The Enhancement of 226Ra in a Tidal Estuary due to the Operation of Fertilizer Factories and Redissolution from Sediments: Experimental Results and a Modelling Study

The presence of 226Ra in a tidal estuary formerly affected by direct discharges from a phosphate fertilizer complex has been investigated. In general, activity levels are lower than those detected when direct discharges were carried out. However, there is still a clear contamination that can be attributed, presumably, to the disposal of Ra-containing phosphogypsum to the river and to the redissolution of radionuclides from the contaminated sediments. A numerical model of the estuary has been developed to investigate these hypothesis. The hydrodynamics are first calibrated and standard tidal analysis is carried out. The dispersion model computes instantaneous currents from the so obtained tidal constants to speed up simulations. The exchanges of 226Ra between water and the bottom sediment have been described in terms of kinetic transfer coefficients. Model results are, in general, in agreement with observations. The simulation results support the previous idea of sediments acting as a source of 226Ra to the water column.