Carbogaseous spring waters, coldwater geysers and dry CO₂ exhalations in the tectonic window of the Lower Engadine Valley, Switzerland

Abstract.

In the region of Scuol-Tarasp in the Lower Engadine Valley in the Eastern Swiss Alps, there are a variety of phenomena related to a geogenetic CO₂ production, including carbogaseous mineral springs, previously active coldwater geysers and dry gas exhalations from the ground via mofettes. Previous isotopic studies revealed that the CO₂ originates from the metamorphic decomposition of carbonate rocks in the crust. This paper presents an inventory of the springs, geysers and mofettes, and proposes a conceptual model on the regional gas and water circulation. Based on hydrochemical criteria, it was possible to identify six main groups of spring waters, three of which are carbogaseous mineral springs. Most of the carbogaseous springs and gas exhalations are bound to the Bündnerschiefer fractured aquifer. The different water types originate from mixing of groundwater and highly mineralised carbogaseous fluids from depth. Near-surface degassing of CO₂ from the fluid phase creates the dry gas exhalations. CO₂ and radon measurements in 178 soil boreholes suggest that the gas exhalations occur at a limited number of point-like anomalies, and there is no evidence for regionally important diffuse CO₂ discharges from the ground.

Resumaziun.

En la regiun da Scuol-Tarasp en l’Engiadina bassa, en l’orientala part dallas alps svizras cumparan antgins fenomens, che coreleschan cun la producziun da CO₂ en la profunditad. Ei dat fontaunas mineralas carbonicas, antruras geisirs activs d’aua freida e sortidas da gas schetg da mofettas. Anteriuras retschercas d’isotops han mussau, ch’il CO₂ deriva dalla decumposiziun metamorfosa da caltschina en la crusta dalla tiara. Questa lavur presenta in inventari da fontaunas, geisirs e moffettas e fa ina proposta per in model concepziunal per la circulaziun regiunala da gas ed aua. Sin fundament da criteris hidrochemics han sis gruppas da fontaunas saviu vegni identificadas, dallas qualas treis gruppas cumprendan fontaunas mineralas carbonicas. Las pliras fontaunas carbonicas neschan egl aquifer dil sfendaglius platter Grischun. Ils differents tips d’aua da fontauna resultan entras la mischeida da liquidas profundas carbonicas fetg mineralisadas cun auas sutterranas giuvnas. Degasaziun dil CO₂ ord la fasa d’aua damaneivel da la surfatscha meina tier l’exhalaziun da gas. Mesiraziuns da CO₂ e radon en 178 ruosnas indicheschan, che l’exhalaziun da gas serestrenscha sin paucas anomalias punctualas. Ei dat negins indezis per ina impurtonta sortida regiunala da CO₂.