Journal of Hydrology 381 (2010) 189–202

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Journal of Hydrology

journal homepage: www.elsevier .com/ locate / jhydrol

Reconstruction of groundwater flows and chemical water evolution in anamagmatic hydrothermal system (La Léchère, French Alps)

E. Thiébaud a,*, M. Dzikowski b, D. Gasquet b, C. Renac c

a Laboratoire Magmas et Volcans, Université Blaise Pascal, UMR-6524. Bâtiment Kessler, 5 rue Kessler, 63 038 Clermont-Ferrand, Franceb Laboratoire Environnements, Dynamiques et Territoires de la Montagne, Université de Savoie, UMR-5204. Campus scientifique,Pôle Montagne, F-73 376 Le-Bourget-du-Lac cedex, Francec Laboratoire de Transferts Lithosphériques, Université Jean Monnet, UMR-6524, 23 rue du Dr. P. Michelon, 42 023 Saint-Etienne, France

a r t i c l e i n f o

Article history:Received 3 April 2009Received in revised form 12 November 2009Accepted 24 November 2009

This manuscript was handled by L. Charlet,Editor-in-Chief, with the assistance ofChong-yu Xu, Associate Editor

Keywords:Orogenic settingThermal conditionsHydrochemistryLa LéchèreAlps

0022-1694/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.jhydrol.2009.11.041

* Corresponding author. Present address: 28 rue Jeyon, France.

E-mail address: e.thiebaud@yahoo.fr (E. Thiébaud)

s u m m a r y

A detailed picture of the hydrogeochemistry of an alpine thermal area (La Léchère, French Alps) wasdrawn up from new and complete analyses of shallow and deep groundwater, including major and traceelements, gases, and stable (d18O, dD, d13C, d34S) and radiogenic isotopes (14C), combined with data on thetrace and stable isotope (d18O, d34S) contents of the aquifer rocks.

Three different groundwater facies were recognized: (1) the shallow groundwater in the carbonate andcrystalline rocks presents a poorly mineralized Ca–HCO3 facies; (2) the shallow groundwater from theevaporitic fault zone presents a mineralized Ca–Mg–SO4 facies; (3) and the deep thermal water presentsa highly-mineralized Ca–Na–SO4 facies. The evolution from facies 2 to facies 3 is controlled by three phe-nomena: (i) the presence or absence of evaporitic deposits along the fault, which governs whether or notthe water can interact with carbonates and dolomites, (ii) interactions with micaschists and Permo-Car-boniferous rocks, which lead to Ca–Na exchanges and enrichment in some trace elements and gases (CO2,CH4 and H2S), and (iii) the long residence time (several thousand years) and high temperature (90 �C) ofthe thermal water.

� 2009 Elsevier B.V. All rights reserved.

Introduction

The most intensively studied thermal sites are those associatedwith active volcanic areas (e.g., Yellowstone National Park, USA,Kharaka et al., 2000). Numerous hydrogeochemical studies havealso been carried out on thermal springs in amagmatic zones. Someof these studies have focused uniquely on the thermal water with-in a single site and have identified mineralization processes, equi-librium temperatures, recharge zones or water residence timesthrough analyses of major and trace element contents (e.g., Chiodi-ni et al., 1995), stable isotopes, such as d18O, dD, d13C, d34S (e.g.,Brugger et al., 2005; Gunn et al., 2006), and/or radiogenic isotopes,such as 3H, 14C, 36Cl (e.g., Lopez-Chicano et al., 2001; Dotsika et al.,2006). Other studies have compared thermal water with shallowgroundwater in order to investigate mixing processes (e.g., Bajjali,2008; Demlie et al., 2008; Karimi and Moore, 2008) or to definemodels of the chemical evolution of groundwater through sedi-mentary basins (Qin et al., 2005; Mayo et al., 2007).

ll rights reserved.

anne d’Arc, 63 140 Chatelgu-

.

In contrast, very little research has been carried out into theevolution and transfer of thermal water in alpine orogenic areas.One of the rare studies in this domain showed the importance ofevaporitic fault zones in controlling flow and mineralization in al-pine settings (Pastorelli et al., 1999).

The La Léchère thermal site is located in an alpine setting thatincludes a wide variety of rock types, including granites, gneisses,micaschists, evaporites and carbonates (Fig. 1). The study area is afault zone that constitutes a continuous aquifer in which both ther-mal water and shallow groundwater circulate. Hydrochemicalstudies of the site’s borehole water (unpublished internal reportsand Dray, 2003) have thrown light onto the mixing between deepand shallow waters in the discharge zone.

The present study extends these earlier results through morecomprehensive analyses of the area’s thermal and shallow ground-water and the aquifer rocks. The groundwater analyses provideddetailed new data for major and trace elements, gases, and stable(d18O, dD, d13C, d34S) and radiogenic (14C) isotopes, which werecombined with major, trace and stable (d18O, d34S) isotope datafor the aquifer rocks. Spa operators have collected a certain amountof data for French thermal sites for regulatory purposes, and asmall number of trace element, gas and isotope analyses have beencarried out. However, the present study is the first to provide a

Fig. 1. Geological setting. (a) Location of La Léchère thermal site in the French Alps. (b) Simplified geologic map (from Barféty et al., 1984; Debelmas et al., 1989; Antoine et al.,1992; Doudoux et al., 1999). Location of water sampling points (Av: Avignons spring, Gl: Glaizette spring, RF: Route de la Forêt spring, Ta: Tartet stream, SI: Iles spring, VS:sulfated outlet, PB: Pont Botto spring, Gu: Guillots stream) and of rock sampling points. (c) Configuration of the thermal discharge zone, location of water sampling points(Nat.: Natacha well, RVII: RVII well, V46, V8 and F2: exploration gallery samples). (d) Representative geological cross-section of the study area.

190 E. Thiébaud et al. / Journal of Hydrology 381 (2010) 189–202

detailed picture of the chemistry of the water and aquifer rocks ofan alpine thermal area.

From these new data, the objectives of this study are to draw upa water–rock interaction model for the chemical evolution of thegroundwater, from system recharge to thermal upwelling. In order

to do this it was necessary to define the system recharge, the typesof groundwater facies, the processes governing the chemical evolu-tion of the shallow groundwater and deep thermal water, the tem-perature of the thermal reservoir, and the residence times of thedifferent groundwater.

E. Thiébaud et al. / Journal of Hydrology 381 (2010) 189–202 191

Site description

Geological setting

The La Léchère thermal site is located on the eastern edge of theexternal French Alps (Fig. 1a). Thermal water rises at the intersec-tion of the Isère Valley and the Léchère Fault Zone (LFZ), whichmarks the contact between the Belledonne crystalline massif (Bin Fig. 1a) and the allochtonous sedimentary cover of the Del-phino-Helvetic Zone (Fig. 1b and d). The Belledonne crystallinerocks, which include the Pussy gneiss, Lauzière granite, Notre-Dame de Briançon granite and Feissons micaschists, are coveredby autochthonous organic-rich Permo-Carboniferous conglomer-ates and sandstones. The allochtonous sedimentary cover is highlyfolded and consists mostly of Mesozoic marls and carbonates.

The SE-dipping LFZ is underlain by discontinuous Triassicdeposits (dolomites, gypsum, cargneules and halite). The dischargezone is sealed by sediments in the Bellecombe glacial basin(Fig. 1b).

Flow pattern

The global flow pattern of the La Léchère thermal system wasdefined using data from previous studies (unpublished internal re-ports) associated to a study that combined geological, hydrody-namic, hydrochemical and numerical approaches (Thiébaud, 2008).

Water infiltration occurs mainly in the fractured micaschists(Fig. 1b) and to a lesser degree in the semi-impermeable carbon-ates of the Delphino-Helvetic cover. This water then drains throughthe LFZ to the Isère Valley. In the LFZ, water circulates mainly in the‘‘decompressed” zones of the slopes, which are 400–500 m-thick(Maréchal, 1998). Numerical models of the system suggest thatonly a small percentage of the shallow water, infiltrated at thehighest points, penetrate to great enough depth to be mineralizedand heated (Thiébaud, 2008). This thermal water then wells upinto the glacial basin of the Isère Valley, where it permeates theQuaternary deposits and mixes with shallow groundwater fromthe decompressed LFZ that has drained into the valley and ground-water from the Isère alluvial aquifer.

Thermal discharge, which was first noted in 1869, is the resultof a collapse which led to the formation of a lake with a water tem-perature of around 20 �C (Fig. 1c). Eleven thermal wells have beendrilled to depths of between 15 and 200 m in a radius of 100 maround the lake. Today, only the Radiana VII (RVII) and Natacha(Nat) wells are still operating (Fig. 1c). RVII, which is used as a con-trol well, pumps water from the base of the Quaternary deposits(between 55 and 80 m deep) at a rate of 0.5 m3 h�1. This waterhas a temperature of 40 �C and a conductivity of 3100 lS/cm.Nat, which has an artesian flow rate of 45 m3 h�1, draws its waterfrom the dolomites and cargneules of the LFZ, at a depth of 200 m.This water has a temperature of around 55 �C and a conductivity of2900 lS/cm. A geological exploration gallery on the left bank of theIsère, which cuts through the LFZ (Fig. 1b), gives access to shallowgroundwater.

Materials and methods

Sampling

GroundwaterIn order to compare the chemical processes affecting shallow

and deep groundwaters, waters emerging from different geologicalformations (Fig. 1b and c) were sampled between September 2005and August 2006. Samples of the different types of water were ta-ken from the following 12 sampling points:

(1) Water from the thermal reservoir: two sampling points –RVII and Nat.

(2) Water from the LFZ: two sampling points at the surface (RFnear the thermal site and VS near Celliers) plus three sam-pling points in the exploration gallery (V46 from the cata-clastic micaschists, V8 from the dolomites and cargneules,and F2 from the carbonates).

(3) Water from the crystalline rocks: four sampling points – Gland SI springs from the micaschists, PB spring from thegneiss and the Ta stream, which flows over gneiss andmicaschists.

(4) Water from carbonate rocks: one sampling point – Av spring.(5) Nat, RVII, RF, V8, V46 and F2 were sampled every month,

whereas the less easily accessible SI, VS, Gl, Ta, PB and Avpoints were sampled every 3 months.

RocksSeveral rock samples representing the area’s different geological

formations were analyzed (Fig. 1):

(1) Crystalline rocks: micaschist, ND de Briançon granite, Lau-zière granite.

(2) Permo-Carboniferous rocks: conglomerate, sandstone, andbituminous schist.

(3) Triassic rocks: quartzite, gypsum, dolomite and cargneules.(4) Jurassic carbonate rocks.

Analytical techniques

GroundwaterPhysico-chemical parameters, and major and trace elements. Tem-perature, conductivity at 25 �C, pH, Eh and alkalinity, determinedby acido-basic titration, were measured in the field. Cation sampleswere acidified during sampling (1% HNO3), in order to prevent pre-cipitation. In the laboratory, cation and anion samples were sub-mitted to ultrasounds and diluted by a factor of between 2 (Gl)and 30 (Nat). Cation (Li+, Na+, NH4+, K+, Mg+, Ca2+, Sr+) and anion(F�, Cl�, NO�3 , Br�, NO�2 , PO2�

4 , SO2�4 ) contents were analyzed using

ionic chromatography (Dionex ICS 90) with conductimetric detec-tion by the Laboratoire de Transferts Lithosphériques (Saint-Eti-enne, France). Measurement errors were up to 15%.

The trace element contents of water from the thermal reservoir,LFZ and crystalline rocks were determined by HR-ICP-MS (Looseret al., 1999) at the Laboratoire de Géologie de l’Ingénieur et del’Environnement (GEOLEP, Lausanne, Switzerland). Detection lim-its were around 0.1 lg/l.

Gases. The gaseous phase of the Nat samples was analyzed for dis-solved gases using gas chromatography. 222Rn and 226Ra contentswere measured by c spectrometry after extraction. The results ofthese gas analyses were compared to data from three neighboringalpine thermal sites: Aix-les-Bains, Brides-les-Bains and Allevard(unpublished data). Sulfide contents (HS�, S2�) were measuredusing IRMS. All these analyses were carried out by the HydroisotopGmbH laboratory at Schweitenkirchen, Germany. Gas sampling hasbeen realized in a steel cell, respecting the procedure communi-cated by the Hydroisotop GmbH laboratory. H2S, S2O�3 , SO�3 andSn2� contents were determined internally using the iodometricmethod (Gallino et al., 2008).

Isotopes. Isotopic compositions (d18O and dD) were determined bythe Laboratoire de Transferts Lithosphériques, using ratio massspectrometry (GV-ISOPRIME) on CO2 and H2 gases. d18O ratioswere measured on CO2 after equilibration of the water with CO2

Fig. 2. dD and d18O of groundwater of the study area compared to the worldmeteoric line (Craig, 1961) and the regional meteoric line, determined at Thonon(Dray, 2003).

192 E. Thiébaud et al. / Journal of Hydrology 381 (2010) 189–202

gas (Majoube, 1971). dD ratios were measured using the uraniumreduction method (Bigeleisen, 1952). d18O ratios have been deter-mined for all the quarterly samples whereas dD ratios have beendetermined only for one sampling campaign. The results are ex-pressed in ‰ compared with the VSMOW international standard(errors: 0.1‰ for d18O and 1‰ for dD).

The d13CCMTD ratio of LFZ shallow groundwater was analyzed byacidification of BaCO3 using 100% H3PO4 (McCrea, 1950). The re-sults are expressed in ‰ compared with the PDB internationalstandard (error: 0.5‰).

For Nat water, d13CCMTD and 14C ratios were determined usingthe acid attack method, and sulfates d18O and d34S and sulfidesd34S were analyzed by IRMS. These carbon and sulfur analyseswere carried out by the Hydroisotop GmbH laboratory.

RocksThe rock samples were analyzed by the Service d’Analyse des

Roches et des Minéraux (SARM) in Nancy, France. ICP-AES wasused to determine major element compositions and ICP-MS wasused for trace elements. In addition, six rock samples were sentto the Centre National de la Recherche Scientifique (CNRS) inLyons, France, for B, Br and Li analyses.

IRMS analyses of gypsum (d18O and d34S) and pyrites (d34S)were performed by the Hydroisotop GmbH laboratory.

Results and discussion

Estimation of the recharge altitude

The isotope composition analyses showed ratios of between�11 and �12.8‰ for d18O and between �82 and �90‰ for dD(Table 1). These values fall on or close to the regional meteoric line

Table 1Analysis of the d18O and dD ratios of the water.

Sample d18O‰ dD‰

13 and 14/09/2005Nat. �12.8RVII �12.6RF �12.4VS �12.7V46 �12.5V8 �12.2F2 �12.3Av. �12.8Gl. �11.0PB �12.3

2 and 3/03/2006Nat. �12.6 �90RVII �12.2 �88RF �11.9 �95VS �12.2 �89V46 �12.1 �87V8 �11.9 �87F2 �11.9 �85Gl. �11.4 �82PB �11.9 �87

31/05/2006Nat. �12.5RVII �12.8RF �12.0VS �12.3V46 �12.6V8 �11.8F2 �12.3Av. �12.5Gl. �11.3SI �12.0PB �12.5

(Fig. 2), indicating that all the groundwaters are of meteoric origin.The H2S content of Nat does not seem to influence the d18O and dDsignatures, therefore these signatures can be used to evaluate re-charge altitudes.

Isotopic gradients (d18O and dD) for the eastern side of the IsèreValley (Fig. 3) were calculated by an approved laboratory as part ofa licensing application to exploit the water (Dray, 2003). Becauseclimatic conditions are similar (Thiébaud, 2008), these isotopic

Fig. 3. Isotopic gradients – d18O (a) and dD (b) – in the NE side of Isère valley (datafrom Dray, 2003). The altitude error bars take as a minimum the altitude ofsampling points and as a maximum the altitude of the closest summit.

Table 2Means (m) and standard deviations (r) of the physico-chemical parameters of the water (2005–2006), DR: range of discharge rate in m3 h�1 (VS corresponds to diffuse superficial flow and Ta. is a stream), n: number of analysis,conductivity at 25 �C in lS/cm, concentrations in mg/l.

DR n t �C cond. pH eH Li Na NH4 K Mg Ca Sr F Cl NO2 Br NO3 PO4 SO4 HCO3

Thermal water Nat. 45 11 m 55.4 2905 7.1 �18.7 0.9 257.6 2.6 17.5 35.7 465.6 10.3 3.0 119.4 0.1 0.9 0.3 0.6 1573.3 84.4r 2.5 32 0.2 6.7 0.0 6.5 1.0 0.4 3.7 11.3 1.3 0.2 6.7 0.3 2.0 0.4 1.9 34.6 12.2

RVII 0.5 11 m 41.8 3098 7.0 �7.0 0.7 201.5 1.4 14.5 47.7 603.8 11.2 2.4 89.0 0.5 2.7 0.6 7.3 1775.3 113.0r 1.3 41 0.1 4.6 0.1 23.2 0.7 2.6 7.7 65.6 1.7 0.6 4.6 1.8 8.6 2.0 23.6 42.1 10.9

‘‘Triassic” water RF 0.5–1 11 m 10.2 1917 7.8 �51.3 0.0 1.8 0.0 0.7 52.9 401.5 8.3 0.2 3.8 0.1 0.0 0.2 0.4 933.8 255.0r 2.6 232 0.3 8.1 0.0 0.3 0.0 0.5 4.9 28.7 2.3 0.0 1.2 0.1 0.0 0.2 0.9 86.0 13.5

F2 0.3–0.6 11 m 11.0 1665 7.1 �9.7 0.0 2.0 0.1 0.8 53.9 308.9 6.5 0.2 4.8 0.1 0.0 0.3 1.2 722.1 298.7r 0.7 61 0.1 7.1 0.0 0.3 0.0 0.2 8.3 28.5 0.9 0.0 2.9 0.2 0.2 0.3 2.5 18.2 9.9

V8 <0.05 11 m 10.2 2610 7.4 �24.3 0.0 2.3 0.1 2.5 76.7 586.4 13.0 0.2 3.1 0.8 0.0 8.6 0.0 1574.6 185.711 r 0.7 60 0.1 3.3 0.0 0.8 0.1 0.8 6.3 15.9 1.0 0.1 1.2 2.4 0.0 26.5 0.0 60.4 32.5

V46 1–2 11 m 11.6 2312 7.1 �12.3 0.0 2.2 0.1 1.5 64.0 520.5 11.8 0.2 3.9 0.0 0.0 0.4 0.0 1267.4 259.4r 0.6 28 0.1 6.9 0.0 0.6 0.1 0.6 5.3 10.5 0.9 0.1 3.2 0.1 0.0 0.4 0.0 34.4 24.2

VS – 4 m 8.2 1905 7.7 �32.5 0.0 1.5 0.0 1.3 25.8 452.3 7.3 0.1 2.6 0.0 0.0 0.5 0.0 1127.8 68.3r 4.1 426 0.3 9.2 0.0 0.4 0.0 0.5 6.0 121.7 2.6 0.1 0.9 0.1 0.0 0.6 0.0 326.4 6.1

‘‘Carb.” water Av. 1–2 3 m 11.3 423 8.1 �66.0 0.0 6.9 0.1 0.6 8.4 72.2 0.4 0.5 2.4 0.2 0.0 3.6 0.1 45.8 188.3r 3.6 82 0.0 7.1 0.0 9.2 0.1 0.6 2.2 9.9 0.2 0.7 2.4 0.1 0.0 2.2 0.2 21.2 21.0

‘‘Crystalline” water PB 5–10 4 m 7.6 104 8.1 �64.0 0.0 1.0 0.1 1.3 1.1 17.3 0.2 0.0 0.4 0.1 0.1 2.1 0.0 4.9 47.6r 3.1 6 0.3 18.4 0.0 0.1 0.1 0.1 0.1 0.8 0.0 0.0 0.2 0.1 0.3 0.5 0.0 0.5 1.4

Gl. 0.5–1 4 m 7.7 260 7.9 �51.0 0.0 2.5 0.1 0.6 2.1 49.6 0.3 0.1 1.2 0.2 0.0 0.6 0.0 23.1 120.5r 5.8 38 0.1 2.8 0.0 0.4 0.1 0.3 0.3 8.4 0.1 0.0 0.2 0.2 0.0 0.3 0.0 0.8 26.2

Ta. 108 2 m 6.7 184 8.2 0.0 2.2 0.2 2.4 2.4 29.9 0.2 0.1 0.6 0.1 0.0 0.8 0.0 23.9 81.7SI 0.5–1 2 m 7.4 406 7.9 �50.5 0.0 2.5 0.3 1.9 8.0 68.3 0.9 0.0 1.3 0.1 0.0 1.9 0.1 114.7 98.8

E.Thiébaudet

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189–202

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194 E. Thiébaud et al. / Journal of Hydrology 381 (2010) 189–202

gradients can be extrapolated to the western side of the valley.Assuming the climatic conditions at the time of infiltration weresimilar to current conditions, the isotopic gradients indicate aninfiltration altitude of between 1400 and 1600 m (Fig. 3); that isto say, at the highest points of the recharge zone. The VS, Gl, PBand Av shallow groundwaters (located on the isotopic gradient)are recharged close to their sampling points and are dischargesof the slope subsystem. In contrast, the RF and exploration gallerygroundwaters (light shading), which were sampled at altitudes of680 and 450 m, respectively, are recharged by water that infiltratesat altitudes of between 1100 and 1500 m. This water seems to bedischarges from the global slope-valley system.

Chemical facies of shallow and thermal waters

The shallow groundwater from the crystalline and carbonaterocks was found to be weakly mineralized (from 100 to 400 lS/cm) and to have high Ca and HCO3 contents, whereas the LFZ shal-low groundwater had conductivities of the order of 2000 lS/cmand a Ca–Mg–SO4 facies (Table 2, Fig. 4). The thermal water is morestrongly mineralized and presents a Ca–Na–SO4 facies.

The shallow groundwater from the crystalline and carbonaterocks only leaches the more soluble elements within the rocksand the LFZ groundwater mostly leaches gypsum. In contrast, thethermal water is highly enriched in Na.

Chemical evolution of groundwater with depth

Major elementsA Principal Components Analysis (PCA) of the major elements

(Fig. 5) revealed three groups of groundwater:

(1) Thermal water (RVII and Nat), which is characterized by hightemperatures (90 �C) and high Cl, K, Na and NH4 contents(axis F1).

(2) LFZ shallow groundwater (exploration gallery, VS and RF),which is characterized by high Mg contents (axis F2).

(3) Crystalline (Gl, Ta and PB) and carbonate (Av) rock shallowgroundwater, which is characterized by high HCO3 contents.

Both the thermal and LFZ shallow groundwaters have high con-ductivities and high Ca and SO4 contents.

These results indicate that the thermal water and LFZ shallowgroundwaters interact with gypsum but only the thermal waterinteracts with halite (Na and Cl contents) and feldspars (K andNa contents). In addition, the high Mg content of the LFZ shallowgroundwater indicates interactions with dolomite, which differen-tiates them from the thermal water.

Closer examination of the results for the LFZ shallow groundwa-ter shows that F2 has the highest HCO3 content, V8 has the highestMg content, and V46 and RF have an intermediate composition.Although all these groundwaters come from the LFZ, their chemicalcompositions depend on their locations with respect to the differ-ent rock types within the LFZ (crushed micaschists, carbonates anddiscontinuous gypsum, dolomites, and halites). F2 water has great-er interaction with carbonates, whereas V8 water has greater inter-action with gypsum. The composition of the V46 samples, whichwere taken in the crushed micaschist area, is the result of gypsumdissolution only. The RF water, sampled in a diffuse spring, is amixture of groundwater from both sides of the LFZ and thereforehas an intermediate chemical composition.

The thermal water is enriched in Cl (Fig. 6a) due to interactionwith halite, which is generally present at depth along alpine strike-slip thrusts (Razack and Dazy, 1990). However, the Na content ofthis water is much higher than would be expected from the halite

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1

E. Thiébaud et al. / Journal of Hydrology 381 (2010) 189–202 195

Fig. 4. Chemical facies of groundwater from the different geological formations in the study area. (a) Piper diagram; (b) Schoeller diagram (concentrations in meq/l) (forsampling point locations, see Fig. 1b and c).

196 E. Thiébaud et al. / Journal of Hydrology 381 (2010) 189–202

dissolution line (Fig. 6a), and the Ca content is lower than would beexpected from the anhydrite dissolution line (Fig. 6b). The magni-tudes of these anomalies were similar. These observations suggestan exchange between Ca and Na due to the presence of a cation ex-changer (André et al., 2005). In the La Lechère system, ion ex-change may be effectuated by clay minerals produced by the

alteration of the cataclastic micaschists or by the organic matter(OM) in the Permo-Carboniferous rocks.

The thermal water has lower HCO3 and Mg contents than theLFZ shallow groundwater (Fig. 6c and d). This may be due to theabsence or rarity of dolomites and carbonate schists at depth alongthe LFZ.

Fig. 5. Principal components analysis of major element compositions for groundwater in the study area (for abbreviations, see Fig. 1b and c).

Fig. 6. Correlation diagrams for major elements (for abbreviations, see Fig. 1b and c). (a) [Cl] vs [Na] + [K]; (b) [SO4] vs [Ca]; (c) [HCO3] vs [Ca]; (d) [Mg] vs [Ca].

E. Thiébaud et al. / Journal of Hydrology 381 (2010) 189–202 197

Trace element compositions of groundwater and rocksTable 5 summarizes the trace element contents of the ground-

water and rocks of the La Léchère area, and compares these valueswith data from the literature (Kilchmann et al., 2004; Muralt, 1999and André, 2002).

Arsenic is only found in groundwater from gneiss (PB), indicat-ing that the As is leached from this rock type. The thermal waterand LFZ shallow groundwater have similar Ba and Sr contents(Table 4), indicating interactions with micaschists (Ba) and evapor-ites (Sr).

The B, Br, Cu, Li, Mn, Rb and Th contents of the thermal waterare an order of magnitude higher than the contents observed inany of the shallow groundwater (Table 3). This difference may bedue to the pressure and temperature conditions of the thermalwater and their long residence times. The B, Br and Mn in theNat water are leached from evaporites (Table 5), whereas the rela-tively high Th content indicates leaching from crystalline rocks.Data from the present study and from the literature (Table 5) indi-

cate that the Cu, Li and Rb contents of groundwater cannot belinked to specific rock types.

Gaseous phaseGases emitted by radioactive decay (222Rn and 226Ra). 222Rn and226Ra are formed by the radioactive decay of the parent elements238U and 234Th, both of which are present in the Lauzière graniteand the Pussy gneiss (Tables 4 and 5).

Water from the Nat well has relatively low 222Rn contents(9 Bq/l) compared with other French thermal water in similar geo-logical settings (Table 6), indicating that Nat water flows close togranites and micaschists containing 238U and 234Th without therebeing substantial liquid–gas exchanges.

Carbon gases (CO2 and CH4). La Léchère thermal water has a CO2

content of 22.7%, which is higher than that of Allevard thermalwater, and a CH4 content of 0.43 ml/kg, which is also relativelyhigh compared with other thermal sites.

Table 7d34S(SO4), d18O(SO4), d34S(H2S) and d34S(S2�) ratios for La Léchère thermal ground-water and for evaporites in the study area.

d34S(SO4) d18O(SO4) d34S(H2S) d34S(S2�)

RocksSG6 (LFZ gypsum) 14.8 13.3SG7 (Belledonne fault zone

gypsum)14.7 11.6

SG8 (median fault zone gypsum) 15.8 16.9SG11 (medain fault zone pyrites) �0.8

WaterNatacha 19.7 16.3 9.4

Table 5Summary table of trace element contents for groundwater and rocks (crosses show the groundwater or rock with the highest concentration for each trace element). (a) In thestudy area, (b) from bibliographic data.

Table 6Dissolved gas contents of Natacha water (sampling in 2007) compared with Allevardthermal water (sampling in 1985) (unpublished data) and to atmosphere. Analysis of222Rn and 226Ra and H2S of Natacha water compared with Brides-les-Bains(sampling in 1997) and Aix-les-Bains (sampling in 1998) thermal water (unpublisheddata).

Nat. (2007) Allevard (1985) Atmopshère

ml/kg % ml/kg % %

H2 <0.01 0 <0.01 0N2 15.3 73.2 35.8 92.2 78O2 <0.01 0 0.2 0.5 21CO2 4.75 22.7 1.6 4.1 0.033CH4 0.43 2.1 0.62 1.6C2H6 0.001 0 0.008 0C3H8 0.001 0C4H10 <0.001 0He 0.1 0.5 0.1 0.3Ar 0.32 1.5 0.49 1.3 0.93

mBq/l Nat. (2007) Brides (1997) Aix (1998)222Rn 9000 1900 37,000226Ra 120

198 E. Thiébaud et al. / Journal of Hydrology 381 (2010) 189–202

CO2 can be produced by deep metamorphism of carbonaterocks, and both CO2 and CH4 can be emitted by mantle degassing(Blavoux, 1995) or by OM alteration, including bacterial sulfatereduction and/or methane production (Clark and Fritz, 1997). TheRn content of the water is indicative of gas circulation from themantle, probably via the regionally important LFZ, which is a po-tential zone of deep CO2 upwelling. In addition, the thermal waterinteracts with fossil OM in the Permo-Carboniferous rocks, whichoccur as folded beds in the micaschists. Metamorphic carbonaterocks are present in the La Léchère area but interaction betweenthe thermal water and these rocks is limited. So, the most impor-tant sources of the CO2 and CH4 in the La Lechère thermal waterappear to be mantle degassing and OM alteration.

Sulfides. The gaseous phase of the thermal water had HS� and S2�

contents of 0.97 mg/l (water sample taken in 2007) and unpub-lished data from the resource owner give an S2� content of0.5 mg/l (water sample taken in 2002); therefore, the HS� concen-tration was approximated to 0.5 mg/l (0.015 mol/l).

In addition, an iodometric analysis of a sample taken in 2005gave a mean S2� content of 12.4�SF or 97 mg/l. This is similar tothe results obtained using the same method for Aix-les-Bains’ ther-mal water (100 mg/l, Gallino et al., 2008), which are valued fortheir sulfur-rich properties.

Sulfides are produced either by the bacterial reduction of sul-fates, which requires the presence of OM, or by the alteration ofpyrite (FeS2). In the thermal aquifer, OM is present in the Permo-Carboniferous rocks and pyrite is present in the carbonate rocksand in the tension faults in the crystalline rocks.

The isotopic signature of pyrite (d34S = �0.8‰) is very differentfrom the isotopic signature of the sulfides in the La Léchère ther-mal water (d34SH2S = 9.4‰) (Table 7). On the other hand, comparedwith the d34S signature of gypsum, the sulfates in the thermalwater are enriched in d34S and the sulfides are depleted in d34S (Ta-ble 7). These observations indicate that the sulfides in the thermalwater are produced by the bacterial reduction of sulfates dissolvedfrom gypsum.

Temperature of the thermal reservoir

The saturation index (SI) were calculated using Phreeqc� soft-ware, version 2.12.5. SI calculations for different temperatures givean equilibrium temperature for Nat water of between 60 and 80 �C(Fig. 7c). As the Nat water does not contain any supersaturated

Fig. 7. Saturation index. (a) Analysis of Natacha water (24/10/04) with silica content (internal data from the resource owner). (b) Saturation index calculated for atemperature of 60 �C (Phreeqc� software, version 2.12.5). (c) Equilibrium temperature of the thermal water based on saturation indices calculated for different temperatures.

Fig. 8. Maximum temperature reached by thermal water as indicated by different geothermometers (Fournier and Truesdell, 1973; Fournier, 1977, 1979; Michard, 1979;Fouillac and Michard, 1981; Giggenbach, 1988).

E. Thiébaud et al. / Journal of Hydrology 381 (2010) 189–202 199

species at 60 �C (Fig. 7b), other geothermometers can also be used(Fournier and Truesdell, 1973; Fournier, 1977; Fournier, 1979; Mi-chard, 1979; Fouillac and Michard, 1981; Giggenbach, 1988)(Fig. 8). Most of these geothermometers gave temperatures of be-tween 60 and 110 �C, thereby corroborating the SI results. How-ever, abnormally high temperatures (200 �C) were obtained whenNa/K and SO4 were used as geothermometers. These anomalous re-sults probably reflect Ca/Na exchanges and the presence of evapor-ites, respectively.

An equilibrium temperature of 90 �C, as indicated by most ofthe geothermometers, and a local geothermal gradient of 1 �C/

26 m, determined from temperature measurements in the Isère-Arc gallery (Bordet, 1961), indicate that the thermal water musthave descended to depths of more than 2000 m.

Residence times

3H and CFCThe low 3H content (3 TU) and the absence of CFCs in samples

taken in 2002 (Dray, 2003) show that the Nat water does not con-tain recent water. Combining their 3H content (9 TU, Dray, 2003)

200 E. Thiébaud et al. / Journal of Hydrology 381 (2010) 189–202

with the rate of radioactive decay gives a residence time for theexploration gallery water of around 10 years.

Carbon isotopesOrigin of the carbon in the thermal water. Carbon isotope analysis ofthe Nat water (samples taken in 2006) gave �7‰ compared withPDB for d13CDIC and 6.6 pmc for 14C. A number of origins are possi-ble for the C contained in the La Léchère thermal water, includinginteractions with soils, dissolution of dolomite and other carbon-ates, interactions with fossil OM and mantle degassing. Becausethe Nat water is reducing water (Table 2), they are not in equilib-rium with atmospheric CO2.

In order to evaluate the contribution of each of the possiblesources of C, the isotopic composition of the Nat water (d13CCMTD

and 14C) was compared with theoretical isotopic compositions.These theoretical isotopic compositions were calculated for aNat-type water (HCO3 = 1.4 meq/l, pH = 7.1 and t = 90 �C) equili-brated with a soil and carbonate matrix (end-member 1, Fig. 9b),deep CO2 (end-member 2, Fig. 9b) and fossil OM (end-member 3,Fig. 9b). End-member values were calculated using Diagrammesoftware (Simler, 2007), which is based on equilibration equationsand temperature-dependant fractionation coefficients.

The isotope composition (d13CCMTD and 14C) of the Nat water isclose to end-member 2 (Fig. 9b), indicating that most of the C is de-rived from exchanges with deep CO2 and from carbonate dissolu-tion, with only a small contribution from exchanges with fossil OM.

Fig. 9. Calculated 14C and d13C for water of La Léchère type (HCO3: 1.4 meq/l, pH: 7.carbonates, end-member 2: deep CO2, end-member 3: fossil OM. (a) Parameters used to ccomposition with respect to these end-members.

Carbon ages. Groundwater residence times can be determinedfrom the fundamental law of radioactive decay by:

t ¼ sln 2

lnft � A14

0 C

A14t C

!ð1Þ

where s is the half-life of 14C (s = 5730 ± 30 years), A0 is the initial14C activity (generally fixed at 100 pmc), At is the measured 14Cactivity of the groundwater sample and ft is the total correction fac-tor. The total correction factor (ft = f1f2f3) takes into account thethree 14C dilution processes – carbonate dissolution (correction fac-tor f1), sulfate reduction (correction factor f2) and incorporation ofdeep CO2 (correction factor f3) (Bajjali et al., 1997; Kattan, 2002).Assuming a total correction factor of 1 gave a non-corrected ageof 22,500 years for the Nat water. When the three dilution processeswere taken into account, corrected ages of between 8000 and12,300 years were obtained.

Indeed, the application of the methodology proposed by Bajjaliet al. (1997) gives a correction factor f2 of 0.0979 and a correctionfactor f3 of 0.519. Then, the effect of carbonate dissolution was cor-rected for using standard corrections from a corrected A0

14C:

A140 Ccorr ¼ f2 � f3 � A14

0 C ð2Þ

The d13C unmodified by sulfate reduction and the incorporation ofdeep CO2 were approximated as being equal to the d13C of sampleF2 (�8.5‰), which results in an A0

14Ccorr of 50.8 pmc. Standard

1 and t �C: 90 �C) interacting with C from three origins: end-member 1: soil andalculate isotopic compositions of the 3 end-members. (b) Location of Natacha water

E. Thiébaud et al. / Journal of Hydrology 381 (2010) 189–202 201

corrections for carbonate dissolution suitable for the La Léchère set-ting were applied. These calculations gave 14C ages of 11,700 years(Tamers, 1960), 8000 years (Pearson and Hanshaw, 1970),11,800 years (Mook, 1976), 12,300 years (Fontes and Garnier,1979) and 9600 years (Salem et al., 1980).

The 14C age corrections, which take into account dilution pro-cesses, do not permit to obtain an absolute age but a range andsuggest that the thermal water extracted at La Lechère was infil-trated after the Würm glacial period, under similar climatic condi-tions to the present.

Conclusion

Most of the groundwater drained by the LFZ circulates in the400–500 m-thick ‘‘decompressed” zone. Only a fraction of thegroundwater that infiltrates at the highest points of the rechargezone reaches the deep system via the cataclastic micaschists. TheLFZ shallow groundwater presents a Ca–Mg–SO4 facies, whereasthe deep groundwater has a Ca–Na–SO4 facies and is enriched inCO2, CH4 and H2S gas. The differences between the chemical com-positions of the shallow groundwater and the deep thermal waterare due to the following processes:

(1) Different rock types are found in the subsurface and atdepth. Halite, for example, is only present at depth alongthe LFZ and it is interactions with this halite that lead tothe thermal water being enriched in Cl. Conversely, Triassiccarbonate deposits are discontinuous at depth, so there isalmost no dissolution of carbonates by the thermal water,which therefore have lower Mg and HCO3 contents thanthe shallow groundwater. Deep circulation allows the ther-mal water to reach the Permo-Carboniferous rocks and tobecome enriched in the CH4 or H2S produced by the bacterialdecomposition of OM. Deep groundwater flows close togneiss and granite, from which it obtains their Rn, and closeto a deep source of CO2.

(2) Shallow and deep waters are subject to different flow condi-tions. The shallow groundwater has residence times ofaround 10 years (3H age), compared with 8000–12,000 years(corrected 14C age) for the thermal water. The thermal waterreaches temperatures of around 90 �C, indicating a circula-tion depth of more than 2000 m (calculated usinggeothermometers).

These different flow conditions allow thermal water to acquiretrace elements such as B, Br and Mn, which are characteristic ofinteractions with evaporites, and Li and Rb, which may be leachedfrom either crystalline or evaporitic rocks. The high sodium con-tent of the thermal water can be attributed to the high temperatureof the water, which facilitates Ca–Na exchanges during interac-tions with clay minerals produced by the alteration of the cataclas-tic micaschists.

Hence, the differences in the chemistry of the shallow ground-water and thermal water at La Léchère are caused by three phe-nomena: (i) the discontinuity of the Triassic deposits along theLFZ, (ii) differences in the flow conditions of the shallow and deepgroundwater (temperature, pressure and residence time) and (iii)deep circulation in the cataclastic crystalline rocks (micaschistsand Permo-Carboniferous rocks).

Acknowledgements

This study was supported by a grant from the ‘‘Valléesd’Aigueblanche” association of district councils. The authors wouldlike to thank B. Moine, V. Berthon, F.Gal and M.C. Gerbe for their

technical advice, and B. Blavoux, M. Dray, C. Emblanch, and G.M.Zuppi for their valuable comments.

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