Overview of isotopic investigations of groundwaters in a fractured aquifer system near Rogaška Slatina

The isotopic investigations of groundwaters stored in the Rogaška Slatina fractured aquifer system were performed in the periods 1978–1985 and 2007–2011 aiming at answering open questions on the groundwater recharge and dynamics, on connections between different types of aquifers and on solute transport. Environmental isotopes 2H, 18O, 3H, 13C of dissolved inorganic carbon and 14C were analysed in mineral, thermo-mineral and spring waters. Results indicated the source and mechanism of groundwater recharge, its renewability, a transit time distribution, hydraulic interrelationships, the groundwater origin and its evolution due to effects of waterrock interaction. They proved the hypothesis that the Boča massif should be a catchment area of the Rogaška mineral waters. The estimates of the mean residence time of mineral waters in the aquifer system are between 7200 and 3400 years, depending on the location and depth. Thermo-mineral water is the oldest water in the study area with the mean residence time of 14000 years.


Introduction
Rogaška Slatina is famous by mineral water, which was discovered in this place in the time of ancient Rome.Numerous investigations of the Rogaška groundwaters were subject to balneology and to the larger exploitation quantities (NosaN, 1975), whereas information essential for the definition of the Rogaška aquifer system and for its protection has been still missing.Questions on the groundwater recharge area and dynamics, on connections between aquifers and on solute transport have remained open, which depends on the field geology and structure.The latter is very complicated -three regional faults intersect in this area, which is folded to anticlinal and synclinal folds.The nature of geological structures, their mutual relations and extent have not been explained at a satisfactory level in many parts of the system.With regard to results of previous hydrogeochemical investigations (NosaN, 1973(NosaN, , 1975;;Pezdič, 1986Pezdič, , 1997) ) it was presumed that the Boč massif near Rogaška Slatina is the catchment area of the Rogaška mineral waters, although geological data did not support this hypothesis (Aničić & Juriša, 1984).
Aiming at answering the discussed open questions also the isotopic investigations of groundwaters stored in the Rogaška Slatina fractured aquifer system were performed.The first studies took place in the period 1978-1985(Pezdič, 1997)), while the last studies were performed during a period 2007-2011 (Trček et al., 2010;Trček & Leis, 2011).The isotopic investigations based on the environmental isotopes of H, O and C, which were used as tracers of geological and hydrogeological processes.
The applications of stable isotope ratios of hydrogen and oxygen in groundwater hydrology are based primarily on isotopic variations in precipitation as the predominant groundwater source.After the infiltration of precipitation into the aquifer, only physical processes, such as diffusion, dispersion, mixing and evaporation, alter the groundwater isotopic composition (Clark & Fritz, 1997).The stable isotope content of water may be considered conservative under low-temperature and low-circulation groundwater systems, as long as the relative amount of water involved in chemical reactions remains limited (Clark & Fritz, 1997;Hoefs, 1997).The exchange with oxygen (possibly also hydrogen) bearing minerals of the host rocks is particularly important in geothermal environment.With low reaction rates in low-temperature environments a long time is needed for a significant exchange to take place and equilibrium will generally not be reached (IAEA, 1983).Another process, which may modify the initial stable isotope content of groundwater, is the isotopic exchange with a gas phase which is not initially in equilibrium with the environmental water (e.g.CO 2 or H 2 S).
The stable carbon isotope composition of dissolved inorganic carbon (δ 13 C-DIC) is not a conservative tracer.Nevertheless, the δ 13 C can trace the carbon sources and reactions for numerous interacting organic and inorganic species.The isotope ratio 13 C/ 12 C is an important tool for quantifying the water-rock interactions, identifying the proportion of different CO 2 sources in water, and determining the initial geological settings of the groundwater recharge (Hoefs, 1997;KeNdall & McdoNNell, 1998).
Groundwater dating is the main field of application of the 14 C and 3 H radioactive isotopes.Their input source functions, which describe the time-varying global fluxes of isotopes deduced from atmospheric, cosmogenic and anthropogenic production, are well known (Mook, 1980;Hoefs, 1997).The measured activity concentrations are compared with the input functions to get fairly informative age determinations over the past several decades. 3H has a half-life of 12.3 years and is a very applicable tracer for determining spring residence times when recharge processes took place within a timescale of less than 50 years.Groundwaters seldom have more than 50 TU today and are typically in the 5-10 TU range (criss et al., 2007; rose, 2007).The recent 3 H study (Kovačič, 2015) indicated that Slovene groundwaters could be divided into four categories -groundwaters that are older than 100 years ( 3 H concentration is below the detection limit), groundwaters with the prevailing old component ( 3 H concentration is between 0 and 2.5 TU), groundwaters with the age between 30 and 60 years ( 3 H concentration is on average 8 TU) and recent groundwaters with age up to 15 years ( 3 H concentration is about 6 TU).
On the other hand 14 C with a half-life of 5,730 years is applicable for dating groundwaters recharged prior to 1,000-2,000 years (Bajjali et al., 1997) and as old as 30,000 years (Clark & Fritz, 1997).

Study area
The study area with its broader surroundings is geologically one of the most complex parts of Slovenia (Trček et al., 2010).It is the juncture of three major regional fault systems, separating three tectonic units (Figs. 1 and 2).The Boč massif belongs to the Southern Karavanke unit.It borders to the south with the Donat line to the narrow tectonic unit between the Donat line and Šoštanj fault close to which the town of Rogaška Slatina is situated.To the north, the Dravinja fault as part of the Periadriatic line separates the Southern Karavanke unit from the Upper Austro-Alpine unit.The ongoing dextral strike-slip zone along the Lavantal fault and the Šoštanj fault (Fig. 2) associates eastward extrusion of the Eastern Alps as a result of the northward shift which are relatively displaced and folded (Figs. 1 and 2).Most of the area is composed of massive limestones and dolomites of the Carnian age.They are capping the Boč mountain crest.The Oligocene and Miocene beds, covering the northernmost part of the territory and the lower slopes north of Rogaška Slatina, are composed of alternating sandstones, sands, shaly claystones and marlstones and conglomerates in the lower parts.A wide belt of volcanic rocks (andesite, its tuffs and volcanic breccias) is also present in within these units.The upper part is almost entirely composed of hard and bituminous marlstones (aničić & Juriša, 1984).
Mineral water is stored in fractured layers of the Oligocene tuff covered by the Upper Oligocene and Lower Miocene beds (Figs. 1 and 2).The discussed water belongs to the magnesium-sodium-hydrogen carbonate-sulphate facies.It was exploited from five boreholes that are 24 to 600 m deep (Fig. 1).Donat Mg is the most famous among Rogaška mineral waters.It has the highest mineral content (Tab. 1) and contains more than 1,000 mg/l of Mg.The content of gaseous CO 2 in the water is normally in the range of 2-30 g/l.Gas is practically pure CO 2 with minor share of nitrogen and negligible concentration of oxygen and methane (Pezdič, 1997).(Trček et al., 2010).Sl. 1. Raziskovalno območje z lokacijami vrtin z mineralno, termomineralno in izvirsko vodo (trček et al., 2010) and the counter clockwise rotation of the Adria microplate.The complexity of the study area is reflected in a lithological heterogeneity as a result of faulting into many small tectonic blocks, Fig. 2. Geological cross section A-A' (see Fig. 1) of the Rogaška Slatina area (Trček et al., 2010 Spring water and thermo-mineral water are also exploited at the study area (Fig. 1, Tab. 1).The former is stored in fractured Triassic carbonate rocks and Miocene sandstones of the Boč massif and the latter in the dolomitized keratophyre at depths between 1,500 and 1,700 m.
The Rogaška mineral waters discharge with a help of a gas lift, thermo-mineral water discharges with a help of a thermo lift, while spring waters of boreholes RSL-4, RSL-8, RSL-10 and RSL-14 are artesian.

Methods
Isotopic investigations were fundamental for studies of groundwater dynamics and solute transport in aquifers with mineral and spring water and among them.The recent investigations included the monitoring of 5 boreholes with mineral water (RSL-2, RSL-3, RSL-6, RSL-7 and RSL-11), 1 borehole with thermo-mineral water (RSL-1), 2 boreholes with spring water from a limestone (RSL-4, RSL-10), 2 boreholes with spring water from a dolomite (RSL-8, RSL-14) and 1 borehole with spring water from a sandstone Fig. 1,Tab. 1).Besides the groundwater CO 2 was sampled and also precipitation at altitudes of 340, 530 and 710 m asl.The previ-ous investigations involved only the sampling of mineral waters and spring water from carbonate rocks (Pezdič, 1997).
Recent isotopic analyses of stable isotopes of H and O were made in Laboratory Centre for Isotope Hydrology and Environmental Analytics, Joanneum Research, Graz, Austria.The oxygen isotopic composition (δ 18 O) of the water was measured by the classic CO 2 -H 2 O equilibrium technique (EpsteiN & Mayeda, 1953) with a fully automated device adapted from Horita et al. (1989) coupled to a Finnigan DELTAplus Mass Spectrometer.Deuterium (δ 2 H) was measured in a continuous flow mode by chromium reduction using a ceramic reactor slightly modified from MorrisoN et al. (2001).Stable oxygen and hydrogen isotopic ratios are reported relative to the VSMOW (Vienna-SMOW) standard with an overall precision of 0.1 and 1 ‰, respectively.
The data of H and O stable isotopic composition in precipitation were applied to define the local meteoric waterline (LMWL) of the Rogaška Slatina area.Due to small number of samples (57) the ordinary linear regression analysis was conducted that based on the method of seasonally weighted mean values (rozanski et al., 1993).
The 13 C content was also measured in the Stable Isotope Laboratory of the Institute of Water Resources Management, Joanneum Research, Graz, mass spectrometrically by Thermo Finnigan DELTA plus XP (CF-IRMS).The values are reported as ‰ vs. V-PDB standard with an overall precision of 0.1 ‰.
Measurements of the radioactive isotopes 3 H and 14 C were performed in the Isotope laboratory HYDROSYS -Water and Environmental Protection Developing Ltd, Budapest, Hungary. 3 H was measured by counting β-decay events in a liquid scintillation counter (LSC).Values are reported in absolute concentrations as tritium units (TU), where one TU corresponds to one 3 H atom per 10 18 atoms of hydrogen 1 H or to an activity of 0.118 Bq/kg in water.
14 C was measured by the super low level liquid scintillation analyser (PerkinElmer Tri-Carb 3170TR/SL) based on ASTM D6866-06 standard.The values are expressed in pmC (Percent Modern Carbon).Groundwater age (t) is determined using the radioactive decay equation (Eq.1), employing the pre-industrial baseline 14 C activity of the soil CO 2 equal to 100 pmC as an initial concentration and the 14 C activity of the DIC (HCO 3 ¯) measured in the groundwater sample.However, the dissolved C mass balance of groundwaters is altered during the groundwater flow path due to geochemical reactions that take place within the aquifer.In such cases, a factor q is incorporated into the radioactive decay equation to take into account the 14 C dilution effect (other than radioactive decay) during the groundwater flow path caused by a dissolution of 14 C-free marine carbonates and incorporation of a geogenic 14 C-free CO 2 of the magmatic or metamorphic source.The groundwater age is thus determined: where t is groundwater age, a 0 14 C is a modern 14 C activity in the soil zone (100 pmC), a t 14 C is a 14 C activity of DIC in the groundwater sample and q is a dilution factor.
The chemical mass-balance correction (CMB model) has been used to quantify the 14 C dilution due to incorporation of 14 C-free geogenic CO 2 , assuming that the carbonate dissolution in the recharge area evolves under the closed system condition.In such cases the 14 C dilution factor of carbonate groundwaters is about 0.5.The subsequent 14 C dilution caused by the incorporation of 14 C-free geogenic CO 2 and resulting geochemical reactions has been determined as a ratio of the HCO 3 -concentration in the recharge area to the HCO 3 -concentration in samples of mineral and thermos-mineral waters.The HCO 3 -concentration in the recharge area is given as an average value of sampled spring waters that equals to 304 mg/l.The total 14 C dilution factor is thus the product of the 14 C dilution ensuing in the recharge area (0.5) and the subsequent 14 C dilution induced by the incorporation of 14 C-free geogenic CO 2 .
The first analyses of O, H and C environmental isotopes were made at Institute Jožef Stefan, Ljubljana (Pezdič, 1997).For δ 18 O determination from water the classic CO 2 -H 2 O equilibrium technique was also applied (EpsteiN & Mayeda, 1953).Hydrogen gas was prepared by reducing water vapor on hot zinc wool at 400 °C or on zinc granules at 490 °C (colemaN et al., 1982).Precipitated DIC and carbonates reacted with 100 % orthophosphoric acid at 55 ºC.Dissolved carbonate species are treated with phosphoric acid to measure just δ 13 C.The isotopic composition of prepared gaseous compounds (H 2 and CO 2 ) were measured on the Varian MAT 250 mass spectrometer.Measuring accuracy exceeds ± 0.05 ‰ for oxygen and carbon and ± 0.5 ‰ for hydrogen.Results were given relative to the SMOW standard for oxygen and hydrogen and PDB for carbon.
The tritium content was measured with a liquid scintillation counter at Institute Jožef Stefan, Ljubljana (Pezdič, 1997).Concentrations were given in TU and Bq.The activity of radiogenic carbon ( 14 C) was measured at Rudjer Boikovic Institute in Zagreb using a gas proportional counter. 14C data were presented as % Modern Carbon (pmC) vs. the activity of standard oxalic acid (Ox) formed before the nuclear period with a correction factor of 0.95 (Pezdič, 1997).

Results and discussion
Average values of sampled water discharges, electroconductivity, mineralisation, temperature and pH during a monitoring period 2008-2010 are presented in Table 1.The level of gaseous CO 2 in water is normally in the range 2-30 g/l, but in some areas is as high as 40 g/1 (NosaN, 1973).Gas analysis revealed that the gas is practically pure CO 2 with a minor share of nitrogen (0.3 %) and negligible concentrations of oxygen and methane (below 0.01 %; Pezdič, 1997).
The relationship between O and H stable isotopic compositions of sampled water is illustrated in Figure 3.The average groundwater O and H isotopic composition approximately equals the weighted average of the precipitation O and H composition as a rule, while surface water is may be enriched more with heavier (more positive) isotopes than precipitation, due to the evaporation process (Clark & Fritz, 1997;rozanski et al., 1992, 1993;trček & zojer, 2010).
Mineral waters have 18 O in the range between -12.2 and -10.3 ‰ and δ 2 H in the range between -80.5 and -63.9 ‰.The lowest δ values refer to waters with the highest mineralization, RSL-2 and RSL-6, and the highest to water of RSL-3 with the lowest mineralisation (Tab.1), which reflects mixing processes of older and younger waters (Fig. 3).
The δ 18 O and δ 2 H values range in spring waters from carbonate rocks is -10.7 to -9.9 ‰ and -78.9 to -65.0 ‰, respectively.It is slightly different and more narrow in spring water from sandstone, -10.9 to -10.6 ‰ and -76.9 to -67.2 ‰, respectively (Fig. 3).The δ 18 O and δ 2 H of thermo-mineral water is similar to spring water from a dolomite.Similar δ 18 O and δ 2 H results were obtained in previous investigations (Pezdič, 1997).Mineral waters had a δ 18 O range -11.2 to -9.55 ‰ and a δ 2 H range -84.0 to -76.9 ‰, while the parameters ranged in spring waters from carbonate rocks from -10.25 to -8.19 ‰ and from -78.6 to -63.6 ‰, respectively.
Craig (1961) firstly described the relationship of discussed isotopes in the precipitation by the global meteoric waterline (GMWL, Fig. 3).The local meteoric waterline (LMWL) of the Rogaška Slatina area slightly differs from the GMWL in both the slope and the intercept (Fig. 3).It is a function of temperature during the secondary evaporation as rain falls from a cloud, which results in effects of the isotopic fractionation with respect to the latitude, altitude, and climate.The isotopic composition of precipitation is affected by the season, latitude, altitude, precipitation amount, and distance from the coast (rozanski et al., 1992, 1993).All sampled waters from recent and previous investigation (Pezdič, 1997)  and their distribution indicate that sampled waters are of a meteoric origin, hence the Rogaška fractured aquifer system is recharged with local precipitation.The δ values of mineral waters fall on the same LMWL, but on its lower part with the depleted heavy isotopes (Fig. 3, Pezdič, 1997), which reflects colder climate conditions during the infiltration processes (Clark & Fritz, 1997;Hoefs, 1997;iaea, 1983).The exception is the shallow RSL-3 mineral water (Tab.1) that is mixed with surface waters (Figs. 3 and 4).The δ values of mineral waters sampled from RSL-2, RSL-6 and RSL-3 slightly deviate from LMWL (Fig. 3).The level of gaseous CO 2 in these waters is the highest (up to 40 g/1; NosaN, 1973), therefore it is assumed that 18 O is depleted due to the exchange of water with the geogenic CO 2 (Clark & Fritz, 1997;Hoefs, 1997;iaea, 1983).
During the period of previous isotopic investigations (1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985) the 3 H contents of 1.9 to 176 TU were measured in mineral waters and of 37 to 150 TU in spring waters from carbonate rocks (Pezdič, 1997).The 3 H data of investigations in July 2008, when almost three half-lives of 3 H has passed, are illustrated in Figure 4.The precipitation had the 3 H content of 10.7 TU, which coincides with the summer values measured in Ljubljana (kovačič, 2015). 3H was not detected in mineral waters from boreholes RSL-6 and RSL-7, hence these waters are not in contact with recent precipitation infiltration.According to kovačič (2015) mineral waters from boreholes RSL-6 and RSL-7 are older than 100 years.The samples of thermo-mineral and mineral waters from boreholes RSL-1, RSL-2 and RSL-11 contain 0.6 TU or less 3 H, which indicates the prevailing old water component (kovačič, 2015).Evidently the recharge of these groundwaters occurred prior to the 1950s, therefore they are relatively unblemished by human activities.Similar findings were published in previous investigations (Pezdič, 1997).
Groundwater samples from RSL-3, RSL-4, RSL-8, RSL-10, RSL-12 and RSL-14 with tritium contents 4.6 to 9.4 TU (Fig. 4) should contain modern water that infiltrated predominantly after the 1960s, suggesting the vulnerability of these groundwater systems to man-made impacts.According to Kovačič (2015) the mean residence time of spring water of shallower boreholes RSL-10, RSL-12 and RSL-14 (Tab. 1) should be around 15 years, around 30 years for spring water of the deeper borehole RSL-4 and up to 60 years for spring water of the deeper borehole RSL-8.
Among listed waters only RSL-3 is mineralized.The water is captured from a depth of 20 m, where it should be mixed with young fresh water.Pezdič (1997) reported that spring waters from carbonate rocks are no more than 15 years old.
During the period 2007-2011 the δ 13 C values of the dissolved inorganic carbon varied between -13 and +2 ‰ in sampled groundwater (Figs. 5 and  6).The parameter values between -2 and +2 ‰ are characteristic for groundwaters that are influenced by the volcanic CO 2 : RSL-2, RSL-3, RSL-6, RSL-7 and RSL-11 (Clark & Fritz, 1997).These waters are highly mineralized, as it is evidenced in Table 1.The RSL-1 water has lower mineralization, which is reflected in δ 13 C-DIC values.On the other hand low δ 13 C values (-12 to -13 ‰) are typical for spring waters, collected from RSL-4, RSL-8, RSL-10 and RSL-14.According to Clark & Fritz (1997) these values closely resemble the δ 3 C content of carbonate groundwater that evolves under a closed system condition.
The main sources of the carbon dissolved in groundwater are soil CO 2 , CO 2 of a geogenic origin or a magmatic CO 2 (from deep crustal or mantle sources), carbonate minerals, an organic matter in soils and rocks, fluid inclusions, and methane.Each of these sources has a different carbon isotopic composition and contribute to the totally dissolved carbon in various proportions.In the studied groundwater systems the total dissolved inorganic carbon (DIC) exists practically all in a HCO 3 -form.The HCO 3 -concentration varies between 224-382 mg/l in groundwater samples from RSL-4, RSL-8, RSL-10, RSL-12 and RSL-14, where the carbonate rocks were dissolute in the reaction with biogenic CO 2 in soil.The HCO 3 -concentration of mineral and thermo-mineral waters from RSL-1, RSL-2, RSL-3, RSL-6, RSL-7 and RSL-11 is increased to about 1950-8280 mg/l (Tab.2).The hydrochemical composition was most probably altered to a great extent by the incorporation of the geogenic CO 2 influx that originates from a volcanic source (Fig. 5).The average δ 13 C of CO 2 in mineral waters is -4.1 ‰ and -9.9 ‰ in thermo-mineral water, respectively.It induced the intensive rock-water interactions and consequently the dissolved solute contents increased significantly.It is well known that the δ 13 C is about -25 ‰ in the soil CO 2 (similar to plants), -7 ‰ in the atmospheric CO 2 , -8 ‰ to -3 ‰ in the CO 2 originating from the geothermal and volcanic systems and about 0 ‰ in marine carbonate rocks (kendall & MCdonnell, 1998).δ values from -2.0 ‰ to +4.1 ‰, with an average of +2.2 ‰ were analysed in Slovene carbonate rocks (koCeli et al., 2013;ogorelec et al., 2000).The relationship between δ 13 C and HCO 3 -concentration in sampled water is illustrated in Figure 6.
The 14 C investigation was done on five groundwater samples of mineral and thermo-mineral waters collected from the boreholes RSL-1, RSL-2, RSL-3, RSL-6 and RSL-7 in 2009.Very low 14 C contents, ranging between 0.9 to 1.4 pmC, have been determined (Tab.2), which reflects very long mean residence times of groundwaters.With the aim to determine the resident time of groundwater according to Equation 1 the geogenic CO 2 was characterised first based on the δ 13 C value of the CO 2 gas phase of water samples.The δ 13 C values show a very wide spread, between -10 and -3 ‰ (Fig. 5).They are within the range of δ 13 C reported in the literature for magmatic CO 2 , -12 to -1 ‰ (truesdell & Huston, 1980;exley et al., 1986;rose & davisson, 1996).Further the chemical mass-balance correction (CMB model) has been used to quantify the 14 C dilution due to incorporation of the 14 C-free volcanic CO 2 , assuming that the carbonate dissolution in the recharge area evolves under a closed system condition.From stoichiometry in such cases the initial 14 C concentration (~ 100 pmC) is expected to become diluted for about 50 %.It follows that the 14 C dilution factor is about 0.5.The subsequent 14 C dilution caused by the incorporation of the 14 Cfree volcanic CO 2 and the resulting geochemical reactions has been determined as a ratio of the HCO 3 -concentration acquired in the recharge zone to the HCO 3 -concentration in the groundwater samples from RSL-1, RSL-2, RSL-3, RSL-6 and RSL-7.The HCO 3 -concentration acquired in the recharge zone is given as an average value from RSL-4, RSL-8, RSL-10 and RSL-14, which is about 304 mg/l.The ages of sampled mineral waters were calculated employing the total 14 C dilution factor, which is the product of the 14 C dilution ensuing in the recharge zone (= 0.5) and the subsequent 14 C dilution induced by the incorporation of the 14 C-free volcanic CO 2 .
Table 2. HCO 3 -concentration, stable C isotopic compositions, 14 C content, dilution factor q total and age estimates of the mineralized waters and thermos mineral water (RSL-1) sampled in 2009.Tabela 2. Koncentracije HCO 3 -, izotopska sestava 13 C, vsebnost 14 C, faktor razredčenja q total in ocena starosti mineralne in termomineralne vode, vzorčene leta 2009.The estimated ages of mineral waters are listed Table 2.The thermo-mineral water, captured from RSL-1 at depths 1500-1700 m exhibits a considerably high mean residence time -14000 years, corresponding to the Pleistocene epoch, the Bølling-Allerød interglacial period that lasted from 14700 to 12900 years ago.During this period glaciers almost disappeared in many European mountain ranges, also in the Alps (palacios et al., 2016;dielforder & Hetzel, 2014;HiPPe et al., 2014;ivy-oCHs, 2015).The wormer climate during the infiltration process of thermo-mineral water is reflected also in the stable isotopic composition of O and H (Fig. 3).

Sampling
The mean residence time of mineral waters RSL-6, RSL-7 and RSL-2 is 7200, 7100 and 3400 years, respectively.The first two correspond to the Holocene epoch, the end of the colder Boreal period that lasted from 8600 to 7200 years ago (roBerts, 2014).According to the 18 O and 2 H isotopic data (Fig. 3) waters from RSL-6 and RSL-2 should be recharged and discharged under similar conditions, but they are captured at different depths (606 and 274 m respectively, Tab. 1) and hence fallow different flow paths, which should result in age differences.It is also worth to note that despite the low 14 C concentration in the groundwater sample from RSL-3, no groundwater age was calculated due to the presence of a higher tritium concentration (4.6 TU) in the groundwater sample.Most likely it represents mixing of recent and old groundwater, thus groundwater age dating using 14 C is not appropriate.
The 14 C was investigated in the RSL-7 mineral water also in 1980s (Pezdič, 1997).A content of 2 pmC was recorded.Two corrections were made to estimate the water mean residence time based on 14 C data: a) according to a dilution factor determined as the ratio between the predicted active and measured total DIC (equals to 0.037), which indicated that the mixture had about 3.7 pmC of initial 14 C activity (ICA) and measured 2 pmC and b) according to estimation that primary mineral water had become mixed with about 20 % of young meteoric water.The recalculation of values indicated the relative 14 C activity of primary RSL-7 mineral water to be approximately 35 pmC.This pointed to an estimated age of about 8000 years, which is compatible with the result of recent investigations.

Conclusions
The results of isotopic investigations of groundwaters in the Rogaška Slatina area performed in the period 1978-1985(Pezdič, 1997) ) and 2007-2011 coupled with available information on a physical hydrogeology and a water chemistry help in understanding the source and mechanism of groundwater recharge, groundwater circulation and its renewability, groundwater transit time distribution, hydraulic interrelationships, the groundwater origin and its evolution due to effects of water-rock interaction.
The δ 18 O and δ 2 H values indicated the meteoric origin of mineral and thermos-mineral waters and proved the hypothesis that the Boč massif near Rogaška Slatina should be a catchment area of the Rogaška mineral waters.They reflected the mixing processes between younger and older waters or mineral and spring waters and hence contributed to the water body vulnerability assessment together with 3 H data.The latter pointed out waters that infiltrated predominantly after the 1960s, suggesting the vulnerability of these groundwater systems to man-made impacts.
The estimations of the mean residence time of mineral, thermos-mineral and spring waters based on 3 H, δ 13 C and 14 C data provided additional information on groundwater recharge processes, its renewability and vulnerability.Thermo-mineral water, captured from RSL-1 at depths 1500-1700 m is the oldest water in the study area with the mean residence time of 14000 years.The mean residence time of mineral waters captured at depth around 600 m from RSL-6 and RSL-7 is estimated to 7100 and 7200 years respectively.However, the residence time of mineral water captured at depth around 270 m from RSL-2 is shorter -3400 years.Nevertheless, the 18 O and 2 H data indicated that mineral waters from RSL-6 and RSL-2 should be recharged and discharged under similar conditions.Based on 3 H data it was estimated that shallow spring waters are around 15 years old, while the age of deeper ones is 30 to 60 years.
The presented findings were integrated with the structural analysis of the study area (trček et al., 2010; Žibret, 2016).The results improved a conceptual hydraulic model of the Rogaška Slatina fractured aquifer system and are essential to determine the optimal balance between environmental protection and economic use of mineral and spring water resources in the study area.