to Vladimir N. Popovichev, the scientific researcher of the DRChB of the IBSS, for his participation in all scientific expeditions and help in the sampling in the salt lakes. The team of authors also express their deep gratitude to Dr. Nikolai V. Shadrin, senior researcher of IBSS, for his scientific advices.

Co-author Oleg Eremin passed away on November 18, 2014 following a tragic accident during a sampling expedition to the Crimean salt lakes. The authors dedicate this paper to his memory.