GEOLOGIJA 63/2, 251-270, 2020 © Author(s) 2020. CC Atribution 4.0 License https://doi.org/10.5474/geologija.2020.019

Synthesis of past isotope hydrology investigations in the area of Ljubljana,

Pregled preteklih izotopskih hidroloških raziskav na območju Ljubljane, Slovenija Klara NAGODE1, Tjaša KANDUČ1, Sonja LOJEN1, Branka BRAČIČ ŽELEZNIK2, Brigita JAMNIK2 & Polona VREČA1 1Jožef Stefan Institute, Department of Environmental Sciences, Jamova 39, SI-1000 Ljubljana, Slovenia; e-mail: [email protected] 2JP 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 , 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 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 obse ž nejše i zotopske ra z i skave, t a ko na č r pa l i šči h t ud i objek ti 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. 252 Klara NAGODE, Tjaša KANDUČ, Sonja LOJEN, Branka BRAČIČ ŽELEZNIK, Brigita JAMNIK & Polona VREČA

Introduction - δ18O and δ2H can be used as conservative As Bowen et al. (2019) states “Earth’s water cy- tracers if the isotope signature is unmodi- cle links solid Earth, biological, and atmospheric fied within a study system, i.e., to identify systems, and it is both pivotal to the fundamental water sources contributing to water sam- understanding of our planet and critical to our pled at a given place; 18 2 13 practical well-being.” In nature, water is bound - δ O, δ H and δ CDIC and their variations in different compartments of the hydrosphere can enable the identification of important (ice, groundwater, surface water, lakes, soil mois- water and carbon cycle processes over- ture reservoirs, oceans, and biomass), biosphere, looked by other methods; lithosphere and the atmosphere, which form part - δ18O and δ2H can link information on the of a global hydrological cycle. The rapid growth history of water as it moves through the hy- in population, coupled with an increased demand drological cycle. for water by agriculture and industry, are putting Isotope methods were introduced into catch- pressure on water resources (Mook, 2001). Al- ment hydrology research to help scientists to un- though the impact that humans are having on the derstand better the geographical origin of water, water cycle is indisputable, there is still a lot un- recharge and discharge processes, biogeochemi- known about how water usage alters regional and cal processes and the sources and mechanisms of global water budgets (Bowen et al., 2019). One of pollution (Clark & Fritz, 1997; Aggarwal et al., the prerequisites for efficient management of wa- 2005; Bowen et al., 2005; Ehleringer et al., 2008; ter resources is having reliable information about 2016; Jameel et al., 2016; Du et al., 2019). the quantity and the quality of the resource that Concerns over climate change and the in- is being exploited (Dansgaard, 1954; Craig, 1961). creasing demand for water in urban areas has Stable water isotopes (1H, 2H, 16O, 17O and 18O) focused research on water supplies and dynamics and carbon isotopes (12C and 13C) in the dissolved within the urban system in order to gain a better inorganic carbon (DIC) occur naturally. They can understanding of the connections between hu- be measured using isotope-ratio mass spectrom- man populations, climate, and water extraction etry (dual-inlet or continuous-flow) (de Groot, (Ehleringer et al., 2016; Zhao et al., 2017; Tipple 2004), laser spectroscopy (Wassenaar et al., 2018), et al., 2017). or by spectrometric imaging methods (Bowen et Water circulates in nature differently than al., 2019). An isotope abundance of an element in urban environments, where the world’s pop- is generally reported in ‰ (per mill = parts per ulation is expected to increase to more than thousand = 10-3) deviations relative to the known 60 % by 2050. Supplying large urban areas with isotope abundance of a standard, δ: (Gat, 1996): high-quality drinking water and providing wa- ter resources in the long term is a major challenge δ (‰) = (R /R −1) × 103 sample standard (Jameel et al., 2016; Ehleringer et al., 2016). In Slo-

were Rsample and Rstandard present isotope ra- venia, drinking water supply is mainly based on tios (2H/1H, 18O/16O, 13C/12C, 15N/14N, 34S/32S) of a groundwater (around 97 % of the drinking water heavy isotope to a light isotope in a sample and supply is from groundwater resources) (Uhan & an international standard, respectively. Because Krajnc, 2003) and in the capital city, Ljubljana, it the numerical values obtained by this equation provides an invaluable drinking water resource are small they are expressed in delta notation (δ). (Tr ek, 2017). Delta values can be negative or positive numbers In Slovenia, only tritium and radon analyses meaning that the isotope ratio of the sample is areč prescribed by drinking water legislation (Of- lower or higher relative to a standard (Gat, 1996; ficial Gazette, No. 74/15), however, if the - para Meier-Augenstein & Schimmelmann, 2019). metric value for tritium is exceeded, it must be Isotopes are an important tool for studying investigated to see if the cause is the presence the water cycle and can be divided into two main of artificial radionuclides. Parametric values for - 2- categories: environmental isotopes (isotope vari- specific basic ions, e.g., NO3 , SO4 and trace el- ations in waters by natural processes) and artifi- ements, e.g., Se, Sb, Pb, Ni, Fe, Cu, Cd, Al, As, cial radioactive isotopes (radioactive isotopes that B in drinking water have also been established are injected into the system under investigation) (Official Gazette, Nos. 19/04, 35/04, 26/06, 92/06, 18 2 13 (Kendall & Doctor, 2003). δ O, δ H and δ CDIC val- 25/09, 74/15, and 51/17), while the regular moni- ues are important in different applications (Gat, toring of stable isotopes of H, O in water and C - - 1996; Clark & Fritz, 1997; Ehleringer et al., 2008; and N in different compounds (e.g., HCO3 , NO3 ) Clark, 2015; Bowen et al., 2019): Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia 253

Site description is not required by legislation. Despite quite a The two most important groundwater aqui- large number of isotope analyses performed in fers for the Slovenia capital Ljubljana and its the past, to date, there has been no comprehen- surroundings are the Ljubljansko polje (LP) and sive research in the use of environmental isotopes Ljubljansko barje (LB) (Fig. 1). The two aquifers in urban water management systems in Slovenia. are separated by the Golovec, Grajski hrib and Here, we review and synthesize past research Rožnik hills (Fig. 1) (e.g., Vižintin et al., 2009; 18 2 13 involving δ O, δ H and δ CDIC to advance our un- Janža, 2015). derstanding of the groundwater characteristics Two rivers bound the LP aquifer (Fig. 1) – the of the Ljubljana aquifers, which can be used as River to the south and the Sava River the basis for future investigations. We focus on to the north (Jamnik et al., 2003; Ogrinc et al., work conducted over the past 40 years. The main 2018). Because of the high velocities (10 m/day) aims of this review were the following: and quite plunder groundwater flow (3–4 m3/s), - make a synthesis of past urban hydrology the quality of groundwater is good (Jamnik et al., investigations of the Ljubljansko polje and 2003; Jamnik & Žitnik, 2020). Hydrological con- Ljubljansko barje aquifers with emphasis ditions in the area are characterized by strong 18 2 13 on the use of δ O, δ H and δ CDIC until 2019; interactions between surface water and ground- - collect information about sampling (loca- water and by the high velocities of groundwater tion, time, type of sampling site) and the flow and pollutant transport: that is, up to 20 m/ analytical methods used; day (Andjelov et al., 2005; Janža et al., 2005). The - identify the main gaps in the previous in- LP is located in the eastern part of the Ljubljana vestigations and propose future activities. basin (Ljubljanska kotlina). It was formed by tec-

Fig. 1. Locations of the studied area with the main wellfields (Kle e, 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. č 254 Klara NAGODE, Tjaša KANDUČ, Sonja LOJEN, Branka BRAČIČ ŽELEZNIK, Brigita JAMNIK & Polona VREČA tonic subsidence in the early Pleistocene together western and central parts of Upper Triassic do- with the main neotectonic fault system that runs lomite and Jurassic limestone, and in northern in an east-west direction. The basin is composed and eastern parts of Triassic and Permo-Car- of Permian and Carboniferous slate claystone boniferous shaly mudstone, quartz sandstone and sandstone (Žlebnik, 1971). The Pleistocene and conglomerate, characterized by low hydrau- and Holocene sediments, accumulated by the lic conductivity. The gravel fans are present on Sava River, form highly permeable of partially the borders of the basins (Mencej, 1988/89; Cer- conglomerated sand and gravel. ar & Urbanc, 2013). The basin was formed by a The thickness of these fluvial sediments in- tectonic depression and filled by alluvial, marshy creases towards the centre of the LP, where it and lacustrine sediments during the Pleistocene even exceeds 100 m (Andjelov et al., 2005). The and Holocene (Mencej, 1988/89). The Ljubljanica aquifer system has an intergranular porosity, River contributes to groundwater storage as well and an unconfined groundwater table, located as the Krimsko-Mokrško hills (ARSO, 2012; Cer- on 20–25 m below the surface (Vrzel et al., 2018) ar & Urbanc, 2013). The wellfield at Brest (Fig. and can fluctuate up to 10 m (source archive JP 1) is an important source of drinking water for VOKA SNAGA d.o.o.). The main recharge of the the southern part of the city of Ljubljana (Bra i aquifer comes from infiltration of precipitation Železnik & Globevnik, 2014). It consists of 13 and the Sava River, which recharge the aquifer wells of different depths (Bra i Železnik, 2016).č č mainly in its north–western part and drains the Water resources in the area are under significant eastern part of the LP. The LP is also recharged pressure, and environmentalč problemsč include via lateral inflow from the LB multi-aquifer sys- water pollution, increasing water demand, flood tem in the south (Jamnik et al., 2000; Vižintin et and drought risk, reduction in retention capac- al., 2009; Vrzel et al., 2018) as well as from the ity, decreasing groundwater levels and terrain Kamniško-Bistriško polje (Jamnik & Urbanc, subsidence (Bra i Železnik & Globevnik, 2014). 2000). However, desethyl-atrazine represents the most Groundwater is exploited at LP from four severe problem forč č the further development of the wellfields: Kle e, Hrastje, Jarški prod and Šentvid Brest water source (Prestor et al., 2017). where drinking water is pumped from 16, 10, 3 The Ljubljana drinking water supply system and 3 wells, respectivelyč (Fig. 1). Anthropogen- ic conditions of the aquifer are characterized by The central Ljubljana water supply system significant pressures of urbanization, industry, consists of five water supply facilities with alto- traffic, agriculture and old environmental bur- gether active 44 wells and more than 1,100 km dens (Jamnik et al., 2012), which occur within the long water supply network supplying 330,000 us- aquifer recharge area (Tr ek, 2017). To date, sev- ers through 43,000 connections. Water supply net- eral different sources of pollutants have been de- work includes different objects (i.e., reservoirs, tected and investigated. Theseč include dispersed water treatment locations, pumping stations) pollution sources where pollutants are consis- (Jamnik & Žitnik, 2020). In the central system, tently present (nitrates from agriculture and some settlements are continuously supplied with sewerage losses, new emerging contaminants in drinking water from a single wellfield (water sup- traces – pesticides from agriculture, plasticizers, ply areas A, C, D and E in Fig. 1), and others from corrosion and fire inhibitors, pharmaceuticals two or more wellfields (water supply areas F, G, from sewage system losses (Jamnik et al., 2009) H and I2 in Fig. 1), depending on water consump- while others originate from past agricultural and tion and pressure conditions in the system. Well- industrial activities (atrazine, desethyl-atrazine, field Hrastje (B) does not represent a unique water chromium (VI), trichloroethene, tetrachloro- supply area (Jamnik & Žitnik, 2020). ethene). Also, the characteristics of plumes and The water from the wells is pumped directly multipoint pollution contamination sources were to consumers or a reservoir for the short-term, recognized (Brilly et al., 2003; Karahodži , 2005; from where it is distributed to the users. Water Prestor et al., 2017). disinfection devices are built-in into the system; The LB aquifer (Fig. 1) extends fromč the however, water does not undergo technical treat- southern part of Ljubljana to the Krims- ments. It is only chlorinated occasionally. For the ko-Mokrško hills. The Barje is a depression with Brest wellfield UV disinfection is used (Jamnik & a stone bedrock that consists in the southern, Žitnik, 2020). Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia 255

Methods itation, and the Sava River. Also, the reference Studies related to the characterization of section of the articles was searched to identify aquifers important for the domestic water supply additional sources. We also inspected the work- in the municipality of Ljubljana were reviewed, ing reports for JP VOKA SNAGA d.o.o. available with a focus on those studies that used δ18O, δ2H, at Jožef Stefan Institute (JSI) including isotope 13 and δ CDIC values for the characterization of wa- data. Studies published in both Slovene and En- ter sources. glish were considered. Information about i) sampling including lo- Study selection criteria cation coordinates, type of sampling location First, we considered articles and reports relat- (groundwater, spring water, precipitation, river) ed to the water cycle and domestic water supply and sampling period; ii) the analytical meth- 18 2 13 investigations for the LP and LB published from ods used for δ O, δ H and δ CDIC analysis, and 18 2 13 1976 to the present (Fig. 2). In the scope of the iii) δ O, δ H and δ CDIC data were collected and review, a comprehensive search of journals was summarised. completed based on several keywords related to the Ljubljana aquifers (Ljubljana/Ljubljansko Results and discussion polje, Ljubljansko barje, Ljubljana groundwater, The initial combined search retrieved 102 re- Ljubljana water, Ljubljana water supply). The cords (Fig. 2). After removing 41 non-relevant search included all studies containing informa- records, the 61 articles remaining were assessed tion about i) sampling, ii) analytical methods, iii) for eligibility. Of these, 24 records were used to the parameters determined, and iv) isotope data. summarize site characteristics, while 41 records 18 2 13 In the second step, we focused on studies re- containing δ O, δ H and δ CDIC data (Table 1) porting the use of δ18O and δ2H to measure, de- were reviewed in detail. Some articles were used scribe or establish the characteristics of the LP in both categories. Information about sampling and LB aquifers. Additionally, we also collected is summarised in subchapter Sampling, followed 13 18 studies involving δ CDIC. Articles on the model- by Analytical methods used for determining δ O, 2 13 ling of LP and LB and other groundwater param- δ H and δ CDIC. Finally, a summary of the isotope eters, e.g., toxic metals in the groundwater and research and the important findings relating to spring waters, electrical conductivity, and phar- the Ljubljana aquifers is presented. maceuticals, and the quantity and quality con- Sampling ditions of groundwater in the Ljubljana aquifers were beyond the scope of this review (Fig. 2). I n for mation col lected about the sampli ng area, sampling locations and type of samples collected Search methods in different investigations for isotope analysis The databases were searched for relevant lit- is presented in Table 1. Isotope investigations of erature published before November 2019 and in- groundwater were first performed in 1976 at LB cluded Google, Google Scholar, Science Direct, (Breznik, 1984) while groundwater and surface Co-operative Online Bibliographic System, and water investigations at LP and on the Sava River Service – COBISS. Included were national and in began in 1997 (Urbanc & Jamnik, 1998). international journals, conference papers, PhD The isotope composition of precipitation in Lju- and Master Theses, reporting data on δ18O, δ2H bljana has been regularly monitored since 1981 13 and δ CDIC in an urban water system, precip- (Pezdi , 1999; Vre a et al., 2008). At the LP, many investigations were performed at the č wellfield č Kle e, followed by the wellfields Hrastje, Jarški prod, and Šentvid (Fig. 1, Table 1). Short-term studies č were performed at the bore- hole LMV-1 (located close to the wellfield Kle e). In contrast, long-term investigations were per- formed in the area of Union Brewery (Table 1).č In LB, sampling was mainly conducted in the well- field Brest (Table 1). Surface waters (e.g., Curn- ovec, Gradaš ica) were also sampled (Urbanc & Jamnik, 2002). On the Sava River, sampling was performed at čTacen, , rnu e, Šentjakob and Fig. 2. Flowchart of study selection for detail review synthesis. Dolsko (see references in Table 1). The Jožef Stefan Č č 256 Klara NAGODE, Tjaša KANDUČ, Sonja LOJEN, Branka BRAČIČ ŽELEZNIK, Brigita JAMNIK & Polona VREČA e č e 11, e 11, č

cesta, L.P. Vodovodna, HMZ Vodovodna, L.P. cesta, e, vodnjak 17, GeoZS, Kle GeoZS, 17, vodnjak e, č Šmartinska e (4, 6, 8a, 11, 12, 14, 15), Hrastje (1a, 5, 8), 8), 5, Hrastje (1a, 15), 14, 12, 11, 8a, 6, e (4, e (V-4, V-6, V-8a, V-11, V-12, V-14, V-15), V-15), V-14, V-12, V-11, V-8a, V-6, e (V-4, entvid (IIa) and Jarški prod (I, III), III), (I, prod entvid (IIa) and Jarški č č Š

ki prod (V-1, V-3), Hrastje (V-1a, V-5, V-8), precipitation-Kle V-8), V-5, Hrastje (V-1a, V-3), (V-1, ki prod entvid (2a) š Š e 8a, Hrastje 3, Navje, Petrol - Petrol Navje, Hrastje 3, e 8a, ica, Ljubljanica, Curnovec, Holocen aquifer (V-1, V-7, V-9, V-10, V-12, V-12, V-10, V-9, V-7, Holocen aquifer (V-1, Curnovec, Ljubljanica, ica, č č e (VIIIa and XII), Hrastje (Ia and V)), V)), Hrastje (Ia and e (VIIIa and XII), ki prod (1, 3), 3), (1, ki prod č š entvid 2A, Kle entvid 2A, entvid (V-2a), Jar entvid (V-2a), groundwater level stations, precipitation station precipitation stations, level groundwater Gradaš Mostec, DBP-9). DBP-6, DBP-5, DBP-4, DBP-2, IŠ-8, IŠ-7, IŠ-6pl, V-13, VD-4gl, PB-1gl, G-12, PB-6gl, PB-4, PB-2gl, OP-1, Upper Pleistocene aquifer (Iš-6gl, DBG-9). DBG-6, DBG-5, DBG-4, DBG-2, IŠ-4gl). A-2gl, A-1gl, P-19gl, PB-5gl, B-1, Pleistocene aquifer (TB-3, Lower RS (Tacen) GeoZS, LP and LB, Ljubljana – JSI Ljubljana – Bežigrad, in Kle wells RS, Mostec, Nadgoriški potok, Jar Union Brewery Union Brewery Lysimeter Ljubljana – Reaktor Ljubljana – JSI, Union Brewery Piezometer Union Brewery Ljubljana – Reaktor Ljubljana – JSI, Kle Hrastje1a, LP (Yulon, Dolsko), Sava RS (Brod, Š Hrastje) Union Brewery Ljubljana – Reaktor Ljubljana – JSI, Ljubljana – Bežigrad, Dolsko), RS (Tacen, Ljubljana – Reaktor Ljubljana – JSI, Ljubljana – Bežigrad, Union Brewery Location Brest of LB The southern part Ljubljana Kle Nadgoriški potok, Mostec, RS (Tacen), Š Ljubljana Bežigrad Kle GW, SW GW, SW P, GW, P G W, S W, P GW (bottled) G W, P P GW (bottled) G W, P P GW SW, GW (bottled) S W, P P GW (bottled) Type of sample GW P, G W P G W, S W, P P GW LB LB LP, Ljubljana LP LP LP Ljubljana LP LP Ljubljana RS LP, LP RS Ljubljana LP Sampling area LB LB LP RS LP, Ljubljana RS LP, DIC DIC DIC C C C 13 13 13 δ δ δ H) 2 H H H, H H H H H, H H H, H H H 2 2 2 2 2 2 2 2 2 2 2 2 2 2 δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ DIC O O, O, O O, O O O O, O, O, O, ( O, O, O, O, O, C O, O, O, 18 18 18 18 18 18 18 18 18 18 18 13 18 18 18 18 18 18 18 18 18 δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ Parameter δ δ δ δ δ δ et al., et al., a, 2005 a, 2006 a, 2007 a, 2010 a, ć č č č č , 2006 , & Vre & Vre & Vre & Vre & č č č č č , 1998 , 1999 , 2003 , i i i i č č č č č č č a et al., 2005 a et al., 2006 a et al., 2008 a et al., č č č ek, 2005 ek, 2006 ek, č č Urbanc & Jamnik, 2002 Urbanc & Jamnik, 2003 & Urbanc, Jamnik Pezdi 2005 et al., Andjelov Bren Tr Vre Bren Tr Vre Kandu Bren 2008 Ogrinc et al., Vre Bren Reference 1984* Breznik, Pezdi Krajcar Broni 1998 1998 Urbanc & Jamnik, Pezdi 2000 & Urbanc, Jamnik Table 1. The list of references reference:of * archive archive the of JP ** d.o.o.; VOKA SNAGA JSI. of = groundwater, (GW P = precipitation, SF = surface related water, TW = tap water, VD = well). to the isotope investigations performed in the area of Ljubljansko polje (LP), Ljubljansko barje (LB), the Sava River (RS), and precipitation (P). Source Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia 257 (1a, 2a, 3), joint 3), 2a, (1a, entvid (1a, 2a, 3), joint 3), 2a, entvid (1a, e Š Šentvid č ki prod (1, 3), Ljubljana – Reaktor, Ljubljana – Reaktor, 3), (1, ki prod e, Brod) e, e, Brod) e, š č č rnu rnu Č Č ki prod (1, 2, 3), VD 3), 2, (1, ki prod , , š Šentjakob Šentjakob kov vrtec; Vrtec Mojca, enota Rozle; OS IG - podruznica Mojca, Vrtec vrtec; kov č ja Dobrava; Vrtec Pedenjped, enota ; Vrtec Visji gaj, gaj, Visji Vrtec enota Zadvor; Pedenjped, Vrtec ja Dobrava; č (1a) e (8, 11, 12), Hrastje (3, 8), Jar 8), Hrastje (3, 12), 11, e (8, č Šentvid ), Kle ), kolin enota Zaj kolin š e (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, 7, 6, 5, 4, 3, 2a, 2, VD Hrastje (1a, 17), 16, 15, 14, 13, 12, 11, 10, 9, 8a, 7, 6, 4, 3, e (2, 7, 6, 5, 4, 3, 2a, 2, VD Hrastje (1a, 17), 16, 15, 14, 13, 12, 11, 10, 9, 8a, 7, 6, 4, 3, e (2, e (4, 8a, 11, 14, 17), VD Hrastje (1a, 3), VD Brest (1, 1a, 2a, 3, 4a, 5, 7, 9), VD Jarški VD Jarški 9), 7, 5, 4a, 3, 2a, 1a, (1, VD Brest 3), VD Hrastje (1a, 17), 14, 11, 8a, e (4, č č č Šentjakob ka vas ka vas š Vrtec Mi Vrtec enota ; Vrtec enota Marjetica; Andersena, Hansa Christiana Vrtec Agip; Bencinski servis enota Kostanj Mladi rod, Vrtec I Stefan Institute at location Jože water Tap PB – 24b/19 PB – 24c/19 PB – 24a/19, LP aquifer, the northern part of LB, the middle part of LB, the southern part of LB – the southern part of LB, the middle part of LB, part the northern LP aquifer, GeoZS Vršaj, and Iški Brest VD-3a LMV-1 LMV-1 Ljubljana – Reaktor VD-3a Kle Ljubljana – Reaktor, Ljubljana – JSI, Ljubljana – Bežigrad, Union Brewery VD Brest-3a RS ( GeoZS RS (Dolsko) VD Kle Jar 9), 8, 7, 6, 4a 5, 4, 3, 2a, 2, (1, VD Brest 8), in tap water fountains, drinking water reservoirs, pumping stations, water exits from RS ( buildings, public and private VD Kle VD 3), 2, (1, prod Jarški 9), 8, 7, 6, 4a 5, 4, 3, 2a, 2, (1, VD Brest 8), in tap water fountains, drinking water reservoirs, pumping stations, water exits from RS ( buildings, public and private V-1A, V-2A, V-3A, V-4A, V-5, V-7, V-8, and V-9, P-23/10 V-9, and V-8, V-7, V-5, V-4A, V-3A, P-23/10 V-9, V-2A, and V-8, V-1A, V-7, V-5, V-4A, V-3A, V-2A, V-1A, VD Kle VD 3), (1, prod TW TW GW GW GW, SW, P SW, GW, GW GW GW P GW P G W, P SW GW, P SW, GW, SW TW SW, GW, TW SW, GW, GW, SW GW, SW GW, SW GW, LB, LP LB, LP LB, LB LB LP, LB LP, LB LP LP Ljubljana LB LP LP LB RS LP, RS RS LB, LP, RS LP, LB, LB LB LB LP, DIC DIC DIC DIC DIC DIC DIC DIC C C C C C C C C 13 13 13 13 13 13 13 13 δ δ δ δ δ δ δ δ DIC H) C 2 H H H, H, H, H, H H H, H H, H H H H, H, 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 13 2 δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ O, O, O, O, O, O, O O O, O, O, O, O, O, O, ( O, O, O, O, O, 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ

ek, 2016 ek, š eleznik et al., eleznik et al., , 2011* , Ž č

i č č i a et al., 2019c** a et al., 2019d** a et al., 2019e ** a et al., 2019f ** a et al., a et al., 2013** a et al., 2014 a et al., 2015** a et al., a & Malen 2019a** a et al., 2019b a et al., a et al., 2011** a et al., č č č č č č č č č č č č ek, 2017 ek, č Vre Vre Vre Vre Cerar & Urbanc, 2013 Cerar & Urbanc, Vre 2014 Mezga, 2014 Mezga et al., Vre Vre Vre Tr Bra 2017 2018 et al., Vrzel 2018 Ogrinc et al., Vre Vre Vre Bren 2012 Urbanc et al., 258 Klara NAGODE, Tjaša KANDUČ, Sonja LOJEN, Branka BRAČIČ ŽELEZNIK, Brigita JAMNIK & Polona VREČA

Analytical methods used for determining stable Institute (JSI) has recorded the isotope com- oxygen, hydrogen and dissolved inorganic carbon position of precipitation since 1981. Samples of isotope composition precipitation were first collected at the synoptic station Ljubljana–Bežigrad located at the Hy- Results of δ18O, δ2H were reported relative to drometeorological Survey of Slovenia (today VSMOW (e.g., Urbanc & Jamnik, 1998; Bren i 13 Slovenian Environment Agency – ARSO), later & Vre a, 2006; Vrzel et al., 2018), while δ CDIC at the JSI (station Ljubljana–JSI) and finally at was reported relative to the VPDB (e.g., Brenčič the Reactor Centre of the JSI (station Ljubljana– & Vre ča, 2006; Kandu , 2006; Vre a et al., 2019e). Reaktor) (Pezdi , 2003; Vre a et al., 2006; Vre a Isotope ratio mass spectrometers (IRMS) wereč č č č 18 č 2 13 & Malenšek, 2016). Precipitation was collected used for the determination of δ O, δ H, and δ CDIC for a short periodč in the areasč of wellfield Kle če, in water except for some precipitation samples Union Brewery and at Geological Survey of Slo- collected at the Ljubljana–Reaktor which were venia (GeoZS) (see references Table 1). č measured by off-axis integrated cavity output The first stable water isotope survey of tap laser spectroscopy, OA-ICOS (Vre a et al. 2017). water in Slovenia, according to our best knowl- Oxygen isotope composition (δ18O) is reported edge, was performed in 2014 (Vre a et al., 2019c). in 40 records (Table 1). In all pastč investigations, In this survey, tap water samples were collect- the authors reported that the δ18O was deter- č ed for O and H isotope analysis at 105 locations mined by the water-CO2 equilibration technique around Slovenia, nine of them at locations in Lju- (Epstein & Mayeda, 1953; Avak & Brand, 1995) bljana and its vicinity (Vre a et al., 2019c). using different IRMS, namely the dual inlet To assess the usefulness of environmental iso- Varian Mat 250 at the JSI (Pezdi , 1998; Urbanc & topes, scientists have been performingč systematic Jamnik, 1998; Jamnik & Urbanc 2000; Andjelov monitoring of the Ljubljana drinking water sup- et al., 2005; Vre a et al. 2005; 2006;č 2008; Ogri- ply system since 2018. The first detailed sampling nc et al., 2008), Finnigan DELTAplus at the Joan- campaign was carried out between 06/09/18 and neum Research (JR)č in Graz, Austria (Bren i & 29/11/18 at 103 locations; 41 wells in five water Vre a 2006; Tr ek, 2017), Finnigan MAT 250 at supply facilities, seven joint exits from the water the Hydroisotop GmbH laboratory in Schweitenč č - pumping station, 22 reservoirs, two water treat- kirchen,č Germanyč (Cerar & Urbanc, 2013; Mezga ment locations, 13 fountains, and 19 taps (see Ta- et al., 2014; Mezga, 2014; Vre a et al., 2015), and ble 1). In addition, samples were collected on the a continuous flow IsoPrime (GV Instruments) at Sava River at Brod, rnu e and Šentjakob (Vre a the JSI (Bra i Železnik et al.,č 2017; Ogrinc et al., et al., 2019a; Vre a et al., 2019b). The first 24-hour 2018; Vrzel et al., 2018; Vre a et al., 2014). Tr ek experiment was performedČ č in the basement č of (2005; 2006) čreportedč that analysis was performed the main buildingč at the Jožef Stefan Institute at the Institute of Groundwaterč Ecology (GSF)č in Ljubljana with emphasis on the hourly isotope in Neuherberg, Germany, but does not state the variability of tap water in April 2019 (Vre a et type of IRMS used for the analysis. The δ18O al., 2019d). analysis of precipitation collected by the JSI at From Table 1, the following sampling locač - the Ljubljana–Reaktor was performed from Feb- tions were identified: ruary 2007 to the end of 2014, using a continuous - Wellfields – Kle e (11 wells), Hrastje (5 flow IRMS IsoPrime (GV Instruments) connected wells), Brest (12 wells), Jarški prod (2 to equilibration system MultiFlow Bio (Vre a et wells), and Šentvidč (1 well) al., 2014). Samples collected since 2015 were mea- - The Sava River – five locations: Brod, sured on a dual inlet Finnigan MAT DELTAč plus

rnu e, Dolsko, Šentjakob and Tacen with CO2-H2O equilibrator HDOEQ48 (Vre a et - Precipitation – six locations: synoptic sta- al., 2019a; 2019b; 2019d; 2019e; 2019f). Čtion č Ljubljana–Bežigrad, JSI-Ljubljana, Hydrogen isotope composition (δ2H) isč re- Ljubljana-Reaktor, Union Brewery, well- ported in 32 records using different analytical

field Kle e, and GeoZS methods, which included H2 generated by the - Other locations – piezometers and spring reduction of water over hot zinc (Pezdi , 1999), č water from the LB, lysimeter and piezom- H2 equilibrated with the water samples using eters at Union Brewery, groundwater in a Pt-catalyst (Horita et al., 1989), reductionč on LMV-1, tap water and different objects of Cr at 800 °C (Gehre et al., 1996; Morrison et al., the drinking water supply system. 2001) or with an OA-ICOS (Wassenaar et al., 2014). Measurements were performed on dif- ferent IRMS including a dual inlet Varian Mat Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia 259

250 at the JSI (Pezdi , 1998; Vre a et al., 2005; aquifer, from the River and other springs in 2006; 2008; Ogrinc et al., 2008; 2018; Vrzel et al., the vicinity. No precise sampling locations with 2018), Finnigan DELTAč plus XP at čthe Joanneum coordinates were reported, and no information Research (JR) in Graz, Austria (Bren i & Vre a, was given about the collection of the samples or 2006; Vre a et al., 2014; Tr ek, 2017), Finnigan where the analyses were performed. They report- MAT 251 at the Hydroisotop GmbH laboratoryč č č in ed values for δ18O between -9.94 and -8.90 ‰ and Schweitenkirchen,č Germanyč (Vre a et al., 2011; -65.8 and -58.9 ‰ for δ2H. From the tritium iso- 2013; 2015; Mezga et al., 2014; Bra i Železnik et tope data, Breznik (1984) concluded that the re- al., 2017). Samples collected from č2015 onwards charge rate of the lower aquifer is very low. were measured on the dual inlet Finniganč č MAT Samples from the southern part of LB were plus DELTA with CO2-H2O equilibrator HDOEQ48 collected in early spring and autumn in 1993. at the JSI (Vre a et al., 2019a; 2019b; 2019d; 2019e; Nineteen sampling points for groundwater and 2019f). Some precipitation samples collected at river base flow measurements were established the Ljubljana–Reaktorč were measured at the Iso- for the determination of groundwater recharge tope Hydrology Laboratory at the International and storage capacity (Pezdi , 1998). Unfortu- Atomic Energy Agency (IAEA) on a Los Gatos nately, the sampling locations are presented only Research OA-ICOS (Vre a et al. 2017). graphically, and the author givesč no exact coor- The carbon isotope composition in the dis- dinates or location names. Precipitation was col- č 13 18 solved inorganic carbon (δ CDIC) is reported in 13 lected in Ljubljana for the determination of δ O 2 18 records and was determined using CO2 collected and δ H values. Pezdi (1998) reported δ O values after the reaction of the water sample with 100 % of springs and surface river water of -9.65 and 2 č H3PO4 on a continuous flow Europa 20-20 IRMS -8.82 ‰, while δ H values ranged from -67.4 to with ANCA-TG separation module for trace gas -61.2 ‰. The weighted means of δ18O and δ2H in analysis (Bren i & Vre a, 2005; 2007; 2010; Mez- precipitation for the year 1993 were -8.07 ‰ and ga, 2014; Vre a et al., 2019a) or a continuous flow -55.6 ‰, respectively. The author concluded that IsoPrime or IsoPrimeč č 100č IRMS with equilibra- the contribution of local precipitation was small tion system č MultiFlow Bio at the JSI (Bren i , and infrequent; however, local precipitation 2011, Bra i Železnik et al., 2017; Vre a et al., could recharge nearby aquifers (Pezdi , 1998). 2011; 2013; 2015; 2019e; 2019f). č č After 1997, Urbanc & Jamnik (2002) performed Only ač fewč articles reported the analyticalč more detailed investigations of the LBč in which errors (Tr ek, 2005; 2006; Bren i & Vre a, 2006; the chemical and isotope composition of ground- 2007; Ogrinc et al., 2008; 2018; Vre a et al., 2008; water was studied. Isotope investigations com- 2018; Cerarč & Urbanc, 2013; Mezgač č et al.,č 2014). bined with hydrogeochemical methods were used Most publications report basic descriptiveč sta- to obtain hydrogeological data on the properties tistics or isotope ranges and only in a few cas- of water in individual aquifers: the Holocene es, whole datasets are publicly available (e.g., aquifer and the upper and the lower Pleistocene Bren i & Vre a, 2006; 2007; Vre a et al., 2008; aquifers. The authors, however, do not provide 2014; Vrzel et al., 2018). any sampling information or at which insti- č č č č tute the analyses were conducted. Also, location History of the stable isotope research in the names are shown only on maps. Surface water catchment area of Ljubljana aquifers and groundwater in wells, piezometers and bore- Here we present a summary of the 41 records holes (Table 1) were sampled between Novem- (Table 1) related to the past stable isotope inves- ber 1999 and February 2002. The authors report tigations in the area of LP and LB aquifers. Ar- mean values for δ18O in surface waters and based ticles usually report the use of δ18O and δ2H in on the isotope data, the mean altitude of individ- water resources investigations; however, it is in- ual water recharge areas (exact numbers were 13 18 teresting, that the δ CDIC was determined in only not provided). The δ O values of groundwater in 13 records. the Holocene aquifer were -8.9 to -8.6 ‰, -9.6 to -8.6 ‰ in the upper Pleistocene aquifer, and -9.5 Ljubljansko barje to -9.2 ‰ in the lower Pleistocene aquifer. Again, The first isotope investigations in the area values were mainly presented graphically, and of Ljubljana aquifers were performed in 1976 numerical values were given only for the lower (Breznik, 1984), as part of the hydrological re- Pleistocene aquifer (Urbanc & Jamnik, 2002). search into the Brest wellfield between 1974 and Since 2010, many isotope investigations at 1976. Water samples were collected at the LB wellfield Brest were performed. In 2011, δ18O, δ2H 260 Klara NAGODE, Tjaša KANDUČ, Sonja LOJEN, Branka BRAČIČ ŽELEZNIK, Brigita JAMNIK & Polona VREČA

13 and δ CDIC values were determined in water sam- To conclude, the data shows a broad range of 18 2 13 ples collected during a pumping test from a 200 m δ O, δ H, and δ CDIC values in groundwater in deep well (VD Brest-3a) to determine the recharge the LB. Historically, isotope investigations were 18 2 13 dynamics, origin and age of groundwater in the rare. In the last years, the δ O, δ H, and δ CDIC dolomite. The investigation began on the 23/05/11 are used more often but still sporadic. Also, dif- when a step-test was performed, followed by a ferent wells in the wellfield Brest yield different one-month-long pumping test. In the third step, isotope compositions. This variation is because the rising of water was investigated. Testing fin- the depths of the wells are not consistent, and ished on 24/06/11 (Bren i , 2011). The δ18O, δ2H the groundwater is captured from different aqui- 13 and δ CDIC were also determined in seven wells at fers. Therefore, careful consideration about how Brest and in one observationč č well (P-23/10). The to implement isotope techniques in the future is values of δ18O ranged between -9.98 and -9.61 ‰ needed for better water resource management of 2 13 and δ H between -64.9 and -61.1 ‰. δ CDIC values the wellfield Brest. were between -12.8 and -11.8 ‰. The isotope com- Ljubljansko polje position of springs near wellfield Brest was also determined. Isotope values were between -9.56 According to available data, isotope investiga- and -6.21 ‰ for δ18O, between -64.4 and -58.8 ‰ tions of groundwater from the LP were not per- 2 13 for δ H and between -9.42 and -18.65 ‰ for δ CDIC formed until 1997. The first samples were collect- (Bren i , 2011). By performing the pumping test, ed between October 1997 and September 1998 at mixing of water from different aquifers, namely, 13 pumping wells in the wellfields Kle e, Hrastje, s h a l lowč č wat e r f r om t he uppe r Holoce ne aqu i fe r a nd Jarški prod and Šentvid (Urbanc & Jamnik, 1998). a lower Pleistocene aquifer in well VD Brest-3a, Samples were collected only for δ18O čanalysis. A was confirmed. A certain amount of deep-water more extensive set of observations (October 1997 was also present; however, the exact amount was to September 1999) is presented by Andjelov et unknown, and its characteristics were not deter- al. (2005). From this data, the authors estimated mined. The isotope composition of the water also the proportion of locally infiltrated precipitation varied during the pumping test, indicating that the and water from the Sava River, but only report- fraction of water of different origin had changed ed the mean values of all measurements obtained (Bren i , 2011; Vre a et al., 2011; Bra i Železnik during the sampling period for selected wells. et al., 2017). In 2013 (from 21/05/13 to 31/05/13), the Reported δ18O values in the groundwater were pumpingč č test was črepeated in well VDč č Brest-3a. between -9.0 and -8.6 ‰ in Kle e (7 wells), -9.1 18 2 13 The δ O, δ H and δ CDIC values ranged from -9.46 and -9.0 ‰ in Jarški prod (2 wells), and -8.9 and and -9.05 ‰, -65.9 and -63.4 ‰, and -14.5 and -8.8 ‰ in Hrastje (3 wells). In Šentvidč , the mean -12.3 ‰, respectively (Vre a et al., 2013; Bra i value of several measurements from a single well Železnik et al., 2017). was -8.8 ‰ (Urbanc & Jamnik, 1998). However, In 2015, another pumpingč test in well VDč č from the figures, it is possible to read the values Brest-3a was performed and the δ18O, δ2H and for specific wells for the entire sampling period 13 δ C DIC values varied between -9.78 and -9.06 ‰, (Urbanc & Jamnik, 1998; Jamnik & Urbanc, 2003; -65.4 and -61.4 ‰ and -12.05 and -11.14 ‰, respec- Andjelov et al., 2005). At the same time, samples tively. The sampling test lasted from 05/06/15 to from the Sava River at Tacen were collected 01/07/15 (Vre a et al., 2015). In 2019, few addi- (Jamnik & Urbanc, 2003). The results, although tional 24-hour pumping tests were performed only shown graphically, confirmed the influence (Table 2). č of human activities on groundwater quality in

18 2 13 Table 2. δ O, δ H, and δ CDIC results (minimum to maximum values) of the sampling performed in 2019 during 24-hour pumping tests. (TA = total alkalinity, EC = electrical conductivity)

Date of Name Parameters identified 18 2 13 Reference δ O δ H δ CDIC sampling 09/04/19- PB–24b/19 18 2 13 -9.59 to -63.9 to -11.1 ‰ δ O, δ H, δ CDIC Vre a et al., 2019e 10/4/19 TA, EC, 3H, 87Sr/86Sr, 88Sr/86Sr -9.50 ‰ -63.1 ‰ (N=2) (N=10) (N=10) č 02/9/19- PB–24a/19 18 2 13 3 -9.49 to -62.8 to -11.4 to δ O, δ H, δ CDIC, TA, EC, H, Vre a et al., 2019f 03/09/19 87Sr/86Sr, 88Sr/86Sr -9.42 ‰ (N=3) -62.5 ‰ -11.1 ‰ (N=3) (N=3) č 03/10/19- PB–24c/19 18 2 13 3 -9.50 to -62.9 to -11.2 to δ O, δ H, δ CDIC, TA, H Vre a et al., 2019f 04/10/19 87Sr/86Sr, 88Sr/86Sr and EC -9.48 ‰ -62.3 ‰ -10.9 ‰ (N=3) (N=3) (N=3) č Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia 261 those wells where the recharge zone extends un- 1, Jarški prod 3, Kle e 8 and Kle e 12. Findings der the city (Urbanc & Jamnik, 1998). were similar to that reported by Urbanc & Jam- In July and October 2003, the Institute for nik (1998). č č Public Health in Maribor collected samples at More detailed investigations (from 2000 to following locations: Yulon, Hrastje 1a, Kle e 17, 2014) in LP were performed in the area of Union GeoZS, Kle e 11, Šentvid 2A, Kle e 8a, Hrastje 3, Brewery where groundwater in Pleistocene fluvi- Navje, Petrol- Šmartinska cesta, L.P. Vodovodna,č al sediments and the lower gravel aquifer is ex- č 13 č HMZ Hrastje, for the δ CDIC and alkalinity mea- ploited by the Brewery (Tr ek 2005; 2006; 2017). 13 surements. The δ CDIC values were ranged from The Union Brewery’s lysimeter was ideal for 13 č -14.7 to -12.2 ‰. The δ CDIC results from LP were studying urban water infiltration and to make graphically presented in Kandu (2006), together accurate measurements of water flow and wa- 13 with δ CDIC values of samples from the Sava Riv- ter balance parameters. It consisted of 42 bore- er to indicate possible biogeochemicalč processes holes drilled into the right and left walls of the in the groundwater-river water system. construction (Juren et al., 2003; Tr ek, 2005). As From March 2010 to December 2011, monthly part of its sustainable groundwater management samples were collected for δ18O and δ2H analy- plan, extensive studies of groundwaterč flow and ses from seven wells at three wellfields: Kle e, solute transport were performed from 2003 to Hrastje, and Jarški prod, and from the Sava Riv- 2014 to predict groundwater flow and contami- er at Šentjakob (Vrzel et al., 2018). Based on δ18čO nant transport through the unsaturated and sat- and δ2H results, the authors determined the pro- urated zone of the urban intergranular aquifer portion of the Sava River in groundwater result- (Tr ek, 2017). ing from periods of low and high precipitation in Actual stable isotope monitoring began in July 2010 and 2011. Numerical values are reported in 2003č (Tr ek, 2005) with the aim to obtain infor- the Supplementary Data and are presented here mation about mixing processes and groundwater as a box plot (Fig. 3). The authors found that both residenceč times in the unsaturated zone and to sources directly influence the groundwater: infil- determine the risk of contamination of drinking tration of local precipitation and recharge from water. From July 2003 to August 2004, monthly the Sava River. Based on average δ18O and δ2H groundwater samples were collected, and δ18O values, it was apparent that groundwater from and δ2H values determined. Tr ek (2005) report- Kle e 11, Hrastje 3, and Hrastje 8 contained only ed δ18O groundwater values between -14.7 ‰ and a low amount of the Sava River water (up to 14 %) -4.5 ‰. All other δ18O values wereč presented as andč was mostly composed of recently infiltrated boxplots, and no values for δ2H are reported. A local precipitation. For comparison, a higher per- synthesis of one-years’ worth of data revealed centage of the Sava River water (up to 86 %) is two types of flow: lateral flow, which has an es- present in the groundwater in wells Jarški prod sential role in the protection of groundwater of

Fig. 3. Box plots of δ18O 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- as part of an extensive survey looking at the or- cal flow, which is the main factor controlling con- igin of groundwater in Slovenia. For the LMV-1, taminant transport towards the saturated zone the authors reported mean values of δ18O of -8.59 2 13 (Tr ek, 2005). ± 0.33 ‰, δ H of -60.4 ± 0.6 ‰ and δ CDIC of -12.7 From July 2003 to June 2004 and from July ± 1.3 ‰ (Mezga et al., 2014). 2004č to June 2005, δ18O and δ2H values in 16 ob- Ljubljansko polje and Ljubljansko barje servation wells (piezometers) were measured simultaneous investigations next to the Union Brewery. The mean values from a single sampling site for δ18O varied between Simultaneous isotope investigations of both -9.21 and -8.70 ‰ (Tr ek, 2006). During the same aquifers are rare. Cerar & Urbanc (2013) stud- period (from July 2003 to June 2005) monthly ox- ied their interactions during two sampling cam- ygen isotope measurementsč of groundwater (ly- paigns in autumn 2010 and spring 2011. They simeter) ranged from -14.7 to -4.4 ‰, while the aimed to obtain a better understanding of how means of single sampling points were between the aquifers interact in order to improve a hydro- -10.7 and -8 ‰ (Tr ek, 2005). In 2017, Tr ek pub- geological conceptual model of the aquifers. In lished the results of the 2004 to 2014 investiga- total, they collected 138 samples at 69 locations tion (Tr ek, 2017). čWater samples were collectedč from 28 wells from the five main wellfields, five daily, weekly or at monthly intervals, although industry wells, two private wells, 29 boreholes, only seasonalč monitoring was performed after and five samples of surface water. Based on the 2010. Samples were collected from 18 observa- hydrogeological and the geographical position of tion points on the right side of the Union Brewery the aquifers they divided LB into three areas: the lysimeter, while precipitation was collected near northern part, middle part and southern part, the entrance to the lysimeter. The δ18O values in including the area of Brest and Iški vršaj (Cerar groundwater from 2004 to 2010 ranged from -16 & Urbanc, 2013). The δ18O in the groundwater of to -6 ‰. In precipitation, δ18O values ranged from the northern part of LB varied between -9.0 and -18 to -3 ‰. Tr ek studied the weighted averages -8.6 ‰. Groundwater from this part of the aqui- of the lysimeter water δ18O values for the peri- fer is enriched in 18O isotope compared to the oth- od 2005-2009 toč get a better insight into the ly- er parts of the aquifers. This enrichment is due to simeter drainage system. Reported values varied the higher influence of local precipitation on the between -9.82 and -7.62 ‰. Again, Tr ek empha- open aquifer. δ18O values in the middle part of the sised the importance of lateral flow and that the aquifer were from -10.0 to -9.1 ‰, while δ18O val- goal for future investigations should čbe directed ues in the southern part (including Brest and Iški towards vertical transport studies of contami- vršaj) were -9.6 to -9.2 ‰. In their final report, nant loads (Tr ek, 2017). Urbanc et al. (2012) report the range of δ18O val- The Union Brewery also produces bottled wa- ues for groundwater from Brest to vary between ter, both still andč flavoured water, which is sold -9.6 and -9.4 ‰ (tabulated values not given). For under the Zala brand. In September 2004, exten- LP, δ18O values in Kle e wells varied from -9.1 to sive research of the general chemistry, δ18O, δ2H -8.7 ‰, -8.9 to -8.8 ‰ in Šentvid, -8.9 to -8.8 ‰ in 13 č and δ CDIC of bottled waters available on the Slo- Hrastje, and from -9.3 to -9.0 ‰ in Jarški prod. vene Market was undertaken (Bren i & Vre a, Jamnik & Urbanc (2000) were the first to study 2005; 2006; 2007; 2010). The authors reported that the connections between LB and LP. They found 2 18 13 č č č δ H, δ O and δ CDIC values of still water were be- that LP is partially recharged with groundwater tween -61 and -60 ‰, -8.90 and -8.95 ‰ and -12.7 from LB. However, Cerar & Urbanc, (2013) also and -12.3 ‰, respectively. For flavoured waters, showed that based on the hydrochemical compo- 2 18 13 - values for δ H, δ O and δ CDIC ranged between sition (Ca/Mg molar ratio and HCO3 concentra- -61 and -59 ‰, -8.95 and -8.80 ‰, and -13.5 and tion) of water, the contribution of groundwater -12.5 ‰ (Bren i & Vre a, 2006; 2007). from LB is of minor importance. The minimal Isotope investigations of groundwater were contribution was detected near the boundary also performedč č at theč pumping station LMV- between the two aquifers. By measuring triti- 1 (located near the Kle e wellfield) from 2009 um activity, they classified groundwater in LP to 2011 (Mezga, 2014). The three-year sampling as “modern waters” with a residence time of up campaign covered threeč annual season cycles: to 10 years, at the interface between the aquifers groundwater at each sampling location was sam- as “submodern waters” with a residence time of pled twice, in spring (March-July) and autumn more than 50 years and in LB as “older waters” (August-November). The samples were collected with residence time between 10 and 50 years. Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia 263

However, increased tritium activities also indi- (Vre a et al., 2019a; 2019b). The results for wells cated “bomb tritium” from nuclear experiments Kle e, Hrastje, Jarški prod and the Sava River in the 1960s (Cerar & Urbanc 2013). Vrzel et al. are presentedč in Fig. 3 together with data from (2018) confirmed “modern” water was mainly Vrzelč et al., (2018). The values for 2018 are low- present in LP and also estimated, using the 3H/ er and less spread, which is a result of a shorter He method, that 10 % of groundwater in Kle e is sampling period (September to November). very old, but additional analyses are needed for The first 24-hour analysis of tap water was precise determinations. č performed from 9:00 on 24/04/19 until 9:00 on In the period from March 2010 to October 2010 25/04/19, with an emphasis on the hourly vari- 13 δ CDIC was measured monthly along with alkalin- ability (Vre a et al., 2019d). The tap water was ity and pH at LP in the following wells: Hrastje 3, sampled in the basement of the main building of 8 (average -12.6 ‰, n = 12), Kle e 8, 11, 12 (average the JSI whereč water from two wellfields (Kle e -12.1 ‰, n = 22), Jarški prod 1, 3 (average -11.3 ‰, and Brest) is mixed. The diurnal variations of č 18 2 13 č n = 13), and the Sava River at Dolsko (average δ O, δ H and δ CDIC were small. However, 24- -10.6 ‰, n = 7) (Kandu , unpublished data). At hour differences in isotope and major and trace LB sampling was performed only in June 2010 at elemental composition suggest that the propor- č 13 wells Brest 1a, Brest 2a and Brest 4a with δ CDIC tion of groundwater from Kle e and Brest water values ranging from -11.3 ‰ to -10.8 ‰ (Kandu , fields changed over 24 hours. unpublished data). To our best knowledge, this Based on the past investigationsč of LP and LB, 13 č was for the first timeδ CDIC was measured at LB. especially 2018 - 2019, the authors selected a sys- Vre a et al., (2019c) were the first to perform a tematic multi-analytical approach that started stable isotope survey (June and July 2014) of tap in 2020. Monthly monitoring of δ18O and δ2H and water č covering Slovenia according to our best multi-element composition in groundwater in five knowledge. The authors determined δ18O and δ2H wellfields (Kle e (4 wells), Brest (4 wells), Hrast- values in nine tap water samples collected in Lju- je (2 wells), Jarški prod (2 wells), and Šentvid (1 bljana and its vicinity. The δ18O and δ2H values well)) was established.č Also, samples from the varied between -9.74 and -9.06 ‰, and between Sava River (Brod and Šentjakob) are collected on -65.2 and -60.1 ‰, respectively. The most nega- the same day and additional tap water investiga- tive values were in tap water from wellfield Brest tions are planned. and the most positive from Kle e. The Sava River A more detailed investigation within the Lju- bljana water supply system startedč in 2018. The Numerous isotope investigations have been 18 2 13 δ O, δ H and δ CDIC values of all objects in the performed along the Sava River basin (e.g., Kan- system (wells, joint exits from water pumping du , 2006; Ogrinc et al., 2008; Bren i & Vre a, station, water reservoirs, water treatment loca- 2016; Torkar et al., 2016; Vrzel et al., 2018; Og- tions, fountains and taps) ranged from -9.53 and rincč et al., 2018). However, only samplingč č locač - -8.68 ‰, -63.6 and -57.8 ‰ and -15.3 and -9.38 ‰, tions close to Ljubljana (Tacen, Brod, Dolsko, respectively. Also, δ2H and δ18O values in samples Šentjakob and Črnuče) are relevant for this re- from Šentvid were less negative, while samples view (Table 1 and 2). Among these studies, ten 13 18 2 13 from Brest had on average lower δ CDIC values reported δ O, δ H, δ CDIC values (Table 2).

18 2 13 Table 3. Values for δ O (‰), δ H (‰) and δ CDIC (‰) for the Sava River at Tacen, Brod, rnu e, Šentjakob and Dolsko perfor- med in different investigations (locations are downstream). Č č 18 2 13 Location δ O δ H δ CDIC Reference Min -10.1 -67.0 / Urbanc & Jamnik, 1998; Andjelov et al., 2005; Ogrinc et Sava Tacen Max -8.51 -57.9 / al., 2008; Urbanc et al., 2012; Cerar & Urbanc, 2013 Min -10.1 -67.0 -10.7 Sava Brod Kandu , 2006; Vre a et al., 2019a; 2019b Max -9.2 -60.4 -8.5 Sava Črnuče -9.39 -62.4 -9.2 Vre a etč al., 2019a;č 2019b Min -9.7 -66.4 Sava Šentjakob Vrzelč et al., 2018; Vre a et al., 2019a; 2019b Max -8.5 -57.6 Min -9.9 -68.0 -12.7 č Sava Dolsko Kandu , 2006; Ogrinc et al., 2008; 2018 Max -8.2 -55.0 -9.9 č 264 Klara NAGODE, Tjaša KANDUČ, Sonja LOJEN, Branka BRAČIČ ŽELEZNIK, Brigita JAMNIK & Polona VREČA

Isotope investigations of the Sava River near in i et al., 2011; Zavadlav et al., 2012; Cerar & Ljubljana began in October 1997, when the first Urbanc, 2013; Markovi et al., 2013; Mezga et al., sampling in Tacen was performed (Urbanc & 2014;č ć Vrzel et al., 2018). The isotope composition Jamnik, 1998). In 2004, Kandu , (2006) under- of precipitation was alsoć monitored at other lo- took a more systematic monitoring programme cations around Ljubljana in the frame of sever- of O, H and C isotopes from Aprilč 2004, Septem- al short-term investigations. For example, the ber 2004 and January 2005 at Brod and Dolsko. precipitation was collected in the wellfield Kle e Ogrinc et al. (2008) also determined δ18O and δ2H from October 1997 to September 1998 (Urbanc in the Sava River watershed at Tacen and Dols- & Jamnik, 1998). The reported δ18O ranged fromč ko in April, September, and December of 2004 -12.0 to -5.5 ‰. Tr ek (2005; 2017) monitored and monthly from January 2005 to August 2006. δ18O values in precipitation from January 2003 The authors used data to provide information to August 2004 and č again from 2004 to 2014 at on hydrological flow paths and to estimate the the Union Brewery. δ18O values were from -15.2 water residence times. The data (Ogrinc et al., to -4.1 ‰ (mean -8.9 ‰) during 2003-2004 and -18 2008) also forms part of the long-term the Global to -3 ‰ during the extended observation period Network of Isotopes in Rivers database (GNIR; (2004 to 2014). Cerar & Urbanc (2013) have also IAEA, 2020), managed by IAEA. The mean resi- reported the monthly composition of precipita- dence times at Tacen and Dolsko of 1.54 and 1.09 tion at the GeoZS in Ljubljana monitored since years, respectively, were estimated by using an 2010; however, the exact sampling period is not exponential model in which precipitation inputs reported. The average monthly δ18O value was are assumed to mix rapidly with resident water. -8.51 ‰ (Cerar & Urbanc, 2013). It was also observed that the Sava River responds quickly to precipitation, which is reflected in the Conclusions isotope composition of the Sava River water (Og- The use of isotopes to characterize water re- rinc et al., 2008). Vrzel et al. (2018) report simi- sources and to track the movement of water in the lar δ18O values in river water at Šentjakob from LP and LB over the past 40 years has significant- March 2010 to December 2011. Monthly isotope ly improved our understanding of groundwater sampling data at Dolsko during 2007 to 2010 re- quality and hydrological processes affecting its vealed a mean residence time of 1.20 years, which recharge and the distribution. Despite this, most is higher than previously estimated (1.09 years) isotope data are a result of intermittent short- in 2004-2006 period (Ogrinc et al., 2018). term studies, and only a few represent long-term monitoring programmes. From all of the anal- Precipitation ysed articles and reports, it is evident that limit- Isotope composition of precipitation was mon- ed sampling and coverage of monitoring of well itored at six different locations in Ljubljana, as networks presents a high risk of, e.g., not detect- reported in 13 records (Table 1). Continuous and ing contamination events (Jamnik et al., 2012). systematic monitoring of the isotope composition The firstδ 18O and δ2H investigations of ground- of monthly composite samples has been carried water in the LB began in 1976, and only later in out in Ljubljana by the JSI since 1981 (Pezdi , 1997 in LP. Also, in 1997 investigations at the 13 1999; 2003; Vre a et al., 2008; 2014; Vre a & Sava River in Tacen started. The first timeδ CDIC Malenšek, 2016). Published data are also includedč was systematically measured at LP was in 2003, in the Global Networkč of Isotopes in Precipitationč while at LB it was only in 2010. Historically, iso- (GNIP) and in the Slovenian Network of Isotopes tope studies were performed in the LP; however, in Precipitation (SLONIP) from 1981 to 2010. In since 2011, isotope data are used more frequently, 1981-2018, the δ18O and δ2H values varied between but still sporadically in the LB. These investiga- -19.40 and -1.65 ‰ (mean -8.65 ‰, n=428) and be- tions mainly involve sampling from wells – sam- tween -147.8 and -7.3 ‰ (mean -59.4 ‰, n=425). pling was most often performed in Kle e, while The data is an important input into GNIP, which other objects in the water supply system were not has been evaluated many times (e.g., Rozanski et well sampled. Five locations on the Savač River al., 1993; Ichiyanagi, 2007; Hughes & Crawford, near Ljubljana were identified. Also, precipita- 2012), and in many hydrological and hydrogeo- tion was monitored for δ18O and δ2H at six differ- logical investigations (e.g., Krajcar Broni et al., ent locations. 1998; 2020; Pezdi , 1999; 2003; Bren i & Vre a, To our knowledge, 102 relevant records were 2006; Vre a et al., 2006; Ogrinc et al., 2008;ć 2018; found and analysed; however, only 41 records Vodila et al., 2011;č Kandu et al., 2012;č č Horvatč - published O, H and C isotope data and underwent č č Synthesis of past isotope hydrology investigations in the area of Ljubljana, Slovenia 265 a detailed review. The highest number of publi- Internet: http://vode.arso.gov.si/hidarhiv/ cations contained δ18O data (40 records), followed pov_arhiv_tab.php (23. 6. 2020) 2 13 by δ H (32 records), while δ CDIC investigations Avak, H. & Brand, W. A. 1995: The Finning MAT were rarely implemented (13 records). Also, long- HDO-Equilibration - A fully automated H2O/ term systemic approach with more frequent (e.g., gas phase equilibration system for hydro- seasonal) monitoring of relevant environmental gen and oxygen isotope analyses. Thermo isotope tracers is missing. In the scope of this re- Electronic Corporation, Application News, view, we would also like to point out that many 11: 1–13. investigations contain an insufficient description Bowen, G. J., Wassenaar, L. I. & Hobson, K. A. of sampling times and exact locations (missing 2005: Global application of stable hydro- coordinates), analytical methods, and reporting gen and oxygen isotopes to wildlife foren- of raw data. In this regards, better use of supple- sics. Oecologia, 143: 337–348. https://doi. mentary material, which should include all ap- org/10.1007/s00442-004-1813-y propriate metadata would be beneficial and nec- Bowen, G. J., Cai, Z., Fiorella, R. P. & Putman, essary for proper comparison in time and space A. L. 2019: Isotopes in the Water Cycle: and would enable tracing isotope changes in wa- Regional- to Global-Scale Patterns and ter resources. Applications. Annu. Rev. Earth Planet. The first stable water isotope survey of tap Sci., 47: 453–479. https://doi.org/10.1146/ water in the City of Ljubljana was performed in annurev-earth-053018-060220 2014. In order to assess the usefulness of environ- Bra i Železnik, B. & Globevnik, L. 2014: mental isotopes more systematically, monitoring Measurement, modelling and analysis of has been performed on the drinking water supply hydrologicalč č and hydrogeological process- system of Ljubljana since 2018. es and trends in a marsh area. In: Daniell, Based on all of the results from previous in- T. (eds.): Hydrology in a Changing World: vestigations of LP and LB, monthly monitor- Environmental and Human Dimensions. ing of δ18O and δ2H in groundwater in five wa- IAHS Publication 363, Montpellier: 413–418. ter supply facilities was established in January Bra i Železnik, B. 2016: Dinamika podzemne 2020. Besides, also the Sava River is sampled at vode sistemov vodonosnikov Iškega vršaja. two locations monthly and additional more de- M.Sc.č č thesis. , Faculty tail sampling of tap water is planned. The results of civil and geodetic engineering, Ljubljana: will be used to prepare guidelines for future iso- 57 p. tope monitoring that will provide a better overall Bra i Železnik, B., en ur Curk, B., Žvab Roži , understanding of water interactions of domestic P., Torkar, A., Vre a, P. & Bren i ., M. 2017: supply important for water managers. Deepč č karstified Čdolomiteč aquifer as a sourceč of drinking water č – isotopic measurements.č č In: EGU General Assembly 2017. https://doi. 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