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Dependence of 137Cs and 90Sr water concentration on sediment radionuclide pollution level in fish breeding ponds of the Tarashcha district, Kyiv region
Exploring the ways to improve specific activity of 137Cs and 90Sr in fish ponds water is an important and urgent radioecology issue. After use, water with dissolved radionuclides from the floodplain and fish ponds is discharged into the river and carries 137Cs and 90Sr downstream, leading to pollution of new territories, waters and aquatic organisms.
The aim of the research was to clarify the relationship between the current contents of 137Cs and 90Sr in bottom sediments and fish ponds water, located in the zone of guaranteed voluntary resettlement. To achieve the goal, it was necessary to determine the radionuclide content in sediments and pond water and conduct mathematical calculations of linear dependence on the content of 137Cs and 90Sr in specified components of aquatic ecosystems.
Sediment plays an important role in the process of accumulation and migration of 137Cs and 90Sr in fish ponds. The mobility of some radionuclides in sediments significantly differs. If 137Cs is held firmly in the upper sediment, 90Sr is very mobile and can be desorbed from top soil, thereby increasing its concentration in water. The lack of a strong relationship with 90Sr bottom sediments cause the desorption of its upper layer, which determines the ability of a large migration of radionuclides in the water. The main route of migration of 90Sr in low flow waters is its sorption or desorption of surface sediments. Ion exchange sorption mechanism involving calcium is typical for 90Sr. Due to the fact that the different sediment is characterized by unequal physical and chemical properties, the concentration of 90Sr is different by area. Strongest 90Sr is held in peaty and silty soils.
The study was conducted on fish ponds of Tarashcha district, Kyiv region in conditions of the third zone of radioactive contamination area. Results of the research activity of 90Sr and 137Cs in the 0 – 20 cm layer of sediment fish ponds, showed that the bottom layer has uneven contamination. Specific activity of 90Sr and137Cs varied within wide limits.
In particular, the lowest activity of 90Sr and 137Cs were sediment pond number 6, wherein the activity of 137Cswas 20.93 Bq / kg and 90Sr – 8,67 Bq / kg.
The content of 90Sr and 137Cs in the sediments increased down the cascade of ponds and the highest activity was determined in the sediment of pond №1: 137Cs – 114,99 Bq/kg and 90Sr – 19,37 Bq/kg. Then the activity of 137Cs and 90Sr in sediment decreased in pond №1b was 28.53 Bq/kg for 137Cs and 10.24 Bq / kg for 90Sr.
We considered whether there is a link between the levels of radioactive contamination of the upper 20-cm layer of sediment and concentration of radionuclides in the water. Concentrations of 137Cs in water ponds located in the resettlement zone were almost the same, but the level of sediments contamination varied within wide limits.
Even pollution of water masses of certain ponds with 137Cs can be explained only by the source of water supply. In addition, according to the different studies, about 96% of 137Cs in sediments exists in a fixed form, which is insoluble in water, and hence the activity of 137Cs in water is 2-5 times lower than 90Sr. Therefore, we did not record any significant effect on sediment 137Cs concentrations in water.
We had marked a significant difference in the content of 90Sr in both separate ponds of water and sediment. In ponds with higher levels of 90Sr activity in bottom sediments, and the higher the level of its activity in the water. The highest activity of 90Sr was in water pool number 1 – 8.49 mBq/l, where activity in the sediments is 19.56Bq/kg, which were the highest compared to other ponds. The lowest activity of 90Sr was in water pond number 6 – 3.31 mBq/l, the activity in the mud was on average 8.67 Bq/kg. Thus, there is a direct linear relationship between the activity of 90Sr in water and sediments.
These features of activity of radionuclides in the pond water were due to the different mobility of 137Cs and 90Sr in bottom sediments. 90Sr, unlike 137Cs, is always moving and doesn’t settle in soil and biota, it can be quite easily desorbed from the bottom sediments, resulting in higher activity in water than 137Cs.
Our research has shown that сontamination of sediment in Tarashcha district Kyiv region fish ponds cascade is uneven. The level of accumulation of 137Cs and 90Sr in the sediments is directly proportional to their type. The activity of 137Cs in the water pond zone of guaranteed voluntary resettlement averaged 1,64 mBq/l, and 90Sr – from 3,31 to 8,49 mBq/l. Sediment significant effect on the activity of 137Cs in the water have been recorded, which can be explained by the fact that the sediments of about 96% 137Cs contained in a fixed form that does not dissolve in water. Thus, the activity of 137Cs in water is much lower than 90Sr. The specific activity of 90Sr in water ponds in this area 2-5 times higher than 137Cs. There were direct linear relationship between the specific activity of 90Sr in water and bottom sediments.
Key words: cesium-137, strontium-90, zone of guaranteed voluntary resettlement, fish ponds, radiation pollution, sediment.
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