Determination of longitudinal dispersion coefficient and velocity of the Ebro river waters (Northeast Spain) using tritium as a radiotracer

Tritium routinely released as low-activity liquid radioactive waste by the Asco nuclear power plant (Tarragona, Spain) was used as a radiotracer to determine the longitudinal dispersion coefficient and velocity of the Ebro river waters (Northeast Spain). Several field experiments, in coordination with the nuclear power plant, were carried out during 1991. After a tritium release, water was sampled downstream at periodic intervals during several hours. Tritium was measured with a low-background liquid scintillation counter and the time evolution of its concentration for different locations was obtained. In order to determine the longitudinal dispersion coefficient and the velocity of the Ebro river waters, experimental data were fitted using an analytical, a box-type and a numerical model to solve the one- dimensional advection-diffusion equation. The observed mixing length occurred about 50 channel widths downstream from the point source, shorter than values predicted from some published expressions (100-300 channel widths). For the river water flows investigated, ranging from 178 to 915 m3 s-1, longitudinal dispersion coefficients ranged from 41 ± 30 to 392 ± 45 m2 s-1 and mean velocities ranged from 0.563 ± 0.011 to 1.278 ± 0.019 m s-1.