Chemical and Isotopic Investigation of Cold and Warm Inflows in the Gotthard Tunnels

Abstract

Groundwaters detected during the crossing of mountains chains by deep tunnel provide valuable information about subsurface flow conditions and rock-water heat transfer. The warm water inflows in the Gotthard tunnels also represent important lowenthalpy geothermal resources, due to their remarkable flow rates. The maintenance and administration building situated at the portals of the Gotthard Highway Tunnel are, in fact heated by these groundwaters. This study is aimed to elucidating the origin of these groundwaters and the processes governing their chemical evolution, within the geological-hydrogeological framework of the Gotthard area. To achieve these objectives, thirty-five water samples were collected and analysed chemically (major and some trace elements) and isotopically (SD and ~180): 21 from the Gotthard Highway Tunnel (GHT) and 14 from the Gotthard Exploration Tunnel of the new Swiss railway project (NEAT) through the Alps. For the GHT water the outflows temperatures range between 8~ to 32~ and the total dissolved solids (TDS) between 50 to 2000 mg/kg and for the GET waters between 11 ~ to 25~ and 220 to 2500 mg/kg. Geologically the GHT is located within the Aar and Gotthard basement massifs, represented mainly of late Variscan granites and Permocarboniferous volcanoclastic sequences with tight synclines of Mesozoic cover composed by evaporates and by dolomitic marbles. The GET crosscut the Lower Pennine nappes of Lucomagno-Leventina made up by orthogneiss and paragneiss and by a Mesozoic cover represented by evaporates and dolomitic marbles grading upward to a calcschist sequence. Sampled water belong to the following hydrogeochemical types (Fig. 1). Type 1:Ca-SO4 intermediate to high-salinity waters with TDS of 660 to 2500 mg/kg and outlet temperatures of 1 I~ to 25~ This type include most