Chemical Geology 265 (2009) 369–380
Contents lists available at ScienceDirect
Chemical Geology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c h e m g e o
High chemical weathering rates in first-order granitic catchments induced by
agricultural stress
A.-C. Pierson-Wickmann a,b,⁎, L. Aquilina a,b, C. Martin b,c, L. Ruiz b,c, J. Molénat b,c,
A. Jaffrézic b,c, C. Gascuel-Odoux b,c
a
b
c
CNRS – Université Rennes 1; CAREN Research Federation – Géosciences Rennes UMR 6118, Campus de Beaulieu, 35042 Rennes Cedex, France
Université Européenne de Bretagne (UEB, European University of Brittany), France
INRA – Agrocampus Ouest; CAREN research federation – Soil Agro and HydroSystem UMR 1069, 65 Rue de Saint-Brieuc, 35042 Rennes Cedex, France
a r t i c l e
i n f o
Article history:
Received 25 July 2008
Received in revised form 11 March 2009
Accepted 27 April 2009
Editor: B. Bourdon
Keywords:
Chemical erosion
Granitic catchment
Agricultural inputs
Cation release
Soil acidification
a b s t r a c t
Chemical erosion rates have been determined on two upland granitic catchments under agricultural pressure
in Brittany, France. Intensive agriculture has been carried out for at least 30 years in this region. The influence
of geochemical processes related to agriculture on the chemistry of streamwaters is determined through a
geochemical mass balance. The elemental export fluxes from these two agricultural catchments are then
compared with other catchments around the world.
The volume and concentrations of the precipitation are taken into account, as well as the inputs of organic
and chemical fertilizers, groundwaters and streamwaters, to estimate the relative influence on export fluxes,
and then evaluate the elemental fluxes released by weathering. The relatively high Si flux of about 1.8 ±
0.9 kmol ha− 1 yr− 1 is directly attributed to the chemical weathering of soil and rock in the catchment
system. However, the Si flux remains comparable to values found in both small and large-sized catchments
under temperate and tropical conditions. On the other hand, extremely high fluxes of major cations (Ca, Na
and Mg) are observed, ranging from 4.2 ± 2.6 to 8.0 ± 4.9 kmol ha− 1 yr− 1, which can be attributed to
chemical weathering. These fluxes remain dramatically higher than those found in granitic catchments
worldwide.
Despite an integrated agriculture, the soil acidification induced by fertilizer application leads mainly to a
release of major cations from the system, by processes of soil ion-exchange leaching as well as weathering of
soil and rock.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Silicate mineral weathering is a natural mechanism in ecosystems
that results in the neutralization of protons and the production of
soluble base cations (Ca, Mg, Na and K), along with aluminium and
silica (Drever, 1988; Likens et al., 1977), which, in turn, sustains
vegetative growth (Taylor and Velbel, 1991; White and Brandley,
1995). The rates of chemical weathering of minerals and rocks in small
catchments can be defined by a geochemical mass-balance method
that describes input–output budgets (Bricker et al., 1994; Garrels and
Mackenzie, 1967; Katz et al., 1985; Paces, 1983; Velbel, 1985). The
input–output budgets reflect a balance between sources (atmosphere,
biomass, exchangeable compounds in the soil, and mineral weath-
⁎ Corresponding author. CNRS – Université Rennes 1; CAREN Research Federation –
Géosciences Rennes UMR 6118, Campus de Beaulieu, 35042 Rennes Cedex, France Tel.: +33
2 23 23 50 56; fax: +33 2 23 23 61 00.
E-mail address: [email protected]
(A.-C. Pierson-Wickmann).
0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemgeo.2009.04.014
ering) and sinks (biomass and exchangeable compounds in the soil,
and streamwaters). The mass-balance approach has been mostly
applied to specific mineral weathering reactions (Blum et al., 1994;
Drever, 1988; Drever and Clow, 1995; Garrels and Mackenzie, 1967;
Katz et al., 1985; Mast et al., 1990; Paces, 1983), and especially on
granitic catchments (i.e. White and Blum (1995), Oliva et al. (2003)).
A number of studies show that chemical weathering is a major sink of
protons (H+) (Driscoll and Likens, 1982; Van Breemen et al., 1984; Van
Breemen et al.,1983). Human activities, through the combustion of fossil
fuels and industrial production, have led to widescale acidification of
rain waters (Schindler, 1988). Similarly, catchment budgets in agricultural systems are significantly influenced by major additional inputs of
agrochemicals or livestock manures. In addition to nitrate pollution,
which has been widely described, nitrogen fertilizers and timber
harvesting result in the net production of acidity in ecosystems, thus
promoting soil acidification. Relatively few catchment studies have
investigated the effects of agricultural practices on element budgets.
Correll et al. (1984) found that ionic outputs were lower for a forested
site than in a catchment on which cereals were grown. Increased output
370
A.-C. Pierson-Wickmann et al. / Chemical Geology 265 (2009) 369–380
Fig. 1. Location map of the Kerbernez and Kerrien catchments (Brittany, France), showing the different sampling sites (wells and streams).
fluxes of nutrients (Collins and Jenkins, 1996; Pereira, 1987), as well as
accelerated rates of chemical weathering, have been attributed to the
addition of fertilizers (Mayorga, 2008; Paces, 1983).
However, the net effect of agriculture on cation fluxes remains
poorly known, as well as the sources of chemical elements in
agricultural catchments. The purpose of this study is to use a massbalance approach to assess the influence of agricultural practices and
applications on weathering rates of silicate rocks in a temperate
environment. This study focuses on two small adjacent catchments in
Brittany, western France. Indeed, Brittany is well known for the
development of intensive agriculture, especially over the last thirty
years. The cationic chemical weathering rates for these catchments
are estimated by monitoring precipitation, groundwater and streamwaters as well as fertilizers (volumes and chemistry) over the last ten
years. The solute fluxes from these two small catchments are then
compared with erosion rates given in the literature.
2. Study site description
The experimental site (Fig. 1), described in previous studies
(Martin et al., 2004; Ruiz et al., 2002a), consists of two adjacent
first-order catchments, Kerbernez (0.12 km 2 ) and Kerrien
(0.095 km2), located in south-western Brittany, France (47°57′N–
4°8′W, about 550 km west of Paris and 10 km from the Atlantic
Ocean). These upland catchments share the same climatic and
lithological characteristics. The regional climate is oceanic. Mean
annual precipitation and potential evapotranspiration calculated for
the last decade are 1179 and 643 mm, respectively (Legout et al., 2005;
Ruiz et al., 2002a). Mean annual temperature is 11.4 °C, with a
monthly minimum of 6.1 °C in January and a maximum of 17.6 °C in
July (Ruiz et al., 2002a). During the period 2000–2005, the average
air/soil temperature was 12.0 °C.
Table 1
Nature and application levels of organic and inorganic fertilizers on Kerbernez and
Kerrien catchments over the 1992-2005 period.
2001-2005
average in
kg/ha
1996-2005
average in
kg/ha
1996-2000
average in
kg/ha
1992-2005
average in
kg/ha
Rate ⁎
Rate ⁎
Rate ⁎
σ
Rate ⁎
Kerbernez catchment
Ammonitrate
98.5
NPK
0
Urea
2.6
CaOMg
5.7
KCl
0
4.7
Slurry (m3/ha)
Manure (T/ha)
0.9
Kerrien catchment
Ammonitrate
NPK
Urea
CaOMg
Trez
KCl
Slurry (m3/ha)
Manure (T/ha)
144.0
39.0
2.7
12.3
236.8
28.4
3.0
4.8
σ
σ
σ
86.9
0
2.8
12.8
0
2.7
0.9
106.8
12.9
2.0
2.8
3.0
4.5
0.7
64.5
29.3
2.9
9.0
4.2
2.4
0.7
115.2
25.9
1.4
0
6.4
4.3
0.5
40.3
39.9
3.1
0
4.0
2.5
0.6
97.0
9.7
1.4
2.0
2.2
5.2
0.5
59.3
25.0
2.6
7.6
3.8
3.3
0.7
71.4
53.5
6.1
27.5
529.6
38.9
2.2
3.1
143.9
31.1
2.5
6.2
118.4
36.7
3.1
3.9
50.5
45.0
5.4
19.5
374.5
49.4
2.0
4.2
143.7
23.2
2.4
0
0
45.0
3.3
3.1
25.1
39.1
5.3
0
0
61.7
2.1
5.3
160.0
29.5
1.8
4.4
84.6
26.2
4.5
3.0
58.5
40.3
4.7
16.5
316.5
44.6
4.0
3.8
⁎: Except for slurry (m3/ha) and manure (T/ha). s represents the standard deviation
over the average period. The application rates are reported for the total catchment area:
12 ha for Kerbernez and 9.5 ha for Kerrien.
A.-C. Pierson-Wickmann et al. / Chemical Geology 265 (2009) 369–380
Table 2
Hydrological context during the study period.
Hydrological period
PET
P
R
QKerbernez
QKerrien
11/2000–10/2001
11/2001–10/2002
11/2002–10/2003
11/2003–10/2004
11/2004–10/2005
2000–2005 average
694
727
746
703
703
714 ± 21
1619
1017
1111
1074
690
1102 ± 333
1043
376
615
444
219
539 ± 315
626
197
278
221
129
290 ± 195
946
179
537
264
71
400 ± 351
Hydrological year is from October 1st to September 31st. A 5-year-average is expressed
with the standard deviation.
Precipitation (P), potential evapotranspiration (PET) and soil water drainage (R) are
calculated for each hydrological year based on daily data and expressed in mm/yr.
Stream specific discharge for both Kerrien (QKerrien) and Kerbernez (QKerbernez) outlets
are also reported for the same period.
The basement of the basin is made up of Paleozoic rocks belonging
to the Plomelin leucogranodiorite (Béchennec et al., 1999). This
coarse-grained rock type consists of quartz (40%), plagioclase (albiteoligoclase) and K-feldspar (50%), muscovite and biotite (10%). Some
clay minerals (kaolinite) are also present in the granitic sandy regolith
(Béchennec et al., 1999). Accessory minerals can include apatite,
garnet and zircon (Béchennec et al., 1999). The fresh granite is
overlain by 1 to 20 m of regolith (Montoroi et al., 2001), which is
slightly thicker at Kerbernez than at Kerrien (Legchenko et al., 2004).
Soils are mainly brown sandy loam (distric cambisol, FAO
classification) developed on a granitic sand. Soil profiles were dug
down to the C or B/C horizon. The weathered granite appears between
0.7 and 1.2 m below the soil surface in Kerrien and Kerbernez (Legout
et al., 2005). The C-horizon represents the weathered bedrock and
contains sand (63%), silt (26%) and clay (11%). The B-horizon
corresponds to a cambic horizon (BW), varying from 40 to 65 cm in
thickness, with low organic carbon content (0.5 wt.% C) and enriched
in silt particles (~64%) relative to sand (19%) and clay (17%). The
sealed macropores are mostly generated by earthworms. The upper
horizons correspond to mineral horizons altered by human-related
activities (Ap) from 10 to 40 cm depth, formed of micaceous sands and
silts with higher organic carbon content (2.8 wt.% C). The soil bulk
density ranges from 1.3–1.5 g cm− 3 (A and B horizons) to 1.7 g cm− 3
(C-horizon). Soils are well drained except in the relatively narrow
bottomlands where hydromorphic soils are found (Ruiz et al., 2002a).
Land use is mainly agricultural (77%), with seven stockbreeding
farms. Most arable fields (43% of cultivated surface-area), growing
maize and cereals in rotation, are farmed intensively, including
importation of pig slurry and cattle manure. Most of the grasslands
(40% of the cultivated surface-area) are grazed intensively by dairy
cows (Ruiz et al., 2002a). Farmers have exhaustively recorded all
agricultural activities over the last decade. These data are presented as
average values in Table 1. The typical total amount of inorganic
371
mineral fertilizer applied varies between the Kerrien and Kerbernez
catchments, and over the years. The inorganic mineral fertilizers
consist of ammonitrates, NPK and KCl for both the Kerrien and the
Kerbernez catchments. Some calcareous soil improvement (liming,
local Trez) has also been carried out on the Kerrien catchment.
Relative to the total catchment area, a higher amount of inorganic
fertilizer is applied on the Kerrien than on the Kerbernez catchment
(164–533 kg/ha as against 30–257 kg/ha). Organic fertilizers (pig
slurry and cattle manure) are also applied on both catchments at
different rates. On average, the Kerbernez catchment receives more
pig slurry and less cattle manure than the Kerrien catchment
(Table 1). These rates are comparable with fertilizer application on
intensively farmed grassland in temperate regions (Oenema, 1990).
Previous studies (Legout et al., 2005; Martin et al., 2004; Ruiz et al.,
2002a) have investigated the relationships between stream water
quality and annual nitrate fluxes, as a function of agricultural practices
and groundwater transfer, as well as the elemental transfer from the
unsaturated to the saturated zone. Both catchments are characterized
by the presence of shallow groundwater in the weathered granitic
material (Molénat et al., 2008). The groundwater feeds the stream
throughout year. In the bottomland, the water table is near the soil
surface and the uppermost layer of the groundwater flows through
the soil. Along hill slopes, the water table is typically 2–8 m below the
land surface (Molénat et al., 2008).
3. Material and methods
3.1. Water sampling
An automatic weather station, located 500 m north of the
catchment outlets in an open field, was used to record daily rainfall
(P) and the different parameters required to calculate potential evapotranspiration (PET) by the Penman formula, assuming that the
parameters are similar for both the Kerrien and the Kerbernez
catchments. Stream discharge was measured continuously at the
two catchment outlets equipped with V-notch gauging stations using
pressure-sensor dataloggers. The monitoring period (in this study)
extended over five hydrological years, from October 2000 to
September 2005, including two very different years (2000–2001
and 2004–2005). The first year (from October 2000 to September
2001) was very wet, with a total precipitation of 1619 mm and a PET of
694 mm, while 2004–2005 was much drier, with a total precipitation
of 690 mm and a PET of 703 mm (Table 2).
Stream- and rainwater-samples were collected monthly over the
two first years, and then every three months. To monitor groundwaters and stream waters, both catchments were equipped with
several piezometers. In the Kerbernez catchment, two transects
were equipped with four wells each (labelled A and B in Fig. 1). In
the Kerrien catchment, eight wells were placed along one transect
Table 3
Average concentrations of elements in precipitation and Kerrien and Kerbernez streamwater during the period from October 1st 2000 to September 31st 2005.
Chemical species Precipitation
Kerrien streamwater
Kerbernez streamwater
Mean (mmol L− 1) Relative standard deviation (%) Mean (mmol L− 1) Relative standard deviation (%) Mean (mmol L− 1) Relative standard deviation (%)
pH
Cl
Ca
Mg
Na
K
Al
Si
Rb
Sr
Ba
U
6.17
0.24
0.02
0.02
0.15
0.01
3.65 × 10− 4
1.01 × 10− 3
5.50 × 10− 6
5.23 × 10− 5
1.09 × 10− 5
1.00 × 10− 7
5.3
4.3
50.5
27.2
31.6
50.7
52.8
103.3
59.0
69.5
104.6
100.3
6.32
0.91
0.27
0.32
0.87
0.13
1.05 × 10− 3
0.274
5.84 × 10− 5
1.15 × 10− 3
1.86 × 10− 4
1.88 × 10− 6
5.6
8.3
20.2
13.3
7.5
23.7
83.7
9.9
21.5
13.8
20.1
45.2
6.05
1.00
0.37
0.43
1.07
0.11
5.83 × 10− 4
0.315
5.51 × 10− 5
1.81 × 10− 3
1.90 × 10− 4
2.93 × 10− 6
3.9
8.6
12.6
10.8
6.8
22.9
59.51
9.1
14.7
8.9
9.6
30.1
372
A.-C. Pierson-Wickmann et al. / Chemical Geology 265 (2009) 369–380
(labelled F in Fig. 1). In this latter catchment, shallow wells were
also placed close to the stream network along two lines (labelled
C and D in Fig. 1). Well depths ranged from 1.5 to 20 m. Wells
consisted of PVC tubes screened over an interval of 0.5 to 1 m at
their base.
4.2. Chemical budget computation
3.2. Chemical analyses
NðiÞ = F ðiÞSO − F ðiÞSI = F ðiÞW + F ðiÞP + F ðiÞAGR F F ðiÞB + F ðiÞS
All water samples were filtered in the field through 0.20 µm
Nylon Millipore filters and then stored acidified (HNO3) and nonacidified in the dark at less than 4 °C. Major and trace cation concentrations were determined using an Agilent Technologies™
HP4500 ICP-MS (Bouhnik-Le Coz et al., 2001; Yeghicheyan et al.,
2001). The instrument was calibrated using both an external calibration based on multi-element synthetic standards and an internal
calibration (indium) in all standards and samples. The international
standard SLRS-3 was used to check the accuracy and reproducibility
of the results (Dia et al., 2000). Typical uncertainties including all
error sources are less than 5% for all trace elements, whereas, for
major anions, the uncertainty lies between 2 and 5%, depending on
the concentration level.
where, per unit of time, Ni is the net accumulation or depletion of
solute i in the system, being the difference between the outputs (F(i)SO)
and the inputs (F(i)SI) of solute i in the system, F(i)W is the mass of solute
i generated by chemical weathering, F(i)P is the mass of solute i from
atmospheric deposition in the system, F(i)AGR is the mass of solute i
from anthropogenic inputs to the system, F(i)B is the mass of the solute i
associated with biological processes in the system, F(i)S is the change in
concentration of solute i on the soil exchange complex in the system.
The chemical mass balance is calculated here over different average
time-periods (2001–2005, 1996–2000, 1996–2005, and 1992–2005),
taking into account the precipitation, streamwater data and fertilizer
data. For this study, the water fluxes are available for the period 2001–
2005. Hence, the same average of water fluxes are used for the four
periods of time considered. The main variable is the amount of fertilizer
recorded over a 14-year period (Appendix A).
4. Chemical budget computation
4.1. Water mass balance
The Kerrien catchment shows a high specific discharge in winter,
but it almost dries out in summer and autumn. By contrast, the
Kerbernez catchment maintains a relatively higher discharge during
low-stage periods, but discharge remains moderate in winter (Martin
et al. (2004). The water budget, calculated as the difference between
precipitation, actual evapotranspiration and stream discharge
(Table 2), indicates a water deficit of ranging from 8 to 30% of the
input precipitation for the different years and outlets. Underflow not
intercepted at the catchment outlets and significant deep losses may
explain this imbalance in the water budget (Ruiz et al., 2002b). This
implies that the measurement of stream discharge underestimates the
total water outflowing from the catchment. To calculate chemical
erosion rates, we need to know the total amount of water outflowing
from the catchment, i.e., the sum of stream discharge, underflow and
deep losses. The outflowing water flux is assumed to be equal to the
soil water drainage, i.e. the water flux through the base of the soil
layer. The soil water drainage is estimated from a simple reservoir
model. The model treats the soil layer as a reservoir whose level is
updated daily by adding rainfall and subtracting PET. When the
reservoir is filled up, soil water drainage occurs at a rate corresponding
to the amount of excess rainfall.
A conceptual model of mass balance, calculated from aqueous
concentrations of solutes (Bricker et al., 1994; Drever, 1988; Mast et al.,
1990), can be represented by the following equation:
ð1Þ
4.2.1. Precipitation budgets
Atmospheric inputs of chemical elements (F(i)P, mol ha− 1 yr− 1)
are estimated using the volumetric measurements of precipitation
together with chemical composition data, according to Eq. (2), where
CP(i) (mmol L− 1) is the concentration of solute i (Table 3, Appendix A),
QP (mm or L m− 2 yr− 1) is the mean annual precipitation.
h
i h
i
−3
4
F ðiÞP = CP ðiÞ × 10
× Q P = 10
ð2Þ
4.2.2. Agricultural inputs
In an agricultural context, the anthropogenic input (F(i)AGR) is
mostly composed of the flux of solute i from the application of
chemical and organic fertilizers and liming. On both catchments, the
main mineral and organic fertilizers (Table 1, Appendix A) are
ammonium nitrate [NH4NO3], cattle manures and pig slurries. More
rarely, KCl fertilizer is also applied to grasslands on the Kerrien
catchment. Based on their chemical formula, ammonium nitrate and
urea [CO(NH2)2] are not considered as base cation sources. The only
base cation sources are represented by KCl fertilizers, cattle manures
and pig slurries, and more occasionally liming. To estimate chemical
fluxes due to agricultural inputs, we compiled a database of solute
Table 4
Element fluxes (F(i)AGR (mol ha− 1 yr− 1)) from fertilizer applications for different periods (2001-2005, 1996-2000, 1996-2005 and 1992-2005) for both Kerrien and Kerbenez
catchments.
FAGR (mol ha− 1 yr− 1)
Kerrien
Chemical species
2001–2005
1996–2000
1996–2005
1992–2005
2001–2005
1996–2000
1996–2005
1992–2005
Cl
Ca
Mg
Na
K
Al
Si
Rb
Sr
Ba
U
452
1724
191
58
653
9
32
1.4) × 10− 1
4.3 × 10− 1
7.5 × 10− 3
1.8 × 10− 3
658
61
74
55
823
9
33
1.1 × 10− 1
2.8 × 10− 1
8.1 × 10− 3
1.3 × 10− 3
555
892
132
56
737
9
32
1.2 × 10− 1
3.5 × 10− 1
7.7 × 10− 3
3.8 × 10− 3
373
608
485
3.2
473
1.7
3.6
6.2 × 10− 2
2.9 × 10− 1
1.2 × 10− 3
1.0 × 10− 3
38
57
91
63
174
11
41
8.7 × 10− 2
8.0 × 10− 2
1.1 × 10− 2
7.4 × 10− 4
53
68
90
60
270
12
42
1.3 × 10− 1
3.2 × 10− 1
1.0 × 10− 2
1.5 × 10− 3
45
63
90
61
222
11
42
1.1 × 10− 1
2.0 × 10− 1
1.0 × 10− 2
1.1 × 10− 3
47
68
102
69
233
13
472
1.2 × 10− 1
1.8 × 10− 1
1.2 × 10− 2
1.1 × 10− 3
Kerbernez
The elemental fluxes are expressed as average values for the different periods.
A.-C. Pierson-Wickmann et al. / Chemical Geology 265 (2009) 369–380
373
Table 5
The chemical budget N(i) (mol ha− 1 yr− 1) of the chemical element supplied by atmospheric inputs (F(i)P) and solutes released from stream at the catchment outlet using the
calculated specific discharge (FSO) for both Kerrien and Kerbernez catchments based on stream discharges for the four different periods of time.
Chemical
species
FP
Cl
Ca
Mg
Na
K
Al
Si
Rb
Sr
Ba
U
4.79 × 103
2.06 × 102
3.12 × 102
2.44 × 103
1.77 × 102
5.95
1.68 × 101
8.64 × 10− 2
9.51 × 10− 1
2.14 × 10− 1
2.03 × 10− 3
FSO
(Kerrien)
FSO
(Kerbernez)
2001–2005
N(i) in Kerrien
1996–2005
1996–2000
1992–2005
2001–2005
N(i) in Kerbernez
1996–2000
1996–2000
1992–2005
4.90 × 103
1.46 × 103
1.73 × 103
4.70 × 103
7.34 × 102
5.64
1.48 × 103
3.15 × 10− 1
6.23
1.00
1.02 × 10− 2
5.42 × 103
2.00 × 103
2.33 × 103
5.80 × 103
5.95 × 102
3.14
1.70 × 103
2.97 × 10− 1
9.76
1.02
1.58 × 10− 2
− 3.50 × 102
− 4.70 × 102
1.23 × 103
2.20 × 103
− 9.54
− 9.60
1.43 × 103
9.23 × 10− 2
4.85
7.82 × 10− 1
6.40 × 10− 3
4.53 × 102
3.62 × 102
1.29 × 103
2.21 × 103
− 1.80 × 102
−9.52
1.43 × 103
1.06 × 10− 1
4.92
7.82 × 10− 1
6.64 × 10− 3
− 5.56 × 102
1.19 × 103
1.34 × 103
2.21 × 103
− 2.66 × 102
− 9.53
1.43 × 103
1.18 × 10− 1
5.00
7.81 × 10− 1
6.86 × 10− 3
− 2.72 × 102
6.46 × 102
1.37 × 103
2.26 × 103
8.46 × 101
− 2.01
1.46 × 103
1.67 × 10− 1
4.99
7.88 × 10− 1
7.15 × 10− 3
5.87 × 102
1.73 × 103
1.93 × 103
3.30 × 103
2.44 × 102
−1.39 × 101
1.64 × 103
1.24 × 10− 1
8.73
7.99 × 10− 1
1.31 × 10− 2
5.80 × 102
1.73 × 103
1.93 × 103
3.30 × 103
1.96 × 102
− 1.43 × 101
1.64 × 103
9.98 × 10− 2
8.61
7.99 × 10− 1
1.27 × 10− 2
5.72 × 102
1.72 × 103
1.93 × 103
3.30 × 103
1.49 × 102
− 1.47 × 101
1.64 × 103
7.59 × 10− 2
8.49
7.99 × 10− 1
1.23 × 10− 2
5.78 × 102
1.72 × 103
1.92 × 103
3.29 × 103
1.85 × 102
− 1.57 × 101
1.64 × 103
9.43 × 10− 2
8.63
7.98 × 10− 1
1.27 × 10− 2
F(i)AGR used for these calculations are presented in Table 4.
A positive value indicates the release of an element and a negative value indicates the storage or consumption of an element.
concentrations measured in organic and mineral fertilizers (Appendix
B), including fertilizers directly applied to the studied catchments, as
well as literature data (McBride and Spiers, 2001; Riou, 1995; Widory
et al., 2001). The database shows relatively homogeneous values for
the major cations. However, there are large differences in trace
element concentrations in NPK fertilizers, which vary by a factor of 10
to 100 compared with the values from McBride and Spiers (2001) and
Riou (1995). Such differences in organic fertilizers could be explained
by the specific nature of regional products. As far as possible, we
preferred to use regional data for budget calculations.
For a given chemical element i, the agricultural input (F(i)AGR, mol
ha− 1 yr− 1) is defined according to Eq. (3), where MAGR (kg ha− 1 yr− 1)
is the annual amount of fertilizer and CAGR(i) is the solute concentration
in fertilizers (g kg− 1 or g L− 1), and M(i) is the molar mass of element i
(g mol− 1). The values of CAGR(i) used for this calculation are given in
Appendix B.
F ðiÞAGR =
X
MAGR × CAGR ðiÞ = MðiÞ
ð3Þ
Table 4 presents the results for the four different periods of time.
4.2.3. Biomass and water storage
In addition to the data on fertilizers, the local farmers provided
exportation data concerning grassland and crop cuts. The dairy cow
uptakes are assumed to be compensated by their excreta. The storage
of water is also considered to be unchanged. F(i)S is thus only related
to concentration changes in the soil exchange complex.
from the catchment, and C(i)SO (mmol L− 1) is the mean annual
stream solute i concentration.
h
i h
i
4
−3
F ðiÞSO = RSO = 10 × C ðiÞSO × 10
ð4Þ
The data are presented in Table 5 for the different periods of time.
The results for the individual years are presented in Appendix C.
4.2.5. Chemical budgets
The chemical mass balance is computed using data presented
previously and reported in Table 5. To determine the flux of elements
released from soil and rock weathering, we assume that the
catchment is at steady-state during a 5-year period. We then define
the net budget (N(i)) as the difference between the solute exports at
the catchment outlets and both the atmospheric and agricultural
inputs, as expressed in Eq. (5):
−1
−1
= F ðiÞW + F ðiÞS = F ðiÞSO − F ðiÞAGR − F ðiÞP ð5Þ
yr
NðiÞ mol ha
In Table 5, negative values for any element indicate storage in the
system, while positive values indicate a release from weathering F(i)W
and/or from soil exchange F(i)S. In Table 6, the Cationic Chemical
Erosion Rate (CCER), commonly expressed in mm/1000 yr, is derived
from the net export of solutes (Table 5) using a measured bedrock
density of 2.65 g cm− 3 (Legout et al., 2007), and normalized to the
catchment area (Drever and Clow, 1995).
4.3. Uncertainties
4.2.4. Solute outputs
Solute output (F(i)SO, mol ha− 1 yr− 1) is estimated according to
Eq. (4), using the mean annual solute concentrations measured in
streamwater, where RSO (mm or L m− 2 yr− 1) is the calculated annual
soil water drainage, assumed to be the sum of all outflowing water
4.3.1. Dry deposition
As the catchments are mainly covered by grass with some trees
lining their borders, throughfall is considered negligible. The
precipitation sampling device, located in an open field, enables the
Table 6
Cationic element release rate (N, mol ha− 1 yr− 1, from Table 5) and cationic chemical erosion rate (mm/1000 yr) for the Kerrien and Kerbernez catchments.
Period
N (Ca + Na + Mg)(mol ha− 1 yr− 1)
CCER (mm/1000 yr)
N(Si) (mol ha− 1 yr− 1)
CCER (mm/1000 yr)
In Kerrien
In Kerbernez
2001–2005
1996–2000
1996–2005
1992–2005
2001–2005
1996–2000
1996–2005
1992–2005
3430 ± 2514
3.9 ± 2.2
1428 ± 819
1.5 ± 0.9
3853 ± 2593
3.7 ± 2.3
1428 ± 822
1.5 ± 0.9
4744 ± 3459
5.0 ± 3.5
1428 ± 821
1.5 ± 0.9
4274 ± 2709
4.3 ± 2.4
1457 ± 841
1.5 ± 0.9
6961 ± 4811
7.4 ± 5.0
1645 ± 954
1.78 ± 1.0
6958 ± 4796
7.4 ± 5.0
1644 ± 952
1.8 ± 1.0
6954 ± 4785
7.4 ± 5.0
1644 ± 951
1.8 ± 1.0
6933 ± 4764
7.4 ± 5.0
1639 ± 945
1.8 ± 1.0
Cationic chemical erosion rate = CCER calculated assuming a rock density of 2.65 g/cm3.
Standard deviations are also shown.
374
A.-C. Pierson-Wickmann et al. / Chemical Geology 265 (2009) 369–380
collection of bulk precipitation, including both wet and dry
deposition.
4.3.2. Biomass uptake and mineralization
Biomass pools may modify the element budget via uptake of
elements such as Ca, Mg, N, P or K from the soil solution and
incorporation into the biomass. As local farmers provided information
on crop exportation from fields (not shown here), we can estimate the
quantity of Ca, Mg, K, Na and Si (F(i)B) removed from the soils by
vegetation as less than 1% of the Net Cation Export (N(i)). This
parameter is considered negligible.
4.3.3. Storm events
In small catchments, the stream discharge can be rather sporadic
with rapid increases of flow during prolonged heavy rains. The
chemical composition of streamwater can vary substantially with the
stream discharge. Previous investigations on the Kerrien and
Kerbernez catchments have indicated that the contribution from
surface waters is generally low (b5%) in small catchments on poorly
permeable bedrock (Molénat et al., 1999). Thus, the measured
chemical concentrations adequately represent the entire compositional range of the stream waters.
calculated as P/(P-PET) ranges from 1.7 to 4.0 over the 5 years,
which implies theoretical concentrations for Cl and Na much lower
than measured in the Kerbernez and Kerrien catchments. This
suggests that, in addition to precipitation, soil exchange and rock
weathering contribute to the chemistry of groundwaters and
streamwaters.
Although water budgets in the Kerrien and Kerbernez catchments
have been measured over the last ten years (Martin et al., 2004; Ruiz
et al., 2002a), no temporal trend can be observed. The chemical
composition of streamwaters is strongly related to that of groundwaters, as the streams are mainly fed by groundwater discharge
(Martin et al., 2004; Ruiz et al., 2002a). Groundwater chemical
compositions (Fig. 2, Appendix C) are rather homogeneous and show
extremely limited variation during the last 5 years of monitoring.
Furthermore, the residence time based on CFC measurements (Ayraud
et al., 2008) yields an apparent groundwater age of 0–10 years in the
upper part of the catchment (5–10 m below surface) and up to 10–
25 years at 10–25 m depth. The streamwater compositions shows
even smaller variations, and are different compared with precipitation, implying that the large variations of chemical fluxes are mainly
related to specific discharge variations.
5.2. Agricultural inputs (F(i)ARG)
5. Results
5.1. Water chemistry
5.1.1. Precipitation (F(i)P)
Table 3 lists the average concentrations and standard deviations of
all cations in samples of precipitation and stream waters over the
studied period. The concentrations in precipitation may show a
significant temporal variation by a factor of 10 for some elements. In
particular, the concentrations of Na, Mg are much higher in winter
than in summer, while Ca has higher concentrations in summer. Due
to the vicinity of the Atlantic Ocean, Na and Cl are the main
components of the precipitation, representing 78 to 96% of the
dissolved solids. Na alone represents 45 to 81% of the total dissolved
cations. The average concentrations calculated over the 5-year study
period, weighted by the volume of precipitation, are 0.24 ± 0.01 mmol
L− 1 for Cl and 0.15 ± 0.04 mmol L− 1 for Na. However, the relatively
low Na/Ca ratios are very different from seawater, indicating reactions
between seasalt aerosols and chemical compounds in terrestrial dust
(Reid et al., 1981).
5.1.2. Stream- and ground-waters
The stream waters (Table 3) are slightly acidic, with a pH within
the range of values found in rainwater. The Na and Cl concentrations
are about 5 times higher in the stream- and ground-waters than in the
precipitation. The concentrations of major cations vary by 7 to 20%
over the last 5 years. This indicates that the streamwater chemical
composition is only slightly sensitive to variations in precipitation or
soil water drainage. The streamwaters of the Kerbernez catchment
display, on average, 10 to 30% higher concentrations of Ca, Mg, Na and
Si and lower K in comparison to Kerrien. In a previous study, Martin
et al. (2004) suggest that the streamwater chemistry is controlled by
mixing between shallow and deep groundwaters in the Kerrien
catchment, while relatively concentrated deep groundwaters make up
the main contribution in the Kerbernez catchment. Kerbernez
streamwaters are indeed generally more concentrated than in the
case of Kerrien. In the data presented in Fig. 2, the streamwater is
bracketed by compositions representing shallow groundwater (3–
8 m) and deep groundwater (15–20 m), which confirms that
streamwater is derived from a mixing of these end-members. As
expected in such catchments, the stream concentration is very close to
groundwaters, which indicates that the stream is directly related to
the groundwater discharge. The evaporative water loss factor
Chemical elements supplied by agricultural inputs are highly
variable from one year to another, due to the changing application of
organic and mineral fertilizers (Table 4, Appendix A). The main
inputs involve K and Cl (in the form of KCl for both catchments),
while NPK fertilizers were not used on Kerbernez during the period
2001–2005, while Ca was twice applied as lime on Kerrien during
the same period. In general, over the last 14 years, the Kerrien
catchment received a larger volume of organic and mineral fertilizers,
as shown in Appendix A.
5.3. Stream solute fluxes and chemical erosion (F(i)SO and N(i))
The dominant precipitation-derived inputs are Na and Cl, reflecting the marine influence on atmospheric deposition in this region. A
large amount of base cations and chloride are exported from the
catchments. During the 2001–2005 period, the following solutes, in
order of decreasing abundance, were exported from the Kerrien and
Kerbernez catchments: (Na, Cl) N Mg N (Ca, Si) N K. There was a larger
export of cations by streamwaters in Kerbernez than in Kerrien, by
about 15–30% depending on the element.
6. Discussion
6.1. Cation release and acidification processes
Even considering the influence of agricultural and atmospheric
inputs on stream solute fluxes, the net budgets (N(i), Table 5) of Si,
Mg, Na and Ca remain particularly high. This suggests that the excess
of cations are derived from a considerable leaching of weathering soil
and rock products. Potassium either remains mainly stored in the
catchment due to the formation of clay minerals in soils or is
consumed by the local biomass. The storage/consumption of K is not
balanced by the release of other cations and silica. This implies
cationic exchange on the soil-clay fraction and/or weathering of rocks
and soil. Along with Si, minor soluble cations such as Ba, Rb and U are
also released by weathering. In view of contribution of atmospheric
and agricultural inputs, up to 88% of Ca, 74–85% of Mg, 97% of Si, 47% of
K and 54–62% of Na can be explained by soil/rock leaching and
weathering.
At the worldwide scale, a coupling between tectonics, erosion and
chemical weathering has been shown from global compilations of
riverine Si and base cation fluxes (Waldbauer and Chamberlain,
A.-C. Pierson-Wickmann et al. / Chemical Geology 265 (2009) 369–380
375
Fig. 2. Major cation and chloride concentrations (µmol.L− 1) in rainwater (black diamonds), streamwater (open diamonds) and shallow groundwater sampled between 3 and 8 m
depth (black squares) and deep groundwater sampled between 15 and 20 m depth (black triangles). The groundwater end-members correspond to the groundwater encountered in
each catchment. All the sampled were collected over the same study period. The error bars represent the standard deviation of the different end-members.
2005), as well as by modelling (West et al., 2005) and field studies
(Riebe et al., 2004). At a more local scale, cation fluxes may depend on
several physical parameters of the catchment such as soil thickness or
topography. Such parameters control water fluxes in the catchments
and may induce enhanced weathering. However, as observed in
studies comparing chemical weathering in small catchments
(b100 ha) (White and Blum, 1995; West et al., 2005; Oliva et al.,
2003), extremely different rock types, topography and climate do not
376
A.-C. Pierson-Wickmann et al. / Chemical Geology 265 (2009) 369–380
generate fluxes as high as those observed in Brittany. Furthermore,
these studies have not emphasized physical parameters as a potential
cause of variations in chemical weathering. Agricultural practices such
as tillage may also enhance the mineral availability and weathering.
Such an effect is relatively closely associated with fertilizer use, but the
processes involved are difficult to distinguish. However, acid loading
due to fertilizer application are extremely high and remain the most
obvious control on chemical weathering.
Acidic deposition related to agricultural fertilizers is defined (Van
Breemen et al., 1984; Van Breemen et al., 1983) as an input/output
−
budget of the NH+
4 /NO3 couple, which can be expressed as:
H
+
+
+
load = NH 4 deposition − NH 4 leaching
−
ð6Þ
−
+ ðNO 3 leaching − NO 3 depositionÞ
Nitrogen budgets for the Kerrien and Kerbernez catchments
indicate mean nitrogen loads of the order of 240 Kg-N/ha/yr. Soil
acidification induced by this level of ammonitrate and urea application
is of the order of 10 to 20 kmol ha− 1 yr− 1. Such a value is even higher
than those found in forested areas under atmospheric acidification,
which give values ranging from 9 to 14 kmol ha− 1 yr− 1 (Van Breemen
et al., 1984; Van Breemen et al., 1983).
6.2. Comparison of Kerrien and Kerbernez catchments
The Kerrien and Kerbernez catchments display many interesting
differences in terms of net cation release. According to our calculations
(Table 5), the Kerbernez catchment releases far more cations during
chemical weathering than the Kerrien catchment. While the export of
Na + Mg + Ca cations in the Kerrien catchment varies from 3.4 ± 2.5 to
4.7 ± 3.4 kmol ha− 1 yr− 1 over the last 14 years, it remains stable in the
Kerbernez catchment during the same period at a level of about 6.9 ±
4.8 kmol ha− 1 yr− 1. Similarly, a release rate of about 1.7 kmol ha− 1 yr− 1
for Ca and Mg in the Kerbernez catchment compares with a rate of less
than 1.2 kmol ha− 1 yr− 1 in the Kerrien catchment. Higher amounts of
potassium are stored and/or consumed in the Kerrien catchment,
whereas we observe a minimum release rate of 1.5 × 102 mol ha− 1 yr− 1
in the Kerbernez catchment.
Two mechanisms are proposed to explain these differences. Firstly,
since the same meteorological parameters are assumed for calculating
cation fluxes from both catchments, we need to consider the existence
of internal differences. Temporal variations of stream discharge reflect
the characteristics and behaviour of groundwaters in each catchment
(Martin et al., 2006). The groundwater transmissivity determined by
geophysical investigations varies over more than one order of
magnitude, with values ranging from 1.8 ± 0.9 × 10− 4 m s− 1 to 2.6 ±
1.3 × 10− 5 m s− 1 in the Kerrien and Kerbernez catchments, respectively (Legchenko et al., 2004). The weathered material is thicker, and
has a higher clay content in the Kerbernez catchment compared to the
Kerrien catchment (Martin et al., 2004), which leads to lower hydraulic
conductivity in the Kerbernez catchment. Such physical differences
may induce a longer residence time of water in the soil and hence
promote weathering.
Secondly, another explanation may be the difference in agricultural
inputs. However, neither the changes in cation export from the
Kerrien catchment, nor the difference between the Kerrien and
Kerbernez catchments are supported by any variation in fertilizer
application rates over the last 14 years. Fertilizers have been used less
on the Kerbernez than on the Kerrien catchment, which nevertheless
shows a lower level of cation exports. However, the difference in
agricultural activities may have occurred decades ago. According to
the local farmers, the Kerbernez catchment received much more
fertilizers in the 1980s. This would imply that high cation export from
the Kerbernez would be a delayed effect of the fertilizers applied 15 to
30 years ago, rather than the result of applications over the last
15 years. The transfer time of elements from the inputs (aquifer
recharge) to the outputs is longer than 10 years, which is in good
agreement with the groundwater ages (Ayraud et al., 2008) and the
strong connection between stream waters and groundwaters. The
underestimation of fertilizer inputs in the past would enhance their
effects on the chemical erosion of local soil and bedrock. Higher
chemical weathering related to agricultural inputs would emphasize
the importance of time-lag due to catchment memory effects
(Kirchner et al., 2000; Molénat et al., 2002; Steinheimer et al.,
1998). We suggest that Kerrien is at nearly steady-state considering
the inputs and outputs of the catchments, while Kerbernez outputs do
not represent the last 14 years of inputs. This interpretation is
supported by the Cl budget for the Kerrien catchment, which is nearly
balanced when corrected for the anthropogenic inputs (−5.56 × 102 to
4.52× 102 mol ha− 1 yr− 1 of Cl), whereas the Kerbernez catchment
shows a net Cl export (5.56 × 102 mol ha− 1 yr− 1). Since the fertilizers
represent a cation-poor source, the underestimation of fertilizer
application does not invalidate the mass-balance conclusions,
although the results should be considered with less confidence for
the Kerbernez catchment.
6.3. Source of exported cations
The neutralization of acidity from the fertilizers can be achieved by
ion exchange reactions in the soil, whereby H displaces base cations
from exchange sites. The leaching of soil during the draining period
releases base cations. The weathering of fresh bedrock in deeper soil
layers, which is less dependent on hydrological fluctuations, also
releases some major elements. Both processes contribute to the major
cation flux from the catchment.
Corrections for atmospheric and agricultural inputs indicate that a
large part of the fluxes of Ca, Mg and Si and half of the Na are due to
rock weathering and/or soil ion-exchange desorption. The relative
proportions of the three main cations (Na, Ca and Mg) in streamwaters are similar to solutes resulting from weathering processes (cf.
soil leaching experiments, Pierson-Wickmann et al., submitted for
publication). This contrasts with the major element composition of
precipitation, indicating that weathering contributes significantly to
streamwaters (Billett and Cresser, 1996; White et al., 1999). We
estimate that chemical weathering contributes more than 76% to the
output fluxes of Ca and Mg, and more than 40% in the case of Na.
Fig. 3. Comparison of the Kerbernez (stars) and Kerrien (hexagons) catchments with UK
catchments from Stutter et al. (2002): mean %Na+ dominance (100x[Na+]/[Na+ +Ca2+ +
Mg2+]) plotted as a log10 function for the some upland sites (open circles) and agricultural
catchments (open squares) vs. the sum of annual weathering rate of Na+, Ca2+ and Mg2+.
Units are molc.m− 2/yr− 1.
A.-C. Pierson-Wickmann et al. / Chemical Geology 265 (2009) 369–380
377
Fig. 4. Si chemical erosion rate (mol.ha− 1.yr− 1) vs. calculated specific discharge for the Kerrien (grey squares) and Kerbernez (open circles). For comparison, small catchments
(b 100 ha) from Oliva et al. (2003) are also shown (black diamonds).
Nevertheless, Na remains the dominant cation in Kerrien and
Kerbernez streamwaters. The main sources of Ca, Mg and Na, as well
as Si, are plagioclase and biotite, which are more easily weathered
than K-feldspar or quartz. Such results are also supported by
strontium isotope analyses presented elsewhere (Pierson-Wickmann
et al., submitted for publication). Even considering the errors in
estimating fertilizer application rates through the residence time of
groundwaters, we cannot explain such high chemical fluxes without a
high chemical weathering component. Fig. 3 compares the Kerrien
and Kerbernez catchments to some agricultural and natural upland
catchments in Scotland (Stutter et al., 2002). The %Na dominance
(calculated as % [Na]/[Na + Ca + Mg] in molc m− 2 yr− 1) increases
and then stabilizes with increasing base cation weathering rate. Our
results are in line with Stutter et al. (2002) indicating that Na
dominance is higher in agricultural catchments as against nonagricultural catchments.
6.4. Chemical fluxes and chemical erosion rates
In a previous study of granite catchments around the world, White
and Blum (1995) showed a linear relationship between silica flux at
catchment outlets and annual runoff on silicate catchments of highly
Fig. 5. Cationic chemical erosion rate (mol.ha− 1.yr− 1) calculated from the flux of Ca, Mg and Na vs. calculated specific discharge for the Kerrien (grey squares) and Kerbernez (open
circles). For comparison, small catchments (b 100 ha) from Oliva et al. (2003) are also shown (black diamonds).
378
A.-C. Pierson-Wickmann et al. / Chemical Geology 265 (2009) 369–380
variable size. Moreover, Oliva et al. (2003) demonstrated that
temperature and runoff are the two main parameters controlling
chemical weathering. However, these studies fail to detect any
anthropogenic effect (i.e. acid rain) on chemical weathering. In our
case, the relatively high silica flux of 1.8 ± 0.9 kmol ha− 1 yr− 1 remains
in the range measured in natural catchments under tropical climates
(India) or Alpine/temperate climates (Europe, North America and
Japan) (Oliva et al., 2003) (Fig. 4). However, the fluxes of dissolved
cations (Ca, Na and Mg) exported from the catchments are
significantly outside the range of values presented in Oliva et al.
(2003) for a comparable runoff (Fig. 5).
The stoechiometry of chemical weathering reactions of silicate
minerals (i.e. feldspars altered into clay minerals), as reflected by
fluxes in other catchments, does not agree with the situation observed
in Brittany, where there is a higher export of cations compared with
silica. This implies that the chemical weathering of primary minerals is
not the only contributor to the cation flux from the catchments, and
we thus require an input from the soil ion-exchange complex. As
suggested above, this contribution is enhanced by soil acidification
caused by fertilizer application. Wright et al. (1988) studied the
reversible effects of acid rain on small granitic catchments in Norway,
showing that the exports of Al, SO4, Ca and Mg are the result of acid
deposition, with a high proportion of Ca and Mg coming from the
leaching of the exchangeable soil fraction and an increase in chemical
weathering rates.
Soil acidification can lead to irreversible clay loss with decreasing
pH. The use of liming to prevent the soil from becoming too acidic is
essential for preventing irreversible degradation of the soil. Essential
nutrients (P, Ca, Mg and Mo) become unavailable at low soil pH, which
leads to a reduction of plant production in farming systems. This
would result in a reduced profitability and an increased reliance on
fertilizers. The long-term effects of agricultural acidification could
become a concern for plant production.
7. Summary and conclusions
Chemical data were collected over a 5 year period on stream-,
rain- and ground-waters, as well as concerning the quantity and type
of agricultural fertilizers applied on two upland granitic catchments
under a temperate climate in Brittany, France. This dataset allows us
to investigate the influence of the application of organic and
inorganic fertilizers on water quality. The region has been under
intensive agriculture for the last 30 years, and the impact of this
system has significant consequences for the water quality of upland
catchments.
Although the Kerrien and Kerbernez catchments share similar
physical, and climatic properties, they react differently to the
agricultural pressures. The Kerrien catchment shows variable cation
export due to chemical weathering of soil and rock depending on the
level of fertilizer application. The Kerbernez catchment does not
exhibit any variation in cation or silica export flux despite variable
levels of fertilizer application.
By comparing these two agricultural catchments in Brittany with
other small and large-sized granitic catchments under diverse climate
conditions, we show that high fluxes of Si are released, and even much
higher in the case of base cations, compared with temperate and
tropical conditions. The results of our study indicate that the role of
agriculture is a major factor in the release of cations as well as the
depletion of soil macronutrients.
Agricultural practices affect cation exports owing to soil acidification. Organic fertilizers, such as urea, or agrochemicals (ammonitrates), provide protons to soil (ion-exchange complex), thus
leading to the leaching of cations from clay minerals and soil ionexchange complexes. High cation export due to chemical weathering is also significant. The resulting depletion of agricultural soils
may lead to an enhanced use of fertilizers and liming in the future.
Such practices would lead to irreversible clay loss and cation
depletion in soils.
Acknowledgments
This work was supported by the French programme ACI “Eau et
Environnement” of the French Ministry of Research, the French
programme EC2CO of the INSU-CNRS and the SYSTERRA programme
of ANR (ANR-08-STRA-01). The authors would like to thank M.
Bouhnik-Le Coz and M. Faucheux for field and laboratory work, the
“Lycée horticole de Kerbernez” and its staff for facilitating access to the
site. We are also grateful to the farmers, who were always trustful and
keen to provide information and allow access to their land. B. Bourdon
is thanked for editorial handling and constructive comments. We are
also much indebted to Bernhard Peucker-Ehrenbrink and an anonymous reviewer for a number of helpful comments on this manuscript. Michael Carpenter post-edited the English style.
Appendix A
Application levels of chemical and organic fertilizers from 1992 to 2005 on the Kerrien and Kerbernez catchments.
1992
1993
Kerbernez catchment
Ammonitrate
84.9
NPK
0
KCl
0
Urea
0
CaOMg
0
Slurry
2.5
Manure
0
24.7
5.0
0
0
0
9.9
0
Kerrien catchment
Ammonitrate
183.2
NPK
0
KCl
0
Urea
0
CaOMg
0
Trez
0
Slurry
0
Manure
0
283.2
41.1
0
0
0
0
10.9
0
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
58.9
0
0
0
0
12.0
0
121.1
1.7
0
0
0
2.9
0
48.5
38.1
5.0
0
0
1.2
0
112.2
89.1
7.5
0
0
6.1
0.5
148.0
2.1
0
0
0
4.0
0.4
145.8
0
10.6
0
0
7.5
0
121.4
0
7.5
7.0
0
2.9
1.4
143.1
0
0
4.7
0
4.9
0
223.4
0
0
6.3
28.5
0
0.9
84.2
0
0
0
0
6.0
0.5
33.0
0
0
0
0
6.0
0.7
8.5
0
0
2.1
0
6.5
2.3
206.7
0
0
0
0
0
6.6
0
128.1
61.0
0
0
0
0
13.1
2.2
167.2
92.3
0
0
0
0
0
1.1
114.6
13.7
0
11.8
0
0
0.6
13.4
172.2
10
0
0
0
0
3.5
1.9
137.0
0
106.6
0
0
0
3.5
0
127.5
0
118.4
0
0
0
5.7
0
142.7
0
71.0
0
0
0
1.9
0
122.7
0
71.0
13.7
61.6
0
6.2
7.4
194.5
0
0
0
0
0
4.1
4.1
37.9
92.2
0
0
0
0
1.3
7.6
222.3
102.7
0
0
0
1184.2
1.3
4.8
Application levels are expressed in kg/ha, except for slurry (m3/ha) and manure (T/ha).
2004
2005
A.-C. Pierson-Wickmann et al. / Chemical Geology 265 (2009) 369–380
379
Appendix B
Solute concentrations measured in mineral and organic fertilizers.
Elements
NPK
NPK
NH4NO3
Urea
Pig Slurry
Pig Slurry
Pig Slurry
Pig Slurry
Pig slurry average
Dairy manure
Dairy manure
References
(1)
(2)
(1)
(1)
(2)
(3)
(4)
(5)
⁎
(1)
(6)
Units
g.kg− 1
g.kg− 1
g.kg− 1
g.kg− 1
g.kg− 1
g.L− 1
g.L− 1
g.L− 1
g.L− 1
g.kg− 1
mg.L− 1
Cl
Na
Mg
Al
Si
K
Ca
Rb
Sr
Ba
U
nm
nm
nm
nm
nm
nm
nm
29.10− 3
37.10− 3
2.10− 3
65.10− 3
26.0
3.0
5.0
18.8
3.5
181.4
25.4
0.04
0.85
nm
8.10− 3
nm
nm
nm
nm
nm
nm
nm
7.10− 4
1.10− 3
1.10− 3
1.10− 4
nm
nm
nm
nm
nm
nm
nm
4.10− 4
1.10− 3
1.10− 3
3.10− 4
15.7
46.7
67.8
4.6
15.2
236.0
40.7
0.18
0.10
nm
1.10− 4
nm
3.5
2.9
nm
nm
16.0
10.1
nm
nm
nm
nm
nm
2.7
11.3
1.2
nm
12.2
42.6
1.10− 2
6.3.10− 2
4.0.10− 2
5.3.10− 3
nm
3.5
14.2
1.2
nm
15.2
35.9
1.3.10− 2
7.3.10− 2
3.6.10− 2
3.0.10− 3
15.7
17.9
28.3
3.9
15.2
89.1
28.9
1.10− 2
0.09
1.8 × 10− 2
2.2 × 10− 3
nm
nm
nm
nm
nm
nm
nm
0.02
0.07
0.05
1.10− 3
152.0
76.1
46.7
0.03
6.35
7.7
99.1
nm
nm
nm
nm
(1): Mc Bride and Spiers (2001), (2): Riou (1995), (3): analyses performed on pig slurry used on Kerbernez and Kerrien catchments (unpublished data), (4): Kerbernez pig slurry
S4LB-161-27, (5): Kerbernez pig slurry S1LB-161-12; (6): Widory et al. (2001), nm: not measured. Pig slurry average represents the average values of pig slurries ((2), (3), (4), (5))
applied to the Kerbernez and Kerrien catchments, used for the computation. Slurry and manure concentrations are based on dry weight.
Underlined values are used in the calculation.
Appendix C
Groundwater end-members used for Fig. 2.
mmol.L− 1
Cl (±σ)
Na (±σ)
Mg (±σ)
Ca (±σ)
Kerrien catchment
Shallow groundwater 0.91 ± 0.13 0.96 ± 0.17 0.34 ± 0.07 0.29 ± 0.09
Deep groundwater
0.95 ± 0.24 1.09 ± 0.28 0.41 ± 0.14 0.24 ± 0.06
K (±σ)
0.15 ± 0.13
0.11 ± 0.06
Kerbernez catchment
Shallow groundwater 0.93 ± 0.20 0.98 ± 0.18 0.35 ± 0.09 0.44 ± 0.11 0.09 ± 0.03
Deep groundwater
1.16 ± 0.14 1.27 ± 0.30 0.51 ± 0.15 0.80 ± 0.50 0.16 ± 0.03
Precipitation
Precipitation
0.20 ± 0.11 0.18 ± 0.12 0.02 ± 0.01 0.01 ± 0.01 0.02 ± 0.03
σ: standard deviation.
The average concentration and standard deviation is determined over the study period
(2000-2005).
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