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Groundwater flow, heat and mass transport in geothermal systems of a Central Alpine Massif. The cases of Lavey-les-Bains, Saint-Gervais-les-Bains and Val d'Illiez.

Abstract : Groundwater flow, heat and mass transport in geothermal or hydrothermal systems locally occurs in the Alps Range where a series of precise conditions are met such as active downflow, permeability at depth, concentrated and fast upflow and favorable geomorphological surface patterns. Advanced studies of the regional and local geology, thermal water chemistry, mixing processes, infiltration area, water-rock interactions, circulation depth, uprising conditions, groundwater residence time, mineralchemical fingerprint, etc. allow the deep flow system to be understood. To carry out these studies, different methods can be employed and combined together such as geological, hydrogeological, geophysical, geochemical, isotopic and numerical investigations. Moreover, these investigations represent a scientific basis providing information required to better manage the short and long term exploitation of the discharged thermal water, qualitatively and quantitatively. They also provide crucial knowledge to prepare future geothermal projects with boreholes and to limit failures that may occur in subsurface and deep geothermal prospection. Due to the high costs for the implementation of a deep borehole, the risk of failure needs to be lowered as much as possible, i.e. it requires a good knowledge of the explored site. This is important for long term sustainable development of geothermal projects. The three selected hydrothermal sites are Lavey-les-Bains and Val d'Illiez in Switzerland and Saint- Gervais-les-Bains in France, where the uprising thermal waters are continuously exploited for various uses such as spa, heating buildings, medical care and cosmetics. Lavey-les-Bains and Saint-Gervaisles- Bains were selected due to several criteria: 1) deep flow systems in the basement, 2) similar chemical and isotopic properties, 3) presence of several end-members with mixing processes, 4) interesting properties of the geothermal reservoirs, 5) many data available and 6) on-going geothermal projects or planned. On the contrary, Val d'Illiez was selected for orther criteria: 1) deep flow system in the autochthonous sedimentary cover of the basement where the hydrothermal systems of Lavey-les-Bains and Saint-Gervais-les-Bains occur, 2) comparison of the chemical and isotopic properties of thermal waters from the cover with those of thermal waters flowing in the basement, 3) history of the site related to the Salanfe Lake and 4) many data available. These three studied sites are located around the Aiguilles Rouges Massif, one of the external crystalline massifs of the Western Alps. Lavey-les-Bains and Saint-Gervais-les-Bains represent the two low-elevation points of the Aiguilles Rouges basement, respectively on the north-eastern and southwestern sides. In contrast, Val d'Illiez is located out of the Aiguilles Rouges basement in a sedimentary domain belonging to the autochthonous cover which outcrops along the north-western edge of the basement. Firstly, the investigations the selected hydrothermal systems were used to study the geological setting and the fracturing conditions for each site. Then, a new sampling campaign was carried out with pumping tests aiming to define the chemical-mineral processes leading to the composition of groundwaters. Finally, two and threedimensional numerical models were established to validate the assumptions formulated from the geochemical investigations, and to represent the deep flow system, the geothermal anomalies and the mixing processes. Groundwater analyses since 1973 at Lavey-les-Bains have revealed a mixing process between a deep Na-SO4 and high-Cl thermal component circulating in the basement (68oC and TDS 1.4 g/L) and cold shallow water from the mountain slope and the Quaternary filling. The production rate of the new deep well P600, installed in 1997, has amplified this mixing process in the well P201, for which a decline in temperature and total dissolved solids has been observed. Numerical hydrogeological two and three-dimensional models of flow, heat and mass transport reproduced the deep flow system and forecasted the long-term exploitation potential of the geothermal resource. Computed temperature of the deep inferred reservoir (100-130oC) is in agreement with the geothermometers, whereas the simulated thermal water flux (5400-9000 m3/d) is probably underestimated. Different fluid production scenarios have been documenting the decline and stabilization phases of temperatures in the hydrothermal ield since 1997. For P201, the mixing ratio calculated before and during the exploitation of P600 is comparable with observed data; the computed temperature tends towards stabilization in P201 at 56oC after 10 to 15 years of production at P600. Another planned new well is likely to reduce the thermal output of the existing wells. The crystalline rocks are not directly outcropping at the Saint-Gervais-les-Bains spa but certainly exist beyond 300 m depth. Uprising waters are pumped from two different aquifers below the Quaternary deposits of the Bon Nant Valley. In the lower Trias-Permian aquifer crossed by De Mey boreholes (27-36oC), the ascending Na-SO4 and high-Cl thermal water from the basement (4.8 g/L) is mostly mixed by a Ca-SO4 and low-Cl cold water circulating in the autochthonous cover of the Aiguilles Rouges basement. The origin of the saline thermal water probably results from infiltration and circulation in the basement until it reaches deep thrust faults where there is leaching of residual brines or there are fluid inclusions at depth. The dissolution of Triassic halite is not possible at Saint-Gervais-les-Bains because the Triassic cold waters have a very low-Cl concentration (< 20 mg/L). For the De Mey Est borehole, gypsum dissolution is occurring with cationic exchanges involving Na, as well as low-temperature Mg dissolution from dolomite in the Triassic formations. The aquifer made of imbricated structures (upper-middle Trias) and crossed by the Lépinay well (39oC) contains thermal waters which are strongly mixed with a low-Cl water, where gypsum dissolution also occurs. The infiltration area for the thermal end-member is in the range of 1700-2100 meters, close to the Lavey-les-Bains hydrothermal system, and corresponds to the average elevation of the Aiguilles Rouges Massif. For the Ca-SO4 and low-Cl end-member, the infiltration area is lower (1100-1300 m), showing circulation from the Mont Joly Massif. The geothermometry method indicates a reservoir temperature of probably up to 65oC but not exceeding 100oC. The deep flow system leading to the thermal springs in the Val d'Illiez occurs at the bottom of the autochthonous cover of the Aiguilles Rouges basement, mainly inside the Triassic formations. The structure of the cover is a great recumbent anticline with an axial plane plunging towards the south-east which would be limited by a basal thrust fault related to the thrust system between the Aiguilles Rouges and Infa-Aiguilles Rouges basements. Mixing processes occur between a Ca-HCO3 cold groundwater and a Ca-SO4 and low-Cl thermal endmember, having a temperature and a total dissolved solids roughly of 30-31oC and 1.8 g/L respectively. The thermal component acquires its mineral composition from the dissolution of gypsum and dolomite occurring in the Triassic formations, in a different way compared to Lavey-les-Bains and Saint-Gervais-les-Bains. The thermal component has an infiltration area close to the elevation of the Salanfe Lake (1900 m) and the reservoir temperature of the deep flow system should probably not exceed 35-40oC at a depth of around 1 kilometre below the Val d'Illiez spring zone. Using the tritium data, the piston-flow model calculated an average residence time for the thermal end-member of around 5 years. A two-dimensional model of groundwater flow and heat transport was carried out to study the relation between the Salanfe Lake and the thermal springs. Four scenarios were tested with different values of water losses from the lake and a long-term cooling of the host rocks was simulated. The natural variations of parameters in the springs indicate that the lake probably does not act on the thermal regime as a single intake. It appears that temperature and chemistry of the thermal waters in the Val d'Illiez have a current steady evolution, while the flow rate seems to vary with the natural variation of the water level of the lake with a time lag of around 140 days. This should indicate that an upper aquifer is situated on the thermal aquifer, without mixing processes, adding a pressure which raises the flow rate of the thermal spring without changing its temperature and chemistry. The understanding gained through this study on groundwater flow, heat and mass transport in the Aiguilles Rouges Massif improves the knowledge for the other external crystalline massifs where hydrothermal sites are present at the front of them. The Mont Blanc Massif is also an area where a large amount of thermal water can discharge from its lowelevation points. The hydrothermal site of Saxon along the Rhone Valley in Switzerland is an interesting site to investigate for this reason. Generally, the density of hydrothermal sites is often higher in the Western Alps where the basement outcrops. Indeed, vertical faults in the basement facilitate the deep infiltration of water leading to deep flow systems, whereas in sedimentary domain the superposition of nappes tends to generate shallower flow systems. Consequently, it would be interesting to investigate other low-elevation zones bordering the external crystalline massifs where the Quaternary filling could mask areas of uprising thermal waters.
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Romain Sonney. Groundwater flow, heat and mass transport in geothermal systems of a Central Alpine Massif. The cases of Lavey-les-Bains, Saint-Gervais-les-Bains and Val d'Illiez.. Geochemistry. Université de Neuchâtel, 2010. English. ⟨tel-00923368⟩



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