GEOLOGIJA 59/1, 7-22, Ljubljana 2016 http://dx.doi.org/10.5474/geologija.2016.001 © Author(s) 2016. CC Atribution 4.0 License Hydrogeochemistry and isotope geochemistry of Velenje Basin groundwater Hidrogeokemija in izotopska geokemija podzemnih vod Velenjskega bazena Tjaša KANDUČ1, Nataša MORI2, Ajda KOCELI3 & Timotej VERBOVŠEK4 Jožef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia; e-mail: tjasa.kanduc@ijs.si 2National Institute of Biology, Večna pot 111, SI-1000 Ljubljana, Slovenia; e-mail: natasa.mori@nib.si 3Podvin 205, SI-3310 Žalec; e-mail: ajda.koceli@gmail.com 4Faculty of Natural Sciences and Engineering, Department of Geology, Privoz 11, SI-1000, Ljubljana, Slovenia; e-mail: timotej.verbovsek@ntf.uni-lj.si Prejeto / Received 25. 1. 2016; Sprejeto / Accepted 11. 4. 2016; Objavljeno na spletu / Published online 11. 8. 2016 Key words: hydrogeochemistry, C isotopes, Sr isotopes, groundwater, Velenje Basin, Slovenia Ključne besede: hidrogeokemija, C izotopi, Sr izotopi, podzemna voda, Velenjski bazen, Slovenija Abstract The geochemical and isotopic composition of groundwater in the Velenje Basin, Slovenia, was investigated between the years 2014 to 2015 to identify the geochemical processes in the major aquifers (Pliocene and Triassic) and the water-rock interactions. Thirty-eight samples of groundwater were taken from the aquifers, 19 in the mine and 19 from the surface. Groundwater in the Triassic aquifer is dominated by HCO3-, Ca2+ and Mg2+ with ¿13Cdic values in the range from -19.3 to -2.8 %o, indicating degradation of soil organic matter and dissolution of carbonate minerals. In contrast, groundwater in the Pliocene aquifers is enriched in Mg2+, Na+, Ca2+, K+, and Si, and has high alkalinity, with ¿13CDIC values in the range of -14.4 to +4.6 %0. Based on the ¿13CDIC values in all the aquifers (Pliocene and Triassic), both processes influence the dissolution of carbonate minerals and dissolution of organic matter and in the Pliocene aquifers, methanogenesis as well. Based on Principal Component Analysis (PCA), and on geochemical and isotopic data we conclude that the following types of groundwater in Velenje Basin are present: Triassic aquifers with higher pH and lower conductivity and chloride, Pliocene, Pliocene 1 and Pliocene 2 aquifers with lower pH and higher conductivity and chloride contents, and Pliocene 3 and Pliocene 2, 3 aquifers with the highest pH values and lowest conductivities and chloride contents. 87Sr/86Sr tracer was used for the first time in Slovenia to determine geochemical processes (dissolution of silicate versus carbonate fraction) in Velenje Basin groundwater of different aquifers dewatering Pliocene and Triassic strata. 87Sr/86Sr values range from 0.70820 to 0.71056 in groundwater of Pliocene aquifers and from 0.70808 to 0.70910 in groundwater of the Triassic aquifer. This indicates that dissolution of the carbonate fraction prevails in both aquifers, while in Pliocene aquifers, an additional silicate weathering prevails with higher 87Sr/86Sr isotope ratios. Izvle~ek Raziskali smo geokemično in izotopsko sestavo podzemnih vod v Velenjskem bazenu, za določitev geokemičnih procesov v glavnih vodonosnikih (pliocenskih in triasnem) in interakcij voda-kamnina med letoma 2014 in 2015. Osemintrideset vzorcev podzemnih vod je bilo odvzetih iz vodonosnikov, od tega 19 iz jame in 19 s površine. V podzemni vodi triasnega vodonosnika prevladujejo ioni HCO3-, Ca2+, Mg2+ , ¿13Cdic vrednosti pa se gibljejo od -19.3 do -2.8 %o, kar kaže na razgradnjo organske snovi in raztapljanje karbonatnih mineralov. Podzemna voda pliocenskega vodonosnika je obogatena z Mg2+, Na+, Ca2+, K+ in Si in ima višje alkalnosti ter vrednosti ¿13CDIC v razponu od -14.4 do +4.6 %0. Na ¿13CDIC vrednosti v vseh vodonosnikih (pliocenskih in triasnem) vplivata raztapljanje karbonatnih mineralov in razgradnja organske snovi, v pliocenskih vodonosnikih pa še dodatno metanogeneza. Na osnovi analize glavnih osi (PCA) lahko zaključimo, da imamo v Velenjskem bazenu na osnovi geokemičnih in izotopskih podatkov sledeče tipe podzemnih vod: triasni vodonosnik z višjimi vrednostmi pH in nižjo elektroprevodnostjo ter klorom, pliocenski, pliocenski 1 in pliocenski 2 vodonosniki z nižjim pH in višjo elektroprevodnostjo in vsebnostjo klora, in pliocenski 3 in pliocenski 2, 3 vodonosniki z višjim pH in nizko elektroprevodnostjo in vsebnostjo klora. V tej študiji smo uporabili nov sledilec izotopskega razmerja 87Sr/86Sr, katerega vrednosti se gibljejo od 0,70820 do 0,71056 v pliocenskem vodonosniku in od 0,70808 do 0,70910 v triasnem vodonosniku. 87Sr/86Sr kaže, da v obeh vodonosnikih prevladuje raztapljanje karbonatov, intenzivnejše pa je raztapljanje silikatnih mineralov v pliocenskem vodonosniku z višjim izotopskim razmerjem 87Sr/86Sr. 8 Tjasa KANDUC, Natasa MORI, Ajda KOCELI & Timotej VERBOVSEK Introduction The Velenje Basin in Slovenia is one of the largest actively mined coal basins in central Europe, producing around 4 million tons of lignite per year (Mihelak, 2010). Large amounts of groundwater are extracted from Velenje Basin aquifers to facilitate underground mining of coal, and coal seam gas outbursts are a serious mine safety concern (Kanduc et al., 2011; Kanduc et al., 2015a). The main problem in the past production years of Velenje lignite mine has been the groundwater standing directly over the main production lignite strata inside the Pliocene's clastic rocks. In the 1970's many groundwater intrusions occurred in the mine. To reduce the hydrostatic pressure during the last four decades, advanced and massive dewatering works have been performed. As a result, constant water pumping in the production area and its surroundings has produced a typical dewatering groundwater cone (Vizintin et al., 2009). Hydrogeochemistry and stable isotope compositions of groundwater provide critical information regarding water-rock interaction along flow paths, and mixing of distinct groundwater bodies (Cartwright et al., 2012). Groundwater geochemistry, especially where aquifers have contrasting mineralogy, is an effective method of determining regional scale inter-aquifer flow and patterns of recharge (Dogramaci & Herczeg, 2002; Edmunds, 2009; Cartwright, 2010). In this study we use major ion geochemistry and environmental isotopes to constrain geochemical processes in groundwater of Velenje Basin. In addition, characterization of geochemical properties of groundwater may further aid in understanding of hydrocarbon entrapment and mineral deposition, and help in identifying the origin of fluids (Aravena et al., 2003). Concentrations of dissolved inorganic carbon (DIC) and its isotopic composition (¿13Cdic) are governed by processes occurring in the water system. Changes of dissolved inorganic carbon concentrations result from addition or removal of carbon from the DIC pool (Atekwana & Krishnamurthy, 1998). The major sources of carbon to the groundwater DIC system are the dissolution of carbonate minerals, and soil CO2 derived from root respiration and from microbial decomposition of organic matter. Constraining water-rock interaction, evapotranspiration, dissolution and precipitation of minerals, ionexchange and groundwater mixing are all required to understand and manage hydrogeological systems (Cartwright et al., 2007). The use of strontium isotopes in hydro-geological studies has many applications. First, minerals that interact with groundwater and surface water exhibit a wide and predictable range of 87Sr/86Sr ratios. 87Sr is produced by the decay of 87Rb with a half-life of 48.8 Ga (Faure, 1991). Strontium derived from K-rich minerals such as biotite and K-feldspar, have high 87Sr/86Sr ratios, minerals such as plagioclase that have lower K/Ca ratios will contain strontium with moderate 87Sr/86Sr ratios, while the strontium in Ca-rich minerals such as calcite or gypsum, have low 87Sr/86Sr ratios that remain essentially unchanged over time (Faure, 1991; Mcnutt, 2000). The geochemical and isotopic composition of carbon and 87Sr/86Sr isotopes of groundwater in the Velenje Basin, combined with mineralogical information, are applied to investigate the groundwater-rock interactions and geochemical processes that contribute to understanding the generation and source of coal seam gases, namely carbon dioxide and methane (Kanduč et al., 2012; Sedlar et al., 2014; Kanduč et al., 2015A). Study area The Velenje Basin and associated coal mines are situated in the NE part of Slovenia (Fig. 1). It is situated at the junction of the WNW-ESE trending Šoštanj fault and the E—W trending Periadriatic zone, bounded to the south by the Smrekovec fault segment. The Šoštanj and Smrekovec faults were generated by the collision of continental plates. In general, two hydrogeological systems can be distinguished in the Velenje coal mine (Fig. 1): (1) Plioquaternary and Pliocene aquifers, each composed of gravel, sand and silt, which are further divided to upper, middle and lower aquifers. Below the latter aquifers, Triassic carbonates are present as a basement of the basin. The upper aquifers are schematically divided into Plioquaternary alluvial sediments (not analyzed in this study) and at least three parts of the Pliocene aquifers (Pliocene 1, Pliocene 2, Pliocene 3) and (2) carbonate aquifers, which are further divided into Lower Triassic (Scythian) limestone and dolomite, Anisian dolomite and limestone, and Miocene Lithotamnium limestone (Vižintin et al., 2009 & references therein). Pliocene and Pliocene 2, 3 aquifers were introduced due to the fact that in some cases, it was not possible to attribute the water to a single aquifer, so a combination of all (Pliocene) or two aquifers (Pliocene 2, 3) was used instead. Hydrogeological properties of these dolomites are presented in detail in the works of Verbovšek & Veselič (2008) and Verbovšek (2008a, 2008b). Groundwater recharging the Velenje Basin is drained by hanging filters to prevent inrush of water into the mine. The average discharge of water from Pliocene sands is 800 l/min, while that from Triassic limestone aquifers is 3400 l/ min (Supovec et al., 2012a, 2012b). In the Velenje coal basin, the shallow aquifers are part of the Quaternary and Pliocene sections. Pliocene aquifers are further classified as aquifers above the coal 0-20 m (Pl-1), aquifers 20-80 m above the coal, Pl-2, and upper Pliocene aquifers (Pl-3) (Fig. 1). Hydrogeochemistry and isotope geochemistry of Velenje Basin groundwater 9 Fig. 1. Geological sketch map (BREZIGAR et al., 1988) of the Velenje Basin with a NNE-SSW cross section. Main aquifers are Pliocene (Pl1, Pl2 and Pl3), and Triassic (T1, T2 and T2 3) dolostones (prevailing) and limestones. Other Triassic and Paleozoic (Pz) lithologies are composed of relatively impermeable strata and therefore do not include significant aquifers. Profiles A-B and C-D show the locations of groundwater sampling in the study of Kanduc et al. (2014). Materials and methods In a previous study (Kanduc et al., 2014), only 14 groundwater samples were taken seasonally in Velenje Basin for geochemical and isotope characterization. Due to complex groundwater flow paths in the Velenje Basin, a more dense sampling network was established for investigation, in detail of spatial variability in hydrogeochemistry and isotope geochemistry. Location of groundwater sampling location is presented on Figure 2. In this study, following locations of groundwater dewatering Triassic and Pliocene strata were sampled: groundwater dewatering Triassic strata (15 locations), Pliocene 2 (10 locations), Pliocene (1 location), Pliocene 1 (9 locations), Pliocene 3 (2 locations), Pliocene 2, 3 (1 location) (Fig. 2). Samples were taken from: a) overflow piezometers from the surface, b) groundwater samples sampled with Solinst sampler and c) mine wells. Groundwater samples were taken wherever possible since the complete Velenje Basin is subsiding due to mining activity and piezometers are sheared. Altogether 38 samples were taken, 19 from the mine and 19 from the surface. Temperature, pH and electrical conductivity were measured in the field. Some groundwater samples (sampling b) were sampled with a Discrete Solinst Sampler (Kanduc et al., 2014, 2015a), modified for depth measurements and for sampling from depths to almost 400 m. Sample aliquots collected for 10 Tjasa KANDUC, Natasa MORI, Ajda KOCELI & Timotej VERBOVSEK Fig. 2. Sampling locations (38 samples) of groundwater between years 2014 and 015 and located in the Velenje Basin: (a) overflow piezometers (red color) (b) groundwater sampling from the surface of Pliocene aquifers (green, yellow, and brown) and (c) from mining wells (purple color). chemical analysis (major, trace elements, 87Sr/86Sr) were immediately passed through 0.45 u PTFE membranes, which were leached with 1-2 ml of dilute clean HCl, then 2-4 ml of MilliQ water (Millipore) and finally 1-2 ml of the sample itself before collecting the sample filtrate. For cations, 87Sr/86Sr and trace element analysis, 30 ml LDPE (Nalgene) bottles were used. The bottles were filled with 10 % HCl leaving some headspace and placed in in a zip lock bag in an oven at 70 °C for one day. The bag was then tightened and left for a second day. The bottles were then rinsed repeatedly with MilliQ water, and filled completely with MilliQ water and capped the bottles. They were finally emptied in a dust free place (fume cupboard) and left until dried. Samples for ¿13CdiC analysis were stored in glass vials filled to the top, with no headspace. Total alkalinity was measured within 24 h of sample collection by Gran titration (Gieskes, 1974) with a precision of ±1 %. Major anions (Cl-, SO42- and NO3-) were analyzed (with ± 2 % precision) with a Dionex Ion Chromatograph (IC) Model 3000, using an AS23 analytical column in the Hydrology and Water Resources Department at the University of Arizona (UA). Major and trace elements (Na+, K+, Ca2+, Mg2+, Sr2+) in water were determined using a quadrupole inductively coupled plasma mass spectrometer (ICP-MS, Agilent 7500ce) with a double-pass spray chamber, PTFE 100 L/min nebulizer, platinum cones and sapphire injector within a quartz platinum-shielded torch. The instrument is located in a filtered air positive pressure lab and sample handling and chemistry was routinely performed on laminar flow benches. External calibration curves for all the elements reported were prepared from 1,000 mg/L single element standard solutions in HNO3 (Inorganic Ventures). Both the samples, blanks and the calibration solutions were prepared in 2.4 % HNO3 (BDH Aristair Plus). Standard Reference Material (SRM) 1643e ("Trace Elements in Water" National Institute of Standards and Technology, USA) was prepared at 1:20 dilution and run together with blanks, samples and calibration curve. Indium at a concentration of 20 ppb was added to the calibration curve, reference material, samples and blanks as internal standard. Samples were diluted in 1:5 and run with inductively coupled plasma mass spectrometer (MC-ICPMS) in blocks of five samples, two blanks and one SRM. This analysis were performed at University of Utah. The stable isotope composition of dissolved inorganic carbon (¿13CdiC) was determined on Europa scientific TG isotope ratio mass spectrometer coupled with a ANCA-TG preparation module. Phosphoric acid (100 %) was added (100-200 ul) to a septum tube, then purged with pure He. The water sample (1 ml) was then injected into the septum tube and CO2 measured directly from the headspace. Two stan2 dard solutions of Na2CO3 Hydrogeochemistry and isotope geochemistry of Velenje Basin groundwater 11 (Carlo Erba and Scientific Fisher) with known ¿13Cdic values of -10.8±0.2 %0 and -4.8±0.2%o were used to calibrate ¿13CdiC measurements (Spotl 2005; Kanduč, 2006). This analysis were performed at Jožef Stefan Institute. Isotopic analysis of strontium was performed using an inductively coupled plasma mass spectrometer (MC-ICPMS) Neptune (ThermoFisher Scientific) at the Department of Geology and Geophysics, University of Utah. Purified fractions, with a strontium concentration around 30 ppb, were introduced in the mass spectrometer through a PTFE nebulizer, quartz double-pass spray chamber, quartz torch and nickel cones. Results and discussion Groundwater geochemistry of Velenje Basin groundwater The pH of groundwater in the Triassic aquifers ranged from 6.8 to 8.7 and, in the Pliocene aquifers from 6.6 to 9.7 (Table 1). Dissolved oxygen (DO) was not measured in this study due to risk of explosion (from elevated methane concentration), but it ranges from 10 to 30 % in groundwater dewatering Pliocene strata and from 60 to 80 % in groundwater dewatering Triassic strata (Kanduc et al., 2014). Conductivity ranged from 205.4 to 4410 ^S/cm in groundwater dewatering Pliocene strata, and from Fig. 3. Piper plot showing the major cation and anion geochemistry of the Velenje Basin groundwater. Principal component analysis (PCA), based on correlation matrix was used to examine the variation of selected environmental variables (temperature, pH, conductivity, Na+, K+, Cl-, Mg2+, Ca2+, nitrate (NO3-), sulphate (SO42-) and HCO3- ions and ¿13CdiC) measured in 38 samples ofVelenje Basin groundwater. The CANOCO software package (Ter Braak & Šmilauer, 2002) was applied, and the results presented as an ordination diagram, on which points represent samples and arrows the measured parameters. Chemical speciation of the carbonate system-e.g. partial pressures of CO2 (pCO2), saturation indices of calcite (SIcalcite) - was evaluated using water composition, pH, alkalinity, and temperature as inputs to the PHREEQC speciation program and employing thermodynamic computations (Parkhurst & Appelo, 1999). 232.8 to 4330 ^S/cm in groundwater dewatering Triassic strata (Table 1). Only in one sample (j.v. 3051/01, Fig 2), from dewatering Pliocene strata, was very high conductivity found (Table 1). The very high alkalinities can be contributed to by the methanogenesis process, that occurs in the Pliocene groundwaters and are discussed below. Concentrations of major and minor ions in Velenje Basin groundwater dewatering Pliocene strata and Triassic strata are presented on Figure 3. Groundwater in the Triassic aquifer is dominated by HCO3-, Ca2+, Mg2+ (mostly water types Ca-Mg-HCO3 and Mg-Ca-HCO3), while groundwater in the Pliocene aquifers is enriched in Mg2+, Na+, Ca2+, K+ and Si. Total alkalinity of Pliocene aquifers ranges from 1.95 to 44.1 mM in groundwater dewatering Pliocene strata and from 1.3 to 10.4 mM in groundwater dewatering Triassic strata (Table 1). The highest alkalinity Table 1. List of sampling locations with geochemical parameters (T, pH. composition of Sr2+ and dissolved inorganic carbon (s7Sr/ssSr and <513CDI(,) , conductivity) and selected major ion concentrations (Ca2+, Mg2+, Na+. inVelenje Basin groundwater during the sampling period 2014-2015. K+, Sr2+, HCO,-, CI", NO,-, SO 2 ) and isot.opic Sample number Date of sampling Geology T(°C) Cond (yiS/cm) pH Total alkalinity-(mM) Total alkalinity-(mg/1) Na+ (mM) Na+ (mg/1) Mg2+ (mM) Mg2+ (mg/1) K+ (mM) K+ (mg/1) Ca2+ (mM) Ca2+ (mg/1) Sr2+ (mM) Sr2+ (mg/1) V 12 t/86 1.4.2014 Pliocene 2 19.4 3340 6.63 42.4 2586.4 4.55 104.65 12.65 307.39 0.25 9.77 3.92 157.19 0.013 1.13 3490 1.4.2014 Pliocene 19.7 2305 6.85 28.0 1708 4.93 113.39 7.14 173.50 0.15 5.86 1.05 42.1 0.003 0.26 j.v. 2346 T/84 1.4.2014 Triassic 16.8 933.6 7.35 5.97 364.17 0.10 2.3 1.39 33.77 0.03 1.17 2.17 87.0 0.003 0.26 j.v. 2370 T/88 2.4.2014 Triassic 17.8 455.0 7.32 1.32 80.52 0.05 1.15 0.55 13.36 0.01 0.39 0.65 26.0 0.002 0.17 j.v.2343/83 2.4.2014 Triassic 16.6 759.6 7.35 5.65 344.65 0.095 2.185 1.66 40.33 0.03 1.17 1.67 66.96 0.005 0.43 BV 28/87 1.4.2014 Pliocene 1 17.3 1117 7.00 12.0 732 1.80 41.4 2.48 60.26 0.11 4.30 1.75 70.17 0.005 0.43 j.v.2391 5/2 2.4.2014 Triassic 14.2 581.4 7.38 5.45 332.45 0.14 3.22 1.43 34.74 0.02 0.78 1.21 48.52 0.002 0.17 j.v. 3051/01 1.4.2014 Triassic 26.7 4330 6.81 10.4 634.4 22.02 506.46 4.94 120.0 0.48 18.76 6.69 268.3 0.044 3.85 V 12 v/87 1.4.2014 Pliocene 1 18.7 2244 6.92 27.2 1659.2 2.41 55.43 6.90 167.67 0.20 7.82 3.35 134.3 0.009 0.78 j.v.2391-2 2.4.2014 Triassic 14.1 581.0 7.42 5.42 330.62 0.13 2.99 1.42 34.51 0.02 0.78 1.50 60.2 0.002 0.17 j.v. 2391-1 2.4.2014 Triassic 14.3 581.3 7.42 5.39 328.79 0.13 2.99 1.39 33.77 0.02 0.78 1.19 47.7 0.002 0.17 j.v. 2391-3 2.4.2014 Triasic 14.1 579.8 7.33 5.09 310.49 0.14 3.22 1.48 35.96 0.02 0.78 0.97 38.9 0.002 0.17 j.v. 2341/83 2.4.2014 Triassic 16.1 1094 7.39 6.30 384.3 0.173 3.979 2.06 50.05 0.03 1.17 2.86 114.7 0.004 0.35 BV 29/87 1.4.2014 Pliocene 2 18.5 2298 6.85 27.1 1653.1 3.31 76.13 7.59 184.4 0.23 8.99 0.72 28.9 0.006 0.52 BV 27/87 1.4.2014 Pliocene 2 16.4 900.7 7.22 9.64 588.04 1.43 32.89 1.92 46.65 0.09 3.51 1.62 64.9 0.005 0.43 V12z/86 1.4.2014 Pliocene 2 18.2 2252 6.89 27.1 1653.1 2.51 57.73 6.75 164.0 0.21 8.21 3.48 139.5 0.008 0.70 3491 1.4.2014 Pliocene 19.9 1848 6.80 21.6 1317.6 4.09 94.07 5.63 136.81 0.12 4.69 1.84 73.8 0.006 0.52 BV 26/87 1.4.2014 Pliocene 2 17.7 1200 7.19 14.2 866.2 1.87 43.01 3.25 78.97 0.12 4.69 1.73 69.4 0.006 0.52 j.v. 3378-K/08 1.4.2014 Pliocene 1 19.3 4410 6.88 44.1 2690.1 6.71 154.33 16.63 404.1 0.32 12.51 1.65 66.2 0.007 0.61 PC-5/83 6.6.2014 Triassic 15.7 616.2 8.14 5.26 320.86 5.78 132.94 0.08 1.94 0.06 2.34 0.21 8.42 0.001 0.08 PB-6/86 6.6.2014 Triassic 14.1 851.1 7.00 8.77 534.97 2.86 65.78 0.88 21.38 0.11 4.30 1.96 78.6 0.005 4.38 PH-13/12 6.6.2014 Triassic 16.2 232.8 8.66 1.50 91.5 0.80 18.4 0.53 12.87 0.067 2.61 0.24 9.6 0.0008 0.07 PT-30/98 6.6.2014 Triassic 14.0 469.9 7.80 4.70 286.7 0.41 9.43 1.21 29.4 0.048 1.87 1.35 54.1 0.006 0.52 PC-7/86 6.6.2014 Triassic 16.6 433.4 7.92 4.22 257.42 0.80 18.4 0.61 14.8 0.040 1.56 1.24 49.7 0.005 0.43 PE-9/10 6.6.2014 Triassic 14.9 388.3 7.57 2.74 167.14 0.16 3.68 0.79 19.19 0.063 2.46 1.18 47.3 0.005 0.43 PH-5/81 25.5.2015 Pliocene 2 16.1 1979 7.50 22.6 1378.6 4.95 113.85 3.02 73.38 0.186 7.27 3.27 131.1 0.011 0.96 PM13/84 26.5.2015 Pliocene 3 16.4 311.1 9.68 3.23 197.03 2.00 46 0.04 0.97 0.017 0.66 0.49 19.6 0.0029 0.25 PE-16/84 26.5.2015 Pliocene 2,3 16.4 205.4 9.68 1.95 118.95 1.82 41.86 0.004 0.097 0.02 0.78 0.05 2.0 0.00034 0.029 PG-5/84 26.5.2015 Pliocene 1 18.0 2276 7.42 27.6 1683.6 6.82 156.86 3.51 85.29 0.19 7.42 4.30 172.4 0.013 1.13 PM-9/67 27.5.2015 Pliocene 2 15.9 1667 8.17 18.9 1152.9 10.2 234.6 1.92 46.65 0.092 3.59 1.36 54.5 0.0125 1.09 Pl-7/68 27.5.2015 Pliocene 3 16.9 639.0 9.55 7.59 462.99 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. PH-6/83 27.5.2015 Pliocene 2 17.6 1061 8.40 11.9 725.9 2.52 57.96 1.67 40.58 0.08 3.12 2.01 80.6 0.007 0.61 Pl-7/91 1.6.2015 Pliocene 1 15.8 1744 8.90 6.29 383.69 3.87 89.01 0.004 0.0972 0.51 19.94 0.36 14.4 0.002 0.17 PF-6/83 1.6.2015 Pliocene 1 17.9 1883 7.73 22.1 1348.1 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.d. n.a. PD-6/83 1.6.2015 Pliocene 1 25.1 1811 7.45 20.6 1256.6 7.39 169.97 1.85 44.95 0.00012 0.00046 3.54 141.9 0.011 0.96 PB-5/86 2.6.2015 Pliocene 1 14.3 2617 7.69 30.5 1860.5 6.17 141.91 3.99 96.95 0.51 19.94 4.71 188.9 0.018 1.57 PD-2/85 2.6.2015 Pliocene 2 15.9 1300 8.00 8.01 488.61 3.30 75.9 0.49 11.9 0.06 2.34 0.90 36.1 0.0003 0.02 PE-4/82 2.6.2015 Pliocene 2 16.9 1117 8.10 12.4 756.4 1.52 34.96 1.02 24.8 0.11 4.30 0.017 0.68 0.008 0.70 H w a > O a p z œ œ w a g O w a a w o o H £ H C H W td O c Ui H W Sample number Date of sampling Geology CI (mM) CI (mg/1) so/- (mM) so/- (mg/1) NO,-(mM) NO,-(mg/1) Pco2 (ppm) ¿13cDIC(%„) 87Sr/86Sr V 12 t/86 1.4.2014 Pliocene 2 3.51 124.6 n.a. n.a. 0.11 6.82 575439.9 -3.4 0.70919 3490 1.4.2014 Pliocene 0.87 30.88 0.31 29.8 n.a. n.a. 245470.9 -0.9 0.70910 j.v. 2346 T/84 1.4.2014 Triassic 0.38 13.49 2.14 205.4 0.005 3.1 16595.9 -9.7 0.70822 j.v. 2370 T/88 2.4.2014 Triassic 0.29 10.29 1.04 99.8 n.a. n.a. 4265.8 -10.9 0.70808 j.v.2343/83 2.4.2014 Triassic 0.20 7.1 1.19 114.2 n.a. n.a. 15848.9 -10.7 0.70818 BV 28/87 1.4.2014 Pliocene 1 0.35 12.4 0.12 11.5 0.005 0.31 75857.8 -3.5 0.70931 j.v.2391 5/2 2.4.2014 Triassic 0.25 8.87 0.32 30.7 0.038 2.35 14125.4 -11.9 0.70846 j.v. 3051/01 1.4.2014 Triassic 3.19 113.2 20.2 1939.2 n.a. n.a. 107151.9 -19.3 0.70869 V 12 v/87 1.4.2014 Pliocene 1 3.13 111.1 0.25 24 0.07 4.34 489778.8 -2.3 0.70925 j.v.2391-2 2.4.2014 Triassic 0.007 0.24 0.33 31.7 0.0006 0.0372 12989.3 -10.3 0.70846 j.v. 2391-1 2.4.2014 Triassic 0.007 0.24 0.32 30.7 0.030 1.86 12589.3 -11.4 0.70845 j.v. 2391-3 2.4.2014 Triasic 0.007 0.24 0.32 30.7 0.045 2.79 14791.1 -12.5 0.70843 j.v. 2341/83 2.4.2014 Triassic 0.56 19.88 3.07 294.7 n.a. n.a. 15848.9 -9.5 0.70842 BV 29/87 1.4.2014 Pliocene 2 1.54 54.67 n.a. n.a. 0.105 6.51 229086.8 -2.8 0.70923 BV 27/87 1.4.2014 Pliocene 2 0.18 6.39 0.06 5.8 0.002 0.124 36307.8 -3.5 0.70938 V12z/86 1.4.2014 Pliocene 2 3.26 115.7 n.a. n.a. 0.063 3.906 208929.6 -1.2 0.70925 3491 1.4.2014 Pliocene 0.53 18.82 0.18 17.3 0.0079 0.4898 213796.2 0.0 0.70907 BV 26/87 1.4.2014 Pliocene 2 0.94 33.4 n.a. n.a. 1.11 6.82 56234.1 -2.0 0.70820 j.v. 3378-K/08 1.4.2014 Pliocene 1 2.71 96.2 0.50 48 0.11 6.82 331131.1 -2.0 0.70924 PC-5/83 6.6.2014 Triassic 0.38 13.5 n.a. n.a. n.a. n.a. 2041.7 -7.9 0.70912 PB-6/86 6.6.2014 Triassic 0.29 10.3 n.a. n.a. n.a. n.a. 44668.4 -2.8 0.70862 PH-13/12 6.6.2014 Triassic 0.18 6.4 0.44 42.2 n.a. n.a. 169.8 -6.9 0.70834 PT-30/98 6.6.2014 Triassic 0.20 7.1 0.21 20.2 n.a. n.a. 3715.4 -8.0 0.70904 PC-7/86 6.6.2014 Triassic 0.19 6.7 0.07 6.72 n.a. n.a. 2691.5 -9.6 0.70868 PE-9/10 6.6.2014 Triassic 0.201 7.1 0.44 42.2 0.0002 0.0124 3801.9 -6.7 0.70835 PH-5/81 25.5.2015 Pliocene 2 0.145 5.1 0.0139 1.33 n.a. n.a. 34673.7 2.6 0.70958 PM13/84 26.5.2015 Pliocene 3 0.024 0.9 0.0961 9.2 n.a. n.a. 46.8 -9.1 0.70972 PE-16/84 26.5.2015 Pliocene 2,3 0.023 0.8 0.002 0.192 n.a. n.a. 15.8 -14.4 0.71056 PG-5/84 26.5.2015 Pliocene 1 0.2431 8.6 0.012 1.152 n.a. n.a. 51286.1 4.3 0.70958 PM-9/67 27.5.2015 Pliocene 2 0.2913 10.4 n.a. n.a. n.a. n.a. 6309.6 1.0 0.71002 Pl-7/68 27.5.2015 Pliocene 3 0.090 3.2 0.0065 0.624 0.003 0.186 398.1 1.0 n.a. PH-6/83 27.5.2015 Pliocene 2 0.0697 2.5 n.a. n.a. n.a. n.a. 2990.9 0.3 0.70975 Pl-7/91 1.6.2015 Pliocene 1 0.58 20.6 0.0059 0.56 n.a. n.a. 398.1 -12.5 0.70867 PF-6/83 1.6.2015 Pliocene 1 0.187 6.6 n.a. n.a. n.a. n.a. n.a. 4.6 n.a. PD-6/83 1.6.2015 Pliocene 1 0.547 19.4 0.00017 0.016 n.a. n.a. 39810.7 1.8 0.70949 PB-5/86 2.6.2015 Pliocene 1 0.098 3.5 0.003 0.28 n.a. n.a. 31623.8 4.3 0.70926 PD-2/85 2.6.2015 Pliocene 2 0.032 1.1 0.00078 0.074 n.a. n.a. 4168.7 -0.3 0.70943 PE-4/82 2.6.2015 Pliocene 2 0.43 15.3 0.0019 0.18 n.a. n.a. 5248.1 1.6 0.70942 K a EE p a o >e o td 03 'Ji 5' OB O £ P a n.a. - not analysed 14 Tjasa KANDUC, Natasa MORI, Ajda KOCELI & Timotej VERBOVSEK Fig. 4. PCA ordination diagram representing differences between groundwater samples taken during the years 2014-2015 (circles) and the gradients in environmental variables (arrows). Colors of the circles indicate the geology of the aquifer - for the legend see Fig. 2). (44.12 mM = 2691.32 mg/l) was observed in well 3378-K/08 (Figs. 2, 3) and the lowest alkalinity of 1.32 mM (80.52 mg/l) at location j.v. 2370 T/88 (Figs. 2, 3). Na+ concentrations range from 0.104 mM (2.38 mg/l) to 10.22 mM (234.0 mg/l), Mg2+ concentrations from 0.004 mM (0.097 mg/l) to 16.63 mM (4074. 1 mg/l), K+ concentrations from 0.0001 mM (0.39 mg/l) to 0.51 mM (19.9 mg/l), and Ca2+ concentrations from 0.01 mM (0.401 mg/l) to 4.705 mM (188.5 mg/l) in Pliocene aquifers (Table 1). In groundwater from Triassic aquifer the following concentrations were obtained: Na+ from 0.056 mM (1.28 mg/l) to 22.02 mM (504.3 mg/l), Mg2+ from 0.078 mM (1.89 mg/l) to 4.94 mM (120 mg/l), K+ from 0.017 mM (0.66 mg/l) to 0.48 mM (18.7 mg/l) and Ca2+ from 0.21 mM (8.42 mg/l) to 6.69 mM (268.2 mg/l) mM (Table 1). SO42- concentrations in groundwater from Pliocene aqu4ifers are low due to reduction conditions in the aquifer (Kanduc et al., 2014) and range from 0.00078 mM (0.074 mg/l) to 0.50 mM (48 mg/l), NO3-concentrations range from 0.0002 mM (0.011 mg/l) to 0.11 mM (6.28 mg/l) mM in groundwater from Pliocene aquifers (Table 1). In addition, SO42- and NO3- concentrations were low in the Pliocene aquifers (up to 0.5 mM (48 mg/l) and up to 0.11 mM (6.82 mg/l)) compared to groundwater from Triassic aquifer indicating sulfate and nitrate reduction (Kanduc et al., 2014), and consistent with microbial methanogenesis (Coetsiers & Walraevens 2009). PCA analysis explained 49.9 % of all variability in data by the first, and an additional 19.4 % by the second axis (Fig. 4). Most of the samples are distributed along the first axis, described mostly as differences in pH and conductivity linked to chloride. Higher conductivity and chloride concentrations and lower pH were observed in the groundwater samples originating from Pliocene strata. The second axis represents mainly a gradient in sulfate and ¿13CDIC. Generally, the groundwater samples dewatering Triassic strata grouped together in the ordination diagram, while samples of groundwater dewatering Pliocene strata are scattered along both axes, indicating greater variation in measured variables. The outlier sample j.v. 3051/01 is from dewatering Triassic strata, having a high concentration of sulfate, 20.18 mM (1937 mg/l) (Table 1). The lowest ¿13CDIC value of -19.3 %o in j.v. 3051/01 indicates greater soil CO2 degradation in the groundwater. The other two outlier samples (V 12 t/86 and j.v. 3378-K/08) have high alkalinities and conductivities (Table 1). Dissolution of calcite and dolomite produces groundwater with the molar ratio (Ca2++Mg2+): HCO3- = 1:2. The weathering of dolomite in carbonate systems contributes the majority of the Mg2+, in which Mg2+ : Ca2+ and Mg2+ : HCO3-molar ratios indicate the relative proportions of calcite and/or dolomite dissolution (Kanduc Hydrogeochemistry and isotope geochemistry of Velenje Basin groundwater 15 Fig. 5. A. Ca2+ + Mg2+ versus alkalinity concentrations. The line indicates the stoichio-metry for weathering of carbonate minerals. B. Mg2+ versus Ca2+ concentrations in Velenje Basin groundwater. B 14 - 12 - 10 - □ Pliocene x Pliocene 1 • Pliocene 2 o Pliocene 3 ■ Pliocene 2,3 A Triassic (mine) A Triassic (surface) Mg2+/Ca2+= 1 „ ■ Mg2/Ca2+ = 0.5_ ,'Î x „ - ~ _________ • % '•* A —:*------ A A»«® __________ ' a - sJi^-----— ____________Mg2+/ce ^ 0.1----- tJfrff —i-1-1-1- 1 2 3 4 5 6 7 Ca2+ (mM) 8 6 4 2 0 9 et al., 2014). Groundwater dewatering Pliocene strata has higher alkalinity and deviates from the 2:1 groundwater line, while the groundwater in the Triassic aquifer falls along the 2:1 HCO3-: (Ca2++Mg2+) line (Fig. 5A). Dissolution of calcite produces waters with a Mg2+ : Ca2+ molar ratio of less than 0.1, a ratio of 0.33 in the case of congruent dissolution of calcite and dolomite, and a ratio equal to 1 if only dolomite is dissolving (Szramek et al., 2011). Groundwater in the Triassic aquifer falls between lines 0.5 and 1, indicating weathering of dolomite (Fig. 5B), while groundwater in Pliocene aquifers deviates from the dolomite line (having a slope of 1) indicating that some other minerals (e.g. albite, plagioclase) contribute to alkalinity, already confirmed in previous studies by X-ray diffraction (Kanduc et al., 2014 and references therein). Carbon cycling and strontium geochemistry of Velenje Basin groundwater Partial pressures of CO2 (pCO2) in groundwater of Pliocene and Triassic aquifers ranged from 15.8 to 575,439.9 ppm and from 169.8 to 107,151.9 ppm, on average 80800 ppm, which is 202 times greater (supersaturated) than that of atmospheric CO2 concentrations (400 ppm). SIcalcite ranged from -0.8 to 1.48 and SI. , ,t ranged from -0.32 to 2.93 in dolomite ° Pliocene aquifer. SIcalcite ranged from -0.75 to 0.52 in groundwater dewatering Triassic aquifer and SIdolomite ranged from -1.54 to 1.06 in groundwater dewatering Triassic aquifer (Fig. 6). Groundwater samples from the Pliocene and Triassic aquifers therefore have calcite saturation indices (SI ) calcite 16 Tjasa KANDUC, Natasa MORI, Ajda KOCELI & Timotej VERBOVSEK Fig. 6. Saturation index of calcite (SI , , ) versus saturaN calcite' tion index of dolomite (SIdolomite) in the groundwater of the Velenje Basin. Photo on figure 6 represents precipitation of carbonates on a probe of "Solinst sampler". generally well above the equilibrium (SIcalcite = 0), indicating that calcite is supersaturated and that precipitation is favored thermodynamically due to high alkalinities, especially in the Pliocene aquifer. Only two samples from mining wells and three samples that overflow dewatering Triassic strata are undersaturated with calcite and dolomite (Fig. 6). We also observed precipitates of dolomite/ calcite as visible incrustations in some of the wells. Such incrustations have formed on the well casing, and were scratched from the casing walls when measuring with the Solinst Discrete Interval Probe (photo on Fig. 6). Values of ¿13CDIC for groundwater of dewatering Pliocene strata ranges from -14.4 to +4.6 %0. Groundwater dewatering Triassic strata ranges from -19.3 to -2.8 %0 (Table 1). Geochemical processes influencing the value of ¿13CDIC in groundwater are presented on Figure 7. Groundwater samples from the Triassic aquifer have generally lower ¿13CDIC and are similar to those investigated previously for surface waters (Kanduc et al., 2014). They fall approximately on the line of carbonate dissolution by carbonic acid produced from the soil zone with a ¿13CCO2 of -26.6 %0 (Kanduc et al., 2007). Groundwater samples from the Pliocene aquifers have higher values of ¿13CDIC (up to +4.6 %0), which are attributed to microbial methanogenesis, causing enrichment in 13C (Kanduc et al., 2012). Isotopic compositions of carbon in methane (¿13CCH4) and of deuterium in methane (¿DCH4), analyzed directly from groundwater ranged from -77.7 to -51.4 %0 and from -246.6 to -162.0 %o, indicating bacterial gas (Kanduc et al., 2014; Kanduc et al., 2015b). Higher ¿13Cdic values are due to methanogenesis within aquifer and are related to microbial methanogenesis associated with coal seam. We also found microbial gas collected from piezometers (as free gas) in previous studies and its ¿13CCH4 have values in the range of -70.5 to -34.2%o (Kanduc et al., 2012; Kanduc et al., 2014). Equilibration lines according to possible geochemical processes were calculated as follows: • Line 1. Considering the isotopic composition of atmospheric CO2 of -7.8 %o (Levin et al., 1987) and the equilibration fractionation with DIC of +9 %o, DIC in equilibrium with the atmosphere should have a ¿13CdiC about +1 %o (Fig. 7). • Line 2. Given the average isotopic composition of carbonates (¿13CCaCO3) with a value of -2 %0 (Kanduc & Pezdič, 2005) and isotopic fractionation (and enrichment in 12C) due to dissolution of carbonates, which is 1.0±0.2 %o (Romanek et al., 1992) (Fig. 7) would give a value of ¿13Cdic of 1.0±0.2 %0. • Line 3. An average ¿13C value of particulate organic carbon (POC) of -26.6 %o was assumed to represent the isotopic composition of POC that was transferred to DIC by in stream respiration. Open system equilibration of DIC with CO2 enriches DIC in 13C by about 9 %o (MOOK et al., 1974), which corresponds to the value of -17.6 %0 (Fig. 7). • Line 4. This represents open system equilibration of DIC with soil CO2 originating from degradation of organic matter with ¿13CCO2 of -26.6 %o (Fig. 7). Hydrogeochemistry and isotope geochemistry of Velenje Basin groundwater 17 Fig. 7. ¿13Cdic versus alkalinity in Velenje Basin groundwater. Variations in ¿13Cdic values of groundwater samples compared to alkalinity concentrations, with lines indicating processes occurring in the Velenje Basin. Arrows show expected trends for a variety of processes. Lines indicate processes in aquifer as follows: 1: open system DIC equilibration with atmosphere (Exchange), 2: dissolution of carbonates with average ¿13CCaCO3 = -2 %o, 3: nonequilibrium carbonate dissolution by carbonic acid produced from soil zone with a ¿13CCO2 of -26.6 %o, 4: open system equilibration of DIC with soil CO2 originating from degradation of organic matter with ¿13C of -26.6 %o. Groundwater in our study is considered as a closed system (Clark & Fritz, 1997), therefore any interactions with atmosphere are not possible. It was found in previous studies that concentrations of dissolved organic carbon (DOC) in the Pliocene and Triassic aquifers in the Velenje basin are low, ranging from 1.54 to 14.69 mg/L (Kanduc et al., 2010), compared to groundwater associated with coalbeds in the Powder River Basin (Orem et al., 2010). In the latter, high DOC concentrations are related to methanogenesis and contact of groundwater with coal, but in Velenje Basin we can conclude that the methanogenesis (Fig. 7) in groundwater is probably related to the presence of low dissolved oxygen (DO), high alkalinity and high ¿13CdiC values, occurring within the Pliocene aquifers (Kanduc et al., 2014). The ratio 87Sr/86Sr ranges from 0.70820 to 0.71056 in groundwater dewatering Pliocene aquifers and from 0.70808 to 0.70910 in groundwater dewatering Triassic strata (Table 1). Figure 8 represents 87Sr/86Sr ratios and indicates characteristic values of 87Sr/86Sr isotopes in different environment with different geological composition e.g. marine limestone 0.711, while silicate fraction has higher 87Sr/86Sr ratio. The relations between 87Sr/86Sr ratios and ¿13Cmn for groundwater in Pliocene and Triassic aquifers for Velenje Basin groundwater are presented on Figure 8. Based on the 87Sr/86Sr ratio in groundwater from both aquifers (Triassic and Pliocene), the carbonate fraction prevails (Fig. 8). The highest 87Sr/86Sr ratio was found in Pliocene 2, 3 strata, implying that more silicate fraction is leached in the aquifer. Generally, in Pliocene aquifers of Velenje Basin groundwater, higher 87Sr/86Sr ratios are observed than in Triassic aquifers (Table 1, Fig. 8). Given that carbonate dissolution rates are higher than those of silicates (Lasaga, 1984) as is the case for the Triassic and Pliocene aquifers of Velenje Basin carbonate dissolution may be important in controlling Sr isotope geochemistry (Harrington & Herzeg, 2003). However in the study of Murray Basin, Australia, 87Sr/86Sr ratios of groundwater lie between 0.71068 and 0.71905 (median = 0.71595), generally higher than values for potential carbonate sources (Cartwright et al., 2007). Trends in 87Sr/86Sr ratios versus ¿13CdiC values for the mixing of groundwater from silicate and carbonate aquifers of Velenje Basin groundwater are shown schematically in Figure 8. Trends in 87Sr/86Sr ratios versus ¿13CdiC values for the mixing of groundwater from silicate and carbonate aquifers or for the progressive dissolution of carbonates are shown in Figure 8. 87Sr/86Sr ratios of the silicate and carbonate fraction of aquifer 18 Tjasa KANDUC, Natasa MORI, Ajda KOCELI & Timotej VERBOVSEK 0.71100 0.71000 0.70900 ^ 0.70800 0.70700 0.70600 0.70500 □ □ -20.0 -15.0 -10.0 -5.0 513Cdic (%„) 0.0 ■ Pliocene 2,3 □ Pliocene XPliocene 1 • Pliocene 2 OPliocene 3 A Triassic mine A Triassic surface 5.0 10.0 Fig. 8. 87Sr/86Sr versus ¿13Cdic in Velenje Basin groundwater. material in Velenje Basin groundwater, rainfall (Ullman & Collerson, 1994), typical 87Sr/86Sr ratios and ¿13CDIC values of marine and terrestrial carbonates (Clark & Fritz, 1997; Faure, 1991), and typical ¿13C values of soil carbon (Clark & Fritz, 1997), are also shown. Conclusions Sampling in this study was performed only where possible due to the subsidence of the area, which destroyed some of the piezometers. New wells were sampled in the following three sampling campaigns during the period 2014-2015: a) overflow piezometers, b) groundwater and c) mine wells. Altogether, 38 wells were investigated for geochemical and isotopic analyses. The major ion geochemistry and isotope geochemistry of the Velenje Basin groundwater are indicated in this study. The major ion geochemistry, isotopic composition of carbon and Sr ratios (¿13CDIC, 87Sr/86Sr) provides a new insight into geochemical processes (water - rock-interactions) in Velenje Basin groundwater. The concentrations of dissolved ions and cations decreases in the order HCO3->Mg2+>Na+>Ca2+ in the Pliocene aquifers and HCO3->Ca2+>Mg2+ in the Triassic aquifer. Multivariate PCA analysis, based on "in situU" measured parameters (conductivity, pH, temperature), major geochemical parameters (Ca2+, Mg2+, K+, Na+, NO3-, Cl-, SO42-) and ¿13Cdic revealed that groundwater samples dewatering Triassic strata were grouped together in the ordination diagram, while the samples of groundwater dewatering Pliocene strata were scattered along both axes due mainly to variations in pH, conductivity, chloride, sulphate and ¿13CDIC. The values of ¿13CDIC and 87Sr/86Sr, together with major ion geochemistry help explain geochemical processes in Velenje Basin groundwater. ¿13CDIC values of groundwater in Pliocene aquifers range from -14.4 to +4.6 %o and in groundwater of Triassic aquifer from -19.3 to -2.8 %o. Alkalinities range from 1.95 to 44.12 mM in Pliocene aquifers and from 1.32 to 10.43 mM in Triassic aquifer. ¿13CDIC of groundwater in the Triassic aquifer indicate carbonate dissolu-tion (dolostones and limestones) and organic matter degradation. In Pliocene aquifers, however methanogenesis (higher ¿13CDIC values up to +4.6 %o) is present in addition to degradation of organic matter and dissolution of carbonates, contributing to the very high alkalinity in these aquifers. Calculated value of pCO2 in groundwater of Pliocene aquifers ranged from 15.8 to 575,439.9 ppm and in groundwater of Triassic aquifer from 169.8 to 107,151.9 ppm. Most groundwater in Pliocene and Triassic aquifers was oversaturated with calcite and dolomite. Pliocene groundwaters have much higher oversaturation values than the Triassic ones, also due to methanogenesis and high alkalinity. The 87Sr/86Sr ratios in Pliocene aquifers range from 0.70820 to 0.71056 and, in groundwater of Triassic aquifer, from 0.70808 to 0.70912. 87Sr/86Sr isotopes in combination with ¿13CDIC, provide added value to the study in deciphering geochemical processes, water-rock interactions and silicate/ carbonate weathering within the aquifers. The carbonate was found to have an 87Sr/86Sr ratio around 0.71120, the silicate fraction being in the range of 0.7120 to 0.7190. Groundwater from Pliocene aquifers has more positive ¿13CDIC values and 87Sr/86Sr ratio, except for two samples located in the Pliocene 3 aquifer (87Sr/86Sr ratio 0.70972, alkalinity 3.23 mM and ¿13CDIC of -9.13 %, PM13/84) and Pliocene 2, 3 aquifer (87Sr/86Sr ratio 0.71056 with low alkalinity of 1.95 mM and