REPOSITORY > RESULTS

Abstract

Radon (222Rn, half-life 3.82 days) is a natural radioactive noble gas which originates from
the radioactive decay of radium (226Ra) in the Earth's crust. It is a known hazard to
humans, due to its radioactivity. Moreover, radon can also be used as a versatile tool in
geophysical research. In order to use radon as a reliable tracer for different geophysical
processes, good knowledge of its transport mechanisms and about the parameters
influencing its exhalation to the atmosphere is of special importance.
In this doctoral thesis, radon transport characteristics in different environments and
spatial and temporal variations of radon levels are presented and discussed. The use of
radon as an earthquake precursor is highlighted and special attention is devoted to the
effects of hydrometeorological parameters on radon concentration in soil gas and to
methods for identifying anomalies in radon concentration caused by seismic activity.
The spatial distribution of radon levels in soil gas was studied in the area of the Ravne
fault and on a fly ash pile. The obtained results showed high spatial variability, which was
(within the same lithological type) mainly controlled by various levels of rock and soil
permeability. Radon concentrations and radon exhalation rates, measured at five profiles
perpendicular and parallel to the Ravne fault, were in the range of 0.9–32.9 kBq m–3 and
1.1–41.9 mBq m–2 s–1, respectively. The concentration of radon in fly ash was in the range
of 0.3–46.9 kBq m–3. The radon exhalation rate was 24 mBq m–2 s–1 on the part of the pile
covered with grass and 37 mBq m–2 s–1 on the part with trees and bushes.
The temporal fluctuation of radon levels in connection to seismic activity was studied
in the thermal waters at Bled and Hotavlje. Several anomalies were identified which
might have been caused by geophysical processes preceding earthquakes. Furthermore,
different responses of radon fluctuation to seismic activity were observed at these two
measurement locations, which indicates a strong dependence on the geological
characteristics of the aquifer. The average radon concentration was 10.5 ± 2.1 kBq m–3 at
Bled (October 2005 to September 2007) and 197 ± 121 kBq m–3at Hotavlje (October
2005 to June 2008).
Four different approaches for differentiating between radon anomalies caused by
seismic activity and those caused solely by hydrometeorological parameters were
compared in order to determine their efficiency. Among these, machine learning methods
were shown to have a great advantage and potential.
Since karst caves represent a boundary layer between the Earth's crust and the
atmosphere, they are often used for scientific research. The temporal and spatial
variability of radon concentration and its transport characteristics were studied on the
basis of continuous measurements of radon concentration at four locations in Postojna
Cave, namely Lepe Jame (Beautiful Caves), the lowest point, Velika Gora (Great
Mountain) and Pisani Rov (Gaily Coloured Corridor). Special attention was devoted to
the use of radon as a tracer for cave ventilation. Radon concentrations measured at three
measurement locations in the tourist part of Postojna Cave (Lepe Jame, the lowest point
and Velika Gora) were in the range of 0.2–1 kBq m–3 and 3–10 kBq m–3, in winter and
summer, respectively. On the other hand, significantly higher radon levels were observed
in the dead-end passage Pisani Rov, with radon levels in summer reaching up to
45 kBq m–3, comparable to the highest concentrations measured in karst caves worldwide.
The most important parameter governing ventilation and thus radon levels in the cave was
found to be the outside air temperature. Two major ventilation regimes were identified,
namely a summer and a winter one. A model for predicting radon concentration on the
basis of outside air temperature was developed for the Velika Gora measurement location.
Besides the temperature gradients between the cave and the outside air, an important
influence on the spatial distribution of radon levels was found to be the geomorphology of
cave passages, as well as the spatial variability of radon exhalation from rock surfaces and
cave sediments.