- Email: [email protected]
Study of the effects of atmospheric parameters on ground radon concentration by track technique
Nucl.
Tracks
ht. .I. Radiat.
Radial. Appl.
Meas,
0191-278X/88
Vol. 14, No. 4, PP. 457-460, 1988
Instrum.,Part
$3.00
+ 0.00
Pergamon Press plc
D
Printed in Great Britain
STUDY OF THE EFFECTS OF ATMOSPHERIC PARAMETERS ON GROUND RADON CONCENTRATION BY TRACK TECHNIQUE ADAMS TIDJANI
Faculte des Sciences, Wpartement
de Physique, Laboratoire des D.S.T.N., Universite de Dakar, Dakar-Fann, Senegal
(Received 15 July 1987; in revisedform 21
March
1988)
Abstract-Radon emanation was continuously monitored for 24 months, accompanied by measurements of atmospheric parameters. Integrated measurements of radon concentrations have been performed with LR-115 cellulose nitrate track detectors. The monitoring was conducted at 16 observation sites distributed on the Dakar University area. Observed changes in radon concentration are interpreted to be caused by
changes in meteorological conditions and ocean tides. 1. INTRODUCTION IN RECENTyears, many studies of radon gas migration in the soil, using different techniques, have been made to address geophysical questions (Fleischer and Mogro-Campero, 1978; Cox et al., 1980; Seidel and Monnin, 1982). However, it has been found that the radon levels in soils can be strongly influenced by the effects of atmospheric parameters. Fluctuation of radon emanation produced by changes in atmospheric parameters has been the subject of numerous investigations, with conflicting results in some cases: for example Fleischer et al. (1980) did not find correlation of high precipitation with either high or low radon readings, contrary to Wakita (1985) who found that normal precipitation does not greatly affect the radon concentration but that a succession of heavy rainfall occasionally causes an increase in radon concentration. For radon concentration changes caused by atmospheric pressure, many authors (e.g. Kraner et al., 1964; Clements and Wilkening, 1974; Schery and Gaeddert, 1982) found that radon concentrations fluctuate inversely with changes in atmospheric pressure. The studies involving atmospheric pressure indicate that changes of the order of 1% may be responsible for variations in the radon flux of as much as 30-90%. Unfortunately, complete studies of effects of atmospheric parameters are very rare. In this paper, we present the results obtained in Dakar, Senegal, on investigations into the effects of several atmospheric parameters on gas migration in the soil, using the well known track-detector technique. Other preliminary data on effects of ocean tides on radon emanometry are reported.
2. EXPERIMENTAL
Camper0 (1978), with some modifications. Two inverted plastic cups were used; the lower one contains a piece of cellulose nitrate on the underside of its top. To prevent water condensation on the detectors, an insulator was placed between the two plastic cups and an anti-220Rn membrane placed at the bottom of the cups, according to the method of Likes et al. (1979); this is an effective way to avoid the temperature of the detector dropping below dew point. In addition, the permeable membrane (PVC foil, 15 pm thick), by delaying the progress of the radon isotopes towards the detection volume beneath the detector, separates the unwanted contribution of thoron gas to the radon track counting (Fleischer and Mogro-Campero, 1978). The experimental apparatus was buried at a depth of 4Ocm, where the detector records alpha decays from 222Rn and its daughters throughout a burial time of one month. The track density of the detector is found to be proportional to the actual concentration of radon and its daughter products in the soil. 2.2. Track counting After field exposure, the detector (LR-115 type II) is chemically etched to enlarge the paths of the alpha particles produced by the decay of radon and its daughters. The etching was performed in a 2.5 M NaOH solution at 60°C during a time previously established to reduce the 12 pm original thickness of the detecting foil to about 6.5 pm. Before counting the detectors with the jumping spark counter (see, e.g., Durrani and Bull, 1987), the thickness of each foil is measured using Millitron 1200 IC comparator; then, the density of tracks can be normalized by using the relationship (Seidel and Tidjani, 1985):
PROCEDURES D, =
2.1. Radon monitoring in the soil The experimental set-up for radon monitoring in the soil is the one described by Fleischer and Mogro457
D, 1 - 0.303 (e - 6.5)
where D, is the track density (cmb2) normalized to a residual thickness of 6.5pm, D, is the track
ADAMS TIDJANI
458 mb 1015c 3(
‘C pressure.....*.. .. ..
.. ....** lOlO-
..a*
2‘ .
.. *..* ... ‘*... ..**.. _:’
*a..
...* ,~,.4.Z.\...~ . . .’
*.*’ ‘\
./ambient
3(
2E
c350
l\
I’ ,’ .*’
l\
/’ 1’
.\
i =\
I’
;/
‘\*_
5 ,I :
,: : 8’ : ,’ :__--
.; .\
.’ .\.I
/./’
.+
\ :\ :
.\
i
l \
temperature
,f .\\ ___I ‘\ :’ soil temperature I, ,*
:
*A
\. \
/’
\
%
).-.-.d. /*
.\ ‘\
.’ .’
*-.*
5‘. .* ‘.*.”
\
.’ .’ 2(
*........... *.**. .* _. . . * . . . . . . . . . 0 . . . . . .* .* ..
,’
,’
,’ -I.. *.
..“I’
,‘&, .\\, *\
,,’
:
‘,I_ a’
:
‘\ ‘.
‘. Ley~.’
8’ a*’
x
8 -300 .z $250 E i200 s 150 a
FIG. 1. Variations of the radon concentration with the ambient temperature, atmospheric pressure, humidity and temperature in the soil over the period 16 March 1985 to 19 March 1987.
density (cmv2) measured with the spark counting device, and e @m) is the residual thickness after etching. To distinguish between the radon contribution from the soil and radon transported by convection, the natural background due solely to diffusion of radon has been measured. For this, below a similar set-up previously described, soil samples from the probe hole of each station were inserted inside a PVC tube (dia., 90 mm; length, 200 mm) up to 100 mm in height; the top of the PVC tube being closed. Afterwards, the background (about 20 tracks cm-’ per month) was subtracted from the data recorded in the field. 2.3. Experimental sites Starting in February 1985, a network of 16 stations was set up on the Dakar University campus. Six of them, aligned at a distance of one metre from each other, have been used to study gas migration in the soil vs atmospheric parameters; the latter data have been registered from the local meteorological station. The ten remaining stations were laid down, beginning
at the sea shore line and running perpendicularly to it; the distance between the stations being variable. 3. RESULTS
AND DISCUSSION
Figure 1 shows radon concentration variation in the soil observed over a 2 yr period while also measuring atmospheric parameters (atmospheric pressure, humidity of the soil, temperature, etc.). The graph of the integrated radon output vs averaged meteorological data over one month is plotted for a 24 month period. It is apparent from Fig. 1 that radon variation patterns are quite complicated. 3.1. Atmospheric pressure Something unexpected occurs with atmospheric pressure; we do not observe any systematic variation of radon concentration with the change of barometric pressure. Nevertheless, according to several papers (previously mentioned in the Introduction), a falling barometric pressure might induce an increase in the flux of the group gases near the soil surface, including radon; conversely, the radon gas migration would decrease when the barometer is rising.
EFFECTS
OF ATMOSPHERIC
PARAMETERS
ON GROUND
RADON
459
60 -
--. ‘t I _._- 0
20
40
SEA-SIDE
FIG. 2. Radon concentration
60
60”
110’77
160
250
400
460
DISTANCE (ItI)
vs distance from the sea-side. The curve is to guide the eye.
The lack of pressure effects on radon gas migration could be explained by the weak variations of atmospheric pressure at Dakar. As diffusion is the dominant transport mechanism which takes place across the interface (Tanner, 1964), we can state that atmospheric pressure changes do not alter the diffusion rate by modifying the UZRn concentration gradient near the surface. Another possible factor would be the composition of the experimental site; being composed of loamy sand to 1 m depth, it has capacity for clays. According to Tanner (1978), clays contain significant proportions of flaky minerals, usually oriented so as to impede vertical movement; they can be expected to retard diffusion and transport. In our case, however important this delay might be, the variations of atmospheric pressure do not display an appreciable effect on radon migration in the soil. 3.2, Soil humidity and precipitations Radon concentration fluctuates with changes in soil humidity; it increases from February to AprilMay, which corresponds to a high percentage of the humidity of the soil in Dakar. Afterwards, we observe a decrease in the radon concentration, with fluctuations during the other months of the year when the humidity of the soil is too high or too low. It is well known that the presence of humidity in the soil facilitates the radon migration in the soil up to a certain percentage which depends on soil porosity; that could explain the increase of radon concentration from February to May. But during the rainy season in Senegal (from June to the end of September) the soil is totally wet; the interstitial spaces are filled up with water which traps and hampers the motion of radon (called “choking effects*‘). This effect could explain the decrease of radon concentration with high humidity (from July to September). From October to December. the mean decrease of the
radon concentration with low humidity in the soil is in agreement with this explanation. 3.3. Temperature There is no correlation between ambient temperature and Rn concentration changes in the soil. This can be due to the fact that our experimental set-up is too deep to be affected by changes of ambient temperature. In contrast, temperatures measured at a depth of 40 cm in the soil seem to affect radon variations. In fact, an increase of this temperature could improve, by evaporation, the circulation of gases in the soil. This phenomenon could explain the high activity of radon at high temperatures. But, in September-October, the radon concentration decreases although the soil temperature is high; in the same period, humidity is maximum. This last observation indicates a higher influence of humidity than of soil temperature on the concentration of radon. 3.4. Ocean tides Figure 2 shows the mean curve of radon concentration vs the distance from the sea. Every point on the curve is obtained by averaging data successively recorded on the site in different months of the year; error bars indicate the average dispersion, which can be due to the influence of the atmospheric parameters, as seen previously. The curve displays a regular decrease of radon activity, with some fluctuations, as the distance from the sea increases. This decrease cannot be explained by a difference in geological structure. Indeed the experimental area, composed of loamy sand, does not present lithological variations; and background measurements carried out in the laboratory by taking one sample of soil from each station indicated that radon contribution from the soil is constant for all stations.
460
ADAMS
The first possible explanation for the high radon concentration in stations set near the sea can be. that it is due to the moisture of the soil. In this case, a logical decrease or increase of radon concentration should be observed by moving away from the ocean. In Fig. 2, some fluctuations in radon concentration can be seen. On the one hand, during the rainy season, when the soil becomes more and more wet, the tendency of radon concentration variation should change; the stations set near the sea should present a lower radon activity. We did not observe this change during our experiments, which lasted over a year. The second possibility, and more probable, is the “pumping effect” which can be induced by the tide motions (Tidjani et al., 1987). Using a piston analogy of the gas movement induced by the tidal motion in the soil, we can expect to obtain high radon activity during high-water and low activity during low-water. But, since our exposure periods of measurements are long, we should have a compensation of these two effects. Nevertheless, a higher radon concentration for the stations set near the sea has been observed: it seems that the effect of high-water is predominant on radon emanometry and impedes the effect of lowwater. On the other hand, fluctuations observed from one station to the other lead us to think that the effects of tidal motions do not affect gas migration in the soil in the same way (Wakita, 1985). To obtain a clear picture of this last influence, more experiments are being carried out.
4. CONCLUSION Radon concentration in the soil, at a depth of 4Ocm, does not seem to be affected by atmospheric pressure and ambient temperature; we can conclude that radon variations do not follow changes of these two parameters. This result corroborates our pre-
vious work (Tidjani, 1984) where radon concentration was measured each hour continuously with electronic detectors; it was accompanied by measurements of ambient temperature and atmospheric pressure. On the other hand, humidity to a certain extent, and soil temperature (with the influence of humidity predominating), promote radon migration and/or radon release in the soil. Experiments carried out in the vicinity of the sea show that ocean tides could be a prominent parameter. In order to confirm this last effect, additional experiments are required.
TIDJANI Acknowledgemenls-We are grateful to J. L. Seidel and M. Monnin of the University of Clermont (France) for valuable discussions and for their logistic support. We thank the Dakar Meteorological Station for providing meteorological information.
REFERENCES Clements W. E. and Wilkening M. H. (1974) Atmospheric pressure effects on radon-222 transport across the earth-air interface. J. geophys. Res. 79, 5025-5029. Cox M. E., Cuff K. E. and Thomas D. M. (1980) Variations of ground radon concentrations with activity of Kilauea volcano, Hawai. Nature 288, 74-76. Durrani S. A. and Bull R. K. (1987) Solid State Nuclear Track Derection, pp. 169-173. Pergamon Press, Oxford. Fleischer R. L., Hart H. R., Jr and Mogro-Camper0 A. (1980) Radon emanation over an ore body: search for long distance transport of radon. Nucl. Insrrum. Math. 173, 169-181.
Fleischer R. L. and Mogro-Camper0 A. (I 978) Mapping of integrated radon emanation for prediction of long dist&ce migration of gases with& the earth: techniaues and mincides. J. aeouhvs. Res. 83, 3539-3549. Kraner ‘H. N., &hr&er G. ‘i. *add Evans i. D. (1964) Measurements of the effect of atmospheric variables on radon-222 flux and soil-gas concentrations. Nat. Rud. Enu. I, 191-215.
Likes R. S., Mogro-Camper0 A. and Fleischer R. L. (1979) Moisture-insensitive monitoring of radon. Nucl. Instrum. Meth. 159, 395-400. Schery S. D. and Gaeddert D. H. (1982) Measurements of the effect of cyclic atmospheric pressure variation on the flux of radon-222 from soil. Geophys. Res. Left. 9, 835-838. Seidel J. L. and Monnin M. (1982) Some radon activity measurements of geophysical significance. In: Proc. 11th Int. ConJ SSNTDs, pp. 517-523. Pergamon Press, Oxford. Seidel J. L. and Tidjani A. (1985) Relation Cpaisseur r&iduelle-densitt de traces comp&s pour le LR-115 type II. Rapport PCCF 85-5, Universite de Clermont. Tanner A. B. (1964) Radon migration in the ground: a review. Nat. Rud. Eno. I, 253-276. Tanner A. B. (1978) Radon migration in the ground: a supplementary review. Nar. Rad. Eno. III, 5-56. Tidjani A. (1984) Etude de sondes pour l’analyse des processus gkophysiques, radon-tmanometrie. Ph.D. Thesis, Universitt de Clermont. Tidjani A., Seidel J. L., Monnin M. and Isabelle D. B. (1987) Realization of a simulator for radon-222 underground migration studies. Nucl. Instrum. Mefh. A255, 423425. Wakita H. (1984) Groundwater observations for earth quake prediction in Japan. Int. Symp. on Seism. and Earrhq. Predict., pp. 494-500. Seismological Press, Beijing.
Tracks
ht. .I. Radiat.
Radial. Appl.
Meas,
0191-278X/88
Vol. 14, No. 4, PP. 457-460, 1988
Instrum.,Part
$3.00
+ 0.00
Pergamon Press plc
D
Printed in Great Britain
STUDY OF THE EFFECTS OF ATMOSPHERIC PARAMETERS ON GROUND RADON CONCENTRATION BY TRACK TECHNIQUE ADAMS TIDJANI
Faculte des Sciences, Wpartement
de Physique, Laboratoire des D.S.T.N., Universite de Dakar, Dakar-Fann, Senegal
(Received 15 July 1987; in revisedform 21
March
1988)
Abstract-Radon emanation was continuously monitored for 24 months, accompanied by measurements of atmospheric parameters. Integrated measurements of radon concentrations have been performed with LR-115 cellulose nitrate track detectors. The monitoring was conducted at 16 observation sites distributed on the Dakar University area. Observed changes in radon concentration are interpreted to be caused by
changes in meteorological conditions and ocean tides. 1. INTRODUCTION IN RECENTyears, many studies of radon gas migration in the soil, using different techniques, have been made to address geophysical questions (Fleischer and Mogro-Campero, 1978; Cox et al., 1980; Seidel and Monnin, 1982). However, it has been found that the radon levels in soils can be strongly influenced by the effects of atmospheric parameters. Fluctuation of radon emanation produced by changes in atmospheric parameters has been the subject of numerous investigations, with conflicting results in some cases: for example Fleischer et al. (1980) did not find correlation of high precipitation with either high or low radon readings, contrary to Wakita (1985) who found that normal precipitation does not greatly affect the radon concentration but that a succession of heavy rainfall occasionally causes an increase in radon concentration. For radon concentration changes caused by atmospheric pressure, many authors (e.g. Kraner et al., 1964; Clements and Wilkening, 1974; Schery and Gaeddert, 1982) found that radon concentrations fluctuate inversely with changes in atmospheric pressure. The studies involving atmospheric pressure indicate that changes of the order of 1% may be responsible for variations in the radon flux of as much as 30-90%. Unfortunately, complete studies of effects of atmospheric parameters are very rare. In this paper, we present the results obtained in Dakar, Senegal, on investigations into the effects of several atmospheric parameters on gas migration in the soil, using the well known track-detector technique. Other preliminary data on effects of ocean tides on radon emanometry are reported.
2. EXPERIMENTAL
Camper0 (1978), with some modifications. Two inverted plastic cups were used; the lower one contains a piece of cellulose nitrate on the underside of its top. To prevent water condensation on the detectors, an insulator was placed between the two plastic cups and an anti-220Rn membrane placed at the bottom of the cups, according to the method of Likes et al. (1979); this is an effective way to avoid the temperature of the detector dropping below dew point. In addition, the permeable membrane (PVC foil, 15 pm thick), by delaying the progress of the radon isotopes towards the detection volume beneath the detector, separates the unwanted contribution of thoron gas to the radon track counting (Fleischer and Mogro-Campero, 1978). The experimental apparatus was buried at a depth of 4Ocm, where the detector records alpha decays from 222Rn and its daughters throughout a burial time of one month. The track density of the detector is found to be proportional to the actual concentration of radon and its daughter products in the soil. 2.2. Track counting After field exposure, the detector (LR-115 type II) is chemically etched to enlarge the paths of the alpha particles produced by the decay of radon and its daughters. The etching was performed in a 2.5 M NaOH solution at 60°C during a time previously established to reduce the 12 pm original thickness of the detecting foil to about 6.5 pm. Before counting the detectors with the jumping spark counter (see, e.g., Durrani and Bull, 1987), the thickness of each foil is measured using Millitron 1200 IC comparator; then, the density of tracks can be normalized by using the relationship (Seidel and Tidjani, 1985):
PROCEDURES D, =
2.1. Radon monitoring in the soil The experimental set-up for radon monitoring in the soil is the one described by Fleischer and Mogro457
D, 1 - 0.303 (e - 6.5)
where D, is the track density (cmb2) normalized to a residual thickness of 6.5pm, D, is the track
ADAMS TIDJANI
458 mb 1015c 3(
‘C pressure.....*.. .. ..
.. ....** lOlO-
..a*
2‘ .
.. *..* ... ‘*... ..**.. _:’
*a..
...* ,~,.4.Z.\...~ . . .’
*.*’ ‘\
./ambient
3(
2E
c350
l\
I’ ,’ .*’
l\
/’ 1’
.\
i =\
I’
;/
‘\*_
5 ,I :
,: : 8’ : ,’ :__--
.; .\
.’ .\.I
/./’
.+
\ :\ :
.\
i
l \
temperature
,f .\\ ___I ‘\ :’ soil temperature I, ,*
:
*A
\. \
/’
\
%
).-.-.d. /*
.\ ‘\
.’ .’
*-.*
5‘. .* ‘.*.”
\
.’ .’ 2(
*........... *.**. .* _. . . * . . . . . . . . . 0 . . . . . .* .* ..
,’
,’
,’ -I.. *.
..“I’
,‘&, .\\, *\
,,’
:
‘,I_ a’
:
‘\ ‘.
‘. Ley~.’
8’ a*’
x
8 -300 .z $250 E i200 s 150 a
FIG. 1. Variations of the radon concentration with the ambient temperature, atmospheric pressure, humidity and temperature in the soil over the period 16 March 1985 to 19 March 1987.
density (cmv2) measured with the spark counting device, and e @m) is the residual thickness after etching. To distinguish between the radon contribution from the soil and radon transported by convection, the natural background due solely to diffusion of radon has been measured. For this, below a similar set-up previously described, soil samples from the probe hole of each station were inserted inside a PVC tube (dia., 90 mm; length, 200 mm) up to 100 mm in height; the top of the PVC tube being closed. Afterwards, the background (about 20 tracks cm-’ per month) was subtracted from the data recorded in the field. 2.3. Experimental sites Starting in February 1985, a network of 16 stations was set up on the Dakar University campus. Six of them, aligned at a distance of one metre from each other, have been used to study gas migration in the soil vs atmospheric parameters; the latter data have been registered from the local meteorological station. The ten remaining stations were laid down, beginning
at the sea shore line and running perpendicularly to it; the distance between the stations being variable. 3. RESULTS
AND DISCUSSION
Figure 1 shows radon concentration variation in the soil observed over a 2 yr period while also measuring atmospheric parameters (atmospheric pressure, humidity of the soil, temperature, etc.). The graph of the integrated radon output vs averaged meteorological data over one month is plotted for a 24 month period. It is apparent from Fig. 1 that radon variation patterns are quite complicated. 3.1. Atmospheric pressure Something unexpected occurs with atmospheric pressure; we do not observe any systematic variation of radon concentration with the change of barometric pressure. Nevertheless, according to several papers (previously mentioned in the Introduction), a falling barometric pressure might induce an increase in the flux of the group gases near the soil surface, including radon; conversely, the radon gas migration would decrease when the barometer is rising.
EFFECTS
OF ATMOSPHERIC
PARAMETERS
ON GROUND
RADON
459
60 -
--. ‘t I _._- 0
20
40
SEA-SIDE
FIG. 2. Radon concentration
60
60”
110’77
160
250
400
460
DISTANCE (ItI)
vs distance from the sea-side. The curve is to guide the eye.
The lack of pressure effects on radon gas migration could be explained by the weak variations of atmospheric pressure at Dakar. As diffusion is the dominant transport mechanism which takes place across the interface (Tanner, 1964), we can state that atmospheric pressure changes do not alter the diffusion rate by modifying the UZRn concentration gradient near the surface. Another possible factor would be the composition of the experimental site; being composed of loamy sand to 1 m depth, it has capacity for clays. According to Tanner (1978), clays contain significant proportions of flaky minerals, usually oriented so as to impede vertical movement; they can be expected to retard diffusion and transport. In our case, however important this delay might be, the variations of atmospheric pressure do not display an appreciable effect on radon migration in the soil. 3.2, Soil humidity and precipitations Radon concentration fluctuates with changes in soil humidity; it increases from February to AprilMay, which corresponds to a high percentage of the humidity of the soil in Dakar. Afterwards, we observe a decrease in the radon concentration, with fluctuations during the other months of the year when the humidity of the soil is too high or too low. It is well known that the presence of humidity in the soil facilitates the radon migration in the soil up to a certain percentage which depends on soil porosity; that could explain the increase of radon concentration from February to May. But during the rainy season in Senegal (from June to the end of September) the soil is totally wet; the interstitial spaces are filled up with water which traps and hampers the motion of radon (called “choking effects*‘). This effect could explain the decrease of radon concentration with high humidity (from July to September). From October to December. the mean decrease of the
radon concentration with low humidity in the soil is in agreement with this explanation. 3.3. Temperature There is no correlation between ambient temperature and Rn concentration changes in the soil. This can be due to the fact that our experimental set-up is too deep to be affected by changes of ambient temperature. In contrast, temperatures measured at a depth of 40 cm in the soil seem to affect radon variations. In fact, an increase of this temperature could improve, by evaporation, the circulation of gases in the soil. This phenomenon could explain the high activity of radon at high temperatures. But, in September-October, the radon concentration decreases although the soil temperature is high; in the same period, humidity is maximum. This last observation indicates a higher influence of humidity than of soil temperature on the concentration of radon. 3.4. Ocean tides Figure 2 shows the mean curve of radon concentration vs the distance from the sea. Every point on the curve is obtained by averaging data successively recorded on the site in different months of the year; error bars indicate the average dispersion, which can be due to the influence of the atmospheric parameters, as seen previously. The curve displays a regular decrease of radon activity, with some fluctuations, as the distance from the sea increases. This decrease cannot be explained by a difference in geological structure. Indeed the experimental area, composed of loamy sand, does not present lithological variations; and background measurements carried out in the laboratory by taking one sample of soil from each station indicated that radon contribution from the soil is constant for all stations.
460
ADAMS
The first possible explanation for the high radon concentration in stations set near the sea can be. that it is due to the moisture of the soil. In this case, a logical decrease or increase of radon concentration should be observed by moving away from the ocean. In Fig. 2, some fluctuations in radon concentration can be seen. On the one hand, during the rainy season, when the soil becomes more and more wet, the tendency of radon concentration variation should change; the stations set near the sea should present a lower radon activity. We did not observe this change during our experiments, which lasted over a year. The second possibility, and more probable, is the “pumping effect” which can be induced by the tide motions (Tidjani et al., 1987). Using a piston analogy of the gas movement induced by the tidal motion in the soil, we can expect to obtain high radon activity during high-water and low activity during low-water. But, since our exposure periods of measurements are long, we should have a compensation of these two effects. Nevertheless, a higher radon concentration for the stations set near the sea has been observed: it seems that the effect of high-water is predominant on radon emanometry and impedes the effect of lowwater. On the other hand, fluctuations observed from one station to the other lead us to think that the effects of tidal motions do not affect gas migration in the soil in the same way (Wakita, 1985). To obtain a clear picture of this last influence, more experiments are being carried out.
4. CONCLUSION Radon concentration in the soil, at a depth of 4Ocm, does not seem to be affected by atmospheric pressure and ambient temperature; we can conclude that radon variations do not follow changes of these two parameters. This result corroborates our pre-
vious work (Tidjani, 1984) where radon concentration was measured each hour continuously with electronic detectors; it was accompanied by measurements of ambient temperature and atmospheric pressure. On the other hand, humidity to a certain extent, and soil temperature (with the influence of humidity predominating), promote radon migration and/or radon release in the soil. Experiments carried out in the vicinity of the sea show that ocean tides could be a prominent parameter. In order to confirm this last effect, additional experiments are required.
TIDJANI Acknowledgemenls-We are grateful to J. L. Seidel and M. Monnin of the University of Clermont (France) for valuable discussions and for their logistic support. We thank the Dakar Meteorological Station for providing meteorological information.
REFERENCES Clements W. E. and Wilkening M. H. (1974) Atmospheric pressure effects on radon-222 transport across the earth-air interface. J. geophys. Res. 79, 5025-5029. Cox M. E., Cuff K. E. and Thomas D. M. (1980) Variations of ground radon concentrations with activity of Kilauea volcano, Hawai. Nature 288, 74-76. Durrani S. A. and Bull R. K. (1987) Solid State Nuclear Track Derection, pp. 169-173. Pergamon Press, Oxford. Fleischer R. L., Hart H. R., Jr and Mogro-Camper0 A. (1980) Radon emanation over an ore body: search for long distance transport of radon. Nucl. Insrrum. Math. 173, 169-181.
Fleischer R. L. and Mogro-Camper0 A. (I 978) Mapping of integrated radon emanation for prediction of long dist&ce migration of gases with& the earth: techniaues and mincides. J. aeouhvs. Res. 83, 3539-3549. Kraner ‘H. N., &hr&er G. ‘i. *add Evans i. D. (1964) Measurements of the effect of atmospheric variables on radon-222 flux and soil-gas concentrations. Nat. Rud. Enu. I, 191-215.
Likes R. S., Mogro-Camper0 A. and Fleischer R. L. (1979) Moisture-insensitive monitoring of radon. Nucl. Instrum. Meth. 159, 395-400. Schery S. D. and Gaeddert D. H. (1982) Measurements of the effect of cyclic atmospheric pressure variation on the flux of radon-222 from soil. Geophys. Res. Left. 9, 835-838. Seidel J. L. and Monnin M. (1982) Some radon activity measurements of geophysical significance. In: Proc. 11th Int. ConJ SSNTDs, pp. 517-523. Pergamon Press, Oxford. Seidel J. L. and Tidjani A. (1985) Relation Cpaisseur r&iduelle-densitt de traces comp&s pour le LR-115 type II. Rapport PCCF 85-5, Universite de Clermont. Tanner A. B. (1964) Radon migration in the ground: a review. Nat. Rud. Eno. I, 253-276. Tanner A. B. (1978) Radon migration in the ground: a supplementary review. Nar. Rad. Eno. III, 5-56. Tidjani A. (1984) Etude de sondes pour l’analyse des processus gkophysiques, radon-tmanometrie. Ph.D. Thesis, Universitt de Clermont. Tidjani A., Seidel J. L., Monnin M. and Isabelle D. B. (1987) Realization of a simulator for radon-222 underground migration studies. Nucl. Instrum. Mefh. A255, 423425. Wakita H. (1984) Groundwater observations for earth quake prediction in Japan. Int. Symp. on Seism. and Earrhq. Predict., pp. 494-500. Seismological Press, Beijing.