Radiation Protection Dosimetry (2014), Vol. 160, No. 1–3, pp. 214 –216 Advance Access publication 17 April 2014
doi:10.1093/rpd/ncu088
ANALYSIS OF PROBLEMS AND FAILURES IN THE MEASUREMENT OF SOIL-GAS RADON CONCENTRATION Martin Neznal* and Mateˇj Neznal RADON v.o.s., Nova´kovy´ch 6, Praha 8 180 00, Czech Republic *Corresponding author: [email protected]
INTRODUCTION Measurements of radon in the soil-gas are used for a wide variety of applications. Different applications are associated with radon risk management (creation of radon potential maps, demarcation of radon-prone areas, characterisation of radon potential of building sites, characterisation of soil contaminated with 226 Ra, choice of appropriate mitigation techniques to be applied in a building, verification of applied mitigation techniques, etc.). Other applications are associated with phenomenological observation (understanding radon transport mechanisms in the soil and from the soil into the building, identification and analysis of radon entry parameters, earthquake prediction, etc.). Also the range of available techniques is large: spot, continuous or integrated measurement methods connected with active or passive soil-gas sampling. The soil-gas radon concentration may vary at the season scale and also from day to day, or even from hour to hour. It also varies in space in the horizontal as well as the vertical dimension. The variations depend on many parameters characterising the soil properties(1 – 3): (1) Geochemical parameters of soils (mainly distribution of uranium and radium in soils and rocks and their localisation influencing the radon emanation), (2) Physical parameters of all present layers of soils (grain size, permeability, porosity and effective porosity, soil moisture and water saturation, and density), (3) Geological situation (thickness of quaternary cover, weathering character of the bedrock, stratification, modification of layers by various anthropogeneous activities), (4) Soil structure (deformation and presence of cracks),
(5) Hydrological and geodynamic processes (transport of gaseous and liquid substances in porous and fractured environment, radium and radon in fissure water), (6) Geomorphological situation (location of the area in a valley, on the slopes or on the top of a hill), (7) Meteorological factors (temperature, pressure and precipitation). Observed temporal variations of soil-gas radon concentration often correspond not only to real variations of the measured physical parameter, but also to fluctuations and errors connected with sampling and measuring techniques(4). MINIMAL DEPTH OF SAMPLING Theoretically, the radon activity concentration in the soil-gas can be defined for any variable depth below the ground surface, and it generally increases with depth below the surface in an ideal homogeneous soil. But there is a minimal depth below the ground surface, at which the parameter can be really measured. The minimal depth depends on the soil properties at a given place and on the measurement method used. In particular, it depends on the volume of the soil-gas sample. When the depth below the ground surface is lower than the above-mentioned minimal depth, the soil-gas sample will be diluted with atmospheric air and the real value of radon activity concentration in the soil-gas will be underestimated. In other words, the capacity of the soil-gas is not unlimited in the soil. If a homogeneous soil environment is assumed, the minimal sampling depth can be derived using a spherical model: r¼
1=3 ð3 Vs Þ ; ð4 p n ð1 sÞÞ
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
ð1Þ
Downloaded from http://rpd.oxfordjournals.org/ at University of Michigan on October 16, 2014
Long-term experience in the field of soil-gas radon concentration measurements allows to describe and explain the most frequent causes of failures, which can appear in practice when various types of measurement methods and soil-gas sampling techniques are used. The concept of minimal sampling depth, which depends on the volume of the soil-gas sample and on the soil properties, is shown in detail. Consideration of minimal sampling depth at the time of measurement planning allows to avoid the most common mistakes. The ways how to identify influencing parameters, how to avoid a dilution of soil-gas samples by the atmospheric air, as well as how to recognise inappropriate sampling methods are discussed.
PROBLEMS AND FAILURES IN THE MEASUREMENT OF SOIL-GAS RADON CONCENTRATION Table 1. Spherical model of the minimal sampling depth.
where Vs is the volume of the soil-gas extracted from the soil during the soil-gas sampling (in cubic metre), r is the radius of the sphere of an homogeneous soil, which contains the volume of soil-gas available for extraction (in metre), s is the water saturation of soil, i.e. the part of soil pores filled with water (in %), n is the soil porosity, i.e. the ratio of the volume of soil pores and the volume of soil (in %), nef is the effective soil porosity, i.e. the ratio of the volume of soil pores filled with air and the volume of soil, nef ¼ n` (1 2 s) (in %). In the homogeneous soil environment, the minimal sampling depth should be larger than the radius r (see Figure 1). As the real soil environment is almost never homogeneous, the minimal sampling depth should be at least twice larger than the radius r under normal field conditions. Informative values of radius r calculated for different values of Vs (0.2, 1, 5, 25 l), for a typical range of water saturation s (from 45 to 90 %) and for a typical range of soil porosity n (from 25 to 40 %), are presented in Table 1. CAUSES OF FAILURES More than 20 y of experience in the field of soil-gas radon concentration measurement, which includes the participation at the international intercomparison exercises, or the organisation of such exercises(5 – 10), allows to identify and describe the most common causes of failures. Dilution of the soil-gas sample by the atmospheric air The soil-gas samples are diluted by the atmospheric air whenever the sampling system is not perfectly sealed. In general, the problem may concern both active and passive sampling systems, but sealing of
n
s
nef
r (m)
0.0002 0.0002 0.0002 0.0002 0.001 0.001 0.001 0.001 0.005 0.005 0.005 0.005 0.025 0.025 0.025 0.025
0.25 0.25 0.4 0.4 0.25 0.25 0.4 0.4 0.25 0.25 0.4 0.4 0.25 0.25 0.4 0.4
0.45 0.9 0.45 0.9 0.45 0.9 0.45 0.9 0.45 0.9 0.45 0.9 0.45 0.9 0.45 0.9
0.1375 0.025 0.22 0.04 0.1375 0.025 0.22 0.04 0.1375 0.025 0.22 0.04 0.1375 0.025 0.22 0.04
0.07 0.124 0.06 0.106 0.12 0.212 0.103 0.181 0.206 0.363 0.176 0.31 0.351 0.62 0.3 0.53
Vs, the volume of the soil-gas extracted from the soil during the soil-gas sampling; n, the soil porosity, i.e. the ratio of the volume of soil pores and the volume of soil; s, the water saturation of soil, i.e. the part of soil pores filled with water; nef, the effective soil porosity, i.e. the ratio of the volume of soil pores filled with air and the volume of soil; r, the radius of the sphere of an homogeneous soil, which contains the volume of soil-gas available for extraction.
the measuring apparatus in the soil is difficult especially in the case of passive systems. The negative influence of the dilution may change with time. Another cause of soil-gas sample dilution is associated with the minimal depth of sampling. If the real sampling depth is lower than the minimal sampling depth, the dilution of soil-gas samples by the atmospheric air cannot be avoided. Moreover, the minimal sampling depth at a given place changes with time, because it depends on the effective soil porosity, which is not constant. Therefore, the level of dilution of the samples, and thus, the underestimation of the real soilgas radon concentration values may vary with time. This is very important when temporal changes of soilgas radon concentrations are the subject of research. Note that the larger is the volume of soil-gas sample, the higher is the risk of sampling from various, shallow or deeper depths, mainly during continuous measurements or repeated measurements. Inappropriate—not entirely suitable sampling methods Some of the methods are inappropriate, or not entirely suitable for soil-gas sampling. If the sampling (measuring) apparatus—for example, small-diameter probe for active sampling, or a passive integrated radon detector, or a passive continuous radon monitor—is inserted into a previously drilled hole with a larger diameter and if the sampling
215
Downloaded from http://rpd.oxfordjournals.org/ at University of Michigan on October 16, 2014
Figure 1. Spherical model of the minimal sampling depth.
Vs (m3)
M. NEZNAL AND M. NEZNAL
Other influencing quantities The influence of all variables that can affect the measurement result should be known and corrected. Attention must be paid to the following parameters: (1) Instrumental background noise, (2) Influence of changes of soil characteristics (moisture and temperature) on the response of the measuring system located in the soil, (3) Influence of any meteorological parameter on the response of the measuring system located on the soil surface, (4) Presence of other gaseous radionuclides alphaemitters or gamma-emitters in the detection chamber, including other radon isotopes and their decay products, especially the presence of thoron (220Rn) and its decay products. CONCLUSION Anyone planning soil-gas radon concentration measurement should first examine the suitability of the method, having regard to the measurement target and to local conditions.
REFERENCES 1. Kemski, J., Klingel, R., Siehl, A., Neznal, M., Neznal, M. and Matolı´n, M. Erarbeitung fachlicher Grundlagen zur Beurteilung der Vergleichbarkeit unterschiedlicher Messmethoden zur Bestimmung der Radonbodenluftkonzentration; Bd.2 Sachstandsbericht ‘Radonmessungen in der Bodenluft – Einflussfaktoren, Messverfahren, Bewertung’. BfS Forchungsvorhaben 3609S10003. 2012, 122 (2012). 2. Neznal, M., Neznal, M., Matolı´n, M., Barnet, I. and Miksˇova´, J. The new method for assessing the radon risk of building sites, Czech Geological Survey Special Papers 16. Czech Geological Survey, Prague, 16, 48 (2004). 3. Barnet, I., Pacherova´, P., Neznal, M. and Neznal, M. Radon in Geological Environment – Czech Experience, Czech Geological Survey Special Papers 19. Czech Geological Survey, Prague, 19, 72 (2008). 4. Neznal, M., Neznal, M. and Sˇmarda, J. Assessment of radon potential of soils—a five years experience. Environ. Int. 22, S819–S828 (1996). 5. Cliff, K. D., Holub, R. F., Knutson, E. O., Lettner, H. and Solomon, S. B. International intercomparison of measurements of radon and radon decay products. Bad Gastein, Austria, September, 29–30, 1991 (National Radiological Protection Board) (1994). 6. Hutter, A. R. and Knutson, E. O. An international intercomparison of soil gas radon and radon exhalation measurements. Health Phys. 74, 108 –114 (1998). 7. Neznal, M., Neznal, M. and Sˇmarda, J. Intercomparison measurement of soil-gas radon concentration. Radiat. Prot. Dosim. 72, 139–144 (1997). 8. Neznal, M. and Neznal, M. International intercomparison measurement of soil-gas radon concentration, of radon exhalation rate from building materials and of radon exhalation rate from the ground, in Radon investigations in the Czech Republic. Barnet I., Neznal M. and Pacherova´ P. Ed. (Czech Geological Survey and RADON v.o.s), 10, 12– 22 (2004). 9. Matolı´n, M., Neznal, M. and Neznal, M. International intercomparison measurement of soil-gas radon concentration (RIM 2010), of radon exhalation rate from building materials and of radon exhalation rate from the ground. In 11th International Workshop on the Geological Aspects of Radon Risk Mapping ( proceedings). Barnet I., Neznal M. and Pacherova´ P. Ed. (Czech Geological Survey and RADON v.o.s), 166 –173 (2012). 10. Gutie´rrez-Villanueva, J. L. et al. International Intercomparison Exercise on Natural Radiation Measurements Under Field Conditions, Saelices el Chico (Spain), May 2011. (PUbliCan Editiones de la Universidad de Cantabria) (2012). 11. Tanner, A. B. A Tentative protocol for measurement of radon availability from the ground. Radiat. Prot. Dosim. 24(1/4), 79– 83 (1988).
216
Downloaded from http://rpd.oxfordjournals.org/ at University of Michigan on October 16, 2014
(measuring) system is sealed only on the top (close to the ground surface), the sampling depth is not well defined. The soil-gas coming to the sampling system (active or passive) ‘uses the easiest way’, i.e. at a given time, it comes preferably from layers with a higher permeability, or with a higher diffusion coefficient. The soil is never homogeneous, and properties of different soil layers, which affect convection and diffusion of gases in the soil, change with time depending on changing meteorological conditions. In this case, measured soil-gas radon activity concentration cannot be related to any sampling depth. Another source of errors was described by Tanner(11): passive detection of radon isotopes depends on diffusion of radon atoms from the sites of their generation to the location of the detecting or collecting device. Because some radon decays en route to a passive detector in soil, the soil-gas radon concentration measured by the detector must be less than the concentrations in those soil pores where it is undiminished by diffusion to the detector cavity. The real soil-gas radon concentration may be significantly underestimated especially in moist soils, or when large detector cavities are used. As the soil moisture usually changes during a 1-y cycle, a degree of underestimating may change with time, again.
doi:10.1093/rpd/ncu088
ANALYSIS OF PROBLEMS AND FAILURES IN THE MEASUREMENT OF SOIL-GAS RADON CONCENTRATION Martin Neznal* and Mateˇj Neznal RADON v.o.s., Nova´kovy´ch 6, Praha 8 180 00, Czech Republic *Corresponding author: [email protected]
INTRODUCTION Measurements of radon in the soil-gas are used for a wide variety of applications. Different applications are associated with radon risk management (creation of radon potential maps, demarcation of radon-prone areas, characterisation of radon potential of building sites, characterisation of soil contaminated with 226 Ra, choice of appropriate mitigation techniques to be applied in a building, verification of applied mitigation techniques, etc.). Other applications are associated with phenomenological observation (understanding radon transport mechanisms in the soil and from the soil into the building, identification and analysis of radon entry parameters, earthquake prediction, etc.). Also the range of available techniques is large: spot, continuous or integrated measurement methods connected with active or passive soil-gas sampling. The soil-gas radon concentration may vary at the season scale and also from day to day, or even from hour to hour. It also varies in space in the horizontal as well as the vertical dimension. The variations depend on many parameters characterising the soil properties(1 – 3): (1) Geochemical parameters of soils (mainly distribution of uranium and radium in soils and rocks and their localisation influencing the radon emanation), (2) Physical parameters of all present layers of soils (grain size, permeability, porosity and effective porosity, soil moisture and water saturation, and density), (3) Geological situation (thickness of quaternary cover, weathering character of the bedrock, stratification, modification of layers by various anthropogeneous activities), (4) Soil structure (deformation and presence of cracks),
(5) Hydrological and geodynamic processes (transport of gaseous and liquid substances in porous and fractured environment, radium and radon in fissure water), (6) Geomorphological situation (location of the area in a valley, on the slopes or on the top of a hill), (7) Meteorological factors (temperature, pressure and precipitation). Observed temporal variations of soil-gas radon concentration often correspond not only to real variations of the measured physical parameter, but also to fluctuations and errors connected with sampling and measuring techniques(4). MINIMAL DEPTH OF SAMPLING Theoretically, the radon activity concentration in the soil-gas can be defined for any variable depth below the ground surface, and it generally increases with depth below the surface in an ideal homogeneous soil. But there is a minimal depth below the ground surface, at which the parameter can be really measured. The minimal depth depends on the soil properties at a given place and on the measurement method used. In particular, it depends on the volume of the soil-gas sample. When the depth below the ground surface is lower than the above-mentioned minimal depth, the soil-gas sample will be diluted with atmospheric air and the real value of radon activity concentration in the soil-gas will be underestimated. In other words, the capacity of the soil-gas is not unlimited in the soil. If a homogeneous soil environment is assumed, the minimal sampling depth can be derived using a spherical model: r¼
1=3 ð3 Vs Þ ; ð4 p n ð1 sÞÞ
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
ð1Þ
Downloaded from http://rpd.oxfordjournals.org/ at University of Michigan on October 16, 2014
Long-term experience in the field of soil-gas radon concentration measurements allows to describe and explain the most frequent causes of failures, which can appear in practice when various types of measurement methods and soil-gas sampling techniques are used. The concept of minimal sampling depth, which depends on the volume of the soil-gas sample and on the soil properties, is shown in detail. Consideration of minimal sampling depth at the time of measurement planning allows to avoid the most common mistakes. The ways how to identify influencing parameters, how to avoid a dilution of soil-gas samples by the atmospheric air, as well as how to recognise inappropriate sampling methods are discussed.
PROBLEMS AND FAILURES IN THE MEASUREMENT OF SOIL-GAS RADON CONCENTRATION Table 1. Spherical model of the minimal sampling depth.
where Vs is the volume of the soil-gas extracted from the soil during the soil-gas sampling (in cubic metre), r is the radius of the sphere of an homogeneous soil, which contains the volume of soil-gas available for extraction (in metre), s is the water saturation of soil, i.e. the part of soil pores filled with water (in %), n is the soil porosity, i.e. the ratio of the volume of soil pores and the volume of soil (in %), nef is the effective soil porosity, i.e. the ratio of the volume of soil pores filled with air and the volume of soil, nef ¼ n` (1 2 s) (in %). In the homogeneous soil environment, the minimal sampling depth should be larger than the radius r (see Figure 1). As the real soil environment is almost never homogeneous, the minimal sampling depth should be at least twice larger than the radius r under normal field conditions. Informative values of radius r calculated for different values of Vs (0.2, 1, 5, 25 l), for a typical range of water saturation s (from 45 to 90 %) and for a typical range of soil porosity n (from 25 to 40 %), are presented in Table 1. CAUSES OF FAILURES More than 20 y of experience in the field of soil-gas radon concentration measurement, which includes the participation at the international intercomparison exercises, or the organisation of such exercises(5 – 10), allows to identify and describe the most common causes of failures. Dilution of the soil-gas sample by the atmospheric air The soil-gas samples are diluted by the atmospheric air whenever the sampling system is not perfectly sealed. In general, the problem may concern both active and passive sampling systems, but sealing of
n
s
nef
r (m)
0.0002 0.0002 0.0002 0.0002 0.001 0.001 0.001 0.001 0.005 0.005 0.005 0.005 0.025 0.025 0.025 0.025
0.25 0.25 0.4 0.4 0.25 0.25 0.4 0.4 0.25 0.25 0.4 0.4 0.25 0.25 0.4 0.4
0.45 0.9 0.45 0.9 0.45 0.9 0.45 0.9 0.45 0.9 0.45 0.9 0.45 0.9 0.45 0.9
0.1375 0.025 0.22 0.04 0.1375 0.025 0.22 0.04 0.1375 0.025 0.22 0.04 0.1375 0.025 0.22 0.04
0.07 0.124 0.06 0.106 0.12 0.212 0.103 0.181 0.206 0.363 0.176 0.31 0.351 0.62 0.3 0.53
Vs, the volume of the soil-gas extracted from the soil during the soil-gas sampling; n, the soil porosity, i.e. the ratio of the volume of soil pores and the volume of soil; s, the water saturation of soil, i.e. the part of soil pores filled with water; nef, the effective soil porosity, i.e. the ratio of the volume of soil pores filled with air and the volume of soil; r, the radius of the sphere of an homogeneous soil, which contains the volume of soil-gas available for extraction.
the measuring apparatus in the soil is difficult especially in the case of passive systems. The negative influence of the dilution may change with time. Another cause of soil-gas sample dilution is associated with the minimal depth of sampling. If the real sampling depth is lower than the minimal sampling depth, the dilution of soil-gas samples by the atmospheric air cannot be avoided. Moreover, the minimal sampling depth at a given place changes with time, because it depends on the effective soil porosity, which is not constant. Therefore, the level of dilution of the samples, and thus, the underestimation of the real soilgas radon concentration values may vary with time. This is very important when temporal changes of soilgas radon concentrations are the subject of research. Note that the larger is the volume of soil-gas sample, the higher is the risk of sampling from various, shallow or deeper depths, mainly during continuous measurements or repeated measurements. Inappropriate—not entirely suitable sampling methods Some of the methods are inappropriate, or not entirely suitable for soil-gas sampling. If the sampling (measuring) apparatus—for example, small-diameter probe for active sampling, or a passive integrated radon detector, or a passive continuous radon monitor—is inserted into a previously drilled hole with a larger diameter and if the sampling
215
Downloaded from http://rpd.oxfordjournals.org/ at University of Michigan on October 16, 2014
Figure 1. Spherical model of the minimal sampling depth.
Vs (m3)
M. NEZNAL AND M. NEZNAL
Other influencing quantities The influence of all variables that can affect the measurement result should be known and corrected. Attention must be paid to the following parameters: (1) Instrumental background noise, (2) Influence of changes of soil characteristics (moisture and temperature) on the response of the measuring system located in the soil, (3) Influence of any meteorological parameter on the response of the measuring system located on the soil surface, (4) Presence of other gaseous radionuclides alphaemitters or gamma-emitters in the detection chamber, including other radon isotopes and their decay products, especially the presence of thoron (220Rn) and its decay products. CONCLUSION Anyone planning soil-gas radon concentration measurement should first examine the suitability of the method, having regard to the measurement target and to local conditions.
REFERENCES 1. Kemski, J., Klingel, R., Siehl, A., Neznal, M., Neznal, M. and Matolı´n, M. Erarbeitung fachlicher Grundlagen zur Beurteilung der Vergleichbarkeit unterschiedlicher Messmethoden zur Bestimmung der Radonbodenluftkonzentration; Bd.2 Sachstandsbericht ‘Radonmessungen in der Bodenluft – Einflussfaktoren, Messverfahren, Bewertung’. BfS Forchungsvorhaben 3609S10003. 2012, 122 (2012). 2. Neznal, M., Neznal, M., Matolı´n, M., Barnet, I. and Miksˇova´, J. The new method for assessing the radon risk of building sites, Czech Geological Survey Special Papers 16. Czech Geological Survey, Prague, 16, 48 (2004). 3. Barnet, I., Pacherova´, P., Neznal, M. and Neznal, M. Radon in Geological Environment – Czech Experience, Czech Geological Survey Special Papers 19. Czech Geological Survey, Prague, 19, 72 (2008). 4. Neznal, M., Neznal, M. and Sˇmarda, J. Assessment of radon potential of soils—a five years experience. Environ. Int. 22, S819–S828 (1996). 5. Cliff, K. D., Holub, R. F., Knutson, E. O., Lettner, H. and Solomon, S. B. International intercomparison of measurements of radon and radon decay products. Bad Gastein, Austria, September, 29–30, 1991 (National Radiological Protection Board) (1994). 6. Hutter, A. R. and Knutson, E. O. An international intercomparison of soil gas radon and radon exhalation measurements. Health Phys. 74, 108 –114 (1998). 7. Neznal, M., Neznal, M. and Sˇmarda, J. Intercomparison measurement of soil-gas radon concentration. Radiat. Prot. Dosim. 72, 139–144 (1997). 8. Neznal, M. and Neznal, M. International intercomparison measurement of soil-gas radon concentration, of radon exhalation rate from building materials and of radon exhalation rate from the ground, in Radon investigations in the Czech Republic. Barnet I., Neznal M. and Pacherova´ P. Ed. (Czech Geological Survey and RADON v.o.s), 10, 12– 22 (2004). 9. Matolı´n, M., Neznal, M. and Neznal, M. International intercomparison measurement of soil-gas radon concentration (RIM 2010), of radon exhalation rate from building materials and of radon exhalation rate from the ground. In 11th International Workshop on the Geological Aspects of Radon Risk Mapping ( proceedings). Barnet I., Neznal M. and Pacherova´ P. Ed. (Czech Geological Survey and RADON v.o.s), 166 –173 (2012). 10. Gutie´rrez-Villanueva, J. L. et al. International Intercomparison Exercise on Natural Radiation Measurements Under Field Conditions, Saelices el Chico (Spain), May 2011. (PUbliCan Editiones de la Universidad de Cantabria) (2012). 11. Tanner, A. B. A Tentative protocol for measurement of radon availability from the ground. Radiat. Prot. Dosim. 24(1/4), 79– 83 (1988).
216
Downloaded from http://rpd.oxfordjournals.org/ at University of Michigan on October 16, 2014
(measuring) system is sealed only on the top (close to the ground surface), the sampling depth is not well defined. The soil-gas coming to the sampling system (active or passive) ‘uses the easiest way’, i.e. at a given time, it comes preferably from layers with a higher permeability, or with a higher diffusion coefficient. The soil is never homogeneous, and properties of different soil layers, which affect convection and diffusion of gases in the soil, change with time depending on changing meteorological conditions. In this case, measured soil-gas radon activity concentration cannot be related to any sampling depth. Another source of errors was described by Tanner(11): passive detection of radon isotopes depends on diffusion of radon atoms from the sites of their generation to the location of the detecting or collecting device. Because some radon decays en route to a passive detector in soil, the soil-gas radon concentration measured by the detector must be less than the concentrations in those soil pores where it is undiminished by diffusion to the detector cavity. The real soil-gas radon concentration may be significantly underestimated especially in moist soils, or when large detector cavities are used. As the soil moisture usually changes during a 1-y cycle, a degree of underestimating may change with time, again.
