Equilibrium-kinetic model of water-rock interaction

Abstract

A computer model was developed for chemical interaction in water-rock systems. The model is based on the concept of partial equilibrium [1] and combines the calculation of chemical equilibria in multicomponent systems with accounting for the kinetics of the congruent dissolution of minerals as a function of pH (zeroth order kinetic reactions). The development of the process in time is simulated as a series of sequential partial equilibria, and the bulk chemical composition of the system is calculated at each time step from the chemical composition of aqueous solution at the beginning of the step and masses of minerals dissolved during time Δt. The dissolution rates of individual minerals are calculated at each time step for the given temperature, current pH value, and the degree of solution saturation with respect to minerals. Variations in the surface area of minerals due to precipitation and dissolution are accounted for. Model application is exemplified by the calculation of chemical equilibria in the water-granite system. The model may be useful for understanding the character of low-temperature interactions in water-rock systems under stagnant conditions, in particular, the multistage development of groundwater chemistry, interaction of liquid radioactive waste injected into underground repositories, etc.