Una anomalía térmica local persistente en el gneis de Ahorn recargada por el agua de deshielo de los glaciares (Austria)

Abstract

In the unlined Tuxbach water transfer tunnel, running between Hintertux (1,500 m asl) and the Schlegeis Reservoir (Austria), a local geothermal anomaly with temperatures up to 14.6 °C exists. These temperatures are around 3 °C higher than expected, considering the tunnel’s shallow depth, together with its surrounding alpine environment and regional heat flow. This is especially noticeable because the temperatures have remained stable since the tunnel’s construction in 1969, although the tunnel is generally cooling the surrounding rock massive. The objective of this investigation is to explain the origin of the anomaly with hydrogeological methods and to evaluate the hydrogeological properties of the gneisses exposed in the tunnel. The anomaly is caused by the high hydraulic conductivity (~2.5∙10−5 m s−1) within a narrow shear-zone core, part of the Tux Shear Zones in the Ahorn Gneiss Core. The zone triggers fast groundwater transport over 1.5 km from both sides towards the tunnel. One reason is that the morphology provides thicker overburden with growing distance from the tunnel and therefore higher temperatures on the same horizontal level in the directions of the fault plane. The second explanation is that the narrowness of the shear zone permits effective heat transfer similar to a heat exchanger. No hydrothermal water share is recognizable; instead, mainly cold glacial melt water and snow contribute to the section of the anomaly and all other runouts of the tunnel. Factually based results show the disproportionately high contribution of snow and glaciers to the groundwater recharge in this alpine hard-rock aquifer.