Natural and artificial radionuclides as tracers in fluvial systems

The environmental pathways of medical 131I have been the subject of several studies presenting its ability as a tracer in lakes, rivers and estuarine systems. In this study 131I, originating from the local wastewater treatment plant (WWTP), is used as a readily available tracer with the aim of obtaining a better understanding on the distribution of radioisotopes in the tidal influenced Weser River. In order to detect low concentration of 131I on Weser River water, a well-defined chemical extraction method needs to be applied. Focused on this aspect, upon investigating 12 different methods, a procedure with a high extraction yield (86 A /- 15)% which targets inorganic and organically bound iodine at the same time was developed, tested and used for bulk water samples. Large amount of water and sediment samples were collected and measured by gamma spectroscopy, for the period autumn 2014 a autumn 2015, during 17 different sampling campaigns. The 131I activity concentration in the river water is strongly dependent on the tidal cycle. Its profile is similar to the one in bank sediments with maximum values directly in the WWTP outflow and lowest in the opposite river bank and upstream direction. A model which computes the 131I behavior in Weser River for routine releases was developed. For assessing the model, water and sediment samples taken from the river are required as well as information on hydrological and morphological data of the river. The model provides realistic radionuclide distributions in water and bank sediments. The simulated currents and 131I activity concentration in the system are, in general, in agreement with observations. Despite the simplifications considered, the assessment of the model indicates its capability of predicting the dispersion of medically derived iodine, originating from the WWTP, in the water body and banks of Weser River. With minor adjustments, one could possibly use the model for the prediction of dispersion for different radionuclides, in different area and in emergency situations.