Journal of Hydrology 365 (2009) 74–78
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Journal of Hydrology
journal homepage: www.elsevier.com/locate/jhydrol
Electron trapping capacity of dissolved oxygen and nitrate to evaluate Mn and
Fe reductive dissolution in alluvial aquifers during riverbank filtration
Monika A.M. Kedziorek, Alain C.M. Bourg *
Environmental Hydro Geochemistry, Dept. Geosciences, University of Pau, BP 1155, 64013 Pau Cedex, France
a r t i c l e
i n f o
Article history:
Received 5 March 2008
Received in revised form 12 November 2008
Accepted 14 November 2008
Keywords:
Alluvial aquifer
Riverbank filtration
Redox reactions
Organic matter
Reduced zone
s u m m a r y
Alluvial aquifers are largely used as groundwater resource since large flow rates can be obtained due to
infiltration from the neighboring river. Reductive conditions in the infiltration flow path, caused by the
degradation of organic matter, can induce effects detrimental to water quality, such as dissolution of
Mn and Fe from the aquifer sediments. In the absence of any direct relationship between the redox potential (Eh) and reductive conditions favorable to manganese and/or iron reduction, we propose a quantitative approach, the electron trapping capacity (ETC). It is calculated using dissolved O2 and NO3
concentrations in groundwater, weighted for the quantity of electrons these two species can trap during
the oxidation of organic matter. This approach, tested on several field and laboratory investigations, indicates that reductive dissolution of manganese and iron oxyhydroxides occurs for an ETC lower than
0.2 mmol L1. Exceptions to that threshold value are observed when Mn-rich groundwater flows too fast
out of a reduced zone into an oxidizing environment to permit equilibrium precipitation of Mn
oxyhydroxides.
Ó 2008 Elsevier B.V. All rights reserved.
Introduction
In Europe a large fraction of drinking water comes from well
fields in alluvial aquifers. The main advantages of this hydraulic
configuration are the proximity of demand areas, the easy access
to the aquifer (shallow water table), the high pumping rate that
can be obtained and the usually good water quality due to river
bank filtration (RBF) (e.g., Hiscock and Grischek, 2002). RBF includes several processes such as solid particle trapping, adsorption,
biodegradation, denitrification, which all improve water quality by
removing suspended particulate matter, trapping pollutants, inactivating viruses, degrading natural organic matter or other anthropogenic organic compounds infiltrated from the river (e.g.,
Schwarzenbach et al., 1983; Jacobs et al., 1988; Von Gunten et al.,
1991, 1994; Bourg and Bertin, 1993; Doussan et al., 1997; Grischeck et al., 1998; Bourg et al., 2002; Hiscock and Grischek,
2002; Ray et al., 2002). However, beneficial effect of RBF can be altered in the presence of a too intense bacterial activity due to the
infiltration of high quantities of organic matter (e.g., Ray et al.,
2002). Under such conditions, reductive dissolution of manganese
and/or iron oxyhydroxides naturally present in the sediments can
occur, leading to problems of water quality (e.g., Richard et al.,
1989; Anderson et al., 1998; Berbenni et al., 2000).
* Corresponding author. Tel.: +33 559 40 74 16; fax: +33 559 40 74 15.
E-mail address: [email protected] (A.C.M. Bourg).
0022-1694/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jhydrol.2008.11.020
The reductive dissolution of Mn and Fe oxides has been extensively studied (e.g., Lovley and Phillips, 1986; Jacobs et al., 1988;
Von Gunten et al., 1991 ,1994; Bourg and Bertin, 1993, 1994; Furrer
et al., 1996; Ludvigsen et al., 1998; Bourg et al., 2002; Petrunic
et al., 2005), and shown to be controlled by bacteria as part of their
energy extraction mechanism for cell growth (e.g., Lovley, 1991;
Cosovic et al., 1996; Hiscock and Grischek, 2002). In alluvial aquifers, redox reactions generally take place in groundwater between
organic matter and dissolved O2 and NO3 but, when more reducing
conditions prevail, other electron acceptors such as manganese or
iron oxyhydroxides, or even sulfate, might be used (Table 1). In
such situations bacteria identified as dissimilatory Fe(III)- or
Mn(IV)-reducing microorganisms (e.g., Shewanella putrefaciens,
Shewanella alga, Geobacter metallireducens, Desulfovibrio sp., etc.)
can develop and use manganese or iron oxyhydroxides as electron
acceptors (e.g., Beliaev and Saffarini, 1998; Dollhopf et al., 2000).
Many well fields in alluvial aquifers satisfy these conditions (i.e.,
continuous infiltration of dissolved organic matter from the river
water leading to locally reductive conditions, water warm enough
to favor bacterial activity). Consequently high dissolved manganese or iron concentrations are not rare in such systems and can
be problematic for the production of drinking water (e.g., Graillat
and Iundt, 1986; Hiscock and Grischek, 2002).
As treatment of manganese and iron present in water is expensive and bacterial activity in the aquifer impossible to control,
being able to identify areas where reductive dissolution of oxyhydroxides could develop is interesting. We show here that simply
M.A.M. Kedziorek, A.C.M. Bourg / Journal of Hydrology 365 (2009) 74–78
75
Table 1
Successive microbial reductive processes in natural waters.
Processes
Respiration
Denitrification
Mn reduction
Fe reduction
Sulfate reduction
a
Number of electrons involved per atom or molecule of electron acceptor
a
CH2O + O2 ? CO2 + H2O
þ
5
! 54CO2 þ 12N2 þ 74H2 O
4CH2 O þ NO3 þ H
þ
1
! Mn2þ þ 32H2 O þ 12CO2
2CH2 O þ MnO2 ðsÞ þ 2H
þ
1
1
CH
O
þ
2H
þ
FeðOHÞ
ðsÞ
! Fe2þ þ 11
2
3
4
4 H2 O þ 4CO2
þ
þ
H
!
HS
þ
2H
O
þ
2CO
2CH2 O þ SO2
2
2
4
4
5
2
1
8
CH2O represents DOC (dissolved organic carbon).
measuring Eh is not adequate to reach that objective and we propose a new approach for characterizing conditions favorable to the
reductive dissolution of Mn and Fe, the electron trapping capacity
(ETC).
The sites investigated
We investigated several situations affected by the reductive dissolution of manganese and/or iron oxyhydroxides. Field data include usually, but not always, on site measurements (pH, Eh,
alkalinity, temperature, dissolved O2) and laboratory analyses
(Cl, dissolved (filterable through 0.45 lm) Mn and Fe, NO
3,
DOC, SO2
4 ).
(1) The well field of Capdenac-Gare is located in the Lot River
alluvial plain (Aveyron, South Western France) and is used
to provide drinking water for about 5000 people. The aquifer
is extensively recharged by the river. The local geology of the
site is gravel and clay lenses overlying marly-limestone and
dolomite from the Lias covering an impermeable Lower Hettangian formation. The saturated and unsaturated zones are
about 7 and 4 m thick, respectively. The water velocity is
0.6–3.3 m d1 (e.g., Bourg and Bertin 1993, 1994; Bertin
and Bourg 1994).
(2) The Glattfelden site, located in North Eastern Switzerland on
the banks of the Glatt River, is not an actual well field. River
water infiltrates under natural gradient as the upper layer of
the local aquifer and remains unmixed for several meters
(where most of the sampling points are located). The aquifer
consists of glacio-fluvial outwash deposits. The water velocity is 2–4 m d1. No data is given for Eh, but nitrate, dissolved oxygen and Mn and some Fe are available (e.g.,
Jacobs et al., 1988; Von Gunten et al., 1991, 1994).
(3) In the Oderbruch polder area in North Eastern Germany, permanent infiltration occurs, due to the hydraulic gradient
existing between the river bordered by a levee and the aquifer (Massmann et al., 2004; Merz et al., 2005). The local geology of the site is Pleistocene glacio-fluvial sands lying on
glacial till and topped by an impermeable alluvial loam.
Massmann et al. (2004) and Merz et al. (2005) have studied
the spatial evolution of redox processes in the infiltration
flow path, including dissolved manganese and iron.
(4) Using d15N analyses in a study of the Torgau aquifer, associated to the Elbe River (Eastern Germany), Grischeck et al.
(1998) demonstrated the occurrence of denitrification in
infiltrating river water. The sand and gravel aquifer provides
a highly productive river bank infiltration scheme. No data is
given for Eh, but nitrate, dissolved oxygen and Mn and Fe are
available.
(5) The aquifer studied by Groffman and Crossey (1999) is
located in the Jemez Mountains (New Mexico, USA). It is
hydraulically connected to the Rio Calaveras but is not
exploited to produce drinking water. Infiltration of river
water into groundwater is due to the site topography and
takes place during snowmelt. It naturally recharges the aquifer formed by colluvial wedges and fluvial material (90%
sand and 10% silt). This site permitted a detailed study of
the spatial and seasonal redox evolution of the groundwater
chemistry, especially of dissolved manganese and iron.
(6) The laboratory studies of Petrunic et al. (2005) refer to the
Fredericton alluvial aquifer (New Brunswick, Canada). This
aquifer, hydraulically connected to the Saint John River, is
composed of eight wells pumping in glacial outwash sand
and gravel. Its drinking water production is about
26,000 m3 d1. Like for the well field of Capdenac-Gare, dissolved manganese concentrations exceeding the drinking
water guideline have been observed in some wells. Petrunic
et al. (2005), using columns filled with Fredericton aquifer
sand, have studied the origin of this dissolved manganese
and tried to model the processes involved. Neither dissolved
oxygen, nor nitrate data are available.
Characterizing redox conditions
Redox conditions in natural waters can be characterized qualitatively by means of presence or absence of redox sensitive species
(dissolved oxygen, nitrate, dissolved manganese or iron and sulfate) as first proposed by Champ et al. (1979) and Berner (1981).
Two approaches are used to characterize these conditions quantitatively, measuring or calculating an electrical potential (Eh) and
estimating a buffering capacity.
The easiest is measuring the electrical potential due to the electron activity, using a platinum and a reference electrodes. This
technique has limitations because the redox couple involved in
the electron transfer measured by the electrodes is not necessarily
the potential determining reaction but rather is controlled by the
redox couple with the fastest electron transfer. Several redox couples (e.g., N(III)/N(V), Fe(II)/Fe(III), Mn(II)/Mn(IV), As(III)/As(V),
S(-II)/S(VI)) can also be used to calculate this potential (e.g., Holm
and Curtiss, 1989) some are accepted as representative of mildly
oxidizing conditions (the N couple), others of strongly reducing
conditions (the S couple), but they are often times not easy to measure as they might involve strict water sampling and conservation
procedures together with analytical determinations of specific
species.
Barcelona and Holm (1991) and Heron et al. (1993) proposed as
a measure of the oxidation capacity of a system, OXC (i.e., capacity
to resist reduction), the sum of the concentrations of electron
acceptors weighted for the number of electrons involved in the
reactions listed in Table 1:
OXC ¼ 4½O2 þ 5½NO3 þ 2½MnðIVÞ þ ½FeðIIIÞ þ 8½SO2
4 þ 4½oxidized C
ð1Þ
Since dissolved oxygen and nitrate play an inhibiting role on the
reductive dissolution of manganese (Figs. 1 and 2) and of iron we
propose here to use only a part of Eq. (1) to determine the potential
76
M.A.M. Kedziorek, A.C.M. Bourg / Journal of Hydrology 365 (2009) 74–78
potentials is about the same for groundwater samples with dissolved manganese (at concentrations greater than 5 lM) (0–
530 mV) as for samples with lower manganese concentrations
(130 mV–530 mV) (Fig. 3).
180
Capdenac-Gare
150
Oderbruch
Thorgau
Mn(µM)
120
Glattfelden
ETC: A better indicator for characterizing conditions favorable to the
reductive dissolution of manganese oxides?
90
60
30
0
0
0.1
0.2
0.3
0.4
O2(mM)
Figure 1. Dissolved (filterable through 0.45 lm) Mn versus dissolved oxygen (for
all sites where data is available).
180
Capdenac-Gare (F)
150
Calaveras (US)
Oderbruch (D)
Mn (µM)
120
Thorgau (D)
Glattfelden (CH)
90
60
30
0
0
0.1
0.2
0.3
0.4
0.5
NO 3- (mM)
Figure 2. Dissolved (filterable through 0.45 lm) Mn versus nitrate (for all sites
where data is available).
of a system to resist Mn and Fe reductive dissolution. This electron
trapping capacity (ETC) represents the quantity of electrons that O2
and NO
3 are capable of trapping as groundwater moves from oxidizing conditions to a Mn and Fe reducing environment (Table 1).
The greater the ETC, the less reductive the conditions and the lower
the probability of dissolution of manganese or iron oxyhydroxides
ETC ¼ 4½O2 þ 5½NO3 In the investigated sites there is a clear trend between ETC and
dissolved manganese (Fig. 4). Most samples affected by manganese
reduction (dissolved concentration greater than 1 lM) are characterized by a poor ETC (lower than 0.2 mM) whereas most of the
samples not affected by this reductive dissolution are characterized by an ETC higher than 0.2 mM. An ETC of about 0.2 mM could
therefore represent a threshold value to discriminate between
unfavorable and favorable conditions for the development of the
reductive dissolution of Mn oxides. This limit is probably related
to the maximal concentration of oxidants (O2 and NO
3 ) tolerated
by dissimilatory Mn(IV)-reducing microorganisms without inhibiting effect on their development and associated redox reactions.
Above this limit, dissolved O2 and nitrate, being used first, inhibit
redox reactions involving manganese or iron oxyhydroxides.
Moderately high dissolved manganese concentrations (between
1 and 10 lM) are observed in some samples despite a reasonably
high ETC (between 0.2 and 2 mM) (Fig. 4). These situations are explained by disequilibrium situations as oxidation of previously dissolved manganese by O2 is a slow process if not catalyzed by
bacteria, dissolved manganese cannot therefore be removed rapidly from solution and can persist in groundwater despite weakly
oxidizing conditions (Diem and Stumm, 1984; Harvey and Fuller,
1998; Groffman and Crossey, 1999; Zhang et al. 2002). The two situations where this was observed are the Glattfelden and Capdenac-Gare sites. In investigations of the Glattfelden site the
sampling points are very close to the river bank (at most a distance
of 7 m). Evidently, at that site dissolution of manganese occurs very
near the river bank (within a few cm) (Von Gunten et al., 1994).
Dissolved manganese in the infiltrating river water increases rapidly (a median value of 16 lM is observed in the interstitial water
of the first cm of the infiltration flow path). Just 2.5 m down gradient from the river bank it decreases but only to a few lM even
though the ETC is greater than 1 mM. The rapid water velocity
(2–4 m d1) and the lack of river- and ground-water mixing explains the relatively long residence time of Mn in the water phase
(12 h to 1 day). In the Capdenac-Gare well field, the high Mn concentrations corresponding to a high ETC (Fig. 4) are observed in a
ð2Þ
200
Validation of the approach
Capdenac-Gare (F)
Fredericton (Can)
Redox potential (Eh) as indicator of conditions favorable for the
reduction of Mn
Calaveras (US)
150
Redox potential is widely used to characterize the redox state of
water as it is easily measured on site. As described by the redox
reaction between MnO2 and organic matter in Table 1, the MnO2/
Mn2+ boundary is pH dependent, dissolution of the oxide starting
for a redox potential of about 470 mV at pH 7.0 (e.g., Drever,
1988). In many studies Eh is not measured (e.g., Grischeck et al.,
1998; Jacobs et al., 1988; Von Gunten et al., 1991, 1994).
In the data we reviewed (see the section on site description) no
simple trend between Eh and dissolved Mn is observed, in other
words there is no simple relation between Eh and the presence
of dissolved Mn. It is not possible to assign a redox potential value
which would be representative of redox conditions favorable to the
reductive dissolution of manganese oxides. The range of redox
100
Mn (µM)
Oderbruch (D)
50
0
-200
0
200
400
600
800
Eh (mV)
Figure 3. Dissolved (filterable through 0.45 lm) Mn versus Eh (for all sites where
data is available).
M.A.M. Kedziorek, A.C.M. Bourg / Journal of Hydrology 365 (2009) 74–78
reductive dissolution of iron oxyhydroxides after manganese
reduction (Massmann et al. 2004).
Mn (µM)
a 180
160
Capdenac-Gare (F)
140
Oderbruch (D)
120
Thorgau (D)
100
Glattfelden (CH)
Conclusion
80
60
40
20
0
0
0.5
1
1.5
2
2.5
3
3.5
ETC (mM)
Mn (µM)
b
20
10
0
0
2
1
ETC (mM)
Figure 4. (a) Relation between dissolved (filterable through 0.45 lm) manganese
and ETC (for all sites where data is available) and (b) enlargement of low Mn
concentrations.
well located 130 m from the river bank. The water pumped in that
well is a mixture of very low Mn waters (<1 lM) with water coming from the vicinity of a well with high Mn and low ETC. The mixing brings in dissolved oxygen and nitrate, but the system is not at
equilibrium again because of slow kinetics of the Mn oxidation
(water velocity of about 3 m d1; Bourg and Bertin, 1994).
ETC and iron reductive dissolution
Iron dissolution is also observed for low ETC. For this element
the threshold value is 0.1 mM (Fig. 5). Conditions needed for iron
dissolution are slightly more reducing than for manganese but
not very different. This is in agreement with the rapid onset of
1200
Capdenac-Gare (F)
1000
Oderbruch (D)
Thorgau (D)
Fe (µM)
800
Glattfelden (CH)
600
400
200
0
0
1
2
3
77
4
ETC (mM)
Figure 5. Relation between dissolved (filterable through 0.45 lm) iron and ETC (for
all sites where data is available).
ETC is a better indicator than Eh for the identification of conditions favoring the reductive dissolution of Fe and Mn. We propose,
on the basis of several field and laboratory investigations, a threshold value for ETC of 0.2 mM. Manganese and iron dissolution takes
place for ETC values lower than 0.2 mM. Above this threshold value
no significant reductive dissolution of oxides is observed, in the
well fields investigated. The reliability of ETC compared to Eh is
certainly due to its focus on elements inhibiting this reduction,
whereas Eh depends on the potential determining redox couple.
This makes ETC a potentially good indicator for characterizing redox conditions favorable to the reductive dissolution of manganese
and iron oxides. Furthermore, ETC is an indicator which is more
practical than the direct determination of dissolved Mn or Fe because it is easier to measure dissolved O2 and nitrate on site.
Being able to simulate ETC in each part of a well field before the
siting of wells, using a model capable of describing the O2 and NO
3
consumption (and evolution of other parameters such as water
temperature and organic matter) could be helpful for predicting
zones to be potentially affected by manganese and iron dissolution.
Such a model could avoid having wells located in unfavorable
zones.
They are exceptions to the threshold value of 0.2 mM for ETC,
however these can be identified (in practical terms pumping down
gradient of reducing zones should be performed so that the water
velocity is not too rapid – below 1 m s1? – to allow the precipitative removal of dissolved Mn). Despite these peculiar situations,
ETC appears to be a powerful tool to characterize redox conditions
that allow the reductive dissolution of manganese oxides.
Acknowledgements
This work was funded by the Conseil Régional (Regional Council) d’Aquitaine, the Département Pyrénées Atlantiques and the
Agence Nationale de la Recherche-Aquitaine. The concept developed here is based on a preliminary idea proposed by S. Geoffriau
(LHGE). We thank B.M. Petrunic, K.T.B. MacQuarrie and T.A. Al for
providing detailed data and E. Silvester for valuable editorial
comments.
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