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SN Applied Sciences

, 1:1518 | Cite as

The effect of geochemical processes on groundwater in the Velenje coal basin, Slovenia: insights from mineralogy, trace elements and isotopes signatures

  • Tjaša KandučEmail author
  • Zdenka Šlejkovec
  • Polona Vreča
  • Zoran Samardžija
  • Timotej Verbovšek
  • Darian Božič
  • Sergej Jamnikar
  • D. Kip Solomon
  • Diego P. Fernandez
  • Christopher Eastoe
  • Jennifer McIntosh
  • Nataša Mori
  • Fausto Grassa
Research Article
  • 137 Downloads
Part of the following topical collections:
  1. 2. Earth and Environmental Sciences (general)

Abstract

This study investigated the mineralogical and isotopic composition of groundwater and precipitation to identify and constrain geochemical processes within stacked Pliocene and Triassic aquifers in the Velenje coal basin. Scanning electron microscopy combined with energy-dispersive X-ray spectroscopic analysis revealed that suspended matter in the Pliocene aquifer consists of feldspars and quartz, while dolomite, calcite and feldspars are present in the aquifer dewatering Triassic strata. The concentrations of trace elements in Triassic and Pliocene aquifers range from highest to lowest Zn > Fe > Ni > Al > Ba > Mn > B>Li > Mo > As with the majority of trace element concentrations below international drinking water health guidelines. Multivariate principal component analysis indicated that concentrations of Mn, Ba, Eu, Cs, Y, Li and T, pH, conductivity and dissolved oxygen in samples were the best chemical parameter for distinguishing the two aquifers. A significant positive correlation (p < 0.05) was found between Ni, Mn, Co, Zn, As and Mo. Groundwater in the Pliocene aquifer likely has an external source of carbon based on the δ13CCO2 values (− 12.3 to − 3.6‰). The groundwater also has detectable levels of dissolved methane with isotopic values (− 77.7 to − 51.4‰ δ13CCH4; − 247 to − 162‰ δ2HCH4) consistent with microbial methanogenesis. The groundwater in the Triassic aquifer has tritium values (up to 4.1 TU 3H) characteristic of modern recharge (< 50 years), while the lack of detectable 3H (0 TU) in the Pliocene aquifer is consistent with longer residence times.

Keywords

Geochemical processes SEM Trace elements Isotopes Velenje coal basin Slovenia 

1 Introduction

To better understand the effects of geochemical processes on groundwater in the Velenje coal basin, it is important to investigate trace element concentrations, which may be of human health concern, in addition to the stable isotope composition of groundwater, which helps to constrain hydrogeological processes in the aquifer. Trace element (Fe, Mn, Co, As, Cu, Cr, Pb, Zn, Ni, etc.) concentrations and stable isotope compositions (e.g., δ13CCO2, δ18O, δ2Η, δ34SSO4, δ18OSO4) of groundwater depend on multiple natural factors including the aquifer lithology, the quality of the infiltrating water, mineral weathering reactions, groundwater residence time and mixing of groundwater bodies [1, 2, 3, 4, 5, 6]. Anthropogenic sources of trace elements include infiltration from contaminated soils, leaching from industrial effluents and the dumping of sewage sludge [6].

The stable carbon isotope composition of dissolved inorganic carbon (DIC) in groundwater (δ13CDIC) is often dependent on vegetation and lithology. For example, groundwater in aquifers containing non-carbonated minerals typically has δ13CDIC values reflecting soil CO2. Chemical reactions in non-carbonate-bearing aquifers such as silicate hydrolysis and ionic exchange do not significantly shift δ13CDIC values. In contrast, dissolution of calcite (CO2 + H2O + CaCO3 → Ca2+ + 2HCO32−) in aquifers containing marine limestone can significantly alter δ13CDIC values. Most marine carbonates have a δ13C value of between − 2 and 1‰, which produces a δ13CDIC of − 11‰ in temperate climates. Crustal rocks underlying the Velenje coal basin have a similar isotopic composition (− 2‰, δ13C) [7].

It is common to find naturally, carbonated water in areas that receive a high CO2 gas supply from deep CO2 from sources other than the atmosphere or soil [7]. Large amounts of deep CO2 are produced by (1) metamorphism of carbonate rocks, which results in δ13CCO2 values (1–2‰) similar to the original marine limestone; (2) diagenesis of buried organic matter in deep sedimentary basins; and (3) mantle-derived magmatic fluids with δ13C values between − 4.7 and − 8‰ [8]. In the present study, we use δ13CDIC values to calculate δ13CCO2 to decipher the source of CO2 within the Pliocene and Triassic aquifers.

It is also common to find methane dissolved in groundwater [9, 10]. Once groundwater concentrations exceed 2 mg/l of methane, the accumulation of methane gas in unventilated areas can exceed the lower explosive limit (LEL) of 4.4% (vol/vol) of CH4 in the air [9]. Recent studies report CH4 concentrations in groundwater as high as 4.72 mg/l in water supply wells in Great Britain [10] and more than 45 mg/l in wells located in the Appalachian basin (USA) [11]. Such high CH4 levels suggest that groundwater can be a significant source of greenhouse gas emissions to the atmosphere. Stable carbon and hydrogen isotopes of natural gas samples can reveal the source of natural gas, from either thermal maturation of organic matter or microbial degradation of organic matter and subsequent methanogenesis [12]. For instance, thermogenic methane has typical δ13C values above − 58 ± 5‰, while microbial methane is below − 58 ± 5‰ [9].

Sulfur and oxygen isotopes of sulfate are useful for distinguishing sources of sulfate in aquifers and redox processes, such as sulfide oxidation and sulfate reduction, that can modify water quality [13, 14, 15]. Rainwater can have δ34S and δ18O values of approximately 6‰ and 7.5‰, respectively, in regions with little marine influence as reported in the Czech Republic and 12‰ (δ34S) and 16‰ (δ18O) in regions with greater marine influence, such as in the British Isles [13]. The average values of δ34S in bedrock evaporites (gypsum/anhydrite) change from 12‰ at the beginning of the Triassic to 17‰ in the Middle Triassic, with a brief high of 27‰ in the Scythian Stage (the so-called Röt event reported by Holser, 1984). These effects are observed both globally [14] and locally in Triassic strata of neighboring north Italy [15], while δ18O values in evaporite sulfate of Lower to Middle Triassic age remain 10 to 18‰ and 16 ± 1‰ during the Röt event [14, 15]. Bacterial sulfate reduction (BSR) can occur in oxygen-free water with available organic matter, leading to increases in both δ34S and δ18O values in residual sulfate, with the relative rates of increase depending on the rate of BSR [16].

Knowing the stable isotopic composition of water (δ18O and δ2H) is important when it comes to defining the extent of evaporation, recharge conditions (e.g., temperature, seasonality and elevation) and groundwater mixing [1, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26]. For instance, oxygen and hydrogen isotopes are conservative tracers in low-temperature aquifer environments [27] and can be used to determine the provenance of water [28, 29, 30, 31, 32, 33]. Tritium and 14C are frequently used to constrain the relative age of groundwater up to ~ 60 years in the case of 3H and ~ 40.000 years (ka) old in the case of 14C [20, 21, 34, 35]. Principal component analysis (PCA) is useful tool for distinguishing different groundwater bodies, for example in cases of assessing natural versus anthropogenic contamination in groundwater [36, 37, 38].

Previous studies in the Velenje coal basin have reported in situ measurements of T, conductivity, pH, dissolved oxygen (DO), major elements (Ca2+, Mg2+, Na+, K+ and Sr2+, Cl, SO42−, NO3 and total alkalinity) and surface water and groundwater interactions using stable and radiogenic isotope tracers (δ13CDIC, δ18O, δ2H, 87Sr/86S, δ34SSO4) [36, 38, 39, 40].

The present study builds upon those previous studies by providing new (1) mineralogical data for the two major aquifers in the Velenje coal basin; (2) trace element composition (Li, B, Al, Ti, Mn, Fe, Co, Ni, Cu, Zn, As, Rb, Y, Ho, Cs, Ba, Eu, Tl, Pb and U) of the two major aquifers; (3) gas molecular (CH4, CO2, N2) and isotopic composition (δ13CCH4 and δ13CCO2) of the two major aquifers; (4) δ18O and δ2H values of precipitation and groundwater; (5) δ34SSO4 and δ18OSO4 values of groundwater; and (6) tritium levels—obtained using the 3He in-growth method, for estimating the age of groundwater [41].

This work was performed as part of several previous research projects and as a part of a much larger project entitled: “Evaluating geological sequestration of CO2 in low-rank coals: Velenje basin as a natural analogue.” It aimed to take an interdisciplinary approach to the study of mineral and coal composition [42, 43], coalbed gases [44, 45] and groundwater [39, 40, 46] in a sedimentary basin.

1.1 Characteristics of the Velenje basin and groundwater from previous studies

The Velenje basin lies in the northeastern part of Slovenia (Fig. 1a, c) at the junction of the WNW–ESE-trending Šoštanj fault and the E–W-trending Periadriatic zone, which is bounded in the south by the Smrekovec fault segment (Fig. 1a, c). The Šoštanj and Smrekovec faults exhibit strike-slip and younger oblique-slip movement due to compression generated by the collision of the continental plate. Figure 1 and SPM—Supplementary material, show the study area including a map of the deep groundwater sampling locations and a geological cross section of the basin. On the NE side of the Velenje fault in the pre-Pliocene basement, Triassic limestones and dolomites are dominant, while Oligocene to Miocene clastic strata, consisting predominantly of marls, sandstones and volcanoclastic, are dominant on the SW side [47]. In the lower portion of the Scythian layers, anhydrite is present, while the upper part consists of gray stratified dolomite, which transitions toward the top to unstratified dolomite of Anisian age [48].
Fig. 1

a Map of the Velenje basin, showing sampling locations for groundwater, surface waters and precipitation. Rivers are denoted by numbers (MP—measuring points): 1. River Toplica, 2. River Pečovnica, 3. River Klančica, 4. River Velunja, 5. Lake Družmirje, 6. River Ljubela, 7. Lake Velenje, 8. River Lepena, 9. Lake Škale, 10. River Paka. Simplified after Basic Geological Map 1:100,000, sheet Slovenj Gradec (Mioč and Žnidarčič [49]): t: Tertiary tonalite, d: Miocene dacite, P3: Upper Permian limestones and dolomites, P1: Lower Permian limestones, T1: Lower Triassic limestones and clastic rocks, T21: Anisian dolomite, T22: Middle Triassic limestones, M21: Miocene clastic rocks, Ol: Oligocene tuff and marls, Q, Pl: Quaternary and Pliocene clay and gravel. b Sampling locations (Ojstro, Bukova gora, Jama Hrastnik, Savski rov, Retje, Sava-polje) for As of Zasavje region with geological map. C, P: shales, sandstones, siltstones and conglomerates, T1: dolomites with clastic intercalations, T2,3: massive dolomites, T32 + 3: Dachstein limestones with transitions into dolomites, M22: Lithothamnium limestones and marlstones. c Geological sketch of the Velenje basin with cross-sectional NNE–SSW. Main aquifers are Pliocene (Pl-1, Pl-2, Pl-3) and Triassic (T1, T2, T3) dolostones (prevailing) and limestones. Other Triassic and Paleozoic (Pz) lithologies are composed of relatively impermeable strata and therefore do not include significant aquifers. Profiles ab and cd are presented in Kanduč et al. 2014 as supplementary material [46]. d Groundwater contour map of Velenje basin. On the figure also underground mining excavation fields are presented

Two hydrogeological systems are distinguishable in the Velenje coal basin; the first is the upper Plioquaternary and Pliocene aquifer system, composed of gravel–sand and silt, which are themselves divided into upper, middle and lower aquifers. The upper aquifer is divided into alluvial, Quaternary and three different Pliocene aquifers (Pl-1, Pl-2 and Pl-3) that lie above the coal strata. The second hydrogeological system is the lower carbonate aquifer that is made up of Miocene Lithothamnium limestone, Middle Triassic (Anisian) dolomite and limestone and Lower Triassic (Scythian) limestone and dolomite [46].

To date, Giammanco et al. [50] investigated the interactions between gas–water–rock in shallow aquifers in the Šaleška valley and used multivariate PCA to determine different geochemical processes affecting groundwater quality. Later, Kanduč et al. [40] performed a geochemical, isotopic characterization of surface waters and deep groundwater in aquifers dewatering Pliocene and Triassic strata. They found that in the Triassic aquifer HCO3, Ca2+ and Mg2+ ions prevail and groundwater δ18O and δ2H values are close to those of surface waters when plotted on the meteoric global meteoric line and that the 3H data indicate recent recharge. The authors also found that waters belonging to the Pliocene aquifers were enriched with Mg2+, Na+, Ca+, K+ and Si and the 3H concentrations, measured using electrolytic enrichment method, were close to the limits of detection, suggesting older groundwater and different recharge area than the Triassic aquifer.

The groundwater in the Velenje basin is not used as a potable water source, but instead for mining processes, e.g., cooling machinery and abating coal dust. The foremost safety concern for the Velenje mine is the presence of standing groundwater in the strata directly over the lignite seams within the Pliocene clastic section. In the 1970s, the mine experienced multiple groundwater intrusions, and since then, it has been lowering the hydrostatic pressure above the lignite deposits by extracting large volumes of groundwater to produce a cone of depression [51]. Water 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 the average discharge from Triassic limestone aquifers is 3.400 l/min. Hydraulic conductivity values (k) are in the range 1.74 × 10−7 to 2.88 × 10−6 m/s [46]. A groundwater-level contour map is presented in Fig. 1d.

2 Sampling and analytical methods

2.1 Sample collection

Wells were not drilled for the purpose of the study, but they already existed in the study area for dewatering purposes of the mine, lowering hydrostatical pressure to prevent water indoors in the mine. Unfortunately, mining activity has caused the basin to subside; thus, many piezometers have been sheared and are unavailable for sampling. In total, 38 groundwater samples were collected from dewatering wells (samples 1–19 were taken from mine, and samples 19–28 were taken from the surface: Tables 1, 2), where possible from the following strata: Triassic (samples = 15), Pliocene 2 (10), Pliocene 1 (9), Pliocene 3 (2) and Pliocene 2, 3 (1) (Fig. 1). The samples were from (a) artesian (overflowing) piezometers, which were taken from surface, (b) deep groundwater samples using a Solinst discrete interval sampler and (c) coal mine dewatering wells through opening the pumps (SPM—Supplementary material).
Table 1

Trace elements of Velenje basin groundwater

No. of wells, original sample ID, date of sampling

Geology

Li (μg/l)

B (μg/l)

Al (μg/l)

Ti (μg/l)

Mn (μg/l)

Fe (μg/l)

Co (μg/l)

Ni (μg/l)

Cu (μg/l)

Zn (μg/l)

Limit of detection (LOD)

 

0.01–0.04

4–15

0.6–6

0.1–0.5

0.1–1

0.2–10

0.01–0.09

0.1–1

0.02–0.1

0.1–20

1. V 12 t/86, 1.4.2014

Pliocene 2

61.1

50

4

10.6

68.1

23.3

0.83

4.1

< 0.5

16.7

2. 3490, 1.4.2014

Pliocene

82.2

30

9

7.5

67.1

7

0.51

1.5

< 0.5

15.8

3. j.v. 2346 T/84, 1.4.2014

Triassic

3.4

< 20

4

2.0

1.5

5.9

0.28

4.4

< 0.5

0.7

4. j.v. 2370 T/88, 2.4.2014

Triassic

63.1

< 20

< 3

< 1

32.8

< 0.8

0.65

3.8

< 0.5

1.4

5. j.v. 2343/83, 2.4.2014

Triassic

3.8

< 20

< 3

2.1

< 0.3

0.9

0.17

1.6

< 0.5

< 0.7

6. BV 28/87, 1.4.2014

Pliocene 1

12.9

20

< 3

6.7

63.8

7.5

0.42

0.9

< 0.5

1.8

7. j.v. 2391 5/2, 2.4.2014

Triassic

1.3

< 20

11

1.6

< 0.3

2.4

0.16

1.4

0.7

2.6

8. j.v. 3051/01, 1.4.2014

Triassic

220.7

430

11

4.6

3.5

2.5

0.54

3.1

< 0.5

5

9. V 12 v/87, 1.4.2014

Pliocene 1

30.8

50

< 3

8.8

83.1

29.9

0.7

1.9

< 0.5

21.7

10. j.v. 2391-2, 2.4.2014

Triassic

1.3

< 20

5

1.7

0.4

1

0.18

1.1

< 0.5

< 0.7

11. j.v. 2391-1, 2.4.2014

Triassic

1.2

< 20

< 3

1.4

< 0.3

< 0.8

0.14

0.8

< 0.5

< 0.7

12. j.v. 2391-3, 2.4.2014

Triassic

1.4

< 20

10

1.4

< 0.3

2.2

0.11

7.4

0.5

< 0.7

13. j.v. 2341/83, 2.4.2014

Triassic

4.3

< 20

< 3

2.5

< 0.3

< 0.8

0.29

1.9

< 0.5

2.4

14. BV 29/87, 1.4.2014

Pliocene 2

37.3

40

< 3

8.3

< 0.3

1.7

0.62

2.1

< 0.5

< 0.7

15. BV -27/87, 1.4.2014

Pliocene 2

8.7

< 20

< 3

5.0

68.5

18.3

0.35

0.8

< 0.5

1.3

16. V 12 z/86, 1.4.2014

Pliocene 2

29.4

50

< 3

9.5

113.4

15.1

0.71

8.8

< 0.5

50.2

17. 3491, 1.4.2014

Pliocene

68.4

30

< 3

8.6

65.3

24.9

0.63

1

< 0.5

53.3

18. BV 26/87, 1.4.2014

Pliocene 2

11.9

20

< 3

7.2

67.9

10.8

0.45

1.1

< 0.5

4.5

19. j.v. 3378-K/08, 1.4.2014

Pliocene 1

72.5

70

40

9.6

0.4

3.5

0.78

4.5

0.5

2.3

20. PC-5/83, 6.6.2014

Triassic

15.0

70

< 6

0.5

5

889

0.22

0.6

0.3

80

21. PB-6/86, 6.6.2014

Triassic

2.0

13

< 6

1.7

53

8

0.56

1.3

0.1

1910

22. PH-13, 6.6.2014

Triassic

10.0

13

< 6

< 0.5

36

< 1

< 0.1

0.4

< 0.02

< 20

23. PT-30/98, 6.6.2014

Triassic

8.0

9

< 6

1.2

11

29

< 0.1

0.6

0.043

160

24. PC-7/86, 6.6.2014

Triassic

0.0

9

16

1.1

68

52

0.28

1.1

< 0.02

50

25. PE-9/10, 6.6.2014

Triassic

6.0

7

< 6

0.1

126

24

< 0.1

1.3

0.1

< 20

26. PH-5/81, 25.5.2015

Pliocene 2

27.5

41

7

3.5

287

816

1.4

3

0.3

2630

27. PM 13/84, 26.5.2015

Pliocene 3

0.8

< 10

13

0.9

5

< 1

0.1

< 1

0.3

< 7

28. PE-16/84, 26.5.2015

Pliocene 2,3

5.2

< 10

4

0.4

< 1

< 1

0.042

< 1

0.6

30

29. PG-5/84, 26.5.2015

Pliocene 1

21.6

29

5

4.0

356

2979

1.7

2

0.6

3480

30. PM-9/67, 27.5.2015

Pliocene 2

12.7

27

6

2.3

24

120

2.6

66

6

200

31. PL-7/68 27.5.2015

Pliocene 3

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

32. PH-6/83, 27.5.2015

Pliocene 2

4.4

< 10

4

2.7

198

224

0.8

3

0.7

4810

33. Pl-7/91, 1.6.2015

Pliocene 1

16.5

< 10

1504

0.4

< 1

26

0.2

5

1.3

470

34. PF-6/83, 1.6.2015

Pliocene 1

n.d.

n.d.

0

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

35. PD-6/83, 1.6.2015

Pliocene 1

10.4

< 10

4.8

3.9

307

427.7

2.6

75

16.2

330

36. PB-5/86, 2.6.2015

Pliocene 1

40.5

59.8

4.8

4.0

264

7530

3.5

24

2.3

1720

37. PD-2, 2.6.2015

Pliocene 2

4.5

< 10

5

2.5

80

530

0.4

4

4.2

520

38. PE-4/82, 2.6.2015

Pliocene 2

7.2

< 10

5

2.0

774

1307

20.1

1951

2.5

8670

Drinking water, WHO or EPA (μg)

 

1000

500

4

 

50

300

 

20

2000

5000

No. of wells, original sample ID, date of sampling

As (μg/l)

Rb (μg/l)

Y(μg/l)

Mo (μg/l)

Cs (μg/l)

Ba (μg/l)

Eu (μg/l)

Tl (μg/l)

Pb (μg/l)

U (μg/l)

Limit of detection (LOD)

0.05–0.1

0.0–0.05

0.002–0.005

0.2–1

0.003–0.02

0.04–0.5

0.002–0.003

0.003–0.02

0.004–0.02

0.001–0.01

1. V 12 t/86, 1.4.2014

2.9

5.8

0.049

< 6

0.26

440.57

0.08

< 0.02

0.032

< 0.01

2. 3490 1.4.2014

< 0.6

4.6

< 0.01

< 6

0.13

19.89

< 0.008

< 0.02

0.033

< 0.01

3. j.v. 2346 T/84, 1.4.2014

2.1

0.6

0.015

6.8

< 0.01

35.4

0.01

0.022

0.056

0.973

4. j.v. 2370 T/88, 2.4.2014

< 0.6

0.9

< 0.01

< 6

< 0.01

19.77

< 0.008

0.039

< 0.02

< 0.01

5. j.v. 2343/83, 2.4.2014

1.8

0.7

0.015

< 6

0.02

44.12

0.02

0.024

< 0.02

0.632

6. BV 28/87, 1.4.2014

4.6

2.2

0.015

< 6

0.1

228.66

0.04

< 0.02

< 0.02

< 0.01

7. j.v. 2391 5/2, 2.4.2014

< 0.6

0.5

0.012

< 6

< 0.01

21.75

0.01

0.086

0.067

0.633

8. j.v. 3051/01, 1.4.2014

< 0.6

20.8

0.136

< 6

0.36

54.81

0.02

< 0.02

0.071

< 0.01

9. V 12 v/87, 1.4.2014

1.6

4.4

0.031

< 6

0.2

508.22

0.08

< 0.02

0.031

< 0.01

10. j.v. 2391-2, 2.4.2014

< 0.6

0.5

< 0.01

< 6

< 0.01

22.91

0.01

0.076

< 0.02

0.66

11. j.v. 2391-1, 2.4.2014

< 0.6

0.5

< 0.01

< 6

< 0.01

21.38

< 0.008

0.084

0.02

0.631

12. j.v. 2391-3, 2.4.2014

< 0.6

0.5

< 0.01

< 6

< 0.01

20.29

0.01

0.1

0.07

0.711

13. j.v. 2341/83, 2.4.2014

2.6

0.8

< 0.01

7.1

< 0.01

39

0.01

0.019

< 0.02

0.876

14. BV 29/87, 1.4.2014

3.1

5

0.025

< 6

0.29

143.77

0.03

0.016

0.025

< 0.01

15. BV -27/87, 1.4.2014

6.3

1.7

0.013

< 6

0.09

184.62

0.03

< 0.02

< 0.02

< 0.01

16. V 12 z/86, 1.4.2014

1.8

4.5

0.029

< 6

0.22

525.48

0.09

< 0.02

< 0.02

< 0.01

17. 3491, 1.4.2014

1.3

3.6

0.023

< 6

0.16

46.27

0.01

< 0.02

< 0.02

< 0.01

18. BV 26/87, 1.4.2014

7.1

2.6

0.024

< 6

0.12

190.28

0.04

< 0.02

0.029

< 0.01

19. j.v. 3378-K/08, 1.4.2014

6.2

7.7

0.031

< 6

0.35

176.48

0.04

< 0.02

0.05

0.083

20. PC-5/83, 6.6.2014

8.3

1.4

< 0.005

40.8

0.05

124

0.01

0.015

0.12

< 0.005

21. PB-6/86, 6.6.2014

1.9

2.2

0.005

0.3

0.07

73

0.01

< 0.005

< 0.04

< 0.005

22. PH-13, 6.6.2014

< 0.05

4.3

< 0.005

15.2

0.08

17

< 0.003

< 0.005

< 0.04

< 0.005

23. PT-30/98, 6.6.2014

3.2

2

0.006

5.9

0.04

122

0.02

< 0.005

< 0.04

0.03

24. PC-7/86, 6.6.2014

17.6

1.8

0.016

0.6

0.03

258

0.03

< 0.005

< 0.04

0.03

25. PE-9/10, 6.6.2014

< 20

2.8

< 0.005

8.7

0.15

16

0.003

< 0.005

< 0.04

< 0.005

26. PH-5/81, 25.5.2015

0.9

10.4

0.02

1.9

0.28

680

0.128

0.34

0.03

0.012

27. PM 13/84, 26.5.2015

2.9

1.1

0.011

3

0.03

23

0.007

0.27

0.1

0.012

28. PE-16/84, 26.5.2015

1.6

2.5

0.004

0.5

0.06

102

0.022

0.17

0.16

0.055

29. PG-5/84, 26.5.2015

0.4

16.9

0.026

0.8

0.56

944

0.174

0.22

0.08

0.013

30. PM-9/67, 27.5.2015

2.1

2

0.034

0.4

0.29

175

0.035

0.16

1.46

0.008

31. PL-7/68 27.5.2015

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

32. PH-6/83, 27.5.2015

1.3

2.9

0.018

< 0.3

0.14

716

0.136

0.25

0.1

0.005

33. Pl-7/91, 1.6.2015

0.7

52.5

0.008

11.2

0.08

59

0.013

0.2

0.52

0.006

34. PF-6/83, 1.6.2015

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

35. PD-6/83, 1.6.2015

8.7

5.4

0.023

9.47

0.54

306

0.061

0.21

0.08

0.027

36. PB-5/86, 2.6.2015

0.75

51

0.028

3.86

0.43

840

0.159

0.2

0.08

0.014

37. PD-2, 2.6.2015

7.8

1.6

0.006

0.9

0.07

64

0.015

0.14

0.22

0.005

38. PE-4/82, 2.6.2015

131.2

11.2

0.036

154.3

0.09

484

0.061

0.16

0.17

0.319

Drinking water, WHO or EPA (μg)

10

  

70

 

700

  

10

15

n.d. not determined

Drinking water, WHO or EPA

Table 2

pH, conductivity, dissolved oxygen, temperature, total alkalinity, sulfate concentration [40], δ13CDIC, δ13CCO2, δ18O, δ2H, 3H, δ34SSO4 and δ18OSO4

Original sample ID

Geology

pH

Conductivity (μS/cm)

Dissolved oxygen (mg/l)

T (°C)

Total alkalinity (mM)

Sulfate concentration (mM)

δ13CDIC (‰) [40]a

δ13CCO2 (‰)

δ 18O (‰)

δ 2H (‰)

3H (TU)

δ 34SSO4 (‰)

δ 18OSO4 (‰)

1. V 12 t/86, 1.4.2014

Pliocene 2

6.63

3340

5.00

19.4

42.38

 

− 3.4

− 8.7

− 11.3

− 76.0

   

2. 3490 1.4.2014

Pliocene

6.85

2305

4.81

19.7

28.04

 

− 0.9

− 7.2

− 11.6

− 79.0

   

3. j.v. 2346 T/84, 1.4.2014

Triassic

7.35

933.6

5.24

16.8

5.97

2.14

− 9.7

− 17.6

− 10.0

− 67.0

 

13.2

− 3.1

4. j.v. 2370 T/88, 2.4.2014

Triassic

7.32

455

4.20

17.8

1.32

 

− 10.9

− 18.6

− 10.3

− 69.0

   

5. j.v. 2343/83, 2.4.2014

Triassic

7.35

759.6

5.82

16.6

5.65

 

− 10.7

− 18.6

− 10.0

− 69.0

   

6. BV 28/87, 1.4.2014

Pliocene 1

7

1117

6.92

17.3

12.02

 

− 3.5

− 10.4

− 12.1

− 82.0

0.03

  

7. j.v. 2391 5/2, 2.4.2014

Triassic

7.38

581.4

5.00

14.2

5.45

0.32

− 11.9

− 20.1

− 10.1

− 68.0

 

6.4

7.1

8. j.v. 3051/01, 1.4.2014

Triassic

6.81

4330

5.30

26.7

10.43

20.2

− 19.3

− 25.0

− 11.2

− 78.0

0.05

21.4

13.9

9. V 12 v/87, 1.4.2014

Pliocene 1

6.92

2244

4.85

18.7

27.19

 

− 2.3

− 8.9

− 11.4

− 80.0

   

10. j.v. 2391-2, 2.4.2014

Triassic

7.42

581

3.41

14.1

5.42

 

− 10.3

− 18.6

− 10.0

− 68.0

   

11. j.v. 2391-1, 2.4.2014

Triassic

7.42

581.3

5.52

14.3

5.39

 

− 11.4

− 19.6

− 10.0

− 68.0

4.17

  

12. j.v. 2391-3, 2.4.2014

Triassic

7.33

579.8

4.12

14.1

5.09

 

− 12.5

− 20.6

− 10.0

− 68.0

   

13. j.v. 2341/83, 2.4.2014

Triassic

7.39

1094

6.36

16.1

6.30

3.07

− 9.5

− 17.5

− 9.9

− 66.0

 

16.3

− 4.4

14. BV 29/87, 1.4.2014

Pliocene 2

6.85

2298

4.00

18.5

27.08

 

− 2.8

− 9.1

− 11.4

− 79.0

0.02

  

15. BV -27/87, 1.4.2014

Pliocene 2

7.22

900.7

4.00

16.4

9.64

 

− 3.5

− 11.1

− 11.0

− 77.0

   

16. V 12 z/86, 1.4.2014

Pliocene 2

6.89

2252

4.50

18.2

27.14

 

− 1.2

− 7.7

− 11.6

− 80.0

   

17. 3491, 1.4.2014

Pliocene

6.8

1848

5.20

19.9

21.61

 

0.0

− 6.0

− 12.3

− 82.0

   

18. BV 26/87, 1.4.2014

Pliocene 2

7.19

1200

4.85

17.7

14.20

 

− 2.0

− 9.5

− 11.6

− 79.0

   

19. j.v. 3378-K/08, 1.4.2014

Pliocene 1

6.88

4410

4.00

19.3

44.12

 

− 2.0

− 8.7

− 11.1

− 74.0

   

20. PC-5/83, 6.6.2014

Triassic

8.14

616.2

7.50

15.7

5.26

 

− 7.9

− 16.7

− 11.8

− 80.5

   

21. PB-6/86, 6.6.2014

Triassic

7

851.1

2.95

14.1

8.77

 

− 2.8

− 9.9

− 11.2

− 76.6

   

22. PH-13, 6.6.2014

Triassic

8.66

232.8

7.30

16.2

1.50

0.44

− 6.9

− 15.7

− 10.3

− 72.0

0.16

36.1

15.7

23. PT-30/98, 6.6.2014

Triassic

7.8

469.9

5.50

14

4.70

 

− 8.0

− 16.8

− 10.0

− 71.7

   

24. PC-7/86, 6.6.2014

Triassic

7.92

433.4

5.90

16.6

4.22

 

− 9.6

− 18.2

− 10.3

− 70.4

   

25. PE-9/10, 6.6.2014

Triassic

7.57

388.3

6.64

14.9

2.74

 

− 6.7

− 15.1

− 10.0

− 69.1

0.10

  

26. PH-5/81, 25.5.2015

Pliocene 2

7.5

1979

2.10

16.1

22.63

 

2.6

− 5.6

− 11.4

− 77–8

   

27. PM 13/84, 26.5.2015

Pliocene 3

9.68

311.1

2.20

16.4

3.23

 

− 9.1

− 17.7

− 11.6

− 82.2

0.05

  

28. PE-16/84, 26.5.2015

Pliocene 2,3

9.68

205.4

2.30

16.4

1.95

 

− 14.4

− 22.9

− 10.3

− 69.8

0.06

  

29. PG-5/84, 26.5.2015

Pliocene 1

7.42

2276

4.00

18

27.57

 

4.3

− 3.6

− 11.4

− 75.9

0.07

  

30. PM-9/67, 27.5.2015

Pliocene 2

8.17

1667

2.50

15.9

18.86

 

1.0

− 7.7

− 11.6

− 77.9

0.08

  

31. PL-7/68 27.5.2015

Pliocene 3

9.55

639

1.50

16.9

7.59

 

1.0

− 7.4

− 11.8

− 83.8

   

32. PH-6/83, 27.5.2015

Pliocene 2

8.4

1061

1.20

17.6

11.9

 

0.3

− 8.4

− 11.7

− 79.9

   

33. Pl-7/91, 1.6.2015

Pliocene 1

8.9

1744

3.00

15.8

6.29

 

− 12.5

− 21.3

− 11.8

− 83.8

   

34. PF-6/83, 1.6.2015

Pliocene 1

7.73

1883

1.80

17.9

22.05

 

4.6

− 3.7

− 11.4

− 79.8

   

35. PD-6/83, 1.6.2015

Pliocene 1

7.45

1811

1.00

25.1

20.57

 

1.8

− 5.5

− 11.6

− 78.0

   

36. PB-5/86, 2.6.2015

Pliocene 1

7.69

2617

3.00

14.3

30.54

 

4.3

− 4.4

− 11.3

− 77.2

0.09

  

37. PD-2/85, 2.6.2015

Pliocene 2

8.00

1300

3.10

15.9

8.01

 

− 0.3

− 9.0

− 10.6

− 73.0

   

38. PE-4/82, 2.6.2015

Pliocene 2

8.1

1117

1.50

16.9

12.4

 

1.6

− 7.1

− 8.3

− 65.4

   

To collect the deep groundwater samples, we used a Zéfal Pump (Husky, high volume and high pressure 16 bar/230PSI) and cylinder filled with air connected to the pressure inlet of the “Solinst discrete interval sampler” (SPM—Supplementary material) to obtain the recommended operating pressure, which depends on the depth below the water level. The discrete interval sampler, which was connected with 381 m of LDPE (low-density polyethylene) tubing, had a capacity of 38 mm × 610 mm and volume of 450 ml. Before sampling, we marked the tubing with depths to know at which depth the sampling should be performed. Sampling was performed following the Discrete Interval Sampler Operating Instructions (Solinst Canada Ltd.). First, we connected the tubing to the discrete interval sampler, assembled the air pump and connected the air pump to the pressure inlet. Then, we turned the pressurize/vent valve to pressurize. Afterward, we performed calculations to ensure that we were operating at the proper pressure. To this end, we used the piezometric depth database from the Velenje coalmine. Afterward, we used the air pump and pressurized the discrete interval sampler to the required pressure. Once the sampler was at the desired sampling depth, we turned the pressurize/vent valve to vent and waited for 1–3 min to allow the discrete interval sample to fill and then we turned the pressurize/vent valve to pressurize and connected the air pump to the pressure inlet on the reel and depressurized the system. Once pressurized, we disconnected the air pump and brought the sampler to the surface. When the sampler was at the surface, and we were ready to retrieve the water sample, we turned the pressure/vent valve to vent. We held the discrete interval sampler over our sample bottle and pressed the sample release device stem up into the lower check ball body until the sample began to flow from the sampler.

Temperature, pH, dissolved oxygen (DO) and electrical conductivity were measured with Multiset 3430 (WTW) in the field and are reported in [40]. For trace elements, all sampling bottles were pre-cleaned with 10% HCl and washed with ultra-pure distilled water. The samples were then filtered through pre-cleaned polyvinylidene fluoride (PVDF) 0.45-μm filters. Samples for δ34SSO4 and δ18OSO4 were collected in plastic bottles (1 l) and after filtration acidified to pH 2. The BaSO4 precipitated with BaCl2. Dissolved gas samples were collected in glass bottles (122 ml) and crimped-sealed in the field with silicon/rubber septa, taking care to avoid bubbles to prevent atmospheric contamination.

Major ion chemistry of Velenje basin groundwater was published in [40, 46]. Additional surface water was analyzed for As in 2018 as part of this study. Surface water samples were filtered on location using PVDF syringe filters and were stored in PE Sarstedt vials (50 ml). Results from this study were compared with mining groundwater from the Zasavje region (Fig. 1b).

2.2 Mineral morphology and composition of particulate matter

The mineralogical composition of the Pliocene and Triassic aquifers was analyzed using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDXS). Two 1 l samples of groundwater were collected from two sites (BV 28 and 2391 5/2). Each sample was filtered through a 0.7-μm glass fiber filter and the dried filtrate coated with a 10-nm carbon layer to ensure sufficient electrical conductivity and to avoid charging under the electron beam. The analysis was performed using a JOEL JSM 5800 SEM equipped with Oxford Instruments LINK ISIS system for EDXS, at an accelerating voltage of 20 kV. Electron micrographs were recorded using secondary electron (SE) and backscattered electron (BSE) imaging modes.

2.3 Geochemical analyses of trace elements

Trace elements (B, Mo, Al, Fe, Ti, Ni, Cu, Zn, As, Se, Mn, V, Ba, Sb, Cr, Cd, Li, Be, Co, Rb, Ag, Nb, Sm, Gd, Dy, Pb, Cs, Ti, Y, La, Ce, Eu, U, Tb, Ho, Lu) in groundwater were determined using an inductively coupled plasma mass spectrometer (ICP-MS, Agilent 7500ce) fitted with a double-pass spray chamber, PTFE 100 μl/min nebulizer, platinum cones and a sapphire injector within a quartz platinum-shielded torch. An external calibration curve was produced by making the appropriate dilutions from a 1000 mg/l single element standard solution (Inorganic Ventures). Samples, blanks and the calibration solutions were all prepared in 2.4% HNO3 (BDH Aristar Plus). The accuracy of the method was checked using the Standard Reference Material (SRM) 1643e (“Trace Elements in Water”, National Institute of Standards and Technology, USA) at a dilution of 1:20. Indium (20 ng/ml) was added as the internal standard. All the samples were diluted 1:5 and analyzed in blocks of five samples, two blanks and one SRM. Three tuning steps were used for different elements: (1) Li, Be, B, Na, Mg, Al, Ti, Co, Zn, Rb, Sr, Y, Mo, Ag, Cd, Sb, Cs, Ba, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Lu, Tl, Pb and U were measured directly; (2) K, Ca, V, Cr, Mn, Fe, Ni and Cu were measured using the collision cell containing 4 mL He/min; and (3) As and Se were measured with the collision cell containing 2 mL He/min + 2 ml H2/min. Twelve replicates were run for each element, with a total acquisition time of 5 min. Samples were loaded by vacuum into a 1-μl PTFE loop and sprayed into the spray chamber via six-way valve at 100 μl/min after which an 80-second rinse (400 μl/min) was performed. The LOD was taken as three times the standard deviation of the blank signal, multiplied by the sample dilution factor (× 5). Several trace elements were measured in three rounds, according to sampling: April 1 and 2, 2014, June 6, 2014, and April 4–June 1, 2015. Because the elements Cr, Se, Ag, Cd, Sb, La, Ce, Nd, Sm, Gd, Tb, Dy, Ho, Lu, V, Yb and Th were LOD > 70% of the samples, they were omitted from further analyses. When calculating the average values, non-detectable concentrations were taken as 70% of the LOD (Table 3).
Table 3

Concentrations of total alkalinity, As and δ18O, δ2H in Velenje basin surface waters and concentration of As in mining waste waters from Zasavje region

 

Type of water

Date of sampling

Total alkalinity (mM) [46]

Total As (μg/l)

δ18O (‰) [46]

δ2H (‰) [46]

1—River Toplica

Surface water

February, 2004

 

6

  

8—River Lepena

Surface water

February, 2004

 

6

  

10—River Paka

Surface water

February, 2004

 

6

  

1—River Toplica

Surface water

February, 2018

4.0

0.38

− 9.5

− 63.0

2—River Pečovnica

Surface water

February, 2018

3.4

0.23

− 9.3

− 62.0

3—River Klančica

Surface water

February, 2018

2.9

0.14

  

4—River Velunja

Surface water

February, 2018

3.1

0.25

− 9.6

− 64.0

5—Lake Družmirje

Surface water

February, 2018

3.3

0.071

− 9.1

− 63.5

6—River Ljubela

Surface water

February, 2018

5.0

0.21

− 9.2

− 62.8

7—Lake Velenje

Surface water

February, 2018

3.1

1.13

− 8.0

− 57.8

8—River Lepena

Surface water

February, 2018

5.1

0.19

− 9.4

− 61.3

9—Lake Škale

Surface water

February, 2018

4.8

0.16

− 9.4

− 67.0

10—River Paka

Surface water

February, 2018

3.9

0.5

− 8.7

− 60.8

Ojstro površinski kop, Zasavje

Mining waste water

February, 2004

 

< 1

  

Ojstro površinski kop Ana, Zasavje

Mining waste water

February, 2004

 

< 1

  

Bukova gora, Zasavje

Mining waste water

February, 2004

 

< 1

  

Jama Hrastnik, Zasavje

Mining waste water

February, 2004

 

< 1

  

Savski rov, Zasavje

Mining waste water

February, 2004

 

< 1

  

Retje deponija jalovine, Zasavje

Mining waste water

February 2004

 

< 1

  

Sava-polje, Zasavje

Mining waste water

February, 2004

 

< 1

  

All samples were measured with FI-HG-AFS method

Arsenic concentrations in surface waters from the Velenje basin and from the Zasavje region (February, 2004) were determined using modified flow injection–hydride generation–atomic fluorescence spectrometry (FI–HG–AFS). Before the HG step, a UV decomposition unit with an alkaline solution of 3% NaOH and 3% K2S2O8 was used to convert arsenic to arsenate [52].

2.4 Statistical processing of trace element data

Multivariate principal components analysis (PCA) was used to investigate variance in measured parameters (temperature, pH, conductivity, DO, 20 trace elements) from 36 groundwater samples collected from different strata (Triassic, Pliocene). Prior to analysis, data were standardized (subtracting the mean and dividing by the standard deviation for each variable) in order to increase the explanatory power of PCA, while a one-way MANOVA (multivariate analysis of variance) was used to test whether or not Triassic and Pliocene groundwater has the same mean concentrations of elements. A Pearson bivariate correlation was used to decipher which trace elements correlate at significance level p < 0.05. The PCA and one-way MANOVA were carried out using the PAST 3.06 software [53], while the Pearson bivariate correlation coefficients were calculated for trace elements using STATISTICA 12.0.

2.5 Gas composition

Gas composition (He, H2, O2, N2, CO, CH4 and CO2) was determined using a Perkin Elmer Clarus 500 GC equipped with Carboxen 1000 columns, TCD–FID double detectors with Ar as the carrier gas. Analytical uncertainties were ± 5%(1σ).

2.6 Water and gas isotopes

For analysis of δ34SSO4 and δ18OSO4, sulfides and sulfates were converted to SO2, the CO2 and H2O were removed and the δ34SSO4 values determined using a continuous-flow isotope ratio mass spectrometer (CFIRMS, ThermoQuest Finnigan Delta Plus XL). δ18OSO4 values were obtained by combusting the samples at 1030 °C with O2 and V2O5 in an elemental analyzer (Costech) coupled to a mass spectrometer. Standardization was based on the international standards OGS-1 and NBS123 and sulfide and sulfate minerals analyzed by different laboratories. The precision was ± 0.15‰ (1σ). Results are reported as δ34S (‰) relative to the Canyon Diablo troilite (δ34SCDT) international standard [54, 55]. The δ18OSO4 was measured as CO gas using CFIRMS (Thermo Electron Delta V). Samples were combusted with excess C at 1350 °C in a thermal combustion elemental analyzer (TCEA, ThermoQuest Finnigan) coupled to a mass spectrometer. Standardization was based on internal standards OGS-1. The precision is ± 0.3‰ or better (1σ) for repeat analyses.

δ13CCO2 was calculated according to enrichment factors of [56, 57, 58] to assess the possible origin of carbon inputs in the Velenje basin aquifer. The isotopic composition of the CO2 (δ13CCO2gas) in equilibrium with groundwater at the sampling temperature (Tk expressed in °K) was computed using Eqs. 14:
$$\delta^{13} {\text{C}}_{{{\text{CO}}2{\text{gas}}}} = \delta^{13} {\text{C}}_{\text{DIC}} - \, \left[ {\varepsilon_{\alpha } *X_{{{\text{HCO}}3}} - \varepsilon_{\beta } *X_{{{\text{CO}}_{{2{\text{aq}}}} }} {-}\varepsilon_{\gamma } *X_{{{\text{CO}}_{3}^{2 - } }} } \right]$$
(1)
where δ13CTDIC is the isotopic composition of the dissolved inorganic carbon species (HCO3, CO2aq and CO32−, respectively) and εα, εβ and εγ are the enrichment factors between each of the three species and CO2gas:
$$\varepsilon_{\alpha } = \delta^{13} {\text{C}}_{{{\text{HCO}}3}} -\updelta^{13} {\text{C}}_{{{\text{CO}}2}} = 9552/T_{k} - 24.1\quad \, \left[ {56} \right]$$
(2)
$$\varepsilon_{\beta } = \delta^{13} {\text{C}}_{{{\text{CO}}2{\text{aq}}}} - \delta^{13} {\text{C}}_{{{\text{CO}}2}} = - 0.91 + 0.0063*10^{ - 6} /T^{2}_{k} \quad \left[ {57} \right]$$
(3)
$$\varepsilon_{\gamma } = \delta^{13} {\text{C}}_{{{\text{CO}}3}} - \delta^{13} {\text{C}}_{{{\text{CO}}2}} = - 3.4 + 0.87*10^{ - 6} /T^{2}_{k} \quad \left[ {57} \right]$$
(4)

XHCO3, XCO2aq and XCO32− are the molar fractions of the three inorganic dissolved carbon species in water (HCO3, CO2aq and CO32− respectively).

Oxygen isotope ratios were measured using CO2–H2O equilibration [28, 59] with a GB-II Thermo peripheral equilibrator connected to a Thermo Delta V IRMS. D/H ratios were obtained using a high-temperature elemental analyzer (Thermo TCEA) connected through a ConFloIV to a Thermo Delta XP mass spectrometer. The data are presented in the delta notation against the V-SMOW international standard. Precision is estimated at ± 0.1 and ± 1.0‰ (1σ) for δ18O and δ2H, respectively.

Linear correlations between δ2H and δ18O were calculated by the reduced major-axis (RMA) regression method [59]. The lines are defined as the local meteoric water line (LMWLRMA), surface water line (SWLRMA) and groundwater line (GWLRMA). The latter was calculated for all the samples including the Pliocene and Triassic aquifer samples separately. The deuterium excess (d = δ2H − 8 × δ18O; [60] was also calculated.

Concentrations of tritium were measured using the 3He in-growth method [41]. The in-growth method enables older groundwater samples from the deeper aquifers to be dated. For this, 500 ml of water was transferred to a 1-l metal flask with a copper stem. The sample was then vacuum-degassed (> 99.9995%) and sealed using a metal pinch clamp. After 60 days, the in-grown 3He was passed through the purification system where it was trapped on charcoal at 10 °K. The 3He was then desorbed and introduced into a large-radius sector-field mass spectrometer (Helix SFT) and quantified using peak height manometry. The LOD for the 60-day holding time is 0.005 TU with an uncertainty of 3% (1σ) at 1 TU and 100% at 0.005 TU. The low LOD for 3H values enables the detection of less than 0.1% of modern water in old water.

The concentration of tritium was determined in one precipitation sample using the electrolytic enrichment method [61, 62, 63]. The precision for 3H measurements (samples determined six times) was ± 0.5 tritium units (TU) with a measurement uncertainty of ± 10%, and LOD of 3 TU. This method is appropriate for waters with high-tritium activities.

3 Results and discussion

3.1 Mineralogy of suspended materials in Triassic and Pliocene aquifers

According to the X-ray diffraction results (Fig. 2A, B), clays and sands in the Pliocene aquifer (Fig. 2Aa–c) are composed of quartz (SiO2), calcite (CaCO3), muscovite (KAl(Si3Al)O10(OH,F)), montmorillonite (CaO2(Al, Mg)2Si4O10), goethite (FeOOH), dolomite (CaMg(CO3)2), albite (NaAlSi3O8), kaolinite (Al2Si2O5(OH)4) and pyrite (FeS2). Suspended matter in the Triassic aquifer is composed of dolomite, silicate and framboidal pyrite (Fig. 2Ba–c).
Fig. 2

A SEM/EDXS microscopy of suspended matter from groundwater dewatering Pliocene aquifer (location BV 28): (a) a “crust” (sediment) with a corresponding spectrum showing the presence of the following elements: O, Fe, Zn, Mg, Al, Si, K, Ca, Ba, Fe, Zn, (b) fragment is composed of K, Si, Al–K feldspars, magn. × 400, and (c) fragment consists of SiO2–quartz (corresponding spectrum), magn. × 200. B SEM/EDXS microscopy of suspended matter from groundwater dewatering the Triassic aquifer (location 2391 5/2), location 2391 5/2, in background are fibers of GF/F filters: (a) general picture, magn. × 200: 2 sp.3—dolomite, 2 sp.4—silicate with a lot of iron, aluminosilicate, 2 sp. 5—quartz, 2 sp. 6—quartz, 2 sp. 7—dolomite, zoom 800 ×, (b) dolomite, zoom 6000 ×, (c) framboidal pyrite with corresponding spectrum indicating reduction conditions, zoom 3000 ×

Figure 2Aa–c shows an SEM image at a magnifications × 200 and × 400 and EDXS spectrum of the suspended matter from well: BV 28 (Fig. 1) in the Pliocene aquifer. Detail description of suspended matter from Triassic aquifer (location 2391 5/2) is presented as EDXS spectra in Fig. 2Ba–c.

3.2 Trace element water geochemistry

Twenty elements (Li, B, Al, Ti, Mn, Fe, Co, Ni, Cu, Zn, As, Rb, Y, Ho, Cs, Ba, Eu, Tl, Pb and U) were measured in groundwater samples from 38 wells collected in 2014–2015. The decreasing trend in trace element content shows the following order (Fig. 3a, b): Zn > Fe > Ni > Al > Ba > Mn > B>Li > Mo > As (Fig. 3a, b). The following mean trace element concentrations ± s (all units are in μg/l) were obtained: Li (25 ± 40), B (52 ± 70), Al (58 ± 240), Ti (4 ± 6), Mn (90 ± 150), Fe (420 ± 1330), Co (1 ± 3), Ni (60 ± 320), Cu (1.1 ± 2), Zn (702 ± 1740), As (6.8 ± 20), Rb (6.6 ± 11), Y (0.02 ± 0.023), Mo (9.537 ± 25), Cs (0.148 ± 0.152), Ba (210 ± 250), Eu (0.04 ± 0.046), Tl (0.082 ± 0.095), Pb (0.107 ± 0.249) and U (0.162 ± 0.294). The results reveal that the levels of Fe, Mn, Cu, Cr, Pb, Zn and Ni are comparable to those in Tamiraparani river basin, South India [6], while in the Velenje basin Cr is below the LOD.
Fig. 3

Trace elements in Velenje basin groundwater; a potentially toxic trace elements, b other trace elements

The two principal components were extracted in a way where the first principal component accounts for the largest possible variance in the data set and the second one for the next highest variance, where it was considered that the latter is uncorrelated with the former. Comparison of samples using PCA (Fig. 4) reveals a strong gradient along the first PCA axis that explains 27.71% of the variance and has the highest negative correlation with dissolved oxygen and highest positive correlation with Mn, Co and Zn. These elements have the most discriminant power separating Pliocene and Triassic strata groundwater. The second axis explains an additional 19.99% of the variance and correlates positively with Li and negatively with pH. Despite clear grouping of aquifers, the two samples were extremely different from others, based on higher concentrations of Co, Ni, As and Zn (sample 38) and higher concentrations of Li, B and Rb and higher temperature and conductivity (sample 8) in the PCA ordination diagram. A MANOVA found no significant difference between the two groups (p = 0.059; Wilks’ lambda = 0.251, df1 = 20, df2 = 15, F = 2.233).
Fig. 4

PCA ordination diagram showing the gradient in the composition of trace elements in the samples from groundwater dewatering Pliocene (square dots) and Triassic (circle dots) strata

Trace element concentrations (Table 1) range from 0.14 to 1.13 µg/l, well below the maximum permissible level of 10 μg/l [64]. In surface waters, trace element concentrations are 0.071 to 0.6 μg/l (Table 2). Mining wastewater in the coal mining parts of the Zasavje region (Fig. 1) has As concentrations below the LOD (< 0.001 μg/l) (Table 2).

Although groundwater in the Velenje coal mine is not for human consumption, most of the water in the aquifers is potable. Only certain major elements, e.g., Mg2+ up to 404 mg/l [46] and SO42− in Lake Velenje (380 mg/l), exceed the limits (250 mg/l) set by the World Health Organization 2006 and Slovenian standards for drinking water [65].

From Table 1, it is clear that the most trace elements are below the threshold values recommended by the WHO, United States EPA and Slovenian legislation. The exceptions are Al that is above the limit in well Pl-7/91 (sampling point 33). Fe is elevated in wells PG-5/84 (sampling point 29), PD-6/83 (sampling point 35) and highly elevated in PB-5/86 (sampling point 36) and PE-4/82 (sampling point 38). Ni is high in dewatering wells: PD-6/83, PE-4/82 and PB-5/86 in Pliocene strata, and Ba is elevated in PB-5/86, PH-6/83 and PG-5/84. Zinc concentrations in PE-4/82 are an order of magnitude higher than the limits set by WHO and US EPA standards (5 mg/l) for drinking water. Uranium and Pb are below the legal limits of 0.015 and 0.01 mg/l, respectively (Official Gazette of the Republic of Slovenia, 2004).

The Pliocene aquifer contains on average 3.11 ± 2.62 µg/l of As, except for one sample PE-4/82 that has 131 µg/l of As, while the Triassic aquifer samples contain 2.69 ± 4.47 µg/l of As. The data reveal that except for the water from PC-7/86 (0.0176 mg/l) and PE-4/82 (0.131 mg/l) most samples are below the WHO drinking water limits (Table 1). The surface waters sampled in 2018 contain much lower As concentrations (0.14–1.13 µg/l), indicating no surface water As pollution.

3.3 Isotopic composition of Velenje basin groundwater

3.3.1 Sources of sulfate

Table 2 shows the sulfate isotope data (n = 5) and corresponding sulfate concentrations for the study area (Fig. 1). The δ34SSO4 values are between 6.4 and 36.1‰ (Table 2, Fig. 5a), indicating different sources of SO42−, from both the atmosphere (with low SO42− concentrations and δ34SSO4 values similar to continental precipitation) and local admixtures of sulfate with higher δ34S values. The maximum SO42− concentration of 3.96 mM (380.2 mg/l) in Lake Velenje can be attributed to the inflow of water that has interacted with nearby coal ash [66, 67]. The lowest δ34SSO4 values in groundwater from dewatering well in Triassic strata are around + 6‰. The δ34SSO4 values in sample j.v. 2391/5/2 resemble surface water with a sulfate content of 0.32 mM (30.7 mg/l) (Table 2, Fig. 5a). δ34SSO4 versus SO42− (Fig. 5b) indicates different origins of sulfate in groundwater: precipitation, sulfide oxidation and anhydrite dissolution.
Fig. 5

a δ18OSO4 versus δ34SSO4, different sulfate sources (i.e., sulfide oxidation, anhydrite dissolution, precipitation) in groundwater wells dewatering Triassic strata. The range of δ34SSO4 values for surface waters is taken from [46]. b δ34SSO4 versus SO42− indicating different sources of sulfate in Velenje basin groundwater

Sulfate in the three groundwater samples with positive δ18OSO4 values likely has a different origin. Precipitation is the most likely source for j.v.2931/5/2. Sample 3051/01 with a high sulfate concentration 1939 mg/l could originate from dissolution of evaporites, combining Röt event and another Triassic marine sulfate source (Fig. 5a). Locally elevated concentrations of sulfate are likely related to lenses of anhydrite and gypsum [48]. PH13 has a δ34S value (+ 36‰) too high for known Triassic evaporites; however, bacterial sulfate reduction BSR of sulfate generated by oxidation of sulfides or dissolution of evaporites (Fig. 5a, b) could produce such a value. Sulfate with negative δ18OSO4 values can form by the oxidation of reduced sulfur [66], and has δ34SSO4 values similar to those of organic sulfur (13.2 to 15.5‰) in the Velenje lignite [67].

3.3.2 Origin of CO2 and evidence of organic matter degradation, methanogenesis and origin of gases

Calculated δ13CCO2 values vary between − 22.9 and − 3.6‰ in the Pliocene aquifer and between − 25 and −9.9‰ in the Triassic aquifer (Fig. 6). The lower δ13C values in Triassic groundwater and surface water correspond to samples where inputs of soil CO2 are dominant. In the vadose zone, root respiration and degradation of organic matter from C3 plants can produce CO2 enriched in light carbon isotopes typically with δ13C values from − 26 to − 18‰ interacting with groundwater during infiltration. For waters hosted in Triassic limestone aquifers, a further contribution of DIC from the dissolution of carbonates (δ13C is − 2% in dolomite rocks in the basement [7]) is also likely.
Fig. 6

Calculated δ13CCO2 versus total alkalinity indicating the origin of CO2 in groundwater. The δ13CCO2 was calculated from δ13CDIC according to physical–chemical laws

Higher δ13CCO2gas values are due to the contribution of inorganic CO2. In sedimentary basins, 13C-rich CO2 generally originates from metamorphic thermal decomposition of carbonate rocks, caused by a magmatic intrusion or from a primary mantle source. Even if there is no evidence of recent magmatism in the study area, a deep geogenic source of CO2 cannot be discounted. For example, some degassing sites at the SE Austria/NE Slovenia border, less than 50 km from the Velenje coal basin, show a helium isotope signature typical for the subcontinental lithospheric mantle (SCLM). In CO2-dominant gas emissions (mofettes) and in the sites with the highest 3He contributions (up to 6.3 Ra, where Ra is the 3He/4He in atmospheric gas), δ13CCO2 values are around − 3.5‰ [68].

Similar, δ13CCO2gas values occur in thermal and mineral springs in the fractured aquifer system near Rogaška Slatina (Slovenia) and bottled waters [69, 70]. Other nearby thermal waters, e.g., Dobrna Topolščica, Hotavlje and Bled, have δ13CDIC values between − 7 and − 2‰, supporting the hypothesis that there is an input of deep, geogenically sourced inorganic CO2 in the Velenje basin (Kanduč unpublished data) but more data on helium isotope ratios are needed to confirm this hypothesis.

Previous studies show that δ13CCO2 and δ13CCH4 values in Pliocene coal beds were highly variable, i.e., − 9.7 to + 0.6‰ and − 70.5 to − 34.2‰, respectively [45]. This means that the source of CO2 is likely from a mixture of deep CO2 (δ13CCO2 = − 3.5‰), as discussed above, and in situ microbial activity (δ13CCO2 from − 26 to −18‰), while the source of CH4 is dominantly from microbial methanogenesis with the possible addition of thermogenic gas from deeper formations. Calcified xylites (authigenic mineralization) enriched with 13C (δ13C values up to + 16.4‰) indicate that microbial methanogenesis was active during sedimentation and basin formation [42].

Groundwater in the Pliocene aquifer has dissolved oxygen (DO) concentrations (1.0 to 6.92 mg/l), while groundwater in the Triassic aquifer contains higher DO (2.95 to 7.50 mg/l) and is suboxic [40]. The SO42− and NO3 concentrations in the Pliocene aquifer were < 0.27 and 0.29 mM, respectively, compared to surface waters and Triassic aquifer waters [40]. These values are consistent with bacterial sulfate reduction and methanogenic redox conditions [71, 72]. It is likely that groundwater with low SO42− concentrations and high δ13CDIC values has elevated δ34SSO4 values from bacterial sulfate reduction, which is only the case for the Triassic aquifer.

In Pliocene aquifer, the relative levels of dissolved gases in groundwater are CO2 > CH4 > N2 > O2 > CO > He > H2 (Table 4, Fig. 7). In the Triassic aquifer samples, only some dissolved gases were detected (CO2, CH4, CO, N2, O2, H2 and He). According to STP conditions, the level of CO2 is 203.29 mg/l, N2 is 13.92 mg/l and CH4 is 8.1 mg/l and the levels of CO2 range from 0.29 to 882.46 mg/l, CH4 is between 8.49 to 43.55 mg/l and N2 is from 3.64 to 23.58 mg/l. In groundwater from dewatering wells in the Pliocene aquifer (Table 4, Fig. 7), concentrations of dissolved gases and the isotopic composition (δ13CCH4) were determined (n = 22) only between April 2014 and June 2014 (Table 4). Methane values range from 8.49 to 43.55 mg/l (recalculated at STP) and are comparable to values (4.2 to 51.2 mg/l) in an Eastern Ontario aquifer [9]. Methanogenesis occurs mainly in the Pliocene aquifers, evidenced by a dissolved CH4 content of 8.8 to 24.9 cc gas l−1. In specific locations, the Triassic groundwater has higher levels of CH4 11.3 to 61.8 cc gas l−1. The δ13CCH4 and δ2HCH4 values in Triassic aquifer range from − 77.7 to − 71.9‰ and from − 231.8 to − 226.2‰. The δ13CCH4 values in Pliocene aquifer are between − 77.7 and − 51.4‰ and δ2HCH4 values are from − 246.6 to − 162‰, respectively, which is indicative of microbial methane production (Figs. 8, 9). Figure 8 shows most of the samples from the dewatering wells are of biogenic origin (microbial CO2 reduction). Only one sample has a higher δ13CCH4 value of − 51.4 (δ2H of − 230.10‰) and is possibly of thermogenic origin. In some groundwater dewatering Triassic aquifer, the levels of methane were too low to be able to measure δ13CCH42HCH4. In specific locations, the Triassic groundwater has higher levels of CH4 (11.3 to 61.8 cc gas l−1). A comparison with groundwater from an aquifer in Eastern Ontario [9] was also made to investigate the origin of methane and concentrations of gases (also higher hydrocarbons). In their study, the gases (N = 13) sampled had a wide range of δ13C values (− 98.4 to − 44.8‰), with an average of − 67.8‰, and δ2H values between − 347 and − 256‰, with an average of − 286‰. Whiticar and Faber [73] defined the range of CO2 reduction as 1.090 < αc (isotope fractionation factor) < 1.055, and for acetate fermentation as 1.055 < αc < 1.040. Velenje basin values are different from the Ontario Basin and are between 1.090 < αc < 1.055, indicating microbial CO2 reduction is the dominant pathway of methanogenesis, as shown in the δ2HCH4 versus δ13CCH4 plot (Fig. 9).
Table 4

Chemical composition of gases (He, H2, O2, N2, CO, CH4, CO2) and isotopic composition of gases (δ13CCH4, δ2ΗCH4) dissolved in Velenje basin groundwater

Location

Geology

Date of sampling

He cc/l STP

H2 cc/l STP

O2 cc/l STP

N2 cc/l STP

CO cc/l STP

CH4 cc/l STP

CO2cc/l STP

δ13CCH4 (‰)

δ2HCH4 (‰)

V 12 t/86

Pliocene 2

1.4.014

b.d.l.

b.d.l.

0.04

4.39

3.06E − 05

9.650805353

398.70

− 68.1

− 218.5

3490

Pliocene

1.04.2014

2.89E − 04

b.d.l.

0.03

3.16

9.38E − 06

17.18486293

282.89

− 67.2

− 243.2

j.v. 2346 T/84

Triassic

1.04.2014

1.37E − 03

b.d.l.

2.15

16.85

1.27E − 04

0.000197368

45.40

n.a.

n.a.

j.v. 2370 T/88

Triassic

2.04.2014

       

n.a.

n.a.

j.v. 2343/83

Triassic

2.04.2014

3.66E − 04

8.48E − 03

3.42

17.25

4.09E − 04

0.000120564

35.35

n.a.

n.a.

BV 28/87

Pliocene 1

1.04.2014

3.30E − 04

b.d.l.

0.05

5.14

2.54E − 04

24.89320373

92.29

− 72.9

− 240.4

j.v. 2391 5/2

Triassic

2.04.2014

4.88E − 04

b.d.l.

3.94

16.87

3.85E − 04

0.000133778

19.19

n.a.

n.a.

j.v. 3051/01

Triassic

1.04.2014

       

n.a.

n.a.

V 12 v/87

Pliocene 1

1.04.2014

2.02E − 04

1.74E − 04

0.01

5.68

9.74E − 05

9.237006115

216.45

− 68.7

− 218.6

j.v. 2391-2

Triassic

2.04.2014

1.88E − 03

b.d.l.

2.39

17.97

2.25E − 04

0.000345616

23.83

n.a.

n.a.

j.v. 2391-1

Triassic

2.04.2014

3.66E − 04

3.99E − 03

1.66

18.71

1.29E − 04

0.000168789

25.04

n.a.

n.a.

j.v. 2391-3

Triassic

2.04.2014

3.22E − 04

5.74E − 04

1.48

13.66

7.65E − 05

0.001636089

32.49

n.a.

n.a.

j.v. 2341/83

Triassic

2.04.2014

4.16E − 04

b.d.l.

2.63

15.55

3.76E − 04

b.d.l.

43.67

n.a.

n.a.

BV 29/87

Pliocene 2

1.04.2014

2.00E − 04

b.d.l.

0.02

5.35

3.14E − 05

13.57265479

207.81

− 68.5

− 208.9

BV -27/87

Pliocene 2

1.04.2014

1.38E − 04

1.50E − 03

0.02

4.73

9.18E − 05

13.01932367

99.40

− 66.4

− 210.6

V 12 z/86

Pliocene 2

1.04.2014

2.43E − 04

1.94E − 04

0.02

6.80

8.99E − 05

8.819907062

202.34

− 65.4

− 162.0

3491

Pliocene

1.04.2014

2.82E − 04

b.d.l.

0.03

2.95

4.73E − 05

20.28303727

169.75

− 66.8

− 232.0

BV 26/87

Pliocene 2

1.04.2014

       

n.a.

n.a.

j.v. 3378-K/08

Pliocene 1

1.04.2014

1.67E − 04

b.d.l.

0.12

4.53

9.38E − 06

14.12189487

359.66

− 65.6

− 228.4

PC-5/83

Triassic

6.06.2014

1.34E − 03

b.d.l.

0.0022692

4.74

7.48E − 04

37.54

2.1

− 77.69

− 231.8

PB-6/86

Triassic

6.06.2014

1.13E − 03

0.001731

0.07

12.40

1.45E − 04

61.82

32.9

− 71.99

− 227.5

PH-13/12

Triassic

6.06.2014

8.54E − 04

b.d.l.

3.0

18.27

5.96E − 05

0.019163

0.151475

n.a.

n.a.

PT-30/98

Triassic

6.06.2014

1.37E − 03

b.d.l.

1.1

19.13

6.42E − 04

0.015507

6.84

n.a.

n.a.

PC-7/86

Triassic

6.06.2014

1.12E − 03

b.d.l.

0.174115

17.12

1.59E − 04

11.29

7.14

− 72.61

− 226.18

PE-9/10

Triassic

6.06.2014

7.12E − 04

b.d.l.

1.88

17.18

2.18E − 05

0.004323

5.1

n.a.

n.a.

PB-5/86

Pliocene 1

14.04.2015

       

− 68.9

− 246.5

PH-5/81

Pliocene 2

25.05.2015

       

− 72.5

− 240.5

PM 13/84

Pliocene 3

26.05.2015

       

n.a.

n.a.

PE-16/84

 

26.05.2015

       

− 69.4

n.a.

PG-5/84

Pliocene 1

26.05.2015

       

− 69.9

− 241.4

PM-9/67

Pliocene 2

27.05.2015

       

− 72.7

− 242.3

PL-7/68

Pliocene 2

27.05.2015

       

− 51.4

− 230.1

PH-6/83

Pliocene 3

27.05.2015

       

− 72.3

− 234.3

Pl-7/91

Pliocene 1

1.06.2015

       

n.a.

n.a.

PF-6/83

Pliocene 1

1.06.2015

       

− 70.7

− 239.9

PD-6/83

Pliocene 1

1.06.2015

       

− 70.5

− 238.0

PB-5/86, repeat, upoštevala to

Pliocene 1

2.06.2015

       

− 68.9

− 246.5

PD-2/85

Pliocene 2

2.06.2015

       

− 72.4

− 245.8

PE-4/82

Pliocene 2

2.06.2015

       

− 69.3

− 246.6

PT-13

  

b.d.l.

b.d.l.

0.04

4.39

3.06E − 05

9.650805353

398.70

− 68.10

− 218.50

b.d.l. below detection limit, STP standard temperature and pressure

Fig. 7

CO2–CH4–N2 trilinear plot for dissolved gases in groundwater. Data are expressed in cc gas l−1 at STP (standard temperature and pressure)

Fig. 8

Cross-plot of δ13CCH4 versus δ2HCH4 for gaseous-phase samples for the Velenje basin and Eastern Ontario aquifer [74]. Fields for microbial (acetoclastic and CO2 reduction) and thermogenic methane defined by [74]

Fig. 9

Cross-plot of δ13CCH4 and coexisting δ13CDIC. The isotope fractionation factors (αc) are shown for methanogenesis by CO2 reduction (1.090 < α < 1.055) and acetate fermentation (1.055 < α < 1.040), derived from [74]

There is also evidence of coal biodegradation in different calcite forms (lenses, impregnations, calcified wood) within coal with δ13CCaCO3 values up to 20‰ [42]. In addition, coalbed gases (CO2 and CH4) are primarily microbial in origin with δ13CCH4 values from − 70 to − 40‰, indicating microbial CO2 reduction and acetate fermentation [44]. Procesi et al. [75] provide an overview and the first global mapping of CO2 and methane in sediment-hosted geothermal systems, i.e., hybrid geological systems, where volcanic and sedimentary domains interact to produce inorganic and organic gases. Biodegradation of organic matter in the Velenje basin is also related to the unexpected organoarsenic compounds found in gelified detrital lignite [46, 76, 77]. Concentrations of arsenic in uncontaminated soils range from 0.2 to 40 mg/kg [78], and the levels in Velenje lignite samples fall within the same range [43, 76].

3.3.3 Isotopic composition of oxygen, hydrogen and tritium (δ18O, δ2H and 3H)

δ18O and δ2H values of precipitation samples vary from − 15.0‰ (January 2013) to − 3.9 and from −108.7 to −28.8‰ (September 2012) (Table 5) during the sampling period 2012–2015.
Table 5

Oxygen and hydrogen isotopic composition (δ18O, δ2H) of precipitation, 2013–2015

Month of sampling

δ18O (‰)

δ2H (‰)

3H (TU)

September 2012

− 3.9

− 28.8

 

October 2012

− 11.7

− 80.3

 

November 2012

− 11.3

− 80.8

 

January 2013

− 15.0

− 108.7

 

February 2013

− 12.0

− 98.3

 

March 2013

− 10.4

− 74.3

 

May 2013

− 10.6

− 76.0

6.4

September 2013

− 7.2

− 44.0

 

October 2013

− 7.8

− 51.8

 

November–December 2013

− 10.7

− 75.0

 

January–February 2013

− 10.5

− 78.0

 

April–May 2014

− 6.8

− 49.7

 

June–July–August 2014

− 7.8

− 55.9

 

August–September 2014

− 8.8

− 69.0

 

November–December 2014

− 9.8

− 69.7

 

February 2015

− 11.8

− 98.4

 

April–May 2015

− 5.8

− 40.2

 
δ18O values range from − 12.3 to − 8.3‰ and 3H from 0.02 to 0.09 TU in groundwater from dewatering wells in the Pliocene aquifer, while δ18O values range from − 11.8 to − 9.9‰ and 3H values from 0.05 to 4.17 TU (Table 2) in groundwater from dewatering wells in the Triassic aquifer. However, the majority of the Pliocene aquifer samples plot in a group with lower δ18O and δ2H values than samples from the Triassic aquifer (Fig. 10). A few samples from the Pliocene aquifer plot within the range of the Triassic aquifer and vice versa.
Fig. 10

δ2H versus δ18O values of groundwater and surface waters shown relative to the global meteoric water line (GMWL; δ2H = 8×δ18O + 10; [79]), local meteoric water line (LMWLRMA) for Ljubljana, period 2012–2015 (δ2H = 7.86 × δ18O + 9.60, n = 24), and LMWLRMA for Velenje 2012–2015 (δ2H = 8.05 × δ18O + 7.33; r = 0.97, n = 17)

The regression lines LMWLRMA (LMWLRMA—local meteoric water line, calculated by the reduced major-axis (RMA) regression method), SWLRMA (SWLRMA—surface water line, calculated by the reduced major-axis (RMA) regression method) and GWLRMA (GWLRMA—groundwater line calculated by the reduced major-axis (RMA) regression method) together with d-excess ranges are presented in Table 6. The Velenje LMWLRMA was compared to the LMWLRMA for Ljubljana for the period 2012–2015 (Table 6). Both the Velenje and Ljubljana LMWLs have similar slopes and intercepts. The d-excess values also vary, with the most extensive range observed for precipitation and the smallest for Triassic groundwater (Table 6).
Table 6

Calculated different water lines calculated by the reduced major-axis (RMA) regression method (local meteoric water line, surface water line, ground water line) in the period 2012–2013

Location

Period

Type of samples

Water line type

d-excess

Velenje

2012–2015

Precipitation

LMWLRMA

δ2H = (8.05 ± 0.45) × δ18O + (7.33 ± 4.46); r = 0.97, n = 17

− 4.0 to 13.6‰

Ljubljana

2012–2015

Precipitation

LMWLRMA

δ2H = (7.86 ± 0.16) × δ18O + (9.60 ± 1.53); r = 0.99, n = 24

7.2 to 15.7‰

Velenje

2015

Surface water

SWLRMA

δ2H = (5.02 ± 1.07) × δ18O − (16.57 ± 9.79); r = 0.77, n = 9

6.2 to 13.9‰

Velenje

2014–2015

GW all

GWLRMA

δ2H = (6.47 ± 0.37) × δ18O − (4.41 ± 4.04); r = 0.94, n = 38

1.0 to 16.4‰

Velenje

2014–2015

GW Triassic

GWLRMA

δ2H = (7.46 ± 0.59) × δ18O + (6.37 ± 6.12); r = 0.95, n = 15

8.3 to 13.9‰

Velenje

2014–2015

GW Pliocene

GWLRMA

δ2H = (5.40 ± 0.49) × δ18O - (16.82 ± 5.54); r = 0.90, n = 23

1.0 to 16.4‰

LMWLRMA local meteoric water line calculated by the reduced major-axis (RMA) regression method; SWLRMA surface water line calculated by the reduced major-axis (RMA) regression method; GWLRMA ground water line calculated by the reduced major-axis (RMA) regression method

The majority of groundwater samples from both the Pliocene and Triassic aquifers plot along the global meteoric water line (GMWL; δ2H = 8 × δ18O + 10; [79] and local meteoric water lines (LMWLRMA, Table 6), within the range of local modern precipitation, indicating they are meteoric in origin with limited evaporation and/or geochemical processes that modify the stable isotope composition of water (Fig. 10). Slight variability around the GMWL may be explained by recharge in the geologic past by precipitation with a different LMWL (slope and intercept). Two samples (3491, BV28/87) from the Pliocene aquifer plot slightly higher left above the GMWL than the rest of the groundwater and surface water samples, enriched in 2H, which may be explained by CO2 exsolution from deeper sources [1].

These findings are in good agreement with higher CO2 concentrations in groundwater from Pliocene aquifers. The δ2H values of groundwater also vary in Pliocene aquifers based on season, with the most 2H-enriched values observed in the summer (Fig. 10). Only one groundwater sample (PE-4/82) deviates to the right of the GMWL. Lower slopes and the intercepts of the groundwater lines (Table 6) show the influence of evaporation and other processes: These are most pronounced in the Pliocene groundwater sample PE-4/82 (d-excess 1‰).

Groundwater in the Triassic aquifer has slightly higher δ18O and δ2H values, compared to the Pliocene aquifer, likely reflecting mixing with precipitation and shallow groundwater. The δ18O and δ2H values of local precipitation, sampled in different seasons from 2012 to 2015, range from − 15.0 to − 3.9‰ and from − 108.7 to − 28.8‰, respectively (Table 6) with lower δ18O and δ2H values in winter months and higher in summer months, overlapping most of the groundwater and surface water values presented in this study. The lower δ18O and δ2H values observed in the Pliocene aquifer may be due to recharge under different (cooler) climatic conditions (e.g., paleo-recharge), recharge of dominantly winter precipitation, or higher elevation recharge.

Figure 11 shows the spatial distribution of the δ18O values in groundwater. The most negative δ18O values (from − 12.3 to − 11.8‰) are in the southwestern part of the basin, where the Pliocene aquifers are located. These groundwater samples have a relatively long residence time based on their tritium values (around 0 TU). Higher δ18O values (from − 11.8 to − 9.9‰) occur in the northeastern part of the basin where Triassic aquifers are present. The recharge area for the Triassic aquifer is in the northeastern part of the basin, in the mountainous region of Paški Kozjak (elevation 638.7 m) [46].
Fig. 11

a Spatial distribution of δ18O values of Velenje basin groundwater, b spatial distribution of δ2H values of Velenje basin groundwater

Figure 11 shows the spatial distribution of deuterium. The isotopic composition of deuterium (δ2H) is similar to oxygen (δ18O). Higher values (− 71.7 to −65.4‰) are characteristic of the Triassic aquifer and local precipitation. Lower values (from − 83.3 to − 76.6‰) are typical of the Pliocene aquifer groundwater.

The activity of geogenic 3H in most groundwater is negligible. Thus, measurable 3H in groundwater samples signifies modern recharge. The useful range for 3H dating is less than 50 years using the enrichment method [21, 80]. This study used the 3H/3He in-growth method for groundwater dating since the enrichment method gave values close to the LOD [46]. Tritium levels in Pliocene aquifer groundwater (0.02 to 0.09 TU) are close to 0 TU, confirming an age greater than 50 years, consistent with previous studies [46]. The Triassic aquifer sample j.v. 2391-1 had a higher 3H value (4.17 TU) and a more positive δ18O value, while the remaining samples had low 3H values (around 0.05 TU). Kanduč et al. [46] recorded 3H values of 3.7 to 6.4 TU for the groundwater using the enrichment method. Such levels are indicative of recent recharge and contact with surface precipitation (5.8 to 7.4 TU). Precipitation in Velenje during the same period had a tritium value of 6.4 TU (enrichment method), similar to local surface water values (Table 6).

4 Conclusions

This study analyzed trace elements (Li, B, Al, Ti, Mn, Fe, Co, Ni, Cu, Zn, As, Rb, Y, Mo, Cs, Ba, Eu, Tl, Pb, U), stable isotopes (δ34SSO4, δ18O, δ2H), tritium (3H) and dissolved gases (CO2, CH4, N2, δ13CCH4, δ13CCO2) in groundwater from different aquifers in the Velenje basin to determine hydrogeochemical processes influencing water quality.

Groundwater from dewatering wells in Pliocene and Triassic strata contains distinct and separate water bodies with different chemical compositions confirming the results from previous studies. New data presented herein show that the groundwater in dewatering wells in Triassic strata was recharged recently and evolved primarily through interaction with carbonate minerals. Data show that groundwater in the Pliocene aquifer is older, influenced by the dissolution of silicate minerals present in sands and clays and impacted by redox reactions, such as bacterial sulfate reduction and microbial methanogenesis.

Elevated concentrations of Fe, Ni, Ba, As and Zn are present in some dewatering wells in the Pliocene strata, a phenomenon related to higher retention of groundwater and the petrographic origin of trace elements.

The δ34SSO4 and δ18OSO4 values indicate several dissolved sulfate sources and processes: anhydrite dissolution, sulfide oxidation and precipitation. Unfortunately, the concentration of SO42− was too low in the groundwater from dewatering wells in Pliocene strata to be able to determine the origin of sulfate. The low sulfate is likely a consequence of bacterial sulfate reduction with low DO and methanogenesis prevalent in the Pliocene aquifer.

Carbon dioxide in groundwater from dewatering wells in Triassic strata and surface waters is likely organic in origin based on the δ13CCO2 values. In contrast, in groundwater from dewatering wells in Pliocene strata the δ13CCO2 values are more positive, suggesting that the CO2 likely originates from the Subcontinental lithospheric mantle. Despite the lack of recent nearby volcanic activity, deep geogenic gas can rise along deeply rooted tectonic lineaments and reach shallower strata in the Velenje basin. Local thermal and mineral springs have similar δ13CCO2gas values. The levels of CH4 are comparable to that reported for an Ontario aquifer, Canada. The δ13CCH42HCH4 values and the αCO2–CH4 fractionation factors suggest methane in the Pliocene aquifer was generated via microbial CO2 reduction.

The δ18O and δ2H values of all groundwater samples plot along the GMWL and LMWLs except for two samples from dewatering wells in the Pliocene strata which may be impacted by CO2 exsolution. The lower δ18O and δ2H values also correspond to older groundwater with longer residence times, confirmed using the 3He in-growth method.

The methodology used in this study could be applied to other regions (e.g., sedimentary coal basins) with similar complexities to evaluate geochemical processes. Furthermore, the applied research useful for active mining areas since knowing the age of groundwater is essential from an engineering perspective, e.g., pumping of groundwater from the mine to lower the hydrostatic pressure to improve safety. The applied research also highlights the use of stable isotopes which are ideal tracers for investigating processes occurring in sedimentary coal basin not just on the local level, but also on the global level.

Notes

Acknowledgements

This study was performed as part of research projects: L1-5451, N1-0054, bilateral cooperation: BI-US/12-13-039 (Fluid dynamics and carbon cycling in sedimentary basins: geochemical characterization, evaluation of biogeochemical processes and modeling) and program funding: P1-0143, P1-0195, P2-0084, financially supported by the Slovenian Research Agency and the Velenje Coalmine d.o.o. Particularly, we thank Mr. Igor Medved for help with sampling and Mr. Nejc Skarlovnik for creating the groundwater contour map. For graphical support, we thank Patrik Kušter. Thanks to MASSTWIN-H2020 Twinning project; Spreading Excellence and Widening Participation in Support of Mass Spectrometry and Related Techniques in Health, Environment, and Food Analysis (Grant Agreement No. 692241).

Compliance with ethical standards

Conflict of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest.

Supplementary material

42452_2019_1561_MOESM1_ESM.docx (668 kb)
Supplementary material 1 (DOCX 668 kb)

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  • Tjaša Kanduč
    • 1
    Email author
  • Zdenka Šlejkovec
    • 1
  • Polona Vreča
    • 1
  • Zoran Samardžija
    • 2
  • Timotej Verbovšek
    • 3
  • Darian Božič
    • 4
  • Sergej Jamnikar
    • 4
  • D. Kip Solomon
    • 5
  • Diego P. Fernandez
    • 5
  • Christopher Eastoe
    • 6
  • Jennifer McIntosh
    • 7
  • Nataša Mori
    • 8
  • Fausto Grassa
    • 9
  1. 1.Department of Environmental SciencesJožef Stefan InstituteLjubljanaSlovenia
  2. 2.Department of Nanostructured MaterialsJožef Stefan InstituteLjubljanaSlovenia
  3. 3.Department of Geology, Faculty of Natural Sciences and EngineeringUniversity of LjubljanaLjubljanaSlovenia
  4. 4.Velenje Coal Mine d.o.o.VelenjeSlovenia
  5. 5.Department of Geology and GeophysicsThe University of UtahSalt Lake CityUSA
  6. 6.Department of GeosciencesUniversity of ArizonaTucsonUSA
  7. 7.Department of Hydrology and Atmospheric SciencesUniversity of ArizonaTucsonUSA
  8. 8.Department of Organisms and Ecosystems ReserachNational Institute of BiologyLjubljanaSlovenia
  9. 9.Istituto Nazionale di Geofisica e VulcanologiaPalermoItaly

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