E. V. Cherkasova

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

AbstractThermodynamic 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.

Chemical composition of the primary aqueous phase of the earth and origin of life

Prokaryotes and cytoplasm of eukaryotes are dominated by K+, whereas the extracellular fluid of most species of multicellular organisms is dominated by Na+. It was substantiated that the K+/Na+ ratio in the salt constituent of the cells of modern organisms qualitatively reflects the proportions between these elements in the aqueous phase, in which the first forms of life and the protocell originated. The same conclusion is done by Armen Y. Mulkidjanian et al. (PNAS 13, 2012, E821-830). The chemical composition of primary aqueous phase of the Earth was reconstructed using thermodynamic numerical simulation of the equilibrium composition of the “carbonaceous chondrite material-water”, “primitive mantle material-water”, “ultramafic rock-water”, “mafic rocks-water” systems that are open with respect to CO2 and CH4.It was shown that at 25°C, total pressure of 1 bar, and partial pressures of CO2 and CH4 10−5–10−8 and 10−2–10−8 bar, respectively, the aqueous phase of the systems with carbonaceous chondrite and primitive mantle has K+/Na+ > 1, which corresponds to the proportions of these elements in the intracellular solution. The aqueous phase is characterized by pH = 8–9, Eh = −450 ± 50 mV, the presence of ammonium nitrogen, and concentrations of K, Na, and Mg close to those in the inferred intracellular fluid. The interaction of water with ultramafic and mafic rocks provides K+/Na+ < 1 in aqueous solution, which corresponds to the chemical composition of the modern natural waters of the Earth’s crust.Simulation results show that the protocell could arise in the primary aqueous phase of the Earth during differentiation of chondritic material into the Earth’s core and mantle, after the formation of the nitrogen atmosphere containing CH4, CO2, NH3, H2, H2S, CO and other gases, but prior to the formation of the modern rocks of the Earth’s crust (first billion years of the planet’s lifetime).

Model for the formation of arsenic contamination in groundwater. 1. Datong Basin, China

Mineral equilibria were analyzed in the system As-bearing rock-meteoric water. It was shown that carbonate rocks are the most probable source of As and Sr in the waters of the Datong Basin (Peoples Republic of China). The reason for groundwater enrichment in As is the shift of the equilibrium FeCO3 (siderite) + H2O = FeOOH(goethite) + CO2(g) + H2(g) to the left (toward siderite formation) owing to organic matter oxidation by atmospheric oxygen and an increase in the equilibrium partial pressure of CO2, while the Eh of the system remains below −0.30 ± 0.06 V.

Estimation of the composition of the Earth’s primary aqueous phase. 2. Synthesis from the mantle and igneous rock material. Comparison with synthesis from the carbonaceous chondrite material

Simulation results of the equilibrium state of systems water-carbonaceous chondrite material, water-primary mantle material, water-ultramafic rock material, and water-mafic rock material open with respect to carbon dioxide and methane at 25°C, 1 bar indicate that highly alkaline reduced aqueous solutions with K/Na > 1 can be formed only if water is in equilibrium with compositions close to those of continental crust and primitive mantle. Yu.V. Natochin’s hypothesis that the living cell can be formed only in an aqueous environment with K/Na > 1 leads to the conclusion that terrestrial life could arise and further evolve on the Earth during the differentiation of primary chondritic material into the Earth’s core and mantle (during the first few million years of the planet’s lifetime) in an alkaline (pH 9–10) reduced (Eh = −400–500 mV) aqueous solution at a temperature of 50–60°C, in equilibrium with an N2-bearing atmosphere, which also contained CH4 (partial pressure from 10−2 to 10−8 bar), CO2 (partial pressure from 10−5 to 10−8 bar), NH3, H2, H2S, CO, and other gases.

Reconstruction of composition of the earth’s primary aqueous phase. Part 1: Formation from carbonaceous chondrite matter

The equilibrium composition was modeled for the water-carbonaceous chondrite matter system open to CO2 and CH4. It was shown that at 25°C, total pressure of 1 bar, and definite proportions between partial pressures of CO2 (10−5–10−8 bars) and CH4 (10−4–10−8 bar), the aqueous phase is characterized by a K/Na ratio of 4–12, which corresponds to the ratios of these elements in the cellular liquid of organisms. Aqueous phase has pH = 8−9, Eh = −450 ± 50 mV, and ammonia nitrogen, and K, Na, and Mg concentrations close to those in the cellular liquid.

Thermodynamic Modeling of Water-Rock Systems to Evaluate Their Generative Potential for Hydrocarbons

The hydrocarbon generative potential of rocks was evaluated by modeling chemical interactions in the systems carbonate rock—seawater, silty sand rock—seawater, argillaceous rock—seawater, basalt—seawater, and granite—seawater at 25, 100, 200, and 300°C and over a pressure range from saturated vapor pressure to the platform thermal gradient. The term seawater is used here to describe the composition of marine solutions at the halite (SW2), epsomite (SW3), and sylvite (SW4) stages of evaporation. The hydrocarbon generative potential of rocks in these systems increases in the order clays > silty sands > carbonates > mafic rocks, while felsic rocks are nongenerative. It was shown that an increase in the rock—water mass ratio (R/W), which can be considered as a proxy for the duration (grade) of metamorphism, led to an increase in the reduction potential (logfH2) and hydrocarbon generative potential of rocks. At R/W → 1, logfH2 for carbonates, clays, and silty sands is equal to −2.74, −2.45, and −2.57 at 100°C; −1.2, −1.1, and −1.0 at 200°C; and −0.5, +0.3, and −1.2 at 300°C, which suggests that at higher temperatures (pressures) clays are more capable of reducing chemical elements in the water—rock system.

Possible reasons for elevated fluorine concentrations in groundwaters of carbonate rocks

The concentration of dissolved F did not change in long-term (four months) experiments on the interaction of crushed limestone and marl with water (at R/W = 1) at room temperature. The comparison of the activity products and equilibrium constants of the mineral dissolution reactions indicates that the experimental solutions were undersaturated with respect to fluorite and oversaturated with respect to F-apatite. The long-term existence of such unequilibrated solutions is explained by the steady state of the system at which the primary F-bearing mineral (fluorite, mica, or palygorskite) is transformed into a secondary one (F-apatite). The dissolved F concentration is controlled under these conditions by the ratios of the dissolution reaction rates for the primary mineral and the precipitation rate of the secondary one.In the experiments with the association fluorite + calcite + dolomite (duration up to seven months), the solution was saturated with respect to these minerals. Their dissolution constants were utilized to derive dependences of the activity of the dissolved F ion on the activities of the Ca, Mg, and carbonate ions in the solutions. The experimental data are consistent with these dependences. If the solution is saturated with respect to gypsum, the activity of the dissolved F ion should also depend on one more parameter: the activity of the sulfate ion.When groundwaters filtrate for a long time or are stagnant, the association F-apatite + calcite + dolomite (+ gypsum) is stable; dependences of the activity of the F ion on the activities of aqueous species in the solution were derived for this association. These dependences are much stronger than those for the association fluorite + calcite + dolomite (+ gypsum).

Model for the formation of arsenic contamination in groundwater: 2. Influence of sorption

Arsenic is accumulated in groundwater in response to As desorption at an increase in the alkalinity of aqueous solution owing to (a) a decrease in the equilibrium concentration of the sorbed H3AsO30 and H3AsO40 species and (b) sorbent (iron hydroxide) decomposition, when Eh decreases below the line of the iron hydroxide-siderite equilibrium.

Iodine and selenium speciation in natural waters and their concentrating at landscape-geochemical barriers

Drinkable waters in Bryansk oblast are generally poor in I and Se. Possible I and Se speciation in the drinkable waters and their means of migration and concentration in soils at geochemically contrasting conditions are analyzed, and the possible reason for the high mobility of I is demonstrated to be predetermined not only by its occurrence in the form of iodide and organic complexes but also by solute mineral species (CaI+ and MgI+), with the former and the latter types of the complexes spread more widely in the polessky and opolny landscape types, respectively. Iodine complexation with alkali-earth cations under reduced neutral-weakly alkaline conditions facilitates, on the one hand, vertical iodine migration and, on the other hand, its precipitation on the carbonate barrier. The predominant solute species of Se in these environments is hydroselenide, which can form FeSe in the presence of significant Fe concentrations and be precipitated on the reduced barrier in soils of hydromorphic landscapes. The generally low total I and Se concentrations in the drinkable waters and the migration of solute compounds of radioactive I in the form of organic and inorganic complexes could likely result in a higher thyroid morbidity rate over the whole territory of Bryansk oblast, including areas contaminated with radioactive I isotopes after the accident at the Chernobyl nuclear power plant.

Formation of dawsonite mineralization at the Zaozernyi deposit, Belarus

Systems of silty sand-water and clayey rock-water were simulated under open-system conditions with respect to CO2 and oxygen at 25°C. It was shown that the dawsonite(+ quartz)+kaolinite assemblage at the Zaozernyi deposit was formed by interaction of terrigenous sediments with chloride-sodium solutions containing NaCl >100 g/kg H2O and saturated in carbonic acid (a partial pressure of CO2 > 0.5 bar). The modeling results do not confirm the possibility of dawsonite formation after primary bauxites (products of kaolinite weathering crusts), which is presumably related to the inconsistent formation parameters of these minerals during weathering of aluminosilciate rocks.