ABSTRACT: The presence of several water points characterised by a high sodium-chloride content, depleted in oxygen18, tritium free and with a very low carbon-14 activity, is very well known in springs in the French-Italian Alps. Nowadays, the geological, hydrological, and geochemical studies, in progress, related to the tunnels for the new high speed railway crossing the French-Italian Alps shows, in the boreholes drilled for this purposes at a mean elevation of 700 msl, the presence of brines (mainly NaCl, with an electric conductivity up to 220 mS.cm-1). Brines appear to be the base of the geological, structural and hydrodynamic systems, and replenish the mort important tectonic contacts. These brines are mobilised in depth by fresh waters coming from the surface. These types of waters reappear on the surface as thermal springs. They are also intercepted by prospection boreholes al different depths and during tunnelling.

1 INTRODUCTION

The presence of water characterized by an high sodium-chloride content, depleted in oxygen-18, varying in tritium content and carbon-14 activity, is very well known in springs in the French-Italian Alps (Vuataz 1982, Cruchet 1983 & 1985, Dazy et al. 1987). Some of them, such as Salins, Brides, Uriage, Saint Vincent, Ceresole Reale, Vinadio, Valdieri, and Bex are located on the verge of external crystalline massifs (Mercantour-Argentera, Belledonne-Pelvoux, Mont Blanc), whereas others are situated along the main tectonic lines (Accidents de Fond de France, de Grand Maison, de Belle Etoile). Numerous water inflows, rich in sodium, poor in tritium, have been recognized in alpine tunnels, at a mean elevation of 1300 msl (Nagra 1988, Maréchal 1999).

The origin of these waters has been the concern of several publications leading to two main hypotheses. The first one is the geochemical origin from a brine similar to that from the Canadian shield already described by Frape & Fritz (1982), Fritz & Frape (1987), and Bottomley et al. (1999); the other one hypothesizes a geological origin called theory of "halocinèse" (Debelmas & Kerkhove 1980, Mascle et al. 1986), based on the presence of deep evaporitic deposits.

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3 DISCUSSION AND CONCLUSIONS

The presence of brines under more than one thousand meters lithological cover bas been kept only at sites where the fracturation is poor and the quantity of fresh water capable of leaching the salts stocked is low.

The high solubility of halite does not allow its identification in the outcropping formations or in the cuttings of boreholes (anhydrite is mentioned in the majority of the geological logs even if the water contains higher quantities of chlorides). Nevertheless, fluid chemistry indicates the chloride leaching to be prevalent with respect to the sulphate one. Anyhow, data exibit a Na enrichment linked to the interactions with silicate minerals, which form the matrix of the geological systems. This reinforces the hypothesis of the structural confinement of the evaporites. In addition to a high sodium chloride, the spring show variable dissolved salt contents with high tritium content and high 14C activity. But salted waters found at different depths during borehole drilling and/or tunnelling reveal no tritium and low 14C activity. This highlights the fact that the percolating water mobilizes brines, trapped within the tectonic structures. The isotopes of the water molecule reflect the mean altitude elevation of the recharge area where precipitation infiltrates to dissolve evaporites. In fact, these springs exhibit variable 18O content between -14.5 %o vs V-SMOW in Brides-les-Bains and 9.0 %o vs V-SMOW in Dignes. The whole set of waters are aligned along the global meteoric line (Fig. 3) without indicating any fractionation coming from thermal waters (Dazy et al. 1987, Vuataz 1982).

Waters from confined aquifers intercepted by deep (from 600 to 1500 m) boreholes show NaCl concentration up to 10 g per litre. These waters show more depleted values in 18O (from -15.0 %o vs V­SMOW to -16.5 %o vs V-SMOW), which do not match the present precipitation (Dray et al. 1998). Deuterium values and 18O values are aligned along the World Meteoric Water Line (Novel et al. 1995, Novel & Zuppi 2000). This isotopic depletion is not linked to an altitude effect but to a temperature effect associated to colder climatical conditions existing in the recharge area probably during the deglaciation following the LGM, about 18000 years BP. The larger water quantity available at that time period allowed a high hydraulic pressure (Blavoux et al. 1993) and, consequently, a per descensum circulation, even at important depths. As quaternary waters mobilise halite deposits they try to move upwards with their high salt content but confined systems prevent them to do so.