The gas flow at mineral springs and mofettes in the Vogtland/NW Bohemia: an enduring long-term increase


Ulrich Koch, Saxon Academy of Sciences at Leipzig, Office Bad Brambach, Badstr. 49, 08648 Bad Brambach, Germany.
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Over the last 15 years, there has been an increase in the 3He/4He ratios (Ra) at gas emission sites including springs (liquid-dominated) and mofettes (gas-dominated) in the Vogtland (Germany) and NW Bohemia (Czech Republic), indicating an active magmatic process. The highest Ra value (6.2) was measured at the mofette Bublák. Continuous CO2 flow records at Bad Brambach mineral springs (Vogtland) and Cheb Basin mofettes show a significant and enduring increase in gas flow. This increase ranges from 1.6% at Schillerquelle spring, Bad Brambach, and 61% per year at mofette Bublák. Calculated gas flow ratios between the individual emission sites not only emphasize the greater increase in gas flow over time at the Cheb Basin mofettes compared with the Bad Brambach springs (factor 2.1 to 2.6) but also show a uniform gas flow behaviour within both emission areas. Using simplified models, it is demonstrated that long-term gas flow measurements reveal a gas-hydraulic parameter, the effective cross-sectional ratio, to compare the near-surface gas-bearing fissures and cracks between the emission sites. Thus, the overall cross-sectional area of gas pathways at the mofette Soos is 13.2 times higher than at the Wettinquelle spring, indicating greater overall fissure widths for gas transport at the Cheb Basin mofettes than at the mineral springs. Comparison between the monthly fluctuations of free gas flow and dissolved CO2 at the Wettinquelle spring shows an inverse dependence of both parameters but without a temporal trend of dissolved CO2. This indicates that the mineral water body acts as a kind of buffer saturated by CO2 and being degassed more or less strongly, depending on the flow rate of the ascending free gas. The results suggest increased activity of the magmatic reservoir below the Cheb Basin, not only from the qualitative (3He/4He) but also from the quantitative (amount of magmatic CO2) perspective.