Factors controlling concentration of dissolved 90Sr and 137Cs in river water in the Chernobyl exclusion zone.

Abstract:

Dynamics of radionuclides, ⁹⁰Sr and ¹³⁷Cs, are still of great interest for mitigating radiological risks in the Chernobyl Exclusion Zone (CEZ) and its downstream area. Previous studies have shown a positive relationship between dissolved ⁹⁰Sr concentration and water discharge, suggesting that ⁹⁰Sr have leached from wetland (Freed, Smith, & Bugai, 2003). However, in practice, the dynamics of radionuclides is closely related not only to the hydrological processes but also to chemical processes and the surrounding environment, and thus cannot be explained by a factor alone.

The objective of this study is to clarify the factors controlling dynamics of radionuclides in the river system in the CEZ, and to evaluate the spatial-temporal variation of dissolved ⁹⁰Sr and ¹³⁷Cs concentrations in a medium catchment area.

The study site was in the Sakhan catchment, locating at approximately 7 km northwest from the Chernobyl Nuclear Power Plant. Water samples were taken at main stream, tributaries and water spring points each season. The discharge rate was determined by cross-sectional survey of flow velocity and water level at the same time as water sampling. Each catchment area was made from digital elevation model (DEM) data and mean ⁹⁰Sr and ¹³⁷Cs inventory for each sub-catchment was determined by distribution maps of radionuclide. In the laboratory, samples were analyzed for dissolved ⁹⁰Sr and ¹³⁷Cs, as well as for dissolved anions and cations.

As a result, two types of sites were identified: 1) the water level fluctuates during the snowmelt and rainy seasons, 2) the water level fluctuates but is still flooded due to damming. It was found that the amount of radionuclide increased with the increase in the discharge rate at the drainage area. This suggests that radionuclides may be leached out in the soil near the river due to changes in the saturation zone of the surrounding soil. On the other hand, it was shown that concentration of radionuclides have a constant or decreasing trend at the sites of springs and waterlogging. At these sites, the concentration change is small and may have reached Kd due to the long period of contact between soil and water. In addition, the concentration of the radionuclide tends to decrease with the increase in the number of cations that compete with the radionuclide, which may be controlled by chemical factors.