URANIUM AND THORIUM CONCENTRATIONS IN AN ESTUARY AFFECTED BY PHOSPHATE FERTILIZER PROCESSING - EXPERIMENTAL RESULTS AND A MODELING STUDY

The Odiel river, in southwest Spain, forms an estuarine system which i s affected by waste disposal from a fertilizer complex. Uranium and th orium concentrations in waters and suspended matter, activity ratios a nd distribution coefficients, k(d), have been measured along the river during two different tidal states. The results have shown that a radi oactive impact is being delivered to the river, as well as a significa nt variability depending on the sampling point and the tidal state. Th us, 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 radiotracers between four phases (water, suspended ma tter and two sediment fractions) and has been designed for non-equilib rium conditions. Thus, radiotracer transfers are designed in terms of kinetic transfer coefficients instead of k(d)s. The model simultaneous ly solves the hydrodynamic equations, the suspended matter equation (i ncluding deposition and resuspension processes) and the equations whic h describe the time evolution of radionuclide concentrations in each o ne of the four phases. The model has yielded good results in predictin g U and Th concentrations in water and suspended matter, distribution coefficients and Th/U mass ratios. (C) 1997 Published by Elsevier Scie nce Limited.