<i>δ</i><sup>13</sup>C, CO<sub>2</sub> ∕ <sup>3</sup>He and <sup>3</sup>He ∕ <sup>4</sup>He ratios reveal the presence of mantle gas in the CO<sub>2</sub>-rich groundwaters of the Ardennes massif (Spa, Belgium)

Abstract. Although natural CO2-rich groundwaters of eastern Belgium have been known for centuries, the exact origin their gas is still unclear. This paper presents results a sampling campaign in (Spa, Stoumont, Malmedy): 30 samples both carbogaseous and non-carbogaseous were analyzed major elements, CO2 content carbon isotopic composition. Among them, 13 also 3He/4He 4He/20Ne ratios. The combination δ13C (between ca. −9 ‰ VPDB1 −2 VPDB), CO2/3He ratio 1.9×108 2.9×109) 0.92 2.70 Ra) shows with high level confidence that groundwater Spa Bru has mantle origin. It can likely be attributed to degassing from neighboring Eifel volcanic fields, located at distance 100 km eastwards. identity nature deep-rooted fractures act as transport pathways surface are clarified, but several thrust faults exist Rhenish Massif could connect fields studied area.