Analysis of accumulation conditions of minor elements in mineral waters: An example of hydrocarbonate sodic waters of the Nagutskoe mineralized groundwater field

Computer simulations of carbon dioxide leaching of Aptian–Albian sandstone at the Nagutskoe groundwater field, Caucasian Mineral Waters, are compared with laboratory experimental data obtained using a high-pressure autoclave under parameters close to conditions under which mineral waters are formed at the Nagutskoe and Essentuki fields (temperatures 20–25 and 65–70°C, carbon dioxide pressure up to 4.04 MPa). The solvents were distilled water and naturally occurring groundwaters from the Caucasian Mineral Waters (CMW) area, individual experimental runs lasted for 2 h, the starting material (rock) was crushed to 0.25 mm, and the gas phase was carbon dioxide. In most of the experiments, the solid: liquid phase (R/W) ratio was 1: 5 and was varied from 1: 10 to 1: 100 in other experiments. Our simulation results indicate that multiple-cycle (10 cycles) leaching leads to an increase in mineralization from 1.3 g/L to 4 g/L and transformation of the geochemical type of the waters from the hydrocarbonate calcic–sodic one (leaching cycle 1) to chloride–hydrocarbonate sodic (cycles 5 and later). The mineralization increased mostly because the and Na⁺ ions are transferred into solution at an insignificant increase in the Cl concentration and a practically unchanging concentrations of the sulfate, calcium, and magnesium ions. With regard for the averaged mineralogical composition of the sandstone (quartz, feldspars, mica, glauconite, magnetite, ilmenite, garnet, rutile, zircon, and tourmaline) used in our thermodynamic simulations, we arrived at the conclusion that the chemical compositions of the waters, including their minor-element compositions, are controlled by (i) the composition of the cement (clay, calcareous, siliceous, limonitic, chloritic, zeolitic, phosphate, sulfate, or mixed) of the rocks, (ii) weight percentages of minerals containing certain elements, and (iii) temperature, at a given composition of the gas phase of the simulated system (silty sandstone–rainwater–CO₂ gas phase).