On the Origin of Carbonated Waters of Kyrgyzstan

The question of the origin of carbonated waters is one of the most controversial in hydrogeology and is pivotal in the problem of mineral water formation.

The hypothesis of volcanic origin of carbonated waters is linked to the indisputable presence of carbon dioxide in volcanic eruptions and in the thermal waters of areas with modern volcanism. The fact that volcanic processes are constantly accompanied by the release of free carbon dioxide eliminates the need to prove its source in mineral waters. Many European geologists accept the volcanic nature of the appearance of CO2 in the mineral waters of the Eastern, Polish, and Czechoslovak Carpathians and even in the Rhenish slate mountains, where carbonated springs are often found, but signs of modern volcanism are completely absent.

Kyrgyzstan also has no modern volcanoes, and it would seem there is nothing to say about the possibility of the formation of mineral water sources in this way. The fact is that there are indisputable cases of magmatic outpourings in the territory of our republic from quite recent (from a geological point of view, of course) past — at the beginning of the latest tectonic stage of its development, at the end of the first (Paleogene) era of the modern (Cenozoic) geological epoch, just over 25 million years ago. Layers of magma that have erupted to the surface can be found along the road from Bishkek to Balakchy in the Boom Gorge, at the foot of the Kungey-Alatoo mountains near the village of Toru-Aigyr on the northern shore of Lake Issyk-Kul, in the Usulek area, in the Torugart-Tau mountains, and in several other places. In this case, while there are no carbonated waters nearby in the first two cases, in the Usulek area, there are highly mineralized waters rich in carbon dioxide, the presence of which can at least spatially be linked to Cenozoic magmatism.

The fact that magma exists at depth not only in places of modern volcanism is recognized by the overwhelming majority of specialists and is confirmed by geophysical research materials. Lacking ways to reach the surface "in person," according to some geologists, it can well release part of its composition, capable of breaking through to the surface due to its high mobility. It is reasonable to assume that such a well-migrating part of the magmatic melt could be carbon dioxide, which, as noted earlier, usually accompanies magmatic outpourings.

A great expert on the mineral waters of the Earth, the outstanding Soviet hydrogeologist A. M. Ovchinnikov (1904–1969), who significantly influenced the development of the Soviet school of mineral water studies, believed that in the modern era, carbonated waters appear only where there are evident or hidden recent intrusions (magmatic bodies), making no exceptions for the territory of Kyrgyzstan, where, despite extensive geological studies involving deep geophysical methods, no recent intrusions have been established, except for the aforementioned erupted magmatic bodies, which, strictly speaking, are considered effusive bodies among geologists, rather than intrusive ones, having cooled unlike the former at depth. However, a recent magmatic intrusion does not necessarily have to release carbon dioxide itself. It is sufficient, some specialists believe, that under the influence of its high temperature, rocks of different composition, such as sedimentary rocks often composed of calcium carbonate, which has a high potential for CO2 release, are heated and begin to release carbon dioxide. In other cases, the leading factor is assumed to be the participation of high pressure, which also contributes to the fundamental transformation of the composition and structure of rocks. This transformation is called metamorphism, and the carbon dioxide released in this process is a product of thermal or dynamic metamorphism.

Alongside deep transformations of rocks at great depths, there are processes of lesser change at relatively shallow depths (hundreds of meters and the first kilometers), which, however, lead to significant changes, such as the transformation of sand into sandstone. This process is called diagenesis by geologists and is quite widespread in areas of young sediment accumulation. During diagenesis, carbon dioxide release has also been established, which, under certain conditions, is likely capable of forming mineral waters. This explanation was accepted as valid by specialists who discovered elevated concentrations of CO2 in the water of a well that penetrated a 900-meter thick layer of Neogene sediments in Slovakia. The main role in recognizing the validity of the diagenetic pathway of gas appearance was played by the fact that as the well deepened, its concentration in the water decreased.

Among the typical hypotheses of near-surface origin of CO2 in groundwater is the hypothesis of its biochemical genesis, developed by the well-known Tashkent hydrogeologist B. A. Beder, primarily based on his research materials in Kyrgyzstan on the southwestern slope of the Fergana Range.

One of the most widespread viewpoints on the formation of free carbon dioxide in the Earth's crust is the chemical hypothesis. A well-known proponent of this hypothesis, geologist and geochemist Academician A. V. Shcherbakov, based on rich personal observations and various literary sources, established that in the modern era, the formation of carbon dioxide in the Earth's crust is often associated with exogenous processes, among which in the zone of oxygen penetration (the so-called oxidation zone), exogenous metamorphism plays a leading role. Its essence lies in oxidation in the broadest sense — in combustion, in the slow (normal) oxidation of bitumens, coal, and sulfides. The author of the hypothesis examined facts about the enrichment of groundwater and the underground atmosphere with carbon dioxide in mining workings and in natural cavities in Kryvorizhzhia, Kuzbass, Donbas, the Urals, Tian Shan, and many other places under circumstances such that the assumption of its exogenous chemical nature often appears to be the only possible one. However, it should also be emphasized that in most of the cases considered by the author, the total amount of generated carbon monoxide is small and, as a rule, does not lead to the formation of carbonated water deposits.

Alongside the hypotheses just discussed, there are other or various variations of these. Most of them have quite a scientific basis and their steadfast supporters. In this situation, it is important to substantiate two statements: first, in nature, there is evidently not one and not even two or three clear processes leading to the formation of mineral waters, and second, the problem of forming these waters is complex and far from a final solution.

Within Kyrgyzstan, mineral waters of various origins are observed, owing their existence to both endogenic and exogenic factors. For example, the Beshbelchir-Arashan deposit is likely due to the influx of energy and part of the composition of mineral waters from very great depths. Evidence for this includes the following circumstances: while discharging in the zone of permafrost, the underground waters still have a temperature of about +20°C; the content of microelements such as lithium and rubidium, considered by some specialists to be typical for waters associated with magmatic sources, is elevated; the gas composition of the mineral waters of this deposit contains an unusually high amount of helium for the mineral waters of Tian Shan — a typical gas from great depths; structurally, the deposit is associated with an area of complex tectonic structure, within the contours of which signs of a ring structure manifest, possibly representing features of the deep structure of the area that contribute to the formation of mineral waters. Given the arguments "for," there are currently no facts denying the deep origin of these mineral waters.

For a large group of carbonated water deposits in Kyrgyzstan, the recognition of the chemical exogenous oxidation by A. V. Shcherbakov is the most acceptable. This refers to the sources of carbonated waters in the axial part of the Fergana Range, where, with the widespread development of coal-bearing rocks, heavily fractured due to tectonic fragmentation, conditions are created for the penetration of oxygen into depth both with rainwater and in air flows. As a result, this leads to the formation of numerous, but small groups of carbonated and ferruginous waters with temperatures elevated relative to background by 2–4 times. The gas composition of such manifestations of mineral waters is characterized by the practical absence of helium and a noticeable amount of nitrogen with a disrupted proportion of its content in the air; oxygen was consumed for oxidation, while nitrogen, as is known, is an inert gas and has been preserved or appeared as a product of biochemical processes.

For some areas of mineral water manifestations, the most likely process for generating carbon dioxide should be recognized as the diagenesis of organic-rich deposits. Such an explanation is appropriate for the group of Karashorin deposits, where, alongside carbon dioxide and nitrogen, methane — a typical gas of the coalification process of such rocks and the transformation of coal into anthracite — is widely represented in the gas composition.

Not all deposits in our republic have their reasons for the appearance of carbon dioxide in mineral waters established more or less convincingly. For example, the gas emissions of the Ak-Suu mineral water deposit consist almost entirely of carbon dioxide with practically no helium. This serves as a sign of its likely metamorphic origin.

And this, of course, is not an isolated case.

Specialists are gathering new and interpreting old facts, obtaining more and more evidence for a more complete and accurate understanding of the phenomena occurring in nature, leading to the revelation of its secrets. There will, of course, come a time when disputes about the origin of carbonated waters in Kyrgyzstan will cease.