The content of radionuclides in the phytoplankton in different sites of the Zaporisky reservoir

Abstract

The radionuclides, which have occurred in the aquatic ecosystem are quickly redistributed there. Most of them come through the water phase to the benthal deposit, while the other part of them accumulates in suspensions and hydrobionts. Even low concentrations of radionuclides in water contributes to its radioactive contamination that poses risks to the environment. The radionuclide contamination of freshwater ecosystems is particularly pressing issue, since water with low mineralization enhances the radionuclides activity in the sediments compared to the water by a factor of 10 or even more, and in the animals by hundreds and thousands of times.

Phytoplankton is the fundamental autotrophic link of aquatic ecosystems. Due to a large total sorption surface, it is able to absorb, retain and transmit radionuclides along the food chains, leading to a deterioration of the environmental position of the reservoir.

Phytoplankton was sampled in the summer of 2019 at 5 sites along the course of the Zaporizke reservoir, which differ in hydrological and hydrochemical conditions: Samara Bay, Festivalnyi pier, Monastyrsky island, creek of the Mokra Sura river and the lower section of the reservoir (near Viyskove village). The specific radioactivity of phytoplankton samples was determined using generally accepted radiochemical methods and gamma spectrometry. The specific radioactivity of radionuclides is given in bequerels per kilogram (Bq/kg) of fresh, natural weight. Statistical processing of research results was carried out using generally accepted methods of variation statistics for a small sample using the Excel 2010 software package.

Purpose of the work. To determine the content of natural (²³²Th, ²²⁶Ra and ⁴⁰K) and artificial (¹³⁷Cs and ⁹⁰Sr) radionuclides in phytoplankton at different sites of the Zaporizke reservoir.

The content of artificial radionuclides in the phytoplankton of the Zaporizke reservoir averaged as follows: ¹³⁷Cs – 7.61±0.501 Bq/kg; ⁹⁰Sr – 1.13±0.242 Bq/kg. The maximum values of the content of artificial radionuclides in phytoplankton were recorded in the area of the Festivalnyi pier and the minimum values were recorded near Viyskove village (¹³⁷Cs) and Samara bay (⁹⁰Sr). In total, phytoplankton has a relatively low content of artificial radionuclides, which is explained by its low content in water.

Hydrobionts receive the main portion of radiation exposure from natural sources of radiation. About 300 natural radionuclides have been identified in the environment; among them ²²⁶Ra, ²³²Th and ⁴⁰K make a significant contribution to the total natural radioactivity. The specific activity of natural radionuclides in the water area of the Zaporizke reservoir on average was following: ²²⁶Ra – 95.45±7,010 Bq/kg; ²³²Th – 85.66±4,013 Bq/kg; ⁴⁰K – 185.26±23.74 Bq/kg. The lowest content of natural radionuclides was observed in the phytoplankton of the Samara bay. The maximum recorded specific activity of ²²⁶Ra and ⁴⁰K was observed in the creek of the Mokra Sura river, which may indicate an increased content of radionuclides in the water of this river, because the river is polluted throughout its length by industrial, agricultural and municipal wastewater.

It has been found that the specific activity of natural radionuclides ²²⁶Ra, ²³²Th and ⁴⁰K was 80-320 times higher than that of the artificial ones (⁹⁰Sr and ¹³⁷Cs).

The distribution of radionuclides in the elements of the freshwater ecosystem is characterized by accumulation coefficients. According to the accumulation coefficients with respect to water, the sequence of accumulation of radionuclides in the phytoplankton of the Zaporizke reservoir has been arranged in descending order: ¹³⁷Cs>²³²Th>²²⁶Ra> ⁴⁰K> ⁹⁰Sr. The accumulation coefficients of ⁴⁰K and ⁹⁰Sr are 11-22 times less than those of ¹³⁷Cs and ²³²Th, which is quite appropriately, because freshwater algae are more able to accumulate ¹³⁷Cs.

For most of the radionuclides studied, the maximum values of accumulation coefficients have been recorded in phytoplankton in the area of the Festivalnyi pier (¹³⁷Cs, ⁹⁰Sr, ²³²Th) and the creek of the Mokra Sura river (²²⁶Ra, ⁴⁰K). The latter is associated with both the hydrochemical and hydrological conditions of these sites, and with increased anthropogenic impact, especially in the area of the river creek.

The data obtained indicate that there is a potential threat of accumulation of radionuclides in phytoplankton and their transfer further along food chains. With reference to the above mentioned, it is important to monitor the content of radionuclides in phytoplankton in order to minimize their transmission through the food chain and preserve the biodiversity of aquatic ecosystems.