Applied Geochemistry 25 (2010) 105–115
Contents lists available at ScienceDirect
Applied Geochemistry
journal homepage: www.elsevier.com/locate/apgeochem
The fate of nitrogen and sulfur in hard-rock aquifers as shown
by sulfate-isotope tracing
Hélène Pauwels a,*, Virginie Ayraud-Vergnaud a,b, Luc Aquilina b, Jérôme Molénat c,d
a
BRGM, Water Division, BP 36009, 45060 Orléans Cedex 2, France
CAREN-Géosciences, UMR 6118, Université Rennes 1-CNRS, Rennes, France
c
CAREN-UMR INRA-Agrocampus Sol Agronomie Spatialisation, Rennes, France
d
IRD-Laboratoire d’étude des Interactions Sol-Agrosystèmes-Hydrosystèmes, UMR INRA-IRD-Supagro, Montpellier, France
b
a r t i c l e
i n f o
Article history:
Received 10 March 2008
Accepted 2 November 2009
Available online 10 November 2009
Editorial handling by Dr. R. Fuge
a b s t r a c t
Stable SO4 isotopes (d34S-SO4 and d18O-SO4), and more occasionally d15N-NO3 were studied in groundwater
from seven hard-rock aquifer catchments. The sites are located in Brittany (France) and all are characterized by intensive agricultural activity. The purpose of the study was to investigate the potential use of
these isotopes for highlighting the fate of both SO4 and NO3 in the different aquifer compartments.
Nitrate-contaminated groundwater occurs in the regolith; d34S fingerprints the origin of SO4, such as
atmospheric deposition and fertilizers, and d18O-SO4 provides evidence of the cycling of S within soil.
The correlation between the d18O-SO4 of sulfates and the d15N-NO3 of nitrates suggests that S and N were
both cycled in soil before being leached to groundwater. Autotrophic and heterotrophic denitrification
was noted in fissured aquifers and in wetlands, respectively, the two processes being distinguished on
the basis of stable SO4 isotopes. During autotrophic denitrification, both d34S-SO4 and d18O-SO4 decrease
due to the oxidation of pyrite and the incorporation of O from the NO3 molecule in the newly formed
SO4. Within wetlands, fractionation occurs of O isotopes on SO4 in favour of lighter isotopes, probably
through reductive assimilation processes. Fractionation of S isotopes is negligible as the redox conditions
are not sufficiently reductive for dissimilatory reduction. d34S-SO4 and d18O-SO4 data fingerprint the
presence of a NO3-free brackish groundwater in the deepest parts of the aquifer. Through mixing with
present-day denitrified groundwater, this brackish groundwater can contribute to significantly increase
the salinity of pumped water from the fissured aquifer.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Groundwater degradation from diffuse pollution is increasing
worldwide and has been a growing concern for the past few decades, posing huge challenges regarding the sustainability of both
agriculture and aquatic ecosystems (Tilman et al., 2002). Today,
soil and geological properties are taken into account (Holman
et al., 2005) for assessing groundwater vulnerability to such pollution. Since the porous and permeable parts of hard-rock aquifers
(composed of granite and metamorphic rock) are mostly restricted
to shallow weathered horizons (0–100 m), they are particularly
vulnerable to diffuse pollution. Such aquifers are characterized
by a strong compartmentalization induced by weathering from
which derive the hydrogeological properties (Taylor and Howard,
2000; Maréchal et al., 2004; Wyns et al., 2004; Dewandel et al.,
2006). From top to bottom, these aquifers consist of two main lay-
* Corresponding author. Tel.: +33 2 38 64 34 06.
E-mail address: [email protected] (H. Pauwels).
0883-2927/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.apgeochem.2009.11.001
ers: (1) saprolite or regolith, a clay-rich material, and (2) a fissured
layer overlying fresh basement that is only locally permeable. With
depth, the fracture- and fissure density decreases, which leads to
highly heterogeneous and localized water circulation (Berkowitz,
2002; Aquilina et al., 2004). Worldwide, such aquifers account
for a considerable resource, since granite and metamorphic rock
underlie large areas of the continents and account for more than
20% of the present continental outcrops. However, their groundwater quality is threatened by intensive agriculture (Krasny and
Hrkal, 2003).
Brittany in western France is representative of hard-rock aquifer areas. It is characterized by intensive agricultural activity that
has developed over the past decades, leading to high NO3 concentrations in surface water and shallow groundwater. Such a situation creates either environmental problems (Menesguen and
Piriou, 1995; Pinay and Burt, 2001) or even forces the abandonment of drinking-water distribution from several supply fields.
The fate of NO3 within the landscape and within the different layers of these hard-rock aquifers has been investigated (Pauwels
et al., 2000, 2001; Pinay and Burt, 2001; Molénat et al., 2002; Ruiz
106
H. Pauwels et al. / Applied Geochemistry 25 (2010) 105–115
et al., 2002), whereby NO3 behaviour was compared to that of
other species such as SO4.
Nitrogen and S have similar biogeochemical cycles: they are
both essential nutrients for plants and bacteria as they are involved
in protein synthesis. Analogies of N and S cycling between the
atmosphere, forest biomass, soil, and seepage water have been recognized for some time (e.g. McGill and Cole, 1981). Stable SO4 isotopes have been used to identify sources and processes
contributing to SO4 concentration in waters. They are particularly
valuable tools for investigating ecosystem acidification related to
air pollution, weathering and biogeochemical reactions, such as organic-matter degradation or sulfide mineral oxidation (Hendry
et al., 1989; Toran and Harris, 1989; Strebel et al., 1990; Van
Stempvoort et al., 1990; Mayer et al., 1995; Feast et al., 1997;
Mitchell et al., 1998; Krouse and Mayer, 2000; Mandernack et al.,
2000; Pauwels et al., 2000; Novak et al., 2003, 2005b; Knöller
et al., 2005). Although comparative aspects of N and S cycling have
been studied in groundwater systems (McGill and Cole, 1981;
Novak et al., 2003) and despite the suitability of SO4 isotopes for
highlighting weathering and biogeochemical processes, only few
studies have discussed the application of SO4 isotopes when investigating the origin and fate of NO3.
The aim of this paper, based on a detailed study of the isotopic
compositions of groundwater from seven sites in hard-rock aquifers in areas with intensive agriculture, was to investigate the suitability of S-SO4 and O-SO4 isotope compositions of sulfates for
tracing and fingerprinting the origin and fate of both SO4 and
NO3 in each aquifer compartment.
where basement rock underlies 27,000 km2 (Fig. 1). These sites
are marked by granite, metamorphic and volcanic rocks, and
show various elevations and topography. They are all characterized by intensive agriculture inducing diffuse pollution. The
selected sites (Table 1) particularly facilitated the sampling of
groundwater circulating through the regolith and the fissured
part of the aquifers.
Denitrification, which depends on either topographical or geological factors, was shown to exist at several of these sites:
(1) Topographical factors: Brittany is relatively flat and the associated gentle slopes allow wetlands to develop in the lowest
parts of the landscape. In these anoxic environments, the
oxidation of organic-matter by NO3 through heterotrophic
denitrification leads to a decrease in NO3 concentrations as
soon as the O is consumed:
5CH2 O þ 4NO3 þ 4Hþ ! 5H2 CO3 þ 2N2 þ 2H2 O
ð1Þ
This process is common in riparian wetlands (Clément et al.,
2002, 2003; Martin et al., 2004; Pauwels and Talbo, 2004).
(1) Geological factors: Sulfide minerals such as pyrite are commonly observed in borehole cuttings from the fissured part
of the aquifer. These minerals are highly sensitive to redox
conditions and promote autotrophic denitrification processes in groundwater, assisted by Thiobacillus denitrificans
(Kölle et al., 1987):
þ
5FeS2 þ 15NO3 þ 5H2 O ! 10SO2
4 þ 5FeOðOHÞ þ 5H
ð2Þ
2. Geological and hydrogeochemical setting of the sites
In order to investigate the fate of NO3 in hard-rock aquifers
and its fingerprinting by SO4 isotopes, the geochemistry of
groundwater was determined at seven sites in Brittany (France),
This process has been observed in different sites around Brittany at depths from 20 to 100 m (Pauwels et al., 2000; Durand, 2005; Tarits et al., 2006). In such cases, the organic-C
content is very low (<1 mg/L) and as a result of autotrophic
Fig. 1. Location of study sites in Brittany in Western France.
Table 1
Site descriptions.
Site
Geology
Weathered layer reached by wells
Pyrite occurrence
Denitrification occurrence (Ref.)
Arguenon
Betton
Gneiss
Schist
Disseminated
Disseminated
Yes (Durand, 2005; Durand et al., 2006)
Yes (Ayraud et al., 2006)
Kerbernez–Kerrien
Lopérec
No
Disseminated
Only in wetlands (Martin et al., 2004; Ayraud, 2006)
Yes (Pauwels et al., 2010)
Naizin
Pleine–Fougères
Granite
Acid and basic volcanic rock
Schist
Schist
Micaschist
Disseminated
Disseminated
Yes Pauwels et al. (1998, 2000)
Yes (Clément et al., 2002, 2003)
Ploemeur
Schist and granite
Fissured
Regolith
Fissured
Regolith
Regolith
Fissured
Fissured
Regolith
Fissured
Regolith
Fissured
Localized
Yes (Tarits et al., 2006)
layer
layer
layer
layer
layer
layer
H. Pauwels et al. / Applied Geochemistry 25 (2010) 105–115
denitrification the NO3 concentration decrease is concomitant with a SO4 concentration increase.
The main characteristics of the seven sites are shown in Table 1.
All have been the subject of previous investigations, but only some
of the available wells were selected for the present SO4 isotopic
investigation.
Ploemeur and Betton are two aquifers pumped for drinkingwater, whereas Arguenon is used for water bottling. Betton and
Arguenon are underlain by schist, whereas the Ploemeur area is
characterized by a complex geology, where a flat-lying contact between granite and micaschist is intersected by a 20°N striking fault
along which a productive aquifer develops (Le Borgne et al., 2004;
Tarits et al., 2006). The Ploemeur area is also characterized by a
high rate of pumping (up to 106 m3/a); a geochemical study (Tarits
et al., 2006) demonstrated the influence of pumping on groundwater geochemistry, and particularly on starting the denitrification
process. At Arguenon, all sampled groundwater was circulating
through the fissured part of the aquifer (Durand, 2005). At Ploemeur and Betton, the groundwater samples come from both the regolith and the fissured part of the aquifer, although in Ploemeur the
shallow sampling depths are actually very close to the interface between the regolith and the fissured layer. At Betton, brackish water
(up the 700 mg/L Cl) was found at depth (Ayraud, 2006).
Naizin, Pleine–Fougères and Kerbernez–Kerrien are experimental sites devoted to investigating the impact of agricultural activities on groundwater chemistry. The bedrock is schist at Naizin,
(Dia et al., 2000; Conan et al., 2003), micaschist at Pleine–Fougères
and granite at Kerbernez–Kerrien. The groundwater samples from
Naizin come from the fissured part of the aquifer (Pauwels et al.,
1998, 2000). The Kerbernez–Kerrien area includes two first-order
watersheds, Kerbernez and Kerrien, both characterized by intensive agriculture but with more pastures on Kerbernez (Ruiz et al.,
2002; Martin et al., 2004). Shallow observation wells are located
along the hill slopes, enabling groundwater collection from the
regolith. One well in a low-lying area provided samples from the
wetland close to the stream. At Pleine–Fougères, a borehole taps
the fissured layer of the aquifer, but most of this site is characterized by a large wetland that was the main focus of earlier studies
(Clément et al., 2002, 2003; Négrel and Pauwels, 2004; Pauwels
and Talbo, 2004); here, shallow observation wells reach both the
regolith and colluvium/alluvium in its uppermost part.
The Loperec site was earlier investigated for Au occurrences. It
is characterized by schist as well as acid and alkaline volcanic rocks
associated with polysulfidic epithermal mineralization. Since mineral exploration ceased, intensive agriculture has been the only
activity in the area. Farm wells and mineral exploration boreholes
provide water samples from both the regolith and the fissured
aquifer (Pauwels et al., 2010).
3. Isotope analyses
Data on SO4 and NO3 isotopes for the Betton, Kerbernez, Loperec
and Ploemeur sites are new, whereas those from the other sites
were taken from existing documents, i.e. Pleine–Fougères (Négrel
and Pauwels, 2004), Naizin (Pauwels et al., 2000) and Arguenon
(Durand, 2005). All new analyses were performed in the BRGM laboratory. Sulfate and NO3 isotopic compositions were determined
using a Delta S mass spectrometer (Thermo Finnigan). The d34S of
sulfates was measured from SO2 obtained from CdS precipitated
after SO4 reduction. Isotopic compositions use the usual d-scale
in ‰ according to
dsample ð%Þ ¼ fRsample Rstandard =Rstandard g 1000;
where R is the
34
S/32S atom ratio.
107
The Canon Diablo Troilite standard was used for S isotopes;
d18O-SO4 was determined from the CO2 produced by the reaction
of BaSO4 with C at 1050 °C, and reported using the usual d-scale
with the V-SMOW standard. The uncertainty for these two analyses was about ±0.3‰. Nitrate was reduced to NH3 and the mass
spectrometry determination of d15N was performed on N2 liberated
by the reaction of NH4Cl with LiOBr. Analytical precision on d15N
was ±0.2‰.
4. Results
Geochemical and isotope data are shown in Table 2 with the
hydrogeological compartment from which the samples were collected. The SO4 and NO3 concentrations vary over a large range
of concentrations (Fig. 2), but independently of each other and of
the sampling site. Though the observed high nitrate concentrations
of up to 110 mg/L are related to the intensive agriculture, the very
low nitrate concentrations have been related to denitrification processes (Pauwels et al., 2000; Clément et al., 2002, 2003; Martin
et al., 2004; Durand, 2005; Tarits et al., 2006). Very high SO4 concentrations clearly exceed concentrations expected from meteoric
and evapotranspiration processes (Clément et al., 2003; Martin
et al., 2004; Ayraud et al., 2006). Groundwater is classified into four
groups according to the hydrogeological compartment in which it
is located and other specifics. The groups are:
(1) Shallow groundwater circulating through the regolith, from
which is distinguished;
(2) Shallow groundwater from wetlands, and;
(3) Groundwater from the fissured part of the aquifer, from
which is distinguished;
(4) Brackish groundwater represented by one point (Pz6 in
Betton).
These four groundwater groups can also be seen on a NO3 vs.
SO4 diagram (Fig. 2). (1) Groundwater from the first group shows
generally low to moderate SO4 concentrations and a clear influence
of N applications with NO3 concentrations of over 25 mg/L and up
to 110 mg/L. (2) Groundwater from a wetland shows low NO3 and
SO4 concentrations. (3) Groundwater located in the fissured part of
the aquifer is generally characterized by low NO3 concentrations
(<5 mg/L) and moderate to high SO4 concentrations, and is commonly subject to autotrophic denitrification as shown by previous
studies (Pauwels et al., 2000; Durand, 2005; Ayraud et al., 2006;
Tarits et al., 2006). (4) Deep brackish groundwater is represented
by only one point (Pz6 from Betton). It is NO3-free with 298 mg/L
SO4, characterized by a lack of anthropogenic gases such as CFC
and SF6 (Ayraud et al., 2008), and dated to more than 10 ka by
14
C (Ayraud, 2006), consistent with the presence of a deep and
old groundwater body. Although at first sight each group can
apparently be distinguished on the basis of geochemical characteristics, some overlaps are observed. For instance, denitrified
groundwater from wetlands and fissured aquifers may have similar
SO4 concentrations, rendering the distinction difficult. Moreover,
groundwater from the fissured compartment may have significant
NO3 concentrations, e.g. the samples from Ploemeur, whereas
groundwater within the regolith may also have SO4 concentrations
over 30 mg/L, which is the case for PZ1 at Betton. The distinction
between groundwater from the regolith and that from fissured
aquifers on the basis of only their chemical composition is further
complicated by partial autotrophic denitrification, by the difficulty
of defining exactly where the interface between regolith and fissured rock lies, as well as by the modifications of natural groundwater circulation induced by pumping (discussed later). Finally,
any mixing between denitrified groundwater from a fissured
108
H. Pauwels et al. / Applied Geochemistry 25 (2010) 105–115
Table 2
Chemical and isotopic data.
Sampling N°
Sampling date
Arguenon
AF2
SW
SE
AF1
SN
TF3
Aquifer compartment
Cl mg/L
NO3 mg/L
SO4 mg/L
d34S %
d18-SO4 %
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
rock
rock
rock
rock
rock
rock
33.0
30.0
37.0
37.0
36.0
34.0
0
0
0
1
0
0
20.0
17.0
34.0
18.0
36.0
20.0
6.3
^1.7
3.9
3.5
1.8
0.3
5.9
1.5
1.5
1.4
3.1
5.3
rock
rock
rock
rock
rock
rock/Old GW
50.1
51.0
34.3
31.7
51.5
49.2
42.0
52.9
49.7
93.5
218.4
28.7
34.3
38.8
690
56.4
62.6
27.2
23.2
51.7
46.2
7.764
43.0
8.599
0.224
0.87
0.036
1.82
0.258
0.0
19.0
18.1
16.0
17.2
19.9
30.7
50.6
202.6
63.3
68.7
98.4
33.0
31.6
41.2
298
11.0
11.3
8.9
9.6
9.5
7.2
7.7
2.1
5.1
11.7
18.5
7.4
7.8
9.4
23.0
4.4
4.6
4.4
6.1
5.0
4.0
2.9
0.6
0.9
7.4
11.9
7.1
5.6
7.1
16.5
Betton
C
C
PZ4
PZ4
PZ3
PZ 1
Pz2
PZ 1
Pzl
Fl
PZ7
PZ8
PZ8
PZ6–1
PZ6
2005
2004
2005
2004
2007
2005
2004
2007
2004
2004
2007
2004
2007
2004
2005
Regolith
Regolith
Regolith
Regolith
Regolith
Regolith
Regolith
Regolith
Regolith
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Kerbernez–Kerrien
B5b
A3
B5a
C2
B4
F5a
F5b
Alb
F4
F2
Flc
2002
2002
2002
2002
2002
2002
2002
2002
2002
2002
2002
Regolith
Regolith
Regolith
Regolith
Regolith
Regolith
Regolith
Regolith
Regolith
Regolith
Wetland
44.6
37.1
38.1
36
51.1
29.5
29.4
44.2
28.4
35.0
32.1
80.8
70.5
71.8
67.1
110.5
59.9
58.7
79.6
56.3
54.8
0
21.4
30.5
21
14.9
26.6
13.8
14.4
16.3
9.3
13.7
17.9
11.9
12.0
13.1
13.3
13.5
14.1
14.6
14.8
16.7
7.4
6.1
7.6
4.8
6.7
4.3
4.1
5.2
4.1
11.5
14.2
Loperec
H4–1
PR
Li
L-AR
L-EX
H2–1
H4–2
2001
2001
2001
2001
2001
2001
2001
Regolith
Regolith
Fissured
Fissured
Fissured
Fissured
Fissured
38.2
23.4
rock
rock
rock
rock
rock
17.1
13.2
20.6
22.6
20.5
20.1
23.5
21.9
14.9
46.4
42.9
50.1
45
61.2
7.1
9.2
9.2
9.1
6.6
3
1.5
4.3
4.8
0.09
1.4
1.3
0.9
1.1
Nalzln
PZ2
Fl
PZ4
F3
DNS3
DNS2
F2
Fl
PZ4
1994
1994
1994
1994
1994
1994
1994
1994
1994
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
rock
rock
rock
rock
rock
rock
rock
rock
rock
23.8
22
17.3
18.4
26.8
25.9
32.1
24.7
17.8
0
0
22.5
21.3
17.5
22.6
25.4
26
20.3
23.9
19.8
19.6
11.4
11.5
11.6
11.9
12.1
13.5
15
15
17.4
15
12.3
11
9.5
10.8
12.4
11.9
11.8
Pleine–Fougeres
Maisonneuve
S7
Sll
Apll
S12
Ap4
S13
S14
Ap6
Ap7
Ap3
S8
Ap3
S15
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
Denitrified GW
Regolith
Regolith
Regolith
Wetland
Wetland
Wetland
Wetland
Wetland
Wetland
Wetland
Wetland
Wetland
Wetland
38.85
54.32
54.33
53.83
38.45
24.47
39.05
29.36
32.26
56.82
26.07
32.66
51.23
55.13
0.00
58.01
13.20
66.10
2.30
2.59
6.00
0.06
0.20
0.20
0.30
2.30
1.20
4.40
44.83
23.22
22.22
18.71
14.31
4.00
18.01
29.22
6.80
22.10
13.81
7.11
20.82
5.80
2.2
8
8.1
8.2
7.3
8.4
9.4
11.8
12.4
13.8
13.9
14.6
15.8
29.8
7
5.9
5.8
4.6
4.9
19.1
10.2
18.3
20.3
18.2
15
12.9
14.6
14
Ploemeur
F17
MF3
F34
F35
2003
2007
2007
2003
Fissured
Fissured
Fissured
Fissured
45.62
34.08
56.66
73.17
33.78
32.08
35.35
34.13
80.39
37.96
11.94
14.35
0.1
5.0
10.9
10.8
4.6
0.9
5.9
7.5
rock
rock
rock
rock
0
0.7
0
2.3
0
0
0
1.9
0
d15N
7.7
7.7
8.9
13.9
10.0
8.7
10.0
9.7
9.7
5.4
5.3
7.8
4.6
5.4
8.9
9.9
17.9
8.4
7.9
109
H. Pauwels et al. / Applied Geochemistry 25 (2010) 105–115
Table 2 (continued)
Sampling N°
Sampling date
Aquifer compartment
Cl mg/L
NO3 mg/L
SO4 mg/L
d34S %
d18-SO4 %
d15N
F35
F9
F9
F20
F21
F26
F13
F13
F37
MF4
F36
F36
MF2
MF2
MF1
F28a
F28b
F31
Fll
PE
PE
2007
2003
2007
2003
2007
2007
2003
2007
2007
2007
2003
2007
2003
2007
2003
2003
2003
2003
2003
2003
2007
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
Fissured
66.50
45.84
47.54
58.85
120.88
103.80
119.40
120.94
71.94
70.69
40.79
37.00
52.35
55.57
53.76
60.19
58.13
64.24
75.06
73.60
78.20
34.28
45.17
50.15
72.65
0.00
5.72
0.00
0.46
2.58
0.29
0.00
2.68
0.00
0.29
0.00
17.23
14.41
8.92
0.00
0.00
1.05
16.85
11.75
11.61
36.73
35.43
552.45
41.42
39.44
57.58
97.97
457.4
454.0
63.81
105.3
159.1
72.21
59.98
58.03
81.27
71.38
73.54
11.3
16.4
15.9
1.1
7
1.2
4.4
5.5
0.9
1.6
10.3
8.5
8
11.2
7.4
6.3
2.3
0.4
5.9
9.4
6.8
7.6
3.9
3.2
4.5
10.5
4.8
7.6
6.9
3.7
3.5
1
0.7
3.8
0.2
0.6
0.1
3.9
6.1
8
10.6
8.2
9.4
7.6
rock
rock
rock
rock
rock
rock
rock
rock
rock
rock
rock
rock
rock
rock
rock
rock
rock
rock
rock
rock
rock
8.3
11.1
8.7
Fig. 2. SO4 vs. NO3 concentration at investigated sites and according to the aquifer compartments.
aquifer and old brackish groundwater, if present, cannot be determined from the chemical composition alone. All these overlaps between groundwater groups justify the use of additional, e.g.
isotopic, tools to distinguish the characteristics of the various
groups.
On Fig. 3, d34S-SO4 is plotted vs. the 1/SO4 ratio (3a) and the NO3
concentration (3b). A clear non-linear trend is seen on Fig. 3a. At
low SO4 concentrations, the d34S-SO4 varies within the (+6‰,
+18‰) range. With a few exceptions, such as Pz6 at Betton, as soon
as the SO4 concentration increases, the d34S-SO4 decreases regardless of the site. As indicated on Fig. 3a, the intercept at origin,
which represents the isotopic composition of the added SO4, would
not be common to all sites, but is always close to zero (Naizin, Betton) or even significantly negative (Ploemeur, Arguenon, Lopérec,
Pleine–Fougères). In the NO3 vs. d34S-SO4 diagram (Fig. 3b) it is
obvious that the d34S-SO4 range at low to very low NO3 concentrations is larger than at higher NO3 concentrations. A comparison of
Fig. 3a and b shows that higher SO4 concentrations and lower
d34S-SO4 correspond also to lower NO3 concentrations.
In a d34S-SO4 vs. d18O-SO4 diagram (Fig. 4), the position and trend
of the data show some site specifics. In particular, points from Ploemeur, Loperec and Arguenon reach the lower d34S-SO4 and d18O-SO4
values, and data from Pleine–Fougères show a large d18O-SO4 range,
with almost constant d34S-SO4 (about +10%).
The four above-mentioned groundwater groups are more
strongly individualized in the d34S-SO4 vs. d18O-SO4 diagrams
(Fig. 4):
(1) Within the regolith, groundwater has a d34S-SO4 that is
homogeneous and relatively high, ranging from 6‰ to
18‰, and a d18O-SO4 between 4‰ and 8‰ (except Pz1 at Betton where occasionally more depleted d34S-SO4 and d18O-SO4
was found).
(2) Groundwater from a wetland shows a high d18O-SO4 (>8‰)
and a d34S-SO4 between 4‰ and 20‰. S15 shows a particularly high d34S-SO4 of around 30‰. The wetland domain is
mainly represented by the Pleine–Fougères site, but it is
worth noting that the only groundwater sampled in a wetland at Kerbernez (F1c) has similar isotopic characteristics.
(3) Within the fissured part of the aquifer, groundwater has a
large range of isotopic values, from 11‰ to +15‰ for
d34S-SO4 and from 1‰ to +15‰ for d18O-SO4, but with a clear
correlation between the two isotopes.
(4) Finally, the old brackish groundwater identified in Pz6 at
Betton has a high d34S-SO4 and d18O-SO4.
Some overlaps are still observable: they occasionally concern
PZ1 at Betton. Overlaps between groundwater from regolith and
fissured aquifers are less visible than from chemical composition,
and concern mainly F9 and to a lesser extent F34 and F35 at Ploemeur. The linear trend observed for groundwater within fissured
rock suggests, as will be discussed later, a mixing process with
brackish groundwater.
The d15N-NO3 values were measured mostly in groundwater
samples from the regolith and only in rare cases from fissured rock,
110
H. Pauwels et al. / Applied Geochemistry 25 (2010) 105–115
Fig. 5. Comparison of d34S-SO4 vs. d18O-SO4 values of shallow NO3-contaminated
groundwater with representative SO4 data within atmospheric input, fertilizers and
soil solutions.
of groundwater within the regolith is 8.23‰, which is lower than
that for groundwater from fissured rock at 12.9‰. This difference
agrees with the earlier statement on denitrification, which is
known to cause isotopic fractionation with higher d15N signatures
of residual NO3 (Mariotti et al., 1988; Aravena and Robertson,
1998).
5. Discussion
Fig. 3. d34S-SO4 of groundwater at each site as a function of (a) 1/SO4 and (b) NO3
concentrations.
Sulfate concentration and isotopic composition in groundwater
are influenced by several sources and processes. They depend
partly on atmospheric deposition, including sea-spray and industrial dust, though industrial activity in Brittany being minor, the
impact from industrial dust is considered as negligible. Since agricultural activities prevail at all study sites, SO4 and NO3 concentrations as well as their isotopic signatures are also largely dependent
on fertilizer application and other agricultural amendments. After
deposition on soil, either unaltered primary SO4 and NO3 directly
infiltrate to deeper levels and into the aquifer, or SO4 and NO3
are involved in different processes occurring within soil, such as
assimilation and mineralization, and even volatilization and denitrification in the case of NO3, before infiltration to groundwater
(Novak et al., 2003). Within the aquifer, mainly water–rock-interaction processes including redox reactions occur, leading either
to dissolution and increase in concentration, or to attenuation of
the compounds. The following discussion focuses on the description of processes involving both NO3 and SO4, from their deposition
on soil to their transfer into the different hydrogeological compartments of hard-rock aquifers. It specifically highlights how SO4 isotopes can contribute to the understanding of these processes.
5.1. Characteristics of groundwater within the regolith
Fig. 4. d34S-SO4 vs. d18O-SO4 diagram at investigated sites and according to the
aquifer compartments. Arrows indicate the mixing trends between denitrified
groundwater with an additional end-member of high d34S-SO4 and d18O-SO4 values
for Betton (full line) and Ploemeur and Naizin (dotted line). Black cross: Present-day
sea-water.
as measurement was limited by low NO3 concentrations. Overall,
the d15N-NO3 varies from 5.4 to 17.9‰. The mean d15N-NO3 value
Regolith is the first groundwater container after soil–water
infiltration. Consequently, d34S-SO4 and d18O-SO4 in such groundwater are controlled by a mixing between atmospheric deposition,
fertilizer input, and secondary SO4 that derives from the mineralization of organic sulfates. The data from such groundwater are
compared to the generally observed values of d34S-SO4 and d18OSO4 in atmospheric SO4, and in sulfates from agricultural amendments as shown in Fig. 5.
H. Pauwels et al. / Applied Geochemistry 25 (2010) 105–115
(i) As stated above, atmospheric sulfates are mainly of marine
origin and the d34S-SO4 of atmospheric SO4 is restricted to
the +15‰ to +25‰ range (Krouse and Mayer, 2000) depending on the proportion of reduced compounds in sea-spray.
The d18O-SO4 of atmospheric SO4 is more variable due to
the various O sources available for reduced S oxidation. For
the diagram in Fig. 5, the (+7‰, +17‰) range was adopted,
which is commonly observed in temperate regions (Mayer
et al., 1995; Krouse and Mayer, 2000), although this range
includes not only the marine origin, but also industrial
sources.
(ii) The domain of SO4 from agricultural amendments can be
restricted to that of SO4 in fertilizers since no S amendments
such as gypsum or elemental S are used in the area. d34S-SO4
and d18O-SO4 values of sulfates in fertilizers have been
reported by several authors (e.g. Moncaster et al., 2000;
Otero and Soler, 2002; Vitoria et al., 2004) and are restricted
to the range (0‰, +10‰) for d34S-SO4 and (+5‰, +15‰) for
d18O-SO4.
Since groundwater tapped in the fissured aquifer from wells F9,
F34 and F35 in Ploemeur has chemical and isotope characteristics
close to that of groundwater in the regolith, it has been included in
this discussion. At first sight, the d34S-SO4 of groundwater within
the regolith can be explained by a simple mixing between (i) atmospheric deposition of marine origin, and (ii) fertilizers. It also appears that atmospheric influence is greater in groundwater with
low SO4 concentrations (Kerbernez site, F9 well in Ploemeur site,
Fig. 5) which are both close to the sea. No other process is observed
from the d34S-SO4 data (Fig. 5). In fact, once deposited in soil, assimilation processes transform both atmospheric and fertilizer sulfates
into organically bound S (C-bonded or sulfate ester). Subsequent
organic-matter mineralization provides a secondary (or organically-cycled) SO4, but fractionation of d34S-SO4 during the assimilation/mineralization cycle is often neglected (Mitchell et al., 1998;
Krouse and Mayer, 2000), or considered minor (2–3‰) (Krouse
and Grinenko, 1991; Novak et al., 2003, 2005a). Sorption/desorption processes that potentially occur within soil are also known
to have a negligible impact on d34S-SO4 (Van Stempvoort et al.,
1990).
Though the d34S-SO4 in groundwater from the regolith indicates
that the SO4 originated from mixing between atmospheric SO4 and
SO4 from fertilizers, the d18O-SO4 (<5‰) values testify that, after
deposition, SO4 was also involved in active S turnover in the soil.
Fig. 5 highlights a clear shift to depleted d18O-SO4 values compared
to a simple mixing, suggesting that O isotope fractionation occurs.
Actually, the assimilation/mineralization cycle is known to affect
d18O-SO4 through the incorporation of O from water molecules,
leading to typical values of d18O-SO4 of soil SO4 within the (10‰
to +5‰) range (Mitchell et al., 1998; Krouse and Mayer, 2000;
Mandernack et al., 2000).
Regardless of the site, the d18O-SO4 in NO3-contaminated shallow groundwater (regolith and F9, F34, F35 at Ploemeur) depends
on land use. Actually, the d18O-SO4 from groundwater below crop
fields is lower than that observed in groundwater under pasture
or forest. The case of the Kerbernez–Kerrien catchment, with a predominance of pasture in Kerbernez and maize and cereal crops in
Kerrien, clearly shows that the difference in d18O-SO4 cannot be explained by different agricultural inputs. In both areas the sampled
groundwater is dated from the present time up to 1990 (Ayraud
et al., 2008) and during this period, despite the difference in agricultural activities, both areas received almost similar agricultural
inputs in terms of both nature and quantity – mainly ammonium
nitrate at 113 and 160 kg/ha for Kerbernez and Kerrien, respectively – as well as minor amounts of pig slurry and cattle manure
(Martin, 2003). Two hypotheses may explain the difference in
111
d18O-SO4, between Kerrien (crops) and Kerbernez (pasture) though
soil analyses would be required to discriminate between them.
Either (i) the proportion of secondary SO4 in water infiltrating at
Kerrien is higher than at Kerbernez, since S turnover in soil is
stronger at Kerrien; or (ii) the organic S formed during assimilation
is different in pastures compared to arable fields. Actually, the
organic S (C-bonded S or ester sulfates) that is formed during
assimilation influences the d18O-SO4 value of secondary sulfates
formed after mineralization, because the mineralization of C-bonded
S requires greater incorporation of atmospheric O or from water
molecules than the mineralization of an SO4-ester that preserves
part of the original SO4.
Still focusing on the Kerbernez–Kerrien area, it must be noted
that the variability observed in d18O-SO4 on Fig. 5 correlates with
that of d15N-NO3 (R2 = 0.71) (Fig. 6). Both areas have received similar
agricultural inputs in terms of nature, but the d15N-NO3 in groundwater varies from +4‰ to +10‰, with higher values at Kerbernez
than at Kerrien. Such d15N-NO3 values are commonly used for estimating the origin of NO3 in water (Mayer et al., 2002), because the
isotopic composition of N is generally different among the possible
NO3 sources, such as atmospheric N2, fertilizer or manure (cf. compilation by Mengis et al. (2001)). The d15N-NO3 of chemical fertilizer
is around 0‰ due to its fabrication process (Vitoria et al., 2004),
but manure spreading and animal droppings can contribute to
increasing the d15N in groundwater. Since the fields received similar N sources in both quality and quantity for several years, the
higher d15N-NO3 at Kerbernez than at Kerrien cannot be explained
by a greater organic origin for NO3 contamination. However, as
mentioned above, fractionation processes are generally neglected
although they can strongly reduce the accuracy of identifying N
sources (Xue et al., 2009). For example, Novak et al. (2003) showed
that assimilation favours the lighter isotope and Kellman (2005)
observed that tile-drain discharge was systematically enriched in
15
N with respect to its N source and this regardless of the source
(fertilizer, manure). Experimental investigations by Choi et al.
(2003) showed that soil moisture is an important parameter for
fractionation. The analogy of N and S cycling between atmosphere,
soil and seepage water (McGill and Cole, 1981; Novak et al., 2003)
and the correlation shown in isotope data from the Kerrien–Kerbernez area (Fig. 6), imply that, if S is involved in the assimilation
and mineralization cycle as demonstrated by d18O-SO4, so must be
N. Correspondingly, any processes depleting 18O-SO4 in groundwater must modify 15N-NO3 through the production of secondary SO4
Fig. 6. d18O-SO4 vs. d15N-NO3 of shallow NO3-contaminated groundwater from the
Kerbernez–Kerrien area.
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H. Pauwels et al. / Applied Geochemistry 25 (2010) 105–115
and NO3 during the assimilation/mineralization cycle in soil before
infiltration to the groundwater. These results strongly suggest that
the variability of 15N-NO3 in groundwater in the Kerbernez–Kerrien
area is more an effect of N isotope fractionation in the soil than of N
sources. This agrees with Legout et al. (2005), who reported denitrification, a fractionating process, at very shallow depth within
soil and the unsaturated zone of this area. The analogy of N and
S cycling and the observed correlation between SO4 and NO3 isotopes thus suggest that combined use of these isotopes could contribute to overcoming the problems encountered when using N
isotopes for discrimination. Fractionation of both SO4 and NO3 isotopes occurs within soil after fertilizer deposition and before NO3
leaching to groundwater.
5.2. Fingerprinting of redox processes in wetlands
In Brittany, catchments have gentle slopes and groundwater
exfiltrates in the lowest parts of the landscape where wetlands
are common. Such wetlands are thus possible receptors of groundwater after circulation within the regolith. In such environments,
reducing conditions generally favour NO3 reduction leading to extremely low NO3 concentrations (Fig. 2). Dissimilatory SO4 reduction may begin soon after NO3 consumption and the influence of
SO4 reduction on S and O isotopes of SO4 is well-documented (Nriagu et al., 1991; Lu et al., 2001; Spence et al., 2001; Berner et al.,
2002; Novak et al., 2005b). Reducing conditions prevail in the
riparian wetland of Pleine–Fougères (Clément et al., 2002), but dissimilatory reduction is not a common process and the isotope composition of sulfates of only the S15 sample is explained by this
process (Négrel and Pauwels, 2004). Sulfate concentration is lower
in wetland than in upstream areas. Rayleigh conditions of SO4 isotopes were reported with a significant fractionation factor for 18O34
S (e = 3.6‰) (Négrel and
SO4 (e = 6.8‰) but a moderate one for
Pauwels, 2004), producing the shift to the right of the ‘‘regolith
domain” in Fig. 4. Therefore, the occurrence of assimilatory SO4
reduction (plant uptake) in the wetland is proposed as the main
process driving the fate of sulfates. A significant decrease of SO4
concentration in water from bogs through plant uptake has also
been reported (Steinmann and Shotyk, 1997) as well as the preferential uptake of light S isotopes (Bartlett et al., 2005), which supports the present conclusion.
In this study, wetlands are located at Pleine–Fougères and in the
Kerbernez sites (F1c) (Table 2 and Fig. 4). It is worth noting that the
Kerbernez point shifts to the right of the (d18O-SO4 d34S-SO4) diagram (Fig. 4) relative to the regolith domain and merges with
points from the Pleine–Fougères wetland, suggesting that at Kerbernez the fate of SO4 in the wetland depends on assimilatory
reduction as well. The wetlands in this area are not continuously
flooded, a condition that is known to limit highly reducing circumstances and thus dissimilatory SO4 reduction processes (Baldwin
and Mitchell, 2000).
The SO4 produced through sulfide oxidation has a d34S-SO4 close
to the d34S-FeS2 value when pyrite is the main oxidized sulfide
(Toran and Harris, 1989; Strebel et al., 1990; Clark and Fritz,
1997; Feast et al., 1997; Krouse and Mayer, 2000), varying between
30‰ and +5‰ (Krouse and Mayer, 2000; Moncaster et al., 2000).
At Naizin, the d34S-FeS2 varies between +4‰ and +6.9‰ (Pauwels
et al., 2000) and from 13‰ to +1.8‰ at Arguenon (Durand
et al., 2006). The d34S-FeS2 values at Lopérec, Betton and Ploemeur
were not determined, but, based on Fig. 3a, values below 10‰
are expected at Lopérec and Ploemeur, while at Betton the d34SFeS2., must be close to that of Naizin.
At Ploemeur, where the coupling between sulfide oxidation and
NO3 reduction is demonstrated, the d15N evolution can be compared to d34S-SO4 (Fig. 7). As the d15N evolution is described by
the classical Rayleigh distillation law, the concomitant evolution
of d15N and d34S-SO4 can be represented by the following system
of equations (Clark and Fritz, 1997):
d15 N ¼ e ln½NO3 =½NO3 0 þ d15 N0 :
34
ð3Þ
34
d SSO4 ¼ ð½SO4 0 =ð½SO4 þ ½SO4 0 Þd S0
þ ð½SO4 =ð½SO4 þ ½SO4 0 Þd34 SFeS2
ð4Þ
In Eq. (3), e is the isotope enrichment factor, and [NO3]0 and d15N0
are the initial NO3 concentration and isotope composition before
the start of denitrification. In Eq. (4), [SO4]0 and d34S0 are the SO4
concentration and isotope composition before the start of denitrification, [SO4] is the concentration of SO4 induced by denitrification,
and d34SFeS2 is the isotope composition of pyrite.
The curve in Fig. 7 was determined for an initial chemical and
isotope composition close to that of F9: [NO3]0 = 45 mg/L,
[SO4]0 = 11 mg/L, d15N0 = + 7.6‰, and d34S0 = + 16.4‰. According
to the trend seen on Fig. 3a, the d34SFeS2 was estimated at 10‰.
Except for F26, the position of which will be discussed below, isotope composition follows the denitrification curve. The position of
F20 along the denitrification curve must be noted. Although it was
collected from the fissured aquifer, the chemical composition of
this sample is close to that of regolith (Fig. 2) and, enrichment of
d15N alone was not sufficient to demonstrate a partial reduction
of the NO3 concentration by denitrification. It is only from the association of both d34S-SO4 and d15N values that the occurrence of denitrification has become clear. At Ploemeur, groundwater is pumped
at a high rate, inducing a modification of the global hydrological
system and in particular an increase of water velocity up to the
5.3. Fingerprinting processes within the fissured aquifer
In Brittany, the N cycle and especially NO3 concentrations are
strongly influenced by bedrock composition. Sulfide-bearing minerals occur in most cases below the regolith, except at Kerbernez–Kerrien and, in a heterogeneous manner, below the
Ploemeur aquifer. They react with nitrates according to reaction
(2), which was demonstrated by earlier investigations at Naizin,
Ploemeur, Betton, Lopérec and Pleine–Fougères (Pauwels et al.,
2000; Négrel and Pauwels, 2004; Tarits et al., 2006; Ayraud et al.,
2006). Nitrate concentrations are low in the groundwater circulating in wetlands (Fig. 2), but according to Eq. (2) the low NO3 concentrations resulting from autotrophic denitrification correlate
with high SO4 concentrations.
Fig. 7. d34S-SO4 vs. d15N-NO3 in groundwater from the Ploemeur area. The curve
represents the evolution of isotopic composition during autotrophic denitrification
(see text for initial conditions and denitrification parameters) along a flow line from
a NO3-contaminated groundwater to a totally denitrified groundwater.
H. Pauwels et al. / Applied Geochemistry 25 (2010) 105–115
pumped zone (Ayraud et al., 2008). In other words, pumping accelerates water transfer from the regolith to the fissured aquifer. This
explains why groundwater with a high NO3 concentration may be
observed in the fissured aquifer (Fig. 2): denitrification is ongoing
but not yet complete through lack of time.
Fig. 4 clearly shows a correlation between d18O-SO4 and d34S-SO4
in groundwater from the fissured aquifer with the lighter values of
d18O-SO4 around 0‰. During autotrophic denitrification, SO4 derives its O and its O isotope composition from NO3. As for NO3, with
the cycling of N within soil observed in groundwater from the regolith, it derives its O and its O isotope composition from soil water.
Therefore, the d18O-NO3 of NO3 must approach typical values within
soil, from about 3‰ to +5‰ (Mengis et al., 2001) and even above
+10‰ according to Kendall and Aravena (2000) or Deutsch et al.
(2005). The denitrification process causes O isotope fractionation
in favour of lighter isotopes and occurs in several steps, though
complete denitrification may lead to a d18O-SO4 of newly formed
SO4 that is not strictly equal to the initial d18O-NO3. Nevertheless,
it appears that samples with the lowest d34S-SO4, i.e. those with a
high contribution of SO4 originating from pyrite, have a d18O-SO4
in the range of soil d18O-NO3 according to Mengis et al. (2001),
which is consistent with the occurrence of denitrification.
The combined use of d34S-SO4 and d18O-SO4 enables the chemical
status of samples from Ploemeur and Betton to be specified (Fig. 4).
Three samples collected in the fissured aquifer of Ploemeur merge
with samples from the regolith that are not denitrified. This lack of
denitrification is probably related to the heterogeneous distribution of pyrite in the rock. However, despite the presence of nitrates
in MF3, F17, F20 and F28, the d34S-SO4 and d18O-SO4 values clearly
indicate that these groundwaters are partly denitrified. The same
result was shown for F20 from the combined use of d34S-SO4 and
d15N. The highly increased rate of water transfer through the fissured aquifer caused by pumping prevents the denitrification from
being complete and NO3 is still present in the groundwater. At Betton, the chemical and isotope compositions of PZ1 vary with time.
Isotopic data from 2005 are consistent with other groundwater
data in the regolith, whereas the 2004 and 2007 data on Fig. 4 plot
close to the fissured aquifer domain. Actually, PZ1 is located in a
low part of the landscape for which, at Naizin and Pleine–Fougères,
it was shown that the regolith can receive a temporary upward flux
of deeper NO3-free groundwater (Pauwels and Talbo, 2004). The
present study shows that this process is also expected to occur at
Betton, and can be traced by d34S-SO4 and d18O-SO4.
In some Ploemeur samples, a significant part of the sulfates
does not derive from denitrification but from another process of
sulfide oxidation (Tarits et al., 2006). Samples F36, MF1 and MF2
are characterized by high and, in the case of F36 (450 mg/L), even
extremely high SO4 concentrations that are associated with high
dissolved-Fe concentrations. This was interpreted as the result of
pyrite oxidation by newly formed FeIII (issued from pyrite oxidation) through the following reaction (Tarits et al., 2006):
þ
FeS2 þ 14Fe3þ þ 8H2 O ! 15Fe2þ þ 2SO2
4 þ 16H
ð5Þ
with FeIII produced by Eq. (2). This reaction clearly indicates that the
18
O of the H2O molecule must impact the d18O-SO4 of newly formed.
SO4. In the absence of atmospheric O, the equation proposed by Van
Everdingen and Krouse (1985) for deriving the O isotopic composition of newly formed SO4, during sulfide oxidation in the presence
of both atmospheric O and O of the water molecule, is summarised as:
d18 OSO4 ¼ d18 OH2 O þ e18 OSO4 H2 O
18
ð6Þ
where e OSO4–H2O, the fractionation factor between isotopes of
water and SO4, is close to +2‰ to +4.1‰ (Toran and Harris, 1989).
With d18OH2O within the (6‰ to 5‰) range, the d18O-SO4 of SO4
formed through reaction 3 must fall within the (4‰ to 1‰) range.
113
The data indicate a good concordance with this range. However, it
must be noted that, given the state of present knowledge, d18O-SO4
cannot help in distinguishing between sulfates issuing from denitrification and sulfates resulting from FeIII oxidation. In addition, no
clear distinction is seen on the isotope diagram of Fig. 4.
5.4. Mixing in the fissured aquifer
Within Pz6 at Betton, an old and brackish groundwater is characterized by high d34S-SO4 and d18O-SO4 values (Fig. 4). Fig. 4 shows
that the groundwater from the fissured aquifer plots along a mixing trend from low d34S-SO4 and d18O-SO4 values resulting from
denitrification, to d34S-SO4 and d18O-SO4 close to those of Pz6. The
mixing trend is particularly clear not only for Betton but also for
Ploemeur and Naizin. At Naizin, a contribution of groundwater
characterized by high d34S-SO4 and d18O-SO4 sulfates has already
been suggested (Pauwels et al., 2000). At Ploemeur, long-term
changes in the chemical composition of water from pumping well
PE, connected to highly producing fractures, have been noticed
since production began. Tarits et al. (2006) report a 97% increase
in Cl concentration, with present-day Cl concentrations exceeding
75 mg/L. The mixing trend observed from the d34S-SO4 and d18O-SO4
values strongly suggests that the salinity increase results from an
increasing contribution of a deep brackish fluid caused by the high
pumping rate. The effect of mixing between the denitrified water
and the brackish water is also seen on the d15N vs. d34S-SO4 diagram
of Fig. 7: F26, which plots in the middle of the mixing line of Fig. 4,
does not plot on the denitrifying curve of Fig. 7, but is significantly
shifted toward higher d34S-SO4 values, highlighting the chemical
composition results from both denitrification and mixing.
The determination of the origin of this brackish water is not the
purpose of the present paper, since it would require further and
different investigations. It is just worth noting that, despite the
location of the Ploemeur site close to the sea, the SO4 isotope composition, particularly d18O-SO4, of this brackish water is distinct enough from that of present-day sea-water to eliminate any
significant sea-water contribution. This might suggest a potential
regional occurrence of brackish water that could occur below several hundred metres depth and might ultimately represent a problem for deep drinking-water exploitation.
6. Conclusions
The fate of N and S was investigated in seven hard-rock aquifers
in Brittany (France) in the context of intensive agriculture, by
applying SO4 isotope (d34S-SO4, d18O-SO4) and occasionally N isotope
(d15N-NO3) tools. A compartment approach had to be used due to
the heterogeneous structure of the hard-rock aquifers, composed
of a highly weathered layer, saprolite or regolith, overlying a fissured layer below which fresh basement is permeable only locally
where joints and fractures occur. Although geological conditions
are different at the investigated sites, the study revealed common
features of the chemistry and isotopes of groundwater, and in particular their dependency on the aquifer structure,
1. Shallow groundwater in the regolith is commonly contaminated by nitrates, and the d34S-SO4, d18O-SO4 and d15N-NO3 values
fingerprint atmospheric deposition, fertilizer application and N
and S cycling through the soil. The d34S-SO4 traces the origin of
fertilizer and manure applications, whereas fractionation during cycling through soil modifies both the d18O-SO4 and d15NNO3 signatures.
2. Wetlands develop in the lowest parts of the landscape and, provided they are preserved, are potential receptacles of shallow
groundwater in the regolith from higher elevations, where het-
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H. Pauwels et al. / Applied Geochemistry 25 (2010) 105–115
erotrophic denitrification occurs. The moderately reducing conditions are probably caused by discontinuous periods of flooding. Nheir d18O-SO4 is clearly differentiated from that in the
regolith and fingerprints the assimilatory SO4 reduction.
3. In the fissured part of the aquifer, autotrophic denitrification
occurs where sulfide minerals are available, unless chemical
and isotope composition is not differentiated from that of
regolith groundwater. Denitrification is almost complete at all
sites, except where water transfer is accelerated by high pumping rates. Since denitrification induces the oxidation of sulfide
minerals, it is observable in both d34S-SO4 and d18O-SO4 values.
4. At depth, an old and brackish fluid has been sampled at one site
(Betton) and SO4 isotopes fingerprint its presence at Ploemeur
and Naizin. This fluid has a clear influence on the chemical
and isotopic characteristics of groundwater in the fissured
layer, and groundwater pumping favours its mixing with
groundwater from the fissured aquifer, thus significantly
increasing water-resource salinity.
In addition to the common features observed within each aquifer compartment regardless of the site, the investigation highlights
how SO4 isotopes provide useful information for land- and watermanagers faced with development problems related to hard-rock
aquifers; They can contribute to a better characterization of a
groundwater body, leading to the implementation and monitoring
of corrective measures to decrease the impact of diffuse pollution.
Such information will in particular:
– Help to show the presence of the denitrification process, particularly within the fissured part of the aquifer. Where NO3 is missing, denitrification cannot be shown by means of the N isotope,
but SO4 isotopes can do this. Where denitrification is incomplete, its impact on N isotopes may be too weak for drawing a
consistent conclusion, but again combined use with SO4 isotopes
can be helpful. Compared to the excess-N method that is proven
to be robust for quantifying denitrification (Singleton et al.,
2007), SO4 isotopes allow identifying the electron donor. Since
sulfide minerals are not inexhaustible and autotrophic denitrification will eventually stop, this evidence is relevant information
for long-term groundwater protection.
– Help to distinguish between a salinity increase caused by diffuse
pollution followed by denitrification, from a salinity increase
caused by the inflow of brackish water, which may require distinct corrective actions.
– Help to separate autotrophic from heterotrophic denitrifying
processes, which may be useful for checking the efficiency of
wetlands if their preservation is intended to protect surface
water from diffuse pollution.
– Help to discriminate NO3 sources by using N isotope and SO4
isotope tools. This is because of the analogy between N and S
cycling within soil, and the observed correlation between d18O15
SO4 and d N in groundwater below fields receiving similar N
applications. Nitrogen fractionation is generally ignored during
such investigations, but it can constrain the accuracy of identifying a N source. It is expected that SO4 isotopes can overcome this
problem, although such methods require further investigations
focusing on the fractionation of both sulfates and nitrates in
soils after fertilizer applications.
Acknowledgments
This paper benefited from the technical assistance provided by
Christine Fléhoc (BRGM Mass-Spectrometer team), who performed
the isotope analyses. This work has been supported by the Region
Bretagne (PRIR-Dateau) and by the French National Research
Agency (ANR) through the VMC programme (Project MOHINI No.
ANR-07-VULN-08). The investigations also benefited from the support of CNRS (Environmental Research Observatories H+ and AgrHys). Two anonymous reviewers and Marinus Kluijver are
warmly acknowledged for improving the manuscript and English
editing.
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