Study on the hydrodynamics of the Ebro river lower course using tritium as a radiotracer

Tritium routinely released as low-activity liquid radioactive waste by the Ascó nuclear power plant (Tarragona, Spain) was used as a radiotracer to study the hydrodynamics of the Ebro river (north-eastern Spain) lower course. In coordination with the plant, a field experiment was carried out in which, after a tritium release, water was sampled downstream from nine locations along the river at periodic intervals. Tritium was measured with a low-background liquid scintillation system and the concentration time evolution for each location was obtained. In order to study the river hydrodynamics, the one-dimensional advection–diffusion equation was used to determine the longitudinal dispersion coefficient and mean velocity of Ebro river waters. Experimental data were fitted using an analytical, a box-type and a numerical model to solve the equation. For the studied river discharge, namely 220 m 3 s −1, the longitudinal dispersion coefficient ranged from 67±20 to 294±36 m 2 s −1 and the mean water velocity ranged from 0.575±0.002 to 0.710±0.014 m s −1, depending on the sampling point. From the analysis of the longitudinal dispersion coefficient it was observed that this river section shows three zones with different hydrodynamics which largely depends on the river morphology.