Assessment of Possible Mechanisms of Decreasing 137Cs Concentrations in River Waters Affected by the Chernobyl NPP Accident

The analysis of possible mechanisms of Cs-137 concentration changes in surface waters was performed in the process of preparation of reliable long-range forecasts of radioactive river contamination after the Chernobyl accident. The following mechanisms were considered: ( 1) radioactive decay; ( 2) advective transport with river waters; ( 3) irreversible sorption; ( 4) vertical migration deep into bottom sediments due to diffusion; ( 5) burial in clean bottom sediments. The data published on Cs-137 monitoring at Dobrush, on the Iput' River in Belarus were used in the analysis. It is shown that the best agreement with the experimental results is achieved when the second, third, and fifth mechanisms are used in calculations. However, a dominating mechanism still cannot be chosen at the present stage of our study. Most probably, all of these mechanisms act simultaneously.