PHYSICOCHEMICAL FACTORS FORMING THE COMPOSITION OF NATURAL WATERS: VERIFICATION OF THE ROCK-WATER MODEL

Abstract

The main physicochemical factors controlling the composition of natural aqueous solutions are as follows: (a) the ratio of the reacting masses of rock and water (R/W); (b) the composition of the rock, particularly, the concentrations of volatile components in it; (c) the openness or closeness of the rock-water system with respect to atmospheric O 2 and CO 2 and the CO 2 partial pressure; and (d) the temperature (pressure) in the system. An increase in the ratio of the rock and water reacting masses (R/W) increases the mineralization, may change the type of the aqueous solution, and controls the equilibrium mineral association in the rock-water system. There is generally no proportionality between the major-oxide rock composition and the proportions of these components in the aqueous solution in equilibrium with this rock. However, the chemistry of the rocks predetermines the specifics and composition of the equilibrium aqueous solution. Silty sandstones and clayey rocks produce waters in the succession HCO 3-Cl-Na-Ca → Cl-HCO 3-SO 4-Na-Ca → Cl-SO 4-Na-Ca → Cl-SO 4-Na → Cl-Na (Cl + HCO 3 > SO 4). Carbonate rocks produce waters in the sequence HCO 3-Ca → HCO 3-SO 4-Cl-Ca-Na → SO 4-Cl-Na-Ca → Cl-SO 4-Na-Ca → Cl-Na (Cl + SO 4 > HCO 3). Crystalline rocks produce waters in the sequence SiO 2-Cl-HCO 3-SO 4-Na-Ca → Cl-SO 4-HCO 3-Na-Ca → Cl-SO 4-Na. A decrease in the O 2 equilibrium concentration in the aqueous solution results in negative Eh values, the appearance of reduced sulfur species, and provides a basis for the stability of equilibria with pyrite. An increase in the Cl extraction degree from the rock increases the mineralization and chloride concentration of the aqueous phase in the rock-water system. A decrease in the equilibrium partial pressure of CO 2 [CO 2 concentration in the solution, CO 2(f)] expands the stability field of the HCO 3-Na-Ca type of waters and produces carbonate waters at P CO 2 > 1. An increase in the temperature (pressure) brings about an increase in the chloride concentration, a decrease in the content of carbonates and sulfates, and an increase in the reduction degree of natural aqueous solutions. The model proposed in this paper can be used for predicting changes in the composition of aqueous solutions under the effect of natural and anthropogenic factors.