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.