The gases of more than 70 mineral springs and mofettes (cold dry CO₂ gas vents) were investigated for gas flux, gas composition and ³He/⁴He ratios in the western part of the Eger Rift. Both regional gas flux and gas composition pattern are controlled by the tectonic structure.
Four separate main gass escape centres could be detected, partly with gas fluxes of more than 150 m³/hr of free gas: Františkovy Lázně/Cheb Basin, Mariánské Lázně, Konstantinovy Lázně and Karlovy Vary. The very similar gases of the gas escape centres are > 99 vol.% CO₂-rich. Isotopically heavy CO₂ and high mantle derived helium proportions indicate the magmatic origin of these CO₂ rich gases. As a result of gas fractionation by CO₂ solution and HCO₃^- formation the N₂ contents in the gas phase increase in the margin areas with lower gas flux. According to a first estimates, the entire gas flux (natural flux) in the western part of the Eger Riftr lies around a minimum of 5.31 million m³/a free gas, including dissolved CO₂ and HCO₃^- of 8.13 million m³a.
The opposite dip of the main faults of the Eger Rift forms a Y-shaped structure and splits the gas flux about 15km below the surface, forming a CO₂-free zone between them. The borders of this zone correspond to the position of the Eger Rift main fault, the southern border to the Litomerie deep fault. The Eger Rift is shifted on younger NNW-SSE striking faults and narrows to the west.
The migration of the magmatic gases is mainly bound to the WSW-ENE striking Eger Rift main faults, while the younger NNW-SSE striking faults only have a distributive function. Southerly directed gas migration along the Horní Slavkov deep fault formed the small gas escape centre of Konstantinovy Lázně in the area of intersection with the Bezdružice deep fault.