© Author(s) 2021. CC Atribution 4.0 License Using stable isotopes and major ions to identify recharge characteristics of the Alpine groundwater-flow dominated Triglavska Bistrica River Uporaba stabilnih izotopov in glavnih ionov za oceno napajalnih značilnosti alpskega rečnega toka Triglavske Bistrice pod vplivom podzemne vode Luka SERIANZ1,2, Sonja CERAR1 & Polona VREČA3 1Geological Survey of Slovenia, Dimičeva ulica 14, SI-Ljubljana, Slovenia; e-mail: luka.serianz@geo-zs.si 2Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jamova cesta 2, SI-Ljubljana, Slovenia; 3Department of Environmental Sciences, Jožef Stefan Institute, Jamova cesta 39, SI-1000, Ljubljana, Slovenia. Prejeto / Received 5. 10. 2021; Sprejeto / Accepted 7. 12. 2021; Objavljeno na spletu / Published online 28. 12. 2021 Key words: groundwater, oxygen and hydrogen isotopes, hydrogeochemistry, recharge area, Alpine aquifer, Slovenia Ključne besede: podzemna voda, kisikovi in vodikovi izotopi, hidrogeokemija, napajalno območje, alpski vodonosnik, Slovenija Abstract Triglavska Bistrica is a typical Alpine river in the north-western part of Slovenia. Its recharge area includes some of the highest peaks in the Julian Alps. The hydrogeological conditions and flow of the river depend largely on groundwater exchange between the karstified aquifer in the carbonate rocks and the intergranular aquifer in the glaciofluvial deposits. The average volume of the river flow is up to several m3/s. In this study, water samples from different locations along the river were analysed for stable isotope ratios of oxygen and hydrogen, major ions, and concentration of tritium activity. The correlation of major ions suggests that the recharge area consists of both limestone and dolomite rocks. The δ18O and δ2H values decrease downstream, implying that the average recharge elevation increases. At the downstream sampling site V-5, located approx. 300 m upstream from the confluence of the Sava Dolinka River, the calculated mean recharge altitude is estimated to be 1,996 m. Izvleček Triglavska Bistrica je tipična alpska reka, ki se nahaja v severozahodnem delu Slovenije. Njeno napajalno zaledje pokriva nekaj najvišjih vrhov v Julijskih Alpah. Hidrogeološke razmere in rečni tok sta v veliki meri odvisna od izmenjave podzemne vode med kraško-razpoklinskim vodonosnikom v karbonatnih kamninah in medzrnskim vodonosnikom v glaciofluvialnih sedimentih. Povprečen pretok reke v spodnjem toku je ocenjen na nekaj m3/s. V tej študiji so bila določena razmerja stabilnih izotopov kisika in vodika, koncentracije glavnih ionov in koncentracija aktivnosti tritija v vzorcih vode na različnih lokacijah nizvodno od izvira Triglavske Bistrice. Na osnovi korelacije osnovnih ionov je možno sklepati, da je napajalno zaledje sestavljeno tako iz apnenca kot tudi dolomita. Vrednosti δ18O in δ2H upadajo dolvodno od izvirnega območja, kar pomeni, da povprečna nadmorska višina napajanja narašča. Na dolvodnem merilnem mestu V-5, ki se nahaja približno 300 m nad sotočjem s Savo Dolinko, znaša izračunana povprečna nadmorska višina napajanja 1.996 m. GEOLOGIJA 64/2, 205-220, Ljubljana 2021 https://doi.org/10.5474/geologija.2021.012 640 m a.s.l. The length of the constant the riv- er flow is about 8 km, and on this stretch the riverbed drops by about 250 m. Under diverse topographical conditions specific hydrogeolog- ical settings are formed, which are typical for alpine river valleys. A river is enriched either with groundwater from the riparian zone or by direct inflow from the rock in the riverbed, as is the case with other alpine rivers in Slovenia (e.g. Introduction Triglavska Bistrica is a small Slovenian river flowing in the Vrata glacial valley and surround- ed by the highest peaks in Slovenia. Its special characteristics are recognizable by the different topographical features of the watershed, which stretches from the highest Slovenian mountain Triglav (2,864 m a.s.l) to the confluence with the Sava Dolinka River at an altitude of about 206 Luka SERIANZ, Sonja CERAR & Polona VREČA Brenčič & Vreča, 2016; Torkar et al., 2016). At the same time, water from the river can infiltrate the banks and surrounding aquifers. These condi- tions change temporally (seasonally) and spatial- ly (lithology, geomorphological processes, etc.). Hydrogeochemical methods are among the most useful approaches to identify such conditions and determine other physiochemical processes. In carbonate reservoirs, the main chemical parame- ters describing carbonate equilibrium in ground- water are calcium (Ca2+), magnesium (Mg2+), their molar ratio (Ca2+/Mg2+), and hydrogen carbonate (HCO3 -). In addition to chemical parameters, sta- ble isotope ratios of oxygen and hydrogen (ex- pressed as δ18O and δ2H) in water can also provide information on recharge areas (Clark and Fritz, 1997), while tritium activity concentration (3H) can provide information on the average residence time of groundwater. A combination of chemical and isotope data has been widely used for hy- drogeological research of alpine water streams (Carey & Quinton, 2005; Thiébauda et al., 2010; Shamsi et al., 2019), and the same techniques have been applied also in Slovenia (Kanduč et al., 2012; Torkar et al., 2016). The Triglavska Bistrica watershed consists of massive limestone and dolomite rocks with frac- tures and karstic porosity. It is therefore very likely that dissolution of carbonate minerals is the most important hydrogeochemical process affecting the chemical components of natural wa- ter flow. There is very little data on the hydroge- ochemistry of the groundwater in the monitoring area, but several chemical analyses are available for a spring near the Peričnik waterfall, which was included in the study of the hydrogeochem- istry of alpine springs from northern Slovenia (Kanduč et al., 2012). These studies suggest that the water of the alpine springs is dominated by HCO3 -, Ca2+ and Mg2+ ions and that most of the springs were near equilibrium in terms of cal- cite. These results can be confirmed by recent hy- drogeochemical studies in similar environments (Mezga, 2014; Torkar et al., 2016; Serianz et al., 2020b). It is commonly observed that precipitation gradually becomes depleted in 18O and 2H iso- topes as altitude increases (Dansgaard, 1964). This phenomenon is commonly referred to as the “altitude effect” and results primarily from the cooling of air masses as they ascend a mountain range, accompanied by the dissipation of excess moisture (Gonfiantini et al., 2001; Kern et al., 2020). In the case of δ18O-precipitation, the glob- al average gradient with altitude is -2.8 ‰/km, and ranges from -1.7 to -5.0 ‰/km; the European average is -2.1 ‰/km (Poage and Chamberlain, 2001). On Slovenian territory, for example, the al- titude effect on precipitation ranges from -0.2 ‰ to -0.3 ‰ δ18O/100 m (Vreča et al., 2006; Brenčič and Polting, 2008). For Croatia and Slovenia combined, these values range from -0.37 ‰ to -0.26 ‰ δ18O/100 m (Horvatinčić et al., 2005). For other countries, such as Austria, the altitude ef- fect is estimated to be -0.21 ‰ δ18O/100 m (Kralik et al., 2003) and the value for Italy is estimated at roughly 0.2 ‰ δ18O/100 m (Longinelli and Selmo, 2003). Based on the fact that the isotopic compo- sition in precipitation is reflected in the isotopic composition of groundwater, Mezga et al. (2013) calculated three elevation effects for groundwa- ter following different patterns of precipitation intensity, ranging from -0.25 ‰ δ18O/100 m for the Alps and the coastal region to 0.33 ‰ δ18O/100 m for the Bela Krajina region (Cerar et al., 2018). Recent studies in the Adriatic-Pannonian region indicate an empirical isotopic altitude effect in modern precipitation for δ18O, which is -1.2 ‰/km and -7.9 ‰/km for the δ2H (Kern et al., 2020). The objective of this research is to identify the source, type, and amount of the different water components of the Triglavska Bistrica River re- charge, to describe their spatial variations using hydrochemical, isotopic, and hydrogeological methods, and to estimate the mean recharge alti- tude of the Triglavska Bistrica River. Study area settings General settings The Triglavska Bistrica River courses through the heart of the Julian Alps in the north-western part of Slovenia and flows on through the Vra- ta Valley. In Mojstrana, a small settlement at the end of the valley, Triglavska Bistrica flows into the Sava Dolinka River, which joins the Sava Bohinjka River to form the Sava River, the larg- est tributary of the Danube by water volume. Tri- glavska Bistrica is a typical Alpine River, which flows in the Vrata glacial valley. It flows under Triglav North, the highest mountain in the coun- try. The largest stream flowing into the river is the Peričnik. The Triglavska Bistrica flows into the Sava Dolinka River about 10 km downstream, with a gradient of about 400 m, and is surrounded by mountains with the greatest number of peaks above 2,500 m a.s.l. in its catchment area. The water percolates out of extensive scree at the foot of the wall (Smolar-Žvanut et al., 2005). On its way to the Sava Dolinka, the Triglavska Bistrica 207Using stable isotopes and major ions to identify recharge characteristics of the Triglavska Bistrica River Fig. 1. The observation area and sampling locations (spring MS-1 is not included due to its distance from the Vrata Valley; coordinates of the spring are given in Table 1). runoff is amplified by lateral inflows. The river discharge is classified as an alpine high moun- tain snow-rain regime (Hrvatin, 1998), and the effects of snowmelt are still evident in late sum- mer; but in the fall the runoff originates just be- low the north face of the Triglav Mountain. In shaded kettles and mountain gorges the snow re- mains all year round. The wider observation area (Fig. 1) is char- acterized by a variety of relief forms, fast and expressive altitude changes due to the geologi- cal base, and process related to the formation of younger mountains. The landscape is character- ized by glacially-formed valleys and rocky high- land ridges, peaks with unusual karstic shapes. They climb over the Gate Valley Stenar (2,601 m), Škrlatica (2,740 m), Kukova Špica (2,427 m) and above the junction of the valley’s Severna Tri- glav wall. Some high peaks can be found also in the ridge that divides Vrata and Kot, for ex- ample Cmir (2,393 m) and Rjavina (2,532 m). The catchment area of the Triglavska Bistrica is overgrown with forest in its lower part. At the foot of the valley, glaciers that probably covered the entire Upper Sava Valley (Serianz, 2016) have left behind well consolidated lateral moraines that cause, together with other hydrological pa- rameters, a slower run-off from the valley slopes (Smolar-Žvanut et al., 2005). The diversity of the Alpine valleys influences the climate there, espe- cially temperature (regime of alpine basins and valleys, ridges, and peaks). Hydrometeorological data The climate of the Upper Sava region belongs to the Alpine region, which is characterized by long and snowy winters and short, moderately warm summers, frequent east winds and abun- dant rainfall. The area along the Vrata Valley is still under the influence of specific climatic condi- tions, which aggravate the mountainous charac- ter of the climate and is a consequence of the alti- tude. Winter usually lasts four to five months. The average minimum daily temperature in January 208 Luka SERIANZ, Sonja CERAR & Polona VREČA Fig. 2. Meteorological data at the Kredarica station for the period September 2019 to September 2020 (ARSO, 2021). is as low as -8 °C, while during the day it can warm above 0 °C. In the warmest month, day- time temperatures rise to 23 °C. Across the val- ley upwards, the microclimatic conditions vary even more. An indicator of this is the thickness of the snow cover, which grows with each meter in altitude. There are also large differences be- tween sunny and shady slopes in winter. Thus, sunny slopes in winter are suitable for trips and walks, because they offer the desired sunlight, and shady slopes protect and preserve the snow blanket. At Kredarica meteorological station, which is located at an altitude of 2,514 m, the cli- mate is even more significantly affected by snow. Here, the snow can be as much as an estimated 7 m deep (Nadbath, 2014). Based on the meteoro- logical data available on the web database of the Slovenian Environmental Agency – ARSO (In- ternet 1), in the period from September 2019 to September 2020, which was representative for the documented hydrogeological investigations, win- ter temperatures dropped to -20 °C, while during the summer they rise to more than 20 °C (Fig. 2). During this period snow was present from No- vember to July, a full 8 months, with snow up to 4 m deep. The specific alpine meteorological con- ditions also affect the Triglavska Bistrica River hydrograph. Data from past observation at the gauging station in Mojstrana (Internet 2) indicate the presence of a snow-rainy river flow regime, with the largest discharges during the melting period in spring (Fig. 3). Hydrogeological settings The Triglavska Bistrica River recharges from the watershed area extending from the south- ern side of the Triglav Mountains and the ridg- es above the Vrata Valley. In this area massive limestones and dolomites and granular dolomites predominate (Jurkovšek, 1987). From a hydro- geological point of view, the carbonate layers, limestones and dolomites form aquifers with fis- sure, karstic, and karstic-fissure porosity. Faults in the Dinaric and Trans-Dinaric directions run through the catchment area, which influence the geometry of the aquifer. In the lower areas, the carbonate rocks are covered with poorly sorted moraine material and sloping sediments, which are sometimes filled with unsorted clay and sand deposits and are mostly presented by good or medium hydraulic conductivity; however, also low hydraulic conductivity can be observed due to the high heterogeneity of the sediment struc- ture (e.g. clay, silt). Intermediate clay inserts represent hydraulic barriers. The sediments are of glaciofluvial origin (Jurkovšek, 1987). Slope sediments and moraines can be found at the bot- tom of the Valley, covering the bedrock slopes. These Quaternary deposits are determined by in- tergranular porosity with good to low hydraulic conductivity. Alluvial deposits of Holocene age up to a few meters thick can also be found along the riverbanks. The alluvial sediments that occur at the bottom of the valley where the Triglavska Bistrica flows represent a highly-permeable and relatively homogeneous intergranular aquifer. 209Using stable isotopes and major ions to identify recharge characteristics of the Triglavska Bistrica River Materials and methods Gauging station M-1 In the past, regular flow measurements were made at the Triglavska Bistrica River. In the period 1953 to 1990, a water gauging station operated on the Bistrica River in the village of Mojstrana. The surface area of the catchment area of the Bistrica to the gauging station of Mojstrana is 47 km2. Since no flow measurements have been made since 1989, a flow tube near the small hydroelectric plant at Mojstrana (M-1) was installed on April 10, 2020, which contains a “diver” that we use to monitor the flow elevation of the Triglavska Bistrica. This monitoring site is located further upstream (~700 m) than the origi- nal gauges Mojstrana and Mojstrana I. Discharge measurements were performed along the stretch where monitoring site M-1 is located. The meas- urements were made using the instant chemical integration method, which is based on the im- mediate injection of tracers into the river. As a follow-up, common table salt was used, which provides a series of quality measurements due to its physicochemical properties (highly soluble in water, harmless to the environment, etc.) and leads to a strong increase in electrical conductiv- ity. The salt concentration was measured based on electrical conductivity using a “Flo-trac- er” measuring device. Based on periodic meas- urements and a comparison with the measured flow height in the pipe at gauging station M-1 a preliminary stage-discharge rating curve was constructed (power type). Currently, only three representative values are available for adjusting stage-discharge rating curve. Water sample collection Sampling of groundwater and surface water from the Peričnik catchment and springs, as well as from the wider area (Fig. 1), was carried out with the aim of determining the hydrogeological conditions of the recharge area of the Peričnik springs, which are important for the water sup- ply for the western part of the municipality of Jesenice. The joint discharge rate of Peričnik springs ranges from 70 to 120 L/s and supplies some 15,000 users (Internet 3). Roughly speaking, the monitoring sites were selected based on preliminary cartographic analysis and knowledge of the hydrogeological conditions of the area. The exact location for each monitoring site was determined in the field (Fig. 1, Table 1). At the first sampling, we selected 10 monitoring sites, 6 of which are represented by Triglavska Bistrica surface water (V-2, V-3, BG, V-4, V-5, M-1), one in the Peričnik stream (P-1), one in the captured Peričnik spring (ZP-1.3), and two surrounding springs (S-1 and MS-1). During the second sampling, we added another monitor- ing site, Triglavska Bistrica (IB), in addition to the existing ones. Sampling was conducted in two campaigns, on April 10, 2020 and May 22, 2020. According to the recorded flows at monitoring site M-1 (Fig. 3), the first sampling was conducted at low water level, and the second sampling was conducted at medi- um water level. Sampling procedures, transport and storage of groundwater samples were carried out in accordance with ISO standards (SIST ISO 5677-11:1996; SIST ISO 5677-03:1996; SIST ISO 5677-6:1996). In-situ measurements of physico-chemical parameters (i.e. temperature (T), pH, and elec- trical conductivity (EC)) were carried out using a waterproof HI98194 multimeter from HANNA instruments Inc. (Hanna instruments, 2020). The reported analytical accuracies of the field meas- urements are ±0.02 for pH, ±0.15 °C for tempera- ture and ±1 % for electrical conductivity. Water samples were collected to determine the isotopic composition of oxygen (δ 18O) and hydro- gen (δ 2H). Based on the results of the first sam- pling and in order to get valuable insight into the hydrogeochemical processes and the natural background of the wider area samples for deter- mination additional parameters were collected during the second sampling. Water samples for basic chemical parameters (i.e. Na+, K+, Ca2+, Mg2+, HCO3 -, SO4 2-, Cl-, NO3 - and total dissolved solids) were collected at 5 locations, and at 3 locations for tritium activity concentration (3H) (Table 1). Precipitation is sampled according to the most rational approach for monitoring isotopes in pre- cipitation as a monthly composite in the frame of the Slovenian Network of Isotopes in Precip- itation (Vreča and Malenšek, 2016, Internet 4) at two meteorological stations (i.e. Kredarica and Zgornja Radovna), which are part of the Sloveni- an National Meteorological Network maintained by the Slovenian Environmental Agency (ARSO). Precipitation samples were collected by ARSO staff from the classical rain gauge collector three times daily (synoptic station Kredarica) or once per day (precipitation station Zgornja Radovna). The volume of collected precipitation is record- ed and the sample is poured into a plastic bot- tle with a tight-fitting cap. In the laboratory, we removed impurities (e.g. dust, particles) from the composite monthly sample by filtration through 210 Luka SERIANZ, Sonja CERAR & Polona VREČA 12–25 μm pore-size ashless filter papers before taking aliquots for different isotope analyses. Samples for the analysis of stable isotopes of hy- drogen and oxygen were stored in glass bottles (minimum 30 mL). Chemical analysis Chemical analysis of major ions was performed by the Slovenian National Laboratory of Health, Environment and Food, Novo mesto laboratory, in accordance with the lab’s methods (accreditation document LP-014, last modification 14 February 2020). For the basic chemical parameters (Na+, K+, Ca2+, Mg2+, SO4 2-, Cl-, NO3 -) the ion chromatography method (SIST EN ISO 10304-1:2009, SIST EN ISO 17294-2:2005, SIST EN ISO 14911:2000) was used, and for HCO3 - the volumetric method was used (EN ISO 9963-1). The measurement uncertain- ty for NO3 - is ±11 %, for SO4 2- ±8 %, for Cl- ±7 %, for Ca2+ ±15 %, for Mg2+ ±12 %, for Na+ ±11 %, for K+ ±12 %, and for HCO3 - ± 3 %. The isotopic composition of hydrogen (δ 2H) and oxygen (δ 18O) was determined at the Jožef Stefan Institute (Ljubljana, Slovenia) using the H2-H2O (Coplen et al., 1991) and CO2-H2O (Epstein and Mayeda, 1953; Avak et al., 1995) equilibration technique. Measurements were performed using a dual inlet isotope ratio mass spectrometer (DI IRMS, Finnigan MAT DELTA plus, Finnigan MAT GmbH, Bremen, Germany) with an auto- mated CO2-H2O and H2-H2O HDOeq 48 Equili- bration Unit (custom-made by M. Jaklitsch). All measurements were performed together with laboratory reference materials (LRM) that are regularly calibrated against primary IAEA cali- bration standards. Water samples were measured as independent duplicates. Results were normal- ized to the VSMOW/SLAP scale using the Labo- ratory Information Management System (LIMS) for Light Stable Isotopes (U.S. Geological Sur- vey) and expressed in standard δ notation (in ‰). For independent quality control, we used a LRM W-45 with defined isotopic values and an estimated measurement uncertainty of δ2H = −60.6 ±0.7 ‰ and δ18O = −9.12 ±0.04 ‰, and the commercial reference materials USGS 45 (δ2H = −10.3 ±0.2 ‰, δ18O = −2.238 ±0.006 ‰) and USGS 47 (δ2H = −150.2 ±0.3 ‰, δ18O = −19.80 ±0.01 ‰). The average sample repeatability for δ2H and δ18O was 0.3 and 0.01 ‰, respectively. Groundwater samples for determination of the tritium activity concentration (3H) were analysed by the Wessling laboratory in Budapest using the IRPA standard (FS-78-15-AKU: 1995) and MSZ 19387:1987 method. This procedure is based on the principal of selective isotopic enrichment us- ing electrolysis. The volumes of the water sam- ples are reduced from 250 mL or 800 mL to 14– 15 mL by electrolytic enrichment, with the factor of tritium enrichment roughly 15–16 or 30–35. The tritium activity of enriched water samples was counted using a liquid scintillation analyser (LD: 0.5 or 0.2 TU, the uncertainty is 5–10 %). Data interpretation models Hydrogeochemistry Graphical analysis classification of ground- water type was performed in an AquaChem® 5.1 (Waterloo Hydrogeologic Inc., Waterloo, Canada), where the ion pattern was defined according to the concentration of dominant dissolved species measured in the groundwater. A trilinear Piper diagram (Piper, 1944) was used to determine hy- drochemical facies using all major ions for classi- fication. In addition, the molar ratio between Ca2+ and Mg2+ in groundwater was used to indicate the relative proportion of rocks in the recharge area. However, values equal to 1 are assumed in the literature to indicate dissolution of dolomite and dolomite prevailing in the recharge area (Mayo and Loucks, 1995). A higher Ca2+/Mg2+ molar ratio (>2) is a consequence of the predominant calcite rocks (Katz, 1998). The saturation indices (SI) of dolomite and calcite were calculated to evaluate the chemical equilibrium using an AquaChem® 5.1, based on the following equation: SI = log (IAP/KT) (1) where IAP is the ion activity product and KT the equilibrium constant at a given temperature. Positive SI values (SI > 0) indicate mineral over- saturation and precipitation, while negative SI values (SI < 0) indicate unsaturated solutions and mineral dissolution. The assumed tolerance equi- librium range with respect to mineral is ±0.1 SI for calcite and ±0.5 SI for dolomite. Stable isotopes The stable isotope composition of groundwa- ter (δ18O and δ2H) was used to obtain information on the characteristics of the recharge area. The δ18O and δ2H values were compared with: - the Global Meteoric Water Line (GMWL) de- fined as δ2H = 8δ18O + 10 (‰) (Craig, 1961), which is very close to the local meteoric water line for the Ljubljana 1981–2010 precipitation isotope record (Vreča et al., 2008 and 2014; In- ternet 4), 211Using stable isotopes and major ions to identify recharge characteristics of the Triglavska Bistrica River Fig. 3. Triglavska Bistrica River hydrography at gauging station M-1 and archive gauging station Mojstrana, including daily precipitation at the Zgornja Radovna meteorological station. - the Eastern Mediterranean Meteoric Water Line (EMMWL) defined as δ2H = 8δ18O + 22 (Gat and Carmi, 1970), and - the precipitation-weighted local meteoric water lines (reduced major axis regression – RMA) for Kredarica defined as δ2H = 8.4δ18O + 19 and for Zgornja Radovna defined as δ2H = 8δ18O + 11 (Internet 4). The δ18O values were also used to determine the mean recharge altitude of the investigated water samples. The calculations used herein were based on: 1. δ18O altitude effect from precipitation in the period 2016–2020 at Kredarica and Zgorn- ja Radovna meteorological stations. Calcu- lations were made separately for snow and total precipitation. The δ18O of snow was de- termined by a detailed analysis of precipi- tation data for each monthly sampling cam- paign at the meteorological station. For δ18O of snow, only those samples where the snow represents 60% or more of total precipitation (Psnow ≥ 0.6 Ptotal) were counted. Spring MS-1 average δ18O from both samplings was used as a representative value for applying pre- cipitation mean altitude effect. 2. archive mean altitude effect calculated for Radovna Valley (Torkar et al., 2016) based on linear model havg= −931.8 δ 18O − 7650.8 and Bled area (Serianz et al., 2020b) based on linear model havg= −939.2 δ 18O − 7518.6. Results and discussion Triglavska Bistrica hydrograph The highest measured discharge at the gaug- ing station M-1 for the preliminary stage-dis- charge rating curve adjustment was 9 m3/s, while measurements at higher water levels were unsuc- cessful due to unfavourable measurement condi- tions. Therefore, the information on some peak discharges above 9 m3/s (Fig. 3) is relatively poor and will be improved in the future. The Triglavs- ka Bistrica hydrograph shows the measurements (average daily flow, maximum daily flow, and minimum daily flow) from 1973 to 1989 (Internet 2). At the same time, the hydrograph shows the flow for 1980, which was chosen as the reference flow for this period of operation at measuring point Mojstrana I. The archive discharges (In- ternet 2) are presented for comparison with new measurement at gauging station M-1. 212 Luka SERIANZ, Sonja CERAR & Polona VREČA The Triglavska Bistrica hydrograph illustra- tion has a specific shape, which can be described with snow-rain regime. It can be divided into two parts: (1) period of snow thaw and (2) autumn rainy period (Fig. 3). The start of the snow-melt period can be detected by the small discharges in early spring, which start with the slow rise of discharges, and during the high-thaw period form a specific shape with high discharge values. A few discharge peaks can be observed from the resulting shape as the result of rainy day(s) in the snow-melt period. Once the highest discharge is reached, usually at the beginning of summer, it starts to decline. Physio-chemical parameters The results of the measurements of field pa- rameters, basic chemical analysis, isotopic com- position of δ18O and δ2H, and tritium activity con- centrations in groundwater and surface water are summarised in Table 1. Water temperatures in the Peričnik catchment (sampling point ZP- 1.3) were 6.0 and 6.1 °C in both sampling cam- paigns, and slightly higher in surface waters and springs, between 5.7 and 8.3 °C (average 7.0 °C) in the first campaign, and between 7.2 and 9.5 °C (average 8.0 °C) in the second sampling cam- paign. The pH value of the groundwater in the Peričnik catchments was constant at 8.0 in the first campaign, and 7.9 in the second campaign. The surface water of the Triglavska Bistrica (sampling point BG) has similar values about 100 m upstream from the Peričnik catchments (8.3 and 8.1, respectively). The pH values in the surface water ranged from 8.2 to 8.4 and from 7.2 to 8.3 in the second campaign. The electri- cal conductivity (EC) of the groundwater in the Peričnik catchment was 192 μS/cm in the first sampling campaign and 187 µS/cm in the second sampling campaign. Similar EC values are also characteristic for the sampling point BG, at 188 μS/cm in the first sampling campaign or slightly lower (173 µS/cm) in the second sampling cam- paign and at spring S-1 (between 195 and 276 µS/ cm). The EC values are lower at the surface water sampling points and range between 110 and 146 μS/cm (average 127.5 μS/cm) in the first sampling campaign. In the second sampling campaign, the values are higher and range between 153 and 201 µS/cm (average 180 µS/cm). In the second sam- pling campaign, the Bistrica spring (IB) was also sampled, where the measured value from EC was 234 µS/cm. The basic chemical parameters were only de- termined in the second campaign, so a compar- ison of the results between the two sampling campaigns is not possible. The Ca2+ concentration in the groundwater in the Peričnik catchment is about 26 mg/L. A similar Ca2+concentration was also measured at sampling point P-1 (26 mg/L), and slightly higher at sampling point BG (28 mg/L). Upstream of the Peričnik catchment (sampling points V-3 and IB), Ca2+ concentrations in surface water are slightly higher, reaching 30 and 42 mg/L, respectively. Mg2+ concentrations indicate the opposite. In the Peričnik catch- ment, slightly higher concentrations are meas- ured in groundwater (8.4 mg/L), while at up- stream sampling points V-3, BG, IB and P-1, Mg2+ concentrations range between 5.7 and 7.6 mg/L and increase downstream towards the Peričnik catchment. Characteristics similar to those of Ca2+ in the water are also reflected in the dis- tribution of HCO3 - concentrations in the water. The highest concentrations of HCO3 - in the wa- ter were measured at the sampling point IB (160 mg/L). The concentrations decrease downstream and reach 129 mg/L at sampling point BG. Simi- lar concentrations were also measured in the wa- ter of the Peričnik catchment (124 mg/L). The values of carbonate ions (Ca2+, Mg2+, and HCO3 -) in the groundwater indicate the car- bonate recharge area. The calculated ratio be- tween calcium and magnesium ions (Ca2+ + Mg2+) and HCO3 - in meq/L is about 1:1. The water sam- ples belong to the Ca-Mg-HCO3 facies, with low K+, Na+, Cl-, NO3 - and SO4 2- ion-content, except for the water at the sampling point IB, which belongs to the Ca-HCO3 facies (Fig. 4). The Ca 2+/ Mg2+ molar ratio for each sampling point shows similar characteristics (Fig. 5a). The Ca2+/Mg2+ ratio is 2.0 for the groundwater in the Peričnik catchment, while the ratio for the surface water upstream of the catchment is slightly higher (be- tween 2.39 and 2.61), and is highest for sampling point IB (4.32). The latter shows that the recharge area of the spring is an aquifer dominated by limestone, while other waters are characterized as mainly fed by aquifers dominated by dolomite over limestone. Figure 5b shows saturation indices (SI) of cal- cite (SIcalcite) and dolomite (SIdolomite) in sampled water. Groundwater at sampling point IB is over- saturated with respect to calcite and dolomite, since SIcalcite and SIdolomite are both positive (SI > 0), which means that the mineral might precipitate but cannot dissolve (Appelo and Postma, 2005). This groundwater was sampled in the recharge area where limestone prevails. 213Using stable isotopes and major ions to identify recharge characteristics of the Triglavska Bistrica River Fig. 4. Graphical analysis of water samples with Piper diagram. Fig. 5. Scatter plots of a) Mg2+ (mmol/L) versus Ca2+ (mmol/L) and b) SIcalcite versus SIdolomite. Water at all other sampling points (ZP-1.3, V-3 in P-1) is unsaturated with respect to calcite and dolomite, since SIcalcite and SIdolomite are both negative (SI < 0) except BG, where SIcalcite is ap- prox. zero. The latter shows that minerals may not be reacting at all or may be reacting reversi- bly, in which case the mineral could be dissolving or precipitating (Appelo and Postma, 2005). The surface water at sampling point V-3 is highly un- saturated with respect to very low SIdolomite values, where also the lowest pH value (7.2) is observed. In this case the mineral might dissolve very slow- ly or not at all, depending on the kinetics of the reaction (Appelo and Postma, 2005). The measured concentrations of other basic chemical parameters are low and do not indicate significant pollution of groundwater and surface water. Nitrate concentrations (NO3 -) in ground- water are below 1.95 mg/L, which is below the expected natural background (5 mg/L) (Serianz et al., 2020a). Sulphate concentrations (SO4 2-) range from 0.76 to 1.40 mg/L and chloride con- centrations (Cl-) range from 0.21 to 0.30 mg/L. No major differences between the sampling point of the Peričnik catchment (ZP-1.3) and surface wa- ter or springs were detected. 214 Luka SERIANZ, Sonja CERAR & Polona VREČA T ab le 1 . S a m p li n g lo ca ti on s a n d r es u lt s fo r b ot h s a m p li n g ca m p a ig n s. L oc at io n Ty pe Y X Z Sa m pl in g da te Q T E C pH T D S H C O 3‒ C a2 + K + C l‒ M g2 + N a+ N O 3‒ SO 42 ‒ H 3 δ1 8 O δ2 H [m a .s. l.] L/ s °C µS /c m / m g/ L m g/ L TU ‰ ±‰ ‰ ±‰ ZP -1 .3 Sp rin g 41 58 29 14 45 26 74 2. 8 10 .0 4. 20 20 6. 0 19 2 8. 0 -1 0. 35 0. 01 -6 9. 6 0. 2 V- 2 R iv er 41 41 88 14 33 57 84 1. 9 10 .0 4. 20 20 9 5. 9 14 6 8. 2 -9 .6 9 0. 00 -6 4. 7 0. 3 V- 3 R iv er 41 55 12 14 43 56 75 1. 7 10 .0 4. 20 20 64 6 6. 8 12 5 8. 3 -1 0. 33 0. 03 -6 9. 4 0. 3 B G R iv er 41 57 44 14 45 30 73 9. 0 10 .0 4. 20 20 76 2 5. 7 18 8 8. 3 -1 0. 40 0. 01 -6 9. 8 / V- 4 R iv er 41 59 71 14 47 67 73 1. 3 10 .0 4. 20 20 71 7 7. 2 12 6 8. 4 -1 0. 36 0. 01 -6 9. 8 0. 4 M -1 R iv er 41 85 36 14 63 92 65 3. 6 10 .0 4. 20 20 1, 40 0 6. 5 12 5 8. 3 -1 0. 47 0. 02 -6 9. 8 0. 1 V- 5 R iv er 41 90 14 14 68 91 64 5. 8 10 .0 4. 20 20 1, 45 0 8. 3 13 3 8. 4 -1 0. 38 0. 01 -7 0. 5 0. 1 P- 1 R iv er 41 55 14 14 43 65 75 2. 0 10 .0 4. 20 20 7. 9 11 0 8. 3 -1 0. 69 0. 02 -7 2. 5 0. 3 S- 1 Sp rin g 41 81 52 14 57 95 67 8. 6 10 .0 4. 20 20 8. 1 19 5 8. 4 -9 .9 3 0. 02 -6 6. 8 0. 2 M S- 1 Sp rin g 42 29 85 13 45 45 1, 21 6. 4 10 .0 4. 20 20 4. 8 16 7 7. 7 -1 0. 07 0. 01 -6 6. 6 0. 2 ZP -1 .3 Sp rin g 41 58 29 14 45 26 74 2. 8 22 .0 5. 20 20 6. 1 18 7 7. 9 15 3. 5 12 4 26 0. 09 0. 23 8. 4 0. 23 1. 95 1. 40 3. 56 -1 0. 30 0. 00 -6 9. 8 0. 1 IB Sp rin g 41 24 07 14 19 84 94 5. 0 22 .0 5. 20 20 8. 0 23 4 8. 4 21 1. 0 16 0 42 0. 18 0. 30 5. 9 0. 32 1. 58 0. 76 4. 49 -9 .6 9 0. 00 -6 5. 7 0. 7 V- 2 R iv er 41 41 88 14 33 57 84 1. 9 22 .0 5. 20 20 20 0 9. 5 20 1 8. 3 -9 .9 7 0. 00 -6 7. 2 0. 1 V- 3 R iv er 41 55 12 14 43 56 75 1. 7 22 .0 5. 20 20 1, 06 0 7. 7 19 2 7. 2 13 7. 5 12 9 30 0. 10 0. 24 7. 6 0. 29 1. 60 1. 22 -1 0. 18 0. 01 -6 8. 5 0. 7 B G R iv er 41 57 44 14 45 30 73 9. 0 22 .0 5. 20 20 2, 25 0 7. 3 17 3 8. 1 17 0. 1 11 6 28 0. 04 0. 23 6. 5 0. 21 1. 49 1. 07 5. 42 -1 0. 52 0. 00 -7 0. 9 0. 2 V- 4 R iv er 41 59 71 14 47 67 73 1. 3 22 .0 5. 20 20 2, 16 0 7. 2 17 4 7. 8 -1 0. 49 0. 02 -7 0. 8 0. 0 M -1 R iv er 41 85 36 14 63 92 65 3. 6 22 .0 5. 20 20 3, 48 0 8. 3 17 8 7. 8 -1 0. 51 0. 00 -7 1. 3 0. 0 V- 5 R iv er 41 90 14 14 68 91 64 5. 8 22 .0 5. 20 20 3, 50 0 8. 0 18 1 8. 0 -1 0. 49 0. 02 -7 0. 8 0. 3 P- 1 R iv er 41 55 14 14 43 65 75 2. 0 22 .0 5. 20 20 7. 2 15 3 8. 0 16 2. 3 10 4 25 0. 04 0. 21 5. 7 0. 23 1. 41 0. 89 -1 0. 93 0. 02 -7 3. 3 0. 3 S- 1 Sp rin g 41 81 52 14 57 95 67 8. 6 22 .0 5. 20 20 8. 5 27 6 8. 0 -9 .9 6 0. 01 -6 7. 4 0. 1 M S- 1 Sp rin g 42 29 85 13 45 45 1, 21 6. 4 22 .0 5. 20 20 5. 5 17 0 8. 3 -9 .5 5 0. 03 -6 3. 1 0. 3 215Using stable isotopes and major ions to identify recharge characteristics of the Triglavska Bistrica River Fig. 6. Isotopic composition of precipitation at the Zgornja Radovna and Kredarica meteorological stations for the period 2016–2020. Table 2: Altitude effect for snow and total precipitation calculated based on samples from Kredarica and Zgornja Radovna meteorological stations. Parameter Equation Slope ‰/1 km δ18O total Y=-0.0008x-8.272 -0.8 δ18O snow Y=-0.0008x-11.561 -0.8 δ2H total Y=-0.0056x-56.426 -5.6 δ2H snow Y=-0.0062x-82.379 -6.2 Stable isotopes δ18O and δ2H in precipitation The lowest isotopic precipitation values can be observed at the Kredarica meteorological station. The average unweighted values for the observation period 2016–2020 are -10.4 ‰, and -70.5 ‰ for δ18O and δ2H, respectively. The low- est value was determined in December 2018 and corresponds to the snow precipitation sample. At the Zgornja Radovna meteorological station the average unweighted values for the observation period 2016–2020 are -8.9 ‰ and -60.7 ‰ for δ18O and δ2H, respectively. The average isotopic com- position of snow at the Kredarica meteorologi- cal station was determined to be -13.5 ‰ for δ18O and -98.1 ‰ for δ2H. Slightly more positive values were observed at Zgornja Radovna: -12.1 ‰ for δ18O, and -87.1 ‰ for δ2H. The altitude effect was calculated for the two selected meteorological stations – Kredarica and Zgornja Radovna – separately, for total precipi- tation and for months when snow prevails in the monthly composite sample. These were select- ed based on a comparison of stable isotope data (Fig. 6) and meteorological parameters (days with snow/rain, amount of precipitation, snow cover thickness, etc.). It was estimated that the altitude effect in precipitation is -0.8‰ δ18O/km. This value is in good agreement with the estimat- ed altitude effect based on spring water δ18O val- ues (-1.1 ‰ δ18O/km) in the Radovna River valley in NW Slovenia (Torkar et al., 2016) and (-1 ‰ δ18O/ km) in Lake Bled and surroundings (Seri- anz et al., 2020b). Isotopic composition of water samples Values for δ18O, δ2H and 3H are presented in Table 1. Values for δ18O in all water samples are between -10.9 ‰ and -9.6 ‰. In general, δ18O val- ues were lower in the second sampling campaign (Fig. 8). Excluding location IB, the differences between both campaigns range from 0.03 ‰ at S-1 up to 0.5 ‰ at MS-1. The first sampling at MS-1 was performed when there was still ap- prox. 50 cm of snowpack in the recharge area, while during the second sampling the snowpack had already melted. The values for δ2H in all wa- ter samples are between -73.3 ‰ and -63.1 ‰. δ2H values are also lower in the second sampling campaign. Excluding location IB, the differenc- es between both campaigns range from 0.3 ‰ at V-5 up to 3.5 ‰ at MS-1. The results of δ18O and δ2H measurements in groundwater and surface 216 Luka SERIANZ, Sonja CERAR & Polona VREČA Fig. 7: Meteorological parameters during sampling (left) and the results of stable isotope analysis, together with Global Meteoric Water Line (GMWL) and East Mediterranean Meteoric Water Line (EMMWL)(right). Fig. 8. Graphical analysis of stable isotope analysis. water are shown in Figure 7b, as compared to the global meteoric line (GMWL) (Craig, 1961). The results show that all water samples are located between LMWLs for Kredarica and Zgornja Ra- dovna. The isotopic composition also allows us to es- timate the mean recharge altitude of the ground- water, namely in conditions where we have refer- ence locations with a known isotopic composition and a correspondingly small recharge area, for which we can estimate the average altitude. As follows from previous research in a wider area (Torkar, 2016; Serianz et al., 2020b), δ18O is par- ticularly suitable for a relevant assessment, and δ2H yields inapplicable results (indicates higher altitudes than in nature). In the given case, we used the spring Mrzli studenec (MS-1) in Poklju- ka (approx. 12 km air distance from the Triglavs- ka Bistrica) and the spring “Na Skedenjcih” (S-1, Fig. 1) as the reference location. Stable isotope analysis has been performed on the MS-1 spring in the past, as this spring, according to the data available so far from the wider area, represents one of the highest-lying springs with a constant flow and a small drainage area (Serianz et al., 2020b). Based on the isotopic composition it is also possible to estimate the mean recharge area of a given water sample using the simple linear rela- tionship between the representative water sam- ple with known recharge altitude and isotopic composition. In this case two springs were se- lected, MS-1 and S-1, based on the assumption that the isotopic signature of spring water is due to the small catchment area equal to the isotop- ic signature of precipitation at the correspond- ing altitude. The results, however, suggest only spring MS-1 is appropriate for such hypothesis, while spring S-1 was not appropriate for con- sideration. Therefore, an alternative approach was used, accounting for average δ18O from both sampling campaigns and a calculated precipita- tion altitude effect of -0.8 ‰ δ18O/km. The results show that the Triglavska Bistrica spring area 217Using stable isotopes and major ions to identify recharge characteristics of the Triglavska Bistrica River Fig. 9. Hydrogeological parameters of Triglavska Bistrica. is of local recharge component, while with the distance downstream the regional groundwater recharge component is much more significant (Fig. 9). At the sampling location V-5, which is approx. 300 m upstream from the Sava Dolinka confluence, the average recharge altitude is esti- mated at 1,996 m a.s.l. (Fig. 9). Mean average re- charge is even higher in the case of linear models from literature (Torkar et al., 2016; Serianz et al., 2020b). The calculated mean recharge altitude of the Triglavska Bistrica at its discharge into the Sava Dolinka was evaluated based on a Digital elevation model and was determined as approx. 1,500 m a.s.l. All water samples analysed for tritium activ- ity concentration can be classified as recent wa- ters (Mezga, 2014). According to the 3H results in the groundwater in the Peričnik catchment (3.5 TU), it is evident that the lower tritium ac- tivity concentration is due to the lower than ex- pected values of tritium, which are characteristic for snow precipitation (Vreča et al., 2013). The re- charge area of the Peričnik stream is represented by a high-altitude karstic-fissured aquifer, where dolomite predominates over limestone. As a re- sult, such an aquifer has a higher storage capac- ity, which is reflected in a longer retention time. Sampling points at Triglavska Bistrica BG and IB, which are located upstream from the Peričnik stream and recharge from both an intermediate flow component in the karstic-fissured aquifer and local flow components in Quaternary aqui- fers, show slightly higher tritium activities (5.4 TU and 4.5 TU, respectively). For a reliable esti- mation of retention times, systematic long-term observation of the geochemical and isotope char- acteristics of all water components (i.e. precipita- tion, snow, surface- and ground-water) is crucial. Conclusions The hydrogeochemistry of the Triglavska Bis- trica is determined by the prevalence of Ca2+ and Mg2+ cations and HCO3 - anions. The hydrogeo- chemical water types are Ca-Mg-HCO3 and Ca- HCO3, suggesting dolomite and limestone in the recharge area. The spatial variations in water carbonate chemistry along the valley are attrib- uted to different lithologies. That means that the recharge area of the Triglavska Bistrica spring is an aquifer or aquifer system dominated by lime- stone, while other waters downstream are char- acterized by the fact that they are mainly fed by aquifers dominated by dolomite over limestone. Analysis of the pH and major ions indicate that the water is alkaline and that carbonate weath- ering as a process dominates in the recharge area. Other major ions are present in low concen- trations and within natural background levels. The δ18O and δ2H values decrease along the Triglavska Bistrica valley, which means that the mean altitude of the recharge area increases downstream. At the spring, where lower altitude recharge prevails, the groundwater is enriched with 18O, while at the downstream locations groundwater becomes increasingly depleted in 18O due to the increasing impact of snow melt water infiltrated at higher altitudes. The mean recharge altitude area of the Triglavska Bistri- ca River was estimated at approx. 1,996 m a.s.l. at sampling point V-5, located approx. 12 km downstream from the spring. The concentrations of tritium activity in groundwater correspond to natural processes related to precipitation. How- ever, a more precise analysis of the residence time would require a systematic series of meas- urements over many years. 218 Luka SERIANZ, Sonja CERAR & Polona VREČA The results of this study provide important and useful new information about the hydroge- ological characteristics of the recharge area of the Triglavska Bistrica, allowing researchers to compare and define groundwater recharge are- as in similar hydrogeological systems. We expect the results of this study could also be used as a basis for different hydrological and hydrogeolog- ical studies, where water sources need to be in- vestigated. The Triglavska Bistrica catchment is highly important as a source of drinking water, as it also represents the recharge of the Peričnik spring, which itself is an important water re- source for part of the municipality of Jesenice. Therefore, it is very important to understand the dynamics of the different flow components in the studied aquifer system. In view of constant cli- matic changes, especially increasing air temper- ature, increasingly shorter snow periods of snow cover, and changing water infiltration, this im- portant resource should be carefully monitored now and in the future. Acknowledgments The authors would like to thank the Municipality of Jesenice for financial support. 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