A 6-PHASE MODEL TO SIMULATE THE CONTAMINATION BY NONCONSERVATIVE RADIONUCLIDES OF SEDIMENTS, SOILS AND PLANTS IN A MARSH AREA - APPLICATIONTO THE ODIEL MARSH IN SOUTHWEST SPAIN

A numerical model to simulate the dispersion of non-conservative radio nuclides in an estuarine system that surrounds a marsh area has been d eveloped. The model includes six phases. water, two types of suspended particles (natural particles in the estuary and contaminated particle s 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 pro cesses, ionic exchanges between water and the solid phases and deposit ion of suspended particles on bottom sediments also occur. Radionuclid es are incorporated in soils in the marsh during the time that they ar e covered by water. Finally, they ale transferred from the soil to the plants. All these processes are represented by a set of partial diffe rential equations. A spatial and temporal discretization is carried ou t and a finite differences scheme is adopted to solve then?. The model must run over long time scales (years) if model results are to be com pared with experimental measurements in the estuary and marsh. Thus, r esidual water circulation is used to solve the advective-diffusive ter ms in the equations. Ionic exchanges are described by kinetic transfer coefficients and the transfer of radionuclides fr om soils to plants by concentration ratios, CR The model is applied to the Odiel marsh. T he Odiel river forms an estuarine system (which surrounds a large mars h area) in which a phosphate fertilizer processing complex releases it s waste. The model yields good results in predicting U-238, Po-210, an d Th-232 concentrations in bottom sediments, soils and plants collecte d from the river and marsh. A predictive study, concerning the process of cleaning of the marsh, has also been carried out. (C) 1997 Elsevie r Science Ltd.