Groundwater of the permafrost regions

In response to repeated changes in the climatic conditions through the Pleistocene and Holocene, which included glaciations and general cooling, fundamental cryogenic transformations occurred within the upper part of the lithosphere. They extended over vast portions (as far as area and depth are concerned) of the Earth's crust, creating quite a specific hydro-geological situation. Groundwater survived only in taliks of various types, below the permafrost base, within the zones of supra- and intrapermafrost groundwater flow and in the form of intrapermafrost water-conducting zones. The rest of the hydrogeological section turned into cryogenic water-confining strata under the effect of great external cooling. A large amount of water was transformed into ice, promoting formation of expanded cryogenic structures as well as ice-rich horizons with numerous ice lenses in the permafrost.

For the territory of the permafrost zone, groundwater is subdivided, with respect to cryogenic water-confining strata, into supra-, intra-, and subpermafrost water, and groundwater between permafrost units. These specific categories of groundwater have a number of inherent features allowing their grouping together for various permafrost conditions and their consideration in a unified context in spite of these hydrogeological differences.

Suprapermafrost water is subdivided into two varieties. These are temporarily existing water of seasonally thawed layers and continuously existing water of closed taliks.

Groundwater between permafrost units is typical of the permafrost zone which has a two-layered structure, and the water is situated between the upper (Holocene or recent) and lower (Pleistocene or 'relict') cryogenic layers. The waters under consideration usually have hydraulic connection with the other (supra- and subpermafrost) groundwater.

Intrapermafrost water is delimited on all sides by frozen ground. It is an uncommon category of groundwater typical of the intensively freezing hydrogeological structures. Such groundwater is excluded from water exchange as a rule.

Subpermafrost water is water found in water-bearing horizons, complexes and zones of jointing, below the base of the frozen strata.

The water of open taliks belongs to a particular category of groundwater within the permafrost zone, connecting all the types of groundwater noted above (except for the intrapermafrost type) into the unified hydraulic system.

Suprapermafrost water of the seasonally thawed layer This exists exclusively during the summer period. This is pore-fissure, pore and fissure water in Quaternary deposits, in crusts of weathering and in exposed bedrock. Its distribution correlates with the position of the permafrost surface and depends on the specific features of topography. The depth of thawing and the thickness of water-saturated horizons depend on soil composition and they increase from north to south. Waters of the seasonally thawed layer carry the greatest amount of heat within the sections where the flow velocity is great. These are sites at the base of slopes composed of well washed-out water-saturated rudaceous formations with good seepage properties of the ground.

Recharge of water in the seasonally thawed layer is by atmospheric precipitation, vapour condensation, thawing of snow patches etc. 'Golets' ice in rudaceous formations is of great importance for this process. Such ice is formed in spring during the infiltration of water from melted snow into frozen rudaceous materials.

Frozen ground is not always the water-confining horizon for waters of the seasonally thawed layers. They can also be underlain by water-impermeable thawed deposits, giving typical vadose water.

For the summer period the groundwater can be classified with respect to recharge sources such as periodically appear after a rainfall (within watersheds), periodically disappear as a result of long absence of rains (upper and middle parts of slopes) and continuously exist on account of inflow of water from the seasonally thawed layer in areas situated hypsometrically higher (lower parts of slopes, valleys).

The water considered above has mainly low mineralization, and a hydrocarbonate composition with variable cation relationships. It is enriched in oxygen, in humic acids and in organic matter. Near the coasts of northern seas as well as in arid zones (Central Yakutiya, Southern Zabaykal'ye)

brackish and saline water is often found in the lower part of the seasonally thawed layer.

This class of water is of no interest as a source for water supply because of short lifetime, variable water regime, predisposition to pollution and unreliability in respect to sanitation as a consequence.

Suprapermafrost water of closed taliks

All the types of groundwater accumulation that persist throughout the year near the permafrost table are included here. This is water in taliks such as the pluvial-radiation type, taliks below or near a river bed, below a lake, as well as pressure-seepage taliks of the subaerial subtype. The thickness of the closed taliks noted above can vary in the range from a few metres to 40-60 m and rarely to 100-120 m, depending on the sources of supply, the character of the profile and its degree of flushing, capacity properties of the water-containing ground and conditions of water exchange. Most of these taliks (except for those situated near a sea coast) are characterized by free water exchange and fresh composition of the groundwater, the recharge of which depends on water of the seasonally thawed layer, atmospheric precipitation and surface water. These sources include soil pore, stratum and fissure water in Quaternary deposits and weathering crusts and zones of jointing in hard and semihard rocks.

Pluvial-radiation taliks are developed mainly in the south of the permafrost zone, with their water coming from rain and snowmelt as well as from condensation of vapour. Wide variations of groundwater levels resulting from the irregularity of precipitation of varying intensity are typical. These waters correspond to atmospheric precipitation and to waters of the seasonally thawed layer in chemical composition and are of low (up to 0.1 g 1 ~1) mineralization.

Water of taliks below or near a river bed represents hydraulically uniform soil pore water in loose valley deposits, as well as fissure waters in the underlying unfrozen hard and semihard rocks. This category of groundwater is the most widespread within the permafrost region, and is of great practical importance for providing a temporary or regular water supply. The replenishment is by surface water courses maintained by atmospheric precipitation and by water of the seasonally thawed layer as well as by subpermafrost groundwater flow as a result of subaquatic pressure discharge. The water regime is unstable throughout the year. Chemical composition corresponds to that of surface water courses. Continuity of the flow below a river bed is broken in winter. Separate basins appear, cryogenic heads are formed and the quality of the groundwater changes. Where

Fig. 13.2. Taliks below a lake within a zone of active water exchange: 1-6 - geological structure (1 - crystalline bedrock; 2 - terrigenous-carbonate rocks; 3 - intrusive rocks; 4 - loose materials of various origins; 5 - tectonic dislocations; 6 - rocks with more than usual jointing and karst processes); 7-11 - hydrogenous (underwater) taliks (7 - infiltration, closed below a lake; 8 - infiltration, open below a lake; 9 - pressure-seepage open, below a lake; 10 - infiltration open, below a river bed; 11 - subaerial pressure-seepage closed); 12 - permafrost boundaries; 13 - direction of groundwater flow.

Fig. 13.2. Taliks below a lake within a zone of active water exchange: 1-6 - geological structure (1 - crystalline bedrock; 2 - terrigenous-carbonate rocks; 3 - intrusive rocks; 4 - loose materials of various origins; 5 - tectonic dislocations; 6 - rocks with more than usual jointing and karst processes); 7-11 - hydrogenous (underwater) taliks (7 - infiltration, closed below a lake; 8 - infiltration, open below a lake; 9 - pressure-seepage open, below a lake; 10 - infiltration open, below a river bed; 11 - subaerial pressure-seepage closed); 12 - permafrost boundaries; 13 - direction of groundwater flow.

subpermafrost water under pressure discharges into the taliks below a river bed the groundwater regime is marked by greater stability.

From a regional perspective, the valley taliks' parameters and the cross-section for seepage flow change, increasing southward and decreasing northward depending on the permafrost conditions, the intensity of surface runoff, type and washout degree of the underlying rocks and the presence of disjunctive dislocations.

Waters of taliks below a lake are formed within various topographic elements. The chemical composition of these waters is practically the same as that of water reservoirs. These are pore-, pore-stratum and fissure waters in the underlying loose and slightly lithified Cenozoic deposits as well as in bedrock (Fig. 13.2). The thickness of these taliks varies from a few metres to

40-80 m and greater, decreasing from south to north under similar conditions. In freezing lake basins, below alases and below lakes that are freezing or drying-up, groundwater mineralization can be as much as 20-80 g 1_1 because of the process of cryogenic metamorphization. Water of closed taliks below a lake often has a stagnant regime and limited storage.

Waters of pressure-infiltration closed taliks are rare and situated within the areas of thick (more than 200-300 m) permafrost with rather high mean annual ground temperatures (up to —3 or — 5°C). Movement of the groundwater of this type occurs within the perennially frozen masses along pipes and channels that are intrapermafrost or situated between two permafrost masses. These are fissure, karst-fissure and vein-fissure waters with an intensive water exchange. The discharge takes place through down-tending sources of groundwater causing formation of small and medium or, rarely, large icings in winter. They are supplied from the atmosphere as well as from surface waters of the seasonally thawed layer. The water is usually fresh. It is often used for supplying installations having a small water consumption.

Groundwater of open taliks

This represents an important link in the system of the existing water exchange between surface water courses, closed taliks, and water of deeper horizons and complexes situated near the permafrost base. There is pore, pore-stratum, fissure, vein-fissure and karst water. These categories are found in practically all these taliks and are subdivided into infiltration and pressure-seepage classes. Downwards or upwards flow of groundwater of this category occurs through the system of thawed fissures and channels in the permafrost. There are differences in specific hydrological features for various subtypes of taliks. Thus, groundwater levels in pluvial-radiation taliks of the infiltration class experience wide, sometimes even spasmodic fluctuations (from 20 to 30 m, or 40 m) in the summer period depending on the precipitation amount and intensity. These fluctuations are gradually damped out away from the talik boundaries. In winter the drop in levels in the upriver infiltration taliks situated below and near a river bed can be as much as tens to a few hundred metres, as the depletion of the surface discharge and of the discharge below the river bed progresses.

Large, or gigantic, icings are usually formed near the pressure-seepage taliks. In large rivers within such places, the warming effect of the subpermafrost groundwater flow being discharged into them leads to the existence of polynias in winter. Along the coasts of northern seas and within the intracontinental regions of the Siberian platform the discharge of highly mineralized water with negative temperatures through taliks below a river bed, produces a cooling effect on the ground.

Subpermafrost groundwater flow

This flow exists near the base of the permafrost and has a significant effect on the cryogenic structures and on the permafrost thickness. There are pore, stratum, fissure, vein-fissure, karst and other types of groundwater. These types are characteristic of the permafrost zone. Formation of the cryogenic water-confining horizons has an effect on the spatial distribution of groundwater flow in various seepage media near the permafrost base. The position of such groundwater results in its definition as subpermafrost, to some extent smoothing over the differences between the separate water-confining horizons, complexes and zones of jointing.

Subpermafrost water can be pressure contacting (in contact with frozen rocks) or non-contacting (separated from frozen rocks by thawed ones) with respect to the permafrost base. The fresh and slightly mineralized water below the permafrost has positive temperatures as a rule, while brines (cryopegs) have negative temperatures. The water differs in chemical composition in accordance with the type of bedrock. For the most part the water has hydrated calcium carbonate perhaps with sodium carbonate, if fresh water, and in salt water sodium chloride perhaps with calcium chloride and sodium and calcium sulphates. Groundwater of the non-contacting type can be under pressure (below the water-impermeable thawed ground) or can have a free water table (below permeable thawed ground). In the course of changes in the climatic situation and repeated raising and lowering of the permafrost base, fissure zones of cryogenic disintegration with higher water content are widely developed and occupy large areas. The total thickness of these layers of cryogenic disintegration near the southern limit of the permafrost zone can be more than 150 m.

The water level regimes of groundwater of the contacting and non-contacting types are different. For the non-contacting type, wide seasonal variations of groundwater levels are noted near the sources of recharge. For the contacting type, the groundwater levels are more stable.

Groundwater between permafrost units and intrapermafrost groundwater

These are typical categories within the permafrost zone and were formed at the closing stages of the cryogenic transformation of the hydro-geological structures.

Groundwater between permafrost units occurs widely within the hydro-

geological structures of the European North of Russia, Western Siberia and in some parts of the intermontane basins of the Baikal type, etc. In addition, the water-bearing strata with fresh and brackish waters occur below draining and freezing thermokarst lakes and hollows, as well as at the sites of ancient water bodies.

The closed infiltration taliks recharging groundwater between permafrost units are found within high terraces and interfluve areas of the Lena and Yilyuy basins, Putorana plateau, etc. Such groundwater is situated below certain lakes, small tributaries and pluvial-radiation taliks. Under climatic cooling the channels conducting water flow inside the permafrost can become separated from the surface being transformed into intrapermaf-rost water-containing cavities which, in the end, freeze.

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  • LUCA
    Is there a ground water at permafrost?
    2 years ago
  • isaac
    How is groundwater in permafrost regions?
    2 years ago

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