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| Titel |
Radon concentration and ventilation in two different passages in the Postojna Cave |
| VerfasserIn |
Asta Gregorič, Janja Vaupotič |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250050283
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| Zusammenfassung |
| The Postojna Cave is the biggest of about 20 show caves in Slovenia and one of the present
day’s most visited show caves in the world. Because of elevated radon concentrations in air it
has been under permanent radon survey since 1995. This cave is a typical horizontal cave
with one large natural entrance, used also as a tourist entrance. Other entrances of the 20.5
km long cave system are scattered on the surface above the cave. Long and branched out
cave system, large entrances at different levels, inflow of the Pivka river and large
variation of the outdoor air temperature and precipitation, make the Postojna Cave
a very complex climatic system in which each part shows different conditions.
The seasonal pattern of radon concentration is mainly a result of air movement
due to differences of the outdoor and cave air densities, determined mostly by air
temperature. The cave temperature is fairly constant the whole year round within 9 to 10
°C.
Radon concentration was measured continuously at two different passages of the Postojna
Cave, in the Beautiful Caves (Lepe jame) and Gay-Coloured Corridor (Pisani rov).
The first site is located in a narrow natural passage in the tourist part of the cave,
characterised by numerous cracks in the faulted rocks, which play an important role in air
ventilation. The second site is at the end of a 500 m long Gay-Coloured Corridor. This
passage deviates from the main passage and has no other connection with the cave
or outdoors. Radon was monitored from January 2006 to December 2010 in the
Beautiful Caves and from February 2010 to January 2011 in the Gay-Coloured
Galleries.
Radon concentration was in the range from 0.2 to 10 kBq/m3 and 0.4 to 40 kBq/m3 in the
Beautiful Caves and Gay-Coloured Corridor, respectively. The lowest radon concentration in
the Beautiful caves is in winter, from November to February, when warm cave air causes a
natural draught of lighter radon rich cave air from the cave upwards through vertical cracks
and channels into the outdoor atmosphere, thus allowing the entry of fresh air into the
cave through the main entrance and other lower entrances (chimney effect). On
the other hand the highest radon concentration is usually in early autumn, when
outside temperature is in the range from 10 to 25 °C and the ventilation in the cave is
minimal or zero. In summer the ventilation is reversed and the air flows from higher
to lower entrances of the cave. However this process can be noticed only when
outdoor air temperatures exceed about 25 °C. Besides seasonal variation also diurnal
variation of radon concentration may be noticed in the Beautiful Caves, usually during
spring or autumn months, when outdoor air temperature varies around 10 °C. In
winter, the ventilation regime in the Gay-Coloured Corridor is similar to that in the
Beautiful caves, causing radon concentration to decrease, whereas in summer, the
ventilation in the Gay-Coloured Corridor stops, allowing radon concentration to build up
to a constant level around 35 kBq/m3. However, a significant decrease in radon
concentration in the Gay-Coloured Corridor was noticed during an extreme flood event in
September 2010. A sudden rise of the Pivka river level presumably lead to higher air
pressure in the cave, compressed the air from the main passage into the Gay-Coloured
Corridor. |
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