Vol. 48, 01, 05ok.qxd 73 ANNALS OF GEOPHYSICS, VOL. 48, N. 1, February 2005 Key words radon – electrical conductivity – ions – thermal springs – earthquakes 1. Introduction Significant variations in geophysical and geo- chemical parameters in soil gas and natural springs have been detected prior to or during earthquakes. The observed geophysical phenome- na include variations in water level in deep wells, hydrostatic pressure, water flow rate from wells, electrical conductivity and water temperature. The most frequently studied geochemical phenomena have been concentrations of dissolved gas and ions in groundwater and variations in concentra- tions of crustal and mantle volatiles in ground gas- es. (Wakita, 1978; King, 1980; Nakamura and Wakita, 1985; Wakita et al., 1985, 1986; Thomas, 1988; King et al., 1994; Koch and Heinicke, 1994; Dongarra et al., 1995; Heinicke et al., 1995; Igarashi et al., 1995; Shimizu et al., 1996; Mon- nin and Seidel, 1997; Bella et al., 1998; Di Bello et al., 1998; Woith et al., 1998; Singh et al., 1999; Zhang, 1999; Biagi et al., 2000; Heinicke et al., 2000; Favara et al., 2001; Koch et al., 2003). Stress/strain fields have been reported to gen- erate geochemical and geophysical anomalies (King, 1978, 1986, 1989; Wakita et al., 1985, 1986; Igarashi et al., 1995; Zhang, 1999; Biagi et al., 2000). Even small strains are able to do so because they can be greatly amplified at pre-ex- isting faults and fractures, where pre-existing stresses may be already near the critical levels and where pore fluids are abundant. The thresh- old strain changes required to cause the observed earthquake-related anomalies were estimated to be ∼ 10 – 8, which is comparable with tidal strain Mailing address: Dr. Andreja Popit, Jožef Stefan In- stitute, Jamova 39, 1000 Ljubljana, Slovenia; e-mail: an- dreja.popit@ijs.si Geochemical and geophysical monitoring of thermal waters in Slovenia in relation to seismic activity Andreja Popit (1), Janja Vaupotič (1) and Tadej Dolenec (2) (1) Jožef Stefan Institute, Ljubljana, Slovenia (2) Faculty of Natural Sciences and Engineering, University of Ljubljana, Ljubljana, Slovenia Abstract Pre-seismic related strains in the Earth’s crust are the main cause of the observed geophysical and geochemical anomalies in ground waters preceding an earthquake. Posočje Region, situated along the Soča River, is one of the most seismically active areas of Slovenia. Our measuring stations close to the Posočje Region were installed in the thermal springs at Bled in 1998 and at Zatolmin in 1999. Since the beginning of our survey, radon con- centration, electrical conductivity and water temperature have been measured continuously once every hour. In May 2002, the number of geochemical parameters monitored was extended to ionic concentration, pH and Eh, which are analysed once a month. Before seeking a correlation between geochemical and geophysical anomalies with seismic events, the influence of meteorological (atmospheric precipitation, barometric pressure) and hy- drological (water table of the Tolminka River) factors on observed anomalies were studied. Results at Zatolmin showed that some radon variation during the period from June to October 2002 may be related to seismic activ- ity and not only to meteorological effects. 74 Andreja Popit, Janja Vaupotič and Tadej Dolenec amplitudes (King, 1986). Wakita et al. (1986) and King (1989) indicated that chemical precursor signals are detectable especially in faults and at intersections of faults. Much evidence demonstrates the major role of crustal fluids of deep origin in the earthquake cycle (Varhegyi et al., 1986; Toutain and Baubron 1999). Geochemical and geophysical anomalies are due to the slow growth of small cracks in the crust caused by stress corrosion by groundwater. Associated micro-fracturing of aquifers may cause mixing of fluids from two or more chemically distinct aquifers (Thomas, 1988; Igarashi et al., 1995; Singh et al., 1999; Heinicke et al., 2000). The exposure of fresh rock surfaces, resulting from mineral fracture, causes leaching of minerals with groundwater and allows the escape of trapped gases from the rock matrix (Thomas, 1988). Thus, geochemi- cal anomalies are strongly dependent on local conditions such as rock type, stress/strain field, degree of water saturation of rock pores, etc. Several thermal springs and wells have been reported to discharge mixtures of deep crustal flu- ids and shallow meteoric components, and pre- cursory changes in fluid composition were shown to be the result of increasing contributions of the deep component to the total flow (Thomas, 1988). Advection of carrier gases, such as CO2 and CH4, may be the main migration process for transport of trace gases (Rn, He) toward the Earth’s surface. In contrast to gas advection, transport mechanisms of endogenous gases by gas diffusion or by water advection (unless un- der rare conditions of high-velocity resurgent water) are usually far too slow to cause the ob- served geochemical anomalies. Bubble move- ment through micro-fractured rocks (fracture aperture of 0.01 to 10 mm at depths of thou- sands of metres) appears to be an effective mode of rapid (gas velocities of the order of 10 to 1000 m per day) and long-distance gas migra- tion. The evolution from bubble regimes to con- tinuous phase flow and vice versa is due to gas pressure and fracture width changes, which pro- vide the most suitable mechanism for explaining surface geochemical anomalies linked to seis- mo-tectonic processes (Varhegyi et al., 1986; Dongarra et al., 1995; Etiope and Martinelli, 2002). A significant correlation between Rn and CO2 in spring gas was found in Vogtland, Ger- many (Heinicke et al., 1995; Koch et al., 2003) and in the Southern Apennines, Italy (Di Bello et al., 1998). Therefore, it should be useful in earthquake prediction research to continuously monitor springs that are abundant in CO2 (Koch and Heinicke, 1994). Geothermal waters are in contact with crustal rocks at various depths. This is why spring gases might be more representative of the local envi- ronment than soil gases. Spring gases are much more enriched with deep gases and only slightly contaminated by atmospheric gases and have been proved to be better earthquake precursors (Toutain and Baubron, 1999). The first geochemical monitoring in Slovenia connected with seismic activity was carried out on thermal waters in the Ljubljana Basin from 1981-1982 by measuring radon concentrations (Zmazek et al., 2000). In 1999, radon measure- ments were extended to other thermal waters in NW and E Slovenia (Zmazek et al., 2002a) and to soil gas along a presumed fault in the Krško Basin (Zmazek et al., 2002b, 2003). In addition to continuous (once per hour) measurements of radon concentration, electrical conductivity and water temperature were monitored, and gas was analysed monthly for He isotopes. Geochemical and geophysical monitoring in thermal water at Zatolmin and Bled (both in NW Slovenia) was extended from May 2002. Besides continuously (once per hour) measured radon concentration, electrical conductivity and water temperature, ionic concentration, pH and Eh in the springs were also analysed once per month. The aim of our investigation was to study changes in geochemical and geophysical parameters in thermal springs in relation to seismic activity. 2. Geotectonic and seismogenic setting of Slovenia The geotectonic position of the Slovenian territory is relatively well understood within the framework of the plate tectonic model. The recent structural pattern is a cumulative result of Tethyan evolution, where recent dynamics is determined by the closure of the Tethys and the collision of several lithospheric units. Slovenia 75 Geochemical and geophysical monitoring of thermal waters in Slovenia in relation to seismic activity lies at the junction of three tectonic plates (Eu- ropean, Adriatic and Tiszian), which were amalgamated during the Tertiary period. The seismicity is not concentrated along the pri- mary plate boundaries, but is rather spread in a broad zone along their deformed rims (Poljak et al., 2000). There is general agreement between the tec- tonic pattern and its geological dynamics with the type of seismicity in a particular area. Thus, seismogenic areas (Eastern Alps, Southern Alps, Friuli Region, external Dinarides and Transdanubian Range) generally coincide with the main geotectonic units (Poljak et al., 2000). Our monitoring stations are installed at two thermal springs (fig. 1). The first is located at Bled and wells on the Bled fault on the eastern side of Lake Bled (46.37N, 14.11E) which be- longs to Southern Alps. The thermal water springs from a deep Triassic carbonate reservoir and rises through Oligocene clay and shallower Quaternary lake and glacial sediment. The ther- mal water at Zatolmin springs from Cretaceous limestone at the fault in the Cave under the Devil’s bridge (46.20N, 13.74E) on the south- ern edge of the Southern Alps, close to the bor- der with the external Dinarides. Both waters be- long to calcium-magnesium-hydrocarbonate- sulphate water type. The Southern Alps are characterised by re- gional thrusts from the north to the south and a number of regional faults in a NW-SE direction. These faults originate from the Paleogene ex- ternal Dinaric tectonic evolution and they are superimposed by the Neogene Southern Alps thrusts. They were reactivated in the post-Neo- gene period. The seismicity of this zone is moderate: the strongest shock occurred in 1895 and damaged the city of Ljubljana (intensity VIII-IX MSK) (Poljak et al., 2000). 3. Analytical methods Radon concentration is measured continu- ously with a Barasol MC 450 probe (Algade, France). Radon enters the detection chamber through a filter, which prevents radon decay products from entering. The detection unit is a solid-state silicon detector. The measurement is carried out by gross alpha counting. The counts were integrated over an hour. In addition to Fig. 1. Geotectonic situation of Slovenia and locations of the thermal springs at Zatolmin and Bled. 76 Andreja Popit, Janja Vaupotič and Tadej Dolenec measurement of radon, water temperature is measured over the same time periods. The sensi- tivity of the radon detector is 50 Bqm–3, and tem- perature is measured to an accuracy of ± 0.01°C. Conductivity and water temperature are measured once per hour with EC 350 Electrical Conductivity Sensor (Greenspan, Australia), employing an electromagnetic field for measur- ing conductivity. An increase in charged ion mobility or concentration causes a decrease in resistivity and a corresponding increase in the output of the EC sensor. Water temperature is monitored by a separate sensor, which provides both a temperature output and a signal to nor- malise and temperature compensate the EC out- put. The sensitivities for the EC and tempera- ture sensor are 0.01µScm–1 and 0.01°C, respec- tively. The temperature measured with the EC sensor was used for the interpretation. Eh and pH are measured by portable redox combination electrodes (Mettler-Toledo) in situ once per month. Water samples for cation analyses are col- lected once per month. Dissolved cation con- centrations are analysed in the ACTLABS labora- tory in Canada by Inductively Coupled Plasma Mass Spectrometry (ICP/MS). Meteorological (average daily air tempera- ture and barometric pressure, daily atmospheric precipitation) and hydrological data (daily aver- age water table of the Tolminka River) were provided by the Environmental Agency of the Republic of Slovenia (ARSO), Office of Mete- orology and Office of Seismology. Seismological data were obtained from AR- SO, Office of Seismology. 4. Results and discussions Geochemical and geophysical monitoring in relation to seismic events was performed in two Slovenian thermal waters. Our research was fo- cused primarily on precursory changes ob- served in radon concentration, water tempera- ture, electrical conductivity, concentrations of dissolved ions, pH and Eh. Non-seismic inter- ferences by meteorological (average daily baro- metric pressure and daily atmospheric precipi- tation) and hydrological parameters (daily aver- age water table of the Tolminka River) were studied. Changes in observed geochemical and geo- physical parameters exceeding two standard de- viations (2σ) from the mean value were consid- ered anomalies (Heinicke et al., 1995; Di Bello et al., 1998; Planinić et al., 2001; Virk and Walia, 2001), according to the normal distribu- tion of measurements, where 95% of the obser- vations are contained in the interval ± 2σ from the mean value. Biagi et al. (2000) labelled each ion or gas concentration signal with amplitude greater than 3σ as an irregularity. 4.1. Measuring station at Zatolmin The thermal spring at Zatolmin is located in a natural cave along the Tolminka River, which overflows it constantly, but more intensively dur- ing heavy rains. This spring is characterised by CO2 bubbling activity. The average water tem- perature and its standard deviation (± σ) between May 2002 and April 2003 was 21.5 ± 0.8°C. Probes for radon and electrical conductivity measurements are installed in the spring-water at the bottom of the cave. The average electrical conductivity and radon concentration (both ± σ) in this thermal spring between May 2002 and April 2003 were 754 ± 47 µScm–1 and 33.4 ± 19 kBqm–3, respec- tively. Variations in radon concentration, elec- trical conductivity and water temperature showed significant temporal fluctuations, re- sulting mainly from atmospheric precipitation. These variations were distinguished from those which could be earthquake related. Electrical conductivity is a function of both ion concentration and water temperature. The correlation coefficients between electrical con- ductivity and calcium, potassium and silicon concentration were 0.88, 0.98 and 0.79, respec- tively. The linear trends of electrical conductiv- ity and water temperature curves (fig. 2) were interrupted during the periods of heavy rain, when the water level of the torrential Tolminka River increased and heavy mixing of both wa- ters occurred. These events are characterised by an instant decrease in water temperature, elec- trical conductivity and dissolved ion concentra- Fig. 2. Geochemical data measured in the thermal spring at Zatolmin between May 2002 and April 2003. The first diagram (starting from the top) shows radon concentration (kBqm–3) and earthquakes (ML). E is the distance between the measuring station and the epicentre, while is D the strain radius. Earthquakes with E / D ≤ 1 are drawn with bold lines and those with E / D >1 with dashed lines. The second diagram shows electrical conductivity (µ S cm–1) and wa- ter temperature (°C), and the third diagram shows rainfall (mm) and the level of the Tolminka River (cm). Table I. List of earthquakes near the thermal spring at Zatolmin and Bled in the period from May 2002 to April 2003. E (km) is the distance between the measuring station and the epicentre, while D (km) is the radius of the precursory manifestation zone. Earthquakes with (E/D ≤ 1) are written bold. Date Time Latitude Longitude ML E D E/D (UTC) 22.05.2002 17:30 46.34 14.09 0.9 4 SW 2 2.0 07.06.2002 12:38 46.32 14.11 1.2 6 S 3 2.0 02.06.2002 13:37 45.64 14.24 3.8 73 SE 43 1.7 19.06.2002 19:07 46.14 13.59 1.8 14 SW 6 2.3 26.06.2002 11:13 46.24 13.70 1.6 5 NW 5 1.0 15.07.2002 09:04 46.29 13.69 3.2 10 NW 24 0.4 22.07.2002 18:19 46.31 13.59 2.0 18 NW 7 2.6 02.08.2002 02:09 46.22 13.76 1.4 4 NE 4 1.0 20.09.2002 12:19 46.12 13.66 2.6 10 SW 13 0.8 30.09.2002 02:48 46.33 13.61 3.5 18 NW 32 0.6 08.10.2002 22:24 46.34 13.63 2.0 17 NW 7 2.4 15.11.2002 09:15 46.12 13.55 2.1 17 SW 8 2.1 11.01.2003 14:32 46.24 13.77 1.6 5 NE 5 1.0 02.02.2003 01:43 46.26 13.48 2.3 21 NW 10 2.1 09.02.2003 23:22 46.25 13.70 1.5 6 NW 4 1.5 24.02.2003 19:38 46.22 13.79 0.6 4 NE 2 2.0 77 Geochemical and geophysical monitoring of thermal waters in Slovenia in relation to seismic activity 78 Andreja Popit, Janja Vaupotič and Tadej Dolenec tion due to dilution of spring water with river water. The correlation coefficient between wa- ter temperature and level of the Tolminka Riv- er was – 0.75. The correlation coefficient be- tween magnesium concentration in the Tolmin- ka River and in the spring water was 0.60 and for silicon, 0.62. The Ca, K, Na, Mg and Si concentrations increased by up to 50% of their initial values during the period from the middle of August to the end of September 2002 (fig. 3), and then decreased to approximately the origi- nal values by November 2002. Electrical con- ductivity showed a similar trend (fig. 3). In the period from the middle of August to the end of September 2002 there was only a small amount of rainfall, while in the period from September to November 2002, precipitation was more in- tense, which is evident also by the increase in Tolminka level (fig. 2). The pH and Eh values measured monthly from March to May 2003 were relatively sta- ble, with average values of 6.9 and 250 mV, re- spectively. Small variations in pH could be ex- plained by dilution of spring water with rain water, or by dissolved CO2, which lowers the pH, and dissolved CH4, which increases pH by stripping CO2. During the monitoring period between May 2002 and April 2003, six earthquakes of magni- tude ML between 1.4 and 3.5 occurred in the vicinity of the thermal spring (fig. 2, table I). The distances from the spring to their epicentres (E) were shorter than the strain radii (D) that define the area in which the effects of the earthquake are, in principle, detectable according to the em- pirical formula: D = 10 exp (0.43 ML), introduced by Dobrovolsky (1979). In the period between June and October 2002 when five of the earth- quakes with E/D ratio equal to or below 1 oc- curred, radon concentration significantly in- creased, while in the period between January and April 2003, with three earthquakes of ML be- tween 1.5 and 2.1 and E/D ratios greater than 1, radon level was much lower. This fact favours the theory of the stress-strain field defined by Do- brovolsky. Two decreases of radon concentration from 27 to 15 kBqm–3, which occurred between May 17 and June 6, 2002, could be explained by di- lution of thermal water with river water as a consequence of rainfall (fig. 2). At the same time water temperature also decreased. Radon concentration correlated positively with water temperature until June 6, 2002. Between June 7 and June 10, 2002 the positive correlation be- tween radon concentration and water tempera- ture changed (fig. 4). Radon concentration in- creased significantly during the heavy precipi- tation, when the level of the Tolminka River in- creased and water temperature decreased. The first radon anomaly of 2σ above the average value occurred twelve days before the earth- quake of ML = 1.6 (fig. 4). The distance from the measuring station to the epicentre corre- sponded to the strain radius (E / D = 1). Dilution of radon concentration due to rainfall was in- significant, so that the radon anomaly might re- flect changes in the regional stress field preced- ing the earthquake. Radon concentration began to decrease soon after it reached the value 2σ above the average and continued to do so until the earthquake, which might be attributed to the stress relaxation prior to the earthquake and Fig. 3. Ion concentrations (mgL–1), electrical con- ductivity (µ cm–1) and water temperature (°C) in the thermal spring at Zatolmin between May 2002 and April 2003. 79 Geochemical and geophysical monitoring of thermal waters in Slovenia in relation to seismic activity some subsequent sealing of micro-cracks lead- ing to decreased radon emanation (King, 1989; Igarashi et al., 1995). A second increase in radon level at the begin- ning of July 2002 started at the same time as the increase in water level and decrease in water tem- perature (figs. 2 and 4). A radon concentration of 1.8σ over the average value was observed nine days before an earthquake of ML = 3.2. The dis- tance from the measuring station to the epicentre was shorter than the strain radius (E / D = 0.4). Soon after the peak in radon concentration was reached, it decreased until the seismic event oc- curred. This decrease might be explained by stress relaxation associated with the earthquakes and not by dilution of thermal water with rainwater, be- cause there was no rainfall during that period. Subsequently, radon concentration increased, in spite of the small amount of precipitation and dilution of spring water with rainwater (figs. 2 and 4). It reached 1σ above the average value at the time of an earthquake of ML = 1.4. The dis- tance from the measuring station to the epicentre corresponded to the strain radius (E / D = 1). A radon anomaly of 1.96σ was reached six days af- ter this earthquake, coinciding with the start of heavy precipitation. Soon, the torrential Tolmin- ka River caused violent mixing of thermal water with cold river water. This is evident from the de- creases in water temperature and radon concen- tration. Radon reached its minimum value one month after the last radon anomaly of 2σ. Radon concentration again started to increase after September 1, 2002, when there was no sig- nificant rainfall, and reached 1σ above the aver- age value at the time of an earthquake of ML = = 2.6 (figs. 2 and 5). The distance from the meas- uring station to the epicentre was shorter than the strain radius (E / D = 0.8). The radon concentra- tion reached its maximum of 1.6 σ five days be- fore an earthquake of ML = 3.2. The distance from the measuring station to the epicentre was shorter than the strain radius (E / D = 0.6). It then decreased until the seismic event took place. That might be explained by stress relaxation as- sociated with the earthquakes but not by dilution Fig. 4. Rn concentration (kBqm–3), earthquakes (ML), rainfall (mm) and the level of the Tolminka River (cm) in the thermal spring at Zatolmin between May and August 2002. Fig. 5. Rn concentration (kBqm–3), earthquakes (ML), rainfall (mm) and the level of the Tolminka Riv- er (cm) in the thermal spring at Zatolmin between Au- gust and October 2002. 80 Andreja Popit, Janja Vaupotič and Tadej Dolenec of thermal water with rainwater, because there was no rainfall during that period. Another two radon peaks of 1.8σ and 1.7σ occurred approximately one month before an earthquake of ML = 2.1 (fig. 2). The measuring station was outside the area, defined by the strain radius (E / D = 2.1). The influence of this earthquake on the radon anomaly was consid- ered less significant. The last radon increase of 1.9σ occurred one and a half months before an earthquake of magnitude ML = 1.6 (figs. 2 and 6). The dis- tance from the measuring station to the epicen- tre corresponded to the strain radius (E / D = 1). No significant variation of radon concentra- tion was found from the middle of January to April 2003 (fig. 2), although four consecutive earthquakes occurred during this period. This could be explained by the fact that the measuring station was outside the area, defined by the strain radius (E / D = 2.1, 1.5, 2.0, 2.3, respectively). Our explanation of radon anomalies and their relation to earthquakes, as well as our in- terpretation of physical processes causing radon variations, will be strengthened after a longer monitoring period. 4.2. Measuring station at Bled Thermal water at Bled wells out in a basin at the Grand Hotel Toplice. It displays CO2 bub- bling activity. The average water temperature between May 2002 and April 2003 was 21.9 ± ± 0.2°C. The water is used for bathing and drinking. Instruments are installed at the bot- tom of the collecting basin, from where the wa- ter is released into the swimming-pool. The average electrical conductivity of spring water between May 2002 and April 2003 was 869 ± 21 µScm–1. No correlation be- tween meteorological parameters and electrical conductivity or water temperature was ob- served (fig. 7). The only two earthquakes dur- ing the monitoring period occurred on May 22, 2002 with a ML of 0.9 and on June 7, 2002 with a ML of 1.2 (table I). In both cases, the measuring station was outside the area, defined by the strain radius (E / D = 2.0). No anomalous values of electrical conductivity or water tem- perature were detected at that time. The electri- cal conductivity suddenly decreased from 895 to 860 µScm–1 in September 2002 and then re- turned to the previous value in April 2003, which cannot be attributed to seasonal varia- tions, because of the rapidity of the changes; on the other hand water temperature changes were smaller and of shorter duration. A longer monitoring period is required to ex- plain this decrease in electrical conductivity. The correlation coefficient between electrical con- ductivity and water temperature was 0.63. The Ca, K, Na, Mg and Si concentrations decreased in the period from December 2002 to March 2003, when no significant precipitation was recorded (figs. 7 and 8). Thus, inflow of Ca, Mg, K, Na and Si to thermal water resevoir from me- teoric water was much reduced during this time. Eh and pH values measured monthly from February to May 2003 were relatively stable, and averaged 220 mV and 6.7, respectively. Fig. 6. Rn concentration (kBqm–3), earthquakes (ML), rainfall (mm) and the level of the Tolminka Riv- er (cm) in the thermal spring at Zatolmin between De- cember 2002 and February 2003. 81 Geochemical and geophysical monitoring of thermal waters in Slovenia in relation to seismic activity The average radon concentration between May 2002 and April 2003 was 9 ± 2 kBqm–3. The radon curve showed a trend of slow de- crease with small fluctuations. Data accumu- lated in a quiescent period is useful as a base- line for recognizing possible precursory changes in the future (Nakamura and Wakita, 1985). 5. Conclusions Much of the variation in radon concentra- tion, electrical conductivity, water temperature, ion concentration, pH and Eh in the thermal spring at Zatolmin could be assigned to non- tectonic causes, including rainfall and the level of the Tolminka River. In the period between June and November 2002, radon concentration was always above 1σ and a few times it even reached 2σ over the mean value. But in the pe- riod between January and April 2003 radon lev- el was constantly up to 1σ below the mean val- ue. If precipitation were the main cause for in- Fig. 7. Geochemical data measured in the thermal spring at Bled between May 2002 and April 2003. The first dia- gram (starting from the top) shows radon concentration (kBqm–3) and earthquakes (ML). E is the distance between the measuring station and the epicentre, while is D the strain radius. The second diagram shows electrical conductivity (µ Scm–1) and water temperature (°C), and the third diagram shows rainfall (mm) and barometric pressure (hPa). Fig. 8. Ion concentrations (mgL–1), electrical con- ductivity (µ Scm–1) and water temperature (°C) in the thermal spring at Bled between May 2002 and April 2003. 82 Andreja Popit, Janja Vaupotič and Tadej Dolenec creased radon level between June and Novem- ber 2002, then we would expect a larger in- crease in radon concentration also during the rainfall in December 2002 and from March to April 2003. Thus, radon behaviour cannot be simply related to meteorological factors. In the period with radon levels exceeding 1σ over the mean value, six earthquakes with E/D ratio equal or below 1 occurred while in the period with radon concentration up to 1σ below the mean value, three earthquakes with E/D ratio above 1 occurred. This fact is in accordance with the theory of the stress-strain field defined by Dobrovolsky. Our interpretation of physical processes causing radon variations will be im- proved after a longer monitoring period. Changes in electrical conductivity and water temperature in the thermal spring at Bled showed no correlation with meteorological parameters. Fluctuations of radon concentration were in- significant due to low seismic activity over the period. Data accumulated in a quiescent period will be useful as a baseline for recognizing pos- sible precursory changes in the future. Continuous monitoring (frequency of sam- pling and analyses once per hour) of radon con- centration, electrical conductivity and water tem- perature in spring waters, together with grab sampling and analyses (once per month or im- mediately after the seismic event) of dissolved ion concentrations constitute an important basis for studying the relationship between the meas- ured parameters and seismic activity. 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