Radiation Protection Dosimetry (2014), Vol. 160, No. 1–3, pp. 222 –225 Advance Access publication 8 April 2014
doi:10.1093/rpd/ncu087
DAILY AND SEASONALVARIATIONS IN RADON ACTIVITY CONCENTRATION IN THE SOIL AIR Monika Mu´´llerova´*, Karol Holy´ and Martin Bulko Department of Nuclear Physics and Biophysics, Faculty of Mathematics, Physics and Informatics, Comenius University, Mlynska´ dolina F-1, 841 04 Bratislava, Slovak Republic *Corresponding author: [email protected]
INTRODUCTION 222
The interest about Rn in the soil air is connected with the fact that it represents a significant source of radon in the indoor atmosphere. Therefore, it is important, before the beginning of the building process, to objectively classify the risk related to soil radon. The interest in radon is also connected with the possibility of its use as the earthquake precursor(1, 2) (here it is important to eliminate the influence of atmospheric pressure and possibly other meteorological parameters, so that this effect could easily be identifiable) and its use as a tracer of the transport of other gases in the soil (CO2, CH4)(3). The knowledge of volume activity variations in 222 Rn is very important for all these applications. SAMPLING PLACE AND SOIL CHARACTERISTICS 222 Rn activity concentration in the soil air has been continuously monitored since 1994. The sampling place is situated in the area of Faculty of Mathematics, Physics and Informatics in Bratislava. The soil of this place is moderately permeable. The average specific activity of 226Ra up to a depth of 1.5 m is equal to 37.5 Bq kg21 in this soil. The emanation coefficient of 222Rn in the surface soil is 14.5 % for the weight content of soil moisture ranging from 5 to 20 %.
METHODS A scintillation cell of Lucas type with a sensitive volume of 125 ml has been used for monitoring 222Rn. The air was sucked from a depth of 0.8 m and dried in the refrigerator (2308C) before entering the Lucas cell. The number of detector counts was recorded every 30 min. 222Rn activity concentration has been calculated
according to the method published by Ward and Borak(4), by the use of the data corresponding to the time interval of 1 h. The monitoring allows us to measure radon activity concentration in the soil air at the level of 10 kBq m23 with a relative error of 1.5 % for 1-h-long measuring interval. This precision is also sufficient for the study of daily radon variations in the soil air. RESULTS AND DISCUSSION The average value of 222Rn activity concentration at a depth of 0.8 m determined from the 19-y-long measurements is 11.6 kBq m23 and the average monthly values vary from 8.0 to 23.4 kBq m23. However, the range of 222Rn concentrations measured in 1-h interval is even greater, from 6.4 to 26.1 kBq m23. Daily and seasonal variations in radon activity concentration were found in the data. Radon measurements were realised at a depth of 0.4 m during 5 months. In this period, a strong influence of precipitation on radon activity in the soil air was observed. Daily variations in radon activity concentration in the soil air at a depth of 0.8 m could be considered as short-term changes from radon half-life point of view. The analysis of the data confirms that regular daily changes in radon activity concentration in the soil air are connected with daily changes in atmospheric pressure. An increase in atmospheric pressure leads to a decrease in radon activity concentration in the nearsurface layer of the soil air, and vice versa (Fig. 1). However, daily changes in atmospheric pressure depend on daily changes of temperature. Therefore, the relation between the daily variations in radon activity concentration in the soil air and daily changes in outdoor temperature was studied. For the purposes of the analysis, it is sufficient to consider the outdoor
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on July 10, 2015
Radon activity concentration in the soil air in the area of Faculty of Mathematics, Physics and Informatics (FMPI) in Bratislava, Slovak Republic, has been continuously monitored since 1994. Long-term measurements at a depth of 0.8 m and short-term measurements at a depth of 0.4 m show a high variability in radon activity concentrations in the soil. The analysis of the data confirms that regular daily changes in radon activity concentration in the soil air depend on the daily changes in atmospheric pressure. It was also found that the typical annual courses of the radon activity concentration in the soil air (with summer minima and winter maxima) were disturbed by mild winter and heavy summer precipitation. Influence of precipitation on the increase in the radon activity concentration in the soil air was observed at a depth of 0.4 m and subsequently at a depth of 0.8 m.
VARIATIONS OF RADON CONCENTRATION IN THE SOIL AIR
temperature because the correlation coefficient between average daily outdoor temperature and soil temperature at a depth of 0.2 m is R 2 0.87. The
Figure 2. Average daily courses of the radon activity concentration in the soil air and of temperature.
Figure 3. Average daily values of the radon activity concentration in the soil air and temperature of the outdoor atmosphere in 2007.
223
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Figure 1. Average daily courses of atmospheric pressure and radon activity concentrations in the soil air.
average daily course of radon activity concentration and temperature for year 2009 is shown in the Fig. 2. For this year there were very good agreement between the average daily courses of the radon activity concentration in the soil air and the temperature with correlation coefficient R 2 0.83. The shape of annual courses of radon activity concentration in the soil air depends mainly on the soil type and on the temperature variations of the outdoor atmosphere during a year, on precipitations and on soil moisture. During the years with dry summers and cold winters, the anti-correlation between radon activity concentration in the soil and atmospheric temperature was observed (Fig. 3). From Fig. 4 it can be seen that typical annual courses of radon activity concentration in the soil air (with summer minima and winter maxima) were disturbed by mild winters. For the soil at the measurement site, a significant influence of precipitation on radon activity concentration in the soil air is typical. For example, as a consequence of an incessant rain in 2006 lasting for several days, noticeable peaks in radon activity concentration in the soil air were recorded, as can be seen in Fig. 5. Figure 5 explains the origin of peak values of radon activity concentration during the summer months. The year 2010 was the rainiest since 1881(5). During 2010, the radon activity concentration was measured at a depth of 0.4 m as well. The time lag between heavy precipitation and the increase in RAC is approximately 1 d and can be clearly seen in Fig. 6. The increases in radon activity concentration are due to larger amount of precipitation (21.7 mm and 47.6 mm). On 1 September 2010 heavy rain (25.9 mm) caused that the soil became saturated with water and even a smaller amount of precipitation (21.7 mm) resulted in a significant increase in radon activity
M.MU´´ LLEROVA´ ET AL.
Figure 5. Influence of long-lasting heavy rain on radon activity concentration in the soil air in 2006.
Figure 6. Influence of rain on the radon activity concentration in the soil air at a depth of 0.4 m in 2010.
224
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Figure 4. Average daily values of the radon activity concentration in the soil air and temperature of the outdoor atmosphere in 2011.
VARIATIONS OF RADON CONCENTRATION IN THE SOIL AIR
concentration. This is the reason why a peak in radon activity concentration was recorded on 9 September 2010—it was likely a result of precipitation that happened a day earlier, on 8 September 2010. The second peak is of different nature. This peak was preceded by a dry period starting from 10 September 2010. This second peak is a consequence of extremely heavy precipitation that fell on 25 September 2010 (as much as 47.6 mm), and that were immediately followed by the increase of radon activity concentration. The increase in the radon activity concentration at a depth of 0.8 m was observed in approximately five days after precipitation. For example the increase in the radon activity concentration on 13 September 2010 after the precipitation that occurred on 8 September 2010, or the increase in the radon activity concentration on 30 September 2010 in the afternoon caused by precipitation that occurred on 25 September 2010 at night (Fig. 7). CONCLUSION The results of long-term continual monitoring of radon activity concentration in the soil air at a depth of 0.8 m show a great variability, with daily and seasonal variations. For the soil type at the measurement site, it is possible to observe a significant influence of precipitation on radon activity concentration. The influence of precipitation is strongest in the lower layers of the soil (0.4 m). It was also found that the time lag between precipitation and increased radon activity
concentration was dependent on the depth at which radon was measured.
FUNDING This study was funded by the Scientific Grant Agency (VEGA) of the Ministry of Education of the Slovak Republic (Grants No 1/0678/09 and No 1/0143/14). REFERENCES 1. Igarashi, G., Saeki, S., Takahata, N., Sumikawa, K., Tasaka, S., Sasaki, Y., Takahashi, M. and Sano, Y. Ground-water radon anomaly before the Kobe earthquake in Japan. Science, New Series 269, 60–61 (1995). 2. Weinlich, F. H., Faber, E., Bousˇkova´, A., Hora´lek, J., Teschner, M. and Poggenburg, J. Seismically induced variations in Maria´nske´ La´zne´ fault gas composition in the NW Bohemian swarm quake region, Czech Republic—a continuous gas monitoring. Tectonophysics 421, 89– 110 (2006). 3. Do¨rr, H. and Mu¨nnich, K. O. 222Rn flux and soil air concentration profiles in West-Germany. Soil 222Rn as tracer for gas transport in the unsaturated soil zone. Tellus. 42B, 20–28 (1990). 4. Ward, D. C. and Borak, T. B. Determination of time varying 222Rn concentrations using flow-through scintillation flasks. Health Phys 61, 799– 807 (1991). 5. Lapin, M. Dlhodoby´ rezˇim u´hrnov atmosfe´ricky´ch zra´zˇok na Slovensku. [online]. Slovak Hydrometeorological Institute (2011) http://www.dmc.fmph.uniba.sk/public_html/climate/ RHurbanovo.htm.
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Figure 7. Influence of rain on the radon activity concentration in the soil air at a depth of 0.8 m in 2010.
doi:10.1093/rpd/ncu087
DAILY AND SEASONALVARIATIONS IN RADON ACTIVITY CONCENTRATION IN THE SOIL AIR Monika Mu´´llerova´*, Karol Holy´ and Martin Bulko Department of Nuclear Physics and Biophysics, Faculty of Mathematics, Physics and Informatics, Comenius University, Mlynska´ dolina F-1, 841 04 Bratislava, Slovak Republic *Corresponding author: [email protected]
INTRODUCTION 222
The interest about Rn in the soil air is connected with the fact that it represents a significant source of radon in the indoor atmosphere. Therefore, it is important, before the beginning of the building process, to objectively classify the risk related to soil radon. The interest in radon is also connected with the possibility of its use as the earthquake precursor(1, 2) (here it is important to eliminate the influence of atmospheric pressure and possibly other meteorological parameters, so that this effect could easily be identifiable) and its use as a tracer of the transport of other gases in the soil (CO2, CH4)(3). The knowledge of volume activity variations in 222 Rn is very important for all these applications. SAMPLING PLACE AND SOIL CHARACTERISTICS 222 Rn activity concentration in the soil air has been continuously monitored since 1994. The sampling place is situated in the area of Faculty of Mathematics, Physics and Informatics in Bratislava. The soil of this place is moderately permeable. The average specific activity of 226Ra up to a depth of 1.5 m is equal to 37.5 Bq kg21 in this soil. The emanation coefficient of 222Rn in the surface soil is 14.5 % for the weight content of soil moisture ranging from 5 to 20 %.
METHODS A scintillation cell of Lucas type with a sensitive volume of 125 ml has been used for monitoring 222Rn. The air was sucked from a depth of 0.8 m and dried in the refrigerator (2308C) before entering the Lucas cell. The number of detector counts was recorded every 30 min. 222Rn activity concentration has been calculated
according to the method published by Ward and Borak(4), by the use of the data corresponding to the time interval of 1 h. The monitoring allows us to measure radon activity concentration in the soil air at the level of 10 kBq m23 with a relative error of 1.5 % for 1-h-long measuring interval. This precision is also sufficient for the study of daily radon variations in the soil air. RESULTS AND DISCUSSION The average value of 222Rn activity concentration at a depth of 0.8 m determined from the 19-y-long measurements is 11.6 kBq m23 and the average monthly values vary from 8.0 to 23.4 kBq m23. However, the range of 222Rn concentrations measured in 1-h interval is even greater, from 6.4 to 26.1 kBq m23. Daily and seasonal variations in radon activity concentration were found in the data. Radon measurements were realised at a depth of 0.4 m during 5 months. In this period, a strong influence of precipitation on radon activity in the soil air was observed. Daily variations in radon activity concentration in the soil air at a depth of 0.8 m could be considered as short-term changes from radon half-life point of view. The analysis of the data confirms that regular daily changes in radon activity concentration in the soil air are connected with daily changes in atmospheric pressure. An increase in atmospheric pressure leads to a decrease in radon activity concentration in the nearsurface layer of the soil air, and vice versa (Fig. 1). However, daily changes in atmospheric pressure depend on daily changes of temperature. Therefore, the relation between the daily variations in radon activity concentration in the soil air and daily changes in outdoor temperature was studied. For the purposes of the analysis, it is sufficient to consider the outdoor
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on July 10, 2015
Radon activity concentration in the soil air in the area of Faculty of Mathematics, Physics and Informatics (FMPI) in Bratislava, Slovak Republic, has been continuously monitored since 1994. Long-term measurements at a depth of 0.8 m and short-term measurements at a depth of 0.4 m show a high variability in radon activity concentrations in the soil. The analysis of the data confirms that regular daily changes in radon activity concentration in the soil air depend on the daily changes in atmospheric pressure. It was also found that the typical annual courses of the radon activity concentration in the soil air (with summer minima and winter maxima) were disturbed by mild winter and heavy summer precipitation. Influence of precipitation on the increase in the radon activity concentration in the soil air was observed at a depth of 0.4 m and subsequently at a depth of 0.8 m.
VARIATIONS OF RADON CONCENTRATION IN THE SOIL AIR
temperature because the correlation coefficient between average daily outdoor temperature and soil temperature at a depth of 0.2 m is R 2 0.87. The
Figure 2. Average daily courses of the radon activity concentration in the soil air and of temperature.
Figure 3. Average daily values of the radon activity concentration in the soil air and temperature of the outdoor atmosphere in 2007.
223
Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on July 10, 2015
Figure 1. Average daily courses of atmospheric pressure and radon activity concentrations in the soil air.
average daily course of radon activity concentration and temperature for year 2009 is shown in the Fig. 2. For this year there were very good agreement between the average daily courses of the radon activity concentration in the soil air and the temperature with correlation coefficient R 2 0.83. The shape of annual courses of radon activity concentration in the soil air depends mainly on the soil type and on the temperature variations of the outdoor atmosphere during a year, on precipitations and on soil moisture. During the years with dry summers and cold winters, the anti-correlation between radon activity concentration in the soil and atmospheric temperature was observed (Fig. 3). From Fig. 4 it can be seen that typical annual courses of radon activity concentration in the soil air (with summer minima and winter maxima) were disturbed by mild winters. For the soil at the measurement site, a significant influence of precipitation on radon activity concentration in the soil air is typical. For example, as a consequence of an incessant rain in 2006 lasting for several days, noticeable peaks in radon activity concentration in the soil air were recorded, as can be seen in Fig. 5. Figure 5 explains the origin of peak values of radon activity concentration during the summer months. The year 2010 was the rainiest since 1881(5). During 2010, the radon activity concentration was measured at a depth of 0.4 m as well. The time lag between heavy precipitation and the increase in RAC is approximately 1 d and can be clearly seen in Fig. 6. The increases in radon activity concentration are due to larger amount of precipitation (21.7 mm and 47.6 mm). On 1 September 2010 heavy rain (25.9 mm) caused that the soil became saturated with water and even a smaller amount of precipitation (21.7 mm) resulted in a significant increase in radon activity
M.MU´´ LLEROVA´ ET AL.
Figure 5. Influence of long-lasting heavy rain on radon activity concentration in the soil air in 2006.
Figure 6. Influence of rain on the radon activity concentration in the soil air at a depth of 0.4 m in 2010.
224
Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on July 10, 2015
Figure 4. Average daily values of the radon activity concentration in the soil air and temperature of the outdoor atmosphere in 2011.
VARIATIONS OF RADON CONCENTRATION IN THE SOIL AIR
concentration. This is the reason why a peak in radon activity concentration was recorded on 9 September 2010—it was likely a result of precipitation that happened a day earlier, on 8 September 2010. The second peak is of different nature. This peak was preceded by a dry period starting from 10 September 2010. This second peak is a consequence of extremely heavy precipitation that fell on 25 September 2010 (as much as 47.6 mm), and that were immediately followed by the increase of radon activity concentration. The increase in the radon activity concentration at a depth of 0.8 m was observed in approximately five days after precipitation. For example the increase in the radon activity concentration on 13 September 2010 after the precipitation that occurred on 8 September 2010, or the increase in the radon activity concentration on 30 September 2010 in the afternoon caused by precipitation that occurred on 25 September 2010 at night (Fig. 7). CONCLUSION The results of long-term continual monitoring of radon activity concentration in the soil air at a depth of 0.8 m show a great variability, with daily and seasonal variations. For the soil type at the measurement site, it is possible to observe a significant influence of precipitation on radon activity concentration. The influence of precipitation is strongest in the lower layers of the soil (0.4 m). It was also found that the time lag between precipitation and increased radon activity
concentration was dependent on the depth at which radon was measured.
FUNDING This study was funded by the Scientific Grant Agency (VEGA) of the Ministry of Education of the Slovak Republic (Grants No 1/0678/09 and No 1/0143/14). REFERENCES 1. Igarashi, G., Saeki, S., Takahata, N., Sumikawa, K., Tasaka, S., Sasaki, Y., Takahashi, M. and Sano, Y. Ground-water radon anomaly before the Kobe earthquake in Japan. Science, New Series 269, 60–61 (1995). 2. Weinlich, F. H., Faber, E., Bousˇkova´, A., Hora´lek, J., Teschner, M. and Poggenburg, J. Seismically induced variations in Maria´nske´ La´zne´ fault gas composition in the NW Bohemian swarm quake region, Czech Republic—a continuous gas monitoring. Tectonophysics 421, 89– 110 (2006). 3. Do¨rr, H. and Mu¨nnich, K. O. 222Rn flux and soil air concentration profiles in West-Germany. Soil 222Rn as tracer for gas transport in the unsaturated soil zone. Tellus. 42B, 20–28 (1990). 4. Ward, D. C. and Borak, T. B. Determination of time varying 222Rn concentrations using flow-through scintillation flasks. Health Phys 61, 799– 807 (1991). 5. Lapin, M. Dlhodoby´ rezˇim u´hrnov atmosfe´ricky´ch zra´zˇok na Slovensku. [online]. Slovak Hydrometeorological Institute (2011) http://www.dmc.fmph.uniba.sk/public_html/climate/ RHurbanovo.htm.
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Figure 7. Influence of rain on the radon activity concentration in the soil air at a depth of 0.8 m in 2010.
