Seasonal variations of natural ventilation and radon-222 exhalation in a slightly rising dead-end tunnel.

The concentration activity of radon-222 has been monitored, with some interruptions, from 1997 to 2005 in the end section of a slightly rising, dead-end, 38-m long tunnel located in the Phulchoki hill, near Kathmandu, Nepal. While a high concentration varying from 6 x 10(3) Bq m(-3) to 10 x 10(3) Bq m(-3) is observed from May to September (rainy summer season), the concentration remains at a low level of about 200 Bq m(-3) from October to March (dry winter season). This reduction of radon concentration is associated with natural ventilation of the tunnel, which, contrary to expectations for a rising tunnel, takes place mainly from October to March when the outside air temperature drops below the average tunnel temperature. This interpretation is supported by temperature measurements in the atmosphere of the tunnel, a few meters away from the entrance. The temporal variations of the diurnal amplitude of this temperature indeed follow the ventilation rate deduced from the radon measurements. In the absence of significant ventilation (summer season), the radon exhalation flux at the rock surface into the tunnel atmosphere can be inferred; it exhibits a yearly variation with additional transient reductions associated with heavy rainfall, likely to be due to water infiltration. No effect of atmospheric pressure variations on the radon concentration is observed in this tunnel. This experiment illustrates how small differences in the location and geometry of a tunnel can lead to vastly different behaviours of the radon concentration versus time. This observation has consequences for the estimation of the dose rate and the practicability of radon monitoring for tectonic purposes in underground environments.

[1]  Z. Mnich,et al.  Seasonal changes in radon concentrations in buildings in the region of northeastern Poland. , 2004, Journal of Environmental Radioactivity.

[2]  A. Tanahara,et al.  Radon distribution and the ventilation of a limestone cave on Okinawa , 1997 .

[3]  T. Przylibski.  Concentration of 226Ra in rocks of the southern part of Lower Silesia (SW Poland). , 2004, Journal of environmental radioactivity.

[4]  D. Patiris,et al.  RADON ACTIVITY LEVELS AND EFFECTIVE DOSES IN THE PERAMA CAVE, GREECE , 2004, Health physics.

[5]  P S Phillips,et al.  Radon anomalies preceding earthquakes which occurred in the UK, in summer and autumn 2002. , 2006, The Science of the total environment.

[6]  D. Gilbertson,et al.  Radon in the creswell crags Permian limestone caves. , 2002, Journal of environmental radioactivity.

[7]  J. Sabroux,et al.  Radon anomaly in the soil of Taal volcano, the Philippines: A likely precursor of the M 7.1 Mindoro earthquake (1994) , 2003 .

[8]  J. Porstendörfer.  Properties and behaviour of radon and thoron and their decay products in the air , 1994 .

[9]  The radon-222 concentration in the atmospheres of mines as a function of the barometric pressure. , 1969, Health physics.

[10]  F. Perrier,et al.  Pressure induced temperature variations in an underground quarry , 2001 .

[11]  P. Linden.  THE FLUID MECHANICS OF NATURAL VENTILATION , 1999 .

[12]  C. Papastefanou.  An overview of instrumentantion for measuring radon in soil gas and groundwaters. , 2002, Journal of environmental radioactivity.

[13]  H. Griffiths.  1990 Recommendations of the International Commission on Radiological Protection. Annals of the ICRP Publication 60 , 1992 .

[14]  M. C. Martín,et al.  Multi-day radon signals with a radioactive decay limb -- occurrence and geophysical significance. , 2006, Applied radiation and isotopes : including data, instrumentation and methods for use in agriculture, industry and medicine.

[15]  S. Sánchez-Moral,et al.  Inorganic deterioration affecting the Altamira Cave, N Spain: quantitative approach to wall-corrosion (solutional etching) processes induced by visitors. , 1999, The Science of the total environment.

[16]  Pierre Genthon,et al.  Microclimates of l'Aven d'Orgnac and other French limestone caves (Chauvet, Esparros, Marsoulas) , 2006 .

[17]  J. Stöcklin.  Geology of Nepal and its regional frame , 1980, Journal of the Geological Society.

[18]  J. Lubin,et al.  Radon measurements in underground dwellings from two prefectures in China. , 1996, Health physics.

[19]  J. Faimon,et al.  Anthropogenic CO2-flux into cave atmosphere and its environmental impact: A case study in the Císarská Cave (Moravian Karst, Czech Republic). , 2006, The Science of the total environment.

[20]  B. N. Upreti,et al.  An overview of the stratigraphy and tectonics of the Nepal Himalaya , 1999 .

[21]  M H Wilkening,et al.  Air exchange and 222Rn concentrations in the Carlsbad Caverns. , 1976, Health physics.

[22]  P. Phillips,et al.  Radon hazards, geology, and exposure of cave users: a case study and some theoretical perspectives. , 2000, Ecotoxicology and environmental safety.

[23]  P. Phillips,et al.  Radon levels in abandoned metalliferous mines, Devon, southwest England. , 2001, Ecotoxicology and environmental safety.

[24]  P. Phillips,et al.  Tidal synchronicity of built‐environment radon levels in the UK , 2006 .

[25]  T. Przylibski.  Radon concentration changes in the air of two caves in Poland , 1999 .

[26]  C. Freitas,et al.  Cave climate: Assessment of airflow and ventilation , 1982 .

[27]  L. Quindós,et al.  Natural ventilation of the Paintings Room in the Altamira cave , 1986, Nature.

[28]  W. Clements,et al.  Atmospheric pressure effects on 222Rn transport across the Earth‐air interface , 1974 .

[29]  J. Toutain,et al.  Gas geochemistry and seismotectonics: a review , 1999 .

[30]  J. Stöcklin,et al.  Geology of Kathmandu area and Central Mahabharat range : Nepal Himalaya , 1977 .

[31]  F. Perrier,et al.  Radon-222 signatures of natural ventilation regimes in an underground quarry (Journal of Environmental Radioactivity 71 (2004) 17-32) , 2004 .

[32]  M. Wilkening,et al.  Transport of radon from fractured rock , 1982 .

[33]  F. Perrier,et al.  Spatial and time variations of radon-222 concentration in the atmosphere of a dead-end horizontal tunnel. , 2004, Journal of environmental radioactivity.

[34]  F. Perrier,et al.  Modelling the effect of air exchange on 222Rn and its progeny concentration in a tunnel atmosphere. , 2005, The Science of the total environment.

[35]  Yan Wang,et al.  A study of daily and seasonal variations of radon concentrations in underground buildings. , 2006, Journal of environmental radioactivity.

[36]  F. Perrier,et al.  Radon-222 signatures of natural ventilation regimes in an underground quarry. , 2004, Journal of environmental radioactivity.

[37]  J. Carretero,et al.  222Rn concentrations, natural flow rate and the radiation exposure levels in the Nerja Cave , 1999 .

[38]  J. Avouac,et al.  Radon emanation and electric potential variations associated with transient deformation near reservoir lakes , 1999, Nature.

[39]  H. Muramatsu,et al.  Seasonal variations of 222Rn concentrations in the air of a tunnel located in Nagano city. , 2002, Journal of environmental radioactivity.