A six-phase model to simulate the contamination by non-conservative radionuclides of sediments, soils and plants in a marsh area. Application to the Odiel marsh in southwest Spain

Abstract

A numerical model to simulate the dispersion of non-conservative radionuclides in an estuarine system that surrounds a marsh area has been developed. The model includes six phases: water, two types of suspended particles (natural particles in the estuary and contaminated particles released from the source of contamination), bottom sediments, soils and plants (Spartina densiflora). Radionuclides in water and suspended matter are transported along the river by advection and diffusion processes, ionic exchanges between water and the solid phases and deposition of suspended particles on bottom sediments also occur. Radionuclides are incorporated in soils in the marsh during the time that they are covered by water. Finally, they are transferred from the soil to the plants. All these processes are represented by a set of partial differential equations. A spatial and temporal discretization is carried out and a finite differences scheme is adopted to solve them. The model must run over long time scales (years) if model results are to be compared with experimental measurements in the estuary and marsh. Thus, residual water circulation is used to solve the advective-diffusive terms in the equations. Ionic exchanges are described by kinetic transfer coefficients and the transfer of radionuclides from soils to plants by concentration ratios, CR. The model is applied to the Odiel marsh. The Odiel river forms an estuarine system (which surrounds a large marsh area) in which a phosphate fertilizer-processing complex releases its waste. The model yields good results in predicting 238U, 210Po and 232Th concentrations in bottom sediments, soils and plants collected from the river and marsh. A predictive study, concerning the process of cleaning of the marsh, has also been carried out.