Geochemical Evidence
for Groundwater-Charging
of Slope Sediments: The Nice Airport
1979 Landslide and Tsunami Revisited
A.J. Kopf, S. Kasten, and J. Blees*
Abstract In October 1979, a period of heavy rainfall along the French Riviera was
followed by the collapse of the Ligurian continental slope adjacent to the airport of
Nice, France. A body of slope sediments, which was shortly beforehand affected
by construction work south of the airport, was mobilized and traveled hundreds of
kilometers downslope into the Var submarine canyon and, eventually, into the deep
Ligurian basin. As a direct consequence, the construction was destroyed, seafloor
cables were torn, and a small tsunami hit Antibes shortly after the failure. Hypotheses
regarding the trigger mechanism include (i) vertical loading by construction of an
embankment south of the airport, (ii) failure of a layer of sensitive clay within the
slope sequence, and (iii) excess pore fluid pressures from charged aquifers in
the underground. Over the previous decades, both the sensitive clay layers and the
permeable sand and gravel layers were sampled to detect freshened waters. In 2007,
the landslide scar and adjacent slopes were revisited for high-resolution seafloor
mapping and systematic sampling. Results from half a dozen gravity and push cores in
the shallow slope area reveal a limited zone of freshening (i.e. groundwater influence).
A 100–250 m wide zone of the margin shows pore water salinities of 5–50% SW
concentration and depletion in Cl, SO4, but Cr enrichment, while cores east or west
of the landslide scar show regular SW profiles. Most interestingly, the three cores
inside the landslide scar hint towards a complex hydrological system with at least two
sources for groundwater. The aquifer system also showed strong freshening after a
period of several months without significant precipitation. This freshening implies
that charged coarse-grained layers represent a permanent threat to the slope’s stability,
not just after periods of major rainfall such as in October 1979.
A. J. Kopf ()
University of Bremen, MARUM Research Centre, 28359 Bremen, Germany
e-mail: akopf@uni-bremen.de
S. Kasten and J. Blees
Alfred Wegener Institute for Polar and Marine Research,
27570 Bremerhaven, Germany
* Present address: University of Basel, 4056 Basel, Switzerland
D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences,
Advances in Natural and Technological Hazards Research, Vol 28,
© Springer Science + Business Media B.V. 2010
203
204
A. J. Kopf et al.
Keywords Submarine landslide • tsunami • hydrology • geohazard • geochemistry • fluid seepage • groundwater charging
1
Introduction
The sedimentary instability of submarine slopes represents a major geohazard and
threatens coastal infrastructure both on- and offshore (Locat and Lee 2002). The
Ligurian Margin, Southern France, represents an area of fluid-charged, metastable
slope deposits, which today pose a geohazard to the French Riviera. This portion of
the Mediterranean coastline receives millions of tourists each year and comprises
valuable infrastructure all along. Understanding the preconditioning factors and
governing trigger mechanisms for near-shore submarine slope instability is one of
the key objectives to be addressed. The Nice airport area represents such a
potentially unstable continental slope where factors favoring instability include
seismicity, groundwater charging, presence of weak minerals, high sediment
accumulation rates, anthropogenic impact by construction, and slope oversteepening.
The hydrological system in the Var Valley, adjacent to the city of Nice, represents
an alluvial aquifer recharged by seepage from the river Var and by subsurface
infiltration from its foreland (Guglielmi and Mudry 1996). Water sources are the
Alps as well as Provencal foreland series, comprising Mesozoic sedimentary rocks,
Pliocene pudding stones (representing an old delta), overlain by Pleistocene gravel
and Holocene clastic series of variable grain size in the Var Valley and river mouth
(Dubar and Anthony 1995). Groundwater from those domains is geochemically
distinctly different, and is migrating oceanwards (along permeable, coarse-grained,
gently southward- dipping beds (Guglielmi 1993). Recent work by Guglielmi and
Prieur (1997) has attested that subsurface pathways are rather complex and result
in three areas of submarine fresh water seepage in the Nice airport area east of the
Var river mouth.
In general, the anthropogenic impact on the Var River system in the twentieth
century had a profound effect on its stability and increased the vulnerability to
hazards such as floods, spill overs, and delta-front slope instability. Land
reclamation for developments such as the Nice international air-port as well as
industrial and administrative estate resulted in bed extraction of gravel and other
deposits in the 1960s and 1970s (Anthony 2007). As a consequence of the
narrowing and deepening of the Var channel, the average bed was lowered by
approximately 10 m, so that the aquifer was lowered too (Guglielmi 1993). Most
importantly, this did not only change the hydrological pathways, but resulted in
saltwater intrusion in the Nice area and increasing risks of flood damage of the
lower channel during periods of higher river discharge (e.g. during spring floods or
the high-precipitation period in fall).
After such a period of heavy rain, a major submarine landslide (∼8.7 × 106 m2)
affected the coastal system offshore Nice on the 16th of October 1979 and resulted
in destruction of an embankment at the Nice airport (Fig. 1a, dashed line), a debris
flow cutting two submarine cables tens of kilometers away from the sliding area
Geochemical Evidence for Groundwater-Charging of Slope Sediments
205
Fig. 1 (a) Combined satellite image and bathymetric data of the Nice airport area and submarine slope
(courtesy of IFREMER), illustrating the failure scar immediately south of the international airport.
Dashed line shows former embankment south of the runways, which collapsed in 1979. (b) Bathymetric
map shows the locations of the Seamonice long-term piezometer, CPTU deployments, gravity cores
used in this study, and water column sampling by Guglielmi and Prieur (1997). See text
and a tsunami wave of 2–3 m height at the nearby coast (Dan et al. 2007). It was
proposed several years ago that overpressuring linked to the hydrogeological condition
could be the trigger mechanism of the Nice Airport failure, and slight freshening
206
A. J. Kopf et al.
of the seawater in parts of this area further suggested fresh groundwater is
released offshore by coastal aquifers (e.g. Guglielmi 1993; Guglielmi and Prieur
1997). The hydrogeological triggering model is also supported by sedimentary and
seismic reflection data indicating permeable layers of sediments may provide aquifer
pathways in the shallow subsurface (e.g. Guglielmi and Mudry 1996), and possibly
down to a maximum depth of 150 m.
The major objective of this study was to decipher whether the slope sediments
off the airport of Nice show evidence for groundwater charging, and if so, in which
horizons these occur. The result is used to assess whether such influx may have
served as a trigger for the Nice airport landslide.
2
Previous Marine Expeditions
An investigation of the superficial marine sediments (max. 30 m subbottom depth)
was recently performed in close collaboration between France (e.g. PRISME cruise
with RV L’Atalante, 2007) and Germany (e.g. M73/1 cruise with RV Meteor,
2007). The study included geophysical acquisition, in situ pore pressure and shear
strength measurements (CPTU devices, Penfeld penetrometer) as well as gravity
coring (Fig. 1). For long- and mid-term measurements, a long-term piezometer,
which acquires the pore pressure at five different depth levels within the sediment,
was installed by IFREMER in 2006. Short-term measurements were carried out
using a marine shallow-water CPTU (cone penetration testing with pore pressure
measurement) probe by MARUM Bremen (Stegmann et al. 2006) and piezometer
instruments by IFREMER, while data down to 30 m depth were acquired using the
Penfeld penetrometer (Sultan et al. 2004, 2008).
The main results at this stage include: (1) The main failure surface of the Nice
airport slide localised in ∼30–50 mbsf and is located in sensitive clays interbedded
with coarse-grained sediment (Sultan et al. 2004). (2) Long-term pore pressure
measurements (Nov. 2006–Nov. 2007 at Seamonice station) in the scar of the 1979
landslide with a piezometer indicate a direct relationship to precipitation events, as
the variability of the measured pore pressure follows the rate of rainfall (N. Sultan,
personal communications, 2008). (3) Mid-term pore pressure records (34 h)
acquired in the landslide scar at different depth levels show contrasting pore pressure evolutions. At 4.25 m below seafloor an increase of pore pressure (∼2 kPa)
over time could be observed, whereas the pressure in the other levels steadily
decreases over time (Sultan et al. 2008). (4) CPTU short-term deployments (25–
310 min.) in the area of the Nice Airport indicate higher than hydrostatic pore pressures in sediments in the upper part of the slope, close to the scar of the 1979
landslide (Kopf et al. 2008). (5) ROV surveys as well as high-resolution geophysical
data indicate that in some portions of the slope, the surface sediment is currently
creeping. (6) Klaucke and Cochonat (1999) further identify slumping as one of the
most fundamental processes in the Var valley and adjacent slopes. (7) Initial shipboard geochemical pore water analyses hint towards groundwater flux in the scar
of the 1979 landslide (Kopf et al. 2008) and were the starting point for our study.
Geochemical Evidence for Groundwater-Charging of Slope Sediments
3
207
Methods
After a multibeam bathymetric survey (Simrad EM710) across the southern margin
of the Nice airport, a total of six gravity cores were taken during cruise M73/1
(Kopf et al. 2008). Five of those cores are used for this predominantly geochemical
study (Fig. 1b). The pH was measured directly in the sediment using a punch-in
electrode before the pore water was extracted. The pore water was then retrieved by
means of rhizons (pore size 0.1 mm) according to the procedure described by
Seeberg-Elverfeldt et al. (2005). The gravity cores were each processed in this way
within a few hours after recovery. Depending on the porosity of the sediments, the
amount of pore water recovered ranged between 4 and 20 ml. Solid phase samples
of the majority of cores were taken for total digestions, sequential extractions and
mineralogical analyses at 25 cm intervals, kept in gas-tight glass- and heavy plastic
bottles under an argon atmosphere and stored at 4°C. Pore water analyses of
ammonium, alkalinity and salinity were carried out onboard. Ammonium was
measured using a conductivity method. Alkalinity was calculated from a volumetric
analysis by titration of either 0.5 or 1 ml of the pore water samples with 0.01 M
HCl. Salinity was measured using a conductivity probe placed directly into the pore
water samples.
In addition, aliquots of the remaining pore water samples were diluted 1:10 and
acidified with HNO3 (suprapure) for determination of cations with an inductively coupled plasma optical emission spectrometer (ICP-OES; Iris Intrepid [Type Duo],
Thermo Nicolet GmbH). Subsamples for sulfate and chloride analysis were diluted
1:50 and measured at the AWI in Bremerhaven using ion chromatography (Metrohm
IC Net 2.3). For solid-phase analyses, sediment samples were taken at 5 cm intervals, freeze-dried and pulverized with an agate mortar and pestle. A subsample of
ca. 50 mg was digested in a CEM Mars microwave system using a concentrated
acid mixture of HNO3 (3 ml), HCl (2 ml) and HF (0.5 ml). Element concentrations
were analyzed by ICP-OES.
For all additional data presented in the context of this paper, in particular Cone
Penetration Testing and pore pressure measurements, refer to cruise reports (Sultan
et al. 2008; Kopf et al. 2008).
4
Results
All six gravity cores taken in the Nice airport area underwent sedimentological
description and basic geotechnical characterization, while geochemical analysis
was done on the pore water for five and on the solid sediment for two of them.
Core GeoB12019 represents a reference core of undisturbed slope deposits west
of the Var river mouth, and comprises silty clay interbedded with layers of silt and
red clayey layers (Fig. 2). Cores GeoB12088, -43 and –42 in the upper northern
portion of the headwall, only a few 10 s of meters south of the airport, show silty
clay to clay with dm-thick gravel deposits close to the seafloor. Core GeoB12003
208
A. J. Kopf et al.
Fig. 2 Visual core description for cores GeoB12003, -42, –43, -72 and –88 taken in the Nice
airport area. Numbers beneath Site labels indicate water depth at each site. See Fig. 1b for location.
For details refer to Kopf et al. (2008)
is located at the eastern rim of the headwall and consists of silty clay without
gravel, but fine sand layers at various depths (Fig. 2). A sixth gravity core east of
the 1979 landslide scar is located in the stable slope, where silty clay and silt dominate the sedimentary succession. Frequent sand layers and turbidites are found in
the interval between 1.5–4.5 m sediment depth (see Fig. 2, right). For a detailed
description of the cores, as well as sediment physical property measurement using
a Multi-Sensor Core Logger, refer to Kopf et al. (2008). Undrained shear strength
using a miniature Vane shear apparatus was determined immediately after core
splitting, and ranged between 2 and 8 kPa for the uppermost silty clays, and rarely
exceeded 20 kPa further down and in the sandy intervals. The only exceptions were
remoulded (so called “marble cake”) intervals and gravel-bearing portions of the
core (see Kopf et al. 2008).
Onboard conductivity measurement of pore waters revealed that the three sampling sites situated inside the landside scar (GeoB12003, -42, -43) are significantly
freshened below 70–90 cm sediment depth. Core GeoB12042 displayed the most
pronounced reduction in salinity, whereas core -43 slightly farther west and -03 at
the eastern margin of the headwall scar show weaker freshening (Fig. 3). This is
best explained by the lower water depth of Core GeoB12042 (Fig. 2) owing to
which material from 10 m lower in the sedimentary section – and thus closer to the
main failure surface – was sampled (see also Ch. 2, and Sultan et al. 2004). Core
GeoB12072 east of the 1979 landslide scar showed regular salinity.
Geochemical Evidence for Groundwater-Charging of Slope Sediments
Sediment depth [m]
Alkalinity
[mmol(eq)/l]
0 4 8 12 16
0
0
1
1
1
2
2
2
3
3
3
4
4
4
5
Sediment depth [m]
0
0
Alkalinity
[mmol(eq)/l]
0 4 8 12 16
5
5
SO4
[mmol/l]
0 10 20 30
Ammonium
[µmol/l]
0
2000 4000
Salinity
[psu]
10 20 30
Cr
[µmol/l]
0 1 2 3 4 5
Cl
[mmol/l]
0 200 400 600
0
0
0
0
0
1
1
1
1
1
2
2
2
2
2
3
3
3
3
3
4
4
Alkalinity
[mmol(eq)/l]
0 4 8 12 16
Sediment depth [m]
Ammonium
[µmol/l]
0
2000 4000
209
4
4
4
SO4
[mmol/l]
0 10 20 30
Ammonium
[µmol/l]
0
2000 4000
Cr
[µmol/l]
0 1 2 3 4 5
Cl
[mmol/l]
0 200 400 600
0
0
0
0
0
1
1
1
1
1
2
2
2
2
2
3
3
3
3
3
4
4
4
4
4
Fig. 3 Pore water concentration profiles for core GeoB12019 (Background core; uppermost
graph) as well as cores GeoB12003 (center) and –42 (lowermost graph) retrieved in the headwall
area. See Fig. 1b for location. While site GeoB12019 shows typical marine depth/concentration
profiles, sites GeoB12003 and -42 display very low chloride (Cl) contents in the lower parts of the
cores. Note the distinctly different Cr contents at these two sites
210
A. J. Kopf et al.
Sulfate and chloride measurements of the pore waters confirmed the initial
shipboard results obtained by means of a conductivity probe. Although only two
samples were available for sulfate and chloride analyses for core GeoB12003 low
concentrations of these pore water constituents clearly indicate a groundwater
charging of sediments below about 70–90 cm sediment depth at sites GeoB12003,
-42 and –43 (Fig. 3).
While these three sites display very similar pore water depth profiles of chloride
and sulfate they substantially differ with respect to dissolved chromium contents.
At station GeoB12003 Cr concentrations are mostly below the detection limit of the
ICP-OES. In contrast, dissolved Cr contents in pore waters of sites GeoB12042 and
-43 increase downcore to reach maximum values of 4.5 mmol/l (Fig. 3). The inverse
correlation of Cr with Cl and SO4 suggests an association with inflowing groundwater; moreover, the variation in Cr implies that this groundwater originates from
more than one source. We will address this point in conjunction with geochemical
data on the solid phase and mineralogical data in the discussion (Ch. 5).
5
Discussion
Submarine discharge of groundwater in the coastal zone of Nice has previously
been reported by Guglielmi and Prieur (1997). Those authors used chemical
variations in samples taken in the water column along two arcuate profiles around
the Nice airport “peninsula” (see Guglielmi and Prieur 1997, their Fig. 2). A subtle
decrease in salinity and Ca2+, Mg2+ and K+ ions, associated with a notable increase
in temperature and silica (the latter related to Pliocene pudding stones) has been
observed in three areas along the profiles. The central of these three areas, station
9 in Guglielmi and Prieur (1997), corresponds to the 1979 Nice airport slide where
we observe the profound freshening.
Guglielmi and Mudry (1996) characterized the chemical composition and estimated
the spatial and temporal variability in the flux of surface and subsurface waters into
the alluvial aquifer of the river Var. They could show that the water of the river Var
contains relatively high levels of sulfate (100–300 mg/l; corresponding to about
1–3 mmol/l) while groundwater in the area typically has lower SO4 contents. In the two
cores from the headwall area presented here sulfate decreases downcore in accordance
with chloride, supporting the presence of groundwater in these sediment levels. Sulfate
contents in the deeper parts of the cores fluctuate around 0.5 mmol/l and thus are lower
than in the upstream area as given by Guglielmi and Mudry (1996). This implies that
either (i) groundwater with even lower sulfate contents infiltrates into the coastal
(submarine) deposits, that (ii) seawater is infiltrating an otherwise groundwatersaturated aquifer in the reclaimed land mass and/or lower Var valley units (i.e. Pliocene
pudding stones and Pleistocene gravel; as discussed in Guglielmi and Mudry 1996), or
that (iii) a portion of sulfate has been consumed in the the degradation of organic
matter by dissimilatory sulfate reduction.
Dissolved Cr contents found in the pore waters of GeoB12042 (Fig. 3) and -43
(not shown here) are unusually high (up to 4.5 mmol/l) and exceed the natural
Geochemical Evidence for Groundwater-Charging of Slope Sediments
211
concentrations of Cr in western Mediterranean seawater (max. ∼4.5 nmol/l, Achterberg
and Van den Berg 1997) by a factor of 1,000. The inverse down-core correlation
between Cr and Cl (and sulfate) further suggests a supply of Cr with the infiltrating
groundwater. Strikingly, Cr contents were not found elevated at the third groundwaterinfluenced site GeoB12003 nearby (see Fig. 1b). These findings raise the question
whether the extremely high Cr contents at GeoB12042/-43 are supplied by
(1) involving groundwater (maybe owing to pollution), (2) desorption of Cr from clay
minerals, or (3) result from a significant difference in mineralogy in the source area.
Either scenario would hint towards a hydrological system with locally different
groundwater sources, with the sources either close to Nice airport (scenarios 1 & 2)
or with a distant source of meteoric water with Cr leached from weathering rocks
farther north (scenario 3). In order to decide which scenario is most likely, we carried
out chemical and mineralogical analyses on the solid phase (Fig. 4). The total element
concentrations of the sediments recovered at sites GeoB12003 and -42 demonstrate
0 .0 8
Cr
[g/kg]
Cr
[g/kg]
0 .0 8
0 .0 4
0 .0 4
0 .0 0
0 .0 0
0
1
2
3
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
0
1
3
2
Sediment depth [m]
4
4
Mg
[g/kg]
Mg
[g/kg]
12
12
8
4
8
4
0
0
2
3
4
200
160
120
80
40
0
0
1
2
3
4
4
2
2
0
0
0
1
2
3
4
60
Al
[g/kg]
60
Al
[g/kg]
200
160
120
80
40
0
4
Ti
[g/kg]
Ti
[g/kg]
1
Ca
[g/kg]
Ca
[g/kg]
0
40
20
40
20
0
0
0
1
2
3
Sediment depth [m]
4
Fig. 4 Total element concentrations of the solid phase recovered in cores GeoB12003 (upper graph)
and –42 (lower graph) near the headwall area of the 1979 landslide. See Fig. 1b for location
212
A. J. Kopf et al.
that Cr closely covaries with Al and Ti representing the typical elements of the
terrigenous sediment fraction. A comparison between the two sites also shows that no
significant difference in total Cr is observed. XRD analyses on two samples from
cores GeoB12003 (220 cm depth) and -42 (260 cm depth) did not reveal any
Cr-bearing minerals (i.e. being present in concentrations above 1 wt%) and did not
reveal any significant differences between the two sites with respect to mineralogical
composition. However, there are significant amounts of phyllosilicates in the
sediment, mirroring source rocks farther upstream of the river Var. Main constituents
include quartz, calcite, chlorite (largely clinochlore), muscovite, corundum, albite and
attapulgite. No garnet-bearing Alpine rocks are found anywhere adjacent to the study
area, so that we exclude direct Cr-sources to the Var river system. However, it should
be noted that in order to obtain the Cr concentrations measured at sites GeoB12042
and -43, relatively low amounts of Cr-bearing minerals would suffice. Also, Cr may
have got adsorbed to clay minerals, muscovite or clinochlore, offering the potential
for desorption and liberation into the pore water. Given that we used rhizons (rather
than a hydraulic press) to extract the water, only in situ desorption needs consideration.
Given the overall low effective stresses in the sediment and the equally low flux rates
from ROV CTD surveys (see Ch. 2), we consider this rather unlikely.
These rather preliminary geochemical and mineralogical investigations therefore favor the first hypothesis, i.e. that the high dissolved Cr contents at sites
GeoB12042/-43 are most likely supplied by seaward groundwater flow. We exclude
infiltration of seawater into the groundwater-charged horizons, because this would
cause elevated concentrations of sulfate and other ions enriched in seawater in the
interstitial water of the sediments. On the other hand, the removal of sedimentary
layers during the landslide event puts seawater in contact with fresh-water saturated
sediments, which were originally buried at some depth below the seafloor. The
sharp gradients at sites GeoB12003 and -42 (Fig. 4) could be advection-diffusion
transients associated with the 1979 event rather than steady-state profiles. Cores
-19 and -72 were taken outside of the landslide scar, which leaves the possibility
that the fresh water simply cannot be reached because the cores are only 5 m long
(but the failure plane is at 30 m; Sultan et al. 2004).
Long-term piezometer measurements (Fig. 1b; Seamonice station) in the 1979
landslide scar attest a more or less direct correlation between precipitation in the
Nice area and transient pore pressure increase in the more permeable, coarse-grained
series where we detected fluid freshening (N. Sultan, personal communications,
2008). Those results indicate a southward-directed submarine groundwater outflow,
which lowers the effective strength of the underground and may contribute to slope
failure, in particular during or shortly after periods of heavy precipitation. The longterm seasonal variability with spring floods in the Var River system (caused by snow
melting, but also a lot of rain usually during the month of May) may have
significantly affected the fluid chemistry during the period of pore water sampling,
in particular since a permeability of 10−8 m/s for the slope sediments south of the
airport (Dan 2007) implies that some of the May 2007 rainfall (>70 mm) still
remained in the system when we sampled in July 2007. This perception is indirectly
supported by elevated pore pressure values at the Seamonice station (Fig. 1b), which
Geochemical Evidence for Groundwater-Charging of Slope Sediments
213
rose approximately simultaneously with higher precipitation around May 22nd,
2007, but then remained at an elevated level until mid-August, although average
rainfall dropped drastically from May (71 mm) via June (21 mm) and July (1 mm) to
August (4 mm) (N. Sultan, personal communications, 2008).
In conclusion, we tentatively propose that the hydrological system at the Var
river mouth and adjacent coastal region near the Nice airport varies on a small
scale. Guglielmi and Prieur (1997) already attested that there are at least three areas
of groundwater flux into the Ligurian submarine slope. In our study area (their station 9), we attest additional complexity and evidence for two different groundwater
sources, one of which shows Cr contents that are three orders of magnitude above
seawater (Fig. 3) and may be explained by anthropogenic impact (e.g. dumping of
scrap metal during 1960s landfill operations, present-day pollution, etc.), either
being released now or originating from in situ desorption from clay. To substantiate
our assumptions, more detailed geochemical studies are needed in an “amphibic”
approach where fountains on land near Nice as well as groundwater seeps south of
the Nice airport are regularly sampled and analyzed to identify seasonal variations
and the regional hydrological variability.
Acknowledgements The authors thank the captain and crew of RV L’Atalante and RV Meteor
during the cruises, and the respective national funding to realize them. Special thanks go to Kara
Bogus, Tim Haarmann and David Fischer for their strong support in geochemical sampling and
analyses both onboard the ship as well as in the laboratory. Christoph Vogt is acknowledged for
semi-quantitative XRD analyses of sediments. The paper benefited from intensive discussion with
Sylvia Stegmann and Nabil Sultan and constructive criticism by reviewers Thom Bogaard and
Pierre Henry. Funds for this study were provided by DFG (to MARUM) and the Helmholtz
Association (to AWI).
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