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Chemical characteristics and equilibration in natural water-felsic rock-C02 system

1972; Elsevier BV; Volume: 36; Issue: 2 Linguagem: Inglês

10.1016/0016-7037(72)90007-5

1872-9533

Tomáš Pačes,

CO2 Sequestration and Geologic Interactions

Weight proportions of the major dissolved species in water from rocks of the Bohemian massif containing alkali feldspars follow the sequence Na+ ≥ HCO3− > Cl− > SO42− > K+ ≷ H4SiO4. For other localities a common order is Na+ ≥ HCO3− > Cl−> H4SiO4 ≅ SO42− > K+. Calcium and magnesium ions change their place in the sequence as the total dissolved solids increase. Cold water samples (<18°C) were in contact with a soil atmosphere in which Pco2 ranges from 10−2 to 10−1 atm. Hot water (18°C to 72°C) was in contact with subsurface atmospheres in which Pco2, varies widely from 10−5 to 101 atm. The high Pco2 was observed in water from the Bohemian massif where CO2 of a deep, possibly magmatic, origin occurs. Ground water is usually not in chemical equilibrium with respect to its rock environment but its chemical composition changes towards an equilibrium state. The equilibration of rain water or snow melt with rock probably includes (1) fast adsorption of hydroxyl ions on surfaces of aluminum silicates, (2) incongruent hydrolysis of primary aluminum silicates, (3) diffusion of cations from the inner structures of aluminum silicates into solution in exchange for hydrogen ions and (4) reversible exchange of silica and probably alumina between the solution and amorphous surface layers of aluminum silicates. The activity of H+ in ground water is probably a function of the rate of hydrogen diffusion into aluminum silicates and the flux of CO2. The activity of sodium ion was found to vary with temperature and Pco2, but residence times of ground waters and the specific surface area of the aquifer can influence the activity significantly. The activity of calcium ions may be controlled by incorporation of calcium ions in the structure of a secondary aluminum silicate such as Ca-montmorillonite. Extremely low activities of calcium ions in some waters from felsic rocks are caused by precipitation of calcium carbonate. Chemical compositions of feldspars having An0–40 are probably not decisive controlling factors of the calcium concentration in the group of ground-water samples studied.