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Solute transport and geochemistry modeling in the vadose zone : application to soil salinisation, alkalinisation and sodification

Condom N., Lafolie F., Hammecker C., Maeght J.L., Vallès V., Marlet S.. 2002. In : World congress of soil science. Bangkok : WCSS, 1 Cd-Rom. World Congress of Soil Science. 17, 2002-08-14/2002-08-20, Bangkok (Thaïlande).

A model is presented for one-dimensional simulation of water flow, solute transport and major ions kinetic reactions suitable for alkaline and sodic soils. The water flow and solute transport model is derived from the model PASTIS proposed by Lafolie (1991). Richards¿ equation is used to model one-dimensional saturated-unsaturated water flow in layered soil profiles. Solute transport has been modeled with the assumption of physical equilibrium. The convection-dispersion equation (CDE) is used to model conservative solute transport. Each aqueous component is defined as its analytical concentration. The geochemical mechanisms of interaction between soil and solution are modeled by the model IRRICHEM (Marlet et al., 1998) which accounts for speciation of the major chemical component, precipitation or dissolution of few minerals and cation exchange. The equilibrium have been previously calibrated and first-order kinetic reactions are considered for mineral precipitation or dissolution and cation exchange. The models are coupled by the introduction of sink terms in the convection-dispersion equations which become non-linear. A numerical procedure is implemented to ensure the convergence of the solution. The time step calculation takes into account the most restrictive phenomenon between transport and geochemistry. The model has been calibrated under a 2 months column experiment with an undisturbed irrigated soil monolith (60 cm high and 23 cm in diameter). Non-reactive solute breakthrough curves (BTCs) show a moderate dispersivity and allows to describe transport using the local physical equilibrium assumption and the convective-dispersive equation. However, reactive solute shows a very high chemical non-equilibrium and kinetically control chemical reactions must be taken into account. The sensitivity of the model has been assessed according to the kinetic of mineral dissolution and cation exchange. The results show that cation exchange plays the main part in short term experiments and the kinetic coefficient of cation exchange has been calibrated as 8x10-5 molc L-1 h-1. The model shows a good agreement with experimental data.

Mots-clés : sol alcalin; sol sodique; modèle de simulation; transport dans le sol; mouvement de l'eau dans le sol; Échange d'ion; cycle biogéochimique; salinisation du sol; alcalinisation; mali; sénégal; sodisation du sol

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