Sodic Groundwaters in the Southern Kuznetsk Basin: Isotopic and Chemical Characteristics and Genesis

The paper summarizes newly obtained data on the chemical and isotopic composition of groundwaters in the southern part of the Kuznetsk Basin. The territory is demonstrated to host groundwaters of three types, which were distinguished according to the chemical, isotopic, and gas composition of the waters. The waters of type 1 are widespread in areas of sluggish water exchange in coal-bearing terrigenous rocks, bear mineralization of 0.2–4.7 g/L, have pH 7–10, and N₂–CH₄ and CH₄–N₂ gas composition. Their source is biogenic (coal) with δ¹³C from –17.1 to –4.2‰. The sodic waters of type 2 were found relatively recently and only locally, are hosted in the same rocks but at more hampered water exchange, and have mineralization of 5–25 g/L, pH 7.7–10, and CH₄ gas composition. These waters are noted for an anomalously isotopically heavy isotopic composition of their carbon δ¹³С(\({\text{HCO}}_{3}^{ - }\)) = 0.2–30.9‰ and δ¹³С(CO₂) = –3.2–22.3‰. The source of their carbon is also biogenic, but long-lasting interaction in the water–coal system resulted in significant carbon fractionation, with isotopically lighter carbon fractionated into CH₄ (δ¹³С = –67.3 to ‒43.3‰) and isotopically heavier one preferably coming to CO₂. The waters of type 3 occur only at certain sites, for example, the Tersinskii CO₂ waters. These waters have mineralization of 2–5.5 g/L, pH 6.2–6.7, and CO₂ gas composition. Their δ¹³С(\({\text{HCO}}_{3}^{ - }\)) = –4.3 to –4.1‰ and δ¹³С(CO₂) = –12.3 to –3.9‰ suggest a mixed source of their carbon. This source was mostly deep-seated, not genetically related to waters. All of the sodic waters belong to the infiltration type according to their δ¹⁸O and δD, with the waters of type 2 showing a positive oxygen shift. The sodic waters of all three types are shown not to be in equilibrium with many primary aluminosilicates but are oversaturated with respect to carbonates, montmorillonites, illite, chlorite, and sometimes even albite and microcline (waters of type 2). The waters of all of the three types were generated by the same mechanism: by dissolving aluminosilicate minerals in coal-bearing rocks (with which the waters are not in equilibrium) and the simultaneous precipitation of carbonates. The reasons for certain differences between the waters are their interaction with rocks and coal (waters of type 2), which increases their mineralization and makes their carbon and oxygen isotopic compositions heavier, or the addition of carbon dioxide from an external source (waters of type 3), which acidifies the waters.