Simulating dissolved ⁹⁰Sr concentrations within a small catchment in the Chernobyl Exclusion Zone using a parametric hydrochemical model

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.