© Author(s) 2022. CC Atribution 4.0 License Rare earth elements and yttrium in cold mineral and thermal (�30-60 °C) waters from Tertiary aquifers in the Mura Basin, north-eastern Slovenia: A review Elementi redkih zemelj in itrij v hladnih mineralnih in termalnih (�30-60 °C) vodah iz terciarnih vodonosnikov Murskega bazena v severovzhodni Sloveniji: pregled dosedanjih raziskav Polona KRALJ Geological Survey of Slovenia, Dimičeva ulica 14, SI-1000 Ljubljana, Slovenia; e-mail: polona.kralj@geo-zs.si. Prejeto / Received 3. 2. 2022; Sprejeto / Accepted 15. 7. 2022; Objavljeno na spletu / Published online 22. 07. 2022 Key words: Rare earth elements, Yttrium, Mineral waters, Thermal waters, Well cycling, Mura Basin Ključne besede: elementi redkih zemelj, itrij, mineralne vode, termalne vode, ·prekomerno izkoriščanje vrtin, Murski bazen Abstract Cold mineral and thermal waters from Tertiary aquifers in the Mura Basin mainly belong to the Ca-(Mg)-(Na)- HCO3 and Na-HCO3 hydrogeochemical facies, respectively, and the concentrations of yttrium (Y) and lanthanides or rare earth elements (REEs) are far below (10-2 – 10-4) the abundances in the aquifer sediments. Mineral waters are high pCO2, and the plots of concentrations of YREEs normalised to Post Archean Australian Shale (PAAS) show fractionation of Y and heavy REEs (HREEs) over light REEs (LREEs), and a significant positive europium (Eu) anomaly. Thermal water from regionally developed aquifer Thermal I (recently also termed the Mura/ Ujfalu Formation aquifer) shows a similar PAAS-normalised pattern with an obvious positive Eu anomaly and the tendency of enrichment with middle REEs (MREEs). The plots of PAAS-normalised YREE concentrations in thermal waters from the underlying low-permeability aquifers with poorly developed fracture porosity and abundant CO2 are flat with insignificant positive Eu anomaly. The abundance and fractionation of YREEs in mineral and thermal waters seems to be mainly controlled by the presence of carbonate complexing ligands, permeability of the aquifers and the related time of water-rock interaction. YREEs have been used for geochemical recognition of overexploitation of the Sob-1 well that yields mixed waters from Thermal I and the underlying low-permeability aquifers. The well overexploitation has resulted in continuous 30-to-80-minute changes in hydrodynamic pressure in Thermal I, and the related change in temperature and chemical composition of abstracted water. Leakage from clayey-silty layers rich in coal and organic matter has been recognised over a several-year time scale by increased abundances of total organic carbon (TOC), YREEs, gallium (Ga), thallium (Tl) and selenium (Se). PAAS-normalised plots of YREE concentrations have shown significant positive anomalies of samarium (Sm), terbium (Tb) and holmium (Ho) and indicate the YHREE complexing ligands could be, beside carbonate species, humic and/or fulvic acids. Izvleček Hladne mineralne vode iz terciarnih vodonosnikov Murskega bazena večinoma pripadajo Ca-(Mg)-(Na)- HCO3 ali Na-HCO3 hidrogeokemičnemu faciesu, koncentracije itrija (Y) in lantanoidov oziroma elementov redkih zemelj (REE) pa so za faktor 10-2 do 10-4 nižje od vsebnosti v prikamnini vodonosnika. Za mineralne vode je značilen visok pCO2, grafi koncentracij YREE, normaliziranih na vsebnosti Po arhajskega avstralskega skrilavca (PAAS) pa kažejo na frakcionacijo, ki se odraža v sorazmerno višji vsebnosti Y in težkih REE (HREE) glede na lahke REE (LREE), ter na značilno visoko pozitivno evropijevo (Eu) anomalijo. Termalne vode iz vodonosnika Termal I, ki je razvit v globinah Murskega bazena, kažejo podobno grafično razporeditev na PAAS normaliziranih koncentracij YREE z jasno izraženo pozitivno Eu anomalijo in težnjo po obogatitvi s srednjimi REE (MREE). Grafi na PAAS normaliziranih koncentracij YREE v termalnih vodah iz vodonosnikov, ki leže pod Termalom I in ki imajo nizko prepustnost in slabo razvito razpoklinsko poroznost ter so bogati s plinom CO2, pa so bolj ravni s komaj zaznavno pozitivno Eu anomalijo. Vsebnosti in frakcionacija YREE v mineralnih in termalnih vodah je najverjetneje odvisna od prisotnosti karbonatnih ligandov, prepustnosti vodonosnika in časa interakcije vode in prikamnine. GEOLOGIJA 65/1, 73-92, Ljubljana 2022 https://doi.org/10.5474/geologija.2022.005 74 Polona KRALJ Introduction The series of lanthanides or rare earth ele- ments (REEs) and yttrium (Y) - here abbreviated as YREEs, have been recognised exceptionally important in various aspects of hydrogeochem- ical research. The early studies refer to ma- rine waters and hydrothermal vents associated with mid-ocean spreading centres (Elderfield & Greaves, 1982; DeBaar et al., 1985; 1988; Klinkhammer et al., 1994; Bau & Dulski, 1999; Douville et al., 1999; 2002) while in continental areas, the abundances of REEs were first investi- gated in thermal waters in France and the Him- alayas (Michard & Albarède, 1986; Michard et al., 1987; Michard, 1989). REEs have been used as geochemical tracers in several studies of ground- water regional flow and mixing (e.g., Johannes- son et al., 1996; 1997; 2000; 2005; Dia et al., 2000; Tweed et al., 2006; Möller et al., 2007; Biddau et al., 2009; Yuan et al., 2014; Liu et al., 2017), explo- ration of power-producing geothermal systems and identification of potential high-level nuclear waste repositories (Möller, 2000; Shannon et al., 2001; van Middlesworth & Wood, 1998; Möller et al., 2003; Wood, 2006). During the past four dec- ades the potential of REEs as environmental pollutants exponentially increased as they have become indispensable in several sectors of mod- ern industry such as clean-energy, electronics or military, and have been utilised in a number of medical, agricultural and zootechnical applica- tions (e.g., Möller et al., 2000; Kulaksiz & Bau, 2011; Hissler et al., 2014; Gonzales et al., 2014; Hatje et al., 2016; Nigro et al., 2018; Aide, 2018; Ladonin, 2019). The utility of REEs as geochemical tracers is based on their conservative behaviour in many natural systems although several processes relat- ed to the REE aqueous chemistry can cause their fractionation. The most important are solubility and precipitation of REE hosting minerals, REE chemical speciation and complexation in solution and on mineral surfaces, ion distribution in the electric double layer attached to mineral sur- faces, the changes in pH, redox conditions and temperature of the aqueous medium, and hydro- geological factors such as flow pathways or resi- dence time (Brookins, 1989; Wood, 1990; Drever, 1997; Möller, 2000). Furthermore, while REEs ex- ist mostly in trivalent state, europium Eu3+ and Ce3+ can be reduced or oxidised to Eu2+ and Ce4+, respectively (Sverjensky, 1984; Smedley, 1991; Bilal, 1991; Bau, 1991; 1999; Johannesson et al., 1996; Liu & Byrne 1998). REEs, especially heavy rare earth elements (HREEs), are known for their affinity to form strong solution complexes with carbonate ions and are commonly relatively en- riched in CO2-rich cold mineral or thermal wa- ters although surface complexation can reduce their abundance in solution (Cantrell & Byrne, 1987; Lee & Byrne, 1993; Michard et al., 1987; Guo et al., 2005; Shand et al., 2005). Absolute concentrations of REEs are highly variable in natural waters and they are custom- ary displayed as plots of normalised values on a logarithmic scale versus atomic number or ionic radius on a linear scale (Coryell et al., 1963). Yt- trium is plotted at about the position of holmium (Ho). Commonly used datasets for normalisation are C1-chondrite (Anders & Grevesse 1989) and Post Archean Australian Shale (PAAS, McLen- nan, 1989) as the use of such reference materials enables easy comparison of large number of anal- yses worldwide. Recently, an improved data set for the European Shale has been proposed for en- vironmental studies in Europe (Bau et al., 2018). Sometimes reservoir rocks are used in order to obtain more specific information on water-rock interaction, pathways of flow or mixing of waters. In Slovenia, the first study on REE abun- dances in natural waters has been related to thermal and mineral waters in the Mura Basin, north-eastern Slovenia (Kralj & Kralj, 2000a), and up to the present, an upgraded and extend- ed research of this topic has been the only avail- able source of data (Kralj & Kralj, 2000b; 2009; 2012; Kralj, 2001; 2004a). In the end of the year 2021, a common Slovenian-Austrian project on geochemistry of water and stream sediment of the river Mur started (https://www.geo-zs.si/ index.php/projekti/drugi-projekti; https://aach. YREE so se izkazale kot dober pokazatelj prekomernega izkoriščanja vrtine Sob-1, kjer so bile zajete vode iz Termala I in slabo prepustnih vodonosnikov pod njim. Prekomerno izkoriščanje vrtine je imelo za posledico zvezno ciklično spreminjanje hidrodinamičnega tlaka v Termalu I, s hkratnim spreminjanjem temperature in kemijske sestave vode v časovnih intervalih 30-80 minut. Izcejanje iz glinasto-meljastih plasti bogatih z lečami premoga in organske snovi smo zaznali v časovnem obdobju več let kot povečanje vsebnosti celokupnega organskega ogljika (TOC), YREE, galija (Ga), talija (Tl) in selena (Se). Grafi na PAAS normaliziranih koncentracij YREE so pokazali visoke pozitivne anomalije samarija (Sm), terbija (Tb) in holmija (Ho) in kažejo, da so bile poleg karbonatnih ligandov v kompleksnih spojinah YHREE najverjetneje pomembne tudi huminske in/ali fulvične kisline. 75Rare earth elements and yttrium in cold mineral and thermal (~30-60 °C) waters from Tertiary aquifers in the Mura Basin... unileoben.ac.at/en/news). The project has been termed MURmap, and the research is focused on classical heavy metal (HM) pollutants and technology-critical elements (TCEs), including YREEs. Therefore, a need for critical review of state-of-the-art of research of YREEs in natural waters and sediments in north-eastern Slovenia occurred, and that is the primary aim of the pres- ent contribution. North-eastern Slovenia forms a part of the Pannonian Mura Basin (Fig. 1) which has been known for centuries for sources of mineral and thermal water. During the past five decades, thermal water from a particular intergranular aquifer named Thermal I (and recently also the Mura/Ujfalu Formation aquifer) has become a basis of economically important spa tourism in the area (Žlebnik et al., 1988a; b; Ravnik et al., 1992; Kralj, 1993; 1994; 1995; 2001; Rman, 2014; Szőcs et al., 2018). Mineral waters are high pCO2 and belong to the Na-Ca-HCO3, Ca-Na-HCO3, Ca-Mg-Na-HCO3 or Ca-Mg-(Na)-HCO3 hydro- geochemical facies. The plots of PAAS-normal- ised concentrations of YREE in mineral waters are characterised by high positive Eu anomalies and the enrichment of heavy REEs (HREEs) over light REEs (LREEs), (Kralj & Kralj, 2000a). Fig. 1. Structural map of the Mura Basin and geographic position of the discussed geothermal wells, and springs and shallow drill-holes capturing mineral waters (modified after Rajver et al., 1994; Kralj & Kralj, 2000a; 2012). Recent geological models (e.g., Šram et al., 2015) use modified naming of geological structures in comparison to those listed in the present contribution. 76 Polona KRALJ Thermal waters are low- to medium-tempera- ture (~30-60 °C) and essentially belong to the Na- HCO3 hydrogeochemical facies. The abundances of YREEs and their PAAS-normalised patterns are not uniform, and for the water from Thermal I they resemble those of mineral waters. Thermal waters from poorly fractured aquifers in the Len- dava, Haloze and Špilje Formations have higher concentrations of YREEs and their PAAS-nor- malised plots are flatter. From the wells Sob-1 and Sob-2, a mixture of thermal waters from Thermal I and several underlying low-yield aq- uifers in the Lendava Formation has been pro- duced, and the plots of PAAS-normalised YREE concentrations are specific owing to temporal variations of chemical composition of abstracted mixture of thermal waters (Kralj, 2004a; b; Kralj & Kralj, 2009; 2012). In the present contribution the significance of YREEs in recognition of water-rock interaction, well overexploitation and mixing of waters from various aquifers in the Tertiary sedimentary suc- cession of the Mura Basin is discussed. Abundant analytical data, normalisation technique using various materials and its interpretation have been included in the article in order to serve as a basis for future studies of YREEs in natural wa- ters and sediments in the Mura Basin and also broader, the catchment area of the river Mur. Analytical techniques The measurements of pressure and temper- ature in the aquifer Thermal I were performed in the Sob-1 well simultaneously with, i) Am- erada-type survey clock with mechanical tem- perature and pressure recorder, and ii) Leutert pressure-temperature gauge having a resolution of 0.007 bar and 0.01 ºC, with measurements tak- en every 10 seconds and computer-managed data output. In the Sob-1 well, the measurements have been performed in the years from 1990 to 1999, at a depth of 600 m, four times a year in a period of one week. Since September 1995, the well-head water temperature (± 0.3 °C), electrical conduc- tivity (± 0.5 % of the measured value), pH (± 0.1 unit), redox potential Eh (± 0.3 mV) and dissolved oxygen DO (± 0.5 % of the measured value) were measured at the well-head using WTW MultiLine P3 pH/LF/Oxymeter portable instrument. In the same week, water samples were collect- ed from the well-head outflow using pre-cleaned polyethylene bottles of one-liter and two-deci- liter volumes for the analysis of major ions and minor ions/elements, and trace elements, respec- tively. The bottles were rinsed three times with the sampled water, and for the analysis of trace elements the samples were acidified by ultrapur HNO3 (Merck) to pH<2 without prior filtration. After collection, the samples were sent immedi- ately in the laboratories. The major ions (Na+, K+, Ca2+, Mg2+, HCO3 -, Cl-, SO4 2-), minor ions/elements (F-, J-, Br-, NH4 +, Fe, Mn), the dissolved CO2-gas and total organic car- bon (TOC) were determined in 95 samples. The concentrations of major cations were analysed by ion chromatography (IC). The concentration of bicarbonate was determined by potentiometric titration using 0.1 N HCl. The chloride and flu- oride ions were analysed by IC and ion-selective electrode (ISE), respectively, and the bromide and iodide ions by Inductively Coupled Plasma Source Emission Spectroscopy combined with Mass Spectroscopy (ICP-MS). The charge bal- ance between the sum of milliequivalent concen- trations (meq/L) of major anions and cations ((Σ[Anions]-Σ[Cations]/(Σ[Anions]+Σ[Cations])) was assessed and the analyses with deviations of <5 % accepted. Trace elements Li, Be, B, Al, Sc, Ga, As, Se, Rb, Sr, Tl, Y and REEs (along with V, Cr, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ag, Cd, In, Sn, Sb, Te, Cs, Ba, Hf, Ta, W, Hg, Pb, Bi, Th and U that are not discussed in this paper) have been determined in 36 samples of thermal water from the Sob-1 well that were collected from May 1997 to June 1999. Trace elements were analysed in X-RAL Acti- vation Services Inc., Ann Arbor, Michigan, the United States of America. The abundances of Y and REE were determined by ICP/MS (inductive- ly coupled plasma source emission spectroscopy combined with mass spectroscopy) techniques without using any preconcentration method such as solid phase extraction, solvent extraction (e.g., Shannon & Wood, 2005) or coprecipitation with iron hydroxide (Zhu et al., 2010). From June 1988 till December 1996, the sam- ples of thermal water from the Sob-2 well have been collected in the same time period, and the sampling and analytical procedure performed in the same manner as for thermal water from the Sob-1 well. Major ions (Na+, K+, Ca2+, Mg2+, HCO3 - , Cl-, SO4 2-), minor ions/elements (F-, J-, Br-, Fe, Mn), the CO2-gas and total organic carbon (TOC) were determined in 53 samples. In October 1998 and June 1999, altogether 8 samples were col- lected for the analysis of major ions, minor ions/ elements, the CO2-gas, TOC and trace elements including Y and REEs. 77Rare earth elements and yttrium in cold mineral and thermal (~30-60 °C) waters from Tertiary aquifers in the Mura Basin... The same well-head measurements of water temperature, pH, Eh, conductivity and DO, and sampling procedure and analytical technique for determination of major ions, minor ions/el- ements, the gases CO2 and H2S, TOC and trace elements as carried out for thermal water from the Sob-1 well were applied for thermal waters from the wells Le-2g and T-4, and mineral wa- ters from shallow drill-holes/walled springs at Benedikt, Nuskova, Ivanjševci, Očeslavci, and Stavešinci. In each thermal well and drill-hole/ walled spring 3 and 1 samples, respectively, were collected. The abundances of major oxides and trace elements in Tertiary sediments including Y and REE were determined in ACME analytical Laboratories, Vancouver, Canada. The analysis of trace elements was performed using ICP/MS and XRF (X-ray fluorescence) techniques. Min- eral composition of aquifer sediments was deter- mined by X-ray diffraction analysis of powdered samples using Philips diffractometer PW 3719 and goniometer PW 1820 owned by the Depart- ment of Geology, Faculty of Natural Sciences and Technology, University of Ljubljana. Digital data were processed with peak-fitting program X’Pert HighScore Plus 4.0. Geological setting The most important tectonic structure in north-eastern Slovenia is the Mura Basin (INA-Projekt Zagreb & Geološki zavod Ljublja- na, 1991) which has been also considered as a part of much larger structural complexes of the southern Pannonian Basin System, namely, of the Mura-Zala Basin (sensu Fodor et al., 2002; Jelen et al., 2006) and the Zala-Drava Basin (e.g., Royden, 1988; Malvić & Velić, 2011), respective- ly. In the west, the Mura Basin terminates along the Pohorje Mountains that belong to the Eastern Alps (Fig. 1) and deepens toward the east where it reaches maximum depths of over 4 km in some isolated compartments near the Slovenian-Hun- garian border. The formation of the Mura Basin begun in Late Ottnangian as a result of continen- tal rifting and the following extension and sub- sidence along two fault zones, more specifically, the Raba Fault in the north and the Ljutomer Fault in the south. The faults have regional ex- tent and they evolved into the Radgona-Vas and Haloze-Ljutomer-Budafa sub-basins (Fodor et al., 2002; Márton et al., 2002; Jelen & Rifelj 2011). The sub-basins have been separated by the Mur- ska Sobota Swell (INA-Projekt Zagreb & Ge- ološki zavod Ljubljana, 1991). Pre-Tertiary basement of the Mura Basin consists of Paleozoic metamorphic rocks, Upper Carboniferous and Lower Permian sedimentary deposits, and rare erosional remnants of Trias- sic dolomite and limestone (Koščec & Jovanović, 1968). The oldest Neogene deposits are Ottnan- gian fluvial-limnic and Lower Badenian marine deposits, and the successions are united in the Haloze Formation. Sarmatian to Early Pannoni- an depositional environment was brackish. Sar- matian deposits are developed as well compacted and partially lithified sand, silt, clayey silt and marl, and Lower Pannonian sediments are dom- inated by silt, siltstone, fine-grained sand and marl. The successions are united in the Špilje Formation (Rijavec et al., 1985; Jelen et al., 2006). During Late Miocene, the Mura Basin became a part of extensive Lake Pannon. The environment gradually changed from brackish to freshwater owing to its isolation from Western Parathetys and advance of fluvial systems draining the East- ern Alps (Rijavec et al., 1985; Piler et al., 2007; Kováč et al., 2017). Deltaic systems developed and fine-grained turbidites fed from delta slopes form the Middle Pannonian to Lower Pontian Lenda- va Formation. Delta front and delta plain depos- its are united in the Upper Pannonian to Upper Pontian Mura Formation. The overlying Dacian and Romanian deposits are related to the systems of alluvial fans and braided rivers (Kralj, 2010), and the successions are united in the Ptuj-Grad Formation (Jelen & Rifelj 2011; Nádor et al., 2012; Šram et al., 2015). Some 3 My ago, alkali basaltic volcanism occurred in the vicinity of the South Burgenland Swell and produced a variety of vol- canic and mixed volcaniclastic-fluvial deposits (Pleničar, 1968; Kralj, 2010). Cold mineral waters spring out along the mar- gins of the Radgona Depression at the place of Ben- edikt and the village of Nuskova (Fig. 1), and in the Ščavnica valley (Stavešinci, Ivanjševci, Očeslavci) just above the westernmost extending of the Mur- ska Sobota Swell (e.g., Kralj & Kralj, 2000; Gabor & Rman, 2016). In the area Badenian and Sarma- tian deposits of the Haloze and Špilje Formations outcrop, and deep-seated faults enable uplift of the CO2-gas from pre-Tertiary basement. In the Mura Basin, thermal aquifers occur in pre-Tertiary basement, and in Haloze, Špil- je, Lendava, Mura and Ptuj-Grad Formations (Fig. 2), (Kralj, 2001; Nádor et al., 2012; Šram et al., 2015). Pre-Tertiary aquifers are commonly fractured or cavernous. In the deepest confined and semi-confined compartments brines of es- sentially Na-Cl composition prevail, and in some 78 Polona KRALJ shallower carbonate aquifers thermal waters of the Na-HCO3, Na-HCO3-Cl or Na-SO4-HCO3 hy- drogeochemical facies have been encountered (Kralj, 1993; 1994; Kralj & Kralj, 2000a). In the Haloze and Špilje Formations low-permeability thermal aquifers occur and chemical composition of waters depends on their depth and paleoenvi- ronmental conditions that locally enabled infil- tration of younger fresh-waters. The amount of total dissolved solids commonly exceeds 10 g/L (Szocs et al., 2013) and the CO2-gas is common (INA-Projekt Zagreb & Geološki zavod Ljubljana, 1991; Kralj, 2001). In the Lendava Formation iso- lated aquifers occur (Szocs et al., 2013), the yield of thermal water is low, and the CO2-gas originat- ing from lithospheric mantle (Bräuer et al., 2016) is abundant. The waters have high amount of to- tal dissolved ions (TDI) that commonly exceeds 10 g/L, and they essentially belong to the Na-HCO3 hydrogeochemical facies (Table 1, T-4). The con- centrations of the Ca2+, K+ and Mg2+ may be much higher than in the waters from pre-Tertiary base- ment (Kralj & Kralj, 2000a; Szocs et al., 2013). Fig. 2. Idealised model of sedimentary formations with thermal aquifers in the Mura Basin and a typical productive well capturing waters from several thermal aquifers (modified from Kralj, 2001). The names of formations are adopted from Jelen et al., 2006. 79Rare earth elements and yttrium in cold mineral and thermal (~30-60 °C) waters from Tertiary aquifers in the Mura Basin... Well Average Ion/Oxide/ Parameter Unit Le-2g* 3 Sob-1* 71 Sob-1 (low-TDI) 1 Sob-1 (high-TDI) 1 Sob-2* 46 Sob-2 (low-TDI) 1 Sob-2 (high-TDI) 1 T-4* 3 Na+ mg/L 280 731 450 970 653 300 1,100 2,087 K+ mg/L 7.8 92 70 105 39 31 52 440 Ca2+ mg/L 5.3 37.0 15 71 12.2 7 18 233 Mg2+ mg/L 7.7 8 5 7 3.7 4.0 2 147 Fe mg/L 0.28 0.15 0.06 0.21 1.12 0.15 0.12 0.047 Mn μg/L 9 13 6 21 8 7 69 72 Br- mg/L 0.11 0.61 0.45 0.92 0.9 0.41 1.52 0.80 J- mg/L <0.01 0.16 0.10 0.09 0.34 0.08 0.67 0.17 F- mg/L 0.53 0.78 0.45 0.75 1.40 0.20 0.55 0.84 Cl- mg/L 3.3 141 95 175 117 85 180 133 HCO3 - mg/L 856 2,214 1,500 2,900 1,645 960 2,600 7,633 SO4 2- mg/L 2 30 12 48 7 13 2 233 CO2 mg/L 1 677 440 869 303 88 713 1,476 SiO2 μg/L 14 18 14 19 15 3 26 32 TOC mg/L 2.4 2.5 2.5 2.0 5.1 2.5 5.5 2.3 TDI mg/L 1,173 3,263 2,165 4,300 2,491 1,407 3,989 10,939 pH 7.30 7.0 6.81 6.85 7.9 7.02 6.78 6.85 Twh °C 58.3 49.0 48.7 49.1 46.6 45.8 47.4 29.8 La μg/L 0.03 0.19 0.04 1.09 0.06 0.02 0.14 0.39 Ce μg/L 0.02 0.27 0.06 2.21 0.08 0.03 0.23 0.82 Pr μg/L <0.01 0.04 0.01 0.27 0.02 0.01 0.05 0.11 Nd μg/L 0.02 0.17 0.06 1.08 0.07 0.02 0.17 0.42 Sm μg/L 0.03 0.11 0.05 0.21 0.10 0.04 0.21 0.18 Eu μg/L 0.04 0.99 0.07 0.08 0.12 0.06 0.20 0.08 Gd μg/L 0.03 0.08 0.03 0.24 0.08 0.02 0.16 0.19 Tb μg/L 0.01 0.13 0.06 0.03 0.02 0.01 0.04 0.01 Dy μg/L 0.03 0.05 0.04 0.13 0.06 0.02 0.09 0.16 Y μg/L 0.03 0.29 0.10 0.75 0.10 0.03 0.19 0.79 Ho μg/L <0.01 0.08 0.01 0.03 0.02 0.01 0.03 0.01 Er μg/L 0.02 0.04 0.03 0.06 0.04 0.01 0.10 0.09 Tm μg/L 0.01 0.01 0.01 0.01 0.02 <0.01 0.03 <0.01 Yb μg/L 0.02 0.03 0.03 0.04 0.05 <0.01 0.11 0.07 ΣYREEs μg/L <0.30 2.48 0.60 6.23 0.84 <0.28 1.75 3.32 *data from Kralj, 2004a and Kralj & Kralj, 2009 Table 1. Average chemical composition of thermal waters; major ions, minor ions/elements, dissolved CO2-gas, dissolved silica (SiO2), total organic carbon (TOC), total dissolved ions (TDI), pH, temperature of water at well-head (Twh) and yttrium and rare earth elements (YREEs). The wells T-4 (Radenci), Sob-1 and Sob-2 (Murska Sobota) and Le-2g (Lendava) tapped thermal aquifers in the Haloze, Špilje and Lendava Formations, Lendava and Mura Formations, and Mura Formation, respectively. For the Sob-1 and Sob-2, low-TDI and high-TDI indicate 30-to-80-minute variations of chemical composition (also referred as well cycling), and low and high pumping rate, respectively. 2012; Kralj, 2001; Rajver et al., 1994) although it extends on the territory of Hungary, Austria, Slovakia and Croatia as a large transboundary thermal groundwater body (TTGWB) occupying a subsurface area of over 22,128 km2 (Nádor et al., 2012; Szocs et al., 2013; Rman, 2014; Šram et al., 2015; Tóth et al., 2016; Rotár-Szalkai et al., 2017; Szőcs et al., 2018). The Mura Formation comprises a subsurface interconnected delta-front sand body that de- veloped into thermal aquifer (Nádor et al., 2012; Šram et al., 2015; Tóth et al., 2016) locally termed Thermal I (INA-Projekt Zagreb & Geološki zavod Ljubljana, 1991; Ravnik et al., 1992; Kralj, 2001; Kralj & Kralj, 2012). In the Mura Basin Ther- mal I encompasses a subsurface area of about 1,372 km2 (Kralj, 1995; Kralj & Kralj, 2000a, b; 80 Polona KRALJ Thermal I outcrops in the west of the Mura Basin near the town of Radenci and deepens to- ward the east reaching a depth of over 1000 m at the Slovenian-Hungarian border. In general, the waters belong to the Na-HCO3 hydrogeochemical facies, TDI commonly ranges from 1,000 – 1,500 mg/L, and free CO2 is practically absent. Isotop- ic studies indicate the Pleistocene age of waters from the Mura Formation and the replacement of formation waters by infiltration has been as- sumed (Szocs et al., 2013). At greater basin’s depths where Thermal I is underlain by thick deposits that still belong to the Mura Formation, the concentrations of the chloride ions are low (Table 1, Le-2g). In places where the underlying deposits are thin and/or tectonically displaced, some thermal waters seem to have naturally ad- mixed minor quantities of waters from the Len- dava Formation, and as a result, the concentra- tions of the chloride ions are higher (Table 1, Sob-1 and Sob-2). In the Sob-1 and Sob-2 wells chemical composition of abstracted water has varied in time owing to the well overexploitation (Kralj & Kralj, 2000b; 2012). Recently, hydrogeo- chemistry, mixing of waters and well cycling and overexploitation during several-year period have been recognised by changes in the abundance of halide ions (Kralj & Kralj, 2020). Ion/Oxide/ Parameter Unit Benedikt Ana Ivanjševci Očeslavci Stavešinci Nuskova Na+ mg/L 51 104 685 162 276 K+ mg/L 21 15 65 13 3 Ca2+ mg/L 800 487 417 375 328 Mg2+ mg/L 180 203 69 32 47 Fe mg/L 7.2 6.7 1.2 2.7 0.9 Mn mg/L 0.170 0.143 0.178 0.359 0.428 J- mg/L <0.01 0.02 0.21 0.05 0.02 F- mg/L 0.04 <0.01 0.78 0.22 0.40 Cl- mg/L 3 7 170 25 32 HCO3 - mg/L 3,400 2,590 2,970 1,690 1,790 SO4 2- mg/L 23 2 118 19 68 CO2 mg/L 1,830 2,500 3,800 3,300 3,300 SiO2 μg/L 43 18 15 17 11 TOC mg/L 1.3 0.9 0.8 0.6 0.9 TDI mg/L 4,536 3,420 4,499 2,321 2,547 pH 6.37 6.22 6.22 6.02 6.00 La μg/L 0.07 0.02 0.06 0.10 0.11 Ce μg/L 0.14 0.01 0.01 0.15 0.19 Pr μg/L 0.02 <0.01 <0.01 0.02 0.02 Nd μg/L 0.08 <0.01 0.09 0.12 0.10 Sm μg/L 0.06 <0.01 0.02 0.02 0.02 Eu μg/L 0.17 0.21 0.04 0.08 0.03 Gd μg/L 0.01 0.01 <0.01 0.05 0.02 Tb μg/L 0.01 <0.01 <0.01 0.01 0.01 Dy μg/L 0.02 <0.01 0.01 0.08 0.02 Y μg/L 0.24 0.70 0.03 1.00 0.28 Ho μg/L 0.01 <0.01 <0.01 0.02 0.01 Er μg/L 0.03 0.01 0.01 0.07 0.04 Yb μg/L 0.02 <0.01 <0.01 0.05 0.02 ΣYREEs μg/L 0.87 <1.03 <0.32 1.84 0.86 Table 2. Chemical composi- tion of mineral waters; ma- jor ions, dissolved CO2-gas, dissolved silica (SiO2), total organic carbon (TOC), to- tal dissolved ions (TDI), pH and yttrium and rare earth elements (YREEs). Data from Kralj & Kralj, 2000. 81Rare earth elements and yttrium in cold mineral and thermal (~30-60 °C) waters from Tertiary aquifers in the Mura Basin... Rare earth elements and yttrium in cold mineral and thermal waters Cold mineral waters Cold mineral waters belong to the Na-Ca- HCO3, Ca-Na-HCO3, Ca-Mg-Na-HCO3 or Ca-Mg- (Na)-HCO3 hydrogeochemical facies. The amount of total dissolved ions (TDI) is variable and rang- es from about 2,300 mg/L to over 4,500 mg/L (Table 2). The waters are high pCO2 and in the analysed samples, the amount of dissolved car- bon dioxide ranges from 1,800 to 3,800 mg/L. The concentrations of the chloride and sulphate ions are generally low and the highest recorded were in the spring Očeslavci where they amounted to 170 mg/L and 118 mg/L, respectively. The content of YREE ranges from <0.32 µg/L to 1.84 µg/L and is not proportional to TDI or the amount of dis- solved CO2-gas. To compare YREE concentrations in cold min- eral waters normalised to PAAS and aquifer sed- iment, Sarmatian calcareous siltstones, the Špil- je Formation, from the well Be-2/03, Benedikt at a depth of 122-124 m, have been used (Kralj et al., 2009). The abundances of YREEs in the aquifer sediment show a proportional depletion with the increase in calcite present either as cement, mi- crofossils or limestone lithic fragments (Table 3). The PAAS-normalised plots are alike (Fig. 3) but well below the shale abundance, and MREEs and HREEs are slightly fractionated over LREEs. The plots of YREE concentrations in mineral wa- ters normalised to PAAS and to the average of 3 aquifer sediments do not differ significantly in the shape but only in the magnitude of about <10-1 (Fig. 4). They are characterised by fractionation of YHREEs over LREEs and a significant posi- tive Eu anomaly, while Ce anomaly is absent or slightly negative. Fig. 3. The plots of PAAS-normalised YREE abundances in aquifer sediments from the wells Be-2/03, Benedikt (B1, B3) and Sob-1, Murska Sobota (S1-S5). For detailed information see Table 3. Sample/ Well Depth Sediment Type La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu B1 Be-2/03 122-124 m Siltstone (30 wt.% CaCO3) 10.0 18.3 1.94 7.9 1.6 0.33 1.62 0.23 1.40 9.0 0.36 0.83 0.13 0.90 0.11 B2 Be-2/03 122-124 m Siltstone (25 wt.% CaCO3) 13.5 25.1 2.78 11.1 2.2 0.47 2.01 0.33 1.74 11.9 0.37 0.96 0.17 1.09 0.14 B3 Be-2/03 122-124 m Siltstone (8 wt.% CaCO3) 31.8 63.1 7.05 27.9 5.6 1.10 4.73 0.64 4.16 20.8 0.81 1.95 0.25 2.10 0.33 S1 Sob-1 633 m Sand 32.9 106.0 7.6 31.6 5.8 1.72 5.2 0.9 4.7 20 0.95 2.6 0.4 2.8 0.40 S2 Sob-1 802 m Sand 38.4 81.0 9.1 36.1 7.4 1.93 6.9 1.0 6.4 28 1.35 3.9 0.6 4.0 0.60 S3 Sob-1 802 m Silt 37.6 80.9 8.6 33.7 6.7 1.56 6.2 1.0 4.8 22 0.96 3.1 0.4 2.8 0.44 S4 Sob-1 853 m Silty sand 34.4 109.0 7.4 33.8 6.4 1.52 5.2 0.9 5.1 20 0.99 3.0 0.3 2.8 0.41 S5 Sob-1 853 m Silt 38.2 82.3 8.5 32.8 7.3 1.67 6.0 1.0 4.8 22 1.05 3.2 0.4 2.8 0.46 PAAS Shale 38.2 79.6 8.83 33.9 5.55 1.08 4.66 0.774 4.68 27 0.991 2.85 0.405 2.82 0.443 Table 3. YREEs in aquifer sediments and PAAS (after McLennan 1989). B1-B3, Špilje Formation, the well Be-2/03, Benedikt; S1-S5, Mura and Lendava Formations, well Sob-1, Murska Sobota. 82 Polona KRALJ Thermal waters from the T-4, Le-2g, Sob-1 and Sob-2 wells Thermal water from the T-4 well in the town of Radenci belongs to the Na-HCO3 hydrogeo- chemical facies and is abstracted from a depth of 400-540 m in the Haloze and Špilje Forma- tions. TDI and the dissolved CO2-gas amount to nearly 11 g/L and 1.5 g/L, respectively (Table 1), and the sum of YREE concentrations averag- es to 3.32 μg/L. The plots of PAAS-normalised YREE abundances in the water have shown that MREEs and YHREEs are slightly fractionated over LREEs, and there are very small negative Ce and moderate positive Eu anomalies (Fig. 5). The well Le-2g in the town of Lendava pen- etrated Thermal I at a depth of 825-950 m and captures some underlying aquifers in the Mura Formation at a depth interval of 970-1,500 m as well. The water belongs to the Na-HCO3 hy- drogeochemical facies, TDI amounts to about 1.2 g/L and the dissolved CO2-gas is practically absent (Table 1). The sum of YREEs averages to < 0.28 μg/L, and the plots of PAAS-normalised values show small negative Ce and moderate pos- itive Eu anomalies, and fractionation of MREEs and HREEs over LREEs (Fig. 5). The wells Sob-1 and Sob-2 reached depths of 870.0 m and 855.8 m and tapped Thermal I at 600-646 m and 605-660 m, respectively, as well as some underlying aquifers in the Mura and Len- dava Formations. The water belongs to the Na- HCO3 hydrogeochemical facies, TDI ranges from <1.4 g/L to over 4.3 g/L and averages to 3.2 g/L and 2.5 g/L for the Sob-1 and Sob-2 well, respec- tively (Table 3). The amount of dissolved CO2-gas is variable and ranges from some 10 mg/L to over 1,500 mg/L (Kralj, 2001). Chemical composition of thermal water from the Sob-1 and Sob-2 wells has been varying in time and often considerably deviated from the av- erage (Table 1). In the Sob-1 well, hydrodynamic pressure and temperature in Thermal I have been commonly changing within the time interval of about 70-80 minutes, although during severe over- exploitation conditions, the intervals have been as short as 30-40 minutes. When the pressure in Ther- mal I increased, the temperature decreased, and vice versa. The maximum recorded difference in pressure and temperature is 0.8 bar and 2.5 °C, re- spectively. The changes are continuously reoccur- ring during the well operation, and for that reason, they have been arbitrarily termed well cycling and the time interval of 30-40 and 70-80 minutes a cy- cle (Kralj, 2004a; Kralj & Kralj 2009; 2012). Chemical composition of abstracted thermal water varied with the changing temperature and hydrodynamic pressure as well. With the increase in temperature TDI, the amount of dissolved CO2 and the concentrations of almost all major ions, minor ions/elements and trace elements including YREEs increased in nearly the same manner (Figs. 6, 7), (Kralj & Kralj, 2000a; 2012; Kralj, 2001). When hydrodynamic pressure in Thermal I increased and the temperature decreased, the composition of abstracted thermal water began to change to- ward lower TDI, lower amount of dissolved CO2, and lower concentrations of nearly all major ions, minor ions/elements and trace elements. As the ab- stracted water is a mixture supplied from Ther- mal I and the underlying aquifers in the Mura and Lendava Formations, the change in hydrodynamic pressure in the aquifers and the change in temper- ature and chemical composition of water indicates the changing supply from the aquifers. Fig. 4. a, The plots of PAAS-normalised YREE concen- trations in mineral waters; b, The plots of concentrations of YREEs in mineral waters normalised to the average of 3 aquifer sediments from the well Be-2/03, Benedikt (122- 124 m). Solid line – Stavešinci, dotted line - Ana (Benedikt), dashed line – Nuskova. Fig. 5. The plots of PAAS-normalised YREE concentrations in thermal waters from the Mura Basin; a, Le-2g; b, Sob-1, low TDI; c, Sob-1, high TDI; d, T-4 (modified after Kralj, 2004a). 83Rare earth elements and yttrium in cold mineral and thermal (~30-60 °C) waters from Tertiary aquifers in the Mura Basin... Fig. 6. Temporal variation of concentrations of some ma- jor ions, minor ions/elements, trace elements and YREEs in thermal water abstracted from the Sob-1 well on November 11, 1997, from 10:00 h to 11:43 h. The peak of concentrations correlates with the decrease in hydrodynamic pressure in Thermal I (data from Kralj & Kralj, 2000a; 2012). Fig. 7. Temporal variation of concentrations of some major ions, trace elements and YREEs in thermal water abstracted from the Sob-1 well on April 7, 1999, from 09:45 h to 11:10 h (data from Kralj & Kralj, 2000a; 2012). 84 Polona KRALJ In thermal water abstracted from the wells Sob-1 and Sob-2, the sum of concentrations of YREEs is, in general, relatively low. For water from Thermal I it amounts to <0.30 μg/L (Table 1; Le-2g and Sob-2, low TDI). The highest sum of 6.23 μg/L has been recorded in water from the Sob-1 well, and the average for the Sob-1 and Sob-2 wells is 2.48 μg/L and 0.84 μg/L, respec- tively. The concentrations of YREEs in thermal wa- ters have been normalised to PAAS rather than the aquifer sediment (Table 3). YREE abundanc- es in silty and sandy aquifer sediments from the Sob-1 well-core show some variations (Fig. 3) re- lated to the grain-size and the amount of heavy minerals such as monazite, allanite and xeno- time (Ce, Sm, Gd, Dy, Tb), or light minerals, in particular plagioclases (Eu). For that reason, the YREE abundances in the aquifer sediments normalised to PAAS commonly show positive Ce anomaly and positive fractionation of MREEs over HREEs. Yet, the abovementioned heavy min- erals have very low solubility in water and there- fore unlikely contribute significant amounts of YREEs in thermal water. The PAAS-normalised YREE concentrations in thermal water from the Le-2g well (Fig. 5) and mixtures of waters from the Sob-1 and Sob-2 wells sourced from or dominated by Thermal I are characterised by fractionation of HREEs and to a lesser extent MREEs over LREEs, with slightly negative Ce and pronounced positive Eu anoma- lies. In mixed waters from the Sob-1 and Sob-2 wells having higher TDI (Fig. 5), MREEs are pos- itively fractionated over LREEs and HREEs, and the Eu anomaly is moderately positive. Similar trends can be seen for temporal var- iations in chemical composition of abstracted thermal water from the Sob-1 well sampled on October 28, 1998 (Fig. 8) and November 11, 1997 (Fig. 9). On April 7, 1999, the concentrations of YREEs were substantially increased and their PAAS-normalised plots show more pronounced negative Ce anomaly, very high positive Sm and Tb anomalies and very high and moderate posi- tive Ho and Tm anomaly, respectively (Fig. 10). Discussion Rare earth elements and yttrium analysed in mineral and thermal waters from Tertiary aqui- fers in the Mura Basin occur in concentrations far below (10-2 to 10-4) the abundances in the aq- uifer sediments. The original idea of the present study was to identify similarities of the YREE Fig. 8. The plots of PAAS-normalised YREE concentrations in low-TDI thermal water from the Sob-1 well sampled on October 28, 1998 (modified from Kralj & Kralj, 2009). Dotted line – sample taken at 13h 00 min; solid line – sample taken at 13h 35 min; dashed line – sample taken at 13h 45 min. Fig. 9. The plots of PAAS-normalised YREE concentrations in high- (H) and low-TDI (L) thermal water from the Sob- 1 well sampled November 11, 1997, from 12:57 h to 13:43 h (data from Kralj & Kralj, 2009). Dotted line – sample taken at 10h 26 min 30 sec; solid line – sample taken at 10h 28 min 45 sec; dashed line – sample taken at 10h 30 min 15 sec; dash- -dotted line – sample taken at 10h 32 min 00 sec. Fig. 10. The plots of PAAS-normalised YREE concentrati- ons in high-TDI thermal water for the Sob-1 well sampled on April 7, 1999 (modified from Kralj & Kralj, 2009). Dashed line – sample taken at 9h 45 min; dash-dotted line – sample taken at 9h 55 min; solid line – sample taken at 9h 58 min; dotted line – sample taken at 10h 00 min. 85Rare earth elements and yttrium in cold mineral and thermal (~30-60 °C) waters from Tertiary aquifers in the Mura Basin... distribution patterns in water and aquifer sed- iment as they have been reported from various hydrological environments worldwide (e.g., Sme- dely, 1991; Gosselin et al., 1992; Johannesson & Lyons, 1996; Fee et al., 1992; Möller, 2000; Zhou et al., 2005; Möller et al., 2007). Yet, the normali- sation to PAAS seems more convenient. The first reason is mineralogical and geochemical diversi- ty of Tertiary sedimentary successions cold min- eral and particularly thermal waters migrated through during their geochemical and thermal evolution. The second reason is carbonate type of waters that influences the solubility and compl- exation reactions of YREEs and is an important control of their fractionation (Lee & Byrne, 1993; Luo & Byrne, 2004; Guo et al., 2005). In the areas of cold mineral water springs, Badenian and Sarmatian sediments of the Haloze and Špilje Formations outcrop. Their deeper compartments comprise low-permeability aquifers with waters of diverse hydrogeochem- ical facies varying inside the composition range of Na-Ca-K-Cl-HCO3-SO4 (Kralj & Kralj, 2000a; 2012; Bräuer et al., 2016; Nádor et al., 2012). Deep waters and carbon dioxide ascend toward the surface through fault systems and mix with shallower groundwaters. Owing to the presence of carbon dioxide, the mixed waters undergo fur- ther geochemical change related to water-rock interaction. The resulting diverse major ion com- position of mineral waters (Table 2) indicates rather individual evolution patterns. The plots of YREE concentrations in cold min- eral waters normalised to PAAS show fraction- ation of Y and HREEs (YHREEs) over LREEs, and positive Eu anomaly. The observed YHREE/ LREE fractionation can be explained by the sol- ubility and complexation of Y and REEs in car- bonate waters as they increase with the increas- ing atomic number or smaller ionic radius (Lee & Byrne, 1993; Luo & Byrne, 2004; Guo et al., 2005). Positive Eu anomalies have been typically encountered in anoxic thermal waters (Michard & Albarède, 1986; Michard et al., 1987; Michard, 1989; Klinkhammer et al., 1994; Douville et al., 1999; 2002) as europium tends to be reduced to more soluble Eu2+ state (Sverjensky, 1984; Bilal, 1991), and when in solution, the large Eu2+ ion is not easily incorporated in precipitating minerals but commonly remains in solution and/or ad- sorbed onto mineral surfaces (Möller, 2000). Low-temperature thermal water (30 °C) from the Haloze and Špilje Formations captured in the well T-4 at Radenci has relatively flat PAAS-nor- malised YREE pattern with insignificant posi- tive Eu anomaly, although MREEs, Y and HREEs are fractionated over LREEs (Fig. 5). Thermal aquifers in the Haloze and Špilje Formations are characterised by very low permeability, the flow of fluids is slow, and the water-rock interaction times are much longer than in fractured aquifers of mineral waters. Similar relationship can be recognised for thermal waters captured in the wells Le-2g, Sob- 1 and Sob-2. The aquifer Thermal I has relatively well-developed intergranular porosity and good permeability. The plots of PAAS-normalised YREE concentrations in thermal waters cap- tured or sourced from or mainly from Thermal I resemble, in general, those of cold mineral waters showing a distinct positive Eu anomaly and frac- tionation of MREEs and HREEs over LREEs (Fig. 5). For mixed waters with a higher proportion of waters from the Lendava Formation the YREE PAAS-normalised plots become flatter with a weak positive Eu anomaly and slightly acquired other MREEs, whilst the fractionation of HREEs over LREEs disappears. In the Lendava Forma- tion the aquifers have similar characteristics as in the Haloze and Špilje Formations, carbon dioxide is abundant, the aquifer sediment and water are more equilibrated, and longer times of water-rock interaction enable LREEs to undergo complexation with carbonate ligands and enter the solution the last, after more soluble HREEs and MREEs. The plots of PAAS-normalised YREE concen- trations obtained for thermal waters produced from the Sob-1 well on April 7, 1999 (Fig. 10) differ from those typical for Thermal I or vari- ous mixtures with waters from the Lendava For- mation. Beside moderate negative Ce anomaly, a pronounced positive Sm, Eu, Tb and Ho anoma- lies have been recognised. In Table 4, six selected analyses of major ions and trace elements from the cycles sampled on November 11, 1997 and April 7, 1999 are shown. The cycle of April 7, 1999, shows a general increase in the concentration of the po- tassium ions (K+), total organic carbon (TOC) and several trace elements, in particular, lithium (Li), boron (B), scandium (Sc), gallium (Ga), selenium (Se), rubidium (Rb), strontium (Sr), caesium (Cs), barium (Ba), thallium (Tl) and YREEs. Chemical composition of thermal water from this cycle and PAAS-normalised YREE concentrations could only be explained by activation of an addition- al source of thermal water with specific chemi- cal composition and possibly low permeability as it has been recognised as late as during severe overexploitation conditions in the well. 86 Polona KRALJ Table 4. Chemical composition of selected samples of thermal water from the Sob-1 well collected on November 11, 1997 (10:00 – 11:43 h) and April 7, 1999, (09:45 – 11:10 h); major ions, dissolved CO2-gas, dissolved silica (SiO2), total organic carbon (TOC), total dissolved ions (TDI), pH, temperature of water at well-head (Twh) and trace elements with yttrium and rare earth elements (YREEs). Sample 180* 184* 190 210 218 221 Date 11.11. 97 11.11.97 11.11.97 7.4.99 7.4.99 7.4.99 Time 1000’ 1028’45’’ 1120’ 945’ 1033’ 1110’ Ions (mg/L) Na+ 490 990 530 620 450 820 K+ 32 98 33 190 70 190 Ca2+ 14 63 21 43 15 57 Mg2+ 8 6 5 19 5 3 HCO3 - 1600 3100 1830 2600 1500 2700 Cl- 120 220 91 159 95 124 F- 0.45 0.92 0.73 0.47 0.45 0.62 J- 0.05 0.07 0.04 0.15 0.10 0.19 Br- 0.41 0.64 0.36 0.60 0.45 0.77 SO4 2- 8 51 8 75 12 48 TDI 2,273 4,530 2,519 3,708 2,148 3,944 Oxides/Dissolved gas SiO2 (µg/L) 18 17 18 13 14 24 CO2 (mg/L) 546 744 637 575 440 712 Parameters TOC (mg/L) 1.2 1.9 1.4 2.3 2.5 12.1 Twh (℃) 48.4 48.7 48.7 48.8 48.6 48.9 pH 6.44 6.67 6.49 6.69 6.81 6.86 Trace elements (µg/L) Li 170 440 180 854 343 700 Be 0.2 1.1 0.3 1.5 0.7 1.3 B 559 1480 678 4000 1100 2800 Al 31 22 17 28 19 75 Sc 1.8 2.2 1.9 7.1 7.6 7.1 Ga 0.09 0.12 0.10 0.62 0.64 0.62 As 2.7 2.9 2.3 3.0 3.8 3.1 Se 2.1 4.8 1.3 6.5 4.4 7.1 Rb 181 668 166 654 258 656 Sr 775 969 827 1470 982 1530 Y 0.05 0.24 0.06 0.76 0.64 0.76 Zr 0.1 0.3 0.1 1.3 0.4 1.0 Cs 6.1 26.8 6.3 34.4 13.2 32.3 Ba 140 190 150 477 292 477 La 0.02 0.24 0.03 0.63 0.09 0.63 Ce 0.05 0.42 0.06 0.26 0.11 0.25 Pr <0.01 0.05 <0.01 0.04 0.02 0.04 Nd 0.04 0.21 0.04 0.18 0.10 0.20 Sm <0.01 0.07 0.01 0.12 0.09 0.12 Eu 0.04 0.05 0.03 0.13 0.09 0.14 Gd 0.03 0.07 0.01 0.10 0.05 0.09 Tb <0.01 0.01 0.01 0.61 0.60 0.61 Dy 0.02 0.06 0.01 0.10 0.06 0.10 Ho <0.01 0.02 <0.01 0.60 0.01 0.60 Er <0.01 0.03 <0.01 0.06 0.04 0.08 Tm <0.01 0.01 <0.01 0.02 0.02 0.02 Yb <0.01 0.02 <0.01 0.06 0.04 0.08 Tl 0.17 0.37 0.11 0.86 0.62 0.79 *data from Kralj & Kralj, 2009 87Rare earth elements and yttrium in cold mineral and thermal (~30-60 °C) waters from Tertiary aquifers in the Mura Basin... The increased concentrations of the K+ ions and Li, B, Rb, Ba and Cs in the water from the assumed additional source indicate a possible or- igin in dissolution of illite and muscovite, which are the principal phyllosilicate minerals of the aquifer sediments in the Sob-1 well (Žlebnik et al., 1988a; Kralj, 2001). Illite contains trace amounts of YREEs and its decomposition could also be the source of YREEs and their fraction- ation. Many studies carried out worldwide have shown that during water-rock interaction LREEs are retained at the site of reaction by adsorption to residual phases while YHREEs are preferen- tially mobilized by solution complexation reac- tions (e.g., Duddy, 1980; Braun et al., 1993; Johan- nesson and Zhou, 1999; Zhou et al., 2005). Guo et al. (2005) have shown for carbonate waters that there is a steady increasing trend of REE solu- bility according to atomic number or decreasing ionic radii for (+3) ions, except for Sm and Ho, and to a lesser extent Tb that show relatively higher solubilities. The observed PAAS-normalised pat- tern in the waters from the cycle of April 7, 1999, could be explained by the process of decomposi- tion of illite and muscovite, and preferential mo- bilisation of HREEs into solution. On the other hand, the increased concentra- tions of TOC, gallium, selenium, thallium and the iodide ions I- also indicate the assumed addi- tional (or another additional) source. This group of elements could have an origin in waters leaked from silty and clayey layers rich in organic mat- ter and coal, that have been activated only during severe overexploitation conditions. In that case the ligands complexing YHREEs could be, beside carbonate species, humic and/or fulvic acids. Conclusions Rare earth elements and yttrium analysed in mineral and thermal waters from Tertiary aqui- fers in the Mura Basin occur in concentrations that are far below (10-2 to 10-4) the abundances in the aquifer sediments. Mineral and thermal wa- ters are carbonate waters, and the concentrations of YREEs and their PAAS-normalised patterns strongly depend on the YREE solubility, compl- exation reactions and the duration of water-rock interaction. In aquifers with moderate to high permeability and high flow rates YREE concen- trations in water indicate lower degree of equi- libration with the aquifer sediment. The plots of YREE concentrations normalised to PAAS show fractionation of YHREEs over LREEs which could be attributed to the higher solubility of YHREEs and their preferential mobilisation into solution by complexation reactions. In low-permeabili- ty aquifers the retention times and water-sedi- ment interactions are longer and equilibration reactions more advanced as the fractionation of YHREEs over LREEs disappears. The same trend has been recognised for the positive Eu anoma- ly which is outstanding in cold mineral waters and thermal waters from Thermal I, and weak in the waters from low-permeability aquifers in the Haloze, Špilje and Lendava Formations. In the Sob-1, uncommonly high concentra- tions of YREEs and their specific PAAS-normal- ised plots indicate the well overexploitation and the consequent leakage from low-permeability clayey and silty lenses rich in organic matter and coal. Here, the YHREE complexing ligands could be, beside carbonate species, humic and/or fulvic acids. Acknowledgements The Slovenian Research Agency ARRS is greatly acknowledged for granting the research (Geotermalna energija - Geothermal Energy L2-7062 and Programme Mineral Resources P-0025). Ms. Staška Čertalič, Geological Survey of Slovenia, is kindly appreciated for technical support. References Aide, M. 2018: Lanthanide soil chemistry and its importance in understanding soil pathways: mobility, plant uptake, and soil health. In: Awwad, N.S. & Mubarak, A.T. (eds.): Lanthanides, IntechOpen. https://doi. org/10.5772/intechopen.79238 Anders, E. & Gravesse, N. 1989: Abundance of elements: meteoritic and solar. Geochim. Cosmochim. Acta 53:197-21. Bau, M. 1991: Rare-earth element mobility du- ring hydrothermal and metamorphic fluid- -rock interaction and the significance of the oxidation state of europium. Chem. Geol., 93: 219-230. Bau, M. 1999: Scavenging of dissolved yttrium and rare earths by precipitating iron hydroxi- des: Experimental evidence for Ce oxidation, Y-Ho fractionation, and lanthanide tetrad effect. Geochim. Cosmochim. Acta, 63: 67-77. Bau, M. & Dulski, P. 1999: Comparing yttrium and rare earths in hydrothermal fluids from Mid-Atlantic Ridge: implications for Y and REE behaviour during near-vent mixing and for Y/Ho ratio of Proterozoic seawater. Chem. Geol., 155: 77-90. 88 Polona KRALJ Bau, M., Schmidt, K., Pack, A., Bendel, V. & Kraemer, D. 2018: The European Shale: an improved data set for normalisation of rare earth element and yttrium concentrations in environmental and biological samples from Europe. Appl. Geochem., 90: 142-149. Biddau, R., Bensimon, M., Cidu, R. & Parriaux, A. 2009: Rare earth elements in groundwater from different Alpine aquifers. Chem. Erde, 69: 327-339. Bilal, B.A. 1991: Thermodynamic study of Eu3+/ Eu2+ redox reaction in aqueous solutions at elevated temperatures and pressures by me- ans of cyclic voltammetry. Z. Naturforschung, 46a:1108-1116. Bräuer, K., Geissler, W.H., Kämpf, H., Niedermann, S. & Rman, N. 2016: Helium and carbon isotope signatures of gas exhalations in the westernmost part of the Pannonian Basin (SE Austria/NE Slovenia): Evidence for active mantle degassing. Chem. Geol., 422: 60-70. Braun, J.J., Viers, J., Dupré, B., Polve, M., Ndam, J. & Muller, J.P. 1998: Solid/liquid REE fra- ctionation in the lateritic system of Goyum, East Cameroon: the implications for the pre- sent dynamics of the soil covers of the humid tropical regions. Geochim. Cosmochim. Acta, 62: 273-299. Brookins, D.G. 1989: Aqueous chemistry of rare earth elements. In: Lipin, B.R. & McKay, G.A. (eds.): Geochemistry and mineralogy of rare earth elements. Rev. Mineral., 21: 201-223. Cantrell, K.J. & Byrne, R.H. 1987: Rare earth ele- ment complexation by carbonate and oxalate ions. Geochim. Cosmochim. Acta, 51: 597-605. Coryell, C.D., Chase, J.W. & Winchester, J.W. 1963: A procedure for geochemical interpre- tation of terrestrial rare earth abundance pa- tterns. J. Geophys. Res., 68: 559-566. DeBaar, H.J.W., Bacon, M.P., Brewer, P.G. & Bruland, K.W. 1985: Rare earth elements in the Pacific and Atlantic oceans. Geochim. Cosmochim. Acta, 49: 1943-1954. DeBaar, H.J.W., German, C.R., Elderfield, H. & Van Gaans, P. 1988: Rare earth element di- stribution in anoxic waters of the Cariaco Trench. Geochim. Cosmochim. Acta, 52: 1203-1219. Dia, A., Gruau, G., Olivié-Lauquet, G., Riou, C., Molénat, J. & Curmi, P. 2000: The di- stribution of rare earth elements in gro- undwaters: Assessing the role of sour- ce-rock composition, redox changes and colloidal particles. Geochim. Cosmochim. Acta, 64/24: 4131-4151. https://doi.org/10.1016/ S0016-7037(00)00494-4 Douville, E., Bienvenu, P., Charlou, J.L., Donval, J.P., Fouquet, Y., Appriou, P. & Gamo, T. 1999: Yttrium and rare earth elements in fluids from various deep-sea hydrothermal systems. Geochim. Cosmochim. Acta, 63/5: 627-634. Douville, E., Charlou, J.L., Oelkers, E.H., Bienvenu, P., Jove Colon, C.F., Donval, J.P., Fouquet, Y., Prieur, D. & Appriou, P. 2002: The rainbow vent fluid (36° 14´N, MAR): the influ- ence of ultramafic rocks and phase separation on trace metal content in Mid-Atlantic Ridge hydrothermal fluids. Chem. Geol., 184: 37-48. https://doi.org/10.1016/s0009-2541(01)00351-5 Drever, J.I. 1997: The geochemistry of natu- ral waters: surface and groundwater envi- ronments. 3rd edn., Prentice-Hall Inc., Upper Saddle River, NJ. Duddy, I.R. 1980: Redistribution and fractionati- on of the rare-earth and other elements in a weathering profile. Chem. Geol., 30: 363-381. Elderfield, H. & Greaves, M.J. 1982: The rare earth elements in sea water. Nature, 296: 214-219. Fee, J.A., Gaudette, H.E., Lyons, W.B. & Long, D.T. 1992: Rare earth element distributi- on in Lake Tyrell groundwaters, Victoria, Australia. Chem. Geol., 96: 67-93. Fodor, L., Jelen, B., Márton, E., Rifelj, H., Kraljić, M., Kevrić, R., Márton, P., Koroknai, B. & Báldi-Beke, M. 2002: Miocene to Quaternary deformation, stratigraphy and paleoge- ography in Northeastern Slovenia and Southwestern Hungary. Geologija, 45/1: 103- 114. https://doi.org/10.5474/geologija.2002.009 Gabor, L. & Rman, N. 2016: Mofettes in Slovenske gorice, Slovenia. Geologija, 59/2: 155-177. https://doi.org/10.5474/geologija.2016.009. Gonzalez, V., Vignati, D.A.L., Leyval, C. & Giamberini, L. 2014: Environmental fate and ecotoxicity of lanthanides: are they uniform group beyond chemistry? Environ. Int., 71: 148-157. https://doi.org/10.1016/j. envint.2014.06.019 Gosselin, D.G., Smith, M.R., Lepel, E.A. & Laul, J.C. 1992: Rare earth elements in chloride-rich groundwater, Palo Duro Basin, Texas, USA. Geochim. Cosmochim. Acta, 56: 1495-1505. Guo, C., Stetzenbach, K.J. & Hodge, V. 2005: Determination of 56 trace elements in three aquifer-type rocks by ICP-MS and approxi- mation of the relative solubilities for these elements in a carbonate system by water-rock concentration ratios. In: Johannesson, K.H. (ed.): Rare Earth Elements in Groundwater 89Rare earth elements and yttrium in cold mineral and thermal (~30-60 °C) waters from Tertiary aquifers in the Mura Basin... Flow Systems. Water Science and Technology Library, 51: 39-65. Hatje, V., Bruland, K.W. & Flegal, A.R. 2016: Increases in anthropogenic gadolinium anomalies and rare earth element concen- trations in San Francisco Bay over a 20 year record. Environ. Sci. Technol., 50/8: 4159- 4168. https:/doi.org/10.1021/acs.est.5b04322 Hissler, C., Stille, P., Guignard, C., Iffly, J.F. & Pfister, L. 2014: Rare earth elements as hydro- logical tracers of anthropogenic and critical zone contributions: a case study at the Alzette river basin scale. Procedia Earth Planet. Sci., 10: 349-352 INA-Projekt Zagreb OOUR KGI & Geološki zavod Ljubljana TOZD GGG 1991: Raziskave nafte in plina v Sloveniji, Vol 1: Murska Depresija (Exploration of oil and gas in Slovenia, Vol. 1: The Mura Depression). Geološki zavod Ljubljana, Ljubljana: 406 p. Jelen, B. & Rifelj, H. 2011: Površinska litostrati- grafska in tektonska strukturna karta obmo- čja T-JAM projekta, severovzhodna Slovenija 1:100,000 = Surface lithostratigraphic and tectonic map of T-JAM project area, northe- astern Slovenia 1:100,000. Geological Survey of Slovenia, Ljubljana. https://www.geo-zs. si/PDF/LitostratigrafskaTektonskaKarta/ Karta_T_Jam_slv.pdf Jelen, B. & Rifelj, H., Bavec, M. & Rajver, D. 2006: Opredelitev dosedanjega konceptual- nega geološkega modela Murske depresije = Definition of current conceptual geological model of the Mura depression in Slovenian. Archive of Geological Survey of Slovenia, Ljubljana. Johannesson, K.H. & Lyons, W.B. 1996: The rare earth element geochemistry of Mono Lake water and the importance of carbonate com- plexing. Limnol. Oceanogr., 39: 1141-1154. Johannesson, K.H. & Zhou, X. 1999: Origin of middle rare earth element enrichments in acid waters of a Canadian High Arctic Lake. Geochim. Cosmochim. Acta, 63/1: 153-165. Johannesson, K.H., Stetzenbach, K.J., Hodge, V.F.& Lyons, W.B. 1996: Rare earth element complexation behaviour in circumneutral pH groundwaters: Assessing the role of carbo- nate and phosphate ions. Earth Planet. Sci. Lett., 139: 305-319. Johannesson, K.H., Stetzenbach, K.J. & Hodge, V.F. 1997: Rare earth elements as geochemi- cal tracers of regional groundwater mixing. Geochim. Cosmochim. Acta, 61/17: 3605-3618. Johannesson, K.H., Zhou, X., Guo, C., Stetzenbach, K.J. & Hodge, V.F. 2000: Origin of rare earth element signatures in groundwaters of cir- cumneutral pH from southern Nevada and eastern California. Chem. Geology, 164: 239-257. Johannesson, K.H., Cortes, A., Ramos Leal, J.A., Ramirez, A.G. & Durazco, J. 2005: Geochemistry of rare earth elements in groundwaters from a rhyolite aquifer, cen- tral Mexico. In: Johannesson, K.H. (ed.): Rare Earth Elements in Groundwater Flow Systems. Water Science and Technology Library, 51: 188-222. Klinkhammer, G.P., Elderfield, H., Edmond, J.M. & Mitra, A. 1994: Geochemical implications of rare earth element patterns in hydrother- mal fluids from mid-ocean ridges. Geochim. Cosmochim. Acta., 58/23: 5105-5113. Koščec, J. & Jovanović, M. 1968: Litostratigrafski profili tercijarnog kompleksa u dubokim bušo- tinama Murske potoline = Lithostratigraphic sections of the Tertiary complex in deep wells of the Mura Basin. (in Croatian). Fond Geol. Dok. INA-Projekt Zagreb, INA-Naftaplin, Zagreb. Kováč, M., Hudáčková, N., Halásová, E., Kováčová, M., Holcová, K., Oszczypko- Clowes, M., Báldi, K., Less, G., Nagymarosy, A., Ruman, A., Klučiar, T. & Jamrich, M. 2017: The Central Paratethys palaeoceanography: a water circulation model based on microfossil proxies, climate, and changes of depositional environment. AGEOS, 9/2: 75-114. Kralj, P. 1993: Optimizacija predterciarnega vi- sokotemperaturnega geotermalnega siste- ma Termal II v severovzhodni in vzhodni Sloveniji = Optimization of pre-Tertiary high- -temperature geothermal system Thermal II in Northeastern and Eastern Slovenia. Geološki zavod Ljubljana, Ljubljana: 56 p. Kralj, P. 1994: Prostorska in ekonomska oce- an izdatnosti predterciarnih vodonosnikov na območju severovzhodne Slovenije, I. faza = Spatial and economic evaluation of capa- city of pre-Tertiary aquifers in the area of Northeastern Slovenia, Phase I. SP Geološki zavod Ljubljana, Ljubljana: 13 p. Kralj, P. 1995: Hidrodinamične in hidrokemij- ske lastnosti termalne vode v geotermalnem sistemu Termal I v Prekmurju, II. Faza = Hydrodynamical and hydrogeochemical cha- racteristics of thermal water in the geother- mal system Thermal I in Prekmurje, Phase II. Geološki zavod Ljubljana, Ljubljana: 26. 90 Polona KRALJ Kralj, P. 2001: Das Thermalwasser-System des Mur-Beckens in Nordost-Slovenien. Mitteil. Ingenieur Hydrogeol., 81: 1-82. Kralj, P. 2004a: Trace elements in medium-tem- perature (40-80°C) thermal waters from the Mura basin (North-Eastern Slovenia). Environ. Geol., 46: 622-629. Kralj, P. 2004b: Chemical composition of low- -temperature (<20-40°C) thermal waters in Slovenia. Environ. Geol., 46: 635-642. Kralj, P. 2010: Eruptive and sedimentary evo- lution of the Pliocene Grad Volcanic Field, North-east Slovenia. J. Volcanol. Geoth. Res., 201: 272-284. Kralj, P. & Kralj, P. 2000a: Thermal and mineral waters in north-eastern Slovenia. Environ. Geol., 39: 488-500. Kralj, P. & Kralj, P. 2000b: Overexploitation of geothermal wells in Murska Sobota, Northeastern Slovenia. In: Proceedings World Geothermal Congress 2000, 837-842. Kralj, P. & Kralj, P. 2009: Rare earth elements in thermal water from the Sob-1 well, Murska Sobota, NE Slovenia. Environ. Earth. Sci., 59: 5-13. https://doi.org/10.1007/ s12665-009-0006-8 Kralj, P. & Kralj, P. 2012: Geothermal waters from composite clastic sedimentary reser- voirs: geology, production, overexploitation, well cycling and leakage – a case study of the Mura Basin (SW Pannonian Basin). In: Yang, J. (ed.): Geothermal energy, technology and geology. Nova Science Publishers Inc., New York: 47-91. Kralj, P. & Kralj, P. 2020: Temporal variati- on of the halide ions (F−, Cl−, Br−, I−) in me- dium-temperature (46–52 °C) thermal waters from the Sob-1 and Sob-2 wells, the Mura Basin, north-eastern Slovenia. Environ. Earth Sci. 79, 283. https://doi.org/10.1007/ s12665-020-09034-y Kralj, P., Eichinger, L. & Kralj, P. 2009: The Benedikt hydrothermal system (north-eastern Slovenia). Environ. Geol., 58/8: 1653-1661. https://doi.org/10.1007/s00254-008-1631-3 Kulaksiz, S. & Bau, M. 2011: Rare earth ele- ments in the Rhine river, Germany: first case of anthropogenic lanthanum a dissolved mi- crocontaminant in the hydrosphere. Environ. Int., 37/5: 973-979. Ladonin, D.V. 2019: Lantanides in soils of the Cherepovets steel mill. In: Awwad, N.S. & Mubarak, A.T. (eds.): Lanthanides. IntechOpen. https://doi.org/10.5772/ intechopen.80294 Lee, J.H. & Byrne, R.H. 1993: Complexation of trivalent rare earth elements (Ce, Eu, Gd, Tb, Yb) by carbonate ions. Geochim. Cosmochim. Acta, 57: 295-302. Liu, H., Guo, H. & Wu, L. 2017: Rare earth ele- ments as indicators of groundwater mixing in the North China Plain: A case study in the area of Hengshui city, China. Procedia Earth Planet. Sci., 17: 396-399. Liu, X. & Byrne, R.H. 1998: Comprehensive in- vestigation of yttrium and rare earth element complexation by carbonate ions using ICP- Mass spectrometry. J. Sol. Chem., 27: 803-815. Luo, Y.R. & Byrne, R.H. 2004: Carbonate com- plexation of yttrium and the rare earth elements in natural waters. Geochim. Cosmochim. Acta, 68: 691-699. Malvić, T. & Velić, J. 2011: Neogene tectonics in Croatian part of the Pannonian Basin and re- flectance in hydrocarbon accumulations. In: Schattner, U. (ed.): New frontier in tectonic research – at the midst of plate convergence, 215-238. InTech, Rijeka, Croatia. https://doi. org/10.5772/21270 Márton, E., Fodor, L., Jelen, B., Márton, P., Rifelj, H. & Kevrić, R. 2002: Miocene to Quaternary deformation in NE Slovenia: complex pale- omagnetic and structural study. J. Geodyn., 34/5: 627-651. https://doi.org/10.1016/ S0264-3707(02)00036-4 McLennan, S.M. 1989: Rare earth elements in sedimentary rocks: influence of provenance and sedimentary processes. In: Lipin, B.R. & McKay, G.A. (eds.): Geochemistry and mine- ralogy of rare earth elements. Rev. Mineral., 21: 169-196. Michard, A. 1989: Rare earth element syste- matics in hydrothermal fluids. Geochim. Cosmochim. Acta, 53: 745-750. Michard, A. & Albarede, F. 1986: The REE con- tent in some hydrothermal fluids. Chem. Geology, 55: 51-60. Michard, A., Beaucaire, C. & Michard, G. 1987: Uranium and rare earth elements in CO2- rich waters from Vals-les-bains (France). Geochim. Cosmochim. Acta, 51: 901-909. van Middlesworth, P.E. & Wood, S.A. 1998: The aqueous geochemistry of the rare earth ele- ments and yttrium. Part 7. REE, Th and U contents in thermal springs associated with the Idaho batholith. Appl. Geochem., 13/7: 861-884. https://doi.org/10.1016/ S0883-2927(98)00019-5 Möller, P. 2000: Rare earth elements and yttrium as geochemical indicators of the source of 91Rare earth elements and yttrium in cold mineral and thermal (~30-60 °C) waters from Tertiary aquifers in the Mura Basin... mineral and thermal waters. In: Stober, I., Bucher, K. (eds): Hydrology of crystalline rocks, Kluwer Academic Press, 227-246. Möller, P., Dulski, P., Bau, M., Knappe, A., Pekdeger, A. & Sommer-von Jarmersted, C. 2000: Anthropogenic gadolinium as a con- servative tracer in hydrology. J. Geochem. Explor., 69-70: 409-414. Möller, P., Dulski, P. & Morteani, G. 2003: Partitioning of rare earth elements, yttrium and selected major elements among source rocks, liquid and steam of the Larderello- Travale Geothermal Field (Tuscany, Central Italy). Geochim. Cosmochim. Acta, 67: 171-183. Möller, P., Rosenthal, E., Geyer, S., Guttman, J., Dulski, P., Rybakov, M., Zilberbrand, M., Jahnke, C. & Flexer, A. 2007: Hydrochemical processes in the Lower Jordan valley and in the Dead Sea area. Chem. Geology, 239: 27-49. MURmap (2022-2024) Hollistic geochemical tracking of inorganic pollutants in the Mur/ Mura River catchment https://www.geo-zs. si/index.php/projekti/drugi-projekti; https:// aach.unileoben.ac.at/en/news Nádor, A., Lapanje, A., Tóth, G., Rman, N., Szőcs, T., Prestor, J., Uhrin, A., Rajver, D., Fodor, L., Muráti, J. & Székely, E. 2012: Transboundary geothermal resources of the Mura-Zala basin: a need for joint thermal aquifer management of Slovenia and Hungary. Geologija, 55/2: 209- 224. https://doi.org/10.5474/geologija.2012.013 Nigro, A., Sappa, G. & Barbieri, M. 2018: Boron isotopes and rare earth elements in the grou- ndwater of a landfill site. J. Geochem. Explor., 190: 200-206. https://doi.org/10.1016/j. gexplo.2018.02.019 Piller, W.E., Harzhauser, M. & Mandic, O. 2007: Miocene Central Paratethys stratigraphy – current status and future directions. Stratigraphy, 4/2: 151-168. Pleničar, M. 1968: Basic geological map SFR Yugoslavia, scale 1:100,000, Sheet Goričko and Leibnitz. Federal geological survey, Belgrade. Rajver, D., Kralj, P., Žlebnik, L., Drobne, F. & Kranjc, S. 1994: Program za učinkovito rabo energije in obnovljivih virov energije, Pridobivanje energije iz obnovljivih virov, Prostorska in ekonomska ocena izdatnosti predterciarnih vodonosnikov na območju se- verovzhodne Slovenije = Programme for effi- cient utilization of energy and renewable re- sources, Extraction of energy from renewble resources, Spatial and economical evaluation of capactiy of pre-Tertiary aquifers in north- -eastern Slovenia. Internal report. Archive of Geological Survey of Slovenia, Ljubljana. Ravnik, D., Rajver, D., Žlebnik, L. & Kralj, P. 1992: Geološke strukture: viri termalnih in mineralnih vod v Sloveniji = Geological structures: the resources of thermal and mi- neral waters in Slovenia. In: Kralj, P. (ed.): Mineralne in termalne vode v gospodarstvu in znanosti Slovenije = Mineral and thermal waters in economy and science of Slovenia. Geološki zavod Ljubljana, Ljubljana: 9-32. Rijavec, L., Bistričić, A. & Jenko, M. 1985: Mura Basin. In: Steininger, F.F., Senes, J., Kleemann, K. & Rogl, F. (eds.): Neogene of the Mediterranean Tethys and Paratethys, Vol 1, 2, Institute of Paleontology, University of Vienna, Vienna. Rman, N. 2014: Analysis of long-term thermal water abstraction and its impact on low- -temperature intergranular geothermal aqu- ifers in the Mura-Zala basin, NE Slovenia. Geothermics, 55: 214-227. https://doi. org/10.1016/j.geothermics.2014.01.011 Rotár-Szalkai, A., Nádor, A., Szőcs, T., Maros, G., Goetzl, G. & Zekiri, F. 2017: Outline and joint characterization of Transboundary ge- othermal reservoirs in the western part of the Pannonian basin. Geothermics 70: 1-16. https:// doi.org/10.1016/j.geothermics.2017.05.005 Royden, L.H. 1988: Late Cenozoic tectonics of the Pannonian basin system. In: Royden, L.H. & Horvath, F. (eds.): The Pannonian Basin, a study in basin evolution. Am. Assoc. Pet. Geol. Mem., 45:1-16. Shand, P., Johannesson, K.H., Chudaev, O., Chudaeva, V. & Edmunds, W.M. 2005: Rare earth element contents of high pCO2 gro- undwaters of Primorye, Russia: Mineral stability and complexation controls. In: Johannesson, K.H. (ed.): Rare earth elements in groundwater flow systems. Water Science and Technology Library, 51: 161-186. Shannon, W.M. & Wood, S.A. 2005: The analysis of picogram quantities of rare earth elements in natural waters. In: Johannesson, K.H. (ed.): Rare Earth Elements in Groundwater Flow Systems. Water Science and Technology Library, 51: 1-37. Shannon, W.M., Wood, S.A., Brown, K. et al. 2001: Behavior of rare eath elements in geothermal sytems: A new exploration/exploitation tool? Exploration Technology, Geothermal Energy R&D Program, 2-31. 92 Polona KRALJ Smedley, P. 1991: The geochemistry of rare earth elements in groundwater from the Carnmenellis area, southwest England. Geochim. Cosmochim. Acta, 55: 2767-2779. Sverjensky, D.M. 1984: Europium redox equili- brium in aqueous solutions. Earth Planet. Sci. Lett., 67: 70-78. Szocs, T., Rman, N., Süveges, M., Palcsu, L., Tóth, G. & Lapanje, A. 2013: The applicati- on of isotope and chemical analyses in ma- naging transboundary groundwater resou- rces. Appl. Geochem., 32: 95-107. https://doi. org/10.1016/j.apgeochem.2012.10.006 Szőcs, T., Rman, N., Rotár-Szalkai, Á., Tóth, G., Lapanje, A., Černák, R. & Nádor, A. 2018: The Upper Pannonian thermal aquifer: cross- -border cooperation as an essential step to transboundary groundwater management. J. Hydrol., 20: 128-144. https://doi.org/10.1016/j. ejrh.2018.02.004 Šram, D., Rman, N., Rižnar, I. & Lapanje, A. 2015: The three-dimensional regional geological model of the Mura-Zala Basin, northeastern Slovenia. Geologija, 58/2: 139-154. https://doi. org/10.5474/geologija.2015.011 Tóth, G., Rman, N., Rotár-Szalkai, Á., Kerékgyártó, T., Szőcs, T., Lapanje, A., Černák, R., Remsík, A., Schubert, G. & Nádor, A. 2016: Transboundary fresh and thermal groundwater flows in the west part of the Pannonian Basin. Renew. Sustain. Energy Rev. 57: 439-454. https://doi.org/10.1016/j. rser.2015.12.021 Tweed, S.O., Weaver, T.R., Cartwright, I. & Schaefer, B. 2006: Behaviour of rare earth ele- ments in groundwater during flow and mixing in fractured rock aquifers: an example from the Dandenong ranges, southeast Australia. Chem. Geol., 234: 291-307. Wood, S. 1990: The aqueous geochemistry of ra- re-earth elements and yttrium. Theoretical prediction of speciation in hydrothermal so- lutions to 350 °C at saturation water vapor pressure. Chem. Geol., 88: 99-125. Wood, S.A. 2006: Rare earth element systematics of acidic geothermal waters from the Taupo Volcanic Zone, New Zealand. J. Geochem. Explor., 89: 424-427. Yuan, J., Mao, X., Wang, Y., Deng, Z. & Huang, L. 2014: Geochemistry of rare-earth ele- ments in shallow groundwater, northe- astern Guangdong Province, China. Chin. J. Geochem., 33: 53-64. https://doi.org/10.1007/ s11631-014-0659-1 Zhou, X., Stetzenbach, K.J., Yu, Z. & Johannesson, K.H. 2005: Origin of rare earth element signa- tures in groundwaters of South Nevada, USA: Implications from preliminary batch leach te- sts using aquifer rocks. In: Johannesson, K.H. (ed): Rare Earth Elements in Groundwater Flow Systems. Water Science and Technology Library, 51: 141-160. Zhu, Y., Inagaki, K., Haraguchi, H. & Chiba, K. 2010: On-line elution of iron hydroxide copre- cipitate carrier for determination of REEs in natural water by mix-gas ICP-MS. J. Anal. At. Spectrom., 25: 364-369. Žlebnik, L., Kralj, P. & Kokol, L. 1988a: Termalne vode na območju Murske Sobote, vrtina Sob-1 = Thermal waters in the Murska Sobota area, The Sob-1 well. Geološki zavod Ljubljana, Ljubljana: 1-24. Žlebnik, L., Žižek, D., Kralj, P. & Kokol, L. 1988b: Termalne vode na območju Murske Sobote = Thermal waters in the Murska Sobota area. Geološki zavod Ljubljana, Ljubljana: 1-42.