GEOCHEMICAL SYSTEMS PRODUCING HIGH-CARBONATE ALKALINE GROUNDWATERS IN UPPER AQUIFERS

Abstract

A synthesis of empirical generalizations of natural observations and the simulation results of geochemical processes indicate that carbonate-rich alkaline waters develop in two successive stages. The preparatory stage involves interaction between atmospheric precipitates with calcium carbonates in a weathering crust and the Ca-water(2+) --> Na-rock(+) A ion exchange. This stage ends with the origin of HCO3-Na waters with limited (<500 mg/l) concentrations of carbonate ions and pH < 8.5. The main stage involves further transformation of the HCO3-Na waters into waters with high concentrations of carbonate ions (HCO3- + CO32- --> concentrations n x 1000) and alkalinity (pH --> 12 and more). Transformations of this type can occur due to (a) the thermodynamic opening, with respect to CO2, of hydrogeochemical systems with high HCO3- concentrations (caused by an increase in P-CO2); (b) vaporization-controlled concentration of HCO3-Na groundwaters with high m(HCO-3) + 2m(CO32-) > 2m(Ca2+) ratios; and (c) dissolution of minerals of the soda group. The development of alkaline waters in semiarid and and zones may be facilitated by atmospheric precipitates, which are able to form Ca concentrations in groundwaters; so high as to initiate ion exchange processes. The latter end with the formation of HCO3-Na waters. Ion exchange is a boundary process that marks the transition from the "calcic" tendency in ground-water metamorphism to its "sodic" tendency, which results in a progressive increase in the concentrations of carbonates and alkalis in the groundwaters. Depending on the actual situations occurring in nature, this increase may be caused by any of the aforementioned processes, leading finally to the same result (the highest concentrations of alkalis and carbonates in the groundwaters).