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Hydrogeology and Environmental Geology



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Hydrogeology

Well types - Permafrost - Hydrolaccolith

Permafrost

Permafrost defines rock or soil in which the temperature remains at or below 0°C for at least two consecutive years (Everdingen 2002). The term is also used to describe perennially frozen ground. It is the result of continuously cold climate as it occurred in the Pleistocene epoch in our latitudes or like it occurs today in higher latitudes in Canada or Russia. Massive ice may form either as buried surface ice (glacier) or as intrasedimental ice (intrusive ice). The main factor of distribution and thickness is the climate but permafrost also depends on vegetation, snow cover, drainage and soil type. Usually a seasonally thawing layer overlies the permafrost. In most areas characteristic geomorphologic features like patterned ground, palsas (ice- cored peat mounds), ice lenses and wedges, frost shattered rock, shallow drained lakes containing taliks (unfrozen areas), and pingos (symmetrical hills in a flat landscape) are visible. The thickness of permafrost ranges from a few meters up to 1500 meters in Siberia. Mostly it occurs in sediments but there are also some examples of permafrost in fractured rock (Gascoyne 2000).
Frozen ground may contain small amounts of liquid water, which is adsorbed onto the surface of soil or sediment particles and is liquid even when the temperature is well below the freezing point. In non-saline soils the amount of unfrozen water tends to be low, especially when the grain size is large. Saline soils normally occur in costal areas. The particle size is smaller there. When permafrost develops in saline soils the ice crystal grows away from the particle into the pore space and rejects dissolved salts into the remaining unfrozen liquid. The soil containing these higher concentrated liquids is called cryopeg. The more saline it becomes, the more the freezing point of the brine decreases. Marine salinity of permafrost is in Russia primarily located at the cost of the Arctic Sea. The other type of saline permafrost is the continental type. In Russia it is located along the mid-course of the river Lena and Aldan and in the west of Lake Baikal. This type is generated in areas where high summer temperatures and a negative balance of soil moisture promote salt accumulation in the soils and underlying strata. The formation of brines is also possible in such non-saline soils if there is a supply of mineralised water. When the soil freezes, its volume expands up to 9% and results in the expulsion of pore water that may accumulate in aquifers or lakes (Gascoyne 2000).
Periglacial environments can also show chemical precipitation due to salt rejection. They are formed because of solubility constraints at lower temperatures. CO2 release or uptake may play a roll when water freezes. The precipitates - for instance Fe, SiO2 or CaCO3 - have been formed on sediment grains during frost and ice formation. Precipitates like calcite can be identified as cryogenic because of their unusual isotopic composition (e.g. 13C up to +17 promille due to non- equilibrium conditions; Gascoyne 2000).
Permafrost is present in all countries around the Arctic regions like Russia or Canada. Approximately half of the territories of Russia are perennially frozen. The following map gives an overview of the permafrost zones in Eurasia.


[Permafrost zones of Eurasia (S.L. Smith, Geological Survey of Canada, 2001)]

There are three types of permafrost zones: continuous, discontinuous and sporadic. Continuous permafrost is described for a geographical region where more than 90 % of the ground is underlain by permafrost. Discontinuous permafrost occurs in a special region, where 30 to 90 percent of the lands surface are frozen ground. If permafrost underlies 5 to 30 percent of the exposed land surface it is called sporadic. Individual areas of frozen ground are surrounded by completely unfrozen ground (Everdingen 2002).
The following sketch gives a rough overview of the distribution of permafrost zones in the area of lake Baikal.


[Distribution of permafrost zones around Lake Baikal]

The red point refers to a hydrolaccolith that was visited during the excursion.

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© B. Merkel, 29.11.2004 http://www.geo.tu-freiberg.de/studenten/Baikal_2004/baikalexcursion/hydrogeology/overview/permafrost.HTM
 
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