Journal of Hydrology 486 (2013) 136–150

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Journal of Hydrology

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Sources and impact of sulphate on groundwaters of Triassic carbonateaquifers, Upper Silesia, Poland

Katarzyna Samborska a,⇑, Stanislaw Halas b, Simon H. Bottrell c

a Institute for Ecology of Industrial Areas, 6 Kossutha St., 40-844 Katowice, Polandb Mass Spectrometry Laboratory, Marie Curie-Sklodowska University, 20-031 Lublin, Polandc Earth Surface Sciences Institute, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK

a r t i c l e i n f o

Article history:Received 20 June 2012Received in revised form 4 January 2013Accepted 22 January 2013Available online 31 January 2013This manuscript was handled by LaurentCharlet, Editor-in-Chief, with the assistanceof Nico Goldscheider, Associate Editor

Keywords:Sulphur isotopesOxygen isotopesCarbonate aquiferLinear mixing modelMonte Carlo method

0022-1694/$ - see front matter � 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jhydrol.2013.01.017

⇑ Corresponding author. Tel.: +48 322546031x120;E-mail address: samborska@ietu.katowice.pl (K. Sa

s u m m a r y

Groundwater within the unconfined or semi-confined parts of Triassic carbonate aquifers in Upper Silesia(Poland) contains high concentrations of sulphate (up to 290 mg/L), sometimes in excess of drinkingwater limits (>250 mg/L). To assess the influence of different possible sulphate sources, isotopic analysesof S and O were performed on groundwater sulphate and potential sulphate sources and combined withliterature data. Three dominant sources of sulphate were delineated, based on the geological and litera-ture study and supported by the mixing relations between inverse concentration of sulphate and its iso-topic compositions. These sources are: (i) sulphate from rainfall; (ii) weathering of sulphide minerals inore deposits in the aquifer-forming carbonate rocks; (iii) dissolution of sulphate evaporites in the Triassicsequence. Fortunately these three sources have distinctive S and O isotope compositions and thus theircontributions to the total dissolved sulphate could be estimated. The application of linear mixing modelsfor three sources in the dual isotope system allowed the impact of the three different sulphate sources onparticular parts of the aquifers to be calculated. The average isotopic composition of sulphate inabstracted groundwater indicates that the most important source of sulphate is sulphide weathering,contributing about 50% of total sulphate. The second most significant source of sulphate input is rainfalland it is characterised by a mean contribution of 30%. Application of Monte Carlo analysis that incorpo-rates the full variability in distributions of isotopic compositions for the three sources and all mixing frac-tions between them gave the most probable ranges of the dissolved in groundwater sulphate. Thisanalysis indicated that the proportion of sulphate derived by sulphide oxidation is comparable withthe estimations based on linear models. This study has shown that the water quality of these importantgroundwater resources is under threat from both natural sources, i.e. metal sulphide oxidation and gyp-sum dissolution. Analysis of the mathematical models analysis shows that the first process is the pre-dominant source of sulphate in groundwater. However, the highest concentrations of dissolvedsulphate are positively correlated with the increasing proportion of sulphate derived from gypsumdissolution. Moreover, one should keep in mind that natural processes might be anthropogenically accel-erated due to variable water demands and groundwater abstraction. Eventually, the statistically second-order source of sulphate – rainfall might contain surface-derived contaminants, and its contribution tothe total load of sulphate might indirectly indicate the vulnerability of aquifers for the pollution.

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1. Introduction

Sulphate is a ubiquitous component of the dissolved load of nat-ural waters and has a wide variety of sources, both natural andanthropogenic. The anthropogenic sources of sulphate are oftenassociated with inputs responsible for water quality deterioration,first of all by introduction of sulphates with fertilisers or sewageand indirectly by precipitation of acid rain. Major natural sources

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of sulphate are cyclic marine salts (in coastal areas), weatheringof sulphide minerals and dissolution of sulphate evaporites (Alperset al., 2000). Weathering reactions can naturally supply large fluxesof sulphate to groundwater (e.g. Drever, 1982), both from sulphidemineral oxidation (Moncaster et al., 2000) and evaporites (Gunnet al., 2006). Recent studies have shown that water table draw-down caused by changes in water abstraction can accelerate sul-phide oxidation reactions. Subsurface conditions become moreoxidizing during intense abstraction of water as shallow water isdrawn into deeper flow paths and ore sulphide dispersed withinthe extended vadose zone is exposed to air (e.g. Andersen et al.,2001). Furthermore, fluctuations of water table level within rocks

K. Samborska et al. / Journal of Hydrology 486 (2013) 136–150 137

containing partly or totally oxidized sulphidic minerals causeseither rewetting and release of sulphate, metal ions and acidity,or drying and precipitation of ephemeral highly soluble sulphateprecipitates. Indeed, in aquifers containing sulphide minerals thevery palpable effects of changes in groundwater abstraction arehigh concentrations of sulphate (Klimas and Gregorauskas, 2002;Bottrell et al., 2008; Samborska and Halas, 2010).

Sulphur is one of the key elements that undergo redox cyclingin earth surface environments and sulphur cycling is intimatelylinked with redox cycles of oxygen, carbon, nitrogen and iron (Bo-lin and Cook, 1983; Turchyn and Schrag, 2006). The key processcontrolling global sulphur isotope distribution between differentenvironmental and geological reservoirs of sulphur is microbialsulphate reduction in marine sediments. This process leads to theproduction of 34S-depleted sedimentary sulphide minerals (pre-dominantly pyrite) and concomitantly enriches the reservoirs ofocean water sulphate and evaporite sulphate in 34S (Claypoolet al., 1980; Bottrell and Newton, 2006). In turn, the oxidative partof the sulphur cycle involves little S isotope fractionation but con-trols the oxygen isotopic composition of sulphate formed duringweathering (Claypool et al., 1980; Krouse and Mayer, 2000; Bottrelland Newton, 2006). This leads to potentially distinctive sulphurand oxygen isotopic compositions associated with different sul-phate sources and thus values of d34S and d18O of sulphate canbe used as a ‘‘fingerprint’’ of different environmental sulphatesources. Hence, the assessment of human and natural inputs onsulphate concentrations may be achieved using these stable iso-tope measurements (Krouse and Grinenko, 1991; Hitchon andKrouse, 1972; Robinson and Bottrell, 1997; Moncaster et al.,2000; Bottrell et al., 2008).

The Upper Silesia region is the area of Poland most heavilyimpacted by human activity as a result of: the largest productionof hazardous waste; production of industrial and municipal sew-age wastes; and the largest emission of dust and gaseous pollu-tants. This densely populated and the most industrialized area(Witkowski et al., 2003) forms part of the ‘‘Black Triangle’’ ofheavy 20th century industrialization in Central Europe. Water re-sources in this region have also been adversely affected by hu-man impact, with resources depleted by intense waterabstraction and mining dewatering, as well as direct entry ofcontaminants to aquifers causing water quality degradation(Kowalczyk et al., 2010).

Previously investigation of sulphate sources has been con-ducted with the confined part of the Olkusz–Zawiercie carbonateaquifer (Samborska and Halas, 2010), where only natural sourcesof sulphate are present and human impact (abstraction, minedewatering) only triggers changes in the natural processes con-trolling the sulphate distribution in groundwater. This paper pre-sents and analyzes sulphate isotopic composition and sulphatesources in groundwater within unconfined part of the carbonateaquifers that are vulnerable to direct influx of surface-derived sol-utes. The samples analyzed were taken from groundwater abstrac-tion wells situated within three Triassic carbonate aquifers(Olkusz–Zawiercie, Gliwice and Chrzanow). This work aims to esti-mate the contribution of possible sulphate sources to the totalload of sulphate dissolved in groundwater in the recharge areasof the Triassic carbonate formation. To achieve this goal, sulphateisotopic data were compiled in the major sulphate sources identi-fied above:

(1) Both archival and a novel data on the isotopic composition ofsulphide from the Upper Silesian lead–zinc ores. Authorsused 175 measurements of d34S in sulphide ore, it includediron, zinc and lead sulphide. Among them 169 results werearchival (measurements taken in the ‘70s) and 6 determina-tions carried out in 2008 as part of this study.

(2) Literature data on the isotopic composition of evaporitesoccurring in the Triassic and Miocene strata, authors usedglobal results; 114 d34S measurements and 69 of d18O inmarine evaporites from Triassic and Miocene strata.

(3) Sulphate in rainfall waters – authors compiled the availableliterature data regarding the isotopic composition of sul-phate in rainwater, it encompasses 54 measurements ofd34S and 48 measurements of d18O, the samples were col-lected in Baton Rouge, Louisiana (Jenkins, 2005) southernGermany (Mayer et al., 1995) and south-western Poland(Jedrysek, 2000). In each of these sites, the sea-salt sulphatecontribution into the rainfall sulphate is negligible or none.However, this area is impacted in a significant degree byhuman activities, and thus sulphate entering to the aquifersfrom precipitation will additionally contain the anthropo-genic contribution to the sulphate load occurring in ground-water. Fortunately, the average isotopic composition ofsulphate in precipitation on the urban areas is similar tothe mean isotopic composition of the sulphate containedin manure, fertilisers or sewage (Moncaster et al., 2000;Bottrell et al., 2008). For instance, the mean values of d34Sfor chicken, pig and cow manures estimated for Susquehan-na River Basin, Pennsylvania study case are 3.35, 3.69 and4.21, respectively. In turn, mean value of d34S for fertilisersin Pennsylvania is 8.22 and sewage sludge is 2.47 (CravottaIII, 1997). The fertilisers examined in England are character-ized by the mean values of d34S equal 5.4 and 0.4 dependingon the type of technology used for the production (Moncast-er et al., 2000).

The attempt to estimate the contribution of three sources (ore,evaporites and rainfall) in a total load of dissolved in groundwatersulphate on the vulnerable, unconfined parts of the Triassic, car-bonate formation has been solved using: a simple linear threesources dual isotope mixing model (referred later as ‘‘first mod-el’’), the mixing model (proposed by EPA http://www.epa.gov/wed/pages/models.htm) which allows the calculation of confi-dence intervals for the proportion of each source taking into ac-count the variability of the isotopic composition of each sources(referred later as ‘‘second model’’) and the Monte Carlo (MC)method. The latter allows comparison of the observed values ofisotopic composition in sulphate with those artificially generatedbased on the collected data for sources according to their statisti-cal distributions and taking into account all possible mixing pro-portions. These three methods (simple linear mixing model,linear mixing model taking into account the variability of sources;MC method) represent different approaches to estimation of thefraction of the three sources in the total load of sulphate ingroundwater. Especially interesting is the assessment the amountof sulphate derived from infiltration of rainwater which can deter-mine the vulnerability of the aquifers for the surface-originatedpollution.

2. Geology and hydrogeology

Five major Triassic carbonate aquifers have been delineatedwithin the Upper Silesia region, occupying an area of about4000 km2. The area studied here is located within the Olkusz–Zaw-iercie, the Gliwice and the Chrzanow aquifers. The first is one of thelargest in the area (1033 km2: Ró _zkowski, 1997) and the mostimportant with respect to the quantity of water resources(561,600 m3/d: Witkowski, 2001). The Gliwice aquifer is the thirdmost important in terms of water resources (Kowalczyk, 2003),whilst the Chrzanow aquifer is the smallest among the five Triassicaquifers, it covers ca. 273 km2 and its resources are estimated to be

138 K. Samborska et al. / Journal of Hydrology 486 (2013) 136–150

slightly over 100,000 m3/d (Kowalczyk, 2003). Groundwater is avery important source of drinking and industrial water in theUpper Silesia region, meeting 27% of water demands in 2008 (Sta-tistical Yearbook of Slaskie Voivodship, 2009). However, thissource is often overexploited and degraded by human activities.Water quality deterioration, among others, high concentrationsof sulphate have been observed in each of the studied aquifers(Samborska, 2009). However, in this area there are sulphide ores(mainly in Triassic carbonate rocks) and gypsum layers among Tri-assic and Miocene strata, which represent potent sources of sul-phate to the groundwater. Indeed, groundwater quality may bechanged and even totally degraded by dissolution of these miner-als. In this area, some groundwater abstractions have been aban-doned due to drastic increases in sulphate concentration andunderstanding the origin of elevated sulphate concentrations canpractically inform groundwater management strategies and thushelp limit further deterioration.

The studied carbonate aquifers are a part of the Silesian–CracowMonocline, they consist of Middle and Lower Triassic (Muschelkalkand Roethian) dolomites, limestones and marls. The aquifers arecharacterised by a triple porosity system, i.e. groundwater residesin pore space in the rock matrix, in fractures and in solutionally-enlarged karstic conduits (Motyka, 1998). All three aquifers arealso crosscut by numerous faults.

The maximum thickness of the carbonate complex within theseaquifers is over 200 m, while minimum is around 12 m. The Olk-usz–Zawiercie aquifer is underlain by impermeable deposits con-sisting of Lower Triassic, Carboniferous and Permian marls, clays,mudstones and siltstones. The Gliwice aquifer is almost entirelyunderlain by impermeable Carboniferous deposits, except in thenorth-eastern part which is underlain by Permian deposits(Ró _zkowski and Wilk, 1980). In turn, the Chrzanow aquifer isunderlain by Permian and Carboniferous deposits (Witkowskiet al., 2003).

The Olkusz–Zawiercie aquifer is semi-confined. Direct rechargetakes place in the western part of the aquifer via outcrops, howeverinfiltration of precipitation also occurs through ‘‘hydrogeologicalwindows’’. The aquifer is covered with largely impermeable UpperTriassic and Lower Jurassic deposits (Fig. 1) but, in the eastern part,overlying Upper Jurassic and the shallow Quaternary aquifers arein hydraulic continuity with the Olkusz–Zawiercie aquifer, so theinfiltration through them comprises the additional indirect re-charge. The Gliwice aquifer is also semi-confined. The unconfined,directly recharged area is situated in the north-eastern and in themiddle part of the aquifer. The confined part is covered by imper-meable Neogene deposits. Hence, recharge occurs on the outcropsof the water-bearing complex or through permeable Quaternarydeposits. Since, on the outcrops the river-beds are not isolatedfrom the aquifer, rivers constitute an additional source of recharg-ing water (Sitek et al., 2009).

Hydraulic conductivity within the Olkusz–Zawiercie aquifer isvery variable. The conductivity of the porous aquifer matrix is inthe range of 3.8 � 10�10–2.6 � 10�8 m/s (Motyka, 1988), whereasthe bulk hydraulic conductivity of fractured rocks varies from2.4 � 10�6 to 3.2 � 10�2 m/s (Wilk et al., 1984; Motyka et al.,1994). The hydraulic conductivity within the Gliwice aquifer is inthe range of 1.2 � 10�5–2.3 � 10�4 m/s (Włostowski et al., 2005).Groundwater flows and water balance within the Olkusz–Zawier-cie aquifer have been influenced by long-term abstraction associ-ated with both public supply and dewatering of lead–zinc mines.Originally, water from the Olkusz–Zawiercie aquifer was dis-charged into rivers. Currently, the northern part is drained mainlyby rivers, whereas in the southern part groundwater flow is forcedby the anthropogenic drainage, i.e. dewatering of mines andgroundwater abstraction by public supply boreholes. In this areathe maximum water-table drawdown exceeds 100 m, and the cone

of depression caused by mining activities encompasses an area ofabout 700 km2 (Fig. 2) (Wilk and Bochenska, 2003).

The hydogeological regime within the Gliwice aquifer is shapedby the following factors: the landform features, the waterwaynetwork and the water abstraction. The Gliwice aquifer is also sub-divided into two zones. In the eastern zone, the Triassic water-bearing complex is open to surface infiltration or is covered byshallow Quaternary aquifer sediments. The western part of theaquifer is overlain by the impermeable deposits of Miocene andthe Pleistocene aquifer. Regional groundwater flow is from thenorth-east to the south-west, the form of the hydrodynamic flowfield is the result of the large difference in water table altitude be-tween the outcrop areas (240–250 m a.s.l.), compared to 138–146 m a.s.l in the most heavily abstracted part of the aquifer, closeto the Gliwice-Łabedy abstraction (Ró _zkowski, 1997).

The Chrzanow aquifer occurs in the Lower Triassic (Roethian)and Middle Triassic (Muschelkalk) sediments. The aquifer is builtof dolomites, limestones and marl interbeddings. The thicknessof these sediments varies from 20 to 150 m, locally exceeding200 m (Witkowski et al., 2003). This aquifer is also semi-confined,the covering deposits are Upper Triassic, Jurasic and Tertiary age.Their total thickness reaches 250 m and they cover an area of about150 km2. Beyond this area, the aquifer sediments crop out at sur-face or they are covered by permeable Quaternary sediments.The hydrodynamic field within this aquifer has been determinedby mining drainage of water in the excavations of the ‘‘Trzebionka’’mine and by water abstraction wells for several tens of years. Thetotal abstraction of water reached 73,000 m3/d in 2002, and almost60% of this amount was connected to mine drainage (Witkowskiet al., 2003). The main source of the recharge is infiltration of rain-water on its outcrops or through the permeable Quaternarysediments.

The Upper Silesian lead–zinc ores occur in the Precambrian,Silurian, Devonian, Carboniferous, Permian, Triassic and Jurassicdeposits (Serafin-Radlicz, 1972). However, the most sulphide-richores are found in the Middle Triassic deposits (in the ore-bearingdolomites and in the Diplopora dolomites) and in Lower TriassicRoethian carbonates. The thickness of the layers containing lead–zinc mineralization varies from several tens of centimetres to sev-eral tens of meters, in places where the sulphide ores occur in thePaleozoic basement the vertical extent of mineralization increasesto a few hundred meters (Górecka, 1993). The ore bodies are fault-associated, especially along the Kraków-Lubliniec fault zone(Fig. 1).

Gypsum layers occur among the Lower Triassic (Roethian)deposits, ranging up to 4.5 m thick (Wyczółkowski, 1978). The lay-ers of gypsum and anhydrites were found also in the Upper Triassicdeposits, in the Lower Gypsum Keuper strata (Grodzicka-Szyman-ko, 1978) and in the Miocene deposits, where their vertical extentis up to 50 m, however the most often the gypsum beds thicknessis in a range of a several meters (Alexandrowicz, 1997).

3. Methods

3.1. Chemical and isotopic analyses

For determination of the sulphate sources on the recharge areasof the Upper Silesian Triassic carbonate formation the archivalchemical and isotopic analyses of groundwater performed as a partof the investigations concerning on the vulnerability of the Triassiccarbonate aquifers (Witkowski, 2001), on the establishing of theGliwice aquifer protection zone (Włostowski et al., 2005) and ontracking of the sulphate sources in the Chrzanow aquifer (Sam-borska, 2011) were used. To allow estimation of the sulphate in-puts from different sources, additional chemical and isotopic

Fig. 1. Geological map (after Kotlicka and Kotlicki (1979) and Kaziuk and Lewandowski (1980)), with the localisation of sampling points within two Triassic carbonateaquifers and the occurrence of lead–zinc ores (after Górecka (1993) and information distributed by the Polish Geological Institute).

K. Samborska et al. / Journal of Hydrology 486 (2013) 136–150 139

analyses were carried out. These analyses were performed in theFaculty of Earth Sciences laboratories at the University of Silesiaand in the Central Laboratory of the Institute for Ecology of Indus-trial Areas in Katowice.

The analyses were nearly complete and encompassed determi-nations of: Ca, Mg, Na, K, NH4, Cl, SO4, HCO3, NO3, Fe, Mn, Zn(Appendix 1). The analyses undertaken by the Faculty of Earth Sci-ences laboratories were performed on filtered water (0.45 lm)samples of 100 mL volume. Unacidified water was used to deter-

mine major ions and N-containing ions by ion-exchange chroma-tography and those fixed with 1 mL HNO3 were used todetermine Fe, Mn, Zn by means of Atomic Absorption Spectroscopy(AAS). The additional chemical analyses were also carried out bythe Central Laboratory of the Institute for Ecology of IndustrialAreas, samples were filtered prior analyzing, anions were exam-ined using ion-exchange chromatography, whilst concentrationsof cations were measured using ICP-OES atomic emission spectrom-eter. The QA/QC requirements included the examinations of

Fig. 2. Groundwater table contour map of Triassic carbonate aquifers, the Gliwice aquifer after Sitek (2008), the Olkusz–Zawiercie aquifer after Samborska (2009) and theChrzanow aquifer after Witkowski et al. (2003).

Fig. 3. The histogram of the d34S values distribution in the sulphide from the UpperSilesian and Cracovian lead–zinc ores.

140 K. Samborska et al. / Journal of Hydrology 486 (2013) 136–150

duplicate and blank samples. Generally, chemical analyses are char-acterized by an error within the ionic balance of ±5% (Appendix 1).During fieldwork, samples for isotopic analysis were taken in plasticcontainers (1.5 L). Aqueous sulphate was recovered for isotopic anal-ysis by precipitation of insoluble barium sulphate. In the laboratory,water samples were acidified to pH� 2 and heated in water bath upto a temperature of �60 �C and then 10% BaCl2 solution was addedand left until BaSO4 precipitated. The precipitate was rinsed withdeionized water to remove undesirable products (chlorides) anddried. BaSO4 samples were decomposed in vacuum into sulphurdioxide for determining d34S(SO4) or into carbon dioxide for deter-mining d18O(SO4) according to the methods described by Halasand Szaran (2001) and Halas et al. (2007). The determinations ofd34S and d18O were carried out on dual inlet and triple collector massspectrometer in the Mass Spectrometry Laboratory at UMCS, Lublin.Analytical precision of both determinations was ±0.08‰. The resultswere reported in terms of delta values – d34S with respect to theVCDT standard, whereas d18O(SO4) with respect to the VSMOWstandard. Samples of sulphide (from core samples) destined for theisotopic examinations were separated from the rocks under a binoc-ular microscope. The sulphides were oxidized in the presence ofCu2O at 1000 �C to SO2 (Robinson and Kusakabe, 1975) and analyzedas described above.

3.2. Methods for the assessment of sulphate origin

3.2.1. The assessment of the average isotopic composition for threesources

The determination of the potential sources involves the estima-tion of the mean isotopic signature for those components. The

archival (Haranczyk and Lis, 1973a,b; Haranczyk, 1978, 1993)and the new data (six samples) of isotopic composition of sulphidefrom the most profitable ores were used to estimate the mean val-ues of d34S for the ore source, (Fig. 3). The mean value of d34S forthe set of data containing 175 numbers is �4‰.

In turn, the estimation of the mean values for d18O in the sul-phate ion which are produced during the oxidation of sulphideore is quite problematic. Recent studies have re-evaluated boththe mechanism of pyrite oxidation (Druschel and Borda, 2006;Pisapia et al., 2007; Mazumdar et al., 2008; Hubbard et al., 2009;Tichomirowa and Junghans, 2009) and values of oxygen isotopeenrichment factors between water and sulphate – eSO4–H2O andoxygen and sulphate – eSO4–O2 (Balci et al., 2007; Mazumdar

Fig. 4. The histograms of the d34S and d18O values distribution in the evaporitesfrom the Triassic and Mesozoic deposits.

K. Samborska et al. / Journal of Hydrology 486 (2013) 136–150 141

et al., 2008; Tichomirowa and Junghans, 2009; Heidel and Ticho-mirowa, 2010, 2011). These studies have also elucidated the frac-tions of molecular oxygen and oxygen derived from the moleculeof water participating in oxidation of sulphide (Balci et al., 2007;Tichomirowa and Junghans, 2009; Heidel et al., 2009; Heidel andTichomirowa, 2010). Regardless of pyrite oxidation path it hasbeen demonstrated that as the process proceeds, d18O(SO4) valuesbecome more similar to the value of d18O(H2O) (Rosso and Vaughan,2006). Thus, it may be inferred that even under aerobic conditionsmost of the oxygen atoms incorporated into sulphate moleculesare derived from water. Indeed, Bottrell et al. (2008) found a rangeof negative d18O values for sulphate produced by oxidation of pyr-ite as a result of aquifer dewatering.

The oxidation of sulphide in the karstic groundwater environ-ment may take place in the presence of atmospheric oxygen duringthe exposure of the rock containing mineralization caused by thewater table level decrease. Accordingly, the oxidation of sulphidemay be continued below the water table level in the presence ofdifferent electron acceptors such as trivalent iron (Stumm andMorgan, 1996), nitrates (Jorgensen et al., 2009) and manganese(Schippers and Jørgensen, 2001).

In the literature one may find a number of models for interpret-ing oxygen isotopes formed during oxidation of pyrite. The ’’gen-eral isotope balance model’’ (Taylor and Wheeler, 1994) ispresented below (Eq. (1)), where x is the fraction of sulphate de-rived from oxidation of pyrite by ferric iron, whereas (1 � x) isthe fraction derived from reaction with molecular oxygen andeSO4–O2 and eSO4–H2O are enrichment factors for these reactions(Taylor et al., 1984; van Everdingen and Krouse, 1985).

d18OðSO4Þ ¼ xðd18OH2O þ eSO4—H2OÞ þ ð1� xÞðd18OO2 þ eSO4—O2Þ ð1Þ

The general isotope balance model is currently widely applied(Haubrich and Tichomirowa, 2002; Knoller et al., 2004; Sraceket al., 2004; Migaszewski et al., 2008).

In order to solve the linear mixing model, recently obtained exper-imental data have been adapted. First linear model requires fixed,average value for d18O in sulphate produced through the oxidationof sulphide. Thus, the ‘‘general isotope balance model’’ (Eq. (1)) hasbeen solved using following data: d18O2 = 23.5‰ (Kroopnick andCraig, 1972), d18O(H2O) =�10‰ (it assumed that groundwater on theoutcrops is Holocene in age, comparable values have been found with-in the studied Triassic formation (Samborska et al., 2012)), eSO4-O2 –�8.4‰ (Heidel and Tichomirowa, 2010), eSO4-H2O – 2.3‰ (Heideland Tichomirowa, 2011) and the proportion of molecular oxygen inoxidation – 11.1% (Heidel and Tichomirowa, 2010).

Hence the general isotope balance model for this case might berewritten as:

d18OðSO4Þ ¼ 0:889� ð�10þ 2:3Þ þ 0:111� ð23:5� 8:4Þ ð2Þ

Eventually, the resulting value of �5.17‰ has been used as a meanfor d18O(SO4) in sulphate derived from the oxidation of sulphide.

Furthermore, the mean signatures of the sulphur and oxygenisotopes in evaporites from Triassic and Miocene deposits wereestimated based on the literature data, these samples were col-lected in north-eastern Mexico (Kesler and Jones, 1981), easternGreenland (Clemmensen et al., 1984), Netherlands, Germany andPoland (Kovalevych et al., 2002), Spain (Alonso-Azcarate et al.,2006) Ukraine (Hryniv et al., 2007), Italy (Longinelli and Flora,2007) and Slovakia (Kasprzyk and Bukowski, 2009). The distribu-tion frequency of evaporite isotopic compositions is presented onthe histograms (Fig. 4), the mean values calculated for the Triassicand Mesozoic evaporites are: 17.65‰ for d34S (114 measurements)and 14.93‰ for d18O (69 measurements).

The third source of sulphate for which the estimation of itsmean isotopic compositions was possible is sulphate entering the

aquifers as recharge from precipitation. This will be comprised oftwo components – a ubiquitous component derived from sulphatein the precipitation and a more variable component derived fromother sources in the groundwater catchment. For this study, the re-sults of investigations conducted into sulphate in precipitationworldwide away from coastal effects were evaluated (Jedrysek,2000; Jenkins, 2005; Mayer et al., 1995). Frequency distribution ofthe d34S and d18O values for sulphate in precipitation is presentedin Fig. 5, the averages for these stable isotope measurements are2.17‰ (54 measurements) and 14.1‰ (48 measurements) respec-tively. This estimate for the ubiquitous sulphate component in therecharge is used for the total sulphate in recharge because isotopiccompositions of sulphate from the likely additional sources aresimilar to these values. In rural areas, isotopic compositions of sul-phate in fertiliser, the major additional sulphate source, are similarto these rainfall values. For example, Bartlett et al. (2010) found an-nual average rainfall sulphate of +4.7‰ and +11.4‰ (d34S andd18O respectively) while fertilizers ranged from +3‰ to +10‰ ford34S and 7‰ to 12‰ for d18O. Moncaster et al. (2000) similarly foundclosely similar sulphur isotopic compositions for sulphate fromfertiliser and rainfall sources. Even in a complex urban rechargeenvironment, Bottrell et al. (2008) found that nearly all other sul-phate sources, including sewage, lay close to the rainfall isotopic val-ues used here; only sulphide oxidation due to groundwaterdrawdown and evaporite sources were distinctive, just as found inthis study.

The three possible sources of sulphate identified thus have dis-tinctively different isotopic ‘‘signatures’’: sulfide oxidation givescharacteristically low d18O and d34S compared to the other sources;recharge waters and evaporite dissolution contribute sulphatewith similar d18O (�15‰) but distinctly different d34S (+3‰ and+18‰, respectively).

Fig. 5. The histograms of the d34S and d18O values distribution in the precipitation.

142 K. Samborska et al. / Journal of Hydrology 486 (2013) 136–150

3.3. The assessment of the contribution of the individual sources (of thethree considered) in dissolved sulphate

3.3.1. Linear modelsStable isotope mixing models have become commonly applied

tools in the environmental sciences. Their use allows determina-tion of the exact contribution of sources to a mixture by taking intoaccount their isotopic signatures. Their application is described innumerous publications (e.g. Schwarcz, 1991; Phillips, 2001; Phil-lips and Gregg, 2003; Moore and Semmens, 2008) and computerprograms such as IsoSource, MixSIR and SIAR.

Three end-member compositions are used to interpret the iso-topic measurements of sulphate dissolved in groundwater of thecarbonate aquifers. The assessment of each sulphate sources con-tribution to the total amount of sulphate dissolved in groundwaterrequires the solution of a three-sources, two-isotopes mixing mod-el (the first model). As described, these sources are sulphides, evap-orites and sulphate delivered from precipitation/recharge and eachsource is characterized by its own range and distribution of bothd34S and d18O values.

The three end-member model has been applied, among others,to determine the components of wolf diet using d13C and d15Nmeasurements (Phillips and Gregg, 2001) and the contributionsof nitrogen and organic carbon from anthropogenic wastes, soilsand bedrock-derived sediments in the Danshuei River (Liu andKao, 2007).

The system of equations in linear mixing model is written in thefollowing form (Phillips and Gregg, 2001; Phillips and Koch, 2002):

dT ¼ fAdA þ fBdB þ fCdC

kT ¼ fAkA þ fBkB þ fCkC

1 ¼ fA þ fB þ fC

8><>:

ð3Þ

where fA, fB and fC are sulphur fractions from the sources A, B and Cin the mixture, dA, dB and dC are the mean delta values of sulphurfrom the respective sources A, B and C, kA, kB and kC are the mean

delta values of oxygen from the respective sulphate sources, dT

and kT are resulting delta values in the mixture.Furthermore, the three sources mixing model developed by EPA

has been applied (http://www.epa.gov/wed/pages/models.htm),the second model allows the calculation of confidence intervalsfor the proportion of each source taking into account the variabilityof the isotopic composition of each sources. This model was de-scribed in detail by Phillips and Gregg (2001).

3.3.2. Monte Carlo methodThe Monte Carlo (MC) method has been widely applied in many

fields of research, including geochemical investigations (Anderson,1976; Cabaniss, 1999; Lofts and Tipping, 2011; Zhu et al., 2003).The MC method utilizes ‘‘repeated calculation of a quantity, eachtime varying the input data randomly within their stated limitsof precision’’ (defined in terms of application to error propagationanalysis by Anderson (1976)). The distribution of the calculatedquantity then illustrates the effect of the imprecision in the inputdata. In environmental studies the MC method is used to producesynthetic datasets against which real data can be compared in or-der to constrain the effect of different variables in complex systems(e.g. application to understanding the effects of different parame-ters on the assessment of contaminant biodegradation; Thorntonet al., 2001). A wider definition (still disputed) may be formulatedas the approximation of the integrals by invoking laws of largenumbers (Anderson, 1999).

Next the MC method has been used for comparing the observedresults of isotopic compositions and randomly generated ones.These comparisons allow for representation of the complex effectsof combined variability in the isotopic composition of the sulphateend-members and also allow us to test whether or not the ob-served compositions follow predicted probability distributions,which may allow identification of systematic bias or non-randombehaviour. In addition, performing of different computational sce-narios reflecting the increasing contribution of sulphate producedthrough oxidation of sulphide allowed more reliable approxima-tion of the fraction of this source recognized as the most importantwithin the confined part of the Triassic formation.

In a first step, using Visual Basic, a macro for generation of theisotopic signatures of sulphate sources was built. This step waspreceded by the construction of histograms representing statisticaldistribution of input data for each source. Hence, the generation ofrandom isotopic composition for sources was done on the basis oftheir probability distributions.

Values of d34S in precipitation were computed using asymmet-ric probability distribution (negative skew) with the minimum,maximum and mode values of �3.9, 6.02 and 4.83, respectively.Furthermore, values of d18O in precipitation and d34S in evaporiteswere computed using normal distribution function. The parame-ters required for this computation were median, standard devia-tion, minimum and maximum. In case of distribution of d18O inprecipitation, it is characterised by median value of 14.08, standarddeviation of 2.65, minimum and maximum values of 8.4 of 19.65,respectively. In turn, the parameters needed for computing of nor-mal distributed data of d34S in evaporites are: 16.85 – median, 5.69– standard deviation, �0.2 – minimum and 38.3 – maximum. Thelast two data; d34S in sulphide and d18O in evaporites are bimodallydistributed. Hence, the calculation of random values for these del-tas was performed using bootstrap function (Chernick, 1999). Theboostrap method is a resampling technique, invented by BradleyEfron (1979), and can numerically simulate results that empiricallywould be impossible to get. Basically, the one available sample isreused to give rise to many others by replication with a replace-ment. In addition the oxygen isotope composition produced duringoxidation of sulphide has been characterized by solving the ‘‘gen-eral isotope balance model’’ assuming a larger spectrum of values

K. Samborska et al. / Journal of Hydrology 486 (2013) 136–150 143

compared to the linear mixing model. For instance, the proportionof molecular oxygen incorporated into the sulphate molecule var-ies from 0% to 30% (Kohl and Bao, 2011, in accordance with otherworks: Mazumdar et al., 2008; Heidel and Tichomirowa, 2010),eSO4–O2 varies from �4.3‰ to �11.4‰ (Balci et al., 2007, in accor-dance with other works: Lloyd, 1967; Oba and Poulson, 2009; Hei-del and Tichomirowa, 2010) and eSO4–H2O varies from 0‰ to 4.1‰

(Balci et al., 2007, in accordance with other works: Mazumdaret al., 2008; Heidel and Tichomirowa, 2010, 2011). Hence, the finalrange for d18O(SO4) is of �10‰ to 1.36‰. Due to the fact that ob-served values of d18O in sulphate formed during the oxidation ofpyrite are skewed towards negative values, the MC calculationsof d18O(SO4) values have been performed assuming that randomlygenerated data should be characterized by the right-skewed distri-bution within the range of �10 to 1.36 and a mode value of �5.16.

From this a first scenario of MC calculations was performed, thisstep includes the generation of artificial data for sulphate sourcesaccording to their statistical distribution, next, a second macrogenerates all possible combinations of three sources (in steps of1%) in a mixture. By multiplying proportions and isotopic signa-tures, the isotopic compositions of sulphate were calculated. Forthe computations, the number of randomly generated isotopic sig-natures for sources, both values d34S and d18O, was 500, the num-ber of all possible combinations is 5151, according to equation:

N:comb ¼ ðð100=iÞ þ ðs� 1ÞÞ=ðð100=iÞðs� 1ÞÞ ð4Þ

where N.comb is number of all combinations, i is increment ofchanges [1%], s is number of sources [3].

Accordingly, 2,575,500 values of d34S and d18O for dissolved ingroundwater sulphate were calculated. For this very large data-set, Python code (version 2.7 – van Rossum and Drake, 2011), to-gether with Matplotlib code (version 1.1 – Dale et al., 2011) wereused to display the results.

Due to the disparity between the observed data-set and the cal-culated most probable values, additional MC scenarios have beencarried out to match those most expected values with the realones. Since sulphide oxidation may constitute the most importantsource of sulphate within the studied aquifers, those scenarios rep-resent the increasing influence of this source on sulphate concen-trations. It has been assumed that the contribution of sulphideoxidation is increasing at 25% of each following scenario, i.e. firstassumed that the contribution of sulphide oxidation is not smallerthan 25%, second that formed sulphate is at least in 50% of sulphideoxidation origin and the last scenario assumed that in at least 75%sulphate is derived from weathering of sulphide ore. Eventually, inthe last scenario, the most probable fraction of weathered sulphidein the total amount of dissolved sulphate has been given.

4. Results and discussion

4.1. Sulphate sources

Groundwater sulphate concentrations in the area of study varybetween 55 mg/L to almost 290 mg/L (this concentration exceedsthe drinking water limit of 250 mgSO4/L) (Appendix 1). The proba-bility distribution histograms for raw data of d34S and d18O in pos-sible sources are presented in Figs. 3–5.

4.2. Groundwater sulphate

Figs. 6 and 7 show the relation between the values of d34S andd18O in dissolved sulphate versus inverse of sulphate concentra-tion. Both plots show sample points shaping of the triangle, thismay indicate the three end-member mixing of sulphate sourceson the recharge areas of the Triassic carbonate formation. The

simultaneous application of isotopic and chemical data for theidentification of three end-member mixing was used to trace thisprocess occurring between river and groundwater containing ura-nium (Osmond and Cowart, 2000) and groundwater of differentages and the salinity (Douglas et al., 2000).

The first of the sulphate sources is characterised by high of d34Sand d18O values, this source may be also the reason for the highconcentrations sulphate in groundwater, over 200 mg/L (point 5in Fig. 6). The second one is distinguished by the relatively low val-ues of both isotopes; sulphur and oxygen, this source is responsiblefor the concentrations of sulphate close to 100 mg/L (points 4, 10 inFig. 6). Accordingly, the isotopic composition of sulphate derivedfrom the third source is characterized by medium d34S valuesand high d18O values, whereas the concentrations of sulphatesoriginated from this source are relatively low, approximately50 mg/L (point 12 in Fig. 6).

The mean d34S and d18O values of the possible sources of sul-phate along with isotopic composition of groundwater sulphatesare displayed in Fig. 8. The groundwater samples are mainly dis-persed in the triangle, whose vertexes correspond to the mean iso-topic signatures of the three sources. However, a few samples suchas: 10, 7, 11, 4 are situated directly on the sides of the triangle,which suggests that in those sampling points (wells) the fractionof sulphate derived from one of the sources may be negligible.

Table 1 presents the results from the first, simple linear modelcomprising the system of linear equations (Eq. (3)) and taking intoaccount only the average isotopic composition for sources. Calcula-tions were carried out for each sample, in a few cases the fractionof sulphate originating from one source is negative; in those casesconcentrations of sulphate are the result rather of two-end mem-ber mixing. Thus, for those samples, this model has been applied.The overall results suggests that sulphate concentrations in the re-charge areas of the Triassic carbonate formation may be controlledby the weathering of sulphide, in half of samples this source isresponsible for more than half of the dissolved sulphate. In aquiferoutcrop areas infiltration of recharge waters may contribute signif-icant sulphate from rainfall to the total sulphate load. It has beencalculated that around 30% of sulphate may be from this surficialsource. The third source is probably gypsum, its average contribu-tion to the total amount of sulphate varies around 18%.

Several patterns can be observed in the results of the first mod-el. First of all, a few groundwater samples were taken from wellssituated within the zinc-lead orefield, i.e. samples no: 6, 11, 16and 19. For these samples the proportion of sulphide-originatedsulphate is quite high: 51.7%, 71.2%, 71%, and 36.2%, respectively.Twelve samples were taken from wells situated within. 2 km ofthe orefield, and they are also characterised by a high contributionof sulphate originating from sulphide oxidation, in majority ofthem this proportion is higher than 40% (Appendix 2). Moreover,the relation between the contribution of the sulphate derived fromthe rainfall and the top screened interval of wells has shown that ina few the shallowest wells, i.e. 17, 9 and 10 the contribution of sul-phate from rainfall is quite high (50.9%, 42.2% and 42.6%, respec-tively) and generally greater than the average contribution thatresulted in the first model (Appendix 3). Furthermore, the solutionof the first, simple model yields the positive correlation betweenconcentration of sulphate dissolved in groundwater and the contri-bution of gypsum as the source of these ions. It means that thisnatural source might be the potential threat for the water qualityeven on the recharge areas, however there are only four samples(7, 9, 10 and 17) that have both high dissolved sulphate (greaterthan 200 mg/L) and a large proportion of gypsum source in the to-tal load of sulphate.

Fig. 9 shows the variability of the isotopic composition (d34S) ofsulphide from the Upper Silesian and Cracovian lead zinc ores andthe range of d18O values for oxygen included in sulphate ions

Fig. 6. d34S(SO4) versus inverse concentrations of sulphate dissolved in groundwater.

Fig. 7. d18O(SO4) versus inverse concentrations of sulphate dissolved in groundwater.

144 K. Samborska et al. / Journal of Hydrology 486 (2013) 136–150

formed during the weathering of sulphide. Furthermore, the simi-lar set of values for the gypsum and sulphate derived from precip-itation displayed on the plot. In the middle part of the Fig. 9 theminimum and maximum values of both isotopic signatures (d34Sand d18O) encountered in studied sulphates along with the meanvalue for those stable isotopes are presented. The plot shows thateach source of sulphate and dissolved ion itself are characterisedby the considerable variability of the isotopic signatures. The sec-ond model proposed by Phillips and Gregg (2001) takes into con-sideration both the influence of the variability of the isotopicsignatures of sources and set of samples representing the mixtureon the estimation of source proportions. Hence, the model was ap-plied to estimate the average, lower and upper confidence intervalfor the fraction of each source of sulphate ion in groundwater. Thiscalculation was performed for the mean isotopic composition ofsulphate dissolved in groundwater on the recharge areas of the Tri-assic carbonate formation. This showed that, on average, sulphideweathering is the most important source of sulphate, contributingbetween 40% and 54% of the total amount of SO2�

4 ions. The second

most important source of dissolved sulphate is precipitation/re-charge water (19–48% of total sulphate with the mean input of34%). The third source, sulphate from dissolution of gypsum con-tributes from 8% to 30% of the total concentration on the rechargeareas of the carbonate aquifers, on the average it is about of 19% ofthe whole dissolved sulphate (Table 2). Generally results of bothlinear models are in a very good agreement, and they demonstratethat recharge water may comprise significant source of sulphate onrecharge areas of the Triassic carbonate aquifers.

Finally, the results of Monte Carlo simulation are presented inFig. 10. This produces a large number of triangles analogous tothe one in Fig. 8; each one generated using end-members ran-domly generated within the distributions described above for eachend-member. As, it was expected the range of calculated values issignificantly broader than observed one. This phenomenon iscaused by extents of input values, i.e. high variability of isotopicsignatures of three sources. In effect this gives a visual representa-tion of the uncertainty associated with calculating end memberproportions for a given observed data point. However the

Fig. 8. The isotopic composition in groundwater sulphates of the recharge area of the Upper Silesian Triassic aquifers with mean delta values of sulphur and oxygen isotopesin sulphate derived from weathered sulphide, dissolved gypsum, and rainwater.

Table 1The detailed proportions originating from three sources: sulphide, gypsum and precipitation in the total amount of sulphate dissolved in groundwater of the Triassic carbonateformation. The results of simple linear mixing model (the first model), taking into account the average values for the three sources and two isotopes system of equations.

Sampling point Number in Figs. 1 and 2 SO4 (mg/L) Depth of the screen (m) Stratigraphy The fraction of source in the total amount ofsulphate (%)

Sulphideoxidation

Gypsumdissolution

Rainfall

Rybna H1 1 68.1 175–187 Roethian 48.80 21.52 29.68Rybna H2 2 76.5 169.5–185.5 Roethian 44.98 23.22 31.80Ptakowice 3 69.3 93–104 Muschelkalk 58.99 12.21 28.80Gliwice 19 4 103.8 88–175 Triassic 69.69 0* 30.31Brzezinka 21 5 289.5 91,5–128,5 Triassic 20.85 51.20 27.95Opatowice 6 66.4 66.6–189.0 Roethian + Muschelkalk 51.72 27.85 20.43Piekło-Szałsza 7 124.7 53.1–165.9 Quartenary + Triassic 50.02 2.95 47.03Gliwice 5a 8 262.7 96.4–177.1 Roethian + Muschelkalk 44.46 41.30 14.24Zawiercie 4bis 9 125.9 24–58 Muschelkalk 46.94 10.89 42.17Zawiercie 1bis 10 88.9 25.6–70 Muschelkalk 57.37 0* 42.63Rokitno Szlch. 11 72.92 185–215 Triassic 71.15 0* 28.85Trzebiesławice 12 55.67 95–116 Roethian 9.55 10.78 79.67Ujejsce 13 70.2 60–90 Roethian 55.98 26.77 17.25Tucznawa 14 214.8 53–140 Roethian + Muschelkalk 46.39 41.42 12.19Sławków 15 96.17 50–100 Triassic 61.28 16.03 22.69Olkusz 16 103.3 50–103 Triassic 71.02 19.13 9.84Jaworzno Dobra I 17 93.3 11.6–101.5 Roethian + Muschelkalk 31.6 17.5 50.90Jaworzno Dobra II 18 223.4 71–83 Roethian 52.03 31.4 16.58Jaworzno

Galmany19 98.2 69.1–85 Ore-bearing dolomites 36.17 13.96 49.87

Jaworzno Bielany 20 77.9 53–78 Muschelkalk 48.02 26.44 25.53Minimum 11.6 – 9.55 0 9.84Maximum 215 – 71.15 51.2 79.67Arithmetic average – – 48.85 19.73 31.42Median – – 49.41 18.32 28.83

* In these wells the two-end member mixing model have been applied taking into account values of d34S in dissolved sulphate.

K. Samborska et al. / Journal of Hydrology 486 (2013) 136–150 145

calculated outcomes (triangles) overlapped significantly, leading tothe relatively small calculated uncertainties in proportions. Withinthe simulated area there are isotopic compositions that are morelikely to occur; to enable identification of these more probableranges, the scatter plot has been redrawn to hexbin (hexagonalbinning) plot (Fig. 11) which gives the probability distribution ofsulphate isotopic composition calculated for all possible mixtureswith end-member compositions randomly assigned within the dis-tributions described above for each.

In Fig. 11, the representation of accumulated computed data isshown together with the values detected in sulphate. As expectedthe zone of highest calculated probability of isotopic compositioncorresponds to the area of overlap of most of the triangles inFig. 10. Some of the observed data are situated outside of the areawhere the most probable values for both isotope compositions oc-cur. This most frequently occurring calculated values are in rangesfrom ca. 2‰ to 12‰ for d34S and from ca. 5‰ to 12‰ for d18O.Whilst about four observed data points lie within this range, the

Fig. 9. The variability of the isotopic composition of sulphate sources and sulphate dissolved in groundwater on the recharge areas of the Triassic carbonate formation.

Table 2Results of calculation of the two-isotopes and three sources mixing model (the second model) conducted for mean isotopes signatures of sulphur and oxygenof both sources and sulphate dissolved in groundwater on the recharge areas of the Triassic formation. Calculations take into account the variability of isotopiccomposition of sources and the results are displayed together with the lower and upper confidence intervals for the input derived from three sources.

Source Lower 95% confidence interval for source input Mean for source input Upper 95% confidence interval for source inputStandard error

Sulphide 0.4 0.47 0.540.03

Rainfall 0.19 0.34 0.480.07

Gypsum 0.08 0.19 0.30.05

Fig. 10. Observed and computed via Monte Carlo simulation data for isotopes indissolved sulphate.

146 K. Samborska et al. / Journal of Hydrology 486 (2013) 136–150

rest lie at lower values of both d34S (by 2–6‰) and d18O (by5–10‰). This difference arises either because there is some non-random ‘‘structure’’ to the mixing process (perhaps one of thesources is more available/abundant then others), or because oneof the input distributions in the calculation is systematicallyunrepresentative of the source that actually contributes to the

observed mixtures in the groundwater. Values of d18O in precipita-tion and evaporites are both strongly positive and their probabilitydistribution functions are overlapping, whereas weathering of sul-phide produces sulphate that is enriched in lighter isotopes. Thus,the occurrence of observed values of d18O may indicate that:

(i) sulphate is formed more predominantly via oxidation of sul-phide than the source model calculated; and more probably

(ii) that the d18O of sulphate formed during sulphide oxidationis systematically depleted in 18O compared to the inputprobability distribution.

Bottrell et al. (2008) found that d18O of sulphate produced bysulphide oxidation during similar aquifer dewatering was indeedvery 18O-depleted. Thus sulphide oxidation at depth in dewateredaquifers appears to be dominated by a mechanism or environmen-tal controls that lead to 18O depletion, likely due to incorporation ofO atoms derived from water molecules rather than direct oxidationby dissolved O2.

In order to verify and to estimate the real contribution of theoxidation mechanism into the total amount of sulphate within re-charge areas of carbonate aquifers a few additional scenarios havebeen tested. Fig. 12 presents the results of simulations assumingvariable fractions of sulphate derived from the oxidation of sul-phide in a mixture (greater than 25% – Fig. 12a, 40% – Fig. 12b,50% – Fig. 12c and 75% – Fig. 12d, respectively, the contributionof the other two sources varied from 0 to 100%). As it is shown,while increasing minimum amount of sulphate derived form

Fig. 11. Concentrated computed values of both isotopes versus observed.

K. Samborska et al. / Journal of Hydrology 486 (2013) 136–150 147

sulphide oxidation from 0% to 25% the observed data-set is shiftingtowards the field of the most expected values, when this minimumamount increased to 50% observed values are rather sticking out ofthis area. Eventually, if the proportion of sulphide in the total loadof sulphate is equal or greater than 75%, observed values of bothisotopes are out of the most expected ranges. The last scenario

Fig. 12. Results of MC simulations taking into account variable (in upward order) proport(d) respectively.

which represents the contribution of the sulphide is greater 40%(i.e. sulphide must constitute at least 40% of dissolved in ground-water sulphate) as well as previous ones indicates that the weath-ering of sulphide may contribute between 40% and 50% of the totalload of dissolved sulphate. Although the results of MC simulationsare also fraught by uncertainty (data for calculating of random val-ues are mainly from literature and observed data-set is small), thismethod is in a good agreement with the results obtained from thelinear models and may be the best way to incorporate the variabil-ity of sources compositions and comparing the obtained broad-spectrum of values with the measured extent.

5. Conclusions

Groundwater of the Upper Silesian Triassic carbonate aquifershave complex geological setting and intense human impact andare affected by very high concentrations of sulphate, sometimesabove drinking water standards. Within and overlying the Triassicsequence there are two geological sulphate sources i.e. gypsumand sulphide. In this study the contribution of the third source ofsulphate, derived from the rainfall was also included. Investigationof the both (S and O) isotopic compositions of these three sourcesshowed that each was characterized by distinctive compositionsand thus contributions of these sources to groundwater sulphatecould be estimated from isotopic data. This gives the possibilityto determine which source of sulphate is responsible for the very

ions of sulphide sources – contribution greater than 25% (a), 40% (b), 50% (c) and 75%

148 K. Samborska et al. / Journal of Hydrology 486 (2013) 136–150

high concentrations, information that can be valuable in informingfuture management of this groundwater resource.

Water samples were taken from areas of outcrop or the directvicinity of ‘‘hydrogeological windows’’ through confining units orhydraulic contacts. First assessment of the contributions of theidentified individual sulphate sources to the total load of sulphateis based on a dual isotope linear mixing model. Solving of this sim-ple model taking into account only the average isotopic signaturesof sources leads to the conclusion that, in half of cases, weatheringof sulphide contributes the majority of the groundwater sulphate(Table 1). However, the mean input of this source for all samplesis slightly greater than 50%. The unconfined character of the stud-ied areas results in about 30% of sulphate being derived from arainfall origin. Of course the calculated contributions of differentsulphate sources should be treated as an approximation, howeverthey do vary coherently between different geological settings. Inaddition, one might observe that in case of the sampling pointswhere high concentrations of sulphate have been observed the nat-ural sources was generally responsible for this phenomenon. Espe-cially interesting is the contribution of gypsum in the total load ofdissolve sulphate in wells that have been affected by deteriorationin water quality. On the other hand calculated mean values of frac-tions for three sources are in good accordance with the results ofthe model that also considers the variability of source isotopiccompositions, which may be assumed as more realistic. The resultsobtained from this model indicated that, for a ‘‘typical’’ isotopiccomposition, from 40% to about 57% sulphate must originate fromsulphide oxidation (Table 2). Second with regard to the amountsource of sulphate is rainfall water with a mean input of 33%.

Eventually, the application of the Monte Carlo method takinginto account the statistical dispersion and distribution of datahas been shown that the most probable isotopic signatures for bothisotopes in sulphate are inconsistent with measured data. How-ever, performing the computational scenarios, first taking into ac-count all possible combinations of fractions and subsequentlychanging the amount of sulphate originating from weathering ofsulphide from 25% to 75%, one may conclude that the averagelycontribution of this source is between 40% and 50%. Thus, the re-sults of the Monte Carlo method lie in the range estimated basedon the previous models.

Although, this research has been conducted on limited numbersof wells, they account for a large proportion of the groundwaterabstraction from the aquifer systems and therefore give a vol-ume-integrated average of sulphate sources and water types with-in the aquifer.

In view of these results, it is clear that human impact on theaquifer might be crucial in controlling of the sulphate distribution.Firstly of all, it may directly result in deterioration of water qualityas natural processes of sulphide oxidation have been acceleratedby water level draw down as a result of human activities (minedewatering and drinking water abstraction. Furthermore, despitethe large input of sulphate from sulphide oxidation there is still alarge flux of sulphate from rainfall and that indicates that ground-water is also under threat from recent sulphate from rainfall. How-ever, the results of the first, simple linear model brought that theincreasing concentrations of SO4 ions are the effect of the bothweathering of sulphide and dissolution of evaporites. The furtherstudy of the carbonate, Triassic formation might validate the ob-tained results.

Acknowledgments

The authors would like to thank to Dr. Andrzej Witkowski, fromthe Silesian University, Katowice, Poland, for permission to publishthe S isotopic data used in this paper and to Dr. Jerzy Cabała fromthe Silesian University, Katowice, Poland for the giving of the rock

samples used to the isotopic examinations of sulphide. This re-search was supported by Grant No. N525 1610 33 ‘‘The hydrogeo-chemical evolution in the northern part of the Triassic carbonateaquifer Olkusz–Zawiercie under the impacts of human activity’’.Last but not least, authors would like to thank to Dr. Marek Korczfor the substantial support and anonymous reviewers, whose re-marks helped to improve the manuscript.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jhydrol.2013.01.017.

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