Chemical composition of natural waters and brines as a result of hydrogeochemical processes in water-rock-gas systems

Thermodynamic computer modeling was carried out to evaluate the formation of the chemical composition of main geochemical types of groundwaters. An explanation was proposed for the geochemical evolution of underground saline waters and brines along the calcic and sodic trends, the inversion of groundwater in the deep horizons of sedimentation structures, and the geochemical diversity of CO₂-rich waters in crystalline rocks. The occurrence of hydrogeochemical processes is controlled by the physicochemical conditions of the state of the water-rock-gas system. The following parameters (boundary conditions) are critical in natural hydrogeologic environments: the mass ratio of interacting rock and water (R/W), the openness (closeness) of hydrogeochemical systems with respect to CO₂ and O₂, the chemical and mineral composition of rocks, and temperature-pressure conditions. The estimation of boundary conditions showed the following.

1. The petrochemical type of rock affects the composition of the aqueous phase through the dissolution rates of minerals, especially volatile-bearing ones. A decrease in water exchange and an increase in R/W (10^?6??10²) are accompanied by an increase in the salinity of the aqueous phase and an increase in the fraction of Cl, Na, and Ca (in a closed system) or HCO₃, Cl, and Na (in a system open to CO₂).
2. The composition of the aqueous phase of water-rock systems is most strongly affected by the abundance in the rock of extractable Cl and reactive organic matter, which controls the geochemical type of the aqueous phase and its position in the Hardie-Eugster diagram.
3. The composition of the aqueous phase is shifted into the calcic field of the Hardie-Eugster diagram at the closure of the water-rock system and into the carbonate field at the opening of the water-rock system to CO₂. Waters showing pH ?? 8.5 are formed in feldspathic rocks with low contents of extractable volatiles. Alkaline waters with pH > 9 are formed in water-rock systems (a) under the influence of organic matter and (b) by the evaporative concentration waters under surface conditions.
4. The higher the degree of seawater concentration and the lower the R/W value, the more significant the effect of seawater composition on the aqueous phase chemistry of the water-rock system. With increasing degree of seawater concentration, the composition of the aqueous phase changes in the sequence Cl-SO₄-Na-Mg- ?? Cl-SO₄-Mg-Na?? Cl-Mg (at low R/W) and Cl-Na ?? Cl-Na-Mg (at high R/W). The influence of the petrochemical type of rock and CO₂ partial pressure, on the geochemical type of the aqueous phase in the seawater-rock system is more significant at high R/W.
5. A temperature increase shifts the acid-base state of the aqueous phase into the alkaline region and its redox state into the reducing region.