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| Titel |
Radon diffusion coefficients in soils of varying moisture content |
| VerfasserIn |
C. Papachristodoulou, K. Ioannides, S. Pavlides |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250023080
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| Zusammenfassung |
| Radon is a naturally occurring radioactive gas that is generated in the Earth’s crust and is free
to migrate through soil and be released to the atmosphere. Due to its unique properties, soil
gas radon has been established as a powerful tracer used for a variety of purposes, such as
exploring uranium ores, locating geothermal resources and hydrocarbon deposits,
mapping geological faults, predicting seismic activity or volcanic eruptions and testing
atmospheric transport models. Much attention has also been given to the radiological
health hazard posed by increased radon concentrations in the living and working
environment.
In order to exploit radon profiles for geophysical purposes and also to predict its entry
indoors, it is necessary to study its transport through soils. Among other factors, the
importance of soil moisture in such studies has been largely highlighted and it is widely
accepted that any measurement of radon transport parameters should be accompanied by a
measurement of the soil moisture content. In principle, validation of transport models in the
field is encountered by a large number of uncontrollable and varying parameters; laboratory
methods are therefore preferred, allowing for experiments to be conducted under
well-specified and uniform conditions.
In this work, a laboratory technique has been applied for studying the effect of soil
moisture content on radon diffusion. A vertical diffusion chamber was employed, in which
radon was produced from a 226Ra source, was allowed to diffuse through a soil column
and was finally monitored using a silicon surface barrier detector. By solving the
steady-state radon diffusion equation, diffusion coefficients (D) were determined
for soil samples of varying moisture content (m), from null (m=0) to saturation
(m=1). For dry soil, a D value of 4.1×10-7 m2s-1 was determined, which increased
moderately by a factor of ~3 for soil with low moisture content, i.e. up to m ~0.2.
At higher water fractions, a decrease in D was initiated and became particularly
pronounced approaching complete saturation; at m =0.9, D was as low as 2×10-9
m2s-1.
A series of field experiments has also been conducted using alpha-track CR-39 detectors
to follow the moisture-dependence of radon diffusion through soil under natural
conditions. Diffusion coefficients were determined as a function of surface soil
moisture assuming a one-dimensional diffusive radon transport model. Comparison
between results obtained by the two methods showed that laboratory studies may
provide a good indication of radon diffusion coefficients to be expected in the field.
However, values determined in the field were systematically lower than those assessed
in the laboratory. This finding could be attributed to soil-dependent parameters,
such as differences in pore space geometry between the soil used in laboratory
experiments and the undisturbed soil. In the latter case, the higher degree of compaction
imposes a more tortuous pathway to soil gas, while at the same time the diffusive
gas flux is hindered by local-scale zones of higher bulk density or water content. |
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