radon migration through soil and bedrock

Geoexploration, 26 (1989) 135-144 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

1:15

Radon Migration through Soil and Bedrock

LENNART MALMQVIST, MATS ISAKSSON and KRISTER KRISTIANSSON

Department of Physics, Unicersity of Lund, Lund (Sweden)

(Received October 31, 1988; accepted after revision April 26. 1989 1

ABSTRACT

Malmqvist. L.. Isaksson, M. and Kristiansson, K., 1989. Radon migration through soil and bedrock. Geoexploration, 26: 135-144.

To investigate radon migration in the ground a series of measurements have been made along a profile of 160 m length. The radon concentration was measured in the soil at different depths with the inverted cup technique. The emission of radon and the gamma emission were measured c,n soil samples from the profile and the exhalation of radon from two drill cores taken right below the profile was investigated. The radon profile agreed better with the drill core emission than with the measurements on the soil samples supporting the hypothesis that streaming transport from the bedrock exists and is important.

INTRODUCTION

The migration of “ig Rn in the ground includes two modes of transportation: diffusion and fluid flow. The diffusional transport is governed by Fick’s law and depends on soil parameters such as porosity and humidity. In dry sand the characteristic diffusion length amounts to about 1 m, whereas it can be as short as a few centimetres in humid clay. The maximum measurable radon transport by diffusion is therefore always restricted to a few meters. If diffusion is the only mode of migration the radon concentration will therefore depend on the local abundance of “i:Ra.

In fluid-flow transport radon is assumed to be carried by flowing gas or water. The transport distance depends on the flow velocity of the carrier and long distance transport is then possible. Both transport mechanisms are exten- sively discussed by Tanner (1964, 1980) in two review articles.

Diffusion always occurs and has for a long time been assumed to be the most important mode of radon transportation in the ground. Especially in the last two decades, however, experimental results have been reported which show that streaming can be important and even dominate the transport.

Enhanced radon concentrations above uranium mineralizations have been

0016-7142/89/$03.50 0 1989 Elsevier Science Publishers B.V.

1:x L. MALMQVIST ET AL.

recorded in spite of the fact that the mineralizations are situated at such depths that diffusional transport is impossible (Fleischer and Mogro-Campero, 1979; Fleischer et al., 1980 and references therein). Mogro-Camper0 and Fleischer (1977) suggested that the radon is carried by a convective gas flow which is driven by differences in the temperature gradient above the ground water table. In connection with earthquakes irregular transport of radon has been observed which is hardly compatible with diffusion (King, 1984). Especially the spike- like radon events require a fast transport which seems to have the properties of flow transport.

Radon measurements in the ground down to about 6 m depth in mining areas in central and northern Sweden with the inverted cup technique showed results which in most cases were consistent with transportation by an upward flowing carrier gas (Kristiansson and Malmqvist, 1982,1984). Below the ground water table the transport was assumed to exist and to take place with the rising bubbles of the carrier gas. Experimental evidence for the existence of bubble transport has recently been found by Somogyi and Len&t (1986) who studied the migration of radon in a long water-filled vertical tube and found the concentration pattern possible to explain only if radon was transported with rising bubbles.

To test the observations of upward flow transport of radon in the ground surface Schery et al. (1984) made a study of the radon depth profile in sandy loam. The radon concentration at different depths was measured in samples pumped from different depths. No evidence was found for the existence of an upward flow mechanism which was strong enough to carry radon faster than diffusion.

The disagreement between the results shows the need for further studies of radon migration in the ground. In this paper we report radon measurements along a profile and compare with the radon emanation both from the soil and from the bedrock.

EXPERIMENT

The experiment was carried out close to Hjulsjii in Vastmanland in the southern part of central Sweden. The geology of the site is characterized by dipping strata of skarn holding a low content of tungsten and enhanced concentration of uranium and thorium. The primary task of the radon measurements was to study the possibility of localizing the skarn formation by radon technique. The radon measurements were made along a profile of 160 m length which traversed two dipping skarn sheets. Along the same profile other measurements were made and two 46 mm drill holes intersected the formation exactly below the profile.

The concentration of radon in the soil was measured with the inverted cup technique with SSNT detectors Kodak LR 115, Type II. In a summer exposure

RADON MIGRATION THROUGH SOIL AND BEDROCK 137

in 1979 measurements were made at depths of 0.5 m and 1.0 m. The cups at each coordinate were placed in separate holes at about 0.5 m distance. At such a distance two measurements do not interfere (Kristiansson and Malmqvist, 1984). The measurements were repeated during the summer of 1980 at a depth of 0.3 m.

Soil samples have been taken along the profile at a depth of 0.4 m for a study of the radon emission of the soil matter. The measurements were made with the plastic detector technique using 200 g of the samples in closed 1000 ml plastic boxes. To normalize the humidity of all samples each sample was dried and subsequently 6 ml of water was added. In this way we equalized the influ- ence of the humidity on the emission as far as possible. The exposure lasted 58 days. The number of counted holes was sufficient to make the total error smaller than 15%.

The emission of gamma radiation from the soil samples was studied in order to determine the total amount of radon in the samples. The gamma measurements were made with a calibrated Ge (Li) detector enclosed in a 15 cm thick lead shield. Of each sample 60 ml was placed in plastic boxes with cover and sealed with tape. The radon concentration was determined by measurements of the 609.3 keV line from the decay of the radon daughter ‘AiBi after radon and radon daughters had reached equilibrium.

In the measurements of the emission of radon from the bedrock two 32 mm drill cores from drill holes exactly below the radon profile were used. The drill cores were split and the 5 cm long semi-cylindrical pieces were enclosed in plastic cups equipped with plastic detectors. The mean weight of the pieces was 60 g. The radon emission was determined in exposures of 4.5 months du- ration. The drill core pieces were slightly different in size and shape. This was corrected for by weighing the pieces and then applying a correction based on the assumption that the emission was proportional to the surface area which in turn was assumed to be proportional to ( mass)2’3. The use of small pieces of rock together with the procedures of measurement and correction will give only a rough estimate of the potential of the local radon emission inside the bedrock but no better method of measurement is available.

RESULTS OF RADON AND GAMMA MEASUREMENTS IN THE SOIL

Figure 1 shows the results of the three radon measurements in the soil layer together with a simplified geological section. The skarn layers reveal the gen- eral strata direction. The radon profiles (a) and (c) show the measurements at soil depths 0.1 m and 0.5 m made in 1979 and profile (b) the measurements in 1980 at depth 0.3 m. The measurements from 1979 and 1980 are not directly comparable in respect of the absolute level of the radon concentration due to a change in the etching procedure of the plastic detectors. [The comparatively high level of the measurements from 1980 may, however, also depend on time fluctuations in the radon concentrations from one year to the next. Such fluctuations are not uncommon (Fleischer et al., 1980). ]

l?lH L. MALMQVIS’I E’I’ AL.

@.I Death 0.1 m 7

“E 100 u

"l

z i .k ; 2000

.

b) Depth 0.3m ~

w c ; lOOO- 0

L i

'; L + 1

'c d : lOOOr c) Depth 0.5m Y

Fig. 1. Radon concentration along the profile at depths 0.1, 0.3 and 0.5 m. The lower part of the figure shows a vertical profile through the bedrock with the skarn layers and the drill holes.

A comparison of the radon profiles shows similarities having significant peaks at about the same profile coordinate in spite of small spatial differences between the positions of the cups at the same coordinate and a one year delay in the 0.3 m depth measurement. The most pronounced radon peaks appear above the skarn outcrop.

The peaks may depend on an enhanced local radon production in the soil, due to a higher skarn content in the basal part of the glacial till close to the outcrop of the skarn formation. Another possible explanation of the radon peaks is that radon or the mother substances of radon are transported to the layer from deeper parts in the bedrock which have an enhanced concentration of the elements in the radioactive series.

To clarify the origin of the radon measured with the cup technique, gamma measurements and radon exhalation measurements were made on the soil samples taken along the profile. The results of the radon exhalation measurements are shown in Fig. 2a; in Fig. 2b the intensity of the 609 keV line in the “AfBi decay is plotted.

It is quite obvious that there is no pronounced radon exhalation peak above the skarn horizons. On the other hand the gamma intensity of one sample taken straight above the skarn is very distinct.


E / E

OE 50E IOOE

Fig. 2. (a) Radon exhalation of soil samples taken along the profile at 40 cm depth. (b ) Decay rate of “AjBi in the soil samples after equilibrium is reached between radon and radon daughters.

I 1 I

b)

IO l

I I / I

50 100 200 500

1

1 Radon concentration in millTracks Icn?.doyl

Fig. 3. Correlation between (a) radon exhalation from the soil samples and mean values of the three inverted cup measurements; (b) gamma intensities of the soil samples and mean values of the three inverted cup measurements.

In Fig. 3a we compare the radon measurements in Fig. 1 with the exhalation measurements in Fig. 2a. The cup data displayed are weighted mean values of the three measurements at different depths in the ground. There does not exist any significant correlation between the in situ measured cup results and the radon exhalation from the soil itself measured in the laboratory in spite of the

140 L. MALMQVIST ET AL.

fact that the soil samples were taken close to the positions of the cup measurements.

A comparison between the gamma measurements of the 609 keV line in the 2A:Bi decay and the cup measurements is displayed in Fig. 3b. The measurements do not show any interdependence between the gamma data and the cup data. The very weak correlations between cup data, exhalation results and gamma intensities agree with results reported by King (1980) and by Kris- tiansson and Malmqvist ( 1984 ) .

The following conclusions can be drawn from Figures l-3. The radon reg- istered in situ in the inverted cup experiments may either be transported to the cups from a non-local source or originate in the decays of local radium close to the cups. If the source is local radium it must be the source not only of the radon in the inverted cup but also of the exhaled radon and of the gamma radiation of the soil sample. In that case positive coefficients of correlation must be found between the three series of measurements. However, the experiments clearly show that there is no significant correlation between the inverted cup data and the results of the soil sample measurements of exhalation and gamma emission. We thus conclude that the radon measured with the inverted cup technique has mainly a non-local origin. The radon atoms are therefore most probably transported to the cups by streaming from greater depth in the ground. This supports our conclusion from 1982 that the major part of the soil gas radon is transported to the soil from greater depth possibly accompanying an upward flux of gas that serves as a carrier (Kristiansson and Malmqvist, 1982 ) .

A special point of interest is the observations at the coordinate 137.5 m where both the ground measurement and the gamma measurement on the soil sample show pronounced peaks. No peak is seen in the radon exhalation measurements of the soil samples. To further study this situation we have measured the gamma activity of one prior element in the decay chain, “$zTh (E,,=63.3 keV), in the samples from the coordinates 132.5,137.5 and 142.5 m to trace a possible radioactive non-equilibrium. The quotients between the gamma intensities from “$$Th and 2AiBi were found to be the same for the three samples within the error of measurements. There is thus no difference in the relative abundance of the elements. This means that the peak at 137.5 m cannot be explained by a local disequilibrium. The anomalous observation coincides, however, with the outcrop of the skarn horizon. The high gamma observation originates therefore probably in an increased amount of radioactive grains with the production of trapped radon not detectable in the exhalation measurements. The maximum in the inverted cup measurement depends probably on enhanced production of radon at larger depths in the bedrock.

RESULTS OF THE DRILL CORE EXHALATION MEASUREMENTS

A comparison between the exhalation of radon from the two drill cores and the inverted cup measurements is made in Fig. 4. The measured exhalation of


z OE 50E 100E 150E m !2

Fig. 4. Comparison between the radon concentration in soil measured with the inverted cup technique and the radon exhalation from the drill cores.

radon is plotted along the drill cores. Each line segment gives the mean value of 5-15 measurements of radon exhalation along a 10 m long drill core section. The inverted cup data are weighted mean values of the three measurements displayed in Fig. 1.

There seems to be a qualitative agreement between the two radon registra- tions in Fig. 4. Both show pronounced maxima in association with the skarn horizons indicating a relationship between the radon production in the bedrock and the concentration measured in the soil.

To make a quantitative comparison between the bedrock radon and the inverted cup radon we have assumed a subdivision of the bedrock into about 9 m thick sections parallel to the two skarn horizons. The width is chosen so that each section contains one of the 10 m long drill core sections whose radon emanation is measured. We have assumed that the radon production rate in

142 L. ~ALMQVIST ET AL.

the whole bedrock section is equal to the production rate in the 10 m drill core section intersecting it. Furthermore we have assumed that a streaming transport of radon exists and that the transport mean free path towards the surface, i.e. the transport velocity, is the same in all sections.

In a first comparison we have assumed that the migration towards the ground surface takes place in a slanting direction parallel to the stratification. In this model the amount of radon transported to the soil is simply proportional to the exhalation of the layer in the bedrock if the above-mentioned model con-

Fig. 5. Display of the factors used in the calculation of the radon concentration at the soil surface if vertical streaming transportation from the bedrock is assumed. In our case 1= 10 m. Together with the angle between ground and drill core and the angle of the strata direction this means t,hat

Ad,=lZm. Adz=...=Ad,=22m; d,=Ad,/2, d,=Ad,$Ad,+...+Ad,/2

In the figure k,,, = 3 which means that

9 C(X,)=const ~clr,+iAdkexp(-dk/J)

k=l

=const[c,~d,exp(-dd,/i)+ci+,Ad,exp(-d,/E,)+c,+,Ad,exp(-d,/i,)]

TABLE I

Correlation between cup data and radon concentration at the soil surface for different values of the number of terms (k) and the mean free path (,%)

__I

k 6 Mean free path, i (m) (m)

it=10 A=30 ii=100

2 6 0.70t0.13 0.66 + 0.14 0.63SzO.16 4 23 0.56+0.19 0.39 L 0.24 6 45 0.48 i: 0.23


ditions are valid. A straightforward calculation of the coefficient of correlation (r) between the exhalation data and the cup data gives r=0.66 t: 0.14. The correlation is thus significant.

In a second comparison we assumed vertical transport through the bedrock. The amount of radon which was expected to reach the soil was computed from a relation based on the above-mentioned assumptions:

C(Xi) =const CC~+~__.I Ad, exp[ - (dJ;l)] k

where C( X, ) is the amount of radon which reaches the ground surface per unit area from the bedrock at the profile coordinate xi. If the radon in the soil measured with the cup technique originates in the bedrock a cup measurement at 3Ci should be proportional to C(xi). ci is the radon exhalation per unit volume in the ith section in the bedrock. A& is the vertical distance through the ith section at the coordinate Xi. dk is the vertical distance from the midpoint of the interval Ad, to the surface at the profile coordinate xi. A is the mean free path of radon transport in the bedrock. The meaning of the different factors in the formula is displayed in Fig. 5.

We have chosen to calculate the coef~cient of correlation between C ( X, ) and the measured cup concentrations along the profile as a function of both an assumed level dk in the bedrock from below which no radon is assumed to reach the ground surface and the mean free path (3L) of vertical radon transport. The results of the calculs ttion are shown in Table 1.

It is obvious that i. ihere is a significant positive correlation between the cup data and the assum d exhalation of radon from the bedrock. It is also obvious that the coefficient ! f correlation decreases comparatively slowly when dk and 1 increase. Even if the uncertainty in the values is large the slow decrease supports the assumption that the radon source is not concentrated to a thin surface layer of the bedrock but extends probably at least to depths of some tens of meters.

There is no significant difference in the coefficients of correlation for the slanting transport and the vertical transport. This makes it impossible to de- cide whether the transport follows the strata direction or is essentially vertical.

DISCUSSION AND CONCLUSIONS

The positive correlation between the inverted cup radon data and the bedrock measurements of exhalation together with the very weak correlation between the cup data and the radon emission from the soil, support the assumption that radon in the soil gas measured with the inverted cup technique originates mainly in the bedrock. Hence the radon atoms must be migrating to the soil layer by streaming through the cracks in the bedrock. The half-life of radon is too short for diffusion to be important. As reported in our previous

144 L. MALMQVIST ET AL

papers we believe that the radon atoms accompany a weak flow of gas bubbles that passes upwards through cracks in the bedrock. This flux has been shown to exist and both the flow rate and the gas composition have been studied (Malmqvist and Kristiansson, 1984). As already mentioned, Somogyi and Len& (1986) have also found experimental evidence for this transport mechanism for radon from a study of the concentration of radon in a long vertical tube in the ground.

We thank Mr. Gunnar Malmqvist for the very extensive analysis of most of the solid state nuclear track detectors. We are also indebted to Boliden Mineral AB for placing geological data at our disposal. The project has been financially supported by the National Swedish Board for Technical Development.

REFERENCES

Fleischer, R.L. and Mogro-Campero, A., 1979. Radon enhancements in the earth: evidence for intermittent upflows? Geophys. Res. Lett., 6: 361-364.

Fleischer, RI,., Hart Jr., H.R. and Mogro-Campero, A., 1980. Radon emanation over an ore body: Search for long-distance transport of radon. Nucl. Instrum. Meth., 173: 169-181.

King, C.-Y., 1980. Episodic radon changes in subsurface soil gas along active faults and possible relation to earthquakes. J. Geophys. Res., 85 (B6): 3065-3078.

King, C.-Y.. 1984. Impulsive radon emanation on a creeping segment of the San Andreas Fault, California. Pageoph., 122: 1-13.

Kristiansson, K. and Malmqvist, L., 1982. Evidence for nondiffusive transport of “ZiRn in the ground and a new physical model for the transport. Geophysics, 47: 1444-1452.

Kristiansson, K. and Malmqvist, L., 1984. The depth-dependence of the concentration of “i;Rn in soil gas near the surface and its implication for exploration. Geoexploration, 22: 17-41.

Malmqvist, L. and Kristiansson, K., 1984. Experimental evidence for an ascending microflow of geogas in the ground. Earth Planet. Sci. Lett., 70: 407-416.

Mogro-Campero, A. and Fleischer, R.L., 1977. Subterrestrial fluid convection: A hypothesis for long-distance migration of radon within the earth. Earth Planet. Sci. Lett., 34: 321-325.

Schery, SD., Gaeddert, D.H. and Wilkening, M.H., 1984. Factors affecting exhalation of radon from a gravelly sandy loam. J. Geophys. Res., 89 (D5) 7299-7309.

Somogyi, G. and LQnPrt, L., 1986. Time-integrated radon measurements in spring and well waters by track technique. Nucl. Tracks Radiat. Meas., 12: 731-734.

Tanner, A.B., 1964. Radon migration in the ground: A review. In: J.A.S. Adams and WM. Lowder (Editors), The Natural Radiation Environment, Univ. of Chicago Press, Chicago. pp, 161- 190.

Tanner, A.B., 1980. Radon migration in the ground: A supplementary review. In: T.F. Gesell and W.M. Lowder (Editors), Natural Radiation Environment, III. Symp. Proc., Houston, Tex.. April 1978. Vol. 1, pp. 5-56.

radon migration through soil and bedrock

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