Increasing temperatures are attributed to climate change at various scales. The Caspian Sea is a valuable place to study climate change cause-and-effect on water levels, because they are related to atmospheric conditions in the North Atlantic Ocean thousands of kilometers to the northwest. The Caspian Sea levels changed in synchronicity with the estimated discharge of the Volga River, which in turn
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Table 18.1 The water levels
Environmental condition of the Caspian Sea
Transgression of the Caspian Sea Rise sea level from -36 m till -34 m Within narrow limits risen sea level -32 m Narrow fallen sea level -33 m Formation delta of the Volga River Sea level risen from -34 till -33 m Risen sea level -29 m Suspended process risen sea level Sea-level fall of 3 m Risen sea level of 3 m -28 m
261-270 311-320 361-370 571-580 871-880
depends on rainfall levels in its catchment. Levels of the Caspian Sea have risen and fallen many times over past centuries (Table 18.1) . The last short-term sea-level cycle started with a sea-level fall of 3 m from 1929 to 1977, followed by a rise of 3 m from 1977 until 1995. Since then shorter oscillations have occurred. In 2004, the water level stabilized at about 28 m below sea level. Our data indicate that temperature is not the only critical environmental factor. Other factors, for example, radionuclide accumulations, are more dangerous in the environment than global temperature change because they may be irreversible.
The most important radionuclide contaminants are those taken up by plants; have high rates of transfer to animal products, such as milk and meat; and have relatively long radiological half-lives. The ecological pathways leading to plant contamination and environmental behaviour of radioactive isotopes are complex. The transport and fate are affected not only by physical and chemical properties of the radionuclide, but also by factors such as soil type and its tillage, species and cropping system of plants, seasonal climatic conditions, and where relevant, biological half-life within plants and animals. Direct radionuclide deposition on plants is the primary source of contamination in several temperature regimes. The 137Cs and 90Sr are relatively immobile in soil, and uptake by roots is of less importance compared with plant deposition. However, soil type (particularly with regard to clay mineral composition and organic matter content) and climate affect their transport to river and groundwater systems.
Contamination of aquatic ecosystems by radionuclide components follow: underwater layer of bottom sedimentation > aquatic hydrobionts > water. Whereas plants up take 137Cs and 90Sr by the same mechanisms as K and Ca respectively, the extent of their uptake depends on the availability of these elements. In contrast to 137Cs, 50-99% of the radionuclide 90Sr migrates in streams as a soluble compound. After the 1986 Chernobyl nuclear power plant (ChNPP) accident, radioactivity contaminated the Dnipro and Pripyat catchments and cooling pond of the ChNPP. The water was contaminated along several pathways by 141Ce, 144Ce, 103Ru, 140Ba, 131I, 95Zr, 95Nb, 140La, 134Cs, 137Cs, 90Sr and other uranium fission and transuranium products. Since 1987, 137Cs, 90Sr and other transuranium radionuclide have contributed to the contamination. Before the Chernobyl accident, the respective concentrations of the 90Sr and 137Cs in the river Pripyat averaged 0,011 and 0,007 Bq/l. Now the respective radionuclide concentrations of 90Sr and 137Cs in water range from 1.59 to 2.70 Bq/l and from 3.35 to 5.95 Bq/l.
The Chernobyl nuclear power plant is located close to the cooling pond and river Pripyat which discharge to the Dnipro river and reservoir system, one of the largest surface water systems in Europe. After an initial period, the radioactivity in rivers and reservoirs was generally below guideline limits for safe drinking water. The longer-lived radionuclides, such as 90Sr, 137Cs and transuranium isotopes, were adsorbed to surface soils preventing their transport to the groundwater system. However, significant transfers of radionuclides to the groundwater have occurred from waste disposal sites in the 30-km zone around Chernobyl. Although there is the potential for transfer of radionuclides from these disposal sites off-site, it is not significant in comparison to current levels of surface-deposited radioactivity washout. In case of migration of 137Cs and 90Sr, runoff played an extremely important role in transferring solid suspensions. When flooding occurred, the high level radioactivity associated with bottom sedimentation became mixed resulting in a multiple-fold increase of river water concentrations. In the 30-km radius surrounding Chernobyl, radioactivity was added by contaminated dust with very small sizes from nanometre to micrometer particles. After a rainfall, contaminated solids increased the radioactivity level of surface soil and water reservoirs which eventually settled as bottom sediment in the Pripyat and Dnipro rivers and cooling pond of the ChNPP. During the flood, the reverse process - transfer of high-sediment-suspension, this leads to a multiple increase in radioactivity of river waters. In this study, estimates and distribution of small-size radionuclide contaminates of the surface plant, soil and water was studied using autoradiography .
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