a geochemical and stable isotope investigation of groundwater/surface-water interactions in the...

A geochemical and stable isotope investigationof groundwater/surface-water interactionsin the Velenje Basin, Slovenia

Tjaša Kanduč & Fausto Grassa & Jennifer McIntosh &

Vekoslava Stibilj & Marija Ulrich-Supovec &

Ivan Supovec & Sergej Jamnikar

Abstract The geochemical and isotopic composition ofsurface waters and groundwater in the Velenje Basin,Slovenia, was investigated seasonally to determine therelationship between major aquifers and surface waters,water–rock reactions, relative ages of groundwater, andbiogeochemical processes. Groundwater in the Triassicaquifer is dominated by HCO3

–, Ca2+, Mg2+ andδ13CDIC indicating degradation of soil organic matterand dissolution of carbonate minerals, similar tosurface waters. In addition, groundwater in theTriassic aquifer has δ18O and δD values that plot nearsurface waters on the local and global meteoric waterlines, and detectable tritium, likely reflecting recent(<50 years) recharge. In contrast, groundwater in thePliocene aquifers is enriched in Mg2+, Na+, Ca2+, K+,

and Si, and has high alkalinity and δ13CDIC values,with low SO4

2– and NO3– concentrations. These waters

have likely been influenced by sulfate reduction andmicrobial methanogenesis associated with coal seamsand dissolution of feldspars and Mg-rich clay minerals.Pliocene aquifer waters are also depleted in 18O and2H, and have 3H concentrations near the detectionlimit, suggesting these waters are older, had a differentrecharge source, and have not mixed extensively withgroundwater in the Triassic aquifer.

Keywords Hydrogeochemistry . Stable isotopes .Groundwater/surface-water relations . Groundwater age .Slovenia

Introduction

The Velenje Basin in Slovenia is one of the largestactively mined coal basins in central Europe, producingaround 4 million tons of lignite per year (Mihelak 2010).Large amounts of groundwater are extracted from VelenjeBasin aquifers to facilitate underground mining of coal,and coal seam gas outbursts are a serious mine safetyconcern (Kanduč et al. 2011). This study analyzed thechemical and isotopic composition of groundwater andsurface water in the Velenje Basin, combined withhydrogeologic information, to investigate groundwater/surface-water interactions, residence times of groundwa-ter, and biogeochemical processes, which aid in under-standing the generation and source of coal seam gases—carbon dioxide (CO2) and methane (CH4)—responsiblefor gas outbursts, and better characterizing the waterresources. This study is also the first systematic seasonalstudy of geochemical (chemical and isotopic) variables ofgroundwater in the Velenje Basin and is part of a majorproject aimed at evaluating the hydrogeology andhydrochemistry of a coal seam gas (CSG) explorationarea.

Hydrochemistry and stable isotope compositions ofgroundwaters provide critical information regardingsources and timing of groundwater recharge, water–rock

Received: 1 April 2013 /Accepted: 9 January 2014Published online: 4 March 2014

* Springer-Verlag Berlin Heidelberg 2014

Electronic supplementary material The online version of this article(doi:10.1007/s10040-014-1103-7) contains supplementary material,which is available to authorized users.

T. Kanduč ()) :V. StibiljDepartment of Environmental Sciences,Jožef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Sloveniae-mail: [email protected].: +386-1-5885238Fax: +386-1-5885346

F. GrassaIstituto Nazionale di Geofisica e Vulcanologia Sezione di Palermo,Via Ugo La Malfa, 153, 90144 Palermo, Italy

J. McIntoshDepartment of Hydrology and Water Resources,University of Arizona, 1133 E. James E, Rogers Way, Tucson, AZ85721, USA

M. Ulrich-Supovec : I. SupovecHGEM D.O.O., Zaloška 143, Ljubljana, Slovenia

S. JamnikarVelenje Coal Mine, Partizanska 78, 3320 Velenje, Slovenia

Hydrogeology Journal (2014) 22: 971–984DOI 10.1007/s10040-014-1103-7

http://dx.doi.org/10.1007/s10040-014-1103-7

interaction along flow paths, and mixing of distinctgroundwater bodies (e.g. Cartwright et al. 2012). δ18Oand δD values may be used to determine the extent ofevaporation, recharge conditions (e.g. temperature orelevation), and groundwater mixing as processes inthe hydrological cycle fractionate 18O:16O and 2H:1Hratios (Gonfiantini 1986; Herzeg et al. 1997; Weaveret al. 1995; Clark and Fritz 1997; Edmunds 2009;Currell et al. 2010; Cartwright et al. 2012). Waterisotopes (δ18O or δD) often behave conservatively inlow-temperature aquifer environments (Kendall et al.1995) and, as a result, researchers have often usedwater isotopes as tracers for determining the waterprovenance (Epstein and Mayeda 1953; Kennedy et al.1986; Mayo and Loucks 1995; Taylor et al. 1992;Katz et al. 1997; Cartwright et al. 2000; Larsen et al.2001). Tritium and 14C are frequently used tracers todetermine the age of groundwater up to ∼40 years and30 thousand years (ka) old, respectively (Clark andFritz 1997; Kallin 2000; Celle-Jeanton et al. 2009;Edmunds 2009).

Stable carbon isotopes are useful indicators of dis-solved inorganic carbon (DIC) sources in groundwatersystems and are used to assess the origin of DIC, which isthe main species in carbonate environments.Concentrations of DIC and its stable carbon isotope ratios(δ13CDIC) are governed by processes occurring in theaquifer system, and these can vary seasonally. Changes inDIC concentrations result from carbon addition orremoval from the DIC pool, while changes in δ13CDIC

values result from the fractionation accompanying trans-formation of carbon or from mixing of carbon fromdifferent sources. The major sources of carbon to aquiferDIC loads are dissolution of carbonate minerals, soil CO2

derived from root respiration, and microbial decomposi-tion of organic matter (Aucour et al. 1999; Li et al. 2005;Kanduč et al. 2007). Methanogenesis in organic-richaquifers may also influence δ13CDIC and alkalinityconcentrations (e.g. McIntosh and Martini 2008). Themajor process removing DIC in aquifer systems iscarbonate mineral precipitation (Atekwana andKrishnamurthy 1998). Stable S isotopes are used to tracethe sources of SO4

2– in groundwater, and to constrainprocesses such as bacterial sulfate reduction (e.g.Dogramaci and Herzceg 2002; Cartwright 2010). Inaddition, characterization of geochemical propertiesof groundwaters may aid in the understanding ofhydrocarbon entrapment and mineral deposition, andhelp in identifying the origin of fluids (Aravena et al.2003).

Some previous studies on the geochemical andisotopic composition of groundwater in Velenje Basinwere reviewed (Mali 1992; Mali and Veselič 1989;Veselič and Pezdič 1998; Urbanc and Lajlar 2002).The molecular and isotopic composition of gases inthe Velenje Basin were previously investigated and itwas determined that CH4 and CO2 in coal seams areof mixed origin. Carbon dioxide is likely sourcedfrom a mixture of in-situ microbial activity (i.e.

biodegradation of organic matter) and external CO2

(endogenic CO2, including CO2 originating fromdissolution of carbonates and hinterland waterrecharging the basin), while CH4 is dominantlysourced from microbial methanogenesis, with possibleaddition of thermogenic gas from deeper formations,and removal by microbial oxidation of methane(Kanduč and Pezdič 2005; Kanduč et al. 2012).Temporal changes in the chemical and isotopiccomposition of “free” seam gases were observed as afunction of the advancement of the working faces(Kanduč et al. 2011). It was also found that areas withhigh CO2 concentrations are associated with lowmethane concentrations and this was related to thevelocity of advancement of the working face (Kandučet al. 2011).

Study area

The Velenje Basin and associated coal mines aresituated in the NE part of Slovenia. It is located atthe junction of the WNW–ESE-trending Šoštanj faultand the E–W-trending Periadriatic zone, bounded tothe south by the Smrekovec fault segment. TheŠoštanj and Smrekovec faults were generated due tothe collision of continental plates. The study area,map of surface water and groundwater samplinglocations and geological profiles of the VelenjeBasin are shown in Fig. 1. The cross-sections A–Band C–D are detailed in the electronic supplementarymaterial (ESM). In general, two hydrogeologicalsystems can be distinguished in the Velenje coalmine: (1) Plioquaternary and Pliocene aquifers com-posed of gravel-sand and silt, which are furtherdivided to upper, interlayer and lower aquifers; upperaquifers are schematically divided into alluvial,Quaternary and at least three parts of Plioceneaquifers (Pl-1, Pl-2 and Pl-3; Fig. 1a); and (2)carbonate aquifers, which are further divided intoSkitian limestone and dolomite, Anisian dolomite andlimestone, and Miocene Lithotamnium limestone(Vižintin et al. 2009 and references therein). Thestudy area belongs to the Southern Karavanke moun-tain range (Fig. 1a). In the pre-Pliocene basement ofthe basin, Triassic limestones and dolomites prevailon the NE side of the Velenje fault. Oligocene toMiocene clastic strata, consisting predominantly ofmarls, sandstones and volcanoclastics prevail on theSW side (Brezigar et al. 1988).

The artificial lakes investigated in this study wereformed due to excavation of coal and subsidence of theterrain. Besides Lake Velenje, which is on average23.4 m deep (maximum 63.2 m), Lake Škale and LakeDružmirje (on average 27.9 m deep and maximumdepth of 86.6 m, (Rošer, PV Invest D.O.O., personalcommunication, 2013) were also investigated withgeochemical analysis.

972

Hydrogeology Journal (2014) 22: 971–984 DOI 10.1007/s10040-014-1103-7

The headwaters for the River Paka, which is the mainriver in the Velenje Basin, are in the Volovica on PohorjeMountains. The streams Pečovnica, Velunja and Toplicacreated the river’s flow path in the Pliocene andQuaternary due to the sinking of the basin. The RiverPaka has a torrential character, with the lowest discharge(0.5 m3/s) in the summer months, and highest (3.5 m3/s) in

the spring. Though the River Paka is pristine at its source,in the Velenje and Šoštanj areas it becomes highlypolluted by input of sewage sludge (Gams and Zupan1994). All tributaries (River Toplica, River Pečovnica,River Klančica, River Velunja, River Ljubela, RiverLepena, River Paka) investigated in this study flow intothe River Paka.

Fig. 1 a Geological sketch map of the Velenje Basin with cross section NNE–SSW. Main aquifers are Pliocene (Pl-1, Pl-2 and Pl−3;bottom half), and Triassic (T1, T2 and T2,3; bottom half) dolostones (prevailing) and limestones. Other Triassic and Paleozoic (Pz)lithologies are composed of relatively impermeable strata and therefore do not include significant aquifers. Profiles A–B and C–D (tophalf) present locations of groundwater sampling in this study. b Map of surface water sampling locations (within the boundary of thelignite seam/coal mine, shown in a). Location numbers (1–10) correspond to Table 1. c Map of groundwater sampling locations withprofiles A–B and C–D. Elevation relative to sea level (m) of sampling locations is presented in Table 2. Exit and delivery roadways arealso indicated in mining areas

973


In the Velenje area, the River Paka is underlain byTriassic limestone, and Pliocene and Quaternarysediments. The headwaters of the River Velunja drainrocks and sediments of the Velunja overthrust belt,composed of sericite and chlorite schist with sand-stones and diabase of Ordovician and Devonian age.In the central part of the River Velunja drainage area,the watershed is composed of rocks of Miocene age,which were deposited on Devonian schist, composedof conglomerate, sandstones and clays. In its lowerreach, the River Velunja drains Pliocene andQuaternary sediments composed of sands, clays andgravels (Mioč and Žnidarčič 1972). The gravels arecomposed of magmatic and metamorphic rocks,which were eroded from the upper part of thewatershed.

The Velenje coalmine is separated into three parts:the Preloge coalmine, Pesje coalmine and Škalecoalmine (closed for mining in 2008; Fig. 1c). Waterrecharging the Velenje Basin is drained by hangingfilters to prevent inrush of water into the mine. The

average discharge of water from Pliocene sands is800 l/min, while the average discharge from Triassiclimestone aquifers is 3,400 l/min (Supovec et al.2012a, b). In the Velenje coal basin, the shallowaquifers are part of the Quaternary and Pliocenesections. Pliocene aquifers are further classified asaquifers above the coal 0–20 m (Pl-1), aquifers 20–80 m above the coal (Pl-2), and upper Plioceneaquifers (Pl-3) (Fig. 1a). The Pliocene aquifers arecomposed of clastic sediments such as sand andgravel. Coefficient of permeability values were obtain-ed through numerous pumping tests performed on thedewatering and observation wells. Hydraulic conduc-tivity values (k) are in the range 1.74×10–7 to 2.88×10–6 m/s (Vižintin et al. 2009). Lithotamnium lime-stone (Miocene age) underlies the northern part of thePreloge coalmine, on the southern side of the Velenjefault and represents a local lens-shaped aquifer, whichis limited by an overlying impermeable barrier. ThePliocene and Lithotamnium aquifers are located in thePreloge coalmine. Triassic strata (Scythian and Anisian

Fig. 1 (continued)

974


ages) are composed of limestone and dolomite andform the Triassic aquifer system, which is of potentialinterest for water-supply management.

Sampling and analytical methods

A preliminary study of geochemical characteristics ofsurface waters and groundwaters in the Velenje Basin wasperformed in 2004 and published by Kanduč et al. (2010).A more systematic study of surface and groundwater wasconducted in 2010 and 2011, as part of this study, duringdifferent sampling seasons (Spring 2010, Summer 2010,Winter 2011 and Spring 2011) to trace seasonal changes ingroundwaters, to compare their geochemical and isotopiccomposition with surface waters, to determine their originand age, and finally to relate their geochemical composi-tion with coalbed gas generation. Sampling locations areshown in Fig. 1b,c. In this study, 10 surface-water bodies(3 lakes, 7 streams; Table 1; Fig. 1b) and 14 groundwaterlocations (10 samples from Pliocene (P1-1 and P1-2)aquifers and 4 samples from Triassic aquifers) wereinvestigated for geochemical and stable isotopic parame-ters (Table 2; Fig. 1c). Groundwater samples werecollected from dewatering wells as follows: V-12* andBV-2* are wells drilled from the surface and attached tothe mine-dewatering network (6 aquifers, all of them inPl-1 and Pl-2 aquifers). Objects labelled with j.v. are minepiezometers (eight total sampled), among which four ofthem are within the Pliocene Pl-1, and the other four aredewatering Triassic strata (Table 2). Temperature and pHwere measured in the field, while electrical conductivityand dissolved oxygen were measured in the laboratory(due to the risk of explosion). Sample aliquots collectedfor chemical analysis were immediately passed through a0.45-μm nylon filter into bottles and kept refrigerated untilanalyzed. Samples for cation (treated with HNO3), anionand alkalinity analyses were preserved in HDPE bottles.Samples for δ13CDIC analyses were stored in glass vials,filled to the top, with no headspace. Water samples forδ18O and δD analyses were collected in HDPE bottleswith no headspace. Samples for sulfur isotope analyseswere collected in 3-l plastic bottles and were acidifiedafter filtration to pH 2. BaCl2 was added to precipitateBaSO4, which was then collected by filtering through a0.45-μm filter. For tritium analyses, 1 l of unfiltered waterwas collected in a plastic bottle at each sampling location.

Total alkalinity was measured by Gran titration(Gieskes 1974) with a precision of ±1 % within 24 h ofsample collection. Major ion chemistry was analyzed inthe Hydrology and Water Resources Department at theUniversity of Arizona (UA). Major cations (Ca, Mg, Na,K, Sr, Si) were analyzed (precision ±2 %) with a Perkin-Elmer Optima 5100DV Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES), and majoranions (Cl, SO4 and NO3) were analyzed (precision ±2 %)with a Dionex Ion Chromatograph (IC) Model 3000, usingan AS23 analytical column. T

able

1Average

annu

alchem

ical

andisotop

iccompo

sitio

nof

surfacewatersfrom

Velenje

Basin,Slovenia

Sam

pling

locatio

nLocation

pHT (°C)

Total

alkalin

ity(m

M)

Con

ductivity

(μS/cm)

Ca2

+

(mM)

Mg2

+

(mM)

Na+

(mM)

K+

(mM)

Si

(mM)

SO42–

(mM)

NO3–

(mM)

SI cal

SI dolpC

O2

(ppm

)Cl–

(mM)

δ13CDIC

(‰)

δ18O

(‰)

δD (‰)

3H (TU)

δ34SSO4

(‰)

1River

Toplica

7.95

12.1

4.00

376

1.26

0.36

0.29

0.05

0.13

0.20

0.10

0.40

0.30

2511.9

0.20

−11.2

−9.5

−63.0

5.8

2River

Pečovnica

8.12

9.8

3.40

330

1.13

0.39

0.42

0.07

0.18

0.19

0.17

0.70

0.90

1584.9

0.33

−11.5

−9.3

−62.0

7.0

5.8

3River

Klančica

7.81

10.4

2.90

ND

1.17

0.48

0.42

0.06

0.26

0.20

0.11

0.40

0.40

1584.9

0.24

−12.8

ND

5.6

4River

Velunja

7.89

10.3

3.10

286

0.98

0.54

0.28

0.04

0.08

0.23

0.07

0.30

0.30

2511.9

0.13

−9.6

−9.6

−64.0

6.2

7.2

5LakeDružm

irje

7.95

12.4

3.30

399

1.19

0.53

0.44

0.11

0.13

0.65

0.04

0.20

0.00

5011.9

0.22

−10.3

−9.1

−63.5

6.3

8.4

6River

Ljubela

8.28

12.2

5.00

443

1.85

0.75

0.30

0.04

0.13

0.16

0.07

1.10

1.90

1000.0

0.30

−10.5

−9.2

−62.8

6.4

6.6

7LakeVelenje

8.08

13.7

3.10

738

3.59

0.73

1.60

0.63

0.04

3.96

0.06

1.10

1.60

1000.0

0.77

−8.8

−8.0

−57.8

7.0

7.2

8River

Lepena

8.33

12.3

5.10

474

1.92

0.94

0.59

0.06

20.07

0.20

0.07

0.90

1.50

1258.9

0.62

−11.6

−9.4

−61.3

7.4

5.0

9LakeŠk

ale

8.05

13.1

4.80

452

1.62

0.94

0.37

0.05

80.05

0.42

0.06

0.70

1.20

1995.3

0.37

−10.8

−9.4

−67.0

6.8

6.5

10River

Paka

8.09

12.9

3.90

786

2.08

0.78

1.00

0.15

0.1

1.23

0.06

0.90

1.30

1258.9

0.82

−9.4

−8.7

−60.8

7.3

9.1

ND

notdeterm

ined

975


Tab

le2

Chemical

andisotop

iccompo

sitio

nof

grou

ndwatersfrom

Velenje

Basin

(spring20

10,summer

2010

,Winter20

10/2011,

spring

2011)

Samplinglocation,

elevationabove

sealevel(m)

Geology

pHT(°C)

Total

alkalin

ity

(mM)

Cond

(µS/cm)

Ca2

+

(mM)

Mg2

+

(mM)

Na+

(mM)

K+

(mM)

Si

(mM)

SO42−

(mM)

NO3−

(mM)

Cl−

(mM)

SI calcite

SI dolomite

pCO2

(ppm

)

δ13CDIC

(‰)

δ18O

(‰)

δD (‰)

3H (TU)

δ34SSO4

(‰)

D.O.

(%)

Spring2010

j.v.783,−7

1.5m

Plio

cene

16.77

21.0

50.75

2.11

14.06

11.27

0.19

1.53

0.24

0.01

0.41

0.35

1.62

416869.4

−0.2

−11.2

−77.0

V-12v,

385m

Plio

cene

1,2

6.40

18.0

34.04

3.75

7.97

3.87

0.19

1.17

0.14

0.00

0.18

1.56

3.51

19952.6

−2.9

−11.5

−79.0

j.v.3404,

−69.1m

Plio

cene

17.15

19.0

36.6

3.63

8.81

6.04

0.15

1.38

0.13

0.00

0.19

0.11

0.67

707945.8

−2.3

−11.7

−79.0

BV-29,

417m

Plio

cene

1,2

6.40

18.0

31.28

3.13

7.10

4.37

0.18

1.19

0.14

0.00

0.15

0.00

0.40

602559.6

−3.6

−11.8

−80.0

V-12z,

387m

Plio

cene

1,2

7.18

18.0

35.95

3.91

8.46

4.54

0.20

1.22

0.13

0.00

0.18

0.90

2.18

112201.8

−2.9

−10.7

−77.0

BV-27,

413m

Plio

cene

1,2

7.40

16.0

12.19

1.75

2.26

1.62

0.09

0.82

0.04

0.00

0.06

0.94

2.00

7585.8

−4.9

−11.0

−76.0

V-11r,372.8m

Plio

cene

1,2

6.80

19.0

44.2

3.50

11.63

7.42

0.18

1.43

0.14

0.00

0.25

0.74

2.06

208929.6

−2.9

−10.6

−77.0

V-11n,

373m

Plio

cene

1,2

6.55

20.0

71.16

4.68

21.01

10.13

0.29

1.48

0.27

0.00

0.62

0.53

1.78

912010.8

−4.8

−10.5

−71.0

j.v.3405,

−91.2

Plio

cene

17.01

20.0

33.92

2.58

8.16

7.79

0.14

1.53

0.23

0.00

0.24

0.57

1.72

162181.0

−2.1

−11.7

−80.0

j.v.3401-K,−22.3m

Plio

cene

17.16

18.5

20.63

2.89

4.07

3.70

0.15

1.34

0.09

0.00

0.09

0.62

1.43

72443.5

−0.8

−11.9

−80.0

j.v.2341,121.6m

Triass-Anisian

7.00

16.0

7.28

3.45

1.77

0.14

0.02

0.16

1.67

0.00

0.15

0.25

0.21

28183.8

−10.4

−10.3

−66.0

j.v.2346,121.0m

Triass-anisian

7.18

17.0

6.99

2.88

1.75

0.10

0.02

0.16

1.35

0.00

0.09

−0.13

−0.46

56234.1

−9.7

−10.0

− 66.0

j.v.2391,85.6

mTriass-Anisian

6.77

14.0

6.18

1.84

1.46

0.11

0.02

0.12

0.33

0.05

0.13

−0.42

−0.98

52480.7

−11.3

−10.1

−67.0

j.v.2343/6,121m

Triass-Anisian

7.11

17.0

6.69

2.33

1.64

0.07

0.02

0.15

0.77

0.00

0.07

0.12

0.11

24547.0

−10.2

−10.3

−72.0

Sum

mer

2010

j.v.783

Plio

cene

17.18

21.0

59.65

4.36

14.47

11.48

0.19

1.58

0.04

0.00

0.42

−0.54

0.52

912010.8

−2.2

−11.3

−70.0

3.7

V-12v

Plio

cene

1,2

6.50

18.0

36.46

3.72

7.88

3.83

0.19

1.17

0.04

0.00

0.18

0.08

0.52

758577.6

−2.3

−11.8

−76.0

3.7

j.v.3404

Plio

cene

16.36

19.0

38.63

3.57

8.77

6.09

0.15

1.43

0.04

0.00

0.19

0.08

0.62

812830.5

−2.1

−11.7

−75.0

3.6

BV-29

Plio

cene

1,2

7.00

19.0

33.63

3.29

7.43

4.43

0.18

1.20

0.04

0.00

0.15

0.65

1.72

165858.6

0.0

−11.8

−74.0

3.6

V-12z

Plio

cene

1,2

6.70

18.0

40.23

3.93

8.67

4.55

0.20

1.21

0.04

0.00

0.19

0.46

1.31

380189.4

−2.3

−11.8

−74.0

3.7

BV-27

Plio

cene

1,2

7.12

16.0

11.54

1.57

1.78

1.31

0.08

0.75

0.05

0.00

0.04

0.12

0.30

44668.4

−4.7

−11.5

−74.0

3.7

V-11r

Plio

cene

1,2

6.50

20.0

47.82

3.47

11.69

7.45

0.19

1.44

0.04

0.00

0.25

0.27

1.15

724436.0

−3.0

−11.5

−71.0

3.6

V-11n

Plio

cene

1,2

7.38

20.0

75.43

4.62

20.63

9.92

0.29

1.46

0.04

0.00

0.52

1.36

3.45

141253.8

−0.8

−11.1

−68.0

3.6

j.v.3405

Plio

cene

16.36

20.0

34.39

2.57

7.88

7.69

0.14

1.54

0.04

0.00

0.15

0.94

2.44

70794.6

−2.2

−11.9

−78.0

3.6

j.v.2341

Triass-Anisian

7.11

16.0

7.82

3.29

1.69

0.14

0.02

0.16

1.57

0.00

0.16

0.31

0.41

32359.4

−9.8

−9.2

−66.0

4.4

j.v.2346

Triass-A

nisian

7.06

17.0

7.84

2.87

1.74

0.10

0.02

0.16

1.26

0.00

0.10

0.17

0.15

34673.7

−9.7

−9.7

−67.0

3.7

j.v.2391

Triass-A

nisian

7.08

14.0

7.45

1.87

1.46

0.13

0.03

0.12

0.33

0.00

0.14

0.46

0.79

9772.4

−11.1

−10.3

−62.0

6.7

j.v.2343/6

Triass-A

nisian

6.66

17.0

7.05

2.27

1.61

0.07

0.02

0.15

0.75

0.00

0.07

−0.36

−0.84

79432.8

−10.2

−9.6

−68.0

4.2

Winter2010/2011

j.v.783

Plio

cene

17.10

20.0

54.80

2.41

12.99

10.52

0.18

1.51

0.00

n.a.

0.21

0.74

2.29

208929.6

−2.7

−10.9

−75.0

V-12v

Plio

cene

1,2

6.50

18.0

34.60

3.32

7.29

3.75

0.19

1.17

0.00

n.a.

0.14

0.15

0.68

524807.5

−2.6

−11.6

−78.0

BV-29

Plio

cene

1,2

6.80

18.5

32.90

2.90

6.83

4.53

0.18

1.22

0.00

n.a.

0.12

0.39

1.19

251188.6

−2.9

−11.5

−84.0

V-12z

Plio

cene

1,2

6.60

17.5

36.60

3.47

7.85

4.42

0.21

1.25

0.00

n.a.

0.15

0.27

0.94

436515.8

−2.5

−11.3

−78.0

BV-27

Plio

cene

1,2

7.22

16.0

11.70

1.60

1.88

1.49

0.08

0.82

0.00

0.29

0.04

0.21

0.50

38018.9

−5.6

−11.1

−74.0

V-11r

Plio

cene

1,2

6.60

20.0

45.00

2.99

10.78

7.61

0.18

1.44

0.00

n.a.

0.15

−0.46

−0.29

74131.0

−2.9

−11.2

−77.0

V-11n

Plio

cene

1,2

6.40

20.0

74.90

4.13

20.59

9.83

0.29

1.45

0.00

0.04

0.34

0.33

1.44

1412537.5

−4.4

−10.7

−70.0

j.v.2341

Triass-A

nisian

7.00

16.0

7.70

3.50

1.67

0.22

0.02

0.17

1.67

0.00

0.16

0.17

0.02

38904.5

−9.5

−9.7

−66.0

j.v.2346

Triass-A

nisian

7.00

16.5

7.43

2.95

1.68

0.20

0.02

0.17

1.29

0.00

0.12

0.09

−0.04

38018.9

−8.7

−9.8

−67.0

j.v.2391/1

Triass-A

nisian

7.10

13.0

6.41

1.80

1.45

0.16

0.02

0.12

0.30

n.a.

0.13

−0.11

−0.35

25118.9

−11.1

−10.1

−73.0

j.v.2343/6

Triass-A

nisian

6.50

16.5

6.83

2.30

1.55

0.14

0.02

0.16

0.74

0.00

0.07

−0.53

−1.22

112201.8

−10.3

− 10.0

−67.0

Spring2011

j.v.783

Plio

cene

16.93

20.0

35.97

3427

3427

0.90

9.64

8.96

0.16

1.23

0.01

0.00

0.27

0.03

208929.6

1.19

−1.7

−10.0

−69.7

n.d.

10.0

V-12v

Plio

cene

1,2

6.86

18.0

19.76

2182

2182

3.30

7.29

3.82

0.20

1.17

0.00

n.a.

0.14

0.32

131825.7

1.03

−2.2

−11.2

−76.3

n.d.

25.0

BV-27

Plio

cene

1,2

7.22

16.0

12.33

942.7

942.7

1.60

1.97

1.57

0.08

0.85

0.00

0.30

0.05

0.26

38018.9

0.60

−3.4

−10.7

−73.6

n.d.

30.0

V-11n

Plio

cene

1,2

7.04

20.0

34.23

4248

4248

2.20

19.51

9.36

0.28

1.41

0.00

0.37

0.13

0.47

144543.9

1.96

−3.4

−10.4

−68.5

n.d.

12.0

j.v.2341

Triass-A

nisian

7.13

17.0

7.77

805.2

805.2

3.20

1.70

0.23

0.02

0.17

1.58

0.19

0.15

0.28

29512.1

0.31

−9.9

−9.5

−63.2

14.7

76.0

j.v.2346

Triass-A

nisian

7.07

17.0

7.66

721.4

721.4

2.80

1.66

0.18

0.02

0.17

1.17

0.11

0.10

0.19

30902.9

0.18

−9.8

−9.7

−65.0

8.5

80.0

j.v.2343/6

Triass-A

nisian

7.30

17.0

7.88

633.6

633.6

2.20

1.54

0.14

0.02

0.16

0.75

0.17

0.07

0.31

20417.4

0.49

−10.6

−9.7

−65.5

10.5

60.0

976


The stable isotope composition of dissolved inorganiccarbon (δ13CDIC) was determined on an IsoPrime GVisotope ratio mass spectrometer coupled with aMultiflowBio preparation module. Phosphoric acid(100 %) was added (100–200 μl) to a septum tube andthen purged with pure He. The water sample (1 ml) wasthen injected into the septum tube and CO2 was directlymeasured from the headspace. Two standard solutions ofNa2CO3 (Carlo Erba and Scientific Fisher) with a knownδ13CDIC value of −10.8±0.2 ‰ and −4.8±0.2 ‰ wereused to calibrate δ13CDIC measurements (Spötl 2005;Kanduč et al. 2007).

Oxygen isotope ratios were measured using CO2–H2Oequilibration (Epstein and Mayeda 1953). Stable hydrogenisotope ratios were determined by H2–H2O equilibration(Coplen et al. 1991). Oxygen and hydrogen isotopicresults are reported in per mil (‰) relative to VSMOW(Vienna Standard Mean Ocean Water) and normalized(Coplen 1994) on scales such that the oxygen andhydrogen isotopic values of SLAP (Standard LightAntarctic Precipitation) are −55.5 and −428 ‰, respec-tively. Precision is estimated at ±0.2 and ±2.0 ‰ for δ18Oand δD, respectively.

The stable isotope composition of sulfur (δ34SSO4) wasdetermined with a Europa Scientific 20–20 continuousflow IRMS ANCA-SL preparation module. About 10 mgof BaSO4 was scraped from the filters and transferred to atin capsule. δ34SSO4 was determined after combustion(1,000 °C) of the capsule and reduction in a Cu tube(600 °C). NBS 127 reference materials for sulfur wereused to relate analytical result to the VCDT standard.Precision is estimated to be ±0.4 ‰.

Concentrations of tritium were determined with theelectrolytic enrichment method (Gröning and Rozanski2003; Villa and Mannjón 2004; Plastino et al. 2007).Precision of 3H measurements (samples determined 6 times)is ±0.5 TU and measurement uncertainty of result is =10 %.Detection limit for tritium measurements is 3 TU.

Thermodynamic computations were used to evaluatechemical speciation of the carbonate system—e.g. partialpressures of CO2 (pCO2), saturation states of calcite(SIcalcite)—using pH, alkalinity, and temperature as inputsto the PHREEQC speciation program (Parkhurst andAppelo 1999).

Results

The pH of surface waters ranged from 7.81 to 8.33 (Table 1),while the pH of groundwater in the Triassic and Plioceneaquifers ranged from 6.5 to 7.3 and 6.4 to 7.4, respectively(Table 2). Dissolved oxygen (DO) values ranged from 10 to30 % in the Pliocene aquifer and from 60 to 80 % in theTriassic aquifer; DO was not measured on surface waters.Alkalinity of surface waters ranged from 2.9 to 5.1 mM,concentrations of Ca2+ ranged from 0.9 to 3.6 mM, Mg2+

from 0.4 to 0.9 mM, and SO42– from 0.2 to 3.9 mM.

Calculated CO2 partial pressures (pCO2) in surface watersranged from 7,585.8 to 912,010.8 ppm (Table 1). δ13CDIC

values of surface waters ranged from −12.8 to −8.8‰, δ18Ovalues from −9.6 to −8.0‰, and δ34SSO4 values from 5.0 to9.6 ‰ (Table 1). Tritium concentrations of surface watersranged from 5.8 to 7.4 TU.

From the geochemical and stable isotope results ofgroundwater in the Velenje Basin (Table 2), two distinctwater groups were identified based on the twomajor aquifers:(1) groundwater in the Pliocene aquifer has relatively highalkalinity values from 6.1 to 75.4 mM. Ca2+ concentrationsranged from 0.9 to 4.7 mM, Mg2+ from 1.5 to 21.0 mM, Na+

from 1.6 to 11.3, Si from 0.8 to 1.5 mM, SO42– from near the

detection limit to 0.3 mM, Cl– from 0.05 to 0.62 mM,δ13CDIC values from −5.6 to 0.0‰, δ18O values from −11.9to −10.5 ‰, and 3H values from 3.6 to 3.7 TU; and (2)groundwater in the Triassic aquifer has lower alkalinityvalues from 6.18 to 7.88 mM, Ca2+ from 1.8 to 3.5 mM,Mg2+ from 1.5 to 2.5 mM, Na+ from 0.07 to 0.22 mM, Sifrom 0.12 to 0.17 mM, δ13CDIC values from −11.3 to −8.7‰, δ18O values from −10.3 to −9.5‰, δ34SSO4 values from8.5 to 14.7 ‰, and 3H values from 3.7 to 6.7 TU.

Concentrations of dissolved organic carbon (DOC) inthe Pliocene and Triassic aquifers in the Velenje basin arelow, ranging from 1.54 to 14.69 mg/l (Kanduč et al.2010), compared to groundwater associated with coalbedsin the Powder River Basin (Orem et al. 2010).

Discussion

Groundwater and surface water geochemistrySurface waters are more dilute and have lower Ca2+, Mg2+,and alkalinity concentrations compared to Velenje Basingroundwaters, as expected (Fig. 2a). Interestingly, ground-water from the Triassic aquifer is only slightly moreconcentrated and plots close to surface waters in terms ofmajor ion chemistry (Fig. 2a), water stable isotopes andtritium (discussed in section ‘Carbon cycling and redoxreactions in groundwater’). This suggests a close connectionbetween surface waters and Triassic groundwater. Surfacewaters are saturated with respect to calcite (SIcalcite indicesrange from 0.2 to 1.1) and dolomite (SIdolomite indices rangesfrom 0.3 to 1.9; Table 1; Fig. 3), haveMg2+:Ca2+ ratios below0.75, and plot along the 2:1 HCO3

–:Ca2+ +Mg2+ line inFig. 2a, demonstrating the predominance of carbonatedissolution controlling water chemistry. Surface waters fromthe Velenje Basin also have δ18O, δD and 3H values withinthe range of local precipitation (Table 1). δ13CDIC valuesindicate that dissolved inorganic carbon (DIC) in surfacewaters is derived from degradation of organic matter,dissolution of carbonates and/or equilibration with atmo-spheric CO2 (Fig. 4a–c). δ34SSO4 values indicate differentsources of SO4

2– in surface waters, both from the atmosphere(with low SO4

2– concentrations) and pyrite oxidation(Fig. 4c). The maximum concentration of 3.96 mM SO4

2–

detected in Lake Velenje could be attributed to inrush ofwater from ash deposit into the lake (Kanduč et al. 2010).

Dissolution of calcite and dolomite produces waterswith a molar ratio of (Ca2++Mg2+):HCO3

–=1:2. Theweathering of dolomite in carbonate systems contributes

977


the majority of Mg2+, in which the Mg2+:Ca2+ andMg2+:HCO3

– molar ratios indicate the relative proportionsof calcite and/or dolomite dissolution (Williams et al.2007; Zavadlav et al. 2013). Dissolution of calciteproduces waters with a Mg2+:Ca2+ molar ratio of lessthan 0.1, 0.33 in the case of congruent dissolution ofcalcite and dolomite, and equal to 1 if only dolomite isdissolving (Szramek et al. 2011). Groundwater in theTriassic aquifer falls along the 2:1 HCO3

–:(Ca2++Mg2+)line (Fig. 2a), similar to the surface waters, indicating thatweathering of carbonates is the major contributor to solute

chemistry. Furthermore, it was found that the Mg2+:Ca2+

ratio in the Triassic aquifer is higher than 0.5, indicatingthat weathering of dolomites is dominant (Fig. 2b).

In contrast, groundwater samples in the Pliocene aquiferdeviate from the 2:1 HCO3

–:Ca2++Mg2+ line (Fig. 2a), andcontain higher K+, Na+ and Si concentrations than theTriassic aquifer, suggesting that solutes are primarily derivedfrom other sources besides carbonate dissolution. Theelevated concentrations of K+, Na+ and Si are probably dueto leaching from feldspars contained in sands and clays in thePliocene aquifer. Mg2+ concentrations were surprisingly

Fig. 2 a Ca2+ + Mg2+ versus alkalinity concentrations. The line indicates stoichiometry for weathering of carbonate minerals. b Mg2+

versus Ca2+ concentrations indicating the importance of dolomite (Mg2+:Ca2+ ratio 1.0) versus calcite (Mg2+:Ca2+ ratio <0.1) weathering ingroundwater; data only shown for groundwater in the Triassic aquifer, and surface waters. Groundwater in siliclastic Pliocene aquifersshows limited evidence for carbonate dissolution

978


high (over 20 mM) in the Pliocene layer (V11-n) and weredetected in all four sampling seasons (spring 2010, summer2010, winter 2011, spring 2011) and can be explainedmineralogically with leaching of Mg from clay minerals.According to the X-ray diffraction results, clays and sands inthe Pliocene aquifer are composed of the minerals 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) (Ranzinger 2003). The

degree to which the mineral phases react with the ground-water depends on the availability of protons (H+), the contacttime and the surface area per unit volume of water (Hem1985). Dissolution of sodium feldspars (albite (NaAlSi3O8))or intermediate plagioclase (Na0.62Ca0.38Al1.38Si2.62O8) isthe source of the Na–HCO3 water type observed in thePliocene aquifer, and produces the secondary mineralproducts kaolinite and/or gibbsite. The alteration of albiteand intermediate plagioclase to kaolinite is given byfollowing Eqs. (1 and 2) as examples:

NaAlSi3O8 þ 2CO2 þ 11H2O ¼ Al2Si2O5 OHð Þ4 þ 2Naþ þ 2HCO3− þ 4H4SiO4

Albite Kaoliniteð1Þ

1:45Na0:62Ca0:38Al1:38Si2:62O8 þ 2CO3 þ 6:6H2O

¼ Al2Si2O5 OHð Þ4 þ 0:9Naþ þ 0:55Ca2þ þ 2HCO3−

þ 1:8H4SiO4 ð2Þ

In summary, the major ion chemistry results suggeststhat the Triassic and Pliocene aquifers contain distinctgroundwater bodies that were likely recharged fromdifferent sources at possibly different times, evolvedthrough water–rock reaction with different host sediments,and have not mixed extensively.

Carbon cycling and redox reactions in groundwaterCalculated CO2 partial pressures (pCO2) for groundwatersamples from the Pliocene and Triassic aquifers variesfrom 7,585 to 1,412,537.5 ppm, which is 19–3,531.3-fold

supersaturated relative to atmospheric CO2 (400 ppm).Groundwater samples from the Pliocene aquifer havecalcite saturation indices (SIcalcite) generally well aboveequilibrium (SIcalcite = 0), indicating that calcite issupersaturated and precipitation is thermodynamicallyfavoured, likely due to high alkalinity values. Incontrast, groundwater in the Triassic aquifers isundersaturated or close to saturation with respect tocalcite (Table 2; Fig. 3).

Figure 4a indicates processes influencing the δ13CDIC

value in groundwater. Groundwater samples from theTriassic aquifer have similar δ13CDIC values as surfacewaters and fall around the line of carbonate dissolution bycarbonic acid produced from the soil zone with a δ13CCO2

of −26.6 ‰ (Kanduč et al. 2007). Groundwater samplesfrom the Pliocene aquifer have higher δ13CDIC values (upto 0.0 ‰), which could be attributed to microbialmethanogenesis, causing enrichment in 13C (Kanduč et

Fig. 3 Saturation indices (SI) of calcite and dolomite for groundwater in the Triassic and Pliocene aquifers

979


Fig. 4 a Variation in δ13CDIC values of groundwater samples compared to alkalinity concentrations, with lines indicating processesoccurring in the Velenje Basin. Arrows show expected trends for a variety of processes (Coetsiers and Walraevens 2009). b SO4

2–

concentrations vs. δ13CDIC values. Arrows show expected trends for sulfate reduction and methanogenesis (Coetsiers and Walraevens2009). c δ34SSO4 vs. δ

13CDIC values for Velenje Basin; δ34SSO4 vs. δ13CDIC values of surface waters from the Sava River Basin, Slovenia,

are indicated in the blue box (Kanduč and Ogrinc 2007) for comparison

980


al. 2012), consistent with high alkalinities and thepresence of coal in the Pliocene section.

Since groundwater represents a closed system, theprocess of open system equilibration with the atmosphereis negligible. Equilibration lines were calculated accordingto possible processes influencing δ13CDIC value asfollows:

Line 1. Given the isotopic composition of atmosphericCO2 of −7.8 ‰ (Levin et al. 1987) and theequilibration fractionation with DIC of +9 ‰,DIC in equilibrium with the atmosphere shouldhave a δ13CDIC of about +1 ‰ (Fig. 4a).

Line 2. Considering the average isotopic composition ofcarbonates (δ13CCaCO3) with a value of −2 ‰(Kanduč and Pezdič 2005) and isotopic fraction-ation (and enrichment in 12C) due to dissolutionof carbonates, which is 1.0 ±0.2 ‰ (Romanek etal. 1992), δ13CDIC would be 1.0±0.2 ‰.

Line 3. An average δ13C value of particulate organiccarbon (POC) of −26.6 ‰ was assumed torepresent the isotopic composition of POC thatwas transferred to DIC by in-stream respiration.Open system equilibration of DIC with CO2

enriches DIC in 13C by about 9 ‰ (Mook et al.1974), which corresponds to the value of−17.6 ‰ shown in Fig. 4a.

Line 4. Represents open system equilibration of DIC withsoil CO2 originating from degradation of organicmatter with δ13CCO2 of −26.6 ‰.

Previous studies have observed the accumulation ofmicrobial methane in Pliocene coalbeds in the Velenje

Basin, likely generated via microbial CO2 reduction(Kanduč and Pezdič 2005; Šlejkovec and Kanduč 2005;Kanduč et al. 2012). Microbial methanogenesis typicallyoccurs in the absence of free O2, NO3

–, and SO42– in

associated groundwater (McIntosh and Martini 2008).Biogeochemical degradation of organic material in theVelenje Basin is also related to the unexpectedorganoarsenic compounds found in gelified detrital lignite(Šlejkovec and Kanduč 2005). The carbon isotope valuesof CO2 (δ13CCO2) and CH4 (δ13CCH4) in Pliocene coalbeds were highly variable, ranging from −9.7 to 0.6 ‰and −70.5 to −34.2 ‰, respectively. Carbon dioxide islikely sourced from a mixture of in-situ microbial activityand external CO2, while CH4 is dominantly sourced frommicrobial methanogenesis with possible addition ofthermogenic gas from deeper formations, and the influ-ence of microbial oxidation of methane (Kanduč et al.2012). Calcified xylites enriched with 13C (δ13C values upto 16.7 ‰) indicate that microbial methanogenesis wasactive during sedimentation and formation of the basin.

Groundwater in the Pliocene aquifer has low DOconcentrations (<30 %) and is likely anoxic to suboxic(Table 2), while groundwater in the Triassic aquifercontains higher DO (60–80 %). In addition, SO4

2– andNO3

– concentrations were low in the Pliocene aquifer (upto 0.27 and 0.29 mM, respectively), compared to surfacewaters and Triassic aquifer waters (Fig. 4b), suggestingbacterial sulfate and nitrate reduction, and consistent withmicrobial methanogenesis (Coetsiers and Walraevens2009). It is typical that groundwaters with low SO4

2–

concentrations and high δ13CDIC values also have elevatedδ34SSO4 values (Dogramaci and Herzceg 2002; Fig. 4b,c),from bacterial reduction. Samples from the Pliocene

Fig. 5 δD versus δ18O values of groundwater and surface waters shown relative to the global meteoric water line (GMWL; δD=8×δ18O+10;Craig 1961) and local meteoric water line (LMWL; δD=8.1×δ18O+9.8, Pezdič 2003)

981


aquifer from the spring 2011 sampling season all had lowSO4

2– concentrations (<0.01 mM); therefore, it waspossible to measure δ34SSO4. Sulfur isotope values ofSO4

2– from the Triassic aquifer range from 8.5 to 14.7 ‰,while surface waters range from 5.0 to 9.6 ‰ (Fig. 4c).These values are within the range of δ34SSO4 valuespreviously observed in the nearby Sava River (Kandučand Ogrinc 2007) and likely indicate oxic conditions inwater systems.

Oxygen, hydrogen and tritium isotopesin groundwater and surface waterGroundwater from both the Pliocene and Triassic aquifersplots along the global meteoric water line (GMWL) andlocal meteoric water line (LMWL), indicating watersoriginated from precipitation with no evidence forevaporation (Fig. 5). The variability in δ18O and δDvalues of Pliocene and Triassic aquifer waters, anddeviation above the GMWL (Fig. 5), may be explainedby recharge in the geologic past by precipitation with adifferent LMWL (slope and intercept), and/or CO2

exsolution from deeper sources (Cartwright et al. 2002).There also appears to be variability in the δD values ofPliocene groundwater based on season, with the most 2H-enriched values seen in the summer (Fig. 5). Groundwaterin the Triassic aquifer has higher δ18O and δD values,compared to the Pliocene aquifer, likely reflecting greatermixing with surface waters and shallow groundwater. Therecharge area for the Triassic aquifer is in the northeasternpart of the Velenje Basin, in the wide mountain area ofPaški Kozjak (elevation 638.7 m). The δ18O values oflocal precipitation, sampled in different seasons, rangefrom −11.8 to −7.8 ‰ (T. Kanduč, Jožef Stefan Institute,unpublished data, 2011), overlapping most of the ground-water and surface-water values measured in this study.The lower δ18O and δD values observed in the Plioceneaquifer may be due to a higher elevation of recharge ordifferent origin of groundwater (e.g. paleorecharge).

Activity of geogenic 3H in most groundwater isnegligible, thus measurable 3H in groundwater samplesvirtually always signifies modern recharge. The usefulrange for dating with 3H is less than about 50 years whenanalyses are performed by the enriched method (Clark andFritz 1997). In this study, 3H concentrations in ground-water samples from the Pliocene aquifer (3.6–3.7 TU) arenear the detection limit of the tritium method used (3.0TU), but slightly above, making it difficult to conclusivelydetermine the age of these waters. Groundwater in theTriassic aquifer had higher 3H values (3.7–6.7 TU), andmore positive δ18O values, likely indicating more recentrecharge (within the past 40–50 years) and contact withsurface waters (range from 5.8 to 7.4 TU 3H).Precipitation, which was sampled in Ljubljana during thesame period as this study, had an average tritium value of6.4 TU (Glavič-Cindro 2011), within the range of surfacewaters in the Velenje Basin (Table 1). Additional age-dating techniques are needed to constrain the residencetime of groundwater in the Pliocene and Triassic aquifers.

Conclusions

The major ion chemistry, redox conditions, stableisotopes (13C, 34S, 18O, 2H), and tritium measured ingroundwater from the Velenje Basin, suggest that thePliocene and Triassic aquifers contain distinct andseparate water bodies. Groundwater in the Triassicaquifer has likely been recently recharged by precipi-tation and surface water and evolved primarily throughinteraction with carbonate minerals, while groundwaterin the Pliocene aquifer is likely older, and impacted byredox reactions, including microbial methanogenesisassociated with coalbeds, and dissolution of silicateminerals in sands and clays.

The concentration of solutes decreases according to thesequence HCO3

–>Mg2+>Na+>Ca2+ in the Pliocene aqui-fer and HCO3

–>Ca2+>Mg2+ in the Triassic aquifer.δ13CDIC values, alkalinity and redox conditions ofgroundwater helped to constrain biogeochemical process-es controlling fluid chemistry. δ13CDIC values of the lakesreached up to −7.5 ‰, which is related to longerequilibration with atmospheric CO2, while river waterhad lower δ13CDIC values similar to those observed in theTriassic aquifer. Higher δ13CDIC values (up to 0.0 ‰) inthe Pliocene aquifer could be attributed to microbialmethanogenesis associated with the coal seam. This wasalso confirmed by the high alkalinity values (up to75.43 mM), and low DO, NO3

– and SO42– concentrations.

The elevated Mg2+, Na+, and Si concentrations in thePliocene aquifer could be attributed to dissolution of clayminerals.

δ13CDIC values, alkalinity (≤10.57 mM) and major ionchemistry of groundwater in the Triassic aquifer indicatethat carbonate dissolution (dolostones and limestone) andorganic matter degradation control fluid chemistry. Mostgroundwater (in both the Pliocene and Triassic aquifers)and surface-water samples were oversaturated with calciteand dolomite. Groundwater in the Triassic aquifer hadhigher δ18O and δD values, more similar to surfacewaters, compared to the Pliocene aquifer, which wasdepleted in 18O and 2H. There were also a fewgroundwater samples with detectable tritium in theTriassic aquifer, while all waters sampled from thePliocene aquifer had tritium values near the detectionlimit. Together, with the other solute and isotope chem-istry, this suggests that groundwater in the Triassic aquiferwas more recently recharged (less than 50 years) and incloser communication with surface waters than ground-water in the Pliocene aquifer, and that the two aquiferscontain distinct water bodies. Alternative methods areneeded to obtain more quantitative groundwater ages forthe Velenje Basin.

The study was performed in the framework of a largerstudy called “Sequestration of CO2 in geological media:criteria and approach for site selection as a response toclimate change” in the Velenje Basin and represents thefirst results of a systematic study of the chemical andisotopic composition of surface waters and groundwatersin this area.

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Acknowledgements This study was financially supported by theprojects Z1-2052 and L1-5451 funded by the Slovenian ResearchAgency (ARRS) and the Velenje coalmine D.D. The authors aregrateful to Mr. Igor Medved, Mr. Stojan Žigon and Mr. Robert Lahfor technical support and assistance with field sampling andanalytical help. The authors are grateful to Mrs. Barbara Svetekfor 3H measurements.

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