CO<sub>2</sub>/<sup>3</sup>He ratios reveal the presence of mantle gas in the CO<sub>2</sub>-rich groundwaters of the Ardenne massif (Spa, Belgium)

Abstract. Natural CO2-rich groundwaters of eastern Belgium have been known for centuries although the exact origin of the gas they contained was still unclear. This paper presents the results of a sampling campaign in the area (Spa, Stoumont, Malmedy) where 30 samples of both carbogazeous and non-carbogazeous groundwaters have been analyzed for major elements, CO2 content and carbon isotopic composition. Among them, 13 samples were analyzed for 3He / 4He and 4He / 20Ne ratios. The combination of δ13C and 3He / CO2 ratios have shown with a high level of confidence that the CO2 in groundwater from springs and boreholes has a mantellic origin, and can very likely be attributed to the degassing of the neighboring and still buoyant Eifel mantle plume, located at a distance of 100 km eastwards. The identity and nature of the deep-rooted fractures that act as CO2 transport pathway to the surface are still to be clarified, but several major thrust faults exist in the Rhenish Massif and could connect the Eifel volcanic field with the studied area.