Sodium-bicarbonate groundwaters in southeastern West Siberia, Russia: Compositions, types, and formation conditions
Introduction
Sodium-bicarbonate HCO3–Na (sodic) groundwaters are of broad natural occurrence and show great diversity in major-ion chemistry, compositions of their gas and organic components, as well as stable isotopes. The definitions and boundary parameters of sodic waters vary in different publications. The waters classified as sodium-bicarbonate in this study have a major-ion chemistry with (HCO3− + CO32-) and Na+ predominant among anions and cations, respectively, high pH (most often above 7.5), and a salinity of >0.6 g/L.
The origin and formation conditions of sodic waters have had a large literature (e.g., Blake, 1989; Matthess et al., 1992; Kimura, 1992; Appelo and Postma, 1994; May 1998; Jankowski and McLean, 2001; Gavrishin, 2005; Shvartsev and Wang, 2006; Shvartsev et al., 2007; Krainov et al., 2012; Popov and Abdrakhmanov, 2013; Christian et al., 2016; etc.) but remain controversial. The controversy can be overcome by viewing the problems in the context of interaction between the waters and the rocks they drain (Garrels and Mackenzie, 1967; Helgeson, 1968; Helgeson et al., 1969, 1984; Pačes, 1972; Aagaard and Helgeson, 1982; Drever, 1982; Helgeson and Murphy, 1983; Arnorsson et al., 1983; Alekseyev et al., 1997; Giggenbach, 1988; Tardy and Duplay, 1992; Nordstrom, Munoz, 1994; Grasby et al., 2000; Putnis, 2002; Hellmann et al., 2003; Fu et al., 2009; Zhu and Lu, 2009). According to the general geochemical aspect of the water-rock interaction theory (Shvartsev, 2008; Shvartsev et al., 2006, 2007, 2016, 2017, 2018), the HCO3–Na chemistry forms at a certain step of the process once water becomes saturated with respect to calcite. Water is never in equilibrium with magmatic minerals but is equilibrated with certain secondary (authigenic) minerals; the nonequilibrium causes dissolution of minerals and maintains continuous evolution of water (Shvartsev, 1991, 1994, 1995, 1997, 2010, 2012, 2013, 2014, 2015, 2016, 2017, 2019). The chemistry of groundwater interacting with rocks records the composition difference between dissolved primary minerals and precipitated authigenic phases and may include also organic compounds and gases. In this respect, HCO3–Na groundwaters widespread in thick Mesozoic-Cenozoic sediments of southeastern West Siberia are of special interest due to their diversity associated with the evolution of the ‘water – rock – gas (CH4 and CO2) – organic matter (coal, peat, etc.)’ system. This study focuses on chemical features of HCO3–Na waters and their formation conditions in southeastern West Siberia from the perspective of water-rock interaction.
Not being unique, the model of S. Shvartsev provides a universal explanation for the geological evolution of the rock-water system and the regional-scale diversity of sodium-bicarbonate waters, while the ion exchange or geological models account for specific mechanisms in this evolution.
Section snippets
Sampling and methods
The study is based on data collected by a joint team from the A.A. Trofimuk Institute of Petroleum Geology and Geophysics (Tomsk Branch) and the National Research Tomsk Polytechnical University (Tomsk) in 2000–2015 and published evidence. Groundswaters were sampled at 157 wells and 118 springs, altogether 460 samples (Fig. 1).
Highly changeable parameters (pH, temperature, and electrical conductivity) were measured in the field using a portable digital AMTAST AMT03 m (USA). In the field, the
Study area
Sodium-bicarbonate waters were studied in the southeastern part of the West Siberian artesian basin (including two smaller basins of Chulym-Yenisei and Middle Ob) and in the northern Altai-Sayan fold area (including the Kuznetsk artesian basin and the Salair and Kolyvan-Tom fold area). The Kuznetsk and Chulym-Yenisei basins (Fig. 1) are especially advantageous for studying these waters.
Chulym-Yenisei basin is located in the southeastern West Siberian artesian basin, mainly within the Tomsk
Results
Sodium-bicarbonate waters are of regional extent in southeastern West Siberia and occur in Mesozoic-Cenozoic sediments at depths from 30 to 300 to 1000–2300 m in the zone of mainly slow water exchange, between fresh HCO3–Ca waters above and saline Cl–Na waters below. They are chemically diverse (with 70–100% NaHCO3), with TDS from 0.2 g/L to 25 g/L; рH from 6.3 to 10.3; the gas phase consisting of methane, nitrogen, and carbon dioxide; high contents of organic components and low dissolved
Discussion
The obtained results were used to model the formation of HCO3–Na waters with regard to specificity of their different types (Fig. 7). Water percolating through rocks continuously dissolves (hydrolyzes) nonequilibrium minerals. These reactions release elements (Cа, Mg, Na, Fe, K, Al, Si, etc.) at different rates and produce assemblages of authigenic phases. Water molecules are likewise involved into hydrolysis: they decompose into H+ and OH− and form a particular chemical environment. H+ binds
Conclusions
1. Sodium-bicarbonate groundwaters occur at depths from 50 to 300 to 1000–2300 m in Mesozoic-Cenozoic sediments throughout southeastern West Siberia. They are compositionally diverse and belong to five different types distinguished according to their compositions, depths, and extent. All waters are of infiltration origin and all have a biogenic source of CO2, except for type V where some CO2 comes from an inorganic source below the sodic water zone.
- 2.
The formation conditions of HCO3–Na waters
Acknowledgment
The paper is dedicated to the memory of Professor Stepan L. Shvartsev whose valuable advice helped me to prepare my doctor thesis and whose ideas lie at the base of this study.
The work was supported by grant 17-17-01158 from the Russian Science Foundation, by grant 20-05-00127 from the Russian Foundation for Basic Research.
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