Abstract
Strontium-90 (90Sr) is the major long-lived radionuclide derived from the Chernobyl accident, and is still being detected in the heavily contaminated catchments of the Chernobyl Exclusion Zone. This study examines the long-term decrease in the dissolved-phase 90Sr concentration and the concentration–discharge (90Sr-Q) relationship in stream water since the accident. We show that the slow decline in 90Sr follows a double-exponential function, and that there is a clear relationship between 90Sr and Q. This study is the first to reveal that the log(90Sr)-log(Q) slope has been gradually decreasing since the accident. This trend persists after decay correction. Thus, it is not caused by the physical decay of 90Sr and environmental diffusion, but implies that the concentration formation processes in stream water have been changing over a long period. We propose a hydrochemical model to explain the time-dependency of the 90Sr-Q relationship. This paper presents a mathematical implementation of the new concept and describes the model assumptions. Our model accurately represents both the long-term 90Sr trend in stream water and the time-dependency of the 90Sr-Q relationship. Although this paper considers a small catchment in Chernobyl, the conceptual model is shown to be applicable to other accidental releases of radionuclides.
Highlights
It has been 34 years since the Chernobyl nuclear power plant (CNPP) accident in Ukraine (Fig. 1), when large amounts of radionuclides were released into the environment
The annual-scale dissolved 90Sr concentration in the Chernobyl Exclusion Zone (CEZ) rivers is known to exhibit a rapid decrease in recently deposited activity followed by slower transfers during subsequent years, and this can be adequately expressed by an exponential function[8,9,10]
The relationship between dissolved ions and/or other substances and the discharge rate (C-Q relationship) is a general phenomenon that has been widely observed for soluble ions and dissolved organic carbon in various river water systems worldwide[16,17,18,19,20,21,22,23]
Summary
It has been 34 years since the Chernobyl nuclear power plant (CNPP) accident in Ukraine (Fig. 1), when large amounts of radionuclides were released into the environment. The tight coupling between 90Sr and the stream discharge rate (Q) in CEZ rivers can be explained by the empirical and theoretical background and parametric modelling developed in previous hydrochemical studies[24,25,26]. Based on these studies, the variations in stream chemistry with respect to discharge www.nature.com/scientificreports can be explained by the variations in water and solute fluxes with respect to depth for a representative soil profile[19,22]. The annual averaged values were used to avoid excessive fluctuations from non-averaged data in the model validation stage
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