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Hydrogeology

Well types - Permafrost - Hydrolaccolith

Hydrogeology, Limnology of Lake Baikal

The lake stretching like a crescent from north to southwest has a maximum length of 636 km and a maximum width of 79,5 km. It is located in Central Asia in the western part of Burjatia and belongs to the Russian Federation. The Baikal is a drainage lake. It has a catchment area of approximately 570000 km2 with more than 300 tributaries, contributing 83 percent of inflow, but only one outflow (Angara River) that makes out 85 percent of the output. Other sources of input are precipitation (13%), groundwater (4%) and evaporation (15%) increasing the outflow.

With a water volume of 23000 km3 and its maximum depth of 1637 m it is the deepest lake, the largest freshwater reservoir of the earth, and the oldest lake with an age of 20 Million years. It contains 20 percent of the non-frozen surface freshwater and 80 percent of Russia's freshwater. In the early stage the lake was divided into several separated basins containing less water than today. The present shape developed at the end of the last ice age when glaciers melted out. Now the lake consists of 3 submarine basins: the northern basin with a maximum depth of 903 metres, the central basin with 1637 metres at its maximum and the 1446 metres deep southern basin. The southern and the central basin are separated by the sediment delta of the Selenga River and the northern basin by a submarine mountain ridge.

The climate of the country around Baikal is distinctly continental. The Baikal causes a climate mitigation effect by its water mass. The variations between summer and winter temperatures are lower around the Baikal compared to the areas that are not influenced by the Lake. The mean annual air temperature is -1,7 °C. The water level varies seasonally about 80 cm (Bukharov et al. 2001).

Because of the low temperatures in winter the water of the Lake freezes. The duration of the ice-covered period varies on average between four and five months. The freezing starts on average in December. Complete release from ice can be expected in May (south) or June (north). Some bays freeze up to one month earlier. The thickness of the ice varies from 40 cm to 120 cm. In more shallow regions like Maloe More (Little Sea) ice thickness can grow up to 150 cm. Springs of thermal water in the lake or at the coast cause regional increases of water temperature. Ice thicknesses are lower there or the water doesn't freeze at all. In summer the water warms up very slowly. The high heat capacity of water and the great volume of the lake cause only a small increase in temperature. A surface temperature of 3 to 4 °C is maintained for a long time. Only in the shallows and in bays does it rise up to 10 to 12 °C. A water temperature up to 16 or 18 °C can be found in July and August along the shores of the southern regions of the lake, for example near the delta of Selenga River.

Water achieves its greatest density at +4 °C. This peculiarity is the main reason for the spring and summer warming of the surface layer of water down to about 300 metres. Dense water at a temperature of +4 °C sinks and mixes with cold water, gradually balancing the temperature in the surface layer. In June the lake goes through a homothermic period, which is a levelling-out of the temperature in the top 300-metre layer. During the state of homothermy, the temperature of the water in the top 300-metre layer gradually rises. In June a period of temperature stratification occurs. A thin, well-warmed surface layer (epilimnion) develops, separated form deep water (hypolimnion) by a steep temperature decrease in the metalimnion. In September, the water of the Baikal water begins to cool and in November it again goes through a state of homothermy. The hypolimnion plays a small role in temperature exchange with the atmosphere. It is characterized by a relatively even temperature, which decreases to +3,1 °C towards the bottom.

The water of the Baikal is exceptionally pure. It has a very low level of mineralisation of up to 96,6 mg/L. Several water samples were taken to investigate parameters like pH, EH, conductivity or to analyse ions and metals.
The water is defined as being of the carbonate calcium type. River water, and especially the water of Selenga influence the concentration of individual ions in the water. Some components vary by location and season. The low level of mineralisation of the Baikal water is due to the low mineralisation of the water of its tributaries. The low mineralisation of the water of the tributaries is caused by the fact that the Baikal basin is composed mostly of metamorphic and magmatic rock and not containing easily solvable minerals. The low mineralisation level is also influenced by the low temperatures in the lake.

The western region of lake Baikal contains an area with increased salinity. For instance the Tasheran lakes near Chernorud are situated in this area. The following picture shows roughly the diversity of marine and continental salinity in Russia originated by permafrost.


[Location of saline permafrost in Russia/Dubikov et al. 1988]

Although the Tasheran lakes are located there, Russian scientists don't derive the salinity of the lakes from the existence of permafrost. They think a basement consisting of clay, a high precipitation and the lack of outflow lead to an increase of the salinity (Bukharov, personal comment).

Russian scientists subdivide groundwater around Baikal depending on their origin into three groups:
1. Massive: This type occurs primarily in the magmatic and metamorphic rocks in the mountains around the lake. They are also called "fissure-type" because water is flowing in the fissures of rock.
2. Basin: Quarternary sediments serve as aquifers. Therefore water of this type exists in valleys or depressions like the Tunka depression near Arshan.
3. Karst: Karstic aquifers develop in easily soluble rock, primarily in carbonates. Such aquifers are in the west and south of lake Baikal. Two regions were investigated during the excursion: Chernorud and Arshan. The Arshan Spring is situated near Chernorud. The karstic rock there is marble. Arshan on the edge of the Tunka depression in the Sajan Mountains is also a karstic region. Investigations there were focussed on the hot springs and thermal waters.

Fissure type waters can be subdivided in three groups depending on their temperature and depth of origin:
1. The origin of these waters is faults with a depth of more than 50 km. The waters are mostly of high temperatures.
2. Faults with a depth up to 40 km provide thermal waters with a high mineralisation of 4 - 4,5 g/L.
3. The third type comprises cold waters from near-the-surface faults up to 10 km. They show low mineralisation. (Auzina, personal comment)

The genesis of thermal waters and their warming depends on the earth's heat. The geothermal gradient is an important parameter to describe these processes. The origin of the waters is still not investigated definitely. There are two theses: On the one hand it is possible that surface water (meteoric water) infiltrating the underground is heated and enriched in minerals. On the other hand juvenile, mantle origin waters enriched in Rb move as well as heated meteoric water along fissures and veins to the surface. Today most scientists believe in both sources of thermal waters. It is accepted, that 98 % of the thermal waters have meteoric origin. So 2 % are mentioned to be juvenile. To analyse the origin of the waters it is also possible to use isotopes. The use of strontium isotopes in hydrogeology is a useful tool to indicate the origin of thermal waters. Because of the enrichment of Sr from sedimentary rock, 87Sr/86Sr of meteoric waters is higher than in juvenile ones.

There is no clear definition according to which one can characterize thermal waters. First, the water's temperature should be higher than the annual temperature. In Germany waters of drillings or spas with a temperature of at least 20 °C (Hölting 1996) are called hot springs. According to Langguth (1984) the temperature of the thermal water should be higher than 35 °C and higher than the water in the surrounding area.

The thickness of the earth crust in the area of the Baikal-Rift-Zone (BRZ) is about 34 km. This is, compared to the thickness of the crust of the Siberian platform (45 km) a distinct difference. The variation of the geothermic gradient is also significant. Although it is relatively constant in the area of the Siberian platform (1.5°C / 100 m) one can observe a non linear decrease in temperature in the BRZ. Down to a depth of 1 km the temperature increases about 20°C (gradient: 2°C /100 m). Two kilometres above the ground there are 50°C and in a depth of 3 km the temperature rises about 100°C. The causes of these anomalies in temperature are not investigated well. In contradiction to this data, Bukharov (2001) mentioned a gradient which behaves more or less linear and higher: 40°C to 50°C in a depth of 1 km, 43°C to 174°C in 3 km and 60°C to 280°C 5 km below the ground. When you look at these data you can see that the range of the temperatures is quite large, which can be interpreted as a spatial dependence of the measurements. Furthermore it is easy to see that there is an overlapping of data. That's why it is almost impossible to give a general statement about the geothermic gradient. The main reason for different geothermic structures in the underground might be the great variety of faults and local variance of lithological units.


[Overview of thermal springs around Lake Baikal]

In the BRZ three types of thermal water are distinguishable according to their chemistry:

The first one are the nitrogen waters, the second there are carbon dioxide waters rich thermal waters and finally the methane waters. Every type of the waters has specific properties in temperature, discharge and degree of mineralisation. During the excursion only springs of the nitrogen type were sampled. The following map shows an overview of the hot springs sampled around Lake Baikal.


[Map of hot springs sampled]

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