Uranium and thorium concentrations in an estuary affected by phosphate fertilizer processing: Experimental results and a modelling study

The Odiel river, in southwest Spain, forms an estuarine system which is affected by waste disposal from a fertilizer complex. Uranium and thorium concentrations in waters and suspended matter, activity ratios and distribution coefficients, kd, have been measured along the river during two different tidal states. The results have shown that a radioactive impact is being delivered to the river, as well as a significant variability depending on the sampling point and the tidal state. Thus, a quantitative study of the distribution of radio-nuclides can be carried out best by means of a mathematical model. The model includes the partition of radio tracers between four phases (water, suspended matter and two sediment fractions) and has been designed for non-equilibrium conditions. Thus, radiotracer transfers are described in terms of kinetic transfer coefficients instead of Kds. The model simultaneously solves the hydrodynamic equations, the suspended matter equation (including depostion and resuspension processes) and the equations which describe the time evolution of radionuclide concentrations in each one of the four phases. The model has yielded good results in predicting U and Th concentrations in water and suspended matter, distribution coefficients and Th/U mass ratios.