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

. Although natural CO 2 -rich groundwaters of eastern Belgium have been known for centuries, the exact origin of their gas is still unclear. This paper presents the results of a sampling campaign in Belgium (Spa, Stoumont, Malmedy): 30 samples of both carbogaseous and non-carbogaseous groundwaters were analyzed for major elements, CO 2 content and carbon isotopic composition. Among them, 13 samples were also analyzed for 3 He / 4 He and 4 He / 20 Ne ratios. The combination of δ 13 C (between ca. − 9‰ VPDB 1 and − 2‰ VPDB), CO 2 / 3 He ratio (between 1 . 9 × 10 8 and 2 . 9 × 10 9 ) and 3 He / 4 He (between 0.92 and 2.70 Ra) shows with a high level

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