Sulfide oxidation and sulfate reduction in a shallow groundwater system (Oderbruch Aquifer, Germany)

Sulfide oxidation and sulfate reduction in a shallow groundwater

system (Oderbruch Aquifer, Germany)

G. Massmanna,*, M. Tichomirowab,1, C. Merzc,2, A. Pekdegera

aDeptartment Of Earth Sciences, Institute for Geological Sciences, Free University of Berlin, Malteserstr. 74-100, 12249 Berlin, GermanybTechnical University Bergakademie Freiberg, Institute of Mineralogy, Brennhausgasse 14, D-09599 Freiberg, Germany

cCentre for Agricultural Landscape and Land Use Research (ZALF e.V.), Muncheberg, Germany

Received 18 June 2002; accepted 14 April 2003

Abstract

Detailed groundwater monitoring was carried out over a period of two years in an anoxic, river recharged aquifer of the

Oderbruch polder, north-eastern Germany. Isotope data from wells located in a 5 km transect along the flow direction was used

to determine sources and sinks of SO224 in the aquifer. The SO22

4 originates from river water infiltration and from oxidative

dissolution of FeS2 within the alluvial loam covering the aquifer sands. A change of confined hydraulic conditions near the river

to unconfined conditions in the central polder effects the hydrochemistry of the aquifer. The confined areas are dominated by

sulfate reduction. Increasing d34S2SO4values suggest continuous but slow (t1=2 ¼ 50 years) sulfate reduction from the beginning

of inflow onwards with d34S2SO4values ranging from þ1.8 to þ44.7‰ versus CDT and an enrichment factor of 233‰. A zone

with a strong sulfate depletion (d34S2SO4of up to þ85.7‰) exists in a shallow microenvironment rich in solid-phase organic

carbon between river and levee. In the unconfined areas of the central polder, a SO224 plume with concentrations exceeding the

original river water content indicates FeS2 oxidation by O2 and/or NO23 within the alluvial loam. The lowered d34S2SO4

value

reflects the input of the isotopically lighter SO224 from the sulfide.

q 2003 Elsevier Science B.V. All rights reserved.

Keywords: Groundwater; Geochemistry; Bank filtration; Sulfur; Stable isotopes; Sulfate reduction; Sulfide oxidation

1. Introduction

Along the river banks of the Oderbruch, a major

part of the groundwater originates from infiltration of

river water into the aquifer. The river as well as

recharge from the arable land are the two principal

sources for solute input into the groundwater. Apart

from the direct input from these two sources, SO224

might be formed by oxidation of reduced sulfur

compounds within the overlying alluvial loam. Since

hydrochemical conditions within the aquifer are

generally reducing, SO224 reduction is likely to be

the major SO224 sink.

Stable isotope analysis of 34S and 18O of has been

used for decades to identify and quantify bacterial

reduction and oxidation of sulfur-species in the field.

0022-1694/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0022-1694(03)00153-7

Journal of Hydrology 278 (2003) 231–243

www.elsevier.com/locate/jhydrol

1 Fax: þ49-3731-393129.2 Fax: þ49-3343-282301.

* Corresponding author. Fax: þ49-30-838-707-42.

E-mail addresses: [email protected] (G.

Massmann), [email protected] (M. Tichomirowa),

[email protected] (C. Merz).

http://www.elsevier.com/locate/jhydrol

The reduction of SO224 is catalysed by bacteria of the

genus Desulfovibrio and others (e.g. Jørgensen, 1982),

producing dissolved sulfide and mineralised carbon

according to the generalised reaction:

2CH2O þ SO224 ! 2HCO2

3 þ H2S ð1Þ

The major part of the H2S produced will react with

dissolved Fe2þ or Fe(III)-oxyhydroxides to form

Fe-sulfide minerals (Appelo and Postma, 1996).

Pursuant to reaction (1), ongoing SO224 reduction

can be recognised by the presence of dissolved sulfide,

a decrease in organic carbon and SO224 content or an

increase in HCO23 : All of these factors can, however,

be influenced by many other processes such as organic

carbon reduction by other electron acceptors, FeS2

precipitation or mixing of different water types. Stable

sulfur isotopes are therefore a useful additional tool to

trace SO224 reduction. It has long been recognised that

reaction (1) is isotopically competitive due to the

preferential consumption of the lighter isotopes

(32S and 16O). It leads to the formation of H2S and

HCO23 depleted in 34S and 18O relative to the original

SO224 : Consequently, the residual SO22

4 fraction will

become heavier, i.e. enriched in 34S and 18O (e.g.

Harrison and Thode, 1958; Kaplan and Rittenberg,

1964; Nakai and Jensen, 1964; Lloyd, 1968; Rees,

1973).

Pyrite oxidation has been the subject of many

studies (e.g. summarised in Nordstrom, 1982 or

Lowson, 1982). Reactions are microbially catalysed

and proceed in several intermediate steps. They can be

summarised as:

FeS2 þ15=4O2 þ7=2H2O!FeðOHÞ3 þ2SO224 þ4Hþ

ð2Þ

Instead of oxygen, nitrate may serve as the electron

acceptor (Kolle et al., 1983; Strebel and Bottcher,

1985; van Beek et al., 1988; Postma et al., 1991;

Engesgaard and Kipp, 1992):

14NO23 þ5FeS2 þ4Hþ!7N2 þ10SO22

4

þ5Fe2þþ2H2O ð3Þ

Various authors have studied isotope fractionation

processes during pyrite oxidation. A review of sulfur

and oxygen isotope composition in both field and

laboratory studies of sulfide oxidation was given by

Toran and Harris (1989) who found that sulfur

isotopes may or may not be fractionated, depending

on the bacterial species and the environment of

oxidation. Fractionation effects are represented as

DSO42S (d34S of sulfate minus d34S of sulfide).

However, in most cases the sulfur isotopic compo-

sition of SO224 resembled the one of the sulfidic sulfur

source. Little or no fractionation during sulfide

oxidation was, for example, reported by Dechow

(1960), Gavelin et al. (1960), Field (1966), Steiner

and Rafter (1966), Taylor et al. (1984) and Strebel

et al. (1990).

The objective of this study was to evaluate the

different sources and sinks of SO224 in the Oderbruch

aquifer system using stable isotopes. The aim was to

prove the assumption that both SO224 reduction within

the aquifer and FeS2 oxidation within the alluvial

loam control the concentration of SO224 in the

groundwater (Massmann et al.,2003b). A suitable

tool appeared to be the analysis of stable isotopes of

SO224 from water sampled at selected wells along a

transect of 5 km length in flow direction (Fig. 1).

2. Study area

The Oderbruch polder is currently the subject of

detailed hydraulic and hydrogeochemical studies. It

is a large polder area in north-eastern Germany

bordered by high-plains in the west and the river

Oder to the east (Fig. 1). Until drainage measures

were initiated about 250 years ago, the region was a

swampy river polder. Nowadays, a levee bordering

the river prevents the flooding of the area and a net

of drainage ditches allows agricultural use of the

fertile loamy soils. Along the river banks, the steep

hydraulic gradient between river and groundwater

leads to infiltration of river water into the shallow

aquifer. The aquifer is of Pleistocene glacio-fluvial

origin and consists of fine to medium sized sands

that tend to get coarser towards the aquifer’s base.

The sands are underlain by a glacial till and topped

by an alluvial loam, which is the relict of regular

flooding during the Holocene. The flow field has

been described in detail with a 3D hydraulic model

(Massmann, 2003). Fig. 2 illustrates the major

hydrological units, the location of the filter screens

along the projection line at Bahnbrucke (Fig. 1) and

G. Massmann et al. / Journal of Hydrology 278 (2003) 231–243232

exemplary flow paths as given by the hydraulic

model. Near the river, the groundwater is confined.

Further towards the central polder, the hydraulic

conditions change to being clearly unconfined and

the water level drops well below the bottom of the

alluvial loam.

A large scale redox sequence proceeding laterally

from oxygen and nitrate consumption towards

sulfate reduction, characterised by the appearance

or disappearance of redox reactants, was identified

with inflow (Massmann et al., 2003a). Inland of the

sulfate reduction zone, the changing hydraulic

conditions (unconfined groundwater) lead to an

enhanced geochemical influence of the seepage

water onto the groundwater. The amount of recharge

is generally comparatively low (50–70 mm a21 at

maximum) and inhibited by the impermeable loam.

However, in the central polder, decelerated flow

velocities, ploughing of the crops and structure

changes (shrinking fissures) in the drained alluvial

loam facilitate vertical flow (Massmann et al.,

2003b).

3. Methods

Water samples were analysed for SO224 by ion

chromatography (Dionex DX 500) while S22 (sum of

H2S and HS2) was measured by photometry (Uvicon

931 Kontron). The sampling procedure as well as

methods of analysis for field parameters and remain-

ing standard ions have been described in detail in

Massmann et al. (2003b).

Water was collected for isotopic analysis from the

river, the main drainage ditch and 12 groundwater

piezometers (six locations with 1 shallow and 1 deep

piezometer each, Fig. 1, Table 1). Between 1 and 20 l

of sample were needed, depending on the individual

SO224 content. Samples were filtered with 0.45 mm

membrane filters to avoid impurities. 2–3 ml Of HCl

Fig. 1. Location of the Oderbruch polder with wells sampled for isotope analysis. Well locations for geochemical analysis indicated.

G. Massmann et al. / Journal of Hydrology 278 (2003) 231–243 233

were added and SO224 was precipitated as BaSO4 in

the laboratory using the reagent BaCl2. The BaSO4

was caught on a filter and freed of the chloride content

by rinsing with warm water. The filtered BaSO4 was

split in samples for d18O and d34S analysis and

subsequently dried at 30 and 80 8C, respectively.

All isotope results are reported as d34S and d18O

values in per mil (‰, Table 1), where:

dð‰Þ ¼ ½ðRsample=RstandardÞ2 1� £ 1000 ð4Þ

for d34S or d18O, R ¼ 34S/32S or 18O/16O, respectively

(Hoefs, 1997).

d34S analyses of BaSO4 and sulfides were

performed after conversion to SO2 in the presence

of V2O5 and SiO2 (Yanagisawa and Sakai, 1983) and

subsequent determination of the sulfur isotope

composition using a Finnigan MAT Delta E mass

spectrometer at the isotope laboratory of the Institute

of Mineralogy in Freiberg. The reproducibility of the

d34S analysis is given in Table 1 (preparation of

SO2 þ mass spectrometer analysis). For routine

measurements an internal standard (SO2), which is

calibrated against the international IAEA-standard

NBS 127, was used. Isotope values are reported

relative to the international standard CDT.

d18O values of BaSO4 were determined on CO2

derived through reaction of BaSO4 with graphite at a

high temperature (Rafter, 1967). Simultaneously

generated CO was converted to CO2 on a Ni-catalyst

at 350 8C. After the complete conversion of CO to

CO2 all CO2 gas was collected and analysed on a

Finnigan MAT Delta plus mass spectrometer. As

internal standard a synthetic BaSO4 with a d18O value

of 12.95‰ calibrated against the international IAEA

standard NBS 19 was used. The reproducibility of the

d18O analysis of BaSO4 is usually better than ^0.2‰

(Table 1). Isotope values are reported relative to the

international standard SMOW.

In addition to the data of the newly built piezo-

meter wells, isotopic values given by Schuring et al.

(2000) were included in some figures. Schuring et al.

(2000) investigated the shallow transect on the flood

plain close to the river, while the present data was

obtained when additional deeper and shallow ground-

water piezometers had been built in a much wider area

behind the levee (Figs. 1 and 2). The time gap

between the present sampling campaign in summer

Fig. 2. Cross section of the field-site Bahnbrucke, filter screens of wells along projection line shown, flow-paths and major hydrological units

indicated (Massmann et al.,2003a).


2001 and the earlier campaign of Schuring et al.

(2000) is irrelevant, because the time series for SO224

are very stable for all wells inland of the ditch (Fig. 3)

and variations are very limited between river and

ditch. But even though the SO224 concentrations might

differ slightly, the general tendencies stay the same.

Sulfide was extracted as acid volatile sulfide (AVS)

using HCl and as pyrite using a Cr(III)Cl solution as

extraction agent. (Cornwell and Morse, 1987; Hsieh

and Yang, 1989; Gagnon et al., 1995). The extraction

with Cr(II)Cl includes pyrite bound S as well as AVS

phases (Canfield et al., 1986). In contrast to

conventional methods, the H2S degassing in the acid

environment was not blown out but transported into

the Zn-acetate trap by passive diffusion over 48 h

(Hsieh and Yang, 1989). After the precipitation of

ZnS in the alkaline solution (pH13), the amount of

sulfide was measured iodometrically (Hsieh and

Yang, 1989; Gagnon et al., 1995). Total carbon and

inorganic carbon content of the sediments were

Table 1

Geographical information of piezometer wells sampled for isotope analysis, mean SO224 -concentration with standard deviations (n between 2

and 9, depending on installation date), f of initial SO224 remaining and isotope data with associated measurement error

Piezometer no. Position Filter depth

(m rel.To

sea-level)

Ground-

water agea

(days)

SO224 f ðSO22

4 Þ d34S

(‰ versus

CDT)

Error ^ d18O

(‰ versus

SMOW)

Error

^

x-Coord. y-Coord. From To mmol l21 ^

River Oder 5447585 5853819 – – – 0.980 0.081 1.000 1.8 0.5 8.4 0.2

Drainage ditch 5447489 5853604 – – – 0.922 0.023 0.941 – – 10.1 0.2

Shallow wells:

9536F 5447413 5853689 21.9 22.9 1179 0.983 0.098 1.003 4.3 0.2 – –

11/99F 5447153 5853619 23.6 24.6 2330 1.018 0.080 1.039 – – –

9561F 5447239 5853217 21.9 22.9 8475 0.985 0.027 1.006 8.5 0.2 13.1 1.5

1/01F 5446326 5852049 23.0 24.0 12834 1.208 0.086 1.232 11.1 0.2 – –

2144F 5445225 5851225 23.8 24.8 50604 0.099 0.006 1.232 11.1 0.2 – –

2/01F 5444489 5850611 22.0 24.0 46178 2.492 0.119 2.543 5.2 0.7 – –

955F 5444050 5850050 21.5 22.5 49873 3.500 0.358 3.572 3.2 0.4 – –

Deep wells:

2/99T 5446750 5853425 214.9 216.9 1458 0.953 0.056 0.972 – – – –

6/99T 5447413 5853689 212.9 214.9 3213 0.951 0.062 0.971 2.4 0.2 10.2 0.2

11/99T 5447404 5853508 212.2 214.2 1161 0.947 0.067 0.966 – – – –

9560T 5447239 5853217 215.8 217.8 4000 0.981 0.033 1.002 5.5 0.2 10.4 0.2

1/01T 5446326 5852049 216.0 218.0 5834 1.135 0.004 1.158 7.3 1.2 – –

2144T 5445225 5851225 216.7 218.7 24955 0.284 0.003 0.290 44.7 0.3 – –

2/01T 5444489 5850611 215.0 217.0 – 1.715 0.004 1.750 10.2 0.2 – –

955T 5444050 5850050 27.5 29.5 – 2.985 0.311 3.046 3 0.6 – –

ShallowTransect near river:

9531F 5447539 5853757 3.8 1.8 20 0.668 0.291 0.682 52.5a – – –

9534F 5447528 5853751 3.8 1.8 27 0.095 0.040 0.096 85.7a – – –

2a 3.2 1.2 37 0.817 0.189 0.834 15a – – –

9531T 5447539 5853757 21.3 23.3 168 0.878 0.049 0.896 3a – – –

9534T 5447528 5853751 20.3 22.3 188 0.674 0.037 0.688 7.5a – – –

a Data by Schuring et al. (2000)

Fig. 3. Time series for SO224 in groundwater piezometers 955F

(shallow) and 955T (deep).


measured with a C–N–S Analyser (Leco) and the

organic carbon content was subsequently calculated.

4. Results and discussion

4.1. Sulfate distribution, sources and sinks

Sulfate concentrations of ground- and river water

as well as the accompanying isotopic composition of

SO224 are given in Table 1. SO22

4 is brought into the

aquifer with the river water. The concentration of

SO224 in the river varied only slightly (between 0.9

and 1.2 mmol l21) over the time-span of sampling

(2 years), mainly as a consequence of water level

fluctuations. The isotopic composition of the river

SO224 was found to be þ1.8‰ for d34S and þ8.4 for

d18O at the time of sampling. It is possible that the

sulfate sources and isotopic composition in the river

may vary with time even though the sulfate content

itself is fairly constant. However, regarding the

combined data set of river and groundwater it

becomes clear that the river represents an end-

member in terms of the sulfate content and isotopic

composition and variations were assumed to be rather

small.

The main potential sulfate sink in the Oderbruch

aquifer is sulfate reduction. The SO224 content of the

groundwater at the field-site Bahnbrucke, illustrated

in Fig. 4, lies within the range observed in the river

water (about 1 mmol l21 of SO224 ). In contrast, a

sudden drop in Eh, O2 and NO23 content plus a gradual

increase of Mn2þ and a little later Fe2þ in solution

(compare Massmann et al., 2003a) indicate anoxic,

reducing groundwater conditions. Even though the

groundwater does not show any sign of SO224

depletion at Bahnbrucke, traces of sulfide were

detected and a faint odour of H2S was occasionally

noted during sampling. Despite the fact that the error

associated with the S22 measurement is relatively

large at these low concentrations, the sulfide content

clearly increases along individual flow-paths (Fig. 5).

Both the d18O and d34S signature of the groundwater-

SO224 reflect a shift towards heavier residual SO22

4

with travel distance (dark boxes in Figs. 4 and 5). The

presence of sulfide in the groundwater as well as its

isotopic signature of SO224 clarify that biogenic SO22

4

reduction does take place at the field-site Bahnbrucke,

even though the SO224 content of the groundwater is

not obviously affected.

In terms of groundwater sulfate and sulfide content,

the shallow wells between river and levee, which were

Fig. 4. d34S values of SO224 from the river and five groundwater wells (9536F, 6/99T, 9561F, 9560T and *9534F (value given by Schuring et al.,

2000)); grey background colours show SO224 concentrations in January 2000; crosses represent wells sampled for geochemical analysis. Boxes

with isotope data are placed at the location of the corresponding filter screen.


placed in a local environment rich in Corg (see

explanation below), represent an exception (Figs. 4

and 5). Here, the SO224 content of the groundwater

locally drops to less than 0.09 mmol l21 (9534F)

while accompanying S22 concentrations are high

(0.02 mmol l21). Likewise, the d34S signature of the

SO224 is very different to that observed throughout the

remaining aquifer. Schuring et al. (2000) measured

d34S2SO4values as high as þ85.7‰ in the very

shallow piezometers and slightly increased values

(þ3 and þ7.5‰) in the shallow wells (Table 1).

With increasing distance to the river, the vertical

flow component initiated by the drainage ditches

behind the levee (Fig. 2), loses importance. Hence,

wells along the projection line can be considered as

lying more or less in flow direction. Regarding the

sulfate and sulfide development in the groundwater in

combination with the d34S2SO4signature on a larger

scale (Fig. 6), the SO224 reduction clearly continues in

a similar manner as near the river (at the field-site

Bahnbrucke): Continuous increase of S22 and

d34S2SO4without any accompanying decrease in the

SO224 content in shallow and deep groundwater wells.

Lowest SO4 concentrations and highest d34SSO4

values are observed at a distance of about 3500 m

from the river where the alluvial loam is particularly

thick (.3 m) and any percolation of water through

the loam impossible. Sulfur isotope composition

indicates the highest degree of SO224 reduction at

this location, where almost the entire sulfate amount

from the groundwater is removed.

Further inland in conditions of an unconfined

aquifer, the increasing sulfate contents are caused by

an additional SO224 input through the unsaturated

zone. The parting line between confined and uncon-

fined hydraulic conditions was added to Fig. 6. Where

the groundwater is unconfined, the formerly hydrous

alluvial loam has been strongly drained and resulting

morphological texture changes, such as shrinking

fissures, enable a preferential and increased down-

ward flow, at least where the loam has a limited

thickness. In addition, flow velocities are much lower

in the central polder as compared to nearer to the river.

The groundwater shows a layering in terms of SO224

content, although there is no dividing impermeable

layer in between the filter screens. The shallow wells

contain about 1/3 more SO224 than the deeper wells.

The values were very stable over the time span of

sampling (Fig. 3). This indicates that the additional

SO224 source is located in the upper sediment layers.

The input of SO224 ; besides other water constituents, is

therefore largely dependent on the hydraulic situation

Fig. 5. d18O values of SO224 from the river, ditch and 3 groundwater wells (9536F, 6/99T, 9561F and 9560T; grey colours in the background

show S22concentrations in January 2000; crosses represent wells sampled for geochemical analysis. Boxes with isotope data are placed at the

location of the corresponding filter screen.


(Massmann et al., 2003b). In the unconfined aquifer

zone (increased sulfate concentrations), d34S values of

groundwater sulfate decrease with lower d34SSO4

values in the shallow wells, pointing out to the fact

that ‘lighter’ SO224 is added from above. The d34SSO4

value of the reduced precursor within the loam should

be lower than 5‰ (Fig. 6), because no or only little

isotopic fractionation occurs during sulfide oxidation

(Field, 1966; Steiner and Rafter, 1966; Taylor et al.,

1984). Since the groundwater contained hardly any

SO224 in the previous zone, the major part of the SO22

4

originates from oxidative dissolution of Fe(II)-sulfide

(mainly pyrite) within the fine-grained, organic rich

alluvial loam by either oxygen and/or nitrate. Nitrate

is likely to be an important electron acceptor, since

more than 90% of the polder area are used for

agricultural purposes. Pore water analysis of 12 cores

near well 955 revealed an increase of SO224 within the

upper soil meter and an accompanying drop of pH as

well as a decrease of NO23 ; if present (Massmann,

1998). Nevertheless, SO224 reduction in the deeper

aquifer parts continues as indicated by S22 contents.

The high pyrite content (0.2–0.7 g kg21) of the

deeper sediment layers measured in a core in

Fig. 6. Average SO224 and S22 concentrations, bars indicate standard deviations (2–9 sampling campaigns, depending on installation date of

well), grey area shows river water concentrations over sampling period; d34S values of SO224 from the river and 12 groundwater wells (numbers

given above) at six locations along projection line given in Fig. 1, bars indicate error associated with measurement.


the central polder confirms that pyrite formation

might occur simultaneously, serving as an additional

SO224 sink.

4.2. Sulfur isotopic behaviour during sulfate

reduction

Under closed system conditions, i.e. when no

additional SO224 is added during SO22

4 reduction, the

reaction follows a Rayleigh-type distillation (Fritz and

Fontes, 1980) and the following equation is valid:

dR ¼ dI þ 1pln f ð5Þ

where dR and dI are the residual and initial d34S values

of SO224 ; f is the fraction of initial SO22

4 remaining

and 1 is the isotopic enrichment factor. Within the

present study, the mean SO224 concentration of the

river Oder ðn ¼ 9Þ represents the initial concentration

used for the calculation of f ðSO224 Þ values and the

accompanying d34S value of SO224 is equivalent to dI :

After Strebel et al. (1990) the enrichment factor 1 can

be determined by plotting d34S of SO224 versus f

ðSO224 Þ on a semi-log plot where 1 is the slope of the

regression line. For the area of the confined aquifer,

this procedure yields an enrichment factor of -33.1‰

(Fig. 7). Data from the area of the unconfined aquifer

in the central polder was not included in Fig. 7,

because it is certain that the system is not a closed

system in terms of SO224 content and the river is not

the exclusive SO224 source. Hence, f ðSO22

4 Þ and dI are

unknown for this region and would strongly differ

from data obtained closer to the river. The regression

line in Fig. 7 shows a reasonable correlation with a

coefficient of determination, r2; of 0.81 ðn ¼ 14Þ:

Samples with highest sulfur isotopic values have

lowest remaining sulfate concentrations and, there-

fore, represent locations with highest reaction pro-

gress (or degree of sulfate reduction). Deviations from

the regression line of Fig. 7 indicate that the aquifer is

not a perfectly closed system. Occasionally, samples

contain slightly more sulfate than the Oder, reflecting

local additional sulfate input from a second source.

This is also responsible for the shift of the regression

line above the river source.

Fig. 7. d34S of SO224 versus fraction ðf Þ of initial SO22

4 remaining (river).


Compared to other studies, the regression line is

rather steep, i.e. the absolute value of 1 is rather high

(233‰). Enrichment factors of 28 to 290‰ have

been reported (e.g. Harrison and Thode, 1958; Kaplan

and Rittenberg, 1964),. Factors between 230 and

260‰ are typical for marine environments with very

high nutrient and SO224 concentrations (Chambers and

Trudinger, 1979), conditions that are clearly different

from those present within the Oderbruch aquifer.

Studies performed in groundwater environments

yielded smaller enrichment factors of 218‰

(Basharmal, 1985, reduction of landfill leachate

derived SO224 ; Borden, Ontario), 215.5‰ (Robertson

and Schiff, 1994, reduction of SO224 below a sandy

forested recharge area, Sturgeon Falls, Ontario) or

29.7‰ (Strebel et al., 1990, reduction of atmospheric

and mineralogical SO224 within the Fuhrberger Feld

aquifer, Germany).

Rye et al. (1981); Bottrel et al. (2000) reported that

the isotopic fractionation during sulfate reduction

(given as D34S ¼ d34SSO4 2 d34SHS) increases as the

rate of reduction decreases and large fractionation

factors (.60‰) are associated to slow reduction

rates. Supposably, the large enrichment factor in the

present study may also be the result of a particularly

slow reduction process.

4.3. Sulfate reduction rate

Since redox processes are microbially catalysed,

availability and consumability of organic carbon

controls the reduction rates. In case of the Oderbruch

aquifer, the reaction rate of SO224 reduction varies

strongly with depth. Combining hydraulic and hydro-

geological considerations, the wells sampled for

isotopic analysis can be subdivided into three zones

with very different SO224 reduction rates, best

illustrated in the plot of age (equivalent to travel

time from the river to the filter screen) versus

d34S 2 SO4 on a double logarithmic plot (Fig. 8.).

The approximate age was derived from the hydraulic

model (Massmann, 2003). A semi-log plot of age

versus SO224 content (Fig. 9) reveals linear decay

curves for each well group, indicating that the

reduction is a first order decay process typical for

many biologically mediated processes. On such a plot,

the slope of each fit represents the half-life of the

reduction process. In the case of the very shallow

wells, the half-life is approximately 5 days, for the

shallow wells near the river 423 days and for the

confined glacio-fluvial aquifer (own data, shallow and

deeper wells) 18550 days (50 years). The number of

data points for each group (4, 3 and 11 points,

including river) is sufficient to give statistically

significant results ðr2 ¼ 0:89Þ only in the latter case.

Some uncertainties exist concerning the age of the

water reaching the very shallow wells. It is not clear

Fig. 8. Age derived from travel times from hydraulic model

(porosity ¼ 0.3) versus d34S of SO224 : Approximate Corg content of

the sediment indicated. In case of deeper aquifer data, mean value of

n ¼ 78 samples taken. For symbol legend see Fig. 7.

Fig. 9. Age derived from travel times from hydraulic model

(porosity ¼ 0.3) versus SO224 : For symbol legend see Fig. 7.


whether the loam continues below the levee (Fig. 2).

If it does, the groundwater flow above the loam

between river and levee might be stagnant and,

consequently, the water might be older than indicated

by the model. However, even though the data set is

very limited and the quality of the plot is therefore

rather poor, (Fig. 9) the general tendencies and

differences become quite clear with the reaction

half-life being in the order of magnitude of days to

months to decades from very shallow to deep wells.

As shown in Fig. 8, the reason for the reduction

rates differing over several orders of magnitude lie

within the different organic carbon contents of

sediments. Whereas the glacio-fluvial aquifer contains

little Corg (,0.1 wt.% Corg, mean value of 78 samples

throughout the entire observation area), both the

shallow and particularly the very shallow wells near

the river were placed in or near a local microenviron-

ment with a much higher sedimentary bound Corg

content (compare loam in Fig. 2). Therefore DOC

concentrations are even exceeding the DOC content

of the river (0.57 mmol l21) despite the commencing

reduction processes. The rate of SO224 reduction of the

aquifer ðt1=2 ¼ 50 years) lies in between those

reported in Canada (Robertson and Schiff, 1994)

with t1=2 ¼ 3 years and Germany (Strebel et al., 1990)

with t1=2 ¼ 75–100 years. At Sturgeon Falls (Robert-

son and Schiff, 1994), the sediment properties of the

homogeneous silty fine sands (Corg ¼ 0.03 wt.%)

were different and SO224 concentrations were much

lower (,0.3 mmol l21). The Fuhrberger Feld aquifer

(Strebel et al., 1990), like the Oderbruch aquifer, was

formed during the Pleistocene ice ages and the sands

and gravel (0.1–1 wt.% Corg) resemble those inves-

tigated in the present study. It seems that the similar

origin and consequently the similar composition of

the Corg content, rather than the total amount present,

determines the rate of reduction when comparing

different aquifer systems.

5. Conclusions

On the basis of previous hydraulic and geo-

chemical evaluations, a set of piezometer wells was

chosen for isotopic analysis. With the help of this

limited but sufficient isotopic data set is was possible

to determine the sources and processes responsible for

the behaviour of sulfur, mainly present as SO224 in the

groundwater, in a natural aquifer system. Within the

Oderbruch aquifer, SO224 is derived from constant

river water infiltration and from oxidative dissolution

of pyrite in the unsaturated zone. The main SO224 sink

is SO224 reduction and possibly mineral precipitation.

While both SO224 production and removal might take

place throughout the entire observation area, zones

with predominance of one or the other process could

be determined. In the plot of d34S2SO4versus SO22

4

(Fig. 10) those areas with domination of SO224

reduction and those with domination of additional

SO224 input due to pyrite oxidation can be differ-

entiated. The SO224 input is only exceeding the

amount of SO224 reduction in the strongly unconfined

areas due to a combination of several factors coming

into effect (soil texture changes, low flow velocities,

extensive application of N-fertilisers). In contrast, the

SO224 removal by reduction is only exceeding the

SO224 production in a local micro-environment rich in

solid-phase organic carbon near the river and at a

location in 3.5 km river distance (2144F þ T) where

the alluvial loam is particularly thick (.3 m) and any

percolation of water through the loam impossible.

A similar interaction between SO224 in- and output

has been described by Brown et al. (2000), for a

coastal plain aquifer. In the case of the Magothy

aquifer, the SO224 reduction is outweighed by SO22

4

Fig. 10. SO224 concentrations versus d34S of SO22

4 : For symbol

legend see Fig. 7.


gain through diffusion from sediments and through

oxidation of FeS2 throughout the entire observation

area. Accordingly, the occurrence of SO224 reduction

could only be proven by isotopic evaluation since

SO224 concentrations increased in flow direction.

Likewise, in the case of the Fuhrberger Feld aquifer

(Strebel et al., 1990) it was impossible to document

SO224 reduction on the basis of SO22

4 concentration

data only. This was due to landuse specific SO224 and

NO23 input- and SO22

4 formation variations both in

space and time. Similarly, in the present case, in some

areas where SO224 concentrations remained constant,

SO224 reduction could only be identified by isotopic

evaluation. The very clear differentiation of the zone

dominated by pyrite oxidation on the large scale was

only possible because of the particular hydrogeologi-

cal situation with the impermeable alluvial loam

strongly restricting recharge to zones with a dried,

ploughed or very thin loam nature.

Acknowledgements

This project was part of the Priority Program 546

“Geochemical processes with long-term effects in

anthropogenically-affected seepage- and ground-

water”. Financial support was provided by the Ger-

man Research Organization (DFG). We would also

like to thank M. Junghans, H. Meinhardt and

R. Liebscher from the University of Freiberg for the

analysis of the isotope samples.

References

Appelo, C.A.J., Postma, D., 1996. Geochemistry, Groundwater and

Pollution, Balkema, Rotterdam.

Basharmal, G.M., 1985. Isotopic composition of aqueous sulphur at

landfill sites. MSc Thesis, University of Waterloo, Waterloo,

172 pp

van Beek, C.G.E.M., Boukes, H., van Rijsbergen, D., Straatman, R.,

1988. The threat of the Netherlands waterworks by nitrate in the

abstracted groundwater, as demonstrated on the well field

Vierlingsbeek. Water Supply 6, 313–318.

Bottrel, S.H., Moncaster, S.J., Tellam, J.H., Lloyd, J.W., Fisher,

Q.J., Newton, R.J., 2000. Controls of bacterial sulphate

reduction in a dual porosity aquifer system: the Lincolnshire

Limestone aquifer. Eng. Chem. Geol. 169 (3–4), 461–470.

Brown, C.J., Schoonen, M.A.A., Candela, J.L., 2000. Geochemical

modeling of iron, sulfur, oxygen, and carbon in a coastal plain

aquifer. J. Hydrol. 237, 147–168.

Canfield, D.E., Raiswell, R., Westrich, J.T., Reaves, C.M., Berner,

R., 1986. The use of chromium reduction in the analysis of

reduced inorganic sulfur in sediments and shales. Chem. Geol.

54, 149–155.

Chambers, L.A., Trudinger, P.A., 1979. Microbiological fraction-

ation of stable sulfur isotopes: a review and critique.

Geomicrobiol. J. 1, 249–293.

Cornwell, J.C., Morse, J.W., 1987. The characterization of iron

sulfide minerals in anoxic marine sediments. Marine Chem. 22,

193–206.

Dechow, E., 1960. Geology, sulfur isotopes and the origin of the

Heath Steele ore deposits, Newcastle, NB, Canada. Econ. Geol.

55, 539–556.

Engesgaard, P., Kipp, K.L., 1992. A geochemical transport model

for redox-controlled movement of mineral fronts in ground-

water flow systems: a case of nitrate removal by oxidation of

pyrite. Water Resour. Res. 28 (10), 2829–2843.

Field, C.W., 1966. Sulfur isotope method for discriminating

between sulfates of hypogene and supergene origin. Econ.

Geol. 61, 1428–1435.

Fritz, P., 1980. Introduction. In: Fritz, P., Fontes, J.Ch. (Eds.),

Handbook of Environmental Isotope Geochemistry, vol. 1.

Elsevier, Amsterdam, pp. 1–19.

Gagnon, C., Mucci, A., Pelletier, E., 1995. Anomalous accumu-

lation of acid-volatile sulphides (AVS) in a coastal marine

sediment, Saguenay Fjord, Canada. Geochim. Cosmochim. Acta

59, 2663–2675.

Gavelin, S., Parwel, A., Ryhage, R., 1960. Sulfur isotope

fractionation in sulfide mineralization. Econ. Geol. 55,

510–530.

Harrison, A.G., Thode, H.G., 1958. Mechanism of the bacterial

reduction of sulphate from isotope fractionation studies. Trans.

Faraday Soc. 54, 84–92.

Hoefs, J., 1997. Stable Isotope Geochemistry, Springer, Berlin,

201 p.

Hsieh, Y.P., Yang, C.H., 1989. Diffusion methods for the

determination of reduced inorganic sulfur species in sediments.

Limnol. Oceanogr. 34, 1126–1130.

Jørgensen, B.B., 1982. Ecology of the bacteria of the sulphur cycle

with special reference to anoxic-oxic interface environments.

Phil. Trans. R. Soc. Lond. B 298, 543–561.

Kaplan, I.R., Rittenberg, S.C., 1964. Microbiological fractionation

of sulphur isotopes. J. Gen. Microbiol. 34, 195–212.

Kolle, W., Werner, P., Strebel, O., Bottcher, J., 1983. Denitrifika-

tion in einem reduzierenden Grundwasserleiter. Vom Wasser

61, 125–147.

Lloyd, R.M., 1968. Oxygen isotope behaviour in the sulfate-water

system. J. Geophys. Res. 73 (18), 6099–6110.

Lowson, R.T., 1982. Aqueous oxidation of pyrite by molecular

oxygen. Chem. Rev. 82, 461–497.

Massmann, G., 1998. Der geochemische Einfluß des Sickerwassers

auf das flachere Grundwasser im nordlichen Oderbruch. Diplo-

marbeit im Fachbereich Geowissenschaften, Universitat Bremen

Massmann, G., 2003. Infiltration of river water into the ground-

water—Investigation and modelling of hydraulic and hydro-

chemical processes in the Oderbruch aquifer (eastern Germany).

PhD Thesis, Free University of Berlin, Berlin, in press


Massmann, G., Pekdeger, A., Merz, C., 2003a. Redox processes in a

river-recharged aquifer in the Oderbruch polder, Germany

(part1): lateral redox sequences during bank-filtration, sub-

mitted for publication

Massmann, G., Liedholz, T., Pekdeger, A., Merz, C., 2003b. Redox

processes in a river-recharged aquifer in the Oderbruch polder,

Germany (part 2): the geochemical influence of the seepage

water onto the groundwater in the polder center, submitted for

publication

Nakai, N., Jensen, M.L., 1964. The kinetic isotope effect in the

bacterial reduction and oxidation of sulfur. Geochim. Cosmo-

chim. Acta 28, 1893–1912.

Nordstrom, D.K., 1982. Aqueous pyrite oxidation and the

consequent formation of secondary iron minerals. Soil Sci.

Soc. Am., Spec. Publ. No 10, 37–56.

Postma, D., Boesen, C., Kristiansen, H., Larsen, F., 1991. Nitrate

reduction in an unconfined sandy aquifer: water chemistry,

reduction processes and geochemical modeling. Water Res. Res.

27 (8), 2027–2045.

Rafter, T.A., 1967. Oxygen isotopic composition of sulfates. Part 1:

a method for the extraction of oxygen and its quantitative

conversion to carbondioxide for isotopic ratio measurements.

New Zealand J. Sci. 10, 493–510.

Rees, C.E., 1973. A steady-state model for sulphur isotope

fractionation in bacterial reduction processes. Geochim. Cos-

mochim. Acta 37, 1141–1162.

Robertson, W.D., Schiff, S.L., 1994. Fractionation of sulphur

isotopes during biogenic sulphate reduction below a sandy

forested recharge area in south-central, Canada. J. Hydrol. 158,

123–134.

Rye, R.O., Back, W., Hanshaw, B.B., Rightmire, C.T., Pearson, F.J.,

1981. The origin and isotopic composition of dissolved sulfide

in groundwater from carbonate aquifers in Florida and Texas.

Geochim. Cosmochim. Acta 45, 1941–1950.

Schuring, J., Schulz, H.D., Fischer, W.R., Bottcher, J., Duijnisveld,

W.H.M. (Eds.), 2000. Redox fronts in aquifer systems and

parameters controlling their dimensions. Redox—Fundamen-

tals, Processes and Applications, Springer, Berlin.

Steiner, A., Rafter, T.A., 1966. Sulfur isotopes in pyrite, pyrrhotite,

alunite and anhydrite from steam wells in the Taupo volcanic

zone. NZ Econ. Geol. 61, 1115–1129.

Strebel, O., Bottcher, J., 1985. Einfluß von Bodennutzung und

Bodennutzungsanderungen auf die Stoffbilanz eines reduzier-

enden Aquifers im Einzugsgebiet eines Forderbrunnens. Wasser

Boden 3, 111–114.

Strebel, O., Bottcher, J., Fritz, P., 1990. Use of isotope fractionation

of sulfate-sulfur and sulfate-oxygen to asses bacterial desulfur-

ication in a sandy aquifer. J. Hydrol. 121, 155–172.

Taylor, B.E., Wheeler, M.C., Nordstrom, D.K., 1984. Isotope

composition of sulphate in acid mine drainage as measure of

bacterial oxidation. Nature 308 (5959), 538–541.

Toran, L., Harris, R.F., 1989. Interpretation of sulfur and oxygen

isotopes in biological and abiological sulfide oxidation.

Geochim. Cosmochim. Acta 53, 2341–2348.

Yanagisawa, F., Sakai, H., 1983. Preparation of SO2 for sulphur

isotope ratio measurements by the thermal decomposition of

BaSO4–V2O5–SiO2 mixtures. Anal. Chem. 55, 985–987.


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Sulfide oxidation and sulfate reduction in a shallow groundwater system (Oderbruch Aquifer, Germany)