With increasing pollution world-wide the need for understanding of the
dynamics of environmental processes has became of primary concern of mankind.
The problems are focused on two, in certain sense opposite areas. These are the
industrially heavily contaminated areas and yet clean but endangered
regions. In the first area questions
regarding the propagation of contaminants, in the second one the transport
processes themselves are in the centre of public and scientific attention,
respectively.
One of the methods suitable to study these processes, which reflect the
interaction of atmosphere, hydrosphere and lithosphere, is the use of tracer
isotopes. Subsurface natural fluids in the majority of cases carry small
amounts of radioactive isotopes. Among them the alpha radioactive 222Rn,
as a member of 238U decay series, is ubiquitously present in the
ground and in the lower atmosphere as well. Through the measurement of radon
concentration in the geological environment, information can be obtained about
the transport processes as well as about penetrated geological structure. This
approach is applicable also in cases of radioactive contaminants, where the
essential question is the retention of transport.
Among the endangered regions
in highly permeable areas, as e.g. karst
is per se, the problems are perhaps
the sharpest, as they serve as natural water reservoirs for mankind. In these areas, owing to the set of
interconnected fractures and voids, the propagation of external effects is very
quick and deep. One of the most appropriate methods to trace transport with
natural radon in these systems is the application of etched track detectors
allowing desirable large-scale in situ
measurements. At the Institute of Nuclear Research of the Hungarian Academy of
Sciences first underground environmental alpha radioactivity studies, based on
the latter technique, were initiated by Dr. G. Somogyi in 1977. I have entered
this step by step widening radon field in 1987. My scientific interest focused
on studying radon transport, giving emphasis to cave investigations.
Caves occur everywhere on
the earth, although mainly in karst areas - as they are formed mostly in
limestone environments. They may appear to men as static environment, where not
only the continual darkness but also the temperature and high relative humidity
is stable. This belief is, however, not justified, because fluid currents can
cause measurable changes in the physical parameters. The underground radon
measurements supplied us with a great amount of data, both spatially and
temporally variable, confirming the awaited diversity of data. The complexity
of environmental transport processes manifests itself everywhere. The influence
of inflowing waters, the presence of underground air circulation as well as the
morphology and structural dependence of transport processes are traceable in
our radon records. Based on these investigations, physical model
interpretations of the observed fluid motions were developed in collaboration
with the Department of Physical Geography, Eötvös Lóránd University (Budapest,
Hungary) and Department of Hydro- and Engineering Geology, Miskolc University
(Miskolc, Hungary). The so-called vertical geogas microbubble radon transport
model was developed in collaboration with Mecsek Ore Mining Ltd (Pécs,
Hungary). Its validity conditions were tested in measurements done in a 270 m
deep karst well at Miskolc University and on a 12 m high model column at the
Laboratory of Hydrogeology, University of Montpellier (Montpellier, France).
These collaborations resulted in developing of a microprocessor controlled
automatic radon-measuring unit in Hungary, which, since 1991, is routinely used
in fieldwork parallel with track etch technique.
The object of this thesis is
to summarise results obtained during the last decade. I have included those
findings, in obtaining of which my role was decisive. As a main part, I will
outline results related to radon concentration measurements in karst caves and
in natural (not necessarily karst) waters. Additionally, I will summarise
results of methodological developments connected to the solutions of tasks of
environmental radon activity concentration measurements. These are the studies of
radon transport through different filter and blocking materials, and the
developments of new measuring techniques for underwater and continuous radon
concentration measurement purposes.
The sources of radon in caves are the bedrock and deposits. Radon levels
in caves are determined primarily by the uranium content of the rock. Limestone
and other sedimentary rocks are found to contain about 1.3 - 2.5 ppm 238U
(16 - 31 Bq×kg-1) on
average. The relatively high values of radon found in caves are due to these
minute quantities of parent substances that occur naturally on and within the
interior surfaces of the caves. Increased 238U concentrations can be
associated either with fluorite mineralizations or hydrocarbons present in the
surrounding limestone.
Uranium can be oxidised and mobilised by groundwater flow. Once reducing
conditions are encountered, the uranium is readily precipitated from solution.
This leaching fixation process leads to the enrichment of uranium in adjacent
deposits. This secondary transport and enrichment process is important in cave
environment, as fractures in rock can increase the surface area interacting
with water. Experimental evidence of this effect in cave environment was found
by Navrátil et al. (1993), who
observed enrichment of uranium on cave walls.
The immediate radon source in caves is the 226Ra content of
the rock. Nazaroff et al. (1988)
reports mean 226Ra content of carbonates to be 25 Bq×kg-1 (range 0.4-233),
which were found to be distributed lognormally. The results of 226Ra determinations of bedrock
and soil samples from the Hungarian caves examined fall in range 0.6-26 Bq×kg‑1 (Hunyadi et al., 1997). The relatively
impermeable soils (deposits), such as clay, do not have sufficient porosity to
allow transfer of significant amounts of soil gas, therefore their contribution
to radon budget is small (Michel, 1987). Accordingly, Burkett (1993) found radon
emitted from the clay to be not sufficient to account for the radon
concentrations measured in the cave.
Subsurface natural fluids in the majority of cases carry small amounts
of environmental isotopes. The behaviour of these elements, and the variation
of their concentration in time and space, is the result of physical, chemical
and biological interactions. These elements, as their physical properties,
concentrations, etc., provide information on flow and kinetics of the carrying substances,
are called natural tracers.
The application of radon as a natural tracer is not yet common and
widespread. Among the natural tracers it would be considered on the one hand as
ideal since it is easily detectable even in small quantities, which do not
modify the characteristic of the environment. On the other hand, unfortunately,
its sources appear everywhere and are spread over in a manner unknown a priory. Therefore, the interpretation
of the concentration data is not straightforward: it needs interdisciplinary
expertise of hydrogeologists, geologists, physicist, radiogeochemists, etc.
Joint efforts have given results in different fields. The observations of
subsurface fluid motions traced by natural radon were followed by new ideas
about the basic transport phenomena and, later, by new interdisciplinary
applications: as, for example, mapping of active faults; investigations of
volcanic and seismic activities; earthquake prediction; hydrogeological
research, etc. (Fleischer, 1988).
In the speleology, similarly to the previously mentioned fields, these
types of measurements have already found their applications, and they give
important contributions to the better understanding of the natural regimes of
caves. Cunningham and LaRock (1991) delineated six microclimate zones in
Lechuguilla cave, Carlsbad caverns, National Park, New Mexico using radon grab
sampling in conjunction with observed airflow data. Atkinson et al. (1983) from a single set of
etched track measurements in the
Castleguard cave, Columbia icefields, Alberta, Canada, identified the effect of
tributary air currents from larger fissures.
The most common and most apparent phenomenon, which was discovered in
the majority of the investigated caves throughout the world, was the annual
change of radon activity concentration. Wilkening and Watkins (1976) identified
temperature gradients favourable to vertical convective transport through
relatively large openings. They identified as well transport of radon by air
movement through cracks and fissures due to pressure gradients (Wilkening,
1980). As karst caves are situated generally in highly fracturized rocks, such
a configuration is favourable for the emergence of air circulation through this
fracture system. The strength of such air motions is taken to be proportional,
to a first approximation, to DT/f, where DT is the temperature
difference between the cave and outside and f is a friction factor
characterising the flow resistance (Wilkening and Watkins, 1976; Atkinson et al., 1983; Quinn, 1988).
The underground radon transport can be described by the following
transport equation:
where
C [m-3] is the radon concentration in pore space, Deff [m2×s-1] is the effective
diffusion coefficient of radon, 
[m×s-1] is the
velocity of the carrying substance, l [s-1] is the
decay constant of radon and f [m-3×s-1] is the source term.
In the equation first term describes diffusion, second term advection, third
term decay and fourth term sources of radon. For the solution of transport
equation, first, it is necessary to know the velocity field (e.g. Navier-Stokes
equation, which by itself is a sufficiently complicated problem); then taking
into account the source term, and the initial and boundary conditions C can be
determined. The emerging phenomena are determined by the form, shape and
structure of the underground void space. The above equation generally can be
solved only numerically.
The realisation of radon transport processes sharply depends on the
configuration of the interconnected underground cavities. In the case of blind
end systems, atmospheric pressure variation are the main control parameter
(Wigley, 1967; Ahlstrand, 1980), which are superimposed by convective air
exchange due to temperature differences in cave systems with vertical
extension. In the case of relatively large entrances, the convective air
exchange due to temperature differences can mostly account for the radon
transport process taking place (Wilkening, 1979). In the case of two or more
entrance systems, where the other ‘entrances’ can be complexes of smaller
fissures and fractures, chimney effect winds may dominantly govern the radon
transport, or in some cases atmospheric winds may do so.
The
interpretation of the data can be affected by the presence of these unknown
(unassumed) ‘entrances’. Yarborough (1980), in a study of nine caves,
identified two general types of physical cave configurations that affect
airflow patterns and radon concentrations. Type 1 caves have most passages at
or above the entrance elevation, Type 2 caves have most passages below the
entrance elevation. When the outside temperature exceeded the cave temperature,
he found that Type 1 caves exhaled, while stagnation occurred in Type 2 caves; when
the outside temperature fell below the cave temperature, both caves inhaled. As
Type 1 cave is horizontal “mirroring” of Type 2 cave the seasonal ventilation
patterns should be alternatives of each other. The clear asymmetry in this
case, however, points to the difference between the air flow-through and blind
end system. This indicates, that
substantial hidden parts of the upward directed systems remained
unrevealed from the descriptive point of view.
The above examples illustrate the problems of interpretation and
modelling. In two special cases, however, the emerging processes are
analytically easy to survey. These are the cases of idealised two-entrance
horizontal flow-through (type A), and one narrow entrance vertical caves (type
B). In the case of the horizontal model cave, the second entrance may represent
the set of vertical fractures, which connect the main passage to the surface
through the overburden.
Figure
1: Schematic horizontal (type A) and vertical (type B) model caves; h is the
elevation difference between the upper and lower entrances (horizontal cave), rin is the air density of inside air, Tout
is the outside temperature, pout is the ambient outside pressure, Vcave
is the cave volume and DV is the volume of air
passing the vertical cave entrance in case of Dp change in the ambient outside pressure.
First, let us consider a schematic horizontal cave (see Fig. 1, left).
In this case, owing to the temperature dependence of air density the pressure
exerted at the lower end of the cave by the outside air will differ from the
inside one. This pressure difference in this case is (Atkinson et al., 1983):
,
where rin is the density of inside air, g is the
gravitational acceleration, h is the height difference between the two entrances,
DT is the temperature difference
between the cave and the outside, and Tout is the outside
temperature. According to this relation, the direction of airflow through the
entrance depends on the season; in warm season air flows out of, and in cold
season air flows into, the cave at the lower entrance.
A more realistic model of fracturized karstic overburden above a
horizontal cave is the model of parallel vertical voids connecting the main
passage to the surface (Scheidegger and Liao, 1972). The variation of 222Rn
activity concentrations in this case is interpreted by an air circulation model
(Géczy et al., 1988). According to
it, in warm season, air will flow into the cave from the direction of the
radon-rich fracture system (and will flow out of the cave through the entrance
at lower altitude) resulting in high radon levels in cave. In cold season, the
direction of the flow is reversed. From the direction of the lower entrance
fresh outside air flows into the cave forming considerably lower radon concentration
levels than in summer
In the case of narrow-entrance vertical caves, the most effective
processes in inducing air motions are the atmospheric pressure changes (Fig. 1,
right). Falling ambient atmospheric pressure drains air from the cave; increasing
atmospheric pressure presses air into the cave through the entrance. The volume
of air passing the cave entrance:
,
where Vcave is the cave volume, pout and Dp are the ambient atmospheric
pressure and the change in the ambient atmospheric pressure, respectively.
Airflows induced by atmospheric pressure changes can be rather quick. At the
entrances of giant caves their speed can reach several tens of km×h-1(Cunningham and
LaRock, 1991).
3 Materials and methods
For the purpose of environmental radon concentration measurements opened
diffusion chambers equipped with LR-115 type II alpha sensitive polymer track
detector were used. The diffusion chamber consisted of a cup (diameter: 5.5 cm,
height 12 cm) with a detector located at the bottom of the cup. The other end
of the diffusion cup was opened to the atmosphere and looked downwards during
the exposure. The discrimination against thoron (220Rn) was obtained
by the delay effect of the diffusion on the basis of the use of a sufficiently
long cup. The sensitivity of the detector is 2.3 alpha-tracks×cm-2/kBq×m-3×h at standard etching conditions
(2.5 N NaOH, 60 °C, 2.5 hours).
Generally, in each cave the track etch detectors were placed along the main
passages of caves more or less equidistantly. There were generally 3-20 regular
measuring sites per cave. Extra observation points were established at
“characteristic” places. Typical exposure time was 1 month. About 10000
observation data were obtained in 31 investigated Hungarian caves during years
1977-1997.
From the beginning of the 1990s, microprocessor controlled automatic
field radon monitors (type Dataqua) were
gradually introduced into cave studies. The single channel type measured only
radon concentration, while the multiparameter version simultaneously
registrated the temperature and air pressure. Radon concentration was measured
in 1 hour cycles using open type
diffusion chamber (delay time » 1000 s) equipped with
alpha sensitive Si based semiconductor detectors. The sensitivities of the
units are 6.7 cph/kBq×m-3×h (detector sensitive area: 1 cm2)
and 17.8 cph/kBq×m-3×h (detector sensitive area: 3 cm2)
with an 0.1 cph initial background (cph stands for count per hour). Generally,
in each cave the continuous radon monitors were placed in the main passages
at characteristic places. There were
1-5 regular measuring sites per cave. A total number of approximately 500000
hourly readings was obtained with 11 monitors in the investigated five caves
during the years of 1991-96.
Measuring sites
The most characteristic results were obtained in the following caves
from the 32 investigated ones:
Bükk Mountains, Bükk National Park, Hungary
- The Létrási-Vizes cave is a multi-level typical swallet cave. The main
passage declines towards the end of the cave, which is located at 85 m depth
from the natural entrance. The cave collects waters from the surrounding area,
but its streamlets often go dry. The cave can be considered as cave of type B.
- The Hajnóczy cave can be considered as cave of type A. Its passages are
located along 3 parallel fault zones. The distance between two farthest points
of the cave is 150 m.
- The Szepessy cave is a cave of type B. The depth of the entrance shaft
is 130 m, the maximum depth of the cave is 165 m.
- The Istvánlápa cave is a cave of type B. It is one of the deepest
caves in Hungary. The depth of the entrance shaft is 210 m, the maximum depth
of the cave is 240 m.
Aggtelek Karst, Aggtelek National Park, Hungary
- The Baradla cave is nearly a horizontal cave located between Aggtelek
and Jósvafő villages. The cave has several entrances located along and at the
ends of the main passage, which length is 6 km. The main passage declines
towards the Jósvafő entrance. The elevation difference between the entrances at
Aggtelek and Jósvafő is 60 m.
- The Vass Imre cave is a cave very similar to the model cave of type A.
It has one entrance and the main passage is situated horizontally. The length
of the cave is 600 m. In the middle part of the cave there is a siphon, which
was closed by water two times during the overall observation period.
Pilis Mountains, Hungary
- The Sátorkő-puszta cave is a vertical cave of type B. The depth of the
cave is 48 m.
Mecsek Mountains, Hungary
- The Abaliget cave is very similar to the model cave of type A. It has
one entrance and the main passage is situated practically horizontally. The
length of the main passage is 500 m.
Buda Mountains, Hungary
- The Szemlő-hegy cave is a horizontal cave with the main entrance
situated at lower altitude, and several other 'entrances' located at higher
altitudes of the hill. The cave passageways are situated in two levels but the cave can be considered as cave of
type A with one dominating large opening to the surface. The length of the
lower passage is 350 m. The end of the main passage is 65 m depth from the
surface, the vertical distance between two levels is 15 m.
Keszthely Mountains, Hungary
- The Cserszegtomaj well cave
is a horizontal maze cave of type B, which was formed in 50 m depth on the
boundary of dolomite and sandstone. It has one 50 m deep artificial vertical
entrance. The whole formation is covered by clay. The radon concentration is
unusually high due to the sandstone environment of high porosity and bad
ventilation.
Lamalou Karst,
France
- The Lamalou cave is a horizontal water cave with a flowing stream
inside the cave. A part of the cave, except floods, is aerated. The aerated
part of the cave has one vertical and one horizontal entrance. The aerated part
of the cave system can be considered as cave of type A. The length of the
vertical entrance shaft is 20 m, the length of the horizontal part of the
aerated passage is 90 m.
4 Summary of results
(Papers referred as [A.. ] containing the new scientific results can be
found in the attachment.)
1. Characterisation of
airborne 222Rn concentration occurrences in
karst caves
1.a
Compiling all the globally available radon concentration data from 220
different caves world-wide I have found, that the distribution of 222Rn
concentrations is lognormal [A1, A2] (GM=1130 Bq×m-3, GSD=6.3), which is in good accordance with the awaited
distribution for a geochemical element. The lower end of the scale is
associated either with big cave chamber volumes or high ventilation rates; the
upper end is characterised by closed, badly ventilated places and uranium-rich
sediments.
1.b For
the 31 examined Hungarian caves I have determined the cave average annual mean
radon activity concentrations, 0.3-20 kBq×m-3,
the characteristic annual maximum/minimum ratios, 2-50, and the periodicity,
which was typically one or half a year [A3]. In the majority of cases the
marked seasonal variations can be characterised with high radon concentration
values in summer and low radon concentration values in winter. In few cases,
reversibly, summer minima and winter maxima were observed.
1.c I have observed a
long-term variation of the annual mean radon activity concentration in all the
studied caves [A1, A4]. The phenomenon shows the effect of slowly
changing environmental parameters on radon transport processes. Such an
environmental parameter may be the annual precipitation. The variation of
annual precipitation, which may due to climatic changes, influences water
content of the cave embedding rocks, which on the other hand affects radon
emanation power of rocks. The long-term variation can be amplified selectively
at different sites in a given cave. This latter effect is markedly shown on
time series taken at two different depths of the Sátorkő-puszta cave. The ratio
of the difference of data pairs to the mean of the same data pairs increases
with years (see Fig. 4. in [A1] ).
2. Influence of karstification and morphology of caves
on airborne 222Rn
concentrations
2.a By model calculations I have shown, that the
saturation value of radon activity concentration in fractures and the radon
exhalation from fractures strongly depend on the size of the fracture. According to calculations, at low airflow
velocity to aperture size ratios, the advective 222Rn transport
along a fracture is strongly reduced by lateral diffusion inside the fracture [A1,
A6]. These theoretical predictions were justified by measurements done in the
Vass Imre and Lamalou caves. In the Vass Imre cave, which can be characterised
by relatively undeveloped fractures with small size openings, I found no daily
variation in the radon records [A1], in spite of the observed daily air flow
variation. On the other hand, in the Lamalou cave, which is embedded in a well
karstified strata, characterised by big solutional openings and fracturized
volumes, strong daily fluctuations of 222Rn activity concentrations
were recorded [A7].
2.b I have found that in horizontal caves the
number of vertical fracture systems communicating the surface affects the width
of the outside temperature interval, which characterises
the transition from the low to the high daily average radon concentration
values [8]. While in the Vass Imre cave the transition width is around 5 °C, it is around 10 °C in the Abaliget
cave. In the Vass Imre cave, only one less developed fracture system, in the
Abaliget cave, more than one more-developed vertical fracture systems exist.
The step by step widening of the transition interval as a function of the
number and size of openings, is further supported by the data obtained in the
Szemlő-hegy cave. Here the step function type curve is practically reshaped to
a linear function (see Fig. 3. in [A8]).
3. Identification of air circulation pathways and
microclimate zones in cave systems based on airborne 222Rn concentration measurements
3.a I have
pointed out, that in a consequence of the seasonally directed underground
transport, enhanced surface exhalation can be expected on karstic terrains
seasonally [A3]. Experimentally the existence of the latter phenomenon was verified
in the Bükk Mountains, Hungary, where the radon time series measured inside the
Hajnóczy cave and in a slit above the Hajnóczy cave were inversely correlated
(see Fig. 6. in [A2]) showing high winter radon concentration values on the
surface. Winter maxima were as well observed on several karstic terrains of
Hungary [A3]. The above phenomenon can serve as an explanation to other
observations published in the literature.
3.b I have identified microclimate zones based on the analysis of temporal
variations of radon concentrations in caves. In the Baradla cave these
zones are [A5]: 1. the entrance region in the Aggtelek part with summer maxima
and winter minima; 2. the middle part from Libanon hill to Vöröstó entrance
with constant radon levels; and 3. the region between the entrances of Vöröstó
and Jósvafő with winter maxima and summer minima. I have pointed out, that the
temporal behaviour of radon activity concentration in the third microclimate
zone of the Baradla cave refer to a definite a connection between the main
passage of the Baradla cave and the Rövid-Alsó cave situated at a lower
altitude. Based on the difference in temporal variation of radon concentrations
three microclimate zones were as well identified in the Létrási-Vizes cave [A2,
A9].
4.
Determination of underground airflow velocity and its relation to cave average
annual mean airborne 222Rn level
4.a I have
determined the propagation speed of outside fresh air intrusions underground
and a volume of a vertical cave from the continuous measurements of radon
concentration [A1, A2]. In horizontal caves fresh air intrusion appears when the
outside temperature falls below the cave air temperature, while in vertical
caves their presence is related to an increase of the ambient outside pressure.
Utilising the time differences among radon falls measured at distinct places,
their propagation speed can be calculated. This speed is about 50 m×h-1 along the main cave
passage of the horizontal Vass Imre cave. As the length of the cave is 600 m,
the whole known volume of the cave is flushed through with outside air in 12
hours. Contrary, the propagation speed of the dilution effect in the vertical
Cserszegtomaj well cave is around only 2
m×h-1 in a 10-20 m region
from the bottom of the entrance well. This low value shows that the cave is
highly unventilated. In average 2 hPa of atmospheric pressure increase flushes
out radon gas from the complete volume of the vertical entrance. Taking into
account the known volume of the entrance well and the ideal gas law, it
indicates, that the volume of the cave is in the order of 10000 m3,
an order of magnitude larger, than it was estimated from the volumes of known
passages.
4.b I have found that cave average annual mean
airborne 222Rn levels depend on the strength of underground air motions [A2, A8]. In the Vass Imre cave the volume of
infiltrated air is naturally controlled by the penetrability of the siphon.
When the siphon is open, there is continuous air flow either in or out from the
cave through the entrance (winter and summer, respectively). When the siphon is
closed, there is no measurable air flow through the entrance. The restriction of the air flows resulted in an
overall drop of 30% in the annual mean
radon concentration level in the cave, accompanied by the decrease of the
amplitude of the seasonal radon concentration variation. This effect can be
explained on the basis of the air circulation through the cave covering strata.
It shows the increase of mean radon levels due to periodically changing flowing
conditions. The absence of seasonal variation in radon levels and the low radon
activity concentration values found in the deepest parts of the narrowest
vertical caves in Hungary (Szepessy, Istvánlápa) may also be attributed to this
phenomenon [A3], as from the point of view of ventilation the deep caves can be
considered as the most closed ones.
5. 222Rn transport in water
5.a I have
found that water inlets can be significant sources of radon in cave. In deeper parts of the
Létrási-Vizes cave the regular seasonal variation of radon levels are
considerably disturbed [A1, A2, A4, A9]. Approaching the endpoint of the cave,
the periodical character of the readings decreases and the variation of
springtime values becomes higher. On the other hand, there is high similarity
between temporal variations of the yield of one of the intermittent streams and
radon concentration measured in the air. As subsurface waters permeating porous
rocks can be significantly enriched in solved radon, this effect is explained
on the way that radon is essentially carried to the place by the latter
intermittent stream.
5.b Using radon as a tracer
I have found that thermal gradient induced convectional mixing is taking place
in the water column of a 270 m deep karst well [A4,
A10]. I have developed a model to calculate the depth dependence of the
vertical transport velocity from the measured vertical 222Rn
concentration profiles. The results indicated continuous upward transport of
radon in the water column with a mean velocity of about 0.7 m×h-1. I have pointed out, that this high value can be
attributed to the vertical thermal gradient induced convectional mixing of
water, and I rejected the possibility of transport by geogas microbubble
theory. The strong effect of vertical mixing on transporting radon was observed
as well in model experiments at the Hydrogeological Department, University of
Montpellier, France, in an 8.5 m high model column [A11]. As the search for new
U ore deposits in Hungary in late 80s was partly based on the interpretation of
the vertical radon profiles taken underwater in shallow drills, the recognition
of underwater vertical mixing processes along bore holes in this respect played
a supplementary role.
6. Methodological developments
6.a Silicon photodiode based Dataqua radon
monitoring devices were developed and brought to successful applications with
my significant contribution
[A12]. The idea of utilising silicon detectors for field radon measurements (by
alpha particle detection) came from Dr. M. Monnin, Hydrogeological Department,
University of Montpellier, Montpellier, France. The hardware of the device
(analog and digital electronics) was developed by the Dataqua Electronic Ltd.
The design of the sensitive volume of the radon head of the instrument and the
electronic testing were done by me at the Institute of Nuclear Research. I have
calibrated the instrument in the radon reference chamber of the Swedish
National Institute of Radiation Protection (Stockholm). The obtained
calibration coefficient, 6.7 cph/kBq×m-3×h for the 1 cm2 surface detector,
coincides well with that I have previously calculated.
6.b A
novel method to measure radon permeability of thin foils using etched track
technique was developed with my significant contribution [A13]. During the
measurement the sample foil separates the radon source volume from the
measuring volume. I have elaborated an analytical model to describe the non
steady state temporal variation of radon activity concentrations in source and
in the measuring volumes as a function of the permeation characteristic of the
investigated foil. This method allows to determine radon permeability
coefficient as well as radon/thoron separation factor of a given foil in few
hours. I have tested the reliability of the method in a series of laboratory
measurements using polyethylene foils of different thickness. The obtained
permeability value, 7.1*10-8 cm2×s-1, is in good agreement with the literature data. The
permeation characteristics of different materials are of interest from the
point of view of sealing homes against invasion by radon. As a practical
application of the method we have examined six types of floor covering
materials, and found, that they can reduce radon entry into dwellings by 2-3
orders of magnitude.
6.c Two experimental
techniques for the determination of the effective diffusion coefficients of
radon in polymer/silicate gels and clay suspensions were developed with my
significant contribution [A16, A17]. Similarly to the previous method, one side of the samples was
exposed to the radon source, the other side was closed by a small measuring
volume. Track etch type radon monitors were used to measure the radon exposures
on both sides of the samples. The diffusion mass transport in the sample was
numerically modelled for different exposure times and sample thicknesses.
Diffusion constant was a fitting parameter to obtain best fit to the
experimentally measured radon exposures. I used Dataqua type continuos radon
monitors to test the experimental arrangement against leaks. The procedure was
based on testing of the variation of the diffusion constant in subintervals of
the full exposure time. The variation in the value of the diffusion constant
was the indicator of leakage. We have determined the
effective diffusion coefficient of radon in polymer/silicate gel-containing
porous media, 3.3*10-6 cm2×s-1, and in Montax/clay suspension, 6.0*10-6 cm2×s-1. The corresponding
values for the bulk phases are comparable to that characteristic in pure aqueous solutions, therefore the sealing
technology applying these materials can be very attractive.
6.d A new method for the determination of the
radium and dissolved radon content of water samples using track etch type radon
monitors was developed with my significant contribution [A14]. During the measurement an immersed small
volume radon monitor is sealed together with the water sample into a container.
The air filled radon monitor is protected from the water by a thin radon
permeable rubber foil. The dissolved radon and the radium content of water
samples is determined by two subsequent exposures. In the first exposure the
sum of dissolved radon and radium concentrations, in the second exposure the
radium concentration itself are measured. I have developed a model, which
describes the temporal variation of the radon concentration in the measuring
volume of the small radon monitor. For the given arrangement I have determined
experimentally the calibration coefficient, 24.1 tracks×cm-2/(d×kBq×m-3) [A15], which value coincided
well with the calculated one. Characteristic for the method is that it not
applies any type of separation procedures, the use of which is often difficult
in field circumstances.
5 Tézisek
(Az új tudományos eredményeket a
[ ]-ben megadott hivatkozások tartalmazzák.)
1. A karsztbarlangok légterében mért
radonkoncentráció értékek jellemzése
1.a Áttekintettem az
elmúlt két évtizedben publikált barlangi radonmérési programok eredményeit s
megállapítottam, hogy a világ 220 barlangjában mért radonkocentráció adatok
eloszlása lognormális [A1, A2] (GM=1130 Bq×m-3, GSD=6,3). Az eloszlás
típusa megegyezik más geokémiai elemek szokásos földi eloszlásával. A skála
alsó részét nagy térfogatú termek ill. erősen szellőző helyek, míg a felső
részt zárt, rosszul szellőző helyek és uránium gazdag mineralizációk jellemzik.
1.b Nyomdetektoros
mérések alapján megállapítottam, hogy a 31 vizsgált magyarországi barlangban az
éves átlagos radon aktivitáskoncentráció a 0,3-20
kBq×m-3
tartományba esik, a mért radon idősorok periodicitása tipikusan egy vagy fél
év. Jellemző az éven belüli 2-50 radonkoncentráció maximum/minimum arány [A3]. Az barlangok többségében nyári
maximumot és téli minimumot tapasztaltam, ritkábban fordítva.
1.c Minden egyes
vizsgált barlangban a mért évi átlagos radon aktivitáskoncentráció hosszú idejű
változását tapasztaltam [A1, A4]. Ez a hosszúidejű trend eltérő lehet egy adott barlang különböző
pontjain. Az utóbbi jelenséget jól mutatja a Sátorkő-puszta-barlang két
különböző mélységében felvett radon idősor (lásd 4. ábra [A1]). A két
adatsornak az adatpárok átlagára normált különbsége időben növekvő tendenciát
mutat. A megfigyelt jelenség tükrözi a lassan változó környezeti paraméterek
hatását a radontranszport folyamatokra. Ilyen paraméter lehet a kőzetek
radonkibocsátási tényezőjének a lassú változása a barlangot befoglaló kőzet
víztartalmának változása miatt. Ez utóbbi paraméter értéke a klimatikus
változások miatti éves csapadékmennyiség változását tükrözheti.
2.
A karsztosodás fokának és a barlang morfológiájának hatása a barlangi levegő
radonkoncentrációjára
2.a Modellszámításokkal kimutattam,
hogy a repedésekben kialakuló telítési radonszintek és a radon exhalációja a
repedésekből erősen függ a repedés méretétől. Kis repedésméret/légáramlási sebesség arányoknál, a
repedés menti advektív radontranszportot erősen csökkenti a keresztirányú
diffúzió hatása [A1, A6] A számítások következtetéseit a Vass Imre- és a
Lamalou-barlangokban folytatott radonkoncentráció mérések eredményeivel
igazoltam. A Vass Imre-barlang egy gyengén fejlett repedésrendszerrel
jellemezhető, a Lamalou karszt, Franciaország, pedig egy erősen karsztosodott
terület nagy oldási üregekkel és töredezett rétegekkel. A Vass Imre barlangban
felvett radon idősorokra a nagyfokú napi stabilitás a jellemző a megfigyelt
erős napi légáramlás változások ellenére. A jelenség a diffúzió erős simító
hatását mutatja. Ezzel szemben a Lamalou barlangban, a felszíni hőmérséklet
napi változásainak megfelelően, erős napi radonszint változásokat találtunk
[A7].
2.b Vízszintes barlangokban a napi
átlagos radonkoncentráció ugrásszerűen változik a napi átlagos felszíni
hőmérséklet függvényében, egy átmeneti tartománnyal az alacsony téli és magas
nyári értékek között. Mérésekkel rámutattam, hogy az átmeneti tartomány szélessége
függ a felszínnel kapcsolatban álló függőleges repedésrendszerek számától [A8]. A Vass Imre-barlangban ennek az
átmeneti tartománynak a szélessége 5 °C, az Abaligeti-barlangban pedig 10 °C. A Vass
Imre-barlangban csak egy, a barlang vége felé elhelyezkedő repedésrendszer
található. Ezzel szemben az Abaligeti-barlangban több mint egy, jobban fejlett
függőleges törészóna található. Az átmeneti tartomány szélesedését a repedések
számának és méretének függvényében a Szemlőhegyi-barlangban mért adatok is alátámasztják.
Itt a megfigyelt görbe gyakorlatilag lineáris (lásd 3. ábra [A8]).
3.
Légáramlási utak és mikroklimatikus zónák azonosítása barlangrendszerekben
radonkoncentráció mérések alapján
3.a Rámutattam, hogy az
évszakosan váltakozó irányú légáramlás következménye a karsztos felszíneken az
évszakosan változó radonexhaláció [A3]. Erre a jelenségre a legmeggyőzőbb kísérleti
bizonyítékot a Hajnóczy-barlangban és a barlang feletti felszínen egy
repedésben mért radonszintek ellenütemű változása szolgáltatta (lásd 6. ábra
[A2]). A felszínen magas téli radonkoncentrációt mértünk, de téli maximumokat
találtunk Magyarország számos más karsztos területén is. A fenti jelenség
magyarázatul szolgálhat az irodalomban publikált további megfigyelésekre is.
3.b Mikroklimatikus zónák jelenlétét
mutattam ki a radon idősorok analízise alapján. A Baradla-barlangban ezek a zónák
[A5]: 1. az aggteleki bejárati szakasz magas nyári és alacsony téli
radonkoncentráció értékekkel; 2. A Libanon-hegy és a vöröstói bejárat közti
rész időben stabil radonszintekkel; valamint a 3. jósvafői és vöröstói
bejáratok közötti rész magas téli és alacsony nyári radonszintekkel. A
radonkoncentráció mérések alapján rámutattam, hogy légkörzés szempontjából
kapcsolat van a Baradla barlang és az alatta elhelyezkedő Rövid-Alsó-barlang
között. Mikroklimatikus zónák jelenlétét a tavaszi radonkoncentráció értékek
szórásának különbözősége alapján sikerült kimutatnom a Létrási-Vizes-barlangban
is [A2, A9].
4. A
felszín alatti légáramlási sebesség meghatározása és kapcsolata az éves átlagos
radonkoncentrációval
4.a Meghatároztam a friss külszíni levegő
barlangba való behatolásának terjedési sebességét és egy vertikális barlang
térfogatát a folyamatos radonkoncentráció mérések alapján. Ez a jelenség vízszintes barlangokban a külső
hőmérsékletnek a barlangi hőmérséklet alá való csökkenésekor, függőleges
barlangokban az atmoszférikus légnyomás meredek növekedésekor tapasztalható. A
detektorok közötti távolságot és a radonszint esések közötti időeltolódását
figyelembe véve a légáramlás sebessége a Vass Imre barlang főjárata mentén kb.
50 m×h-1 [A1, A2]. Mivel a
járat hossza 600 m, ezért külszíni hatások 12 óra alatt érik el a barlang
végpontját. Ezzel szemben a Cserszegtomaji-kútbarlangban a külszíni hatások
terjedési sebessége a kút aljától 10-20 m-re csak kb. 2 m×h-1.
Ez az alacsony érték mutatja, hogy a barlang szellőzése rossz. A mérések
szerint átlagosan 2 hPa légnyomás emelkedés hatására terjed ki a friss levegő
radonkoncentráció hígító hatása a bejárati kút teljes térfogatára. Az ideális
gáztörvény figyelembevételével a barlang térfogata eszerint kb. 10000 m3,
ami egy nagyságrenddel nagyobb, mint az ismert barlangi járatok térfogatának
becsléséből számított érték.
4.b Kimutattam, hogy kapcsolatot van a
felszín alatti évi átlagos radonkoncentráció és felszín alatti légáramlások
erőssége között [A2,
A8]. A Vass Imre-barlangon átáramló levegő mennyiségét a barlangban található
szifon természetes módon szabályozza. Ha a szifon zárva van, nincs mérhető
légáramlás a barlangban. A légáramlás természetes korlátozása éves szinten
mintegy 30%-os átlagos radonkoncentráció csökkenést okoz, egyben csökkentve az
évszakos radonkoncentráció változások nagyságát. Ez az jelenség a fedő
rétegeken keresztüli légkörzéssel magyarázható s a periodikusan változó
áramlási feltételek átlagos radonkoncentrációt növelő hatását mutatja. A
legmélyebb magyarországi függőleges barlangok (Szepessy, Istvánlápa) mélyén
talált alacsony radonkoncentráció szintek és a radonkoncentráció szezonális változásainak
a hiánya [A3] szintén a fenti jelenségnek tulajdonítható, mivel szellőzés
szempontjából a mély barlangok általában a legzártabbak közé tartoznak.
5. Radontranszport vizekben
5.a Kimutattam, hogy a szellőzési
folyamatokon túl, esetenként, a barlangi vízfolyások is befolyásolják a
barlangi levegő radonszintjét [A1, A2, A4, A9]. A Létrási-Vizes-barlang mélyebb részein a radonszintek
évszakos periodikus változása jelentősen módosul, a barlang végpontja felé a
periodikus jellege csökkenése mellett nő a tavaszi értékek szórása. Ezzel
szemben nagy hasonlóság van a barlang végpontján fakadó egyik időszakos forrás
vízhozama és a levegőben mérhető radonszint időbeli változásai között. Mivel a
felszín alatti vizek oldott radontartalma jelentősen nőhet a porózus kőzeteken
való áthatolásuk során, így a hasonlóság azzal magyarázható, hogy a radont ez
az időszakos patak szállítja a helyszínre.
5.b Egy 270 m mély fúrt karsztkútban
folytatott radonkoncentráció profil mérése alapján keveredési folyamatokat
azonosítottam a függőleges vízoszlopban [A4, A10]. Az adatok értelmezéséhez kifejlesztettem egy
eljárást, amely kapcsolatot teremt a mért radonkoncentráció profil és a
mélységfüggő vertikális transzportsebesség között. Az eredmény a radonnak a
függőleges vízoszlop menti 0,7 m×h-1 átlagos sebességű
felfelé irányuló mozgását jelzi. Megállapítottam, hogy a megfigyelt nagy
vertikális sebesség a vertikális termikus gradiensek indukálta konvekciónak
tulajdonítható, a geogáz mikrobuborékok
által gyorsított radon feláramlás
lehetőségét elvetettem. A vertikális keveredés erős hatását egy 8,5 m
magas modelltornyon (Montpellieri Egyetem, Franciaország) lefolytatott
kísérletsorozat is igazolta [A11]. Mivel a 80-as években Magyarországon az urán
kutatás részben a sekélyfúrásokban felvett víz alatti radonkoncentráció
profilok értelmezésén alapult, az utóbbi eredmény felhasználást nyert a módszer
ipari alkalmazásában.
6. Módszerfejlesztések
6.a Jelentősen hozzájárultam a
szilícium fotodiódán alapuló Dataqua radonmérő eszköz kifejlesztéséhez [A12]. A fotodióda
alkalmazhatóságának ötlete terepi radonkoncentráció mérésekre (alfa részecskék
detektálása útján) Dr. M. Monnin-tól (Montpellieri Egyetem, Hidrogeológiai
Tanszék, Montpellier, Franciaország) származik. A műszer hardwerét és szoftverét
a Dataqua Elektronikai Kft. fejlesztette ki. A mérőfej érzékeny térfogatának
tervezését modellszámításokkal, az elkészült mérőfejet a mérőtérfogat és az
elektronikus beállítások változtatásával kísérletileg teszteltem. A Svéd
Nemzeti Sugárvédelmi Intézet stockholmi referencia radonkamrában folytatott
méréseim alapján meghatároztam a műszer kalibrációs állandóját, 6,7 cph/kBq×m-3×h, mely
jó egyezésben volt a számításaimmal. A Dataqua márkanevű műszer prototípusát
terepi körülmények között a Mecsekurán kft. (Pécs) munkatársa, Dr. Várhegyi
András tesztelte.
6.b. Jelentősen hozzájárultam a radon
vékony fóliákon belüli diffúziós állandójának nyomdetektor technikán alapuló
mérését szolgáló új eljárás kifejlesztéséhez [A13]. A mérés során a vizsgálandó fóliát egy radonforrás és egy mérőkamra közé
helyezzük. Kidolgoztam a módszer hátterét képező modellt, amely leírja a
radonkoncentráció időbeli változását a mérőkamrában a mérendő diffúziós állandó
függvényében. Az eljárás lehetővé teszi a vizsgálati idők csökkentését néhány
órára. A módszer alkalmazhatóságát különböző vastagságú polietilén fóliákkal
laboratóriumi körülmények között teszteltem. A kapott érték, 7,1*10-8
cm2×s-1, jól egyezik az
irodalomban publikáltakkal. A különböző anyagok radonáteresztő képessége a
radont a lakásokból kizáró szigetelő anyagok iránti igény miatt érdekes. A
módszer gyakorlati alkalmazásaként megvizsgáltuk néhány padlóburkoló műanyag
radonáteresztő képességét. Úgy találtuk, hogy mindegyik anyag 2-3
nagyságrenddel csökkenheti a talajeredetű forráshoz köthető radonszintet a
lakásokban [A13].
6.c Jelentősen hozzájárultam a radon polimer/szilikát gélekben és agyag szuszpenziókban
való effektív diffúziós állandójának mérését szolgáló két további
eljárás kifejlesztéséhez [A16, A17]. Hasonlóan az előbbi eljáráshoz, a vizsgálandó anyagot egy
radonforrás és egy mérőkamra közé helyeztük. Nyomdetektorokkal mértük a
radonexpozíciót a minták két oldalán. A mintán belüli diffúziós
tömegtranszportot numerikus modelleztük különböző mintavastagság és besugárzási
idők esetére. A diffúziós állandót, mint paramétert, a mért expozíciókhoz
tartozó legjobban illeszkedő modellezés eredménye szolgáltatta. A kísérleti
elrendezés zártságát Dataqua típusú folyamatos radonmérőkkel felvett idősorok
elemzése alapján ellenőriztem. Az elemzés a besugárzási idő
rész-időintervallumaira meghatározott diffúziós paraméter állandóságának
vizsgálatán alapult. Úgy találtuk, hogy a vizsgált polimer/szilikát gélt
tartalmazó porózus közegben az effektív diffúziós állandó 3,3*10-6
cm2×s-1, míg a Montax/agyag
szuszpenzióban 6,0*10-6 cm2×s-1. Az eredmények alapján
a tiszta fázishoz tartozó értékek összemérhetők a pusztán vizes oldatokat
jellemző adattal, ezért az ezeket az anyagot alkalmazó szigetelési eljárások
nagyon vonzóak lehetnek.
6.d.
Jelentősen hozzájárultam a vízminták rádium és oldott radon tartalmának mérését
szolgáló, nyomdetektor technikán alapuló új módszer kidolgozásához [A14]. A mérés során a vízmintát egy kis térfogatú,
gumimembránba csomagolt radon monitorral együtt egy üvegedénybe zárjuk. A
gumimembrán átengedi a radont de kizárja a vizet a radonmonitor
mérőtérfogatából. A vízminta oldott radon és rádium tartalmát két független
méréssel határozzuk meg. Az első méréssel a rádium és oldott radon együttes
koncentrációját, a második méréssel a rádium koncentrációt határozzuk meg.
Kidolgoztam a kis térfogatú radon monitor mérőtérfogatán belüli a
radonkoncentráció időbeli változását leíró modellt. Az általunk használt mérési
elrendezésre laboratóriumi mérésekkel meghatároztam modellben szereplő
kalibrációs állandó értékét, 24,1 nyom×cm-2/nap×kBq×m-3
[A15], mely jó egyezésben volt számításaimmal. A kidolgozott módszerre
jellemző, hogy semmilyen elválasztási technikát nem alkalmaz, melyek használata
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A3. Hakl J., Hunyadi I., Csige I., Géczy G., Lénárt L.,
Törőcsik I. (1992) Outline of natural radon occurences on karstic terrains of
Hungary. Radiation Protection Dosimetry,
45, pp. 183-186
A4. Hunyadi I., Hakl J.,
Lénárt L., Géczy G., Csige I. (1991) Regular subsurface radon measurements in
Hungarian karstic regions. Nuclear Tracks
and Radiation Measurements, 19, pp. 321-326
A5. Hakl J., Hunyadi I. and Törőcsik I. (1991)
Radon measurements in the Baradla cave. In: Conference
on the karst and cave research activities of educational and research
institutions in Hungary, (eds. L. Zámbó, M. Veres), pp. 109-115, ISBN
9637173692, Berzsenyi College, Szombathely
A6. Hakl J. (1992) A radontranszport dinamikájának
vizsgálata a Vass Imre-barlangban és a
Cserszegtomaji-kútbarlangban végzett mérések alapján. Karszt és barlang, I-II,
pp. 15-20
A7. Hakl J., Hunyadi I., Várhegyi A, Morin, J.P., Seidel,
J.L. and Monnin, M. (1994) Experimental and theoretical studies on radon
transport based on monitoring in caves. Atomki Annual Report 1993, pp. 110-111
A8. Hakl J., Csige I., Hunyadi I., Várhegyi A. and Géczy G.
(1996) Radon transport in fractured porous media - experimental study in caves.
Environment International, 22, pp. S433-S437
A9. Lénárt L., Somogyi Gy., Hakl J. and Hunyadi I. (1989) Radon mapping in caves of Eastern
Bükk region. In.: Proceedings of the 10th. International Congress of
Speleology, Vol. 2 (ed. Kósa A.),
pp. 620-622, Magyar Karszt- és Barlangkutató Társulat, Budapest
A10. Hakl J., Lénárt L. and Somogyi Gy.(1989) Time
integrated radon measurements performed in a karstic well water. In.:
Proceedings of the 10th. International Congress of Speleology, Vol. 2 (ed. Kósa A.), pp. 618-619, Magyar
Karszt- és Barlangkutató Társulat, Budapest
A11. Várhegyi A, Hakl J.,
Monnin M., Morin J.P. and Seidel J.L. (1992) Experimental study of radon transport
in water as test for a transportation microbubble model. Journal of Applied Geophysics,
29, pp. 37-46
A12. Várhegyi A. and Hakl
J. (1994) A silicon sensor based radon monitoring device and its use in
environmental geophysics. Geophysical
Transactions, 39, pp. 289-302
A13. Hakl J., Hunyadi I., and Tóth-Szilágyi M. (1991) Radon
permeability of foils measured by SSNTD technique (Non-equilibrium approach). Tracks and Radiation Measurements, 19,
pp. 319-320
A14. Hakl J., Hunyadi I., Varga K. and Csige I. (1995) Determination
of radon content of water samples by SSNTD technique. Radiation Measurements, 25,
pp. 657-658
A15. Hakl J., Hunyadi I., Csige I., Vásárhelyi A., Somlai
J., Faludi G. and Varga K. (1995) Determination of dissolved radon and radium
content of water samples by track etch method. Environment International, 22,
pp. S315-S317
A16. Csige I, Hakl J.
and Lakatos I. (1995) Measurement of effective diffusion coefficient of radon
in porous media with etched track monitors. Radiation
Measurements, 25, pp. 659-660
A17. Lakatos I., Bauer K., Lakatos-Szabó J., Csige I., Hakl J. and Kretzschmar H.-J. (1997)
Diffusion of radon in porous media saturated with gels and emulsions. Transport in Porous Media, 27, pp. 171-184
