Journal of Hydrology 414–415 (2012) 211–219

Contents lists available at SciVerse ScienceDirect

Journal of Hydrology

journal homepage: www.elsevier .com/ locate / jhydrol

Variable infiltration and river flooding resulting in changing groundwaterquality – A case study from Central Europe

Konrad Miotlinski a,⇑, Dieke Postma b, Andrzej Kowalczyk a

a University of Silesia, Faculty of Earth Sciences, 41-200 Sosnowiec, Polandb GEUS – Geological Survey of Denmark and Greenland, Ø. Voldgade 10, DK-1350 Copenhagen K, Denmark

a r t i c l e i n f o s u m m a r y

Article history:Received 31 January 2011Received in revised form 9 September 2011Accepted 29 October 2011Available online 7 November 2011This manuscript was handled by LaurentCharlet, Editor-in-Chief, with the assistanceof Peter Wolfgang Swarzenski, AssociateEditor

Keywords:Groundwater qualityPyrite oxidationNickel mobilisationFloodingClimate change

0022-1694/$ - see front matter Crown Copyright � 2doi:10.1016/j.jhydrol.2011.10.034

⇑ Corresponding author. Present address: CSIRO LHealthy Country Flagship, Private Bag 2, Glen Osmond8 8303 8742.

E-mail addresses: konrad.miotlinski@csiro.au (K.mail.com (D. Postma), andrzej.kowalczyk@us.edu.pl (

The changes in groundwater quality occurring in a buried valley aquifer following a reduction in ground-water exploitation and enhanced infiltration due to extensive flooding of the Odra River in 1997 wereinvestigated. Long-time series data for the chemical composition of groundwater in a large well fieldfor drinking water supply indicated the deterioration of groundwater quality in the wells capturing waterfrom the flooded area, which had been intensively cultivated since the 1960s. Infiltration of flooded riverwater into the aquifer is suggested by an elevated chloride concentration, although salt flushing from therewatered unsaturated zone due to the enhanced recharge event is much more feasible. Concomitantlywith chloride increases in the concentrations of sulphate, ferrous iron, manganese, and nickel implythe oxidation of pyrite (FeS2) which is abundant in the aquifer. The proton production resulting from pyr-ite oxidation is buffered by the dissolution of calcite, while the Ca:SO4 stoichiometry of the groundwaterindicates that pyrite oxidation coupled with nitrate reduction is the dominant process occurring in theaquifer. The pyritic origin of SO2�

4 is confirmed by the sulphur isotopic composition. The resultant Fe2+

increase induces Mn-oxide dissolution and the mobilisation of Ni2+ previously adsorbed to Mn-oxide sur-faces. The study has a major implication for groundwater quality prediction studies where there are con-siderable variations in water level associated with groundwater management and climate change issues.

Crown Copyright � 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction

Pristine aquifers are often considered as a reliable source of po-table quality water, which will inevitably lead to enhanced exploi-tation of these aquifers. However, the variations in groundwaterquantity affected by changes in exploitation, prolonged droughts,or events of enhanced recharge including river floods may easilyresult in long term deterioration of groundwater quality. In addi-tion, climate change may affect recharge and changes in rechargerates of groundwater bodies in recent years have already been re-ported in various areas around the world (Loaciga et al., 2000; Maet al., 2004; Hanson et al., 2006; Holman et al., 2009; Crosby et al.,2010). However due to a delayed response between changing re-charge rates and the resultant groundwater quality, the influenceof climate change on the quality of groundwater resources has re-ceived little attention in the literature to date.

Reduced or enhanced recharge results in induction of geochem-ical processes including pyrite oxidation (e.g. Błaszyk and Górski,

011 Published by Elsevier B.V. All

and and Water, Water for a, SA 5064, Australia. Tel.: +61

Miotlinski), dieke.postma@g-A. Kowalczyk).

1981; Kinniburgh et al., 1994; Larsen and Postma, 1997; Prommerand Stuyfzand, 2005), precipitation and dissolution of carbonates(Hanshaw and Back, 1979; van Breukelen et al., 1998), sulphates(Postma, 1983; Kinniburgh et al., 1994), iron (Magaritz and Luzier,1985) and manganese oxides (Larsen and Postma, 1997).

This paper demonstrates the results of a detailed study in ahighly cultivated agricultural region of the Odra Valley in Racibórz,Poland where the quality of groundwater has dramatically deteri-orated over a number of years. Most water quality changes oc-curred after an extreme precipitation event and the flood of theOdra River in July 1997 (Kundzewicz et al., 1999). Prior to thisaquifer recharge occurred via infiltration of precipitation followedby inflow from the surrounding aquifers. The rising water tableassociated with reduced abstraction, enhanced recharge, and theflooding event resulted in groundwater contamination in abstrac-tion wells. The objective of this study is to elucidate the geochem-ical processes that are responsible for groundwater qualitychanges.

2. Hydrogeology

The study area is located in the Upper Odra River Valley (Fig. 1)near the city of Racibórz in southern Poland. Fig. 2 shows a cross

rights reserved.

Fig. 1. Sketch of the study area presenting land use and the location of the selected wells with the capture zones.

Fig. 2. Hydrogeochemical cross-section (September–December, 2005): 1 – water level in the Quaternary aquifer, 2 – boreholes, 3 – well permeable units, 4 – poorlypermeable units, 5 – semi-permeable units, 6 – directions of groundwater flow, 7 – directions of groundwater seepage, 8 – boundary between Quaternary and Neogene(Kotlicka, 1978), 9 – Pie diagram with a borehole number and a scale of diameter representing Total Dissolved Solids (TDS).

212 K. Miotlinski et al. / Journal of Hydrology 414–415 (2012) 211–219

section through the buried valley deposits. One can distinguish twoaquifers in the profile: a Pleistocene and a Miocene (Sarmatian)aquifer (Miotlinski, 2008). The Pleistocene aquifer, which is of par-ticular interest here, comprises a heterogeneous mixture of coarseand fine-grained material rich in organic matter (Kotlicka, 1978;Fabianska et al., 2008). The aquifer can be subdivided into the threehydrofacies; an upper (Upper Pleistocene) and a lower (LowerPleistocene) outwash facies composed of sands and gravels depos-ited by the glacial meltwater, and the intervening silt-rich

semiconfining unit (Middle Pleistocene). In the central part of theburied valley, the thickness of the fine-grained layer seldom ex-ceeds 10 m, and therefore a hydraulic connection exists betweenthe two coarse-grained layers (Miotlinski, 2008). Perpendicular tothe axis of the valley, the fine-grained layers outweigh the sandylayers in the upland areas towards the west. In these areas, the out-wash deposits of the Lower Pleistocene, that form the most perme-able part of the buried valley aquifer, do not occur. The aquifer,varying in thickness from about 10 m to more than 40 m, is

K. Miotlinski et al. / Journal of Hydrology 414–415 (2012) 211–219 213

covered by a poorly permeable layer and is underlain by the Mio-cene clay. The hydraulic conductivity of the permeable depositsranges from 1.6 � 10�5 m/s to 9.2 � 10�3 m/s. The lower values re-flect the upland areas in which the glacial sandy deposits containclay-rich layers, whereas the higher values are typical of the out-wash gravel layers of Lower and Upper Pleistocene ages(Miotlinski, 2008). All pumping wells penetrate the lowest partof the aquifer (�10 m), where hydraulic conductivity is the highest.The capture zone of the Boguminska well field is the largest in thearea covering a broad range of land use activity including urbanand residential areas, agricultural fields, and the area flooded bythe river in July 1997 (Fig. 1). The Miocene (Sarmatian) aquifer inthe area consists of the one layer of sandy deposits with an approx-imate thickness of 20–30 m. The hydraulic conductivity of thislayer varies in the range of 4 � 10�5–5 � 10�4 m/s. This aquiferplays a minor role in groundwater provision in the area of Racibórz(Witczak et al., 2007).

Extensive groundwater abstraction from the Pleistocene aquiferduring the twentieth century resulted in a water level decline ofabout 20 m. A maximum drawdown was observed in the 1980sin the Boguminska well field. At that time the extraction fromthe aquifer was in excess of 25,000 m3/d. Since 1994, a recoveryof the groundwater level has been observed (Fig. 3) which can beattributed to a gradual decrease in groundwater exploitation andan increase in rainfall over the preceding years. Groundwaterextraction reduced to 16,700 m3/d and 11,600 m3/d in 1997 and2005, respectively. The mean annual precipitation also increasedfrom 530 mm (1982–1994) to 680 mm (1995–2001). At the begin-ning of July 1997 some areas of the Odra catchment received over450 mm of rainfall within a couple of days (Kundzewicz et al.,1999; Fenske et al., 2001) which resulted in an extensive floodevent in Racibórz (Fig. 1). The concomitant increase in the ground-water table (Fig. 3) indicates major infiltration into the aquifer. In2005, the drawdown in the Boguminska well field had reduced toless than 12 m. Since 1997 the area has not been flooded. The studyarea is not irrigated.

A transient groundwater flow model (1991–2006) constrainedby isotopic (18O, 2H, 3H, 14C, 13C and 4He) data indicates that thePleistocene groundwater system is primarily recharged by precip-itation (�85%), although ascending flow from the Miocene aquifercontributes to recharge as well (�15%; Miotlinski, 2008). Stream-flow loss from the Odra River was taking place for the flooding per-iod solely while is insignificant under normal water levels in theriver. Net recharge rates as a part of the annual precipitation vary

Fig. 3. Groundwater level elevation at the Boguminska well field in the period of 1991–2Fig. 1.

from 5% in the upper parts of the catchment to 25% and in thefloodplain areas where the transmissivity of the overlying depositsis the lowest (Miotlinski, 2008).

3. Methods

3.1. Water sampling and analysis

Targeted groundwater sampling was undertaken during largefield campaigns in October 2002, September 2005, September–October 2006, December 2006, and July 2007. Measurements ofpH, electrical conductivity, Eh, O2 and temperature were carriedout in line on unfiltered groundwater with electrodes in a flow cellto prevent exposure to the atmosphere. Alkalinity and acidity weredetermined shortly after sampling by the Gran titration method(Stumm and Morgan, 1996). Samples for cations: Ca2+, Mg2+, Na+,K+, NH4

+ were filtered through 0.42-lm cellulose acetate filters,collected in 100-ml polyethylene bottles and preserved with 2%(v/v) 7 M HNO3 solution, whereas samples for anions: SO2�

4 , Cl�,NO�3 , NO�2 , F�, PO3�

4 were collected in 100-ml polyethylene bottleswithout filtering. Samples for the analysis of Fe, Mn, Ni, Cd, Cu, Co,Zn, Al, As and Si were filtered through 0.42-lm filters and pre-served with 1 ml of a Suprapure 65% v/v Merck HNO3. For ferrousiron, an unfiltered subsample and one pure-water sample (blanksolution) were reacted in the field with an acetic acid and phe-nantroline, and Fe2+ was measured by the spectrophotometric Fer-rozine method (Stookey, 1970) immediately after transferring tothe laboratory. A similar procedure was also applied for sulphide(Cline, 1967). Samples for 34S and 18O in sulphate were collectedin 1.5 l polyethylene bottles. All water samples collected in thefield were refrigerated at 4 �C until analysis.

To obtain water samples in a depth profile, four observationwells were drilled: #10 and #15 in September 2005 and #12 and#17 in December 2006. Groundwater was sampled during breaksin drilling using a submersible pump lowered in a short-screen0.1 m pipe.

The water samples were analysed for Ca2+, Mg2+, Na+, K+, NHþ4 ,SO2�

4 , Cl�, NO�3 , NO�2 and F� concentrations by ion chromatography(IC). Analyses of Fe2+, Mn2+, Ni2+, Cd2+, Cu2+, Co2+, Al3+ and Zn2+

were performed by flame atomic absorption spectrometry (AAS).These metals as well as As and Si were additionally determinedby using ICP–OES. Concentrations of ferrous iron, sulphide andphosphate were quantified using spectrophotometry. Tritium mea-surements were performed by liquid scintillation coating of water

005. The location of piezometers is depicted in the inserted map of the well field in

214 K. Miotlinski et al. / Journal of Hydrology 414–415 (2012) 211–219

after electrolytic enrichment of 3H (Eichinger et al., 1980). Prior tothe examination of d34S, d18O isotopes of sulphate, BaSO4 was pre-cipitated. The BaSO4 was ground together with Cu2O and quartzglass and reduced to SO2 by heating at 1120 �C, as described byColeman and Moore (1978). The oxygen isotope composition ofthe sulphate was determined on CO2 prepared by reduction ofBaSO4 with spectrographic graphite heated inductively in molyb-denum crucibles (Mayer and Krouse, 2004).

In addition, monthly monitoring data for the Racibórz Water-works’ wells since 1991 are included in this paper. The long-timeseries renders it possible to identify temporal trends in groundwa-ter chemistry. Determination of Ca2+, Mg2+, Na+, K+, NHþ4 , SO2�

4 , Cl�,NO�3 , NO�2 concentrations was undertaken by spectrophotometryand alkalinity by Gran titration (Stumm and Morgan, 1996) bythe Racibórz Waterworks. The local laboratory, analysing thesesamples, does not measure the concentrations of Na+ and K+, andthus the accuracy of the chemical analyses could not be evaluatedby calculation of charge balance. The accuracy of the analyses pro-vided by the Racibórz Waterworks was tested on several occasionsby inter-laboratory comparison. The variation in the concentra-tions of Ca2+, Mg2+, HCO�3 , SO2�

4 and Cl� measured at the two labo-ratories was less than 5%.

3.2. Sediment sampling and analysis

Sediments were sampled in December 2006 (during drilling ofwells #12 and #17) and the analyses were performed from Januaryto May 2007 at the Faculty of Earth Sciences of the University ofSilesia.

The extraction of Mn-oxides, Fe-oxides and sulphides wasundertaken via a three-step procedure. A sample of frozen sedi-ment (5 g) was first extracted by shaking it with 35 mL of a0.2 M hydroxylamine hydrochloride (HA) solution in a v/v 25% ace-tic acid for 1 h (Chester and Hughes, 1967; Chao, 1972; Larsen andPostma, 1997). The suspension was centrifuged at 3500 rpm for20 min, and the supernatant was removed. The sediment was thenrinsed twice with distiled water, immersed in 35 ml of a 6 Mhydrochloric acid (HCl) solution, and again shaken for 1 h (Chesterand Hughes, 1967). The suspension was centrifuged at 3500 rpmfor 20 min, and the supernatant was removed. The sediment wasthen rinsed twice with distiled water and placed into a glass vessel.Subsequently, the sample was boiled in a concentrated nitric acid(HNO3) (Huerta-Diaz and Morse, 1990). The supernatant of eachextraction step was filtered through a 0.42-lm membrane filterand acidified with 0.5 ml of a Suprapure 65% v/v Merck HNO3.The samples were stored at 4 �C until analysis. The metals includ-ing Ni2+ were analysed with flame AAS.

Sulphide and sulphur were determined in sediments using amodified Cr(II) reduction method (Canfield et al., 1986). Chromiumreducible sulphur was measured spectrophotometrically, insteadof by iodometrical titration as proposed by Canfield et al. (1986).Also chromium powder dissolved in HCl was used as an alternativeto using chromium chloride solution (Sullivan et al., 2000).

Water soluble sulphates were determined after displacement ofthe pore water. Initially the frozen sediment samples were rinsedwith ethanol at room temperature to displace pore water and theremaining ethanol was removed by evaporation overnight. The driedsamples were then shaken for 2 h in demineralised water and centri-fuged at 3500 rpm for 15 min. In the filtered (0.42-lm) supernatant,Ca2+ and SO2�

4 were determined by ion chromatography.Pyrite and other heavy metals were extracted from selected

sand samples for microscopy and isotope analyses by density sep-aration. Bromoform (CHBr3) with a density of 2.8 g/cm3 was usedfor the separation. The sulphur isotopic composition of pyritewas determined from SO2 prepared from the oxidation of pyriteby Cu2O at 1100 �C (Taylor et al., 1984).

4. Results

4.1. Water chemistry

The major features of the studied aquifer are distinct temporalvariations in water quality. Those changes are observed chiefly inthe Boguminska well field (wells #11, 13, and 14) after 1997 as de-picted in Fig. 4. Prior to 1995, sulphate and Fe2+ were low (0.8–1 mM and 0.025 mM, respectively) and the Cl� was around0.75 mM. From 1995 to the 1997 flooding event a small increasein SO2�

4 was visible evident in well #13, and to a lesser extent alsofor Cl�. After 1997 the increases in SO2�

4 and Cl� were more dra-matic followed by the increases in Fe2+, Mn2+ and Ni2+. The in-creases in SO2�

4 and Cl� correspond to declines in alkalinity andpH. Remarkably, the alterations in Fe2+, Mn2+ and Ni2+ are signifi-cantly delayed in comparison to sulphate. As a result of thesechanges, the water types evolved from a Ca–HCO3 type towards aCa–SO4–HCO3 water type. Other indicators of the water qualitychanges include the gradual evolution towards the (a) undersatu-ration with respect to calcite and (b) negative d34S(SO2�

4 ) values.The tritium concentration in well #11 indicates that part of the ex-tracted water must have recharged the aquifer after the 1960s.

The changes in water chemistry observed since 1995 result inspatial variability within the aquifer. The changes along the regio-nal direction of groundwater flow (from the east to the west) cross-ing the Boguminska well field in the year of 2005 are shown as piediagrams in Fig. 2. Groundwater in the well field is characterised bythe highest TDS which is reflected by the area of the circle. Thiswater is also enriched in Ca2+, SO2�

4 , and Cl�, among the major ionsand depleted in HCO�3 . Such high concentrations do not occur inother wells located in the central part of the buried valley (see well#1 in Table 1). Hence, the following broad factors must be respon-sible for the spatial variability of water quality: (a) the geology(permeability of the formation overlying the aquifer, geochemistry,heterogeneity), (b) the hydrodynamics (the areas contributing torecharge to the individual pumping wells as well as the upwardmovement of the water table in the Boguminska due to enhancedrecharge rates including flooding in 1997 and diminishing ground-water abstraction), and (c) the anthropogenic influences (the landuse including agriculture and urban areas).

The depth profiles, that were sampled at the Boguminska wellfield, in 2005 and 2006, also show significant changes over depth(Fig. 5). Boreholes #12 and #17 display the water chemistry ofthe rewatered part of the saturated zone, whereas the much deeperwells #10 and #15 mainly represent the water chemistry of thedeeper part of the aquifer. The water chemistry of the rewateredzone (Fig. 5, # 12 and 17) is oxic with up to 0.22 mM of O2, andup to 0.05 mM NO�3 . Further downward the O2 and NO�3 concentra-tions decrease while there is a significant Fe2+ increase and theSO2�

4 concentration is around 3 mM. The deeper groundwater(Fig. 5, #10 and 15) is anoxic with only traces of nitrate but con-tains both ferrous iron and manganese, reaching 244 lM Fe2+

and 44 lM Mn2+. Sulphate concentrations are between 1 and2 mM and show no clear depth trend.

4.2. Sediment chemistry

The sediment chemistry of the rewatered zone (wells #12and #17) is shown in Fig. 6. Borehole #17 exhibits enhancedconcentrations in particular for Fe, Mn and Ni in the depth range9–14 m. The 6 M HCl and hydroxylamine hydrochloride extrac-tions both show high Fe concentrations from 10 to 14 m. HA re-duces poorly crystalline iron oxyhydroxides and the similardistribution for HCl-extractable-Fe suggests that 6 M HCl extractsmore crystalline iron oxyhydroxides (e.g. goethite). However, the

Fig. 4. The variations in selected ions concentrations in groundwater over time.

Table 1Chemical parameters of groundwater and river water from Racibórz. In the heading the name of sampling point and date is given.

Well #1 Well #11 Well #13 Well #13 River water*

27/09/2006 11/12/2006 27/05/1991 16/06/2005 26/11/2004

Hydrochemical type Ca–HCO3 Ca–SO4–HCO3 Ca–HCO3 Ca–HCO�3 SO4 Na–ClTDS (mg/L) 525 810 550 640 950pH 7.14 6.52 6.43 6.71 7.25O2 (mM) <0.003 <0.003 n/a <0.003 0.35Ca2+ (mM) 2.49 3.35 2.64 2.72 3.2Mg2+ (mM) 0.68 0.96 0.73 0.87 1.27Na+ (mM) 0.41 0.93 0.41 1.01 6.91K+ (mM) 0.03 0.15 0.04 0.19 0.22

SO2�4 (mM) 0.29 3.8 0.7 1.86 1.33

NO�3 (mM) <0.02 <0.02 <0.02 <0.02 0.23HCO�3 (mM) 5.8 2.25 5.03 3.8 2Cl� (mM) 0.37 1.03 0.75 1.13 8.46Fe2+ (mM) 0.02 0.26 0.025 0.2 <0.002Mn2+ (mM) 0.002 0.004 0.002 0.013 0.01Ni2+ (lM) <0.017 3.8 n/a <0.017 <0.017SIcalcite �0.01 �1.03 �0.77 �0.65 �0.59SIsiderite 0.08 0.22 �0.52 0.54 �10.8SIrhodochrosite �0.63 �1.33 �1.33 �0.38 �0.19SIgypsum �2 �0.9 �1.62 �1.23 �1.9d34S(SO4) (‰) 6.12 �5.44 n/a n/a 3.02

* Note that the river water sample represents a non-flooding period; n/a – value not available.

K. Miotlinski et al. / Journal of Hydrology 414–415 (2012) 211–219 215

amount of iron extracted with hydrochloric acid is far greaterthan by HA. Hydroxylamine hydrochloride also reduces Mn fromthe sediment, indicating the presence of Mn-oxides and againthe concentration extracted is approximately an order of magni-tude greater in the extraction with HCl than for HA. In contrast,oxidative dissolution by HNO3 which will dissolve pyrite,

releases little Mn. The leaching of Ni by hydroxylamine hydro-chloride suggests a Ni liberation during reductive dissolution ofFe- or Mn-oxides and the amount of Ni2+ released by HCl iscomparable to the HA extraction. Up to ten times more Ni2+ isreleased by HNO3, suggesting that pyrite is the primary sourceof this metal.

Fig. 5. The variations in selected ions concentrations in groundwater over depth.

Fig. 6. Geochemical properties of sediments. Symbols represent the solution being used for the extraction: HA (hydroxylamine hydrochloride), HCl (hydrochloric acid), HNO3

(nitric acid), CrCl2 (chromium (II) chloride), H2O (pure water). n/a–not applicable.

216 K. Miotlinski et al. / Journal of Hydrology 414–415 (2012) 211–219

SEM analysis revealed the persistence of euhedral pyrite in therewatered zone (Miotlinski, 2008) indicating that all iron sulphideswere not completely oxidised when the water table was low. Ingood agreement (Fig. 6) the concentration of Cr(II)-reducible sul-phur, comprising pyrite, ranges from 75 to 100 mM/kg S. The gyp-sum content was assayed by measuring H2O-extractable sulphateand concentrations of up to 6 mM/kg S were measured (Fig. 6).

5. Discussion

5.1. Hydrogeology and water quality of enhanced infiltration

The regional groundwater flow field indicates that the vastmajority of groundwater extracted in the Boguminska well fieldcomes from the upland areas (Fig. 2). Nevertheless, the water de-

K. Miotlinski et al. / Journal of Hydrology 414–415 (2012) 211–219 217

rived directly from the upland areas is of a reasonably good quality(e.g. wells #1 and #6 in Table 1). The tritium data and particletracking modelling indicate that at least part of this water musthave recharged the aquifer during the period of intensive agricul-tural fertilisation since the 1960s (Miotlinski, 2008).

Prior to the flooding event, there was little variation in thechemical composition of groundwater which may suggest thatthe flooding event from 1997 was responsible for the changingwater quality. This hypothesis finds a confirmation in the suddenrise of the water table following the flood. Wells #13 and #14,whose capture zones were covered by the flood waters in1997, show an increase in the Cl� concentration (Fig. 4), whichcould be considered to be of the river origin, as river water isgenerally higher in chloride than the groundwater (Table 1).The midpoints of the chloride breakthrough curves in Fig. 4 oc-curred in October 1999 (Well #13) and April 2000 (Well #14),corresponding to travel times for infiltration of river water of27 and 33 months respectively. Estimating a horizontal distanceof 500 m from the flooded area and a vertical distance of 40 mtowards the well screens, the breakthrough curves correspondto a high estimate for the average groundwater velocity of240 m/year. A linear groundwater velocity in this range seemsto be unlikely, because the superficial and upper sections ofthe aquifer consist of fine grained material of relatively lowhydraulic conductivity. Moreover, the chloride signature of theriver water is believed to be diluted during the flooding eventsdue to very large runoff rates associated with the episodes ofheavy precipitation (Fenske et al., 2001).

Concomitantly with Cl�, the sulphate concentration increases inwells #13 and 14 (Fig. 4) which points towards a related mecha-nism for increase. For the same timespan as the chloride break-through with a rise of about 0.75 mM Cl�, the increase in SO2�

4

are is up to 1.9 mM. However, in the river water the Cl/SO4 ratiois higher than 6 (Table 1). The implication is that the river watercannot be the main source of SO2�

4 for the observed increase inSO2�

4 after flooding. More likely the infiltrating river water and dif-fuse recharge from rainfall are flushing the products of agriculturalactivity, which had accumulated here during the relatively dryconditions before 1995, from the unsaturated zone. Many authors(e.g. Postma et al., 1991; Böhlke and Denver, 1995; Tesoriero et al.,2000) have previously reported that in addition to nitrate loading,agricultural activity may result in the increases of sulphate, chlo-ride or calcium in groundwater. Also, some water of elevated sul-phate concentration may have accumulated in a relatively poorpermeability zone of the upper part of the saturated zone priorto 1997 and have been washed out with increased recharge follow-ing the flood event. In addition wells #13 and #14 showed an in-crease in SO2�

4 before flooding (Fig. 4) in response to a rise in thegroundwater table of a few metres as a result of the reduction inpumping.

A possible source of sulphate to be flushed from the rewateredunsaturated zone is gypsum that is being dissolved. The presenceof gypsum in the unsaturated zone of dewatered aquifers has pre-viously been reported elsewhere. In the London basin aquifer up to1.7 weight percent of gypsum was found (Kinniburgh et al., 1994)and up to 6 mM/kg of water-soluble sulphur was measured in theBeder well field (Larsen and Postma, 1997). In both cases the disso-lution of gypsum was associated with recovery of water level.Although the oversaturation of groundwater with respect to gyp-sum has been never confirmed in Racibórz, traces of gypsum wereidentified in the unsaturated zone (Fig. 6). The dissolution of gyp-sum will release Ca2+ and SO2�

4 in a 1:1 stoichiometry according tothe reaction:

CaSO4 � 2H2O $ Ca2þ þ SO2�4 þ 2H2O ð1Þ

5.2. Pyrite oxidation

The increases in the concentration of sulphate and iron, follow-ing the flooding event (Fig. 4) suggest that pyrite oxidation is in-volved or at least the flushing of the oxidation products of pyriteoxidation from the unsaturated zone takes place. Pyrite oxidationas a result of groundwater exploitation and a corresponding de-cline in the water level is a well documented issue in many aqui-fers (Błaszyk and Górski, 1981; Kinniburgh et al., 1994; Larsenand Postma, 1997).

Pyrite oxidation by reduction of oxygen may either proceedwith oxidation of both sulphur and iron:

FeS2 þ 15=4O2 þ 7=2H2O! FeðOHÞ3 þ 2SO2�4 þ 4Hþ ð2Þ

or, when insufficient electron acceptor is present, by oxidation ofsulphur only:

FeS2 þ 7=2O2 þH2O! Fe2þ þ 2SO2�4 þ 2Hþ ð3Þ

Pyrite oxidation may also be coupled to nitrate reduction inaquifers (Postma et al., 1991; Böhlke and Denver, 1995; Tesorieroet al., 2000; Broers, 2004). When nitrate is the electron acceptor,the reactions are:

5FeS2 þ 15NO�3 þ 10H2O! 5FeðOHÞ3 þ 15=2N2 þ 10SO2�4 þ 5Hþ

ð4Þ

5FeS2 þ 14NO�3 þ 4Hþ ! 5Fe2þ þ 7N2 þ 10SO2�4 þ 2H2O ð5Þ

In the presence of CaCO3 and at near neutral pH, these reactionsare buffered by the reaction:

CaCO3 þHþ ! Ca2þ þHCO�3 ð6Þ

Combining reactions (2)–(5) with (6) leads to:

FeS2 þ 15=4O2 þ 7=2H2Oþ 4CaCO3

! FeðOHÞ3 þ 2SO2�4 þ 4Ca2þ þ 4HCO�3 ð7Þ

With the stoichiometric release ratio Ca: SO4 = 2: 1,

FeS2 þ 7=2O2 þH2Oþ 2CaCO3 ! Fe2þ þ 2SO2�4 þ 2Ca2þ þ 2HCO�3

ð8Þ

With the ratio Ca: SO4 = 1: 1,

5FeS2 þ 15NO�3 þ 10H2Oþ 5CaCO3

! 5FeðOHÞ3 þ 15=2N2 þ 10SO2�4 þ 5Ca2þ þ 5HCO�3 ð9Þ

With the ratio Ca: SO4 = 1:2,

5FeS2 þ 14NO�3 þ 4Ca2þ þ 4HCO�3

! 5Fe2þ þ 7N2 þ 10SO2�4 þ 2H2Oþ 4CaCO3 ð10Þ

Note that reaction (5) is proton consuming and therefore reac-tion (10) may precipitate CaCO3. The corresponding stoichiometricratio is Ca:SO4 = �1:2.5.

From the foregoing it becomes clear that the predominant pyr-ite oxidation reaction can be identified from the Ca:SO4 ratio evenwhen the electron acceptor is no longer present. Nonetheless, theapproach presented for identifying the reaction path in the pyriteoxidation process is based on the following assumptions:

1. The amount of sulphate originating from pyrite must exceed theamount derived from agricultural sources. The latter could beconfirmed by the isotopic examination of sulphur.

2. The uptake and release of calcium by ion exchange must besmall.

Fig. 7. Relation between dissolved Ca2+ and SO2�4 in groundwater and correspond-

ing stoichiometry of major reactions.

218 K. Miotlinski et al. / Journal of Hydrology 414–415 (2012) 211–219

Using the Ca2+ and SO2�4 contents of the pristine groundwater

(Table 1, well #1) as starting point the lines corresponding to thesedifferent stoichiometries are shown in Fig. 7 together with datafrom the different wells that were sampled over time (Fig. 4).Clearly most data-points fall within the nitrate reduction window.Since the amount of Fe2+ released, as compared to sulphate, is low-er than predicted by reaction (10) the overall reaction stoichiome-try must be somewhere between reactions (9) and (10), as is alsoapparent from Fig. 7. On the other hand, pyrite oxidation by oxygendoes not appear to be important in the development of the ground-water chemistry as data points do not plot in the oxygen reductionwindow of Fig. 7. The ratio Ca:SO4 = 1:1 is predicted for partial oxi-dation of pyrite (reaction (8)) as well as for dissolution of gypsum(reaction (1)). Although there are a few data that plot near this line,in general neither of the two processes has a major influence on thegroundwater composition. The only well that indicates an influ-ence of pyrite oxidation by oxygen is well #11. Nevertheless, thiswell was drilled after the flood (in 2001) and might contain a sig-nificant admixture of water from shallower parts of the aquifer.Otherwise its data plot between the oxygen and nitrate windowswhich would imply that both processes contribute in the capturezone of this well.

An intriguing question is the source of nitrate required forpyrite oxidation according to reactions (9) and (10). Their stoi-chiometry predicts that to produce 3 mM SO4 about 4.5 mMNO3 is needed. Inspection of the composition of the presentday river water (Table 1) shows that this is an insufficientsource. Similarly, the present day superficial groundwater islow in nitrate (Fig. 5). However, historically the lands in the areahave been intensively fertilised. Before the collapse of

Fig. 8. Relations between trace metal

communism in Poland in 1989 around 200 kg N/ha/year wasused while the current level is 120 kgN/ha/year. McMahonet al. (2006) studied the effect of irrigation on the behaviourof nitrogen and chloride in thick unsaturated zones beneath cul-tivated areas, in the high plains of central USA, and concludedthat the induced infiltration results in the mobilisation of 60%of nitrogen (mostly in the form of NO�3 ) and as much as 80%of chloride previously stored in the profile. Most probably theflooding alongside enhanced recharge from precipitation hasflushed nitrate, or the oxidation products of pyrite oxidation,that had accumulated in the soil and unsaturated zone.

5.3. Nickel mobilisation

Following the recharge event the quality of groundwater alsodeclined with respect to nickel, with concentrations in the Bogu-minska well field up to 340 nM (Fig. 4) or up to eight times the rec-ommended EU drinking water limit. The increase in the Ni2+

concentration suggests that Ni mobilisation is related to pyrite oxi-dation as has been reported from other well fields (Larsen andPostma, 1997; Broers, 2004). The high content of Ni extracted byHNO3 in the sediments at Racibórz (Fig. 6) suggests that the pyriteis rich in Ni and therefore probably is the primary source of dis-solved Ni2+ in groundwater. However, Fig. 4 shows that there isno simple relation between the groundwater sulphate and Ni2+

concentrations and an additional process affecting the mobilisationof Ni2+ in the aquifer must be operative.

Fig. 8 displays that the concentrations of Ni2+ and Co2+ in thegroundwater correlate well with that of Mn which suggests acoupled mobilisation process, most likely the reduction of Mn-oxides. Mn-oxides are known to be important scavengers oftrace metals due to their high surface area and a large negativesurface charge (McKenzie, 1980; Tonkin et al., 2004). An almostcomplete uptake of both Ni2+ and Co2+ in the presence ofMn-oxides was observed by Kay et al. (2001) during a columnexperiment. Larsen and Postma (1997) found a strong associa-tion between Ni2+ and Mn-oxides in aquifer sediments. Probablythe Ni2+ released during pyrite oxidation has predominantly be-come adsorbed on the surfaces of Mn-oxides. Subsequent reduc-tive dissolution of the Mn-oxides will cause the mobilisation ofNi2+ as well (Stollenwerk, 1994; Larsen and Postma, 1997; Kayet al., 2001). In particular, during a rising water table, the mobi-lisation of Fe2+, for example by partial oxidation of remainingpyrite, can drive the reaction (Postma, 1985; Larsen and Postma,1997; Postma and Appelo, 2000):

2Fe2þ þMnO2 þ 4H2O ! 2FeðOHÞ3 þMn2þ þ 2Hþ ð11Þ

The time series displayed in Fig. 4 support this scenario, show-ing how Fe2+ builds up followed by the increases of Mn2+ and Ni2+.

s and Mn2+ in the shallow wells.

K. Miotlinski et al. / Journal of Hydrology 414–415 (2012) 211–219 219

6. Conclusions

The reduction in groundwater extraction, an increase in re-charge, and the 1997 flooding event of the Odra River renderedthe recovery of the water level in the Quaternary aquifer at Raci-bórz, Poland. The infiltration event is reflected in the groundwaterquality of the pumped wells by sharp increases in the Cl� and SO2�

4

concentrations.The enhanced infiltration resulted in increases in dissolved sul-

phate and iron and decreases in pH and alkalinity. These changeswere attributed to flushing of oxidation products from pyrite.The calcium-sulphate-stoichiometry suggests nitrate accumulatedin the unsaturated zone from fertilizer application to be the mainelectron acceptor for pyrite oxidation.

In addition the upward movement of the water level has re-sulted in increases in the Ni2+ concentration. The rise in Ni2+ isstrongly correlated to an increase in Mn2+ and it is inferred thatNi2+ is mainly present adsorbed on the surfaces of Mn-oxide. Sub-sequent reductive dissolution of Mn-oxide is presumably causedby reaction with Fe2+ and releases Ni2+ to the groundwater.

Acknowledgements

The project was funded by the Ministry of Science and HigherEducation of Republic of Poland (Research Project N 521 001 31/0335) under the PhD scholarship project of the first author. Weappreciate the constructive comments by two anonymous review-ers. It is a pleasure to thank Joanne Vanderzalm from CSIRO Landand Water for her comments and linguistic guidance. Jacek Wróbeland Alina Niewiadomska are acknowledged for field and laboratoryassistance.

References

Błaszyk, T., Górski, J., 1981. Ground-water quality changes during exploitation.Ground Water 19 (1), 28–33.

Böhlke, J.K., Denver, J.M., 1995. Combined use of groundwater dating, chemical, andisotopic analyses to resolve the history and fate of nitrate contamination in twoagricultural watersheds, Atlantic coastal plain, Maryland. Water Resour. Res. 31,2319–2339.

Broers, H.P., 2004. Nitrate reduction and pyrite oxidation in the Netherlands. In: IAHSelected Papers, No 5, Balkema, Rotterdam, pp. 141–147.

Canfield, D.E., Raiswell, R., Westrich, J.T., Reaves, C.M., Berner, R.A., 1986. The use ofchromium reduction inorganic sulfur in sediments and shale. Chem. Geol. 54,149–155.

Chao, T.T., 1972. Selective dissolution of manganese oxides from soils andsediments with acidified hydroxylamine hydrochloride. Soil Sci. Soc. Am.Proc. 36, 764–768.

Chester, R., Hughes, M.J., 1967. A chemical technique for the separation of ferro-manganese minerals, and adsorbed trace elements for pelagic sediments. Chem.Geol. 2, 149–262.

Cline, J.D., 1967. Spectrophotometric determination of hydrogen sulfide in naturalwaters. Limnol. Oceanogr. 14, 454–458.

Coleman, M.L., Moore, M.P., 1978. Direct reduction of sulfates to sulfur dioxide forisotopic analysis. Anal. Chem. 50, 1594–1595.

Crosby, R.S., McCallum, J.L., Walker, G.R., Chiew, F.H.S., 2010. Modelling climate-change impacts on groundwater recharge in the Murray-Darling Basin,Australia. Hydrol. J. 18, 1639–1656.

Eichinger, L., Forster, M., Rast, H., Rauert, W., Wolf, M., 1980. Experience Gathered inLow-level Measurements of Tritium in Water. TECDOC-246. IAEA, Vienna, pp.43–64.

Fabianska, M., Miotlinski, K., Kowalczyk, A., 2008. Geochemical features of re-deposited organic matter occurring in fluvioglacial sediments in the Racibórzregion (Poland); a case study. Chem. Geol. 253, 151–161.

Fenske, C., Westphal, H., Bachor, A., Breitenbach, E., Buchholz, W., Jülich, W.-D.,Hensel, P., 2001. The consequences of the Odra flood (summer 1997) for theOdra lagoon and the Beaches of Usedom: what can be expected under extremeconditions? Int. J. Hyg. Environ. Health 203, 417–433.

Hanshaw, B.B., Back, W., 1979. Major geochemical processes in the evolution ofcarbonate aquifer systems. J. Hydrol. 43, 287–312.

Hanson, R.T., Dettinger, M.D., Newhouse, M.W., 2006. Relations between climaticvariability and hydrologic time series from four alluvial basins across thesouthwestern United States. Hydrogeol. J. 14 (7), 1122–1146.

Holman, I.P., Tascone, D., Hess, T.M., 2009. A comparison of stochastic anddeterministic downscaling methods for modelling potential groundwaterrecharge under climate change in East Anglia, UK: implication forgroundwater resource management. Hydrol. J. 17, 1629–1941.

Huerta-Diaz, M.A., Morse, J.J., 1990. A quantitative method for determination oftrace metals concentrations in sedimentary pyrite. Mar. Geol. 29, 119–144.

Kay, J.T., Conklin, M.H., Fuller, C.C., O’Day, P.A., 2001. Processes of nickel and cobaltuptake by a manganese oxide forming sediment in Pinal Creek, Globe MineDistrict, Arizona. Environ. Sci. Technol. 35, 4719–4725.

Kinniburgh, D.G., Gale, I.N., Smedley, P.L., Darling, W.G., West, J.M., Kimblin, R.T.,Parker, A., Rae, J.E., Aldous, P.J., O’Shea, M.J., 1994. The effects of historicabstraction of groundwater from the London Basin aquifers on groundwaterquality. Appl. Geochem. 9, 175–195.

Kotlicka, G.N., 1978. Stratygrafia osadów czwartorzedowych w dolinie Odry kołoRaciborza. [Stratigraphy of Quaternary deposits in the Odra Valley nearRacibórz]. Biuletyn Instytutu Geologicznego 300, 303–387 (In Polish,extended abstract in English).

Kundzewicz, Z.W., Szamałek, K., Kowalczak, P., 1999. The great flood of 1997 inPoland. Hydrol. Sci.-J.-des Sci. Hydrol. 44, 855–870.

Larsen, F., Postma, D., 1997. Nickel mobilization in a groundwater well field: Releaseby pyrite oxidation and desorption from manganese oxides. Environ. Sci.Technol. 31, 2589–2595.

Loaciga, H.A., Maidment, D.R., Valdes, J.B., 2000. Climate-change impacts in aregional karst aquifer, Texas, USA. J. Hydrol. 227, 173–194.

Ma, T., Wang, Y., Guo, Q., 2004. Response of carbonate aquifer to climate change innorthern China: a case study at the Shentou karst springs. J. Hydrol. 297, 274–284.

Magaritz, M., Luzier, J.E., 1985. Water-rock interactions and sea-water freshwatermixing effects in the coastal dunes aquifer, Coos Bay, Oregon. Geochim.Cosmochim. Acta 49, 2515–2525.

Mayer, B., Krouse, H.R., 2004. Procedures for sulfur isotope abundance studies. In:Handbook of Stable Isotope Analytical Techniques, Elsevier, Amserdam, pp.538–596.

McKenzie, R.M., 1980. The adsorption of lead and other heavy metals on oxides ofmanganese and iron. Aust. J. Soil Res. 18, 61–73.

McMahon, P.B., Dennehy, K.F., Bruce, B.W., Böhlke, J.K., Michel, R.L., Gurdak, J.J.,Hurlbut, D.B., 2006. Storage and transit time of chemicals in thick unsaturatedzones under rangeland and irrigated cropland, High Plains, United States. WaterResour. Res. 42, W03413. doi:10.1029/2005WR00441.

Miotlinski, K., 2008. Hydrogeochemical Evolution in the Buried Valley in theRacibórz area. PhD Dissertation. University of Silesia, Faculty of Earth Sciences,Sosnowiec, Poland.

Postma, D., 1983. Pyrite and siderite oxidation in swamp sediments. J. Soil Sci. 34,163–182.

Postma, D., 1985. Concentration of Mn and separation from Fe in sediments – I.Kinetics and stoichiometry of the reaction between birnessite and dissolvedFe(II) at 10 C. Geochim. Cosmochim. Acta 49, 1023–1033.

Postma, D., Appelo, C.A.J., 2000. Reduction of Mn-oxides by ferrous iron in a flowsystem: column experiment and reactive transport modelling. Geochim.Cosmochim. Acta 64, 1237–1247.

Postma, D., Boesen, C., Kristiansen, H., Larsen, F., 1991. Nitrate reduction in anunconfined sandy aquifer: water chemistry, reduction processes, andgeochemical modeling. Water Resour. Res. 27, 2027–2045.

Prommer, H., Stuyfzand, P., 2005. Identification of temperature dependent waterquality changes during a deep well injection experiment in a pyritic aquifer.Environ. Sci. Technol. 36, 2200–2209.

Stollenwerk, K.G., 1994. Geochemical interactions between constituents in acidicgroundwater and alluvium in an aquifer near Globe, Arizona. Appl. Geochem. 9,353–369.

Stookey, L.L., 1970. Ferrozine – a new spectrophotometric reagent for iron. Anal.Chem. 42, 779–781.

Stumm, W., Morgan, J.J, 1996. Aquatic Chemistry. Chemical Equilibria and Rates inNatural Waters, third ed. Wiley and Sons.

Sullivan, L.A., Bush, R.T., McConchie, D.M., 2000. A modified chromium reduciblesulphur method for reduced inorganic sulphur: optimum reaction time for acidsulphate soil. Aust. J. Soil Res. 38, 729–734.

Taylor, B.E., Wheeler, M.C., Nordstrom, D.K., 1984. Stable isotope geochemistry ofacid mine drainage: experimental oxidation of pyrite. Geochim. Cosmochim.Acta 48, 2669–2678.

Tesoriero, A.J., Liebscher, H., Cox, S.E., 2000. Mechanism and rate of denitrification inan agricultural watershed: electron and mass balance along flow paths. WaterResour. Res. 36, 1545–1559.

Tonkin, J.W., Balistrieri, L.S., Murray, J.W., 2004. Modeling sorption of divalent metalcations on hydrous manganese oxide using the diffuse double layer model.Appl. Geochem. 19, 29–53.

van Breukelen, B.M., Appelo, C.A.J., Olsthoorn, T.N., 1998. Hydrogeochemicaltransport modelling of 24 years of Rhine water infiltration in the dunes of theAmsterdam Water Supply. J. Hydrol. 209, 281–296.

Witczak, S., Szklarczyk, T., Kmiecik, E., Szczepanska, J., Zuber, A., Ró _zanski, K.,Dulinski, M., 2007. Hydrodynamic modelling, environmental tracers andhydrochemistry of a confined sandy aquifer (Kedzierzyn-GłubczyceSubtrough, SW Poland). Geol. Quart. 51 (1), 1–16.