We present a study of the regional distribution pattern (gas composition and isotope ratios of CO₂ and He) of 25 degassing locations (mineral springs and mofettes) in the East Eifel (EE) and 2 in the South Eifel (SE). In the EE, the CO₂ concentrations in the free gas phase range from > 99 vol.% in the surroundings of the Laacher See to about 2.8 vol.% at the southern periphery (Bad Kreuznach). At the locations with more than 99 vol.% CO₂, δ¹³C values range between − 6.3 and − 3.0‰ and ³He/⁴He ratios are between 1.3 and 5.6 Ra. At the two locations sampled in the SE, the gas contains also more than 99% CO₂, with a δ¹³C signature of − 2.0‰ and a ³He/⁴He ratio of ~ 4 Ra.

Detailed investigations of the gas composition and the isotope compositions of C, N, He, Ne, and Ar were carried out at least yearly at two locations in the SE and two locations in the EE area for five and ten years, respectively. As a result, these locations are characterised by robust average values of their most significant fluid signatures, which we use to argue for the existence of at least two different magmatic reservoirs beneath the Eifel area. The fluid signatures of the monitored Eifel locations are compared with that of Bublák (NW Bohemia, Czech Republic), where ongoing magmatic activity was recognized by long-term studies during the last 20 years. Based on the stable isotope signature of the major magmatic component CO₂ in the free gas phase and on the geological background, we propose that at the SE locations and at Bublák mantle-derived volatiles are supplied from magmatic reservoirs with a similar melt composition. In general, the results of our time series studies have provided robust data for the reservoir discussion and underscore the potential of such a multi-isotope approach of free gases for the evaluation of the geodynamic situation as a whole.