Carbon dioxide in Postojna Cave (Slovenia): spatial distribution,seasonal dynamics and evaluation of plausible sources and sinks |
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| Authors: | Mitja Prelovšek Stanka Šebela Janez Turk |
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| Institution: | 1.Karst Research Institute ZRC SAZU,Postojna,Slovenia;2.Slovenian National Building and Civil Engineering Institute,Ljubljana,Slovenia |
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| Abstract: | The CO2 concentration of the air in Postojna Cave (400–7900 ppm) is found to be induced by CO2 sources (human respiration contributing?~?20,000–58,000 ppm per breath, outgassing of dripwater and water seeping from the vadose zone/epikarst with a pCO2 values of 5000–29,000 ppm, and underground Pivka River having pCO2 at 2344–4266 ppm) and CO2 dilution (inflow of outside air with a CO2 concentration of?~?400 ppm). Measurements show that sinking Pivka River has the lowest CO2 concentration among plausible CO2 sources but still continuously exceeds the surrounding cave air CO2 concentration. During the winter months, intensive ventilation reduces the cave air CO2 concentration to outside levels (~?400 ppm), even in the centre of the cave system. CO2 dilution is less pronounced in summer (CO2(min)?≈?800 ppm), since the ventilation rate is not as strong as in winter and the outside air that enters the cave through breathing holes and fractures is enriched with soil CO2. During spring and autumn, the daily alternation of the ventilation regime with a smaller rate of air exchange results in yearly cave air CO2 peaks of up to?~?2400 ppm. Some dead-end passages can be much less affected by ventilation, resulting in a cave air CO2 concentration of up to 7900 ppm. The strongest diurnal CO2 peaks due to human respiration were recorded during the spring holidays (increase of up to 1300 ppm day?1), compared to considerably smaller summer peaks despite peak visits (increase of?~?600 ppm day?1). |
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