Considering the structural characteristics of soil and the chemical reactivity of contaminants

the kinetic model of contaminant transportation in the soil-water system is established

in which the immobile water and equilibrium/non-equilibrium adsorption-desorption are considered. The model is solved using the quasi-ADI method; and the corresponding program is used to simulate a paddy field

which is in the pesticide application period. The law of the contaminant transportation and transformation in soil is studied; and the effect of immobile water and non-equilibrium on concentration distribution is analyzed. The result from numerical simulation shows that the concentration hysteresis exists both in immobile water phase and in non-equilibrium phase; so it retards the pollution in prior period

and increases the pollution in latter period. The existences of the two phases both have important effect on the contaminant transportation. So they must be sufficiently considered in numerical simulation. The results of numerical simulation and analysis can provide quantitative basis for environmental evaluation

and have important theoretical and practical significance for controlling of contamination.