The rock - fluid interaction on the solid - liquid interface is a complex process comprising diffusion, adsorption, reaction and desorption. According to experimental results of hydrothermal reactions, the leaching layer where specific elements are depleted and secondary minerals are precipitated was formed on the rock surface. The transition of the surface condition plays an important role in the quantitative evaluation of the rock - fluid interaction in natural systems.<BR>The zonal structure of hydrothermal alteration, and leaching and enrichment process in the subsurface region were simulated using the T(thermo)-H(hydro)-C(chemical) coupled mathematical model which is included in the chemical reaction and dynamic function of the solid-liquid interface and overall systems. As an example of zonal structure, the cation exchange of montmorillonite from Ca to Na, and illitization via the illite/montmorillonite mixed layer in a drillhole were analyzed by this model. Particularly, cation exchange reaction is sensitive to the mass transport factor of the solid - liquid interface due to the exchangeable cation in montmorillonite being directly reflected on the in-situ solution chemistry. The transition from Ca- to Na- montmorillonite in the field could be simulated well using the mathematical model under relatively high groundwater velocity and the present geothermal gradient condition. On the other hand, the numerical analysis for illitization did not coincide with the field data under the present geothermal gradient irrespective of dynamic function. This also denotes the capability of cooling of underground temperature.