Geochemical evidence for regional and long-term topography-driven groundwater flow in an orogenic crystalline basement (Aar Massif, Switzerland)

Wanner, Christoph; Waber, H. Niklaus; Bucher, Kurt (2020). Geochemical evidence for regional and long-term topography-driven groundwater flow in an orogenic crystalline basement (Aar Massif, Switzerland). Journal of hydrology, 581, p. 124374. Elsevier 10.1016/j.jhydrol.2019.124374

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Detailed knowledge about the circulation of meteoric water in non-magmatic, orogenic belts is fundamental for assessing the potential of such settings for geothermal power production, as well as their use as potential groundwater resources. To get more general insight into these hydrological processes, we have conducted regional (20 × 10 × 9 km) thermal-hydraulic-chemical (THC) simulations of meteoric water circulation in the orogenic, crystalline basement of the Aar Massif in the Central Alps, Switzerland. Model results were compared to numerous geochemical and isotopic analyses of groundwater discharging into the longest and deepest tunnel of the world, the Gotthard railbase tunnel located within the model domain. Explicitly considering the surface topography in our model was sufficient to reproduce all key characteristics of the tunnel inflows (salinity, temperature, δ18O values, and up- and downward directed flow zones inferred from geochemical constraints). This quantitatively confirms that surface topography operates as the governing control on fluid flow in orogenic crystalline basements with meteoric water infiltration occurring at high altitude and resulting upward directed flow zones along major valleys. Owing to low flow rates below 2 m year−1, computed residence times of the longest flow paths were above 100 k years, confirming that groundwater and/or porewater in orogenic crystalline basements may act as an archive for palaeohydrologic variations. Moreover, simulation results show that down to the lower model boundary at 9 km depth, penetration of meteoric water is not limited by the decrease in permeability with depth that is typically observed in granitic rocks. This suggests that advective fluid transport in orogenic crystalline basements may reach the brittle ductile-transition zone and that infiltrating meteoric water can attain temperatures well above 150 °C. We conclude that orogenic geothermal systems are promising plays for geothermal power production.

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