Next Article in Journal
Effects of Fertilizer Reduction and Straw Application on Dynamic Changes of Phosphorus in Overlying and Leaching Water in Rice Fields
Next Article in Special Issue
Time-Series Analysis of Isotope Composition of Precipitation in Zagreb, Croatia
Previous Article in Journal
Mitigation Techniques for Water-Induced Natural Disasters: The State of the Art
Previous Article in Special Issue
Characterizing the Groundwater Flow Regime in a Landslide Recharge Area Using Stable Isotopes: A Case Study of the Urbas Landslide Area in NW Slovenia
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

An Integrated Approach to Characterising Sulphur Karst Springs: A Case Study of the Žvepovnik Spring in NE Slovenia

1
Department of Geology, Faculty of Natural Sciences and Engineering, University of Ljubljana, Aškerčeva 12, SI-1000 Ljubljana, Slovenia
2
IRGO Consulting d.o.o., Slovenčeva 93, SI-1000 Ljubljana, Slovenia
3
Department of Environmental Sciences, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
4
Research Centre of the Slovenian Academy of Sciences and Arts, Karst Research Institute, Titov trg 2, SI-6230 Postojna, Slovenia
5
UNESCO Chair on Karst Education, University of Nova Gorica, Glavni trg 8, SI-5271 Vipava, Slovenia
6
Regional Unit Celje, Institute of the Republic of Slovenia for Natural Conservation, Vodnikova ulica 3, SI-3000 Celje, Slovenia
*
Author to whom correspondence should be addressed.
Water 2022, 14(8), 1249; https://doi.org/10.3390/w14081249
Submission received: 4 March 2022 / Revised: 7 April 2022 / Accepted: 8 April 2022 / Published: 13 April 2022

Abstract

:
We present an integrated approach to characterizing the Žvepovnik sulphur spring, comprising detailed basic geological (mapping), geochemical (physico-chemical, elementary), isotopic (δ2H, δ18O, δ13CDIC, δ34S and 3H), and microbiological analyses. We used a multi-parameter approach to determine the origin of the water (meteoric or deeper infiltration), the origin of the carbon and sulphur, and water retention times. Our special research interest is the origin of the sulphur, as sulphur springs are rare and insufficiently investigated. Our results show that the Žvepovnik spring occurs along the fault near the contact between the dolomite aquifer and overlying shales and volcanoclastic beds. The spring water is the result of the mixing of (1) deeper waters in contact with gypsum and anhydrite and (2) shallow waters originating from precipitation and flowing through the surface carbonate aquifer. The results of δ2H and δ18O confirm local modern precipitation as the main source of the spring. δ13CDIC originates from the degradation of organic matter and the dissolution of carbonates. We therefore propose four possible sources of sulphur: (1) the most probable is the dissolution of gypsum/anhydrite; (2) barite may be a minor source of sulphur; (3) the microbial dissimilatory sulfate reduction; and (4) the oxidation of pyrite as the least probable option.

1. Introduction

Understanding karst springs in terms of their origin and dynamics poses important research challenges. Characteristics of spring water are the result of the flow of older groundwater and inflows of younger waters from the surface. Therefore, the interaction of groundwater with the aquifer host rocks as well as the characteristics of surface water sources constitute the basis for understanding their origin [1,2]. The karst springs often respond very quickly and intensively to meteorological phenomena; therefore, it is essential also to understand the impact of the composition of precipitation on spring water. Understanding the exchange of water within the geospheres is vital to addressing environmental issues linked to water resource management [3]. Exchanges within the geosphere are also important in karst environments due to the higher degree of vertical connectivity of these systems with the atmosphere and deeper aquifer units through faulting [4,5,6].
To date, only a few natural sulphur springs have been found and characterized in Slovenia, where karst comprises almost half (i.e., 43%) of the country’s territory. The Žvepovnik sulphur spring, locally known as Žviponik, is one of five known natural sulphur springs documented in Slovenia. The name originates from the term “žveplo”, which means sulphur in the Slovenian language. While some sulphur springs such as Žveplenica [7,8] and Žvepleni studenec pri Rihtarju have been known in Slovenia for decades and are protected for their natural value, information on the Žvepovnik sulphur spring was only first published in a tourist guide of the Municipality of Ljubno in 2013. At the initiative of the Municipality of Ljubno, the first expert inspection of the studied spring was carried out in 2016 as part of a research study on sulphur springs in Slovenia. As an exceptional and rare natural phenomenon in the country, the Žvepovnik sulphur spring was also determined to be of protected natural value in 2019. The locations of sulphur springs in the territory of the Republic of Slovenia can be found under the Nature Conservation Atlas (https://www.naravovarstveni-atlas.si: accessed 30 January 2022) Natural Values tab: Žveplenica (ID 686), Žvepleni studenec pri Rihtarju (ID 502), and the Žvepovnik sulphur spring researched herein (ID 80218). In recent years, two more sulphur spring locations are under investigation: submarine and coastal sulphur springs near the city of Izola [9] and Smrdljivec [10].
The integration of different investigative methods is crucial for a comprehensive understanding of spring dynamics. A fundamental geological understanding (lithological and structural characteristics [11,12]) of the wider area provides important information on the backgrounds of the springs for the further planning of water sampling and are crucial for final interpretations. Hydrogeochemistry and isotopic composition provide information about the sources and residence times; water–rock interaction along the flow path helps us understand the sources of the dissolved constituent in water within the aquifer; and the mixing of individual groundwater bodies is also key [13].
Isotopes in the water molecule determined as the stable isotope composition of hydrogen (δ2H) and oxygen (δ18O) serve as natural fingerprints and allow us to trace the natural origin of the water, which is reflected in its isotope composition as the water circulates through the water cycle [14]. They are commonly used to characterize and trace the source of water, the influence of different processes during infiltration, and the flow of water through the water body or to quantify exchanges of water, solutes, and particulates between hydrological compartments according to different hydrological processes [15,16]. Therefore, δ2H and δ18O are powerful tools that can be used to track the path of water molecules from precipitation to surface and groundwater, and further to drinking water supply. Many authors have reported on the multi-parameter approach and combine the isotope and other geochemical parameters to address specific questions (e.g., [17] and related papers).
Carbon isotopes are used to assess the origin of dissolved inorganic carbon (DIC), which is the main component in waters draining carbonate watersheds. Concentrations of DIC and its isotopic composition of carbon (δ13CDIC) are governed by processes occurring seasonally in the soil–aquifer system. Changes in DIC concentrations result from the addition or removal of carbon from the DIC pool, whereas changes in δ13CDIC result from the fractionation accompanying the transformation of carbon or from the mixing of carbon from different sources. The major sources of carbon to aquifer DIC loads are the dissolution of carbonate minerals and soil CO2 derived from root respiration and from the microbial decomposition of organic matter. The major processes removing DIC in aquifer systems is carbonate mineral precipitation [18]. δ13CDIC for deciphering geochemical processes in combination with hydrogeochemcal data and other isotope tracers (e.g., δ18O, δ2H) have been investigated in many freshwater springs worldwide in different climates [19,20,21,22].
One of the major anions in karst groundwater other than HCO3 is sulphate (SO42). Its origin in groundwater varies, from precipitation, dissolution of evaporites, oxidation of sulphides, and anthropogenic inputs [23]. Different sources of sulphate can be distinguished by the isotope signatures of δ34SSO4 and δ18OSO4 [24,25].
A detailed characterization of the environmental parameters is important to understand the accompanying biota. Underground sulphur habitats can be considered as hotspots of biodiversity [26,27]. The microbiota that colonizes terrestrial springs likely originate from groundwater but may also originate from the surface. The presence and distribution of microorganisms is likely controlled by spring geochemistry and surface and subsurface transport mechanisms [28].
The aim of the present study was to determine geochemical and biological processes occurring in the Žvepovnik spring using an integrated approach comprising basic geologic, geochemical, isotopic, and microbiological analyses. We combined the use of several isotope analyses to determine water origin (meteoric or deeper infiltration), origin of carbon and sulphur in water, and water retention times. Of special research interest was the origin of sulphur, as sulphur springs are rare, remain insufficiently investigated, and are often interesting to the public owing to their characteristic rotten-egg smell. A holistic approach, including the scientific and public conservation legislative approach, is therefore necessary to characterize the springs properly; here, we present one such example.

2. Materials and Methods

2.1. Location and Basic Characteristics of the Spring

The Žvepovnik sulphur spring is located near the town of Ljubno ob Savinji in the Savinja Valley (NE Slovenia). The spring is located in the forested area on the right bank of the stream of Rajserjev graben (WGS84 coordinates: 46.3500° N, 14.8452° E; local national D96/TM system: e = 488,093 m, n = 134,568 m and Z = 460 m) (Figure 1). The spring water was obtained via a plastic pipe into a wooden drinker from where it flows into the Rajserjev graben stream a few meters on. The smell of the spring cannot be detected from a distance, while the distinctive odor of hydrogen sulfide can be detected in its vicinity (from a distance of a few meters). Directly at the outflow of the spring, white and pale-yellow organic sediments and attached microbial biofilm were observed.

2.2. Geological Characterisation

The geological characterisation of the Žvepovnik spring was performed using several methods. The geological setting was compiled from previous work on regional structure, stratigraphy, and basic geological maps (scale 1:100,000). The surroundings of the spring were further mapped (approximately 0.5 km2) at 1:5000 scale using a combination of boundary tracing and the interpolation of point outcrops. The contact between Anisian dolomite (basement aquifer rock) and Oligocene beds (cover impermeable rock) was studied in detail. The outcrop of approximately 10 m2 was analysed, two short laterally positioned sedimentological sections (scale 1:50) were logged, and diverse lithologies were sampled. From rock samples, thin sections were made for further microfacies analysis that was manifested under a polarized Zeiss Axioplan2 (Carl Zeiss AG, Oberkochen, Germany) and a Nikon Eclipse E600 (Nikon Instruments Inc., Melville, NY, USA) petrographic microscope. Thin sections were partly stained with Alizarin Red.
In order to analyse the mineral composition of the rock, samples were powdered in agate mortar and analysed using X-ray diffraction using a Philips PW3710 X-ray diffractometer (PAN-Analytical, Almelo, Nederland) with CuKα1 radiation (with wavelength 1.54060 Å) and a secondary graphite monochromator. The data was collected at 10 kV with a current of 10 mA at a speed of 3.4° 2θ per minute at a range of 2θ 3.01° to 70.01° with in steps of 2θ 0.02°.

2.3. Field Sampling, Physico-Chemical Measurements, Geochemical and Isotopic Analyses of Groundwater

The first field observations and physico-chemical measurements were carried out in March 2016, while detailed research was performed between November 2017 and July 2018 (Table 1). Geochemical and isotopic analyses were performed up to two times, while the measurements of physico-chemical parameters (temperature (T), pH, specific electroconductivity (EC), and dissolved oxygen in mg/L and % (D.O. and D.O.%) were carried out repeatedly from November 2017 to July 2018 every month with a WTW Multi 3430 Multiparameter probe (WTW GmbH, Weilheim, Germany).
Groundwater sampling for geochemical analyses was performed once (Table S2). Water samples for geochemical analysis were preserved in 50 mL HDPE bottles and kept refrigerated until analysed in the laboratory. For cations, the water was filtered through a 0.45 μm nylon filter and pre-treated with HNO3 to pH = 2 immediately on site.
Samples for isotopic analyses (δ18O, δ2H, δ13CDIC) and total alkalinity were collected twice (Table S3). For the analysis of total alkalinity and isotope composition of dissolved inorganic carbon (δ13CDIC), water was filtered through 0.45 µm pore-sized membrane filters and stored in 30 mL high-density polyethylene (HDPE) bottles for total alkalinity and 12 mL Labco glass vials with septum, without headspace for δ13CDIC analyses. Samples for oxygen and hydrogen isotopic composition were collected in 50 mL HDPE bottles that were rinsed with sample water prior to filling. Additional water samples for some ions, hardness, and alkalinity (Table S4) were collected for analyses on a portable field YSI 9300 spectrophotometer (YSI, Yellow Springs, OH, USA).
For tritium (3H) (Table S3), the water was sampled once in 1 L HDPE bottles.
The groundwater sample was taken for analyses of δ34SSO4 and δ18OSO4 (Table S3). The water was filtered through 0.45 µm pore-sized membrane filters, treated with 2N HCl solution to keep water at acidic conditions (pH = 2) at all times, and stored in 50 mL HDPE bottles. The BaSO4 precipitated with BaCl2.
Water samples were collected aseptically in sterile bottles for microbiological analysis and transported to a laboratory in a refrigerated box.
Dissolved hydrogen sulphide in the spring water was determined immediately after sampling on site according to the manufacturer’s instructions (YSI).

2.3.1. Hydrogeochemistry

Measurements of major, minor, trace elements, and REE were performed at the ActLabs accredited laboratory (Activation Laboratories Ltd., Ancaster, ON, Canada). Elements were analysed (see Table S2) by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) (ActLabs Code 6 ICP/MS—Hydrogeochemistry), major anions (SO42, NO3, Cl, F) were determined by Ion Chromatography (IC), while some of the major cations (e.g., Ca2+, Mg2+) were measured using an Overrange Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) owing to their excessively high concentrations for ICP-MS.
Limits of detection for the ICP-MS, ICP-OES, and IC methods were generally in the range of a few μg/L and are reported for each element in detail on the ActLabs homepage (https://actlabs.com/downloads/: accessed 30 November 2021 → Geochemistry Schedule of Services and Fees → Actlabs—Schedule of Services—Euro → Hydrogeochemistry Section). A blank sample was also analysed, and all its measured values were below the detection limit. A duplicate sample was also analysed in the laboratory (not discussed in this study, but was measured in the same batch, yielding good quality measurements).
Additional chemical analyses for hardness, alkalinity, K+, NH4+, NO2, NO3, Cl, H2S, SO42, PO43, Fe(tot), SiO2, Cu2+, Cr(tot), Mn2+, Ni2+, Zn2+ were performed using a YSI 9300 spectrophotometer according to the manufacturer’s instructions (YSI) (see Table S4).

2.3.2. Geochemical Modelling

The PHREEQC for Windows program [30] was used to calculate saturation indexes (SI) for minerals and partial pressure (PCO2). In this program, the LLNL database was used; compared to other databases, it contains the highest number of species considered.

2.3.3. Determination of Isotope Composition of Hydrogen and Oxygen

The isotope composition of hydrogen (δ2H) and oxygen (δ18O) was determined using the H2–H2O [31] and CO2–H2O [32,33] equilibration technique. Measurements were performed on a dual inlet isotope ratio mass spectrometer (DI IRMS, Finnigan MAT DELTA plus, Finnigan MAT GmbH, Bremen, Germany) with an automated CO2–H2O and H2–H2O HDOeq 48 Equilibration Unit (custom built by M. Jaklitsch). The temperature of the water bath was 18 °C. The water vapour trap was cooled to −55 °C. The CO2 (Messer 4.5, Messer, Ruše, Slovenia) and H2 (IAEA) gases were used as working standards for water equilibration. Samples of water were measured as independent duplicates. All measurements were performed together with laboratory reference materials (LRM) calibrated periodically against primary IAEA calibration standards VSMOW2 and SLAP2 to VSMOW-SLAP scale [34]. The results are expressed in the standard δ notation (in per mil, ‰) as deviation of the sample (sp) from the standard (st) as:
δyX (‰) = (Rsp/Rst −1) × 1000
where δyX is hydrogen (δ2HVSMOW-SLAP, abbreviated as δ2H) or oxygen (δ18OVSMOW-SLAP, abbreviated as δ18O) and R the 2H/1H or 18O/16O, respectively. Results were normalized to the VSMOW-SLAP scale using the LIMS programme. In order to normalize results, we used LRMs, namely W-3869 with defined isotope values and estimated measurement uncertainty δ2H = +2.5 ± 0.7‰ and δ18O = +0.36 ± 0.04‰ and W-3871 with values of δ2H = −148.1 ± 0.7‰ and δ18O = −19.73 ± 0.02 ‰. For independent quality control, we used LRM W-45 with defined isotope values and an estimated measurement uncertainty of δ2H = −60.6 ± 0.7‰ and δ18O = −9.12 ± 0.04‰, and commercial reference materials USGS 45 (δ2H = −10.3 ± 0.2‰, δ18O = −2.238 ± 0.006‰) and USGS 46 (δ2H = −235.8 ± 0.4‰, δ18O = −29.80 ± 0.02‰). The average sample repeatability for δ2H and δ18O was 0.2 and 0.02‰, respectively.

2.3.4. Determination of Total Alkalinity after Gran

In order to measure alkalinity, the water sample was passed through a 0.45 m nylon filter into an HDPE bottle and kept refrigerated until analysed. First, the pH was measured in the lab with a pH meter (Mettler Toledo AG 8603, Schwerzenbach, Switzerland). The total alkalinity was then determined within 24 h after sample collection using the Gran titration method [35] with a precision of ±1%. Approximately 8 g of the water sample was weighed into a plastic container and placed on a magnetic stirrer. A calibrated pH electrode (7.00 and 4.00 ± 0.02) was placed in the sample and the initial pH was recorded. Reagencon HCl 0.05 N (0.05 M) was used for titration. The titration performed using a CAT titrator (Ingenierbüro CAT, M. Zipperer GmbH Ballrechten-Dottingen, Germany). The method has been described in detail by Zuliani et al. [36].

2.3.5. Determination of Isotopic Composition of Dissolved Inorganic Carbon

Samples were stored in glass serum bottles filled with no headspace and sealed with septa caps immediately after collection on the field. The δ13CDIC values were then determined using continuous flow IRMS (Europa Scientific 20–20) with an ANCA-TG preparation module (Sercon Limited, Crewe, UK) for the sample taken in July 2018, while with IsoPrime 100 coupled with the Multiflow preparation module (Elementar, Manchester, UK) for the sample that was taken in November 2017. Phosphoric acid (100%) was added (100–200 µL) to a septum-sealed vial, which was then purged with pure He. The sample (6 mL for ANCA-TG 20–20 and 1 mL for MultiflowBio preparation modules) was then injected, and the headspace CO2 measured (modified after [37,38]). In order to determine the optimal extraction procedure for bottled water samples, a standard solution of Na2CO3 (Carlo Erba reagents, Val de Reuil, France) and a Scientific Fisher sample with a known δ13CDIC value of −10.8 ± 0.2‰ and −4.8 ± 0.1‰ were used. Messer reference gas for the calibration of measurements with known δ13CCO2 −35.4 ± 0.2‰ was also used. The reference material (Carlo Erba solution) was used to convert the analytical results to the Vienna Pee Dee Belemnite (VPDB) scale. The average sample repeatability was 0.2‰.

2.3.6. Determination of Tritium

The tritium (3H) was measured with a liquid scintillation counting (LSC) TriCarb 3170 TR/SL (PerkinElmer, Waltham, MA, USA). Before analyses, the sample was treated and prepared using electrolytic enrichment, which consists of primary distillation, electrolyte enrichment, and secondary distillation. The error of the analysis was ±0.5 TU, with a detection limit of 3 TU.

2.3.7. Determination of Isotopic Composition of Sulphur and Oxygen in Sulphate

The isotopic composition of sulphur and oxygen were measured in precipitate that was obtained from water samples according to standard procedures. Measurements of δ34SSO4 in δ18OSO4 were performed at the University of Arizona in the USA (https://www.geo.arizona.edu/node/153: accessed 30 November 2021). For analysis of isotopes, sulphides and sulphates were converted to SO2, and the CO2 and H2O were removed. The δ34SSO4 was analysed with a continuous-flow isotope gas-ratio mass spectrometer (CFIRMS, ThermoQuest Finnigan Delta PlusXL, Thermo Fisher Scientific Inc., Waltham, MA, USA). Samples were combusted at 1030 °C with O2 and V2O5 using an elemental analyzer (Costech) coupled to the mass spectrometer. Standardization of the method was based on several other sulfide and sulfate materials from different laboratories and international standards OGS-1 and NBS123, which were used to convert the analytical results to the Vienna-Canyon Diablo Troilite (VCDT) for sulphur in sulphate and Vienna Standard Mean Ocean Water (VSMOW) for oxygen in sulphate. The precision was ± 0.15‰ (1σ) based on repeated internal standards. δ18OSO4 was measured as CO with CFIRMS (Thermo Electron Delta V, Thermo Fisher Scientific Inc., Waltham, MA, USA). Samples were combusted with excess C at 1350 °C using a thermal combustion elemental analyzer (ThermoQuest Finnigan, Thermo Fisher Scientific Inc., Waltham, MA, USA) coupled to the mass spectrometer. Precision was ± 0.4‰ (1σ), based on repeated internal standards.

2.4. Microbiological Analyses

Ready-to-use microbiological media Compact Dry plates (Nissui Pharmaceutical, Tokyo, Japan) were used to determine the concentration of heterotrophic aerobic bacteria (Compact Dry TC), coliforms and Escherichia coli (Compact Dry EC), and enterococci (Compact Dry ETC) directly from 1 mL of the water sample. EC, ETC, and TC plates were incubated at 37 °C for 48 h; in addition, TC plates were incubated at 20 °C for 7 days. Concentrations of bacteria were expressed as Colony-Forming Units (CFU) per millilitre.
AquaSnap Total (Hygiena, Santa Ana, CA, USA) was used to measure total ATP content in a water sample to determine both microbial ATP (living cells and particulate matter) and free ATP (non-microbial or dead cells). Results were expressed in Relative Light Units (RLU), where 1 RLU equals 1 fmol ATP per millilitre.
Table 1. Summary list of monitoring parameters, sampling periods, and methodologies.
Table 1. Summary list of monitoring parameters, sampling periods, and methodologies.
ParameterSampling PeriodMethod, Instrument Used
Physico-chemical
parameters
Monthly, November 2017–July 2018WTW Multi 3430 Multiparameter probe (WTW GmbH, Weilheim, Germany)
Geochemical analyses (anions, cations)21 November 2017, 23 July 2018Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), Ion Chromatography (IC), Overrange Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) (for details see https://actlabs.com/: accessed 30 November 2021) and YSI 9300 spectrophotometer (YSI, Yellow Springs, OH, USA)
Dissolved hydrogen sulphide 21 November 2017 and 23 July 2018YSI 9300 spectrophotometer (YSI, Yellow Springs, OH, USA)
Isotopic composition of hydrogen and oxygen (δ18O, δ2H)21 November 2017, 23 July 2018H2–H2O [31] and CO2–H2O [32,33] equilibration technique; dual inlet isotope ratio mass spectrometer (DI IRMS, Finnigan MAT DELTA plus, Finnigan MAT GmbH, Bremen, Germany) with an automated CO2–H2O and H2-H2O HDOeq 48 Equilibration Unit (custom built by M. Jaklitsch)
Isotopic composition of dissolved inorganic carbon (δ13CDIC)21 November 2017, 23 July 2018Continuous flow IRMS (Europa Scientific 20–20) with an ANCA-TG preparation module (Sercon Limited, Crewe, UK)
IsoPrime 100 coupled with the Multiflow preparation module (Elementar, Manchester, UK)
Total alkalinity after Gran21 November 2017CAT titrator (Ingenierbüro CAT, M. Zipperer GmbH Ballrechten-Dottingen, Germany), pH meter
(Mettler Toledo AG 8603, Schwerzenbach, Switzerland)
Tritium (3H)21 November 2017Liquid scintillation counting (LSC) TriCarb 3170 TR/SL (PerkinElmer, Waltham, MA, USA)
δ34SSO4 and δ18OSO4 in sulphate21 November 2017Continuous-flow isotope gas-ratio mass spectrometer (ThermoQuest Finnigan Delta PlusXL and Thermo Electron Delta V; Thermo Fisher Scientific Inc., Waltham, MA, USA)
Microbiological
analysis
23 July 2018Biomass (total ATP content), cultivation of microbial indicators (heterotrophic aerobic bacteria, E. coli/coliforms, enterococci)

3. Results and Discussion

3.1. Geological Characterisation of the Žvepovnik Spring

The spring is situated in the Upper Savinja Valley at the eastern outskirts of the Kamnik–Savinja Alps. Structurally, the spring area belongs to the easternmost segment of the Southern Alps formed by Miocene south-directed thrusting and post-thrusting, eastward-oriented strike-slip movements [39,40]. Today, this segment of the Southern Alps is bounded by the Periadriatic fault zone to the north and the Sava fault to the south. The area between these W–E to WNW–ESE major strike-slip fault zones is characterized by NE–SW-oriented connecting faults that show a relative lowering of the eastern tectonic blocks [41]. A cross-cutting of these tectonic directions is evident also in the wider area of the Žvepovnik spring (Figure 2).
The Kamnik–Savinja Alps are dominated by a thick carbonate succession that originated from the Upper Permian to the end-Triassic. The carbonates built up a near-continuous succession with just a few formations in the Lower and Middle Triassic that additionally contain interlayers of fine-grained clastics [42,43,44]. The local occurrence of Middle Triassic volcanic rocks has also been reported [45,46]. With the prominent disconformity, Mesozoic carbonates are overlain by Oligocene beds. They begin with a locally present Okonina conglomerate that passes upwards into the widespread, deep-marine shales and volcanic rocks of the Smrekovec Volcanic Complex [47,48].
Due to its tectonic structure, Mesozoic succession dominates in the central part of the Kamnik–Savinja Alps, whereas, towards the east dolomite, outcrops in the fault-bounded carbonate massifs are surrounded by Oligocene clastic and volcanic rocks [41,49]. Such a situation is also observed in the Žvepovnik spring area that is dominated by Oligocene fine clastics, though close to the spring Anisian Dolomite outcrops in a small erosional window. Further to the NE, carbonates form the extensive Golte massif [47].
In the near vicinity of the Žvepovnik spring, a sulphidic ore deposit of proposed hydrothermal origin is present (Figure 2). The Lepa njiva antimony deposit, where stibnite is associated with barite and chert, is well studied [50,51]. In older publications, the ore is considered to occur in Upper Permian limestone, but Mlakar [52] pointed out that it also spreads to the Triassic beds close to the Oligocene volcanic and clastic rocks. Associated barite enrichment is documented also in the closely situated Anisian carbonates (Figure 2) [47].
Our detailed mapping of the surroundings of the Žvepovnik spring revealed that Anisian dolomite is in erosional contact with the overlying Oligocene beds that are dominated by fine-grained clastic rocks in this area. The boundary is stratigraphic in the north and the west, whereas, in the south and the east, there are fault contacts. Tectonically disturbed Oligocene beds at the microlocation of the Žvepovnik spring indicate another W–E fault situated there. It runs parallel to the faults that mark the southern margin of the Anisian Dolomite. No cinematic indices were recognized in this fault; however, considering the kinematic behaviour of the parallel faults, a relative downward displacement of the southern block is most probable. A normal fault seems more probable, but strike-slip movements (with beds dipping with strikes different from the strike of a fault plane) cannot be excluded.
Sedimentological study of the contact between Anisian dolomite and Oligocene clastics yields important information on the potential source of the sulphur. The study revealed that the uppermost part of the dolomite is a clast-supported dolomite breccia composed solely of dolomite clasts equal to the underlying massive dolomite. In some clasts, a primary limestone structure is still recognizable. They belong to peloidal/bioclastic wackestone and calcimicrobic boundstone with fenestrae, both microfacies typical for Anisian carbonates [53]. Several foraminifera were recognized, i.e., Earlandia sp., Aulotortus sinuosus, Reophax sp., and Citaella dinarica, the latter indicating the Anisian age of the dolomite (dolomite breccia). Oncoids were also detected. Matrix in breccia is microsparitic dolomite, locally sparitic dolomite, and calcite. Breccia contains veins filled with saddle dolomite and calcite with pronounced twin lamellae. Anisian dolomite originated in shallow seas and was later dolomitized. The uppermost part of the succession was brecciated, presumably due to long exposure prior to sedimentation of overlying beds. The transport of dolomite clasts was negligible. Dolomite and calcite veins probably mineralized from the hydrothermal waters.
Above the disconformity, an Oligocene fine clastic succession material was deposited. These beds consist largely of marl/shale that contain fine quartz grains, muscovite, and plankton foraminifera. Silt laminae and thin beds also occur. A thin, matrix-supported breccia lies in the laterally discontinuous, i.e., channelized, beds (up to 20 cm) that are intercalated within fine clastic rocks. The matrix in breccia is shale/marl with quartz and muscovite. Clasts are polimict and belong to diverse carbonates such as bioclastic packstone, dolostone, and ooidal grainstone, the latter of which typical for the Lower Triassic Werfen formation [53]. Additionally, plasticlasts occur and indicate the erosion of fine-clastic sediment. These clasts show partial dolomitization. The matrix and clasts are replaced by pyrite (proved by XRD analysis) that comprises up to 15% of the rock volume. It occurs in microcrystals that are dispersed inside the matrix, concentrated in clusters, or that replace (partially or completely) particular clasts in breccia. We note that pyrite impregnation is observed solely in basal meters of the Oligocene beds. Calcite and dolomite veins similar to those of the underlying dolomite also occur. The age of the Oligocene beds was assessed in previous studies [48]. This formation originated in a deep marine environment, fine clastic by pelagic sedimentation, and probably by diluted turbidites, or breccia by debris flows. Clasts in breccia indicated that older formations (than Anisian dolomite) were also exposed at the time of the deposition laterally from the investigated outcrop. Carbonate veins indicate hydrothermal waters that run through the cracks in these beds. Hydrothermal origin is also implied for the intense pyrite impregnation of the basal Oligocene beds.
Figure 2. Geological characteristics of the Žvepovnik sulphur spring: (a) Geological map of the wider area with the location of the spring and Sb mineralisation (simplified from [47]), (b) detailed geological map and geological A–B cross-section of the Žvepovnik spring of the nearby surroundings (author: T.P.), (c) photograph and sketch of the sedimentologically analysed contact between Anisian dolomite/dolomite breccia (violet) and Oligocene clastics (light brown—shale/marl, blue—breccia) with positions of investigated samples (red stars); outcrop is located approximately 100 m upwards in the same valley as the Žvepovnik sulphur spring.
Figure 2. Geological characteristics of the Žvepovnik sulphur spring: (a) Geological map of the wider area with the location of the spring and Sb mineralisation (simplified from [47]), (b) detailed geological map and geological A–B cross-section of the Žvepovnik spring of the nearby surroundings (author: T.P.), (c) photograph and sketch of the sedimentologically analysed contact between Anisian dolomite/dolomite breccia (violet) and Oligocene clastics (light brown—shale/marl, blue—breccia) with positions of investigated samples (red stars); outcrop is located approximately 100 m upwards in the same valley as the Žvepovnik sulphur spring.
Water 14 01249 g002

3.2. Physico-Chemical Parameters of the Žvepovnik Spring

The measurements of physicochemical parameters are presented in Table S1 and Figure 3. The error of the analysis is very low, at 0.26%. The groundwater temperature at the spring was very constant during the complete measuring period, ranging from 10.4 °C to 10.8 °C (on average 10.6 °C), and therefore showed no significant seasonal fluctuations. pH varied from 7.14 to 7.44, with an average value of 7.30. The specific electroconductivity was rather constant, ranging from 635 μs/cm to 723 μs/cm. The only significantly lower value (380 μs/cm) was measured on 12 December 2017—one day after heavy rain with a spring discharge three times higher than usual (Table S1), indicating mixing with precipitation water. Values for dissolved oxygen were relatively low, mostly around 50% (again, the exception is the day with the highest discharge, when saturation was close to 100%; Table S1). However, the values were much higher than in the Žveplenica spring, where very low oxygen saturation was observed (0.93 mg/L or 10% [8]).
Anoxic conditions, i.e., D.O. < 0.5 mg/L, were not observed in the water of the Žvepovnik spring during our investigation (Table S1); on the contrary, the water was oxygenated to some extent, but still contained a detectable amount of hydrogen sulphide (Table S4).

3.3. Hydrogeochemistry of the Žvepovnik Spring

The major geochemical results for the Žvepovnik groundwater are presented in Tables S2 and S3 (supplementary material). Spring groundwater is dominated by HCO3, SO42, Ca2+, and Mg2+ ions. The Ca2+/Mg2+ molar ratio (1.4) indicates that dolomite (the major dissolving rock in the catchment area) weathering can be a source of major solutes within the aquifer; however, this mineral is not the only one dissolving, as the ratio greater than 1 indicates higher calcite content, corresponding also to limestone and/or gypsum/anhydrite dissolution. The SO42 ion concentration is relatively high (109–116 mg/L). Most trace elements were found to be below the detection limit (Table S2).
In the Piper diagram (Figure 4), the geochemical composition of the Žvepovnik spring is placed very close to the 201 m-deep borehole TO-1 (located approximately 3 km southeast of the spring [54]) and slightly farther than the Žveplenica spring [8], indicating a much higher source of sulphate ion and slightly higher Ca2+ content than in the Žveplenica spring. The water in the Žvepovnik spring therefore comes not only from the dissolution of dolomite, as was expected from the geological mapping. We supposed four sources of sulphur, which are discussed further at the end of this chapter, along with the isotopic data results: (1) oxidation of pyrite (enrichment detected at the disconformity between Triassic and Oligocene beds), (2) microbial-driven dissimilatory sulphate reduction, (3) dissolution of gypsum/anhydrite (occurring along with the Upper Permian carbonates and in Lower Triassic rocks) in our case study, and (4) dissolution of barite. The latter is possible locally due to occurring mineral deposits of barite (explained in the geological setting). In Figure 5, the concentrations of Ba2+ and SO42 ions fall directly among the published water samples [55], making the origin of sulphur from this mineral feasible.
There are some interesting differences between the chemical analyses we conducted in November 2017 and July 2018. In the July 2018 sampling, ammonium was present in the spring, chloride levels were higher, and chlorine was also detected (Table S4). The results of the chemical analyses indicate that the spring may be impacted to some degree by anthropogenic factors or agricultural activities. There is a pasture with grazing livestock in the vicinity.

3.4. Geochemical Modelling of the Žvepovnik Spring

The calculated saturation index (SI) for the most abundant minerals shows a slight undersaturation of calcite and oversaturation of dolomite (Table 2). The partial pressure of CO2 in the water was 10−1.95 bar (equal to 11,220 ppm), which is 27 times the saturation of atmospheric CO2 (410 ppm = 10−3.39 bar). Relatively large undersaturation values were recorded for gypsum and anhydrite, therefore the Žvepovnik groundwater is far from an equilibrium state for these two minerals and the origin of the sulphur cannot be determined from these two minerals alone (Table 2).
Basic geochemical modelling in PHREEQC software was performed for six equilibrium scenarios: S1: water equilibrium with dolomite only, S2: calcite + dolomite, S3: dolomite + gypsum, S4: calcite + dolomite + gypsum, S5: dolomite + barite, and S6: calcite + dolomite + barite (Table 2). Other minerals were not considered due to their relatively low abundance and low solubility, and thus cannot contribute greatly to the major dissolved ions in groundwater. Saturation indices from the Žveplenica spring [8] were also included in this table for comparison. From the results, it is possible to observe that none of the scenarios modelled can definitively explain the minerals in equilibrium, but some models are more realistic than others. For instance, dolomite in the spring water is oversaturated with no calcite or dolomite precipitating. There are many reasons for the non-precipitation of dolomite [56], with the most important ones related to kinetics and crystal structure. The problematic precipitation rates of dolomite compared to calcite are referred to as “the dolomite problem”, with most being described in sedimentological references (a good review was given by [57]). In comparison to the vast amounts of dolomite beds in former geological periods, recent dolomites are forming in very restricted areas. The reasons for this are many, but most authors point to the slow reaction kinetics [58,59]. Consequently, dolomite can be far more oversaturated than the calcite before precipitating. Therefore, the most probable of the four scenarios (Table 2) seems to be the dissolution of dolomite and/or calcite (scenarios S1 and S2). This corresponds to the geological setting and molar ratio of Ca to Mg, which is approximately 1.4, indicating more calcitic dolomites than pure dolomites. The SIs of gypsum and anhydrite in scenario S1 are most similar to the actual SIs recorded in the Žvepovnik spring, with all being quite undersaturated.
However, the dissolution of gypsum and anhydrite cannot be excluded, despite their undersaturation, as discussed further on. The undersaturation of both minerals can be explained by the mixing of deeper-sourced waters, which are in contact with the sulphate-bearing minerals (Upper Permian and/or Lower Triassic), where the water reaches equilibrium with both minerals, and of shallow waters, which are infiltrated as meteoric waters and flow through the Middle Triassic carbonate, predominantly dolomite aquifer. The latter are undersaturated with gypsum and/or anhydrite, though are in equilibrium with calcite/dolomite. As a result, deeper sulphate-dissolving waters are mixed with shallow carbonate-equilibrated waters.
Differences in the partial pressure of CO2 (Table 2) could be related to biological activity and not only to the dissolution or precipitation of carbonates. A comparison with the Žveplenica sulphur spring shows lower undersaturation with gypsum/anhydrite in the Žvepovnik spring and higher sulphate concentrations. Žveplenica water is also related to greater dolomite dissolution.

3.5. Isotopic Characteristics of the Žvepovnik Spring

The isotopic composition of hydrogen and oxygen of the Žvepovnik spring was determined during two different seasons, namely in autumn 2017 and summer 2018 (Table 1 and Table S3). No significant difference was observed between the seasons (Figure 6), with mean δ2H and δ18O values of −60.3‰ and −8.85‰, respectively, which fit along the global meteoric water line (GMWL; δ2H = 8 × δ18O + 10; [60]). No monitoring of isotopes in precipitation is performed regularly in this area, but some data do exist for the nearby Velenje basin for the period 2012–2015 [61]. The isotopic values of the Žvepovnik spring fall into the range of Velenje precipitation and were very similar to the average weighted values of Velenje precipitation (−64.9‰ and −9.14‰ for δ2H and δ18O, respectively). Results indicate slightly modified local modern precipitation as a source of the Žvepovnik without modifications due to water evaporation or other geochemical processes that would change the hydrogen and oxygen isotopic signal. We also compared our results with other available data for similar sulphur springs [7,8,10] and groundwater from karst-fissured aquifers from central Slovenia [62] and Pliocene and Triassic boreholes from the Velenje area [63]. Isotopic data from the Žvepovnik spring were similar to the mean data values from karst-fissured aquifers in central Slovenia [62] and the Sovra artesian borehole [8] and are typical for groundwater in shallow Slovenian aquifers where precipitation represents the primary source of recharge [64].
Boreholes made in Pliocene sediments from the Velenje area [63] typically have more negative values that plot slightly higher left above the GMWL and may be due to recharge under different (cooler) climatic conditions (e.g., paleorecharge), recharge of predominantly winter precipitation, or from higher elevations [61]. δ2H and δ18O groundwater values from Triassic boreholes from Velenje area were slightly more negative than our data indicated [63], which likely reflects the mixing of precipitation and shallow groundwater. In contrast, average δ2H and δ18O values of the Žveplenica sulphur karst spring [8], the Studenec karst spring [7], the Izola submarine and terrestrial sulphur springs [9], and the Smrdljivec sulphur spring [10] (located in the western part of Slovenia, Figure 1) are typically more positive and also plot all along the GMWL, indicating the predominant influence of local precipitation that changes across Slovenia in space and time [65,66].
δ13CDIC for groundwater from the Žvepovnik spring was measured in two seasons: November 2017 (autumn) and July 2018 (summer). Alkalinity (measured only in November 2017) was measured as 5.4 mM and δ13CDIC value was −12.5‰. Alkalinity and δ13CDIC values (Figure 7) indicate that the Žvepovnik spring has a δ13CDIC value characteristic of karstic and fractured aquifers in Central Slovenia [63]. The Žveplenica sulphur karst spring from the Trebuša valley had similar δ13CDIC values as the Žvepovnik spring, which was enriched with 12C isotope by 0.6‰ and had a higher alkalinity (about 1 mM) than the Žveplenica karstic spring [8]. The Izola sulphur submarine and terrestrial springs sampled in 2020–2021 in spring and autumn yielded different δ13CDIC and alkalinity values (Figure 7); the submarine springs originated in carbonate rocks, while the terrestrial springs originated in flysch rocks [9]. Biogeochemical processes were calculated as follows: line 1 (with a value of 1.2‰): dissolution of carbonates according to the average δ13CCaCO3 (2.2‰) value—predicted value [19] resulting in 1‰ ± 0.2 enrichment in 12C in DIC [67], line 2 (with a value of −12.5‰) non-equilibrium carbonate dissolution by carbonic acid produced from soil zone CO2 [19], and line 3 (with a value of −18.2‰) open-system equilibration of DIC with soil CO2 originating from the degradation of organic matter with δ13Csoil = −27.2‰ [19]. The Žvepovnik groundwater falls within line 2 (Figure 7) due to the non-equilibrium carbonate dissolution caused by the carbonic acid produced from soil zone CO2.
The tritium (3H) concentration was 3.85 TU, which is close to the detection limit of the method. The value indicates the mixing of recent and older water, which means a deep source of water mixed with precipitation. The recorded 3H concentrations were comparable with those measured at Žveplenica [8].
Sulphate concentrations in the Žvepovnik spring ranged from 109 to 116 mg/L. Sulphate isotopic data were measured once. δ34SSO4 and δ18OSO4 values were 17.7‰ and 9.4‰, respectively. We compared our results with the available data for similar sulphur springs in Slovenia [8,10,68] and data from the nearby Velenje basin [61]. The results from previous studies, such as the Žveplenica spring, indicated lower sulphate concentrations and negative δ34SSO4 values compared to Žvepovnik and revealed that the sulphur in the sulphate is generated via the oxidation of igneous sulphate [8]; the results of δ34SSO4 and δ18OSO4 from the springs at Izola generally yielded values characteristic of seawater, and some can be associated with anaerobic bacterial activity [10,68]. δ34SSO4 values of Velenje basin groundwater returned a wide range of isotopic concentrations, indicating different sources of sulphate [61]. The δ34SSO4 and δ18OSO4 values of the Žvepovnik spring, however, fell into the range of groundwater values from the Velenje basin, and these values offer several possible explanations for the sulphur and the mixing of waters (deep and shallow) in the spring. The deep water at the Žvepovnik spring gives us information about the origin of lithospheric sulphur. The isotopic values of sulphate fell within the range characteristic for the dissolution of gypsum/anhydrite (evaporites) [69] occurring along with the Upper Permian carbonates and in Lower Triassic rocks in our case study (Figure 8).

3.6. Microbiological Characteristics of the Žvepovnik Spring

ATP as a criterion for microbial biomass indicated concentrations of microorganisms in the range of 105 CFU/mL (24 RLU) based on Hygiena Biomass Estimation (Table 3). Isolates indicative of enterococci were detected in the spring water. Their presence in water usually indicates poor sanitary conditions, but they can also be found on plants and are capable of reproducing in extra-enteric environments [70]. Future studies should provide a more extensive insight into the microbiology of the spring, focusing on faecal indicators and possible measures to protect the spring recharge area.

3.7. The Origin of Sulphur in the Žvepovnik Spring

Complementing the regional geological, geochemical, and isotopic data, the results show the main source of the Žvepovnik spring to be local precipitation, which undergoes different infiltration. Therefore, the spring water represents a mixing of (1) deeper waters, which are in contact with the gypsum and/or anhydrite (proven by the results of δ34SSO4 and δ18OSO4), as both minerals appear regionally in both Upper Permian carbonate rocks and Lower Triassic carbonate–clastic successions [42,71], and (2) shallow waters, which flow through the Middle Triassic carbonate aquifer, which outcrops on the surface in the immediate proximity of the Žvepovnik spring. Both types of water are likely in geochemical equilibrium with gypsum and/or anhydrite in the deeper aquifer and also with dolomite (and calcite) in the shallow carbonates and become undersaturated with gypsum and/or anhydrite when mixed near the surface, as the shallow carbonates are more permeable [72,73,74]. Carbon in the water comes from the non-equilibrium carbonate dissolution by carbonic acid produced from soil zone CO2, as confirmed by δ13CDIC values. The dissolution of barite is also a possible source of sulphur, as the concentrations of both Ba and SO4 plot directly among the samples of water-dissolving Ba as a function of dissolved sulphate [55]. Barite was found in the Lepa njiva ore deposit, which is expected to occur on a larger scale in carbonate rocks just under oligocene vulcanoclastes, including areas closer to the Žvepovnik spring [50,51,52] (Figure 9). Some hydrogen sulphide in the spring can also be linked to microbial dissimilatory sulfate reduction. One direct proof of the sulfate reduction process is the formation of reduced inorganic sulfur compounds (e.g., as sulfide minerals FeS2) which were found in the geological profile (Figure 1). The fourth possible origin of the sulphur is the oxidation of pyrite into sulphate. We assumed that the last two might contribute to the total amount of sulphur compounds in smaller quantities, despite the large amounts of pyrite found in the outcrops and detected in thin sections.

4. Conclusions

Based on the holistic approach used, including the geological, geochemical, isotopic, and microbiological data, the characteristics of the Žvepovnik spring were determined. The results represent an important upgrade in our understanding of the complexity of groundwater flow and mixing within carbonate environments. Namely, the Žvepovnik spring indicates a permanent, deep sulphur-rich source that is diluted by precipitation-responsive, near-surface dolomitic groundwater.
The Žvepovnik spring occurs near the contact between the Triassic dolomite (carbonate aquifer) and the overlying Oligocene shales and volcanoclastic beds (barrier). The microlocation of the spring is bound to what we assume to be a normal fault of unknown displacement, but one with a hydrogeological role (either as a barrier or a zone of increased permeability).
The physico-chemical parameters of the Žvepovnik spring are generally constant. The mixing of sulphide-containing water with the near-surface groundwater is indicated by lower specific electroconductivity and higher oxygen saturation after a rain event.
Geochemically, the Žvepovnik spring indicates that dolomite (and/or calcite) weathering could be the main source of the major solutes within the aquifer. Based on the geochemical modelling, we can conclude that the spring groundwater is a mixture of sulphate-dissolving waters (the water is in equilibrium with gypsum and/or anhydrite) with carbonate-equilibrated waters (undersaturated waters with both minerals, far from equilibrium).
δ2H and δ18O values indicate local precipitation as the main source of water for the Žvepovnik spring. δ13CDIC originates from the degradation of organic matter and the dissolution of carbonates. 3H concentrations indicate the mixing of recent and older water. The isotopic values of sulphate (δ34SSO4 and δ18OSO4) indicate the dissolution of evaporites.
We therefore propose four possible sources of the sulphur compounds in the Žvepovnik spring. The first and most likely is the dissolution of gypsum/anhydrite, which occurs in the Upper Permian and Lower Triassic succession. As a result of dissolution from barite mineral deposits (known in the wider area), the barite may represent a second source of sulphur—possible, though contributing only small amounts. Although significant quantities of pyrite were detected in the outcrops around the spring there, are two potential sources for the sulphur compounds—microbial driven dissimilatory sulphate reduction and oxidation of pyrite into sulphate. Considering both the geological and isotopic compositions, all four alternatives are possible.
The coordinated and integrated geochemical investigation of the Žvepovnik sulphur spring provided crucial information about the spring’s properties that will serve science and the local community. It is important to conduct such investigations as soon as such natural features are discovered—especially in vulnerable karst areas—in order to adopt appropriate management plans.

Supplementary Materials

The Microsoft excel file is available online at https://www.mdpi.com/article/10.3390/w14081249/s1, Table S1: Physico-chemical measurements, spring discharge, and the odour of the sampled water. Q: Spring discharge, D.O.: Dissolved oxygen, EC: Electroconductivity, TDS: Total Dissolved Solids; Table S2: Results of hydrogeochemical analysis, sampled on 21 November 2017; Table S3: Results of total alkalinity after Gran and isotopic analysis, sampled on 21 November 2017 and 23 July 2018; Table S4: Results of additional chemical analyses.

Author Contributions

Conceptualization, P.Ž.R.; methodology, P.Ž.R., T.V., B.R., T.K., J.M. and P.V.; software, T.V.; formal analysis, T.P., T.K., J.M. and P.V.; investigation, P.Ž.R., T.P., T.V., B.R. and J.M.; data curation, P.Ž.R., T.V., B.R., T.K., J.M. and P.V.; writing—original draft preparation, P.Ž.R., T.V., B.R., T.K., J.M., P.V., L.S. and T.P.; writing—review and editing, P.Ž.R.; visualization, T.V., B.R., T.K., P.V., J.M. and P.Ž.R. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully acknowledge the financial support of the Slovenian Research Agency (research core fundings No. P1-0195, P6-0119 and P1-0143) and the project J1-1712: “Record of environmental change and human impact in Holocene sediments, Gulf of Trieste)” which was also financially supported by the Slovenian Research Agency.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available in supplementary material in this article.

Acknowledgments

Special thanks to Mojca Zega for introducing us to the investigative problem and encouraging us to do the research. Many thanks also to Alojzij Gluk for providing fieldwork assistance, to Franc Dobrovc who allowed us to undertake research in his own territory, to Sara Skok and Stojan Žigon for laboratory support, and to Jeff Bickert for assistance with English language editing.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. White, W.B. Springwater geochemistry. In Groundwater Hydrology of Springs: Engeneering, Theory, Management, and Sustainability; Kresic, N., Stevanovic, Z., Eds.; Butterworth-Heinemann: Burlington, NJ, USA, 2010; pp. 231–268. [Google Scholar] [CrossRef]
  2. Khadka, K.; Rijal, M.L. Hydrogeochemical assessment of spring water resources around Melamchi, Central Nepal. Water Pract. Technol. 2020, 15, 748–758. [Google Scholar] [CrossRef]
  3. Yang, D.; Yang, Y.; Xia, J. Hydrological cycle and water resources in a changing world: A review. Geogr. Sustain. 2021, 2, 115–122. [Google Scholar] [CrossRef]
  4. Allshorn, S.J.L.; Bottrell, S.H.; West, L.J.; Odling, N.E. Rapid karstic bypass flow in the unsaturated zone of the Yorkshire chalk aquifer and implications for contaminant transport. In Natural and Anthropogenic Hazards in Karst Areas: Recognition, Analysis and Mitigation; Parise, M., Gunn, J., Eds.; Special Publications; Geological Society: London, UK, 2007; Volume 279, p. 111. [Google Scholar] [CrossRef]
  5. Maurice, L.; Farrant, A.R.; Mathewson, E.; Atkinson, T. Karst hydrogeology of the Chalk and implications for groundwater protection. In The Chalk Aquifers of Northern Europe; Farrell, R.P., Massei, N., Foley, A.E., Howlett, P.R., West, L.J., Eds.; Special Publications; Geological Society: London, UK, 2021. [Google Scholar] [CrossRef]
  6. Keim, D.M.; West, L.J.; Odling, N.E. Convergent flow in unsaturated fractured chalk. Vadose Zone J. 2012, 11. [Google Scholar] [CrossRef]
  7. Mulec, J.; Oarga, A.; Schiller, E.K.; Perşoiu, A.; Holko, L.; Šebela, S. Assessment of the physical environment of epigean invertebrates in a unique habitat: The case of a karst sulfidic spring, Slovenia. Ecohydrology 2015, 8, 1326–1334. [Google Scholar] [CrossRef]
  8. Zega, M.; Rožič, B.; Gaberšek, M.; Kanduč, T.; Žvab Rožič, P.; Verbovšek, T. Minerlogical, hydrogeochmical and isotopic characteristics of the Žveplenica sulphide karstic spring (Trebuša Valley, NW Sllovenia). Environ. Earth Sci. 2015, 74, 3287–3300. [Google Scholar] [CrossRef]
  9. Šušmelj, K.; Verbovšek, T.; Kanduč, T.; Vreča, P.; Zuliani, T.; Nagode, K.; Čermelj, B.; Žvab Rožič, P. Hidrogeokemične in izotopske karakteristike žveplenih izvirov pri Izoli. In Geološki Zbornik 26, Proceedings of the 25th Meeting of Slovenian Geologists, Ljubljana, Slovenia, 8 October 2021; Rožič, B., Ed.; Oddelek za Geologijo: Ljubljana, Slovenia, 2021; pp. 133–136. [Google Scholar]
  10. Mulec, J.; Oarga-Mulec, A.; Skok, S.; Šebela, S.; Cerkvenik, R.; Zorman, T.; Holko, L.; Eleršek, T.; Pašić, L. Emerging Ecotone and Microbial Community of a Sulfidic Spring in the Reka River near Škocjanske jame, Slovenia. Diversity 2021, 13, 655. [Google Scholar] [CrossRef]
  11. Medici, G.; Smeraglia, L.; Torabi, A.; Botter, C. Review of modeling approaches to groundwater flow in deformed carbonate aquifers. Groundwater 2021, 59, 334–351. [Google Scholar] [CrossRef]
  12. Medici, G.; West, L.J. Groundwater flow velocities in karst aquifers; importance of spatial observation scale and hydraulic testing for contaminant transport prediction. Environ. Sci. Pollut. Res. 2021, 28, 43050–43063. [Google Scholar] [CrossRef]
  13. Cartwright, I.; Weaver, T.R.; Cendón, D.I.; Keith Fifield, L.; Tweed, S.O.; Petrides, B.; Swane, I. Constraining groundwater flow, residence times, inter-aquifer mixing, and aquifer properties using environmental isotopes in the southeast Murray Basin, Australia. Appl. Geochem. 2012, 27, 1698–1709. [Google Scholar] [CrossRef]
  14. Dansgaard, W. Stable isotopes in precipitation. Tellus 1964, 16, 436–468. [Google Scholar] [CrossRef]
  15. Clarke, I.D.; Fritz, P. Environmental Isotopes in Hydrogeology; Lewis: New York, NY, USA, 1997; p. 328. [Google Scholar]
  16. Aggarwal, P.K.; Froehlich, K.F.; Gat, J.R. Isotopes in the Water Cycle; Springer: Dordrecht, The Netherlands, 2005; p. 382. [Google Scholar]
  17. Vreča, P.; Kern, Z. Use of Water Isotopes in Hydrological Processes. Water 2020, 12, 2227. [Google Scholar] [CrossRef]
  18. Atekwana, E.; Krishnamurthy, R. Seasonal variations of dissolved inorganic carbon and δ13C of surface waters: Application of a modified gas evolution technique. J. Hydrol. 1998, 205, 265–278. [Google Scholar] [CrossRef]
  19. Kanduč, T.; Mori, N.; Kocman, D.; Stibilj, V.; Grassa, F. Hydrochemistry of Alpine springs from North Slovenia: Insights from stable isotopes. Chem. Geol. 2012, 300–301, 40–54. [Google Scholar] [CrossRef]
  20. Chandrajith, R.; Jayasena, H.A.H.; van Galdern, R.; Barth, J.A.C. Assessment of land subsidence mechanisms triggered by dolomitic marble dissolution from hydrogeochemistry and stable isotopes of spring waters. Appl. Geochem. 2015, 58, 97–105. [Google Scholar] [CrossRef]
  21. Mohammadi, Z.; Vaselli, O.; Muchez, P.; Claes, H.; Capezzuoli, E.; Swennen, R. Hydrogeochemistry, stable isotope composition and geothermometry of CO2-bearing hydrothermal springs from Western Iran: Evidence for their origin, evolution, and spatio-temporal variations. Sediment. Geol. 2020, 404, 105676. [Google Scholar] [CrossRef]
  22. Knuth, J.M.; Potter-McIntyre, S.L. Stable isotope fractionation in a cold spring system, Utah, USA: Insights for sample selection on Mars. Astrobiology 2021, 21, 235–245. [Google Scholar] [CrossRef]
  23. Nriagu, J.; Rees, C.; Mekhtiyeva, V.; Lein, A.Y.; Fritz, P.; Drimmie, R.J.; Pankina, R.G.; Robinson, B.W.; Krouse, H.R. Hydrosphere. In Stable Isotopes in the Assessment of Natural and Anthropogenic Sulphur in the Environment; Krouse, H., Grinenko, V., Eds.; John Wiley and Sons: Chichester, UK, 1991; pp. 229–230. [Google Scholar]
  24. Pearson, F.J.; Rirhtmire, C.T. Sulphur and oxygen isotopes in aqueous sulfur compounds. In Handbook of Environmental Isotope; Fritz, P., Fontes, J.C., Eds.; Elsevir: Amsterdam, The Netherlands, 1980; pp. 179–226. [Google Scholar]
  25. Pu, J.; Yuan, D.; Zhang, C.; Zhao, H. Hydrogeochemistry and possible sulfate sources in karst groundwater in Chongqing, China. Environ. Earth Sci. 2013, 68, 159–168. [Google Scholar] [CrossRef]
  26. Brad, T.; Iepure, S.; Sarbu, S.M. The Chemoautotrophically Based Movile Cave Groundwater Ecosystem, a Hotspot of Subterranean Biodiversity. Diversity 2021, 13, 128. [Google Scholar] [CrossRef]
  27. Zgonik, V.; Mulec, J.; Eleršek, T.; Ogrinc, N.; Jamnik, P.; Ulrih, N.P. Extremophilic Microorganisms in Central Europe. Microorganisms 2021, 9, 2326. [Google Scholar] [CrossRef]
  28. Headd, B.; Engel, A.S. Biogeographic congruency among bacterial communities from terrestrial sulfidic springs. Front. Microbiol. 2014, 5, 473. [Google Scholar] [CrossRef] [Green Version]
  29. Jarvis, A.; Reuter, H.I.; Nelson, A.; Guevara, E. Hole-filled Seamless SRTM Data V4, International Centre for Tropical Agriculture (CIAT). 2008. Available online: https://srtm.csi.cgiar.org (accessed on 22 December 2021).
  30. Appelo, C.A.J.; Postma, D. Geochemistry, Groundwater and Pollution, 2nd ed.; A.A. Balkema Publishers: Amsterdam, The Netherlands, 2005; p. 649. [Google Scholar]
  31. Coplen, T.; Wildman, J.; Chen, J. Improvements in the gaseous hydrogen-water equilibration technique for hydrogen isotope ratio analysis. Anal. Chem. 1991, 63, 910–912. [Google Scholar] [CrossRef]
  32. Epstein, S.; Mayeda, T. Variations of 18O content of waters from natural sources. Geochim. Cosmochim. Acta 1953, 4, 213–224. [Google Scholar] [CrossRef]
  33. Avak, H.; Brand, W.A. The Finning MAT HDO-Equilibration—A fully automated H2O/gas phase equilibration system for hydrogen and oxygen isotope analyses. Thermo Electron. Corp. Appl. News 1995, 11, 1–13. [Google Scholar]
  34. International Atomic Energy Agency. Reference Sheet for VSMOW2 and SLAP2 International Measurement Standards. IAEA, Vienna, 8 pp. (Rev 1 dated 2017-07-11). Available online: https://nucleus.iaea.org/sites/ReferenceMaterials/Shared%20Documents/ReferenceMaterials/StableIsotopes/VSMOW2/VSMOW2_SLAP2.pdf (accessed on 31 January 2022).
  35. Gieskes, J.M. The total alkalinity—Total carbon dioxide stem in seawater. In Marne Chemistry of the Sea; Goldberg, E.D., Ed.; John Wiley ad Sons: New York, NY, USA, 1974; pp. 123–151. [Google Scholar]
  36. Zuliani, T.; Kanduč, T.; Novak, R.; Vreča, P. Characterization of bottled waters by multielemental analysis, stable and radiogenic isotopes. Water 2020, 12, 2454. [Google Scholar] [CrossRef]
  37. Miyajima, T.; Yamada, Y.; Hanba, Y.T. Determining the stable isotope ratio of total dissolved inorganic carbon in lake water by GC/C/IRMS. Limnol. Oceanogr. 1995, 40, 994–1000. [Google Scholar] [CrossRef]
  38. Spötl, C. A robust and fast method of sampling and analysis of δ13C of dissolved inorganic carbon in groundwaters. Isot. Environ. Health Stud. 2005, 41, 217–221. [Google Scholar] [CrossRef]
  39. Placer, L. Contribution to the macrotectonic subdivision of the border region between Southern Alps and External Dinarides. Geologija 1998, 41, 223–255. [Google Scholar] [CrossRef]
  40. Placer, L. Principles of the tectonic subdivision of Slovenia. Geologija 2008, 51, 205–217. [Google Scholar] [CrossRef]
  41. Vrabec, M.; Fodor, L. Late Cenozoic tectonics of Slovenia: Structural styles at the Northeastern corner of the Adriatic microplate. In The Adria Microplate: GPS Geodesy, Tectonics and Hazards—NATO Science Series, IV, Earth and Environmental Sciences; Pinter, N., Grenerczy, G., Weber, J., Stein, S., Medak, D., Eds.; Springer: Dordrecht, The Netherlands, 2006; pp. 151–168. [Google Scholar] [CrossRef]
  42. Dozet, S.; Buser, S. Triassic. In The Geology of Slovenia; Pleničar, M., Ogorelec, B., Novak, M., Eds.; Geological Survey of Slovenia: Ljubljana, Slovenia, 2009; pp. 161–214. [Google Scholar]
  43. Celarc, B. Geological Structure of the Northwesternpart of the Kamnik Savinja Alps. Ph.D. Thesis, University of Ljubljana, Ljubljana, Slovenia, 2004. [Google Scholar]
  44. Miklavc, P.; Celarc, B.; Šmuc, A. Anisian Strelovec Formation in the Robanov kot, Savinja Alps (Northern Slovenia). Geologija 2016, 59, 23–34. [Google Scholar] [CrossRef]
  45. Jurkovšek, B. Langobardske plasti z daonelami in pozidonijami v Sloveniji. Geologija 1984, 27, 41–95. [Google Scholar]
  46. Trajanova, M.; Grafenauer, S. Triasni Vulkanizem—Triassic Volcanism. In Geologija Slovenije = The Geology of Slovenia; Pleničar, M., Ogorelec, B., Novak, M., Eds.; Geology Survey of Slovenia: Ljubljana, Slovenia, 2009; pp. 479–490. [Google Scholar]
  47. Mioč, P.; Žnidarčič, M. Tolmač za List Ravne na Koroškem: L 33-54: Socialistična Federativna Republika Jugoslavija, Osnovna Geološka Karta, 1:100.000; Zvezni Geološki Zavod: Beograd, Yugoslavia, 1983. [Google Scholar]
  48. Kralj, P. Terciarne vulkanske formacije = Tertiary Volcanic Formations. In Geologija Slovenije = The Geology of Slovenia; Pleničar, M., Ogorelec, B., Novak, M., Eds.; Geološki Zavod Slovenije: Ljubljana, Slovenia, 2009; pp. 503–516. [Google Scholar]
  49. Buser, S. Geological map of Slovenia 1: 250.000; Geological Survey of Slovenia: Ljubljana, Slovenia, 2010. [Google Scholar]
  50. Bidovec, M. Antimony ore deposit at Lepa njiva. Geologija 1980, 23, 285–313. [Google Scholar]
  51. Drovenik, M.; Pleničar, M. The origin of Slovenian ore deposits. Geologija 1980, 23, 1–157. [Google Scholar]
  52. Mlakar, I. On the lithologic, stratigraphic and structural control of mineralization and the age of the Lepa Njiva antimony deposit. Geologija 1990, 33, 353–395. [Google Scholar] [CrossRef]
  53. Ogorelec, B. Mikrofacies mezozojskih karbonatnih kamnin Slovenije = Microfacies of mesozoic carbonate rocks of Slovenia. Geologija 2011, 54 (Suppl. S2), 1–136. [Google Scholar] [CrossRef]
  54. Juvančič, V.; Sadnikar, J.M. Geotermalna Raziskava v Okonini. 2. Faza, Poročilo; GEOKO, Podjetje za Geološko Svetovanje in Raziskave d.o.o.: Ljubljana, Slovenia, 1994; 10p. (In Slovenian) [Google Scholar]
  55. Hanor, J.S. Barite–Celestine geochemistry and environments of formation. Rev. Mineral. Geochem. 2000, 40, 193–275. [Google Scholar] [CrossRef]
  56. Tucker, M.E.; Wright, V.P. Carbonate Sedimentology; Blackwell: Oxford, UK, 1990; 482p. [Google Scholar]
  57. Warren, J. Dolomite: Occurence, evolution and economically important associations. Earth Sci. Rev. 2000, 52, 1–81. [Google Scholar] [CrossRef]
  58. Morrow, D.W. Diagenesis 1. Dolomite—Part 1. The chemistry of dolomitization and dolomite precipitation. Geosci. Can. 1982, 9, 5–13. [Google Scholar]
  59. Usdowski, E. Synthesis of dolomite and geochemical implications. In Dolomites. The International Association of Sedimentologists Special Publication; Purser, B., Tucker, M., Zenger, D., Eds.; The International Association of Sedimentologists: Cambridge, UK, 1994; Volume 21, pp. 345–360. [Google Scholar]
  60. Craig, H. Isotopic variation in meteoric waters. Science 1961, 133, 1702–1703. [Google Scholar] [CrossRef]
  61. Kanduč, T.; Šlejkovec, Z.; Vreča, P.; Samardžija, Z.; Verbovšek, T.; Božič, D.; Jamnikar, S.; Mori, N.; Grassa, F. The effect of geochemical processes on groundwater in the Velenje coal basin, Slovenia: Insights from mineralogy, trace elements and isotopes signatures. SN App. Sci. 2019, 1, 1518. [Google Scholar] [CrossRef] [Green Version]
  62. Verbovšek, T.; Kanduč, T. Isotope geochemistry of groundwater from fractured dolomite aquifers in central Slovenia. Aquat. Geochem. 2016, 22, 131–151. [Google Scholar] [CrossRef]
  63. Kanduč, T.; Mori, N.; Koceli, A.; Verbovšek, T. Hydrogeochemistry and isotopic geochemistry of the Velenje basin groundwater. Geologija 2016, 51, 7–22. [Google Scholar] [CrossRef]
  64. Brenčič, M.; Vreča, P. Identification of sources and production processes of bottled waters by stable hydrogen and oxygen isotope ratios. Rapid Commun. Mass Spectrom. 2006, 20, 3205–3212. [Google Scholar] [CrossRef]
  65. Vreča, P.; Malenšek, N. Slovenian Network of Isotopes in Precipitation (SLONIP)—A review of activities in the period 1981–2015. Geologija 2016, 59, 67–84. [Google Scholar] [CrossRef]
  66. Kern, Z.; Hatvani, I.G.; Czuppon, G.; Fórizs, I.; Erdélyi, D.; Kanduč, T.; Palcsu, L.; Vreča, P. Isotopic ‘altitude’ and ‘continental’ effects in modern precipitation across the Adriatic-Pannonian region. Water 2020, 12, 1797. [Google Scholar] [CrossRef]
  67. Romanek, C.S.; Grossman, E.L.; Morse, J.W. Carbon isotopic fractionation in synthetic aragonite and calcite: Effects temperature and precipitation rate. Geochim. Cosmochim. Acta 1992, 46, 419–430. [Google Scholar] [CrossRef]
  68. Šušmelj, K.; Žvab Rožič, P.; Vreča, P.; Kanduč, T.; Verbovšek, T.; Nagode, K.; Zuliani, T.; Čenčur Curk, B.; Rožič, B.; Čermelj, B. Hidrogeokemične in izotopske raziskave podmorskih in kopenskih izvirov pri Izoli. In Raziskave s Področja Geodezije in Geofizike 2021; Slovensko združenje za geodezijo in geofiziko: Ljubljana, Slovenia, 2022; pp. 55–64. [Google Scholar]
  69. Krouse, H.R.; Mayer, B. Sulphur and Oxygen Isotopes in Sulphate. In Environmental Tracers in Subsurface Hydrology; Cook, P.G., Herczeg, A.L., Eds.; Springer: Boston, MA, USA, 2000; pp. 195–231. [Google Scholar] [CrossRef]
  70. Boehm, A.B.; Sassoubre, L.M. Enterococci as indicators of environmental fecal contamination. In Enterococci: From Commensals to Leading Causes of Drug Resistant Infection; Gilmore, M.S., Clewell, D.B., Ike, Y., Shankar, N., Eds.; Massachusetts Eye and Ear Infirmary: Boston, MA, USA, 2014. [Google Scholar]
  71. Skaberne, D.; Ramovš, A.; Ogorelec, B. Middle and Upper Permian. In The Geology of Slovenia; Pleničar, M., Ogorelec, B., Novak, M., Eds.; Geological Survey of Slovenia: Ljubljana, Slovenia, 2009; pp. 161–214. [Google Scholar]
  72. Verbovšek, T. Diagenetic effects on the well yield of dolomite aquifers in Slovenia. Environ. Geol. 2008, 53, 1173–1182. [Google Scholar] [CrossRef]
  73. Verbovšek, T. Hydraulic conductivities of fractures and matrix in Slovenian carbonate aquifers. Geologija 2008, 51, 245–255. [Google Scholar] [CrossRef]
  74. Verbovšek, T.; Veselič, M. Factors influencing the hydraulic properties of wells in dolomite aquifers of Slovenia. Hydrogeol. J. 2008, 16, 779–795. [Google Scholar] [CrossRef]
Figure 1. Geographical location of the Žvepovnik spring (red star) with other described sulphur springs in Slovenia (see text for citations). Source of SRTM digital elevation model [29].
Figure 1. Geographical location of the Žvepovnik spring (red star) with other described sulphur springs in Slovenia (see text for citations). Source of SRTM digital elevation model [29].
Water 14 01249 g001
Figure 3. Field measurements of Žvepovnik spring water.
Figure 3. Field measurements of Žvepovnik spring water.
Water 14 01249 g003
Figure 4. Piper plot diagram of the Žvepovnik spring with geochemical data of the Žveplenica spring [8], Smrdljivec spring [10], and the TO-1 borehole [54].
Figure 4. Piper plot diagram of the Žvepovnik spring with geochemical data of the Žveplenica spring [8], Smrdljivec spring [10], and the TO-1 borehole [54].
Water 14 01249 g004
Figure 5. Comparison of some water samples with different concentrations of sulfates and Ba (modified after [55]). The Žvepovnik spring is marked with the blue cross (X).
Figure 5. Comparison of some water samples with different concentrations of sulfates and Ba (modified after [55]). The Žvepovnik spring is marked with the blue cross (X).
Water 14 01249 g005
Figure 6. The δ2H versus δ18O values of the Žvepovnik spring shown relative to the global meteoric water line (GMWL; δ2H = 8 × δ18O + 10; [60]) and compared to other Slovenian springs such as the Žveplenica sulphur karstic spring [8], the Sovra artesian borehole [7], the Studenec spring water [7], the Izola submarine and terrestrial sulphur springs [9], and the Smrdljivec spring [10], as well as to groundwater from karstic and fractured aquifers in central Slovenia [62] and Pliocene and Triassic boreholes from the Velenje area [63].
Figure 6. The δ2H versus δ18O values of the Žvepovnik spring shown relative to the global meteoric water line (GMWL; δ2H = 8 × δ18O + 10; [60]) and compared to other Slovenian springs such as the Žveplenica sulphur karstic spring [8], the Sovra artesian borehole [7], the Studenec spring water [7], the Izola submarine and terrestrial sulphur springs [9], and the Smrdljivec spring [10], as well as to groundwater from karstic and fractured aquifers in central Slovenia [62] and Pliocene and Triassic boreholes from the Velenje area [63].
Water 14 01249 g006
Figure 7. δ13CDIC versus alkalinity in Žvepovnik, Savinja valley, with lines indicating biogeochemical processes with a comparison of δ13CDIC versus total alkalinity of spring water from this and other studies: the Žveplenica sulphur karstic spring [8], tap water from central Slovenia [62], mining groundwater wells (Pliocene and Triassic) from Slovenia [63], and the Izola submarine and terrestrial sulphur springs [9]. See text for explanation of dotted lines 1, 2, and 3.
Figure 7. δ13CDIC versus alkalinity in Žvepovnik, Savinja valley, with lines indicating biogeochemical processes with a comparison of δ13CDIC versus total alkalinity of spring water from this and other studies: the Žveplenica sulphur karstic spring [8], tap water from central Slovenia [62], mining groundwater wells (Pliocene and Triassic) from Slovenia [63], and the Izola submarine and terrestrial sulphur springs [9]. See text for explanation of dotted lines 1, 2, and 3.
Water 14 01249 g007
Figure 8. Values of δ34SSO4 and δ18OSO4 on the diagram with commonly observed isotopic ranges for sulphate of various origins (modified after [69]). The Žvepovnik spring is marked with a blue cross (X).
Figure 8. Values of δ34SSO4 and δ18OSO4 on the diagram with commonly observed isotopic ranges for sulphate of various origins (modified after [69]). The Žvepovnik spring is marked with a blue cross (X).
Water 14 01249 g008
Figure 9. Schematic model of the Žvepovnik spring: sulphur-rich groundwater from the deep sulphidic or sulphatic (more probable) source is mixing with the shallow groundwaters.
Figure 9. Schematic model of the Žvepovnik spring: sulphur-rich groundwater from the deep sulphidic or sulphatic (more probable) source is mixing with the shallow groundwaters.
Water 14 01249 g009
Table 2. Six PHREEQC scenarios (S1 to S6) and spring data with calculated saturation indices for calcite, dolomite, anhydrite and gypsum, and partial pressure of CO2 gas, given as 10SI bar. Saturation indices of these minerals and CO2 (g) are also presented for the speciation of Žvepovnik groundwater. Data for the Žveplenica spring are taken from Zega et al. [8].
Table 2. Six PHREEQC scenarios (S1 to S6) and spring data with calculated saturation indices for calcite, dolomite, anhydrite and gypsum, and partial pressure of CO2 gas, given as 10SI bar. Saturation indices of these minerals and CO2 (g) are also presented for the speciation of Žvepovnik groundwater. Data for the Žveplenica spring are taken from Zega et al. [8].
ScenarioMineralsSICALSIDOLSIANHSIGYPSIBARSICO2(g)
S1Dol−0.460−1.94−1.620.54−1.67
S2Cal + Dol00−1.73−1.40.54−1.8
S3Dol + Gyp0.040−0.3201.29−1.63
S4Cal + Dol + Gyp00−0.3201.3−1.62
S5Dol + Bar−0.460−1.94−1.620−1.67
S6Cal + Dol + Bar00−1.73−1.40−1.8
/Rajserjev graben0.181.31−1.91−1.580.51−2.03
/Žveplenica−0.170.71−2.87−2.55−0.53−2.1
Table 3. Results of microbiological analyses (date of sampling 23 July 2018).
Table 3. Results of microbiological analyses (date of sampling 23 July 2018).
ParameterUnit
Adenosine triphosphate-totalRLU24
Bacteria (37 °C)CFU/ml33
Bacteria (20 °C)CFU/ml142
E. coliCFU/ml0
ColiformsCFU/ml7
EnterococciCFU/ml3
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Žvab Rožič, P.; Polenšek, T.; Verbovšek, T.; Kanduč, T.; Mulec, J.; Vreča, P.; Strahovnik, L.; Rožič, B. An Integrated Approach to Characterising Sulphur Karst Springs: A Case Study of the Žvepovnik Spring in NE Slovenia. Water 2022, 14, 1249. https://doi.org/10.3390/w14081249

AMA Style

Žvab Rožič P, Polenšek T, Verbovšek T, Kanduč T, Mulec J, Vreča P, Strahovnik L, Rožič B. An Integrated Approach to Characterising Sulphur Karst Springs: A Case Study of the Žvepovnik Spring in NE Slovenia. Water. 2022; 14(8):1249. https://doi.org/10.3390/w14081249

Chicago/Turabian Style

Žvab Rožič, Petra, Teja Polenšek, Timotej Verbovšek, Tjaša Kanduč, Janez Mulec, Polona Vreča, Ljudmila Strahovnik, and Boštjan Rožič. 2022. "An Integrated Approach to Characterising Sulphur Karst Springs: A Case Study of the Žvepovnik Spring in NE Slovenia" Water 14, no. 8: 1249. https://doi.org/10.3390/w14081249

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop