Interaction of groundwater with the permafrost and types of cryohydrogeological structures

Permafrost affects the groundwater condition through the mechanism of mutual interaction. The water loses energy to the frozen rock, tending to thaw it, while the frozen rock, on the contrary, tends to freeze the water by taking its heat energy. The permafrost zone currently existing is derived from this complex process proceeding in various directions.

Partial or full freezing of water-bearing horizons, complexes, and zones of jointing has occurred as a result of such interaction. They have been excluded from natural water circulation to a significant depth of the geological section and even entirely in some places. The particular conditions of exchange of the ground, atmospheric and surface waters through the system of taliks discussed above have arisen in this way. Storage capacities of the bedrock have been reduced and the groundwater circulation has narrowed spatially. At the same time separation of the hydrodynamic systems previously interconnected becomes typical of the upper part of the lithosphere. The interrelations in the primordially existing water-bearing horizons, complexes and zones of jointing have been disturbed and the directions and intensity of groundwater flow near the permafrost base have been changed.

Layers of frozen ground (with ice), cryotic (without ice) and cooled (with cryopegs) were formed. In the vertical section there can be one-, two- and even three-layered cryogenic units within the various parts of the lithosphere. At the same time the total thickness of the cryogenic water-confining horizons can vary from the upper few metres in the south of the permafrost zone, to 500-700 m and even more than 1000 m in the most severe conditions of highly dissected mountain structures.

In the hydrogeological structures changed during the course of freezing, abnormally high heads were formed. The initial structures were separated into individual basins with enormously concentrated run-off at times and with extremely restricted water exchange at other times. In the course of the thawing of the hydrogeological structures during warming periods, zones of abnormally low heads developed.

The initially fresh composition of groundwater underwent cryogenic metamorphism as a result of the interaction with the perennially frozen water-confining horizons. The main point is that in the course of transition of water into ice some portion of the dissolved salts precipitates. The content of easily dissolvable components increases in the freezing solution and is pressed out by the growing ice inclusions into the lower part of the section promoting cryogenic concentration. During warmings when degradation of the cryogenic water confining horizons takes place, not all the precipitated salts are redissolved. As a result, the water-bearing systems that are reestablished in the course of thawing have lower salt content (are cryogenically desalted).

Formation of cryogenic water-confining horizons affects the development of salt waters with negative temperature (cryopegs) and it has caused the formation of fissure zones of cryogenic disintegration with higher than usual water content near the permafrost base, thus giving entirely new (cryogenic) hydrogeological structures and accumulation basins (in depressions and fissure zones) and groundwater flow (along valleys and tectonic zones) of a particular kind.

At the same time the existence of groundwater beneath penetrating permafrost actively counteracts the freezing process and this must have an effect on the character of the developing cryogenic section. As this takes place, the spatial involvement of ground in the freezing changes considerably, not only in the wide regional sense but also locally, depending on the groundwater discharge, its heat content and the character of water exchange in the hydrological system. Thus, the very severe freezing conditions in high latitudes and within mountain regions had an effect on the formation of the very thick (up to 800-1000 m) cryogenic water-confining horizons found within Baikal-Stanovoy, Verkhoyansk-Kolyma and other folded regions. Sizable areas here have no significance at all as far as hydrogeology is concerned. The zones of regional exogenic jointing are fully frozen within these areas. The groundwater contained in them before the process of lithosphere cooling began was either carried away or transformed into ice, having an effect on the permafrost ice content. Within the areas situated around the highly elevated rock massifs with entirely frozen zones of jointing excluded from water exchange, there exist places with extremely concen trated groundwater flow along cavities, pores, fissures and veins in the rocks. Such groundwater concentration occurs along dislocations with a break of continuity, along neotectonic depressions and the network of valleys and it changes the temperature conditions of the frozen rocks, their thickness and continuity. At the same time in the immediate neighbourhood one can observe areas of continuous, ultradeep freezing (more than 300500 m) with vast areas of discontinuous and massive-island development of the cryogenic water-confining horizons and even areas where the permafrost is absent. Such a pattern is typical, for example, of a number of hydrogeological structures within the Baikal-Stanovoy folded region. Rock massifs with ultradeep freezing (along the main watersheds of Severo-Muyskiy, Udokan, Kodar and other ridges) are adjacent here to areas where the permafrost is absent. These areas are highly water-saturated and alternate with permafrost massifs and islands, the thickness of which is one third to one fifth of that in the areas of continuous freezing situated around them.

Salt groundwater and brines assume negative temperatures and are transformed into cryopegs in the course of climatic cooling. Remaining liquid, they affect the intensive rock cooling, increasing the thicknesses of the cryogenic part of the section abnormally because of their cooling ability. Within the Siberian Platform, the Verkhnevilyuysk, Verkhnemarkhinsk and Turukhansk uplifts and Putorana volcanogenic massif, etc., show such distinctive cooling involving cryopegs.

The mutual effect of groundwater and cryogenic water-confining horizons on each other can be shifted sharply in either direction, as a result of natural as well as artificial disturbances in the surface conditions, in spite of the apparent relative equilibrium of this interaction. These disturbances can arise as a result of general climatic changes as well as of geotechnical activity. At the same time, any change in the equilibrium towards warming causes a usually greater and more efficient effect of the groundwater on the permafrost. As a consequence, it very quickly becomes the leading factor in permafrost degradation.

The heat exchange between groundwater and permafrost is revealed in various ways by groundwater flow. If one follows the groundwater flow within the permafrost zone from recharge to discharge, in the most general terms it is obvious that the character of the thermal interaction with the permafrost changes. Where downward flow along the open infiltration taliks occurs, the rocks of the upper part of the section are warmed. Closer to the base of the permafrost the temperatures of groundwater and bedrock equalize and become lower than that of the thawed ground near the lower boundary of the cryogenic water-confining horizons. Within this interval

Fig. 13.3. Main types of hydrogeological structures changed by cryogenesis: 1-6 - geological structure (1 - crystalline bedrock; 2 - terrigenous-carbonate and carbonate-terrigenous rocks; 3 - intrusive rocks; 4 - loose and slightly lithified materials - of various origins; 5 - tectonic dislocations; 6 - rocks with increased jointing and karst processes); 7-9 - boundaries (7 - of permafrost; 8 - of hydrogeological structures of various types; 9 - of areas of deep and ultra-deep freezing without groundwater); 10 - direction of groundwater flow. Main types of cryohydrogeological structures: CGM - cryogeological massif; HGM -hydrogeological massif; AB - artesian basin ; HAM - hydrogeological admassif; ADB - adartesian basin; CB - cryogenic basin of pressure-fissure waters; CGB -cryogeological basin.

Fig. 13.3. Main types of hydrogeological structures changed by cryogenesis: 1-6 - geological structure (1 - crystalline bedrock; 2 - terrigenous-carbonate and carbonate-terrigenous rocks; 3 - intrusive rocks; 4 - loose and slightly lithified materials - of various origins; 5 - tectonic dislocations; 6 - rocks with increased jointing and karst processes); 7-9 - boundaries (7 - of permafrost; 8 - of hydrogeological structures of various types; 9 - of areas of deep and ultra-deep freezing without groundwater); 10 - direction of groundwater flow. Main types of cryohydrogeological structures: CGM - cryogeological massif; HGM -hydrogeological massif; AB - artesian basin ; HAM - hydrogeological admassif; ADB - adartesian basin; CB - cryogenic basin of pressure-fissure waters; CGB -cryogeological basin.

the groundwater flow acts as a cooling factor resulting in the permafrost thickness increasing. In the course of more or less horizontal subpermafrost flow the strata are heated by the heat from the Earth's interior. As this flow is parallel with the isolines of the temperature field, the groundwater exerts a neutral effect on the thermal state of the ground. As the groundwater flows toward the places of discharge (upward flow) its temperature is higher than that of the surrounding frozen material, on which its exerts a warming effect. As a consequence, the open pressure-seepage taliks remain stable.

Within the permafrost zone as well as within the regions free of permafrost, hydrogeological massifs and artesian basins are separated in the course of hydrogeological regionalization (Fig. 13.3). Protrusions of crystal line basement rocks with fissure and vein-fissure types of groundwater occur in the massifs. Artesian basins are basins, depressions and platforms filled by horizontal sedimentary rocks of various age and composition with pore, pore-stratum, stratum-fissure and karst-fissure waters. In addition, the artesian structures of the intermediate type (between massifs and basins), share a number of traits with both of them. For example, the adartesian basins are similar to the artesian ones as far as their geological-structural position is concerned; however, their water-containing rocks are folded into synclines. The adartesian massifs are characterized by the wide development of anticlines and represent positive topographic structures. In either case the fissure and stratum-fissure groundwater accumulations prevail as is the case in the hydrogeological massifs. The flow of the groundwater is centripetal in adartesian basins and centrifugal in adartesian massifs.

The names of the types of hydrogeological structures (probably it is more correct to call them cryohydrogeological or hydrogeocryological structures) should reflect the cryogenic characteristics considered as a result of the process of groundwater interaction with the cryogenic water-confining horizons: for example, hydrogeological structures of continuous ultradeep freezing (more than 300-400 m); of continuous deep freezing (200-300 m); of not very deep freezing (100-150m); of discontinuous intermittent freezing (frozen massifs comprise 50-75% of the total area); of discontinuous massive-islands (25-50%) and of islands and sporadic freezing (5-25%).

Water-bearing horizons, complexes and zones of jointing are the main taxonomic units in the cryohydrogeological structures. One is forced to characterize them additionally in the context of cryogenic changes that have taken place in them: for example, frozen entirely, dry; frozen partly, across the section; mainly frozen with local water saturation, etc.

Rocks in cryohydrogeological massifs and admassifs are water-saturated near the base of cryogenic water-confining horizons and along taliks of various types. At the same time, separation of a single system of pressurized water in massifs into a number of systems in the basins of river runoff, with localization of recharge sources and formation of cryogenic heads (pressures) takes place. The most deeply frozen parts in such structures are 1500-2000 m higher than the present river beds and they are characterized by the total absence of groundwater in the liquid phase (except for the suprapermafrost water during the summer period). Such structures are termed cryogeological.

In the course of freezing of the large Meso-Cenozoic artesian basins, when the permafrost thickness becomes greater than that of the fresh water belt, there are only cryopegs in the lower part. Such structures are termed cryoartesian basins. Within the more shallow artesian structures corresponding to Cenozoic superimposed basins, the complete freezing of water-containing rocks of the artesian cover often takes place. This causes transformation of these artesian structures into the so-called cryogeological basins. Within these basins the ground waters are contained in faulted rocks of the basement as well as in open and closed infiltration-, pressure seepage-and ground seepage taliks.

In the course of cryogenesis particular hydrogeological structures were formed when groundwater was concentrated near the permafrost base or within the zones of cryogenic disintegration. These are cryogenic basins of pressurized-fissure water. They are characterized by the continuous extent of the permafrost water-confining horizons. The layer of cryogenic disintegration with the pressurized-groundwater does not usually exceed 1520 m.

A particular type of cryogenic basin appears near the southern limit of the permafrost. Permafrost water-confining horizons in these basins are discontinuous while the thickness and the number of layers of cryogenic disintegration increases by an order of magnitude. Fissure water in the zones of cryogenic disintegration has water tables that change throughout the year.

Water exchange in the far from complete list of hydrogeological structures discussed above and subjected to cryogenesis, is termed 'open', 'closed' or 'partly open'. Recharge and discharge of groundwater in open structures takes place directly within them. Within partly open structures only one process (either recharge or discharge) proceeds, while the missing element of the water exchange cycle is implemented through the nearest structure. In the hydrogeological structures closed by the cryogenic or lithological water-confining horizons the recharge and discharge of the groundwater do not take place inside the structure. They are implemented by groundwater transfer and flow from the basement rocks or from the cover of the nearest structures.

Regional features and evolution of permafrost

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