Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia Pregled preteklih izotopskih hidroloških raziskav na območju Ljubljane, Slovenija Klara NAGODE 1 , Tjaša KANDUČ 1 , Sonja LOJEN 1 , Branka BRAČIČ ŽELEZNIK 2 , Brigita JAMNIK 2 & Polona VREČA 1 1 Jožef Stefan Institute, Department of Environmental Sciences, Jamova 39, SI-1000 Ljubljana, Slovenia; e-mail: klara.nagode@ijs.si 2 JP VOKA SNAGA d.o.o., Vodovodna 90, 1000 Ljubljana, Slovenia Prejeto / Received 5. 5. 2020; Sprejeto / Accepted 2. 11. 2020; Objavljeno na spletu / Published online 7. 12. 2020 Key words: water, stable isotopes, hydrogen, oxygen, carbon, Ljubljansko polje, Ljubljansko barje Ključne besede: stabilni izotopi, vodik, kisik, ogljik, Ljubljansko polje, Ljubljansko barje Abstract Water isotope investigations are a powerful tool in water resources research as well as in understanding the impact that humans have on the water cycle. This paper reviews past hydrological investigations of the Ljubljansko polje and Ljubljansko barje aquifers that supply drinking water to the City of Ljubljana, with an emphasis on hydrogen, oxygen and carbon stable isotope ratios. Information about the methods used and results obtained are summarised, and the knowledge gaps identified. Overall, we identified 102 records published between 1976 and 2019. Among them, 41 reported stable isotope data of groundwater, surface water and precipitation and were further analysed. Isotope investigations of the Ljubljansko barje began in 1976, while groundwater and surface water investigations of the Ljubljansko polje and along the Sava River began as late as 1997. Isotope investigations of carbon started even later in 2003 in the Ljubljansko polje and in 2010 in the Ljubljansko barje. These investigations were performed predominantly in the frame of short-term groundwater research projects at five main wellfields and sites along the Sava River. Almost no large-scale, long-term stable isotope studies have been conducted. The exceptions include groundwater monitoring by the Union Brewery in Ljubljana (2003- 2014) and precipitation in Ljubljana since 1981. Since 2011, more detailed surveys of the Ljubljansko barje were performed, and in 2018, the first extensive investigation started at wellfields and objects that form part of the domestic water supply system. Given the number of available studies, we felt that publishing all the numerical data and appropriate metadata would allow for a better understanding of the short and long-term dynamics of water circulation in the urban environment. In the future, systematic long-term approaches, including the appropriate use of isotopic techniques, are needed. Izvleček Izotopske raziskave se uporabljajo za proučevanje vodnih virov ter človeškega vpliva na vodni krog. V članku podajamo pregled preteklih izotopskih hidroloških raziskav na območju ljubljanskih vodonosnikov s poudarkom na uporabi razmerij stabilnih izotopov vodika, kisika in ogljika do leta 2019. Zbrali smo podatke o metodah in rezultatih ter identificirali glavne pomanjkljivosti preteklih raziskav. V sklopu pregleda smo zbrali različne vire (skupno 102) z informacijami, ki se nanašajo na karakterizacijo vodonosnikov, pomembnih za oskrbo z vodo na območju mestne občine Ljubljana. Med zbranimi viri je 41 takšnih, ki smo jih podrobneje pregledali, saj poročajo o izotopskih raziskavah podzemne in površinske vode ter padavin. V Sloveniji so bile izotopske raziskave kisika in vodika v podzemni vodi prvič izvedene na Ljubljanskem barju leta 1976, medtem ko so se raziskave na Ljubljanskem polju ter na reki Savi pričele šele 1997. Izotopske raziskave ogljika v podzemni vodi so se pričele kasneje: na Ljubljanskem polju leta 2003 ter na Ljubljanskem barju leta 2010. Spremljanje izotopske sestave se je na obravnavanem območju v preteklosti izvajalo večinoma v sklopu različnih raziskav podzemne vode v glavnih petih črpališčih ter na reki Savi. Raziskave so potekale pretežno v sklopu različnih kratkotrajnih projektov ter so redko vključevale večje območje (npr. Ljubljansko polje in barje). Daljše zvezne izotopske raziskave podzemne vode so potekale od 2003 do 2014 na območju Pivovarne Union, spremljanje padavin pa poteka v Ljubljani od leta 1981. Od leta 2011 so potekale podrobnejše izotopske raziskave na Ljubljanskem barju, leta 2018 pa so bile opravljene pr ve obsežnejše i zotopske ra ziskave, tako na čr pal išči h kot tud i objekti h, k i so del jav nega vodovod nega sistema. Ugotovili smo, da je objavljanje numeričnih podatkov in ustreznih metapodatkov pomembno. Pregled razpoložljivih virov kaže, da bi objava vseh numeričnih podatkov in ustreznih metapodatkov omogočila boljše razumevanje kratke in dolgoročne dinamike kroženja vode v urbanem okolju, zato so v prihodnosti potrebni sistematični dolgoročni pristopi, ki bodo vključevali tudi ustrezno uporabo izotopskih tehnik. GEOLOGIJA 63/2, 251-270, Ljubljana 2020 https://doi.org/10.5474/geologija.2020.019 © Author(s) 2020. CC Atribution 4.0 License 252 Klara NAGODE, Tjaša KANDUČ, Sonja LOJEN, Branka BRAČIČ ŽELEZNIK, Brigita JAMNIK & Polona VREČA Introduction As Bowen et al. (2019) states “ Earth’s water cy- cle links solid Earth, biological, and atmospheric systems, and it is both pivotal to the fundamental understanding of our planet and critical to our practical well-being.” In nature, water is bound in different compartments of the hydrosphere (ice, groundwater, surface water, lakes, soil mois- ture reservoirs, oceans, and biomass), biosphere, lithosphere and the atmosphere, which form part of a global hydrological cycle. The rapid growth in population, coupled with an increased demand for water by agriculture and industry, are putting pressure on water resources (Mook, 2001). Al- though the impact that humans are having on the water cycle is indisputable, there is still a lot un- known about how water usage alters regional and global water budgets (Bowen et al., 2019). One of the prerequisites for efficient management of wa- ter resources is having reliable information about the quantity and the quality of the resource that is being exploited (Dansgaard, 1954; Craig, 1961). Stable water isotopes ( 1 H, 2 H, 16 O, 17 O and 18 O) and carbon isotopes ( 12 C and 13 C) in the dissolved inorganic carbon (DIC) occur naturally. They can be measured using isotope-ratio mass spectrom- etry (dual-inlet or continuous-flow) (de Groot, 2004), laser spectroscopy (Wassenaar et al., 2018), or by spectrometric imaging methods (Bowen et al., 2019). An isotope abundance of an element is generally reported in ‰ (per mill = parts per thousand = 10 -3 ) deviations relative to the known isotope abundance of a standard, δ: (Gat, 1996): δ (‰) = (R sample /R standard −1) × 10 3 were R sample a n d R standard p r e s e n t i s o t o p e r a - tios ( 2 H/ 1 H, 18 O/ 16 O, 13 C/ 12 C, 15 N/ 14 N, 34 S/ 32 S) of a heavy isotope to a light isotope in a sample and an international standard, respectively. Because the numerical values obtained by this equation are small they are expressed in delta notation ( δ). Delta values can be negative or positive numbers meaning that the isotope ratio of the sample is lower or higher relative to a standard (Gat, 1996; Meier-Augenstein & Schimmelmann, 2019). Isotopes are an important tool for studying the water cycle and can be divided into two main categories: environmental isotopes (isotope vari- ations in waters by natural processes) and artifi - cial radioactive isotopes (radioactive isotopes that are injected into the system under investigation) (Kendall & Doctor, 2003). δ 18 O, δ 2 H and δ 13 C DIC val- ues are important in different applications (Gat, 1996; Clark & Fritz, 1997; Ehleringer et al., 2008; Clark, 2015; Bowen et al., 2019): - δ 18 O and δ 2 H can be used as conservative tracers if the isotope signature is unmodi- fied within a study system, i.e., to identify water sources contributing to water sam- pled at a given place; - δ 18 O, δ 2 H and δ 13 C DIC a n d t h e i r v a r i a t i o n s can enable the identification of important water and carbon cycle processes over- looked by other methods; - δ 18 O and δ 2 H can link information on the history of water as it moves through the hy - drological cycle. Isotope methods were introduced into catch- ment hydrology research to help scientists to un- derstand better the geographical origin of water, recharge and discharge processes, biogeochemi- cal processes and the sources and mechanisms of pollution (Clark & Fritz, 1997; Aggarwal et al., 2005; Bowen et al., 2005; Ehleringer et al., 2008; 2016; Jameel et al., 2016; Du et al., 2019). Concerns over climate change and the in- creasing demand for water in urban areas has focused research on water supplies and dynamics within the urban system in order to gain a better understanding of the connections between hu- man populations, climate, and water extraction (Ehleringer et al., 2016; Zhao et al., 2017; Tipple et al., 2017). Water circulates in nature differently than in urban environments, where the world’s pop- ulation is expected to increase to more than 60 % by 2050. Supplying large urban areas with high-quality drinking water and providing wa- ter resources in the long term is a major challenge (Jameel et al., 2016; Ehleringer et al., 2016). In Slo - venia, drinking water supply is mainly based on groundwater (around 97 % of the drinking water supply is from groundwater resources) (Uhan & Krajnc, 2003) and in the capital city, Ljubljana, it provides an invaluable drinking water resource (Tr ček, 2017). In Slovenia, only tritium and radon analyses are prescribed by drinking water legislation (Of- ficial Gazette, No. 74/15), however, if the para- metric value for tritium is exceeded, it must be investigated to see if the cause is the presence of artificial radionuclides. Parametric values for specific basic ions, e.g., NO 3 - , SO 4 2- and trace el - ements, e.g., Se, Sb, Pb, Ni, Fe, Cu, Cd, Al, As, B in drinking water have also been established (Official Gazette, Nos. 19/04, 35/04, 26/06, 92/06, 25/09, 74/15, and 51/17), while the regular moni- toring of stable isotopes of H, O in water and C and N in different compounds (e.g., HCO 3 - , NO 3 - ) 253 Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia is not required by legislation. Despite quite a large number of isotope analyses performed in the past, to date, there has been no comprehen- sive research in the use of environmental isotopes in urban water management systems in Slovenia. Here, we review and synthesize past research involving δ 18 O, δ 2 H and δ 13 C DIC to advance our un- derstanding of the groundwater characteristics of the Ljubljana aquifers, which can be used as the basis for future investigations. We focus on work conducted over the past 40 years. The main aims of this review were the following: - make a synthesis of past urban hydrology investigations of the Ljubljansko polje and Ljubljansko barje aquifers with emphasis on the use of δ 18 O, δ 2 H and δ 13 C DIC until 2019; - collect information about sampling (loca- tion, time, type of sampling site) and the analytical methods used; - identify the main gaps in the previous in- vestigations and propose future activities. Site description The two most important groundwater aqui- fers for the Slovenia capital Ljubljana and its surroundings are the Ljubljansko polje (LP) and Ljubljansko barje (LB) (Fig. 1). The two aquifers are separated by the Golovec, Grajski hrib and Rožnik hills (Fig. 1) (e.g., Vižintin et al., 2009; Janža, 2015). Two rivers bound the LP aquifer (Fig. 1) – the Ljubljanica River to the south and the Sava River to the north (Jamnik et al., 2003; Ogrinc et al., 2018). Because of the high velocities (10 m/day) and quite plunder groundwater flow (3–4 m 3 /s), the quality of groundwater is good (Jamnik et al., 2003; Jamnik & Ž it n i k, 2020). Hydrological con- ditions in the area are characterized by strong interactions between surface water and ground- water and by the high velocities of groundwater flow and pollutant transport: that is, up to 20 m/ day (Andjelov et al., 2005; Janža et al., 2005). The LP is located in the eastern part of the Ljubljana basin (Ljubljanska kotlina). It was formed by tec - Fig. 1. Locations of the studied area with the main wellfields (Kle če, Hrastje, Brest, Jarški prod and Šentvid) and correspon- ding water supply areas in the Municipality of Ljubljana (wellfield Hrastje does not represent a unique water supply area). Source of topography: Geodetska uprava RS. 254 Klara NAGODE, Tjaša KANDUČ, Sonja LOJEN, Branka BRAČIČ ŽELEZNIK, Brigita JAMNIK & Polona VREČA tonic subsidence in the early Pleistocene together with the main neotectonic fault system that runs in an east-west direction. The basin is composed of Permian and Carboniferous slate claystone and sandstone (Žlebnik, 1971). The Pleistocene and Holocene sediments, accumulated by the Sava River, form highly permeable of partially conglomerated sand and gravel. The thickness of these fluvial sediments in- creases towards the centre of the LP, where it even exceeds 100 m (Andjelov et al., 2005). The aquifer system has an intergranular porosity, and an unconfined groundwater table, located on 20–25 m below the surface (Vrzel et al., 2018) and can fluctuate up to 10 m (source archive JP VOKA SNAGA d.o.o.). The main recharge of the aquifer comes from infiltration of precipitation and the Sava River, which recharge the aquifer mainly in its north–western part and drains the eastern part of the LP. The LP is also recharged via lateral inflow from the LB multi-aquifer sys - tem in the south (Jamnik et al., 2000; Vižintin et al., 2009; Vrzel et al., 2018) as well as from the Kamniško-Bistriško polje (Jamnik & Urbanc, 2000). Groundwater is exploited at LP from four wellfields: Kle če, Hrastje, Jarški prod and Šentvid where drinking water is pumped from 16, 10, 3 and 3 wells, respectively (Fig. 1). Anthropogen- ic conditions of the aquifer are characterized by significant pressures of urbanization, industry, traffic, agriculture and old environmental bur- dens (Jamnik et al., 2012), which occur within the aquifer recharge area (Tr ček, 2017). To date, sev- eral different sources of pollutants have been de- tected and investigated. These include dispersed pollution sources where pollutants are consis- tently present (nitrates from agriculture and sewerage losses, new emerging contaminants in traces – pesticides from agriculture, plasticizers, corrosion and fire inhibitors, pharmaceuticals from sewage system losses (Jamnik et al., 2009) while others originate from past agricultural and industrial activities (atrazine, desethyl-atrazine, chromium (VI), trichloroethene, tetrachloro- ethene). Also, the characteristics of plumes and multipoint pollution contamination sources were recognized (Brilly et al., 2003; Karahodži č, 2005; Prestor et al., 2017). The LB aquifer (Fig. 1) extends from the southern part of Ljubljana to the Krims- ko-Mokrško hills. The Barje is a depression with a stone bedrock that consists in the southern, western and central parts of Upper Triassic do- lomite and Jurassic limestone, and in northern and eastern parts of Triassic and Permo-Car- boniferous shaly mudstone, quartz sandstone and conglomerate, characterized by low hydrau - lic conductivity. The gravel fans are present on the borders of the basins (Mencej, 1988/89; Cer- ar & Urbanc, 2013). The basin was formed by a tectonic depression and filled by alluvial, marshy and lacustrine sediments during the Pleistocene and Holocene (Mencej, 1988/89). The Ljubljanica River contributes to groundwater storage as well as the Krimsko-Mokrško hills (ARSO, 2012; Cer- ar & Urbanc, 2013). The wellfield at Brest (Fig. 1) is an important source of drinking water for the southern part of the city of Ljubljana (Bra či č Železnik & Globevnik, 2014). It consists of 13 wells of different depths (Bra či č Železnik, 2016). Water resources in the area are under significant pressure, and environmental problems include water pollution, increasing water demand, flood and drought risk, reduction in retention capac- ity, decreasing groundwater levels and terrain subsidence (Bra či č Železnik & Globevnik, 2014). However, desethyl-atrazine represents the most severe problem for the further development of the Brest water source (Prestor et al., 2017). The Ljubljana drinking water supply system The central Ljubljana water supply system consists of five water supply facilities with alto- gether active 44 wells and more than 1,100 km long water supply network supplying 330,000 us- ers through 43,000 connections. Water supply net - work includes different objects (i.e., reservoirs, water treatment locations, pumping stations) (Jamnik & Žitnik, 2020). In the central system, some settlements are continuously supplied with drinking water from a single wellfield (water sup - ply areas A, C, D and E in Fig. 1), and others from two or more wellfields (water supply areas F, G, H and I2 in Fig. 1), depending on water consump- tion and pressure conditions in the system. Well- field Hrastje (B) does not represent a unique water supply area (Jamnik & Žitnik, 2020). The water from the wells is pumped directly to consumers or a reservoir for the short-term, from where it is distributed to the users. Water disinfection devices are built-in into the system; however, water does not undergo technical treat- ments. It is only chlorinated occasionally. For the Brest wellfield UV disinfection is used (Jamnik & Ž it n i k, 2020). 255 Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia Methods Studies related to the characterization of aquifers important for the domestic water supply in the municipality of Ljubljana were reviewed, with a focus on those studies that used δ 18 O, δ 2 H, and δ 13 C DIC values for the characterization of wa- ter sources. Study selection criteria First, we considered articles and reports relat - ed to the water cycle and domestic water supply investigations for the LP and LB published from 1976 to the present (Fig. 2). In the scope of the review, a comprehensive search of journals was completed based on several keywords related to the Ljubljana aquifers (Ljubljana/Ljubljansko polje, Ljubljansko barje, Ljubljana groundwater, Ljubljana water, Ljubljana water supply). The search included all studies containing informa- tion about i) sampling, ii) analytical methods, iii) the parameters determined, and iv) isotope data. In the second step, we focused on studies re- porting the use of δ 18 O and δ 2 H to measure, de- scribe or establish the characteristics of the LP and LB aquifers. Additionally, we also collected studies involving δ 13 C DIC . Articles on the model- ling of LP and LB and other groundwater param - eters, e.g., toxic metals in the groundwater and spring waters, electrical conductivity, and phar- maceuticals, and the quantity and quality con- ditions of groundwater in the Ljubljana aquifers were beyond the scope of this review (Fig. 2). Search methods The databases were searched for relevant lit- erature published before November 2019 and in- cluded Google, Google Scholar, Science Direct, Co-operative Online Bibliographic System, and Service – COBISS. Included were national and international journals, conference papers, PhD and Master Theses, reporting data on δ 18 O, δ 2 H and δ 13 C DIC i n a n u r b a n w a t e r s y s t e m , p r e c i p - itation, and the Sava River. Also, the reference section of the articles was searched to identify additional sources. We also inspected the work- ing reports for JP VOKA SNAGA d.o.o. available at Jožef Stefan Institute (JSI) including isotope data. Studies published in both Slovene and En- glish were considered. Information about i) sampling including lo- cation coordinates, type of sampling location (groundwater, spring water, precipitation, river) and sampling period; ii) the analytical meth- ods used for δ 18 O, δ 2 H and δ 13 C DIC analysis, and iii) δ 18 O, δ 2 H and δ 13 C DIC data were collected and summarised. Results and discussion The initial combined search retrieved 102 re- cords (Fig. 2). After removing 41 non-relevant records, the 61 articles remaining were assessed for eligibility. Of these, 24 records were used to summarize site characteristics, while 41 records containing δ 18 O, δ 2 H and δ 13 C DIC d a t a ( T a b l e 1 ) were reviewed in detail. Some articles were used in both categories. Information about sampling is summarised in subchapter Sampling, followed by Analytical methods used for determining δ 18 O, δ 2 H and δ 13 C DIC . Finally, a summary of the isotope research and the important findings relating to the Ljubljana aquifers is presented. Sampling Infor mation collected about the sampling area, sampling locations and type of samples collected in different investigations for isotope analysis is presented in Table 1. Isotope investigations of groundwater were first performed in 1976 at LB (Breznik, 1984) while groundwater and surface water investigations at LP and on the Sava River in Tacen began in 1997 (Urbanc & Jamnik, 1998). The isotope composition of precipitation in Lju- bljana has been regularly monitored since 1981 (Pezdi č, 1999; Vre ča et al., 2008). At the LP, many investigations were performed at the wellfield Kle če, followed by the wellfields Hrastje, Jarški prod, and Šentvid (Fig. 1, Table 1). Short-term studies were performed at the bore- hole LMV-1 (located close to the wellfield Kle če). In contrast, long-term investigations were per- formed in the area of Union Brewery (Table 1). In LB, sampling was mainly conducted in the well- field Brest (Table 1). Surface waters (e.g., Curn - ovec, Gradaš čica) were also sampled (Urbanc & Jamnik, 2002). On the Sava River, sampling was performed at Tacen, Brod, Črnu če, Šentjakob and Dolsko (see references in Table 1). The Jo žef Stefan Fig. 2. Flowchart of study selection for detail review synthesis. 256 Klara NAGODE, Tjaša KANDUČ, Sonja LOJEN, Branka BRAČIČ ŽELEZNIK, Brigita JAMNIK & Polona VREČA Table 1. The list of references related to the isotope investigations performed in the area of Ljubljansko polje (LP), Ljubljansko barje (LB), the Sava River (RS), and precipitation (P). Source of reference: * archive of the JP VOKA SNAGA d.o.o.; ** archive of JSI. (GW = groundwater, P = precipitation, SF = surface water, TW = tap water, VD = well). Reference Parameter Sampling area Type of sample Location Breznik, 1984* δ 18 O LB GW Brest Pezdi č, 1998 δ 18 O, δ 2 H LB P, G W The southern part of LB Krajcar Broni ć et al., 1998 δ 18 O, δ 2 H LP P Ljubljana Urbanc & Jamnik, 1998 δ 18 O LP , RS G W, S W, P RS (Tacen), Mostec, Nadgoriški potok, Kle če (V-4, V-6, V-8a, V-11, V-12, V-14, V-15), Šentvid (V-2a), Jarški prod (V-1, V-3), Hrastje (V-1a, V-5, V-8), precipitation-Kle če Pezdi č, 1999 δ 18 O, δ 2 H Ljubljana P Ljubljana Bežigrad Jamnik & Urbanc, 2000 δ 18 O LP , RS GW Kle če (VIIIa and XII), Hrastje (Ia and V)), Šentvid (IIa) and Jarški prod (I, III), groundwater level stations, precipitation station Urbanc & Jamnik, 2002 δ 18 O LB GW , SW Mostec, Gradaš čica, Ljubljanica, Curnovec, Holocen aquifer (V-1, V-7, V-9, V-10, V-12, V-13, IŠ-6pl, IŠ-7, IŠ-8, DBP-2, DBP-4, DBP-5, DBP-6, DBP-9). Upper Pleistocene aquifer (Iš-6gl, OP-1, PB-2gl, PB-4, PB-6gl, G-12, PB-1gl, VD-4gl, DBG-2, DBG-4, DBG-5, DBG-6, DBG-9). Lower Pleistocene aquifer (TB-3, B-1, PB-5gl, P-19gl, A-1gl, A-2gl, IŠ-4gl). Jamnik & Urbanc, 2003 δ 18 O LP , LB GW, P, SW LP and LB, GeoZS, RS (Tacen) Pezdi č, 2003 δ 18 O, δ 2 H Ljubljana P Ljubljana – Bežigrad, Ljubljana – JSI Andjelov et al., 2005 δ 18 O, δ 2 H LP G W, S W, P Nadgoriški potok, Mostec, RS, wells in Kle če (4, 6, 8a, 11, 12, 14, 15), Hrastje (1a, 5, 8), Jarški prod (1, 3), Šentvid (2a) Bren či č & Vre ča, 2005 δ 18 O, δ 2 H, δ 13 C DIC LP GW (bottled) Union Brewery Tr ček, 2005 δ 18 O, (δ 2 H) LP G W, P Lysimeter Union Brewery Vre ča et al., 2005 δ 18 O, δ 2 H Ljubljana P Ljubljana – JSI, Ljubljana – Reaktor Bren či č & Vre ča, 2006 δ 18 O, δ 2 H LP GW (bottled) Union Brewery Tr ček, 2006 δ 18 O, δ 2 H LP G W, P Piezometer Union Brewery Vre ča et al., 2006 δ 18 O, δ 2 H Ljubljana P Ljubljana – JSI, Ljubljana – Reaktor Kandu č, 2006 δ 18 O, δ 2 H, δ 13 C DIC LP , RS SW , GW RS (Brod, Sava Dolsko), LP (Yulon, Hrastje1a, Kle če, vodnjak 17, GeoZS, Kle če 11, Šentvid 2A, Kle če 8a, Hrastje 3, Navje, Petrol - Šmartinska cesta, L.P . Vodovodna, HMZ Hrastje) Bren či č & Vre ča, 2007 δ 13 C DIC LP GW (bottled) Union Brewery Ogrinc et al., 2008 δ 18 O, δ 2 H RS S W, P RS (Tacen, Dolsko), Ljubljana – Bežigrad, Ljubljana – JSI, Ljubljana – Reaktor Vre ča et al., 2008 δ 18 O, δ 2 H Ljubljana P Ljubljana – Bežigrad, Ljubljana – JSI, Ljubljana – Reaktor Bren či č & Vre ča, 2010 δ 18 O, δ 2 H, δ 13 C DIC LP GW (bottled) Union Brewery 257 Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia Vre ča et al., 2011** δ 18 O, δ 13 C DIC LB GW , SW V-1A, V-2A, V-3A, V-4A, V-5, V-7, V-8, and V-9, P-23/10 Bren či č, 2011* δ 18 O, δ 2 H, δ 13 C DIC LB GW , SW V-1A, V-2A, V-3A, V-4A, V-5, V-7, V-8, and V-9, P-23/10 Urbanc et al., 2012 δ 18 O LP , LB GW , SW VD Kle če (4, 8a, 11, 14, 17), VD Hrastje (1a, 3), VD Brest (1, 1a, 2a, 3, 4a, 5, 7, 9), VD Jarški prod (1, 3), VD Šentvid (1a) Cerar & Urbanc, 2013 δ 18 O LP , LB GW , SW , P LP aquifer, the northern part of LB, the middle part of LB, the southern part of LB – Brest and Iški Vršaj, GeoZS Vre ča et al., 2013** δ 18 O, δ 2 H, δ 13 C DIC LB GW VD-3a Mezga, 2014 δ 18 O, δ 2 H, δ 13 C DIC LP GW LMV-1 Mezga et al., 2014 δ 18 O, δ 2 H LP GW LMV-1 Vre ča et al., 2014 δ 18 O, δ 2 H Ljubljana P Ljubljana – Reaktor Vre ča et al., 2015** δ 18 O, δ 2 H, δ 13 C DIC LB GW VD-3a Vre ča & Malenšek, 2016  δ 18 O, δ 2 H LP P Ljubljana – Bežigrad, Ljubljana – JSI, Ljubljana – Reaktor, Kle če Tr ček, 2017 δ 18 O, (δ 2 H) LP G W, P Union Brewery Bra čič Železnik et al., 2017 δ 18 O, δ 2 H, δ 13 C DIC LB GW , SW VD Brest-3a Vrzel et al., 2018 δ 18 O, δ 2 H LP , RS GW , SW , P RS (Šentjakob), Kle če (8, 11, 12), Hrastje (3, 8), Jarški prod (1, 3), Ljubljana – Reaktor, GeoZS Ogrinc et al., 2018 δ 18 O, δ 2 H RS SW RS (Dolsko) Vre ča et al., 2019a** δ 18 O, δ 2 H LP , LB, RS GW , SW , TW VD Kle če (2, 3, 4, 6, 7, 8a, 9, 10, 11, 12, 13, 14, 15, 16, 17), VD Hrastje (1a, 2, 2a, 3, 4, 5, 6, 7, 8), VD Brest (1, 2, 2a, 3, 4, 4a 5, 6, 7, 8, 9), Jarški prod (1, 2, 3), VD Šentvid (1a, 2a, 3), joint exits from water pumping stations, reservoirs, drinking water fountains, tap water in public and private buildings, RS (Šentjakob, Črnu če, Brod) Vre ča et al., 2019b δ 18 O, δ 2 H, δ 13 C DIC LB, LP , RS GW , SW , TW VD Kle če (2, 3, 4, 6, 7, 8a, 9, 10, 11, 12, 13, 14, 15, 16, 17), VD Hrastje (1a, 2, 2a, 3, 4, 5, 6, 7, 8), VD Brest (1, 2, 2a, 3, 4, 4a 5, 6, 7, 8, 9), Jarški prod (1, 2, 3), VD Šentvid (1a, 2a, 3), joint exits from water pumping stations, reservoirs, drinking water fountains, tap water in public and private buildings, RS (Šentjakob, Črnu če, Brod) Vre ča et al., 2019c** δ 18 O, δ 2 H LB, LP TW Vrtec Miškolin enota Zaj čja Dobrava; Vrtec Pedenjped, enota Zadvor; Vrtec Visji gaj, enota Kozarje Bencinski servis Agip; Vrtec Hansa Christiana Andersena, enota Marjetica; Vrtec Vodmat; Vrtec Mladi rod, enota Kostanj čkov vrtec; Vrtec Mojca, enota Rozle; OS IG - podruznica Iška vas Vre ča et al., 2019d** δ 18 O, δ 2 H LB, LP TW Tap water at location Jože Stefan Institute Vre ča et al., 2019e ** δ 18 O, δ 2 H, δ 13 C DIC LB GW PB – 24b/19 Vre ča et al., 2019f ** δ 18 O, δ 2 H, δ 13 C DIC LB GW PB – 24a/19, PB – 24c/19 258 Klara NAGODE, Tjaša KANDUČ, Sonja LOJEN, Branka BRAČIČ ŽELEZNIK, Brigita JAMNIK & Polona VREČA Institute (JSI) has recorded the isotope com- position of precipitation since 1981. Samples of precipitation were first collected at the synoptic station Ljubljana–Bežigrad located at the Hy- drometeorological Survey of Slovenia (today Slovenian Environment Agency – ARSO), later at the JSI (station Ljubljana–JSI) and finally at the Reactor Centre of the JSI (station Ljubljana– Reaktor) (Pezdi č, 2003; Vre ča et al., 2006; Vre ča & Malenšek, 2016). Precipitation was collected for a short period in the areas of wellfield Kle če, Union Brewery and at Geological Survey of Slo- venia (GeoZS) (see references Table 1). The first stable water isotope survey of tap water in Slovenia, according to our best knowl- edge, was performed in 2014 (Vre ča et al., 2019c). In this survey, tap water samples were collect- ed for O and H isotope analysis at 105 locations around Slovenia, nine of them at locations in Lju - bljana and its vicinity (Vre ča et al., 2019c). To assess the usefulness of environmental iso- topes, scientists have been performing systematic monitoring of the Ljubljana drinking water sup- ply system since 2018. The first detailed sampling campaign was carried out between 06/09/18 and 29/11/18 at 103 locations; 41 wells in five water supply facilities, seven joint exits from the water pumping station, 22 reservoirs, two water treat- ment locations, 13 fountains, and 19 taps (see Ta- ble 1). In addition, samples were collected on the Sava River at Brod, Črnu če and Šentjakob (Vre ča et al., 2019a; Vre ča et al., 2019b). The first 24-hour experiment was performed in the basement of the main building at the Jožef Stefan Institute in Ljubljana with emphasis on the hourly isotope variability of tap water in April 2019 (Vre ča et al., 2019d). From Table 1, the following sampling loca- tions were identified: - Wellfields – Kle če (11 wells), Hrastje (5 wells), Brest (12 wells), Jarški prod (2 wells), and Šentvid (1 well) - The Sava River – fi v e l o c a t i o n s : B r o d , Črnu če, Dolsko, Šentjakob and Tacen - Precipitation – six locations: synoptic sta - tion Ljubljana–Bežigrad, JSI-Ljubljana, Ljubljana-Reaktor, Union Brewery, well- field Kle če, and GeoZS - Other locations – p i e z o m e t e r s a n d s p r i n g water from the LB, lysimeter and piezom- eters at Union Brewery, groundwater in LMV-1, tap water and different objects of the drinking water supply system. Analytical methods used for determining stable oxygen, hydrogen and dissolved inorganic carbon isotope composition Results of δ 18 O, δ 2 H were reported relative to VSMOW (e.g., Urbanc & Jamnik, 1998; Bren či č & Vre ča, 2006; Vrzel et al., 2018), while δ 13 C DIC was reported relative to the VPDB (e.g., Bren či č & Vre ča, 2006; Kandu č, 2006; Vre ča et al., 2019e). Isotope ratio mass spectrometers (IRMS) were used for the determination of δ 18 O, δ 2 H, and δ 13 C DIC in water except for some precipitation samples collected at the Ljubljana–Reaktor which were measured by off-axis integrated cavity output laser spectroscopy, OA-ICOS (Vre ča et al. 2017). Oxygen isotope composition ( δ 18 O) is reported in 40 records (Table 1). In all past investigations, the authors reported that the δ 18 O was deter- mined by the water-CO 2 equilibration technique (Epstein & Mayeda, 1953; Avak & Brand, 1995) using different IRMS, namely the dual inlet Varian Mat 250 at the JSI (Pezdi č, 1998; Urbanc & Jamnik, 1998; Jamnik & Urbanc 2000; Andjelov et al., 2005; Vre ča et al. 2005; 2006; 2008; Ogri- nc et al., 2008), Finnigan DELTA plus at the Joan - neum Research (JR) in Graz, Austria (Bren či č & Vre ča 2006; Tr ček, 2017), Finnigan MAT 250 at the Hydroisotop GmbH laboratory in Schweiten- kirchen, Germany (Cerar & Urbanc, 2013; Mezga et al., 2014; Mezga, 2014; Vre ča et al., 2015), and a continuous flow IsoPrime (GV Instruments) at the JSI (Bra či č Železnik et al., 2017; Ogrinc et al., 2018; Vrzel et al., 2018; Vre ča et al., 2014). Tr ček (2005; 2006) reported that analysis was performed at the Institute of Groundwater Ecology (GSF) in Neuherberg, Germany, but does not state the type of IRMS used for the analysis. The δ 18 O analysis of precipitation collected by the JSI at the Ljubljana–Reaktor was performed from Feb- ruary 2007 to the end of 2014, using a continuous flow IRMS IsoPrime (GV Instruments) connected to equilibration system MultiFlow Bio (Vre ča et al., 2014). Samples collected since 2015 were mea - sured on a dual inlet Finnigan MAT DELTA plus with CO 2 -H 2 O equilibrator HDOEQ48 (Vre ča et al., 2019a; 2019b; 2019d; 2019e; 2019f). Hydrogen isotope composition ( δ 2 H) i s r e - ported in 32 records using different analytical methods, which included H 2 generated by the reduction of water over hot zinc (Pezdi č, 1999), H 2 e q u i l i b r a t e d w i t h t h e w a t e r s a m p l e s u s i n g a Pt-catalyst (Horita et al., 1989), reduction on Cr at 800 °C (Gehre et al., 1996; Morrison et al., 2001) or with an OA-ICOS (Wassenaar et al., 2014). Measurements were performed on dif- ferent IRMS including a dual inlet Varian Mat 259 Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia 250 at the JSI (Pezdi č, 1998; Vre ča et al., 2005; 2006; 2008; Ogrinc et al., 2008; 2018; Vrzel et al., 2018), Finnigan DELTA plus XP a t th e J o ann e um Research (JR) in Graz, Austria (Bren či č & Vre ča, 2006; Vre ča et al., 2014; Tr ček, 2017), Finnigan MAT 251 at the Hydroisotop GmbH laboratory in Schweitenkirchen, Germany (Vre ča et al., 2011; 2013; 2015; Mezga et al., 2014; Bra či č Železnik et al., 2017). Samples collected from 2015 onwards were measured on the dual inlet Finnigan MAT DELTA plus with CO 2 -H 2 O equilibrator HDOEQ48 at the JSI (Vre ča et al., 2019a; 2019b; 2019d; 2019e; 2019f). Some precipitation samples collected at the Ljubljana–Reaktor were measured at the Iso- tope Hydrology Laboratory at the International Atomic Energy Agency (IAEA) on a Los Gatos Research OA-ICOS (Vre ča et al. 2017). The carbon isotope composition in the dis- solved inorganic carbon ( δ 13 C DIC ) is reported in 13 records and was determined using CO 2 collected after the reaction of the water sample with 100 % H 3 PO 4 on a continuous flow Europa 20-20 IRMS with ANCA-TG separation module for trace gas analysis (Bren či č & Vre ča, 2005; 2007; 2010; Mez- ga, 2014; Vre ča et al., 2019a) or a continuous flow IsoPrime or IsoPrime 100 IRMS with equilibra- tion system MultiFlow Bio at the JSI (Bren či č, 2011, Bra či č Železnik et al., 2017; Vre ča et al., 2011; 2013; 2015; 2019e; 2019f). Only a few articles reported the analytical errors (Tr ček, 2005; 2006; Bren či č & Vre ča, 2006; 2007; Ogrinc et al., 2008; 2018; Vre ča et al., 2008; 2018; Cerar & Urbanc, 2013; Mezga et al., 2014). Most publications report basic descriptive sta- tistics or isotope ranges and only in a few cas- es, whole datasets are publicly available (e.g., Bren či č & V r e ča, 2006; 2007; Vre ča et al., 2008; 2014; Vrzel et al., 2018). History of the stable isotope research in the catchment area of Ljubljana aquifers Here we present a summary of the 41 records (Table 1) related to the past stable isotope inves- tigations in the area of LP and LB aquifers. Ar- ticles usually report the use of δ 18 O and δ 2 H in water resources investigations; however, it is in- teresting, that the δ 13 C DIC was determined in only 13 records. Ljubljansko barje The first isotope investigations in the area of Ljubljana aquifers were performed in 1976 (Breznik, 1984), as part of the hydrological re- search into the Brest wellfield between 1974 and 1976. Water samples were collected at the LB aquifer, from the Iška River and other springs in the vicinity. No precise sampling locations with coordinates were reported, and no information was given about the collection of the samples or where the analyses were performed. They report - ed values for δ 18 O between -9.94 and -8.90 ‰ and -65.8 and -58.9 ‰ for δ 2 H. From the tritium iso- tope data, Breznik (1984) concluded that the re- charge rate of the lower aquifer is very low. Samples from the southern part of LB were collected in early spring and autumn in 1993. Nineteen sampling points for groundwater and river base flow measurements were established for the determination of groundwater recharge and storage capacity (Pezdi č, 1998). Unfortu- nately, the sampling locations are presented only graphically, and the author gives no exact coor- dinates or location names. Precipitation was col- lected in Ljubljana for the determination of δ 18 O and δ 2 H values. Pezdi č (1998) reported δ 18 O values of springs and surface river water of -9.65 and -8.82 ‰, while δ 2 H values ranged from -67.4 to -61.2 ‰. The weighted means of δ 18 O and δ 2 H in precipitation for the year 1993 were -8.07 ‰ and -55.6 ‰, respectively. The author concluded that the contribution of local precipitation was small and infrequent; however, local precipitation could recharge nearby aquifers (Pezdi č, 1998). After 1997, Urbanc & Jamnik (2002) performed more detailed investigations of the LB in which the chemical and isotope composition of ground- water was studied. Isotope investigations com- bined with hydrogeochemical methods were used to obtain hydrogeological data on the properties of water in individual aquifers: the Holocene aquifer and the upper and the lower Pleistocene aquifers. The authors, however, do not provide any sampling information or at which insti- tute the analyses were conducted. Also, location names are shown only on maps. Surface water and groundwater in wells, piezometers and bore- holes (Table 1) were sampled between Novem- ber 1999 and February 2002. The authors report mean values for δ 18 O in surface waters and based on the isotope data, the mean altitude of individ- ual water recharge areas (exact numbers were not provided). The δ 18 O values of groundwater in the Holocene aquifer were -8.9 to -8.6 ‰, -9.6 to -8.6 ‰ in the upper Pleistocene aquifer, and -9.5 to -9.2 ‰ in the lower Pleistocene aquifer. Again, values were mainly presented graphically, and numerical values were given only for the lower Pleistocene aquifer (Urbanc & Jamnik, 2002). Since 2010, many isotope investigations at wellfield Brest were performed. In 2011, δ 18 O, δ 2 H 260 Klara NAGODE, Tjaša KANDUČ, Sonja LOJEN, Branka BRAČIČ ŽELEZNIK, Brigita JAMNIK & Polona VREČA and δ 13 C DIC values were determined in water sam- ples collected during a pumping test from a 200 m deep well (VD Brest-3a) to determine the recharge dynamics, origin and age of groundwater in the dolomite. The investigation began on the 23/05/11 when a step-test was performed, followed by a one-month-long pumping test. In the third step, the rising of water was investigated. Testing fin - ished on 24/06/11 (Bren či č, 2011). The δ 18 O, δ 2 H and δ 13 C DIC were also determined in seven wells at Brest and in one observation well (P-23/10). The values of δ 18 O ranged between -9.98 and -9.61 ‰ and δ 2 H between -64.9 and -61.1 ‰. δ 13 C DIC values were between -12.8 and -11.8 ‰. The isotope com- position of springs near wellfield Brest was also determined. Isotope values were between -9.56 and -6.21 ‰ for δ 18 O, between -64.4 and -58.8 ‰ for δ 2 H and between -9.42 and -18.65 ‰ for δ 13 C DIC (Bren či č, 2011). By performing the pumping test, mixing of water from different aquifers, namely, sha l low water f rom the upper Holocene aqu i fer a nd a lower Pleistocene aquifer in well VD Brest-3a, was confirmed. A certain amount of deep-water was also present; however, the exact amount was unknown, and its characteristics were not deter- mined. The isotope composition of the water also varied during the pumping test, indicating that the fraction of water of different origin had changed (Bren či č, 2011; Vre ča et al., 2011; Bra či č Železnik et al., 2017). In 2013 (from 21/05/13 to 31/05/13), the pumping test was repeated in well VD Brest-3a. The δ 18 O, δ 2 H and δ 13 C DIC values ranged from -9.46 and -9.05 ‰, -65.9 and -63.4 ‰, and -14.5 and -12.3 ‰, respectively (Vre ča et al., 2013; Bra či č Železnik et al., 2017). In 2015, another pumping test in well VD Brest-3a was performed and the δ 18 O, δ 2 H and δ 13 C DIC values varied between -9. 78 and -9.06 ‰, -65.4 and -61.4 ‰ and -12.05 and -11.14 ‰, respec- tively. The sampling test lasted from 05/06/15 to 01/07/15 (Vre ča et al., 2015). In 2019, few addi- tional 24-hour pumping tests were performed (Table 2). To conclude, the data shows a broad range of δ 18 O, δ 2 H, and δ 13 C DIC v a l u e s i n g r o u n d w a t e r i n the LB. Historically, isotope investigations were rare. In the last years, the δ 18 O, δ 2 H, and δ 13 C DIC are used more often but still sporadic. Also, dif- ferent wells in the wellfield Brest yield different isotope compositions. This variation is because the depths of the wells are not consistent, and the groundwater is captured from different aqui - fers. Therefore, careful consideration about how to implement isotope techniques in the future is needed for better water resource management of the wellfield Brest. Ljubljansko polje According to available data, isotope investiga - tions of groundwater from the LP were not per- formed until 1997. The first samples were collect - ed between October 1997 and September 1998 at 13 pumping wells in the wellfields Kle če, Hrastje, Jarški prod and Šentvid (Urbanc & Jamnik, 1998). Samples were collected only for δ 18 O analysis. A more extensive set of observations (October 1997 to September 1999) is presented by Andjelov et al. (2005). From this data, the authors estimated the proportion of locally infiltrated precipitation and water from the Sava River, but only report- ed the mean values of all measurements obtained during the sampling period for selected wells. Reported δ 18 O values in the groundwater were between -9.0 and -8.6 ‰ in Kle če (7 wells), -9.1 and -9.0 ‰ in Jarški prod (2 wells), and -8.9 and -8.8 ‰ in Hrastje (3 wells). In Šentvid, the mean value of several measurements from a single well was -8.8 ‰ (Urbanc & Jamnik, 1998). However, from the figures, it is possible to read the values for specific wells for the entire sampling period (Urbanc & Jamnik, 1998; Jamnik & Urbanc, 2003; Andjelov et al., 2005). At the same time, samples from the Sava River at Tacen were collected (Jamnik & Urbanc, 2003). The results, although only shown graphically, confirmed the influence of human activities on groundwater quality in Date of sampling Name Parameters identified δ 18 O δ 2 H δ 13 C DIC Reference 09/04/19- 10/4/19 PB–24b/19 δ 18 O, δ 2 H, δ 13 C DIC TA, EC, 3 H, 87 Sr/ 86 Sr, 88 Sr/ 86 Sr -9.59 to -9.50 ‰ (N=10) -63.9 to -63.1 ‰ (N=10) -11.1 ‰ (N=2) Vre ča et al., 2019e 02/9/19- 03/09/19 PB–24a/19 δ 18 O, δ 2 H, δ 13 C DIC , TA, EC, 3 H, 87 Sr/ 86 Sr, 88 Sr/ 86 Sr -9.49 to -9.42 ‰ (N=3) -62.8 to -62.5 ‰ (N=3) -11.4 to -11.1 ‰ (N=3) Vre ča et al., 2019f 03/10/19- 04/10/19 PB–24c/19 δ 18 O, δ 2 H, δ 13 C DIC , TA, 3 H 87 Sr/ 86 Sr, 88 Sr/ 86 Sr and EC -9.50 to -9.48 ‰ (N=3) -62.9 to -62.3 ‰ (N=3) -11.2 to -10.9 ‰ (N=3) Vre ča et al., 2019f Table 2. δ 18 O, δ 2 H, and δ 13 C DIC results (minimum to maximum values ) of the sampling performed in 20 1 9 during 24-hour pumping tests. (TA = total alkalinity, EC = electrical conductivity) 261 Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia those wells where the recharge zone extends un - der the city (Urbanc & Jamnik, 1998). In July and October 2003, the Institute for Public Health in Maribor collected samples at following locations: Yulon, Hrastje 1a, Kle če 17, GeoZS, Kle če 11, Šentvid 2A, Kle če 8a, Hrastje 3, Navje, Petrol- Šmartinska cesta, L.P. Vodovodna, HMZ Hrastje, for the δ 13 C DIC and alkalinity mea- surements. The δ 13 C DIC values were ranged from -14.7 to -12.2 ‰. The δ 13 C DIC results from LP were graphically presented in Kandu č (2006), together with δ 13 C DIC values of samples from the Sava Riv- er to indicate possible biogeochemical processes in the groundwater-river water system. From March 2010 to December 2011, monthly samples were collected for δ 18 O and δ 2 H analy- ses from seven wells at three wellfields: Kle če, Hrastje, and Jarški prod, and from the Sava Riv- er at Šentjakob (Vrzel et al., 2018). Based on δ 18 O and δ 2 H results, the authors determined the pro - portion of the Sava River in groundwater result- ing from periods of low and high precipitation in 2010 and 2011. Numerical values are reported in the Supplementary Data and are presented here as a box plot (Fig. 3). The authors found that both sources directly influence the groundwater: infil - tration of local precipitation and recharge from the Sava River. Based on average δ 18 O and δ 2 H values, it was apparent that groundwater from Kle če 11, Hrastje 3, and Hrastje 8 contained only a low amount of the Sava River water (up to 14 %) and was mostly composed of recently infiltrated local precipitation. For comparison, a higher per - centage of the Sava River water (up to 86 %) is present in the groundwater in wells Jarški prod 1, Jarški prod 3, Kle če 8 and Kle če 12. Findings were similar to that reported by Urbanc & Jam- nik (1998). More detailed investigations (from 2000 to 2014) in LP were performed in the area of Union Brewery where groundwater in Pleistocene fluvi - al sediments and the lower gravel aquifer is ex- ploited by the Brewery (Tr ček 2005; 2006; 2017). The Union Brewery’s lysimeter was ideal for studying urban water infiltration and to make accurate measurements of water flow and wa- ter balance parameters. It consisted of 42 bore- holes drilled into the right and left walls of the construction (Juren et al., 2003; Tr ček, 2005). As part of its sustainable groundwater management plan, extensive studies of groundwater flow and solute transport were performed from 2003 to 2014 to predict groundwater flow and contami- nant transport through the unsaturated and sat- urated zone of the urban intergranular aquifer (Tr ček, 2017). Actual stable isotope monitoring began in July 2003 (Tr ček, 2005) with the aim to obtain infor- mation about mixing processes and groundwater residence times in the unsaturated zone and to determine the risk of contamination of drinking water. From July 2003 to August 2004, monthly groundwater samples were collected, and δ 18 O and δ 2 H values determined. Tr ček (2005) report- ed δ 18 O groundwater values between -14.7 ‰ and -4.5 ‰. All other δ 18 O values were presented as boxplots, and no values for δ 2 H are reported. A synthesis of one-years’ worth of data revealed two types of flow: lateral flow, which has an es - sential role in the protection of groundwater of Fig. 3. Box plots of δ 18 O values taken from Vrzel et al., 2018 (period 2010/2011) and from research per- formed in autumn 2018 for wells in Kleče, Hrastje, Jarški prod and the Sava River (Vreča et al., 2019a; 2019b). 262 Klara NAGODE, Tjaša KANDUČ, Sonja LOJEN, Branka BRAČIČ ŽELEZNIK, Brigita JAMNIK & Polona VREČA the Pleistocene alluvial gravel aquifer, and verti- cal flow, which is the main factor controlling con - taminant transport towards the saturated zone (Tr ček, 2005). From July 2003 to June 2004 and from July 2004 to June 2005, δ 18 O and δ 2 H values in 16 ob- servation wells (piezometers) were measured next to the Union Brewery. The mean values from a single sampling site for δ 18 O varied between -9.21 and -8.70 ‰ (Tr ček, 2006). During the same period (from July 2003 to June 2005) monthly ox- ygen isotope measurements of groundwater (ly- simeter) ranged from -14.7 to -4.4 ‰, while the means of single sampling points were between -10.7 and -8 ‰ (Tr ček, 2005). In 2017, Tr ček pub- lished the results of the 2004 to 2014 investiga- tion (Tr ček, 2017). Water samples were collected daily, weekly or at monthly intervals, although only seasonal monitoring was performed after 2010. Samples were collected from 18 observa- tion points on the right side of the Union Brewery lysimeter, while precipitation was collected near the entrance to the lysimeter. The δ 18 O values in groundwater from 2004 to 2010 ranged from -16 to -6 ‰. In precipitation, δ 18 O values ranged from -18 to -3 ‰. Tr ček studied the weighted averages of the lysimeter water δ 18 O values for the peri- od 2005-2009 to get a better insight into the ly- simeter drainage system. Reported values varied between -9.82 and -7.62 ‰. Again, Tr ček empha- sised the importance of lateral flow and that the goal for future investigations should be directed towards vertical transport studies of contami- nant loads (Tr ček, 2017). The Union Brewery also produces bottled wa- ter, both still and flavoured water, which is sold under the Zala brand. In September 2004, exten- sive research of the general chemistry, δ 18 O, δ 2 H and δ 13 C DIC of bottled waters available on the Slo- vene Market was undertaken (Bren či č & V r e ča, 2005; 2006; 2007; 2010). The authors reported that δ 2 H, δ 18 O and δ 13 C DIC values of still water were be - tween -61 and -60 ‰, -8.90 and -8.95 ‰ and -12.7 and -12.3 ‰, respectively . For flavoured waters, values for δ 2 H, δ 18 O and δ 13 C DIC ranged between -61 and -59 ‰, -8.95 and -8.80 ‰, and -13.5 and -12.5 ‰ (Bren či č & Vre ča, 2006; 2007). Isotope investigations of groundwater were also performed at the pumping station LMV- 1 (located near the Kle če wellfield) from 2009 to 2011 (Mezga, 2014). The three-year sampling campaign covered three annual season cycles: groundwater at each sampling location was sam- pled twice, in spring (March-July) and autumn (August-November). The samples were collected as part of an extensive survey looking at the or- igin of groundwater in Slovenia. For the LMV-1, the authors reported mean values of δ 18 O of -8.59 ± 0.33 ‰, δ 2 H of -60.4 ± 0.6 ‰ and δ 13 C DIC of -12.7 ± 1.3 ‰ (Mezga et al., 2014). Ljubljansko polje and Ljubljansko barje simultaneous investigations Simultaneous isotope investigations of both aquifers are rare. Cerar & Urbanc (2013) stud- ied their interactions during two sampling cam- paigns in autumn 2010 and spring 2011. They aimed to obtain a better understanding of how the aquifers interact in order to improve a hydro - geological conceptual model of the aquifers. In total, they collected 138 samples at 69 locations from 28 wells from the five main wellfields, five industry wells, two private wells, 29 boreholes, and five samples of surface water. Based on the hydrogeological and the geographical position of the aquifers they divided LB into three areas: the northern part, middle part and southern part, including the area of Brest and Iški vršaj (Cerar & Urbanc, 2013). The δ 18 O in the groundwater of the northern part of LB varied between -9.0 and -8.6 ‰. Groundwater from this part of the aqui- fer is enriched in 18 O isotope compared to the oth - er parts of the aquifers. This enrichment is due to the higher influence of local precipitation on the open aquifer. δ 18 O values in the middle part of the aquifer were from -10.0 to -9.1 ‰, while δ 18 O val- ues in the southern part (including Brest and Iški vršaj) were -9.6 to -9.2 ‰. In their final report, Urbanc et al. (2012) report the range of δ 18 O val- ues for groundwater from Brest to vary between -9.6 and -9.4 ‰ (tabulated values not given). For LP, δ 18 O values in Kle če wells varied from -9.1 to -8.7 ‰, -8.9 to -8.8 ‰ in Šentvid, -8.9 to -8.8 ‰ in Hrastje, and from -9.3 to -9.0 ‰ in Jarški prod. Jamnik & Urbanc (2000) were the first to study the connections between LB and LP. They found that LP is partially recharged with groundwater from LB. However, Cerar & Urbanc, (2013) also showed that based on the hydrochemical compo- sition (Ca/Mg molar ratio and HCO 3 - concentra- tion) of water, the contribution of groundwater from LB is of minor importance. The minimal contribution was detected near the boundary between the two aquifers. By measuring triti- um activity, they classified groundwater in LP as “modern waters” with a residence time of up to 10 years, at the interface between the aquifers as “submodern waters” with a residence time of more than 50 years and in LB as “older waters” with residence time between 10 and 50 years. 263 Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia However, increased tritium activities also indi- cated “bomb tritium” from nuclear experiments in the 1960s (Cerar & Urbanc 2013). Vrzel et al. (2018) confirmed “modern” water was mainly present in LP and also estimated, using the 3 H/ He method, that 10 % of groundwater in Kle če is very old, but additional analyses are needed for precise determinations. In the period from March 2010 to October 2010 δ 13 C DIC was measured monthly along with alkalin - ity and pH at LP in the following wells: Hrastje 3, 8 (average -12.6 ‰, n = 12), Kle če 8, 11, 12 (average -12.1 ‰, n = 22), Jarški prod 1, 3 (average -11.3 ‰, n = 13), and the Sava River at Dolsko (average -10.6 ‰, n = 7) (Kandu č, unpublished data). At LB sampling was performed only in June 2010 at wells Brest 1a, Brest 2a and Brest 4a with δ 13 C DIC values ranging from -11.3 ‰ to -10.8 ‰ (Kandu č, unpublished data). To our best knowledge, this was for the first time δ 13 C DIC was measured at LB. Vre ča et al., (2019c) were the first to perform a stable isotope survey (June and July 2014) of tap water covering Slovenia according to our best knowledge. The authors determined δ 18 O and δ 2 H values in nine tap water samples collected in Lju - bljana and its vicinity. The δ 18 O and δ 2 H values varied between -9.74 and -9.06 ‰, and between -65.2 and -60.1 ‰, respectively. The most nega- tive values were in tap water from wellfield Brest and the most positive from Kle če. A more detailed investigation within the Lju- bljana water supply system started in 2018. The δ 18 O, δ 2 H and δ 13 C DIC val ues o f all objects in the system (wells, joint exits from water pumping station, water reservoirs, water treatment loca- tions, fountains and taps) ranged from -9.53 and -8.68 ‰, -63.6 and -57.8 ‰ and -15.3 and -9.38 ‰, respectively. Also, δ 2 H and δ 18 O values in samples from Šentvid w er e l e ss n e g a t i v e , w hil e s am p l e s from Brest had on average lower δ 13 C DIC values (Vre ča et al., 2019a; 2019b). The results for wells Kle če, Hrastje, Jarški prod and the Sava River are presented in Fig. 3 together with data from Vrzel et al., (2018). The values for 2018 are low- er and less spread, which is a result of a shorter sampling period (September to November). The first 24-hour analysis of tap water was performed from 9:00 on 24/04/19 until 9:00 on 25/04/19, with an emphasis on the hourly vari- ability (Vre ča et al., 2019d). The tap water was sampled in the basement of the main building of the JSI where water from two wellfields (Kle če and Brest) is mixed. The diurnal variations of δ 18 O, δ 2 H and δ 13 C DIC w e r e s m a l l . H o w e v e r , 2 4 - hour differences in isotope and major and trace elemental composition suggest that the propor- tion of groundwater from Kle če and Brest water fields changed over 24 hours. Based on the past investigations of LP and LB, especially 2018 - 2019, the authors selected a sys- tematic multi-analytical approach that started in 2020. Monthly monitoring of δ 18 O and δ 2 H and multi-element composition in groundwater in five wellfields (Kle če (4 wells), Brest (4 wells), Hrast - je (2 wells), Jarški prod (2 wells), and Šentvid (1 well)) was established. Also, samples from the Sava River (Brod and Šentjakob) are collected on the same day and additional tap water investiga- tions are planned. The Sava River Numerous isotope investigations have been performed along the Sava River basin (e.g., Kan- du č, 2006; Ogrinc et al., 2008; Bren či č & V r e ča, 2016; Torkar et al., 2016; Vrzel et al., 2018; Og- rinc et al., 2018). However, only sampling loca- tions close to Ljubljana (Tacen, Brod, Dolsko, Šentjakob and Črnuče) are relevant for this re- view (Table 1 and 2). Among these studies, ten reported δ 18 O, δ 2 H, δ 13 C DIC values (Table 2). Location δ 18 O δ 2 H δ 13 C DIC Reference Sava Tacen Min -10.1 -67.0 / Urbanc & Jamnik, 1998; Andjelov et al., 2005; Ogrinc et al., 2008; Urbanc et al., 2012; Cerar & Urbanc, 2013 Max -8.51 -57.9 / Sava Brod Min -10.1 -67.0 -10.7 Kandu č, 2006; Vre ča et al., 2019a; 2019b Max -9.2 -60.4 -8.5 Sava Črnu če -9.39 -62.4 -9.2 Vre ča et al., 2019a; 2019b Sava Šentjakob Min -9.7 -66.4 Vrzel et al., 2018; Vre ča et al., 2019a; 2019b Max -8.5 -57.6 Sava Dolsko Min -9.9 -68.0 -12.7 Kandu č, 2006; Ogrinc et al., 2008; 2018 Max -8.2 -55.0 -9.9 Table 3. Values for δ 18 O (‰), δ 2 H (‰) and δ 13 C DIC (‰) for the Sava River at Tacen, Brod, Črnu če, Šentjakob and Dolsko perfor- med in different investigations (locations are downstream). 264 Klara NAGODE, Tjaša KANDUČ, Sonja LOJEN, Branka BRAČIČ ŽELEZNIK, Brigita JAMNIK & Polona VREČA Isotope investigations of the Sava River near Ljubljana began in October 1997, when the first sampling in Tacen was performed (Urbanc & Jamnik, 1998). In 2004, Kandu č, (2006) under- took a more systematic monitoring programme of O, H and C isotopes from April 2004, Septem- ber 2004 and January 2005 at Brod and Dolsko. Ogrinc et al. (2008) also determined δ 18 O and δ 2 H in the Sava River watershed at Tacen and Dols- ko in April, September, and December of 2004 and monthly from January 2005 to August 2006. The authors used data to provide information on hydrological flow paths and to estimate the water residence times. The data (Ogrinc et al., 2008) also forms part of the long-term the Global Network of Isotopes in Rivers database (GNIR; IAEA, 2020), managed by IAEA. The mean resi- dence times at Tacen and Dolsko of 1.54 and 1.09 years, respectively, were estimated by using an exponential model in which precipitation inputs are assumed to mix rapidly with resident water. It was also observed that the Sava River responds quickly to precipitation, which is reflected in the isotope composition of the Sava River water (Og- rinc et al., 2008). Vrzel et al. (2018) report simi- lar δ 18 O values in river water at Šentjakob from March 2010 to December 2011. Monthly isotope sampling data at Dolsko during 2007 to 2010 re- vealed a mean residence time of 1.20 years, which is higher than previously estimated (1.09 years) in 2004-2006 period (Ogrinc et al., 2018). Precipitation Isotope composition of precipitation was mon- itored at six different locations in Ljubljana, as reported in 13 records (Table 1). Continuous and systematic monitoring of the isotope composition of monthly composite samples has been carried out in Ljubljana by the JSI since 1981 (Pezdi č, 1999; 2003; Vre ča et al., 2008; 2014; Vre ča & Malenšek, 2016). Published data are also included in the Global Network of Isotopes in Precipitation (GNIP) and in the Slovenian Network of Isotopes in Precipitation (SLONIP) from 1981 to 2010. In 1981-2018, the δ 18 O and δ 2 H values varied between -19.40 and -1.65 ‰ (mean -8.65 ‰, n=428) and be- tween -147.8 and -7.3 ‰ (mean -59.4 ‰, n=425). The data is an important input into GNIP, which has been evaluated many times (e.g., Rozanski et al., 1993; Ichiyanagi, 2007; Hughes & Crawford, 2012), and in many hydrological and hydrogeo- logical investigations (e.g., Krajcar Broni ć et al., 1998; 2020; Pezdi č, 1999; 2003; Bren či č & V re ča, 2006; Vre ča et al., 2006; Ogrinc et al., 2008; 2018; Vodila et al., 2011; Kandu č et al. , 20 12 ; Horva t - in či ć et al. , 20 1 1 ; Za vadla v et al. , 20 12 ; Cerar & Urbanc, 2013; Markovi ć et al., 2013; Mezga et al., 2014; Vrzel et al., 2018). The isotope composition of precipitation was also monitored at other lo- cations around Ljubljana in the frame of sever- al short-term investigations. For example, the precipitation was collected in the wellfield Kle če from October 1997 to September 1998 (Urbanc & Jamnik, 1998). The reported δ 18 O ranged from -12.0 to -5.5 ‰. Tr ček (2005; 2017) monitored δ 18 O values in precipitation from January 2003 to August 2004 and again from 2004 to 2014 at the Union Brewery. δ 18 O values were from -15.2 to -4.1 ‰ (mean -8.9 ‰) during 2003-2004 and -18 to -3 ‰ during the extended observation period (2004 to 2014). Cerar & Urbanc (2013) have also reported the monthly composition of precipita- tion at the GeoZS in Ljubljana monitored since 2010; however, the exact sampling period is not reported. The average monthly δ 18 O value was -8.51 ‰ (Cerar & Urbanc, 2013). Conclusions The use of isotopes to characterize water re- sources and to track the movement of water in the LP and LB over the past 40 years has significant - ly improved our understanding of groundwater quality and hydrological processes affecting its recharge and the distribution. Despite this, most isotope data are a result of intermittent short- term studies, and only a few represent long-term monitoring programmes. From all of the anal- ysed articles and reports, it is evident that limit- ed sampling and coverage of monitoring of well networks presents a high risk of, e.g., not detect- ing contamination events (Jamnik et al., 2012). The first δ 18 O and δ 2 H investigations of ground - water in the LB began in 1976, and only later in 1997 in LP. Also, in 1997 investigations at the Sava River in Tacen started. The first time δ 13 C DIC was systematically measured at LP was in 2003, while at LB it was only in 2010. Historically, iso- tope studies were performed in the LP; however, since 2011, isotope data are used more frequently, but still sporadically in the LB. These investiga- tions mainly involve sampling from wells – sam - pling was most often performed in Kle če, while other objects in the water supply system were not well sampled. Five locations on the Sava River near Ljubljana were identified. Also, precipita- tion was monitored for δ 18 O and δ 2 H at six differ- ent locations. To our knowledge, 102 relevant records were found and analysed; however, only 41 records published O, H and C isotope data and underwent 265 Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia a detailed review. The highest number of publi- cations contained δ 18 O data (40 records), followed by δ 2 H (32 records), while δ 13 C DIC investigations were rarely implemented (13 records). Also, long- term systemic approach with more frequent (e.g., seasonal) monitoring of relevant environmental isotope tracers is missing. In the scope of this re- view, we would also like to point out that many investigations contain an insufficient description of sampling times and exact locations (missing coordinates), analytical methods, and reporting of raw data. In this regards, better use of supple- mentary material, which should include all ap- propriate metadata would be beneficial and nec- essary for proper comparison in time and space and would enable tracing isotope changes in wa- ter resources. The first stable water isotope survey of tap water in the City of Ljubljana was performed in 2014. In order to assess the usefulness of environ - mental isotopes more systematically, monitoring has been performed on the drinking water supply system of Ljubljana since 2018. Based on all of the results from previous in- vestigations of LP and LB, monthly monitor- ing of δ 18 O and δ 2 H in groundwater in five wa- ter supply facilities was established in January 2020. Besides, also the Sava River is sampled at two locations monthly and additional more de- tail sampling of tap water is planned. The results will be used to prepare guidelines for future iso- tope monitoring that will provide a better overall understanding of water interactions of domestic supply important for water managers. Acknowledgements This review paper was prepared in the frame of the programme P1-0143, Young research program (PR-09780) and IAEA CRP contract No. 22843 - Use of Isotope Techniques for the Evaluation of Water Sources for Domestic Supply in Urban Areas (F33024). We thank also the reviewers for all valuable comments and D. Heath for linguistic corrections. References Aggarwal, P. K., Gat, J. R. & Froehlich, K. F. 2005: Isotopes in the water cycle. 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