Hydrogeology Journal (2015) 23: 1761–1779
DOI 10.1007/s10040-015-1298-2
Contribution of time-related environmental tracing combined
with tracer tests for characterization of a groundwater conceptual
model: a case study at the Séchilienne landslide, western Alps (France)
A. Vallet & C. Bertrand & J. Mudry & T. Bogaard &
O. Fabbri & C. Baudement & B. Régent
Abstract Groundwater-level rise plays an important
role in the activation or reactivation of deep-seated
landslides and so hydromechanical studies require a
good knowledge of groundwater flows. Anisotropic and
heterogeneous media combined with landslide deformation
make classical hydrogeological investigations difficult.
Hydrogeological investigations have recently focused on
indirect hydrochemistry methods. This study aims at
determining the groundwater conceptual model of the
Séchilienne landslide and its hosting massif in the western
Alps (France). The hydrogeological investigation is streamlined by combining three approaches: a one-time multitracer test survey during high-flow periods, a seasonal
monitoring of the water stable-isotope content and electrical
conductivity, and a hydrochemical survey during low-flow
periods. The complexity of the hydrogeological setting of
the Séchilienne massif leads to development of an original
method to estimate the elevations of the spring recharge
areas, based on topographical analyses and water stableisotope contents of springs and precipitation. This study
shows that the massif supporting the Séchilienne landslide is
characterized by a dual-permeability behaviour typical of
fractured-rock aquifers where conductive fractures play a
major role in the drainage. There is a permeability contrast
between the unstable zone and the intact rock mass
supporting the landslide. This contrast leads to the definition
Received: 26 August 2014 / Accepted: 12 July 2015
Published online: 9 August 2015
* Springer-Verlag Berlin Heidelberg 2015
A. Vallet ()) I C. Bertrand I J. Mudry I O. Fabbri I B. Régent
UMR6249 Chrono-Environnement—Université Bourgogne
Franche-Comté, 16 route de Gray, 25030, Besançon cedex, France
e-mail: aurelien.vallet@univ-fcomte.fr
T. Bogaard
Water Resources Section,
Delft University of Technology, Stevinweg 1, 2628 CN, Delft,
The Netherlands
C. Baudement
CEREGE (UMR7330), Aix-Marseille University, CNRS-IRD,
Europôle Méditerranéen de l’Arbois, Avenue Louis PhilibertBP 8013545, Aix-en-Provence cedex 04, France
of a shallow perched aquifer in the unstable zone and a deep
aquifer in the intact massif hosting the landslide. The
perched aquifer in the landslide is temporary, mainly
discontinuous, and its extent and connectivity fluctuate
according to the seasonal recharge.
Keywords Conceptual models . Tracer
tests . Hydrochemistry . Landslide . France
Introduction
Gravity is known to be the main factor in landslide
motion, and water plays a prominent triggering role.
Groundwater-level rise plays an important role in the
(re)activation of deep-seated slope movements (Van Asch
et al. 1999; Iverson 2000; Rutqvist and Stephansson
2003). Many studies have attempted to characterize the
relationships between water infiltration and landslide
destabilization (Alfonsi 1997; Hong et al. 2005;
Helmstetter and Garambois 2010; Abellán et al. 2015;
Belle et al. 2014; Bernardie et al. 2014) or to couple
hydromechanical models (Cappa et al. 2004, 2006;
Corominas et al. 2005; Guglielmi et al. 2005; Bonzanigo
et al. 2007; Sun et al. 2009). However, to be accurate and
to reflect field conditions, such approaches must be based
on relevant and realistic groundwater conceptual models.
In the Alps or in the northern Apennines, most deepseated landslides occur in crystalline bedrocks (Barla and
Chiriotti 1995; Agliardi et al. 2001; Ronchetti et al. 2009).
The hydrological response of a fractured-rock slope depends
on the geometry and hydraulic connectivity of the
discontinuity network, on individual discontinuity properties, and on the intact rock properties (Maréchal 1998;
Cappa et al. 2004; Bogaard et al. 2007). In addition,
vertical gradients of the hydraulic conductivity can occur
with values ranging from 10−11 m/s at depth to 10−5 m/s
towards the decompressed surface areas (Maréchal and
Etcheverry 2003). These permeability contrasts may be
sufficient to support perched aquifers (Vengeon 1998;
Tullen 2002; Cappa et al. 2004; Binet 2006).
Hydrogeological data strongly depend on the scale of
observation (Clauser 1992), and local measurements are
1762
rarely representative of the overall behaviour of the
aquifers. Heterogeneous, anisotropic and discontinuous
properties of fractured rocks are accentuated by the
landslide deformation and by the presence of weathering
products (colluvium, clay) filling the open fractures (Cappa
et al. 2004; Binet 2006).
Although piezometers can give significant, yet local,
insights on the groundwater system (Kosugi et al. 2011;
Padilla et al. 2014), their cost, their short lifespans in
unstable areas and their poor representativeness make
piezometers seldom used in landslide studies (Michoud
et al. 2013). Consequently, recent studies focus on indirect
methods such as hydrochemistry surveys by monitoring
springs for natural and artificial tracers (Bogaard et al.
2007). For example, recent studies have demonstrated that
the recharge area of a landslide can be larger than the
landslide itself (Guglielmi et al. 2002) or that the landslide
groundwater variations can be significantly controlled by
deep groundwater flows (Ronchetti et al. 2009). In
addition, hydrochemical methods can discriminate waters
which flowed through the stable rock masses from those
which flowed through the unstable rock masses (Binet
et al. 2009). Methods are mainly based on chemical
analyses of major elements (Cappa et al. 2004; Binet et al.
2007a), but additional parameters can be used to identify
and quantify specific processes. These parameters include
water stable isotopes (Guglielmi et al. 2002; Lin and Tsai
2012), trace elements (Cervi et al. 2012), natural fluorescence (Charlier et al. 2010) and artificial tracers (Bonnard
1988; Binet et al. 2007b). However, hydrochemistry
surveys are time-consuming, expensive and require
specific expertise. Consequently, many landslide studies
are based on a few field visits and do not provide anything
more but a snapshot of the hydrogeological conditions
(Vengeon 1998; Guglielmi et al. 2002; Binet et al. 2007a).
This contribution aims at characterizing the groundwater conceptual model of the Séchilienne landslide and its
hosting massif. This case study aims at streamlining the
hydrogeological investigation methods by combining
three complementary approaches: (1) a one-time multitracer test survey during a high-flow period, (2) a seasonal
monitoring of the water stable isotope content and the
water electrical conductivity, and (3) a hydrochemical
survey during low-flow periods. These three complementary approaches allow for determination of: (1) flowpaths
and residence times of the groundwater, (2) average
recharge elevations and hydrodynamic behaviour of
springs at season time-steps, and (3) spring chemical
clusters and water origin.
Materials and methods
Investigation strategy background
It is crucial to characterize the groundwater-flow pattern
during high-flow periods since it is during these periods
that the landslide destabilisation triggered by large
amounts of recharge is the strongest. Moreover, under
high-groundwater-level conditions, flowpaths not existing
Hydrogeology Journal (2015) 23: 1761–1779
in the drier seasons can occur as well as higher flow rates,
thus allowing increase in the probability of tracer
restitution at springs. Fluorescent tracers are used in this
study as they show a high sensitivity analysis, low
detection limits, and low toxicity levels (Leibundgut
et al. 2011). Tracer test settings can be used to characterize
(1) the extension of the recharge area of the unstable
slope, (2) groundwater flowpath hypotheses by investigating the contribution of a spatially constrained area, (3) the
presence or not of a perched drainage in the disturbed
zone, (4) the role of the main fractures involved in the
groundwater flow and (5) groundwater flow velocities.
The recovery rate of the tracer can further help quantify
the hydraulic properties of the aquifer (Cappa et al. 2004;
Binet et al. 2007b).
The dual-permeability of fractured reservoirs involves
complex hydraulic connections between rare strongly
conductive fractures and numerous poorly conductive
fractures (Cappa et al. 2004). The poorly conductive
fractures are hereafter referred to as micro-fissured matrix.
The seasonal fluctuation of the chemistry of a spring can
inform on the proportion of the spring that is recharged
through the conductive fractures (reactive medium) and/or
the micro-fissured matrix (inertia of the aquifer promoting
water–rock interaction; Pili et al. 2004). Springs supplied
by conductive fractures are thus expected to show high
seasonal variations in water chemistry compared to
springs supplied by the micro-fissured matrix. In addition,
the fluctuation of the saturated-zone depth, controlled by
the seasonal recharge variability, can modify the hydraulic
connections and the groundwater flowpaths of the fractured reservoir. Springs supplied by a well-developed and
well-connected network of conductive fractures will show
a significant seasonal variation related to the elevation of
their average recharge area, whereas the springs supplied
by the micro-fissured matrix will show less seasonal
variation. Lastly, the recharge areas of springs supplied
through extended networks of conductive fractures will be
significantly larger than the recharge areas of springs only
supplied by the micro-fissured matrix. For all these
reasons, the analysis of seasonal variations of chemistry
and of recharge-area elevation of springs (seasonal
pattern, dispersion, amplitude …) can be used to qualitatively characterize the hydrodynamic behaviour of the
aquifer.
During low-flow periods, the groundwater flow is
mainly driven by the aquifer drainage, with low external
disturbance (low recharge). As a consequence, the
chemical content of the groundwater is controlled by the
mineral dissolution rates and the water–rock interaction
duration, and is independent from the recharge influence
(Hilley et al. 2010). The hydrochemical analysis enables
one to distinguish various water chemical groups and to
compare them with the massif lithology. This comparison
provides information about water origin, residence time
and flowpaths. The groundwater chemistry can also be
influenced by the landslide hydromechanical processes,
thus distinguishing water which flowed through the
unstable zone from the water which did not flow through
DOI 10.1007/s10040-015-1298-2
1763
the unstable zone (Binet et al. 2009). In addition,
geochemical inverse modelling allows one to characterize
solid phases involved in the water–rock interaction
processes, to estimate the mass transfers and to validate
the flowpath hypotheses with the results of tracer tests and
δ18O values (Cervi et al. 2012).
Study site
Study area
The Séchilienne landslide is located to the south-east of
Grenoble (France), on the right (north) bank of the
Romanche River, on the southern slope of the Mont-Sec
massif (Fig. 1a). The landslide is located in the Belledonne
crystalline range and is composed of micaschists. The
micaschists are characterized by a N–S-trending vertical
foliation. Carboniferous to Liassic sedimentary deposits
unconformably cover the micaschists along the massif
ridgeline, above the unstable zone. Locally, glacio-fluvial
deposits overlie both the micaschists and the sedimentary
deposits. Micaschists mainly consist of quartz, biotite,
phengite and chlorite, with occurrences of quartz-albite
granoblasts, carbonate veins, and disseminated pyrite
(Vengeon 1998). Liassic deposits consist of limestones with
intercalation of breccia-rich layers containing fragments of
coal, micaschists and dolomites, while Triassic deposits are
represented by sandstone, quartzite, dolomite and minor
intercalation of black shales and argillites. Carboniferous
deposits are characterized by micaceous black shales,
sandstones and conglomerates with intercalations of anthracite (Barféty et al. 1972; Vengeon 1998). Fluvio-glacial and
colluvial deposits contain reworked materials from the
previously cited formations.
Séchilienne unstable slope
The Séchilienne landslide is limited eastwards by a N–S
fault scarp and northwards by a major head scarp of
several hundred meters wide and tens of meters high
below the Mont Sec. Rare geomorphological evidence
allows for precise definition of the western and southern
boundaries of the unstable area. The Séchilienne landslide
is characterized by a deep progressive deformation
controlled by the network of faults and fractures. A
particularity of the Séchilienne landslide is the absence of
a well-defined basal sliding surface. The landslide is
affected by a deeply rooted (about 100–150 m) toppling
movement of the N50–70° slabs to the valley (accumulation zone) coupled with the sagging of the upper slope
(depletion zone) beneath the Mont Sec (Vengeon 1998;
Durville et al. 2009; Lebrouc et al. 2013). Lastly, a very
actively moving zone, where high displacement velocities
are measured (from 150 to 300 cm/year on average), is
distinguishable from the unstable slope (from 2 to 15 cm/
year on average). The Séchilienne landslide is characterized by a good correlation between antecedent cumulative
precipitation and average displacement velocities (Rochet
et al. 1994; Alfonsi 1997; Durville et al. 2009; Chanut
Hydrogeology Journal (2015) 23: 1761–1779
et al. 2013). Helmstetter and Garambois (2010) showed a
weak but significant correlation between rainfall signals
and rockfall micro-seismicity. Vallet et al. (2015) showed
that the Séchilienne displacement rates are better correlated to the recharge than to the precipitation, reinforcing the
significant role of groundwater flow in the Séchilienne
destabilization. Because of the mountainous location of
the studied landslide, precipitation consists of rain and
snow. Annual snow amount is 7-fold lower than rainfall.
Unlike the groundwater recharge which shows high
seasonal contrasts (dry summers vs. wet winters), precipitation does not show any pronounced seasonal tendencies. Since groundwater controls the Séchilienne
destabilisation, the landslide velocity also shows seasonal
contrasts (low velocities in summer vs. high velocities in
winter, Fig. 1b).
Hydrogeological background
The landslide is highly fractured and shows a much higher
hydraulic conductivity than the intact underlying bedrock
(Vengeon 1998; Meric et al. 2005; Le Roux et al. 2011),
thus leading to a perched aquifer in the landslide
(Guglielmi et al. 2002). Besides this, a deep saturated
zone at the base of the slope is hosted by the fractured
metamorphic bedrock over the whole massif bearing the
landslide, with a thick (about 100 m) vadose zone above
it. Although the Séchilienne landslide has already been
investigated by two distinct hydrochemistry snapshot
surveys (Vengeon 1998; Guglielmi et al. 2002), groundwater flow mechanisms responsible for recharge of the
perched aquifer in the landslide (hereafter referred to as
landslide perched-aquifer) are still debated.
Vengeon (1998) showed that the landslide perchedaquifer is recharged by water-level rises of the deep
saturated zone, whereas Guglielmi et al. (2002) showed
that the main recharge originates from a perched aquifer in
the sedimentary cover (hereafter referred to as sedimentary perched-aquifer). Conclusions are mainly based on
rough water-balance estimation, geological observations
and limited hydrogeology data. Indeed, the Séchilienne
site presents a spatially sparse hydrogeological network.
First, no springs are located in the unstable zone. Second,
surrounding springs are scattered in the massif and are
subject to a strong anthropic pressure (water diversion)
which makes spring flow measurements impossible and
hinders the analyses.
Monitoring sites
Hydrochemistry data and the conceptual models of
Vengeon (1998) and Guglielmi et al. (2002) were used
as a baseline to design the monitoring network (Fig. 1a)
and to select the injection locations of the artificial tracers.
Table 1 details the location name and the type of each
monitoring network point. Most of the monitored spring
flows (Fig. 1a) are (1) the overflow leftover from water
withdrawals (S5, S6, S9, S11, S13, S15, S18, S19, S20,
S21, S25), (2) single points in areas with multiple
DOI 10.1007/s10040-015-1298-2
1764
a
5°48'0"E
5°49'0"E
5°50'0"E
45°6'0"N
5°47'0"E
G900
G710
900
G585
G1
G3
G4
G5
800
G2
P1
I3
G670
1 000
Metres
Luxembourg
Luxembourg
France
I4
S10
300
y
an
rm
Ge
Belgium
C4
S9
Italy
I2
S20
800
S24
S21
S25
700
50
0
S2
S3
S4
ry
EDF galle
S8
S16
0
40
S22
River Romanche
Depletion zone
Liassic
Chemistry - δ O
Triassic-Liassic
Precipitation collector (18O)
Carboniferous
Accumulation zone
High motion zone
Fault/fracture
Piezometer
Crystalline units
Stream
Gallery
Internal crystalline units
Topographic elevation (m asl)
Legend
Tracer survey
Moraine
Unstable
slope
Alluvium
External massif
Tracer injection
18
S26
45°3'0"N
S7
0
60
S5 S6
S14
nt
ya
ru
B
f
Ri
S19
S15
900
am
re
St
0
100
S18
C3
S13
Spain
S1
00
11
C2
Mont-Sec
Séchilienne
00
12
S11
itz
Sw
Grenoble
00
14
S17
S12
UK
C5
45°5'0"N
200
Metres
45°4'0"N
250 500
50 100
Séchil
ienne
fault
0
Peak Oeilly
Sabo
t fau
lt
C1
700
0
S23
00
13
I1
b
140
120
100
80
60
40
20
0
mm of water
120
Displacement (mm)
100
20
15
80
10
60
40
5
20
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Rain +
Snow = Precipitation
Recharge
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
High motion zone
Accumulation zone
0
Fig. 1 Séchilienne site. a Landslide site map and monitoring network. b Climatic and landslide kinematic settings based on the monthlycumulated amounts of rainfall, snowfall, recharge and displacements averaged over the years from 1993 to 2013. The unstable slope
boundary is defined according to the geophysical survey of Le Roux et al. (2011). Recharge was computed according to the computation
workflow of Vallet et al. (2015) based on temperature and precipitation records of the Mont-Sec meteorological station
resurgences, (S1, S2, S3, S4, S10, S12, S14, S24, G5) and
(3) water collected by vinyl sheets in galleries, which
Hydrogeology Journal (2015) 23: 1761–1779
constitutes the mean water leakage/inflow from those
galleries (G1, G2, G3, G4).
DOI 10.1007/s10040-015-1298-2
1765
Table 1 List of monitoring network locations and type
Location code
Location name
Type
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S18
S19
S20
S21
S22
S23
S24
S25
S26
C1
C2
C3
C4
C5
G1
G2
G3
G4
G5
I1
I2
I3
I4
P1
Vizille
Vizille
Vizille
Vizille
Dhuy
Reine
Gallery EDF
Romanche
Cornier
Chemin Mont-Jean
Mont-Jean
Pleney
Fonafrey
Thiebaud-EDF
Thiebaud-Lavoir
Noyer-Chute
Clobasse
Clos-Bénit
Finet
Les Mathieux
La Bathie
Romanche
Mulet
Les Aillouds
Mounier
Rif-Bruyant
Gallery 710
Madeleine
Fonafrey
Rochassier
Pic-Oeilly
Gallery 710-1
Gallery 710-2
Gallery 585-1
Gallery 585-2
Gallery 585-3
Gallery 900
Crevasse
Gallery 585
Rochassier
Gallery 710
S
S
S
S
S
S
G
R
S
S
S
S
S
S
S
S
S
S
S
S
S
R
S
S
S
R
Rc /L
Rc / L
Rc
Rc
Rc
G
G
G
G
G
IS
IS
IS
IS
P
G gallery outlet or water inflow, IS infiltration structures such as
crevasse/fractures/sinkholes, L lysimeter, S spring, R river, Rc rain
collector, P piezometer
Among the 30 springs identified in the Séchilienne
massif, none are located in the unstable zone. However,
three former mining galleries, situated at 900, 670 and
585 m asl (G900, G670, G585), and a gallery excavated to
survey the landslide at 710 m asl (G710), are located
within the unstable zone, with lengths of 60, 88, 240 and
240 m, respectively (Fig. 1a). These galleries have a
north–south orientation, except G670 which is oriented
N155 and which intersects the unstable zone. Galleries
G585 and G710, both located in the unstable zone, show
water inflow and/or leakage suitable for monitoring.
Unfortunately, G585 is no longer accessible and was only
sampled by Vengeon (1998), and monitored as part of
tracer test surveys. Another gallery (EDF gallery, French
Electricity Company) is also located at the foot of the
landslide slope at an elevation of about 425 m asl. Except
for the EDF gallery where some highly fractured zones are
covered with concrete, these galleries show bare rock.
Hydrogeology Journal (2015) 23: 1761–1779
A piezometer (P1) is located near the G710 gallery,
which was active from November 2009 to April 2010,
before being clogged. For the water isotope survey, five
hermetically closed rain collector tanks (C1, C2, C3, C4
and C5) were buried about 1 m deep and wrapped with an
isotherm cover to avoid isotope fractionation due to the
fluctuations of the atmosphere temperature. Rain collector
points were installed every 200 m at elevations from 640
to 1,520 m asl, covering most of the massif hosting the
landslide (Fig. 1a). Two rain collectors (C1 and C2) were
installed in woodland and in grassland (the two main
vegetation types of the landslide recharge area), respectively, and were each coupled with a lysimeter.
The EDF gallery outlet (S7), the Romanche River (S8
and S22) and the Rif Bruyant stream (S26) were
occasionally added to the monitoring program.
Additionally, the EDF gallery was also monitored for a
unique water chemistry survey (March 2002) during
maintenance of the EDF gallery. The G900 gallery was
used only for tracer test surveys. Demolition works of a
hydropower plant, after the tracing survey, modified the
flowpath of S16 spring, which is now flowing directly to
the alluvial aquifer. S23 and S17 were only monitored
with tracer test surveys.
Methodology
Tracer test surveys were performed in two campaigns, in
April 2001 and in March 2002. The March 2002 survey
was coupled with a hydrochemical survey of the water
inflows in the EDF gallery. Hydrochemical surveys were
conducted from September 2010 to September 2012. To
take account of seasonal variations, the surveys included:
(1) four isotopic campaigns (26 spots), performed quarterly, coupled with 12 water electrical conductivity (EC)
surveys, and (2) five hydrochemistry surveys (21 spots)
performed in low-flow periods, from early June to late
September. The four isotopic campaigns were performed
at 3-month intervals: December 2011, March 2012,
June 2012 and September 2012. Hydrochemical data of
the G585 gallery (Vengeon 1998), were also integrated
into this study. Lastly, water inflows observed in the five
galleries (Vengeon 1998) were analysed in order to
identify saturated and unsaturated zones inside the
landslide body and to infer the depth to the water level
within the unstable slope.
Tracer tests
A four-tracer test survey was implemented in April 2001
in the Séchilienne unstable slope recharge area. Different
fluorescent tracers were injected at four carefully chosen
locations (Fig. 1a; Table 2).A sinkhole (I4) located along
the Sabot fault (Fig. 1a) was selected as a representative
infiltration location of the top sedimentary cover to
evaluate the extent of the recharge area. A crack located
in the depletion zone (I2) was chosen to represent the
infiltration on the summital landslide area. A depression
zone (I1) located in the accumulation zone near the mine
DOI 10.1007/s10040-015-1298-2
1766
Table 2 Settings of the multi-tracer test of April 2001 and single tracer test of March 2002
Injection location
Tracer
Date
Tracer amount (kg)
Injected water volume (m3)
I2
I1
I4
I3-A
I3-B
Rhodamine
Sodium naphtionate
Uranin
Eosin
Uranin
10-April-2001 10:00
10-April-2001 13:00
10-April-2001 17:00
11-April-2001 12:00
13-March-2002 8:30
5
6
8
5
5
8
30
3
N/A
N/A
gallery G900 was chosen to test the hydraulic conductivity
of the N70 near-vertical fractures. A sink in the mine
gallery G585 (I3) was chosen as an injection spot to test
the infiltration on the N–S fault zone bordering the rapidly
moving landslide zone. Since the EDF gallery was flooded
at the time of the survey, detailed monitoring of the water
inflow points was not possible. A second tracer test was
performed in March 2002 when the gallery was emptied
for maintenance. Due to the impossibility of measuring
the flow rate of springs, the tracer velocities were obtained
by dividing the distance (in a straight line) between
injection and spring points by the shortest tracer transit
times.
Special instruments and sample rates were implemented for the two tracer tests (in 2001 and 2002). For the
multi-tracing survey performed in April 2001, monitoring
lasted for 2 months, and a sample rate was adapted
according to survey time by decreasing the sampling
resolution. G5, S5 and S6 springs were monitored with an
automatic sampler, while activated charcoal packets were
implemented and manual water samples were taken at S1,
S4, S9, S11, S13, S15, S18, S19 and S20. Only charcoal
monitoring was performed on the remaining springs. For
the reiterated tracer test from gallery G585 (I3-B test),
only hand sampling was performed in all water inflows of
the EDF gallery, between kilometric points 5 and 7 during
the 2 days following the injection day (Fig. 3).
Seasonal analysis and spring mean recharge-area
elevation
In order to distinguish the main springs flowing through
conductive fractures from those flowing through the
micro-fissured matrix, analyses of the seasonal variations
of chemistry and of recharge-area elevation were undertaken. These analyses are based on the water electrical
conductivity (EC) and on the water stable isotope ratio
(δ18O). EC is representative of the water total mineralization; δ18O allows for the estimation of the recharge-area
elevation of a spring (see next section).
For each spring, the seasonal variations of EC and
recharge-area elevations were investigated based on (1)
the seasonal scattering (based on the standard deviation
(SD) of the samples of the seasonal campaigns) and on (2)
the seasonal patterns. In addition, the average elevation of
the recharge area for each spring was estimated from the
elevation values deduced from the four water sampling
campaigns. The difference between the average elevation
of the recharge area and the elevation of the springassociated ridgeline point (see next section), expressed in
Hydrogeology Journal (2015) 23: 1761–1779
percent, will distinguish springs having local recharge
areas from those having remote recharge areas.
A spring is considered to be mainly supplied by the
conductive fractures if the seasonal variations of EC and
elevation of the recharge area show a high scattering and
clear seasonal pattern (that is, high EC values in summer
and low EC values in winter, and low elevation values in
summer and high elevation values in winter). This
interpretation can be reinforced if a geological structure
(e.g., fault) links up the average elevation of the recharge
area and the spring location. In this case, a remote
recharge area is expected. Conversely, a spring is
considered to be mainly supplied by the micro-fissured
matrix if the seasonal variations of EC and elevation of the
recharge area show low scatterings and unclear seasonal
pattern. This interpretation can be reinforced if no
geological structure links up the average elevation of the
recharge area and the spring location. In this case, a local
recharge area is expected. Intermediary spring behaviour
can be identified between these two groups and others
factors can influence the spring signal (surface network,
local perched aquifer, etc.).
Estimation of the mean elevation of the spring recharge
area from δ18O measurements
Water stable isotope fractionation is thermo-dependent,
leading to a series of effects on the isotope fractionation
(Clark and Fritz 1997). At the scale of a landslide site,
only the elevation effect and seasonal effects have a nonnegligible impact on the isotope fractionation (Leibundgut
et al. 2011). The determination of the local elevation
gradient (elevation=a×δ18O+b) enables the determination
of the average recharge elevation of the sampled springs.
Generally, the seasonal effect, which can significantly
influence the elevation effect, makes it necessary to
characterize the local elevation effect for each sampling
campaign, as was done in this study.
In mountainous areas, a local calibration of the isotopic
gradient is generally possible, using springs with welldefined recharge areas. In the case of the Séchilienne
landslide, no such springs allow for estimation of the
isotope elevation gradient. Instead, five rainfall collectors
were settled to characterize the δ18O signal (Fig. 1a). The
infiltration isotopic signals can be modified from the
precipitation initial signal through the soil layers by
evaporation processes, leading to δ18O enrichment (Gat
1996). Two rainfall collectors were therefore coupled with
a lysimeter in order to evaluate the evaporation impact on
the actual δ18O infiltration signal.
DOI 10.1007/s10040-015-1298-2
1767
Because the hydrodynamic properties of the aquifers
can delay or buffer the infiltration signal, the spring water
does not generally correspond to the last rainfall period;
therefore, instantaneous synchronization of δ18O water
sampling for both springs and rainfall collectors would not
be relevant. To take into account the transit through the
aquifer, the spring water δ18O signal was compared with
the δ18O signal of a cumulated amount of precipitation
that fell during the period between two consecutive spring
sampling campaigns (about 3 months); thus, the isotopic
signal of the rain collectors corresponds to the precipitation δ18O signal weighted by the 3-month rainfall amount.
The 3-month period is based on the study of Vallet et al.
(2015) which shows that the best R2 coefficient of
determination between the cumulative groundwater recharge and the landslide velocity is obtained for periods
from 68 to 132 days. As the landslide velocity is mainly
controlled by the pore-water pressure, these periods can be
considered as representative groundwater residence times.
The isotope elevation gradient of precipitation is determined
with the δ18O data of the five rain collectors. The isotope
elevation gradient of infiltration is considered identical to the
slope of the isotope elevation gradient of precipitation and
only the intercept is adjusted to fit to the lysimeter δ18O
measurements, assuming that evaporation is homogeneous
(Fig. 2a). The elevation of the spring recharge area is
estimated from the infiltration isotope gradients for the
April–June and July–September sampling campaigns, for
a
-9
-9.5
δ18O (‰)
-10
-10.5
Lysimeter
shift
-11
-11.5
-12
-12.5
600
800
1000
1200
Elevation (masl)
1400
Precipitation isotope elevation gradient
δ18O rainfall collector
δ O lysimeter
b
Legend
13
00
11
00
1700
1500
700
900
500
Hydrochemical analysis
The chemical analysis of samples from low-flow periods
enables distinction between water end-members flowing
from sedimentary, unstable and/or stable fractured reservoirs, and the mix between these end-member reservoirs.
Groundwater chemistry is discussed using major ion
concentrations. The nature and origin of the various
springs is established by using a Durov diagram, which
allows for characterization of the relationships between
the chemical type (function of encountered rock types)
and the electrical conductivity (degree of mineralization).
Hydrochemistry is complex as the alteration of each of the
three distinct rocks (moraines, carbonates and micaschists)
produces the same ions (Table 3).
Major ion combinations are used to characterize the
water origin and the flowpaths: Ca vs. HCO3 for the
sedimentary cover and Na vs. Cl to distinguish a
precipitation origin from alteration of the micaschist
Infiltration isotope elevation gradient
18
300
which actual evaporation is significant, and from the
precipitation isotope gradient for the October–December
and January–March sampling campaigns.
In order to locate the recharge area of the springs, the
steepest slope line is drawn for each spring, up to the
ridgeline, based on a digital elevation model with a 25-m
resolution (Fig. 2b), which allows for the definition of a
ridgeline point associated to each spring. If the δ18Oderived elevation of a spring recharge area falls between
the spring elevation and the spring-associated ridgeline
point elevation (materialized by the steepest slope straight
line, Fig. 2b), then the recharge area will be considered as
local. In this case, the groundwater flow is thought to
follow the topographic gradient without any influence or
disturbance linked to geological features such as fractures
or faults (hereafter referred to as topographic control).
Otherwise, the δ18O-derived elevation of the spring
recharge area will be placed along the ridgeline, at the
nearest corresponding elevation, and the recharge area of
the spring will be considered as remote. In this case, the
groundwater flow is thought to be controlled by geological
features (hereafter referred to as structural control).
0 100 200
400
Metres
Spring
Ridge line
Steepest slope line
Associated ridge line point
Steepest slope straight line
Local
δ18O elevation of
Remote the recharge area
Local straight line
Remote straight line
Topographic elevation (m asl)
Fig. 2 Illustration of the methods implemented for the δ18O
analysis. a Determination of the isotope elevation gradient for
precipitation and infiltration. b Spatial representation (map) of the
δ18O elevation of the spring recharge area
Hydrogeology Journal (2015) 23: 1761–1779
Table 3 Origin of minerals according to rock types from Vengeon
(1998) and structural formulae established from the LLNL database
(Parkhurst and Appelo 1999)
Lithology
Mineral
Structural formula
Water ion
release
B
B
B
NaAlSi3O8
KAl3Si3O10(OH)2
Mg5Al2Si3O10(OH)8
Na, HCO3
K, HCO3
Mg
B
B (veins)/SC
SC
Albite
Muscovite
Clinochlore14A
Phlogopite
Calcite
Dolomite
KAlMg3Si3O10(OH)2
CaCO3
CaMg(CO3)2
B
B
Goethite
Pyrite
FeOOH
FeS2
Mg, K
Ca, HCO3
Ca, Mg,
HCO3
Fe
Fe, SO4, H
B bedrock; SC sedimentary cover
DOI 10.1007/s10040-015-1298-2
1768
(albite). In addition, the water content variations for each
identified hydrochemical facies are investigated with Stiff
diagrams. The hydrochemical facies deduced from the
previous analyses are then spatially compared with the
distribution of the geological formations. A geochemical
inverse model is performed on the spring chemical
contents and rock mineral phases, using PHREEQC
3.1.1 code (Parkhurst and Appelo 1999) and the LLNL
database (Lawrence Livermore National Laboratory thermodynamic database, llnl.dat). In this study, inverse
modelling is used first to identify mineral phases and
mass transfers involved in the water–rock interaction
processes and, second, to validate flowpath hypotheses
deduced from tracer tests and δ18O analyses.
The inverse model is tested for springs representative
of both the stable and unstable zones. The initial water
chemistry of the inverse model (Table 4) matches with the
average composition of Alpine rainwater (Atteia 1994).
Minerals representative of the sedimentary cover and
micaschist bedrock (Table 4) were chosen based on the
work of Vengeon (1998); also taken into consideration
were gaseous phases (O2 and CO2) in the inverse model.
Sample analysis
Survey of the tracer tests
Artificial tracers (Uranin, Eosin, Rhodamin B and Sodium
naphtionate) were extracted from charcoal adsorbents
using an eluent (ethanol mixed with ammonia) by a
fluorescence spectrometer (Perkin-Elmer LS 30 UVspectrometer). Accuracy depends on the natural organic
matter content, which is highly variable through precipitation events. Analyses by fluorescence spectrometry were
performed at the Chrono-Environnement laboratory at the
University of Franche-Comté, France.
Oxygen stable isotope
Water samples were collected in glass vials with caps with
an additional parafilm to prevent any possible evaporation.
The oxygen stable isotope was analyzed with a liquid
water isotope analyzer method (LWIA) using an off-axis
integrated cavity output spectroscope (OA-ICOS), model
DLT-100, manufactured by Los Gatos Research Inc. For
more details about method accuracy, precision and
repeatability, see Penna et al. (2010) and Lis et al.
(2008). Isotopic analyses were performed at the Faculty
of Civil Engineering and Geosciences at the Delft
University of Technology in the Netherlands.
Field measurement
The pH, water electrical conductivity and temperature
were measured on site with a WTW apparatus, model
LF30 (a Xylem Inc. branch). The probes were calibrated
before each campaign with standard buffer solutions.
Measurements are reduced to the standard temperature of
25 °C with a respective accuracy of 0.1 pH units and
0.1 μS/cm.
Water chemistry analysis
Water samples were collected in polyethylene bottles and
were filtered at 0.45 μm. Analyses of Na+, Ca2+, K+, Mg2+
were performed by atomic absorption spectrometry (AA
100 Perkin–Elmer) with detection limits of 0.01, 0.5, 0.1
and 0.1 mg/L, respectively. Analyses of SO42−, NO3− and
Cl− were performed by high-pressure ion chromatography
(Dionex DX 100) with detection limits of 0.1, 0.05 and
0.1 mg/L, respectively. The concentrations in HCO3− were
measured by acid titration with N/50 H2SO4, within at
most 48 h after sampling, with 1 % accuracy. For the
Séchilienne hydrochemical conditions (pH between 6 and
8.5), total and carbonate alkalinity can be considered as
equalling HCO3− concentration. Only analyses which
have a charge balance lower than 10 % were taken into
account. Silica was analyzed with a spectrophotometer
(Spectroquant, Pharo 300, Merck) using a silica-test kit
(Merck) with 3 % accuracy. Chemistry analyses were
performed at the Chrono-Environnement Laboratory at the
University of Franche-Comté.
Table 4 Initial water rainwater composition from Atteia (1994)
used in the inverse model. aH+ and ae− are respectively hydrogen
and electron activities
Results and discussion
Parameter
Value
Temp (°C)
pH=−log aH+
pe=−log ae−
Ca (meq/L)
Mg (meq/L)
Na (meq/L)
K (meq/L)
Fe (meq/L)
Al (meq/L)
SO4 (meq/L)
Cl (meq/L)
HCO3 (meq/L)
SiO2 (mmol/L)
20
6
4
3.85E−02
4.42E−02
7.39E−03
1.25E−03
5.45E−05
1.15E−04
1.50E−02
Equilibrate
2.41E−02
5.30E−01
For the I4 test (Fig. 3a), the G1 water inflow displays the
highest tracer velocity (3 km/day). Numerous springs
(S13, S16, S20 and S21) located along the N20 Sabot
fault are positive for tracer detection, with velocities
ranging from 0.45 to 0.84 km/day. The same is observed
for the outlet S7 of the EDF gallery (0.88 km/day). The
velocity contrast between G1 and the other positive
springs indicates that the drainage by the unstable slope
bypasses the N20 faults and is supported by near-surface
drainage between the sedimentary cover and the unstable
slope perched aquifers. This test also demonstrates the
prominent role of the Sabot fault and the EDF gallery in
the drainage of the slope.
Hydrogeology Journal (2015) 23: 1761–1779
High-flow periods: tracer test survey
DOI 10.1007/s10040-015-1298-2
1769
5°50'0"E
Sabo
t fau
lt
0.7
4
0.84 0.45
0
100
0
80
0
60
00
11
0.55
700
400
700
Sabo
t fau
lt
00
11
00
11
06
0.
900
S7
S1
0.2 G5 (I3)
0.0
7
3
0.1
00
12
0.49
S2
I3
d
I3-B test
Gallery 585
Uranin 700
0.1
4
2.33
S7
S8
0
60
2.36
2.3
S4
S16
0
80
700
0.16 0.16
0.09 0.1
S4
400
0
60
I1
S16
8
0.8
S8
0.44
I3
0.38
KP 5.28
KP 5.5
KP 6.40
500
9 0.4
0.3
KP 6.32
400
0 50 100
e
5°48'0"E
5°48'20"E
5°48'40"E
200
Metres
5°49'0"E
Sedimentary cover
EDF gallery
Unstable slope extent
Fault/fracture
Stream
Topographic elevation (m asl)
Tracer survey
Breakthrough spot
Pathway (speed km/day)
Legend
Tracer injection
00
14
0
100
300
0
80
700
400
45°4'0"N
00
14
500
I3-A test
Gallery 585
Eosin
00
12
900
300
45°3'40"N 45°3'0"N
b
0
100
600
45°3'30"N
2 000
Metres
Sabo
t fau
lt
1 000
00
13
45°3'0"N
500
500
45°5'0"N
0
I1 test
Gallery 900
Sodium napthionate
c
0
60
S16
a
S6
0
80
0.08
G1
S7
S2
0
100
00
13
400
S20
S21
I2
00
14
00
12
S13
G1
S1
5°50'0"E
00
11
2.9
7
00
12
900
S13
00
14
300
900
300
45°4'0"N
I2 test
Mont Sec crevasse
Rhodamine
00
13
88
0.
5°49'0"E
500
I4
5°48'0"E
0.55
I4 test
Sinkhole Rochassier
Uranin
5°47'0"E
Sabo
t fau
lt
5°49'0"E
500
45°5'0"N
5°48'0"E
00
13
5°47'0"E
Fig. 3 Tracer-test analysis of April 2001 and March 2002 campaigns with a I4 test, b I2 test, c I1 test, d I3-A test and e I3-B test
For the I2 test, only G1 and S13 are positive (Fig. 3b).
A large velocity difference can be observed between the
Hydrogeology Journal (2015) 23: 1761–1779
down-slope flows, 7-fold faster than the lateral flow
towards the N20 fault (0.55 vs. 0.08 km/day). This
DOI 10.1007/s10040-015-1298-2
1770
Hydrogeology Journal (2015) 23: 1761–1779
the three tracer tests (I2, I1, I3-A), a rather homogeneous
velocity (0.07–0.14 km/day) is observed for S16 and S13.
These slow flows, compared to the one observed during
the I4 test, demonstrate flow through micro-fissured
matrix drained toward the east by N20 faults. Globally,
the groundwater flows are fast and poorly hierarchized
(tracers appear by intermittent pulses, without any
characteristic restitution curve).
Analysis of seasonal variations and of spring mean
recharge-area elevation
The isotopic abundance ratio of a water sample is
expressed as δ18O in ‰ with respect to the Vienna
Standard Mean Ocean Water (V-SMOW). The isotopic
elevation gradients show significant seasonal variation
(−0.12 ‰/100 m for November–December, −0.25 ‰/
100 m for January–March, −0.21 ‰/100 m for April–June
and −0.18 ‰/100 m for July–September, Fig. 4). These
values confirm the effect of season on the isotope
fractionation and are in accordance with commonly
observed values, from −0.1 to −0.36 ‰/100 m
(Leibundgut et al. 2011). Some springs show either
unrealistic values of elevation of the recharge area or lack
of data. The δ18O measured in S18 water sampled during
the low-flow period indicates an elevation of the recharge
area higher than the highest peak of the massif (Peak
Oeilly). This inconsistency can be explained by a spring
recharged by water depleted in 18O. This depletion may be
related to former water infiltrated during cold months
(Martelloni et al. 2012; Cervi et al. 2015). The δ18O
measured in G1 water sampled during the low-flow period
indicates an elevation of the recharge area lower than the
spring itself. This inconsistency can be explained by 18O
enrichment due to an extended air–water contact (evaporation). Lastly, δ18O data are lacking for some springs (dry
springs: S9, S5, S11, S24; no sampling at S12 and S13).
The elevations of the recharge areas of the S5 and S4
springs are similar and are about 1,800 m above the
highest local massif peak elevation (Peak Oeilly, 1,500 m)
and below the Romanche River (Fig. 5a), but show a
distinct seasonal variability (SD of 32 m for S5 and 315 m
Apr-Jun
2012
Jul-Sep
2012
-9
-9.5
δ18O (‰)
difference can be attributed to the dense opened-fractures
network and to the highest hydraulic gradient along the
steepest slope. This test demonstrates the significant
drainage role of the unstable slope and also the role of
the N20 Sabot fault.
The I1 test highlights a main westward drainage
component, except for S16 and G5 springs (Fig. 3c). In
this part of the unstable zone, the groundwater flow is very
likely supported by the dense network of N70 fractures
and by the EDF gallery. In contrast, the southeast drainage
seems to be supported by the Sabot fault. The average
velocities range between 0.06 and 0.16 km/day. The first
test in gallery G585 (I3-A test, Fig. 3d) is detected at the
EDF gallery outlet (S7), confirming the drainage role of
the EDF gallery. High velocities (0.49 to 2.36 km/day) are
observed for the western springs, contrary to the eastern
S16 spring (0.14 km/day).
The second G585 test (I3-B test, Fig. 3e) is positive at
four kilometric points (KP): 5.28 and 5.50 to the east,
demonstrating the influence of the N20 Sabot fault, and
6.32 and 6.40 to the west, confirming the role of the EDF
gallery in the collection of a part of the water flowing
from the N70 fractures.
A comparative analysis of the four tracer tests
improves the characterization of groundwater flow in the
massif. S7 showed high velocity (2.33 km/day) during the
I3-A test (Fig. 3d) and the EDF gallery water inflows
show low velocities (about 0.4 km/day) during the I3-B
test. For both tests, first tracer arrival times are similar
(about 1 day). The velocity contrast between the two tests
can be explained by the quasi-instantaneous transit along
the EDF gallery, which leads to overestimating groundwater velocities to the EDF gallery outlet S7. The S7
tracer velocity for the I3-A test can be estimated
independently of the gallery transit at 0.4 km/day instead
of 2.33 km/day. This estimated velocity is similar to the
EDF gallery tracer velocities observed during the I3-B
test. The I1 and I3 tracer tests indicate the same velocity
magnitudes for S7 and for some of the western springs (S2
and S4), suggesting that these western springs are partly
recharged by water from the EDF gallery. The EDF acts as
a by-pass of the groundwater flow. The high speeds of the
western springs (S2 and S4) during the I3-A test, similar
to the S7 outflow, show that these springs are likely
influenced by the EDF gallery outflow.
The I2 test shows a high velocity toward G1 (0.55 km/
day) as does the I1 test toward G5 (0.2 km/day). These
high velocities reveal a high transmissivity of the unstable
zone promoted by a dense fracture network. The low
number of tracer positive points during the I2 test
compared to the other tracer tests could be a consequence
of the actual groundwater flowpath; however, it could also
reflect the disadvantages of using Rhodamine in fractured
rock media (i.e., high retardation coefficient and high
sorption coefficient, Leibundgut et al. 2011).
The Romanche River (S8) is positive for two tracer
tests (I1 and I3-A), meaning that the EDF gallery collects
only a part of the slope groundwater, the remaining
reaching the river through the alluvium aquifer. Among
y = -2.09E-3 x - 7.76
R² = 0.88
y = -1.85E-3 x - 7.99
R² = 0.94
-10
-10.5
-11
Oct-Dec y = -1.18E-3 x - 7.29
2011 R² = 0.98
Jan-Mar y = -2.55E-3 x - 5.26
2012 R² = 0.97
-11.5
600
800
1000
1200
Elevation (m asl)
1400
Fig. 4 Isotope elevation gradients for the four seasonal water
sampling campaigns. Solid lines stand for precipitation isotope
gradient and dashed lines stand for infiltration isotope gradients
DOI 10.1007/s10040-015-1298-2
1771
a
2500
Spring
River δ18O average
Massif peak
Associated ridge line point
δ18O seasonal sample
δ18O average
disregarded / missing values
Spring recharge area
Romanche
Elevation (m asl)
2000
Peak Oeilly
1500
1000
Rif Bruyant
Ridge line
500
Elevation
difference (%)
b
Electrical conductivity
(μS/cm)
c
0
25
20
15
10
5
0
-5
32
315
157
139
16
142
45
55
25
71
60
154
107
68
Standard deviation (m)
124
56
71
1000
600
900
500
800
400
700
300
200
Romanche
100 Rif Bruyant Standard deviation (μS/cm)
14
15
12
15
5
0
x axis
S5
S4
S9
S25
S10
a, b ,c
Oct-Dec 2011
6
16
63
21
17
19
19
39
17
7
S24
S11
G1
S21
S15
S14
S20
S13
S18
S12
Jan-Mar 2012
Apr-Jun 2012
Jul-Sep 2012
Fig. 5 Seasonal analysis of δ18O and electrical conductivity of springs. a Seasonal δ18O estimated elevation of springs plotted relative to
spring elevation, associated ridgeline elevation, and δ18O estimated elevation of surface network (Romanche and Rif Bruyant). b Difference
between the average spring recharge-area elevation and the associated ridgeline point elevation. c Seasonal analysis of electrical
conductivity plotted relative to surface network signal (Romanche and Rif Bruyant). d Spatial representation (map) of the δ18O elevation of
the spring recharge area
for S4). In addition, these springs show low values and a
low variability of EC (340 μS/cm with SD of 14 μS/cm,
Fig. 5c). These two springs seem to be influenced by both
the massif slope groundwater and the Romanche water.
The Romanche water smoothes the amplitude and the
Hydrogeology Journal (2015) 23: 1761–1779
variations of the EC and increases the elevation of the
recharge area and its seasonal variability (high sensitivity
of drainage surface network from seasonal variations). S4
cannot be recharged by the Romanche alluvium aquifer,
thus indicating that the open EDF gallery, which
DOI 10.1007/s10040-015-1298-2
Hydrogeology Journal (2015) 23: 1761–1779
0.14
0.17
0.16
0.16
0.15
0.16
0.12
0.23
n.a
n.a
0.14
0.12
0.12
0.13
0.12
0.12
0.12
0.12
0.16
0.41
1.62
1.12
1.06
0.75
5.88
3.18
1.03
2.29
3.92
1.36
2.30
0.80
1.58
1.36
1.97
1.93
0.86
1.59
4.28
3.84
3.75
3.04
3.43
0.61
2.64
2.92
3.47
2.05
3.30
2.51
3.25
3.87
3.58
3.40
0.03
0.06
0.06
0.04
0.04
0.08
0.05
0.05
0.33
0.07
0.06
0.18
0.03
0.09
0.09
0.13
0.06
0.06
0.01
0.01
0.02
0.02
0.02
0.02
0.04
0.05
NA
NA
NA
0.02
0.02
0.01
0.02
0.01
0.02
0.01
0.09
0.15
0.19
0.16
0.14
0.18
0.26
0.21
NA
NA
NA
0.21
0.10
0.15
0.13
0.10
0.12
0.12
0.21
0.93
3.16
2.32
2.32
1.87
4.79
1.80
2.09
3.04
5.07
0.68
1.46
1.07
1.42
1.21
1.41
1.33
0.88
1.05
2.69
2.59
2.50
1.95
5.10
1.98
1.55
2.03
2.22
2.88
4.20
2.31
3.53
4.06
4.25
3.99
120.0
202.8
538.8
455.8
440.5
367.0
872.5
384.3
314.0
446.0
629.7
354.8
522.4
332.2
465.2
481.8
521.2
499.7
10.22
10.30
12.10
11.47
9.90
13.66
11.75
12.10
10.10
10.55
10.10
11.22
8.62
11.50
11.24
8.92
10.66
12.00
4
3
S12
S24
S9
S10
S11
S25
G1
G2
G3
G4
G5
S4
S13
S14
S15
S18
S20
S21
1
2
Ca–HCO3
Mg–Ca– HCO3
Mg–Ca– HCO3
Mg–Ca– HCO3
Mg–Ca–- HCO3
Mg–Ca– HCO3
Mg–Ca–SO4– HCO3
Mg–Ca–SO4
Mg–Ca– HCO3–SO4
Mg–Ca– HCO3–SO4
Mg–Ca–SO4– HCO3
Ca–Mg–HCO3–SO4
Ca–Mg–HCO3–SO4
Ca–Mg-HCO3–SO4
Ca–Mg–HCO3–SO4
Ca–Mg–HCO3–SO4
Ca–Mg–HCO3–SO4
Ca–Mg–HCO3–SO4
5
5
4
4
2
5
4
4
1
2
3
5
5
5
5
5
5
3
7.72
7.57
7.52
7.94
7.75
7.86
8.08
7.67
7.05
7.44
7.37
7.56
7.85
7.77
7.54
7.63
7.50
7.70
SO4
meq/L
HCO3
meq/L
Cl
meq/L
K
meq/L
Na
meq/L
Mg
meq/L
Ca
meq/L
EC
(field)
pH
(field)
Temp.
(field)
SN
Water Type
Station ID
The chemical composition of the monitored springs is
detailed in Table 5. The Durov and Stiff diagrams
(Fig. 6a,b) highlight four distinct water chemical groups
reflecting groundwater flow through various geological
units. Group 1 corresponds to Ca–HCO3-rich waters (S12)
with low EC values (about 100 μS/cm). Group 2
Group
Low-flow periods: hydrochemistry survey
Table 5 Physico–chemical parameters and major ions of the monitoring springs in low-flow periods, from early June to late September (2010–2012)
withdraws water from the Romanche River, recharges the
slope aquifer in its western part. S25 and S24 springs
show a behaviour similar to that of S4 with low values
and a low variability of EC (370 and 200 μS/cm with SD
of 5 μS/cm). The hydrodynamic behaviours at seasonal
time-steps of S25 and S24 are probably due to the
recharge of the slope aquifer by the Rif Bruyant stream.
The dilution by Rif Bruyant stream (recharge-area
elevation of 1,010 m asl) certainly caused an underestimation of the recharge-area elevation of these springs.
The elevation of the recharge area is thus assumed to be
near the Peak Oeilly from where groundwater is drained
by the main Séchilienne fault (Fig. 5d)
S21, S15, S14, S20, and S13 springs show mean
elevations higher than the elevation of their associated
ridgeline points (16 % higher on average, Fig. 5b). These
springs further show high seasonal variabilities (SD of
100 m and 22 μS/cm on average) with the lowest EC
values and recharge-area elevations observed during the
high-flow period and the reverse for the low-flow periods
(Fig. 5a,c). These springs are interpreted to be supplied by
rapid water flow through fractures during high-flow
periods (dilution) and drainage through the microfissured matrix during low-flow periods, mobilizing
remote recharge areas. In contrast, S9, S10, and S11
springs show mean recharge-area elevation close to their
associated ridgeline points (at most 4 % higher) with low
seasonal variations. These springs show moderate EC
variabilities (SD: 15 μS/cm) with no clear seasonal
patterns. These results show that their recharge areas are
mainly local and have little sensitivity to the seasonal
variations (Fig. 5a,b,c), meaning that the spring flow is
controlled by the micro-fissured matrix component. S18
spring is interpreted as showing an intermediate hydrodynamic behaviour at seasonal time-steps between the two
previous groups.
Spring G1 was only sampled twice and it is not
possible to assess clearly its seasonal variation. However,
the recharge area seems mainly local as the two high-flow
period surveys show a mean recharge area (1,143 m asl)
close to the elevation of the associated ridgeline point. G1
shows the highest EC values (840 μS/cm) and the highest
EC seasonal variability (SD: 63 μS/cm). These results are
interpreted as generated by water–rock interactions between groundwater and the unstable slope rocks (Binet
et al. 2009). Spring S12 has a very low conductivity
(155 μS/cm), probably due to low water–rock interaction
time. It also shows a scattered δ18O signal. One possible
explanation is that this spring is recharged by rapid water
flow through the sedimentary cover.
SiO2
meq/L
1772
DOI 10.1007/s10040-015-1298-2
1773
Cl
a
80
c
6
20
60
5
40
60
C
Ca (meq/L)
H
40
4
80
SO
O3 g
+C M
O3
20
80
60
3
2
ite
lom
Do
1
40
Na
+
K
te
lci
Ca
4
20
0
0
1
2
3
4
HCO3 (meq/L)
d
80
5
0.3
60
40
C
200 400 600 800 1000
200
ric
ct
e
El
al
400
nd
co
0.2
ric
teo
e
M
0.1
ity
tiv
uc
600
Na (meq/L)
a
20
(u
5°49'0"E
0.2
S21
0.1
0.15
Cl (meq/L)
S20
S14
S13
S4
0.05
Group 4
Group 3
5°50'0"E
Ca
HCO3
Ca
HCO3
Mg
SO4
Mg
SO4
Na+K
6
4
45°4'0"N
G5
G4
G3
G2
G1
S25
Group 2
5°48'0"E
45°5'0"N
b
5°47'0"E
S24
S9
S10
S11
S12
Group 1
0
S18
)
cm
0
1000
S15
S/
800
Group 1
2 (meq/L) 2
4
Cl Na+K
6 6
4
Group 3
2 (meq/L) 2
4
Cl
6
Ca
HCO3
Ca
HCO3
Mg
SO4
Mg
SO4
Na+K
6
4
Group 2
2 (meq/L) 2
4
Cl Na+K
6 6
4
Group 4
2 (meq/L) 2
4
Cl
6
Fig. 6 Hydrochemistry analysis with a Durov diagram, b spatial representation (map) and Stiff diagrams, and scatter plots of chemical
concentrations: c Ca vs. HCO3 and d Na vs. Cl
corresponds to Mg–Ca–HCO3-rich waters (S24, S25, S11,
S10 and S9) where Ca and Mg are in the same proportions
and EC is from 200 μS/cm (S24) to 540 μS/cm (S9).
Water composition of group 3 has various chemical
signatures and varies from Mg–Ca–HCO3–SO4-rich waters (G4 and G3), with intermediate Mg–Ca–SO4–HCO3rich water (G5 and G1), to Mg–Ca–SO4-rich waters (G2).
The EC values vary from 300 μS/cm (G4 and G3) to 850
μS/cm (G1). Group 4 corresponds to Ca–Mg–HCO3–SO4rich waters (S4, S14, S15, S18, S21, S20 and S13) with
EC values varying from 350 μS/cm (S4 and S14) to 520
μS/cm (S21, S20 and S13).
Hydrogeology Journal (2015) 23: 1761–1779
Stable zone (groups 1 and 2)
The low concentrations of Ca and HCO3 of group 1 (S12),
located on the calcite equilibrium line, is representative of
water circulating in a carbonate or calcite dominant cover
(limestone or moraines including carbonate materials)
with a short residence time (Fig. 6c). For this group,
inverse modelling has been tested for the S9, S10 and S11
springs.
The results show that 91 % of the Mg concentrations
come from dolomite and 9 % come from phlogopite,
while 87 % of the Ca is derived from the dolomite and the
remaining 13 % from the calcite. Therefore, the high Mg
DOI 10.1007/s10040-015-1298-2
1774
concentrations are explained by water transit through the
carbonate materials; however, these spring compositions
are above the dolomite equilibrium line (Fig. 6c), which
can be explained by the alteration of pyrite which releases
protons (Eq. 1), which can then consume HCO3 produced
by dissolution of carbonate minerals (Eq. 2), thus
releasing CO2 and H2O (Eq. 3). The δS34 analyses
performed by Vengeon (1998) show that SO4 has a
sulfurized origin, due to alteration of pyrite. Pyrite is
present only in the micaschist bedrock.
FeS2 þ
15
7
þ
O2 þ H2 O↔FeðOHÞ3 þ 2SO2−
4 þ 4H
4
2
ð1Þ
ðCa; MgÞðCO3 Þ2 þ 2H2 O þ 2CO2 ↔Ca2þ þ Mg2þ þ 4HCO−3
ð2Þ
HCO−3 þ Hþ ↔H2 O þ CO2
ð3Þ
This succession of chemical reactions explains the
range of pH (between 7 and 8) measured in the
Séchilienne waters, despite the production of protons,
and results in a depletion of HCO3 ions with respect to Ca
and Mg, the latter being balanced by SO4 (Table 5). The
springs of group 2 are, therefore, recharged by water
which circulated through carbonates and basement formations. The low electrical conductivity values of S24 and
S25 are explained by the influence of the Rif Bruyant
stream, as shown by the seasonal analysis (see ‘Analysis
of seasonal variations and of spring mean recharge-area
elevation’ section).
Unstable zone (group 3)
Group 3 (G5, G4, G3, G2, G1) is characterized by the
highest SO4 concentrations (Fig. 6; Table 5), which can be
explained by two mechanisms related to the slope
deformation. First, the opening and/or closing of fractures
lead to a new flowpath through unaltered pyrite-bearing
zones (Calmels et al. 2007) and, second, the friction and
grinding along fractures due to the movement of rock
masses cause a refreshment of pyrite reactive surfaces,
increasing the weathering rate (Binet et al. 2009). Lastly,
the alteration of the pyrite promotes the alteration of
carbonate and silicate (Gaillardet et al. 1999; Dongarrà
et al. 2009). G1 shows high Ca and Mg concentrations
(Fig. 6b,c), as well as high Na concentrations in the G710
gallery (G1 and G2, Fig. 6d). For this group, inverse
modelling was tested for G1. The result shows that 58 %
of the Ca concentration comes from calcite and 42 % from
dolomite. Mg contents are explained by water–rock
interactions mainly in the basement with a Mg origin of
37 % from dolomite and 63 % from chlorite. Water
mineralization results mainly from the interaction with
the basement minerals. In particular, the SO4 content is
Hydrogeology Journal (2015) 23: 1761–1779
clearly a marker of the unstable zone. The variety of the
water mineralisation characteristics between the various
gallery points is due to the strong mineralogical heterogeneities within the basement and within the fracture
network of the unstable zone. Nevertheless, water
interaction with carbonate rock is necessary to explain
water chemistry content. Ca and Mg ions can originate
either from the major tension cracks filled with
colluvial deposits rich in limestone fragments or from
water flowing from the sedimentary cover. Although
the I4 tracer test indicates a supply from the sedimentary
cover, the δ18O analysis indicates a predominantly local
recharge.
Mixing zone (group 4)
The group 4 (S4, S14, S15, S18, S21, S20 and S13) is
distinguishable from other springs by its concentrations of
Ca, Na and SO4. The Ca concentrations fall near the
calcite equilibrium line (Fig. 6c), reflecting a significant
transit in carbonate materials. The SO4 concentrations that
are up to 25 % higher than the springs of the stable zone
(group 2) can be explained by groundwater flow through
the stable bedrock. A mixing test was performed with
water of the carbonate sedimentary cover (S12) and water
of the unstable zone (G1) to quantify the contribution of
each component. The waters of group 1 are represented
only by a single spring (S12), having a very small
recharge area with a limited residence time, inducing a
very low mineralization. This group is considered as
representative of the sedimentary cover. In the modelling,
group 1 mineralization has been increased 4-fold in order
to account for of a longer flowpath between the
sedimentary cover and group 4 (as indicated by artificial
tracers, Fig. 3) and, hence, for a longer residence time.
The composition of group 4 could be explained by mixing
30 % unstable-zone water and 70 % sedimentary-cover
water. Lastly, S4 water is also a mixture of the Romanche
River water, the stable-zone water and the unstable-zone
water through drainage of the EDF gallery.
Gallery water inflows survey
Five galleries and one piezometer (P1) were surveyed
(Fig. 7a). The highest gallery (G900) is always dry. The
G710 gallery shows two leakages, one quasi-permanent
almost dry in summer (G1), at 150 m from the entrance,
and one temporary (G2) at 80 m from the entrance. Two
leakage zones were also identified for the G670 gallery at
25 and 70 m from the entrance. G585 gallery shows a
temporary water inflow (202 m, G3) and a permanent
water inflow (160 m, G5), and a localized leakage area
(170 m, G4). Water inflows of the galleries are localized
near fracture-damaged zones (Vengeon 1998), reinforcing
the prominent role of the fracture network in the
groundwater flowpath. Although the piezometer P1 is
currently clogged, the water level fluctuations were
recorded between 590 and 602 m asl from November
2009 to April 2010 (Fig. 7d). P1 piezometer did not reach
DOI 10.1007/s10040-015-1298-2
1775
a
5°47'0"E
5°48'0"E
45°4'0"N
110
0
5°49'0"E
5°50'0"E
0 125 250
Y
500
Metres
1000
45°3'45"N
900
0
50
Chamoussière
G900
800
G585
Les Rivoirands 700
45°3'30"N
4 km
G670
G710
5.5 km
6 km
6.5 km
7 km
4.5 km
5 km
600
400
River Romanche
7.5 km
X
Unstable zone (G1)
SO4 mg/l
b
Stable zone (S9)
c
800 Unstable zone drainage
700
600
500
400
300
200
100
0
7
6.5
Hydrochemistry survey - March 2002
6
5.5
GEDF gallery kilometric point
X
5
4.5
4
d
Y
30
Precipitation
Recharge
mm
1050
10
850
0
Piezometer
G710
650
G2
G1
605
G670
G5
600
G585
River
Romanche
G4 G3
Piezometer P1
water-level
m asl
Elevation (m asl)
G900
20
595
450
Gallery EDF
590
Nov-2009 Dec
250
0
Legend
Jan
Feb
Mar
Apr May -2010
Distance (m)
500
Piezometer
Gallery
Fault/fracture
Unstable
slope limit
Leakage zone
Localized water inflow
1000
Gallery cemented
part
Colluvial deposits
Fracture
drainage
Micro-fissured block
drainage
Sub-surface
drainage
Water-level
Fig. 7 Gallery survey. a Spatial representation (map) of water inflows. b SO4 concentration of the EDF gallery water inflows. c Crosssection of water inflows. d Plot of the water level recorded in the piezometer P1 with precipitation and recharge. The cross-section of the
unstable zone is modified after Lebrouc et al. (2013). The unstable slope boundary is defined according to the geophysical survey of Le
Roux et al. (2011). Recharge was computed according to the computation workflow of Vallet et al. (2015)
the stable zone below the landslide body and, thus, P1
water levels are associated with rock/matrix within the
landslide body. These water levels are considered as
representative of high-groundwater-level conditions
(Fig. 7c).
The EDF gallery, in particular, shows bare rock with
limited cemented sections (revealing heavily fractured
zones) with water flowing unconfined by gravity from an
upstream Romanche withdrawal point. During the excavation of the EDF gallery, perennial springs located at Les
Rivoirands and Chamoussière villages were permanently
dried up (Fig. 7a). Numerous and scattered water inflows
and leakages occur in the EDF gallery between 5.5 and
7 km, matching with the dense fracture network of the
Hydrogeology Journal (2015) 23: 1761–1779
landslide (Fig. 7a). Between 3.9 and 4.5 km, the
Séchilienne fault has completely damaged the zone and
many water inflows occur, revealing the prominent
drainage role of the fault. To a lesser extent, the Sabot
fault seems to have a more localized impact on the EDF
gallery water inflows. The hydrochemistry survey shows
that SO4 is the marker of the unstable slope—see section
‘Unstable zone (group 3)’. From kilometric points 4 to
5.80, SO4 content is similar to S9 and is representative of
the stable zone, whereas from kilometric points 6 to 7, the
SO4 signal increases and even exceeds the concentration
of G1 representative of the unstable zone (Fig. 7b). The
SO4 content confirms the prominent role of the EDF
gallery in the drainage of the unstable slope.
DOI 10.1007/s10040-015-1298-2
1776
Groundwater conceptual model
Stable zone: fractures vs. micro-fissured matrix
Two main groundwater flow types are identified: (1) rapid
and reactive water flow in fractures (S4, S13, S14, S15,
S20, S21, S24 and S25; seasonal analysis, Fig. 5) which
bypass the bulk of the less pervious and (2) inertial water
flow from micro-fissured matrix, on which springs S9,
S10 and S11 depend (seasonal analysis, Fig. 5). These
groundwater flows are characteristic of the dualpermeability of fractured reservoirs (Maréchal 1998;
Cappa et al. 2004). Intermediate hydrodynamic behaviour
between micro-fissured matrix and fractures is also
observed (S18). Lastly, groundwater flow occurs in the
perched aquifer located in the sedimentary cover (S12,
Guglielmi et al. 2002). The main N20 Sabot fault plays a
major role in the massif drainage, with a rapid transfer
from the sedimentary cover, but this fault also drains
the unstable zone with much lower velocities (tracer
test and seasonal analysis, Figs. 3 and 5). This is
confirmed by the hydrochemistry survey which showed
that the springs located along the Sabot fault match
with mixed water that flowed through the sedimentary
cover and the unstable slope (Fig. 6, group 4);
therefore, the Sabot fault has a significant spatial
influence, draining out the deep aquifer below the
unstable slope, likely because of a lowering of the
groundwater level which, in turn, causes a west to east
hydraulic gradient (tracer test, Fig. 3). Flow velocities
(tracer test, Fig. 3), water mixing (hydrochemistry,
Fig. 6) and the lack of fractures indicate that the
unstable slope drainage is done mainly through the
micro-fissured matrix and is minor in influence relative
to the Sabot fault drainage axis (Fig. 8a).
The Séchilienne fault also acts as a major drainage
axis and even promotes water infiltration from Rif
Bruyant stream (seasonal analysis, Fig. 5). Outside of
the major fault or fracture zones, the groundwater
behaviour is more inertial, being characterized by a
longer residence time. This is testified by the hydrodynamical behaviour of the springs of the northern slope
of the Mont Sec massif. The hydrodynamic behaviour of the
S18 spring is typical of a spring supplied by the microfissured matrix with very small influence from conductive
fractures.
Unstable zone: landslide perched aquifer
During the low-flow period, the water level of the
piezometer P1 dropped below 590 m asl and therefore
below the boundary of the unstable zone. Unsaturated
zones have been observed in all galleries (gallery survey,
Fig. 7). In contrast, during almost all the high-flow
periods, the water level of P1 (about 600 m asl) suggests
a saturated zone at the base of the unstable zone below the
G710 gallery. Moreover, during high-flow periods, a
shallow continuous drainage (tracer tests, Fig. 3) occurs
through the landslide, draining water from the sedimentary
cover (I4 tracer test, Fig. 3a). This recharge mechanism
Hydrogeology Journal (2015) 23: 1761–1779
involves a hydraulic connection of the unstable zone with
the sedimentary perched aquifer which bypasses the deep
aquifer (Fig. 8), which indicates that, in high-flow periods,
the recharge area is much larger than the landslide surface
(Guglielmi et al. 2002; Cappa et al. 2004). However, the
water from the G710 gallery has mainly a local origin
(seasonal analysis, Fig. 5) with a characteristic SO4
concentration explained by mechanical weathering rather
than by variations of residence time (hydrochemistry,
Fig. 6), which shows that the recharge of the unstable
perched aquifer is mainly local and that the contribution of
the remote groundwater from the sedimentary cover is
limited (Fig. 8).
Heterogeneous, anisotropic and discontinuous properties of the landslide (Binet 2006) lead to a perched
discontinuous fractured reservoir slope which is better
described by numerous disconnected saturated zones
rather than by a single one (Cappa et al. 2004, Figs. 7c
and 8b). This discontinuous reservoir is temporary since it
is rapidly drained by the dense fracture network as
demonstrated by intra-annual variability of the leakage
flow and water chemistry content of the landslide galleries
(seasonal analysis and gallery survey, Figs. 5 and 7). The
recharge is essentially local, enhanced by the trenches and
the counterscarps which tend to limit the runoff and to
facilitate groundwater infiltration in the landslide area
(Binet et al. 2007a). During high-flow periods, the waters
infiltrated in the landslide flow perpendicularly to the
slope through the dense network of near-vertical N70
conductive fractures down to a basal continuous perched
aquifer (vertical drainage), which is maintained as
saturated by the high recharge amount. When the recharge
amount is sufficient, the numerous disconnected saturated
zones can become temporarily connected, leading to a
groundwater flow parallel to the slope down to the
landslide (horizontal drainage). This near-surface drainage, above the basal perched aquifer, occurs in the
unsaturated zone and is controlled by the N70 fractures
nearly parallel to the slope. In contrast, during the dry
season, the recharge amount is not sufficient to maintain a
continuous perched aquifer in the landslide and only
temporarily disconnected saturated zones can occur
after storm events. The numerous disconnected saturated zones likely result from the high vertical gradient
of the hydraulic conductivity (Maréchal 1998) and the
heterogeneity of the landslide, particularly the tension
cracks filled by colluvial deposits and altered materials
(Cappa et al. 2004). An interpretative position of the
perched-aquifer water level during the high-flow periods is
suggested along the cross-sections XY in Fig. 7c and XYZ in
Fig. 8b.
Unstable zone: slope deep aquifer
The EDF gallery acts as a main drainage structure for the
slope (tracer test, Fig. 3), and especially beneath the
unstable zone, where the fracture network is very dense
(gallery survey, Fig. 7b). The EDF gallery imposes a
constant head of the deep aquifer at about 425 m asl
DOI 10.1007/s10040-015-1298-2
1777
5°47'0"E
5°48'0"E
5°49'0"E
5°50'0"E
Legend
Spring
12
00
45°5'0"N
Stream
S12
Unstable slope extent
Sedimentary cover
S11
400
250 500
1100
00
13
0
80
Gallery
Z
I4
S9
Fault/fracture
0
0
140
S17
S10
Peak Oeilly
1500
Piezometer
yant
if Bru
am R
e
r
t
S
1 000
Metres
S13 S18
Y
S19
45°4'0"N
I2
S20
S15
S14
S24
50
0
S25
S2
S3
S4
45°3'30"N
S21
G900/I1
S1
S5
900
600
45°4'30"N
5°51'0"E
Séchil
ienne
fault
5°46'0"E
Sabo
t fau
lt
45°5'30"N
a
700
G710
G585/I3
S16
S6
X
S7
River Romanche
b
1450
X
Y
Elevation (m asl)
Perched aquifer
in landslide
1050
S13 S18
G900
850
650
Perched aquifer in
sedimentary cover
Hydraulic connection
1250
Z
Sabot fault
S14
Piezometer
Piezometer
G710
River
G670
Romanche
G585
S15 S21
Deep slope aquifer
450
Gallery EDF
250
0
Fracture
drainage
500
1000
1500
Micro-fissured block
drainage
2000
2500
Sub-surface
drainage
3000
Surface
water
Distance (m)
3500
4000
Water-level
Fig. 8 Sketch of the groundwater conceptual model. a Spatial sketch. b Cross-section sketch. The cross-section of the unstable zone is
modified after Lebrouc et al. (2013). The unstable slope boundary is defined according to the geophysical survey of Le Roux et al. (2011).
S14, S15, S21, S13 and S18 springs are projected on the cross-section
(Fig. 7c). The deep-aquifer water level is also controlled
by the constant water head of the Romanche alluvium.
The EDF gallery on its western side recharges the
deep aquifer with water from the Romanche River and
water inflow from the unstable zone (seasonal analysis
and hydrochemistry, Figs. 5 and 6). The EDF gallery
acts as a major east–west drain, whose influence is
mainly controlled by the N70 crossing fractures
(Fig. 8a). Groundwater which is not collected by the
EDF gallery flows toward the Romanche alluvium
aquifer or to the springs located westward (tracer-test,
Fig. 3c,d). All this information allows for the proposal
of an interpretative position of the deep-aquifer water
level (Figs. 7c and 8b).
Hydrogeology Journal (2015) 23: 1761–1779
Conclusion
This study highlights the dual-permeability properties of
fractured-rock reservoirs with preferential water flow in
fractures bypassing most of the less pervious and inertial
micro-fissured matrix. The major faults or fractures play a
key role in the massif drainage. The EDF gallery also acts
as a major drain in the massif. This survey highlights the
contrast in hydraulic properties between the unstable zone
and the intact rock mass outside the landslide. This vertical
heterogeneity leads to a two-layer aquifer, with a shallow
perched aquifer localized in the unstable zone and a deep
aquifer in the whole massif. The landslide perched aquifer is
temporary and mainly discontinuous, and its extent and
DOI 10.1007/s10040-015-1298-2
1778
connectivity fluctuate according to short-term recharge
variations. The perched aquifer is almost dry during the
low-flow periods. The groundwater flows mainly through a
dense network of widely opened fractures. The recharge area
of the landslide perched aquifer is mainly local although, in
high-flow periods, the recharge area may become larger than
the landslide surface and may include the remote sedimentary cover perched aquifer.
Seasonal monitoring of natural and artificial tracers
enables characterization of the groundwater scheme of
unstable slopes and their surrounding stable massif. The
results obtained in the case of the Séchilienne landslide
show that this method is able to solve various important
issues for hydromechanical studies (Cappa et al. 2014).
Due to the general scarcity of hydrogeological monitoring
networks in landslide sites, the proposed method could be
suitable to conceive outstanding flow schemes for other
landslide types.
Acknowledgements This research was funded by SLAMS
(Séchilienne Land movement: Multidisciplinary Studies), the
program of the Agence Nationale de la Recherche. The meteorological and displacement data were supplied by CEREMA Lyon.
The authors gratefully acknowledge the support of Jean-Pierre
Duranthon and Marie-Aurélie Chanut (CEREMA Lyon). The
authors are also very grateful to Christophe Loup (UMR ChronoEnvironnement) for the chemical analyses. The implementation of
the monitoring network would not have been possible without the
cooperation of Mrs. and Mr. Aymoz, Patrick Boyer from the Office
National des Forêts, and Gérard Cret, mayor of Séchilienne. The
manuscript was improved by detailed and constructive comments
from the editor Jiu Jimmy Jiao, the associate editor Jürgen
Mahlknecht, and reviewers Stéphane Binet, Federico Cervi, Boris
Matti and one anonymous reviewer.
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