The Hartousov mofettas are a significant landscape feature in the Karlovy Vary Region in the Cheb District. Everywhere in the meadows we can see gas bubbling from the swamps. However, the gas also bubbles through the bed of the Plesná River. The Hartousov mofettas are located on the road from Hartousov to Hněvín, south of the settlement of Vackovec in the Plesná river floodplain near Hajský mlýn. The mofettas occur on both sides of the road, where wooden walkways have been built, which bypass several springs. Information panels "Uneasy Chebsko" with texts by geologist Petr Rojík are located in two places. The texts on the information panels provide detailed information about the mofettas, natural phenomena that are typical of the Cheb region. Access to the mofettas is completely free, but it is recommended to walk on the built walkways, because the terrain here is very waterlogged. Approximately 1.2 km north is a similar mofetta field called "Bublák".
Bubbling Hartousov mofettas
The subject of protection is the mofette field in the vicinity of the Plesná River. The Holocene alluvium of the floodplain is underlain by sediments of the upper sandy-clayey Cypris Formation of the Cheb Basin. In places, the alluvium is missing and claystones of the Cypris Formation emerge on the surface. In 1958, an exploratory well was drilled here, which demonstrated a large inflow of dissolved and free carbon dioxide. The amount of the gas phase many times exceeds the amount of groundwater that the gas displaces from deeper horizons. Due to the high amount of carbon dioxide, its content is difficult to measure. During strongly turbulent separation of the gas phase, its intense turbulent release occurs. From inaccurate measurements, the content of free carbon dioxide was determined to be 2000 to 2500 mg/l. The analyses were carried out by the Research Institute of Balneology in Mariánské Lázně in cooperation with the Research Institute for Balneology in Bad Elster (Saxony).From the geological bedrock along the so-called Hrzín Fault, gas flows into the mofettas from depths of tens of kilometers, which is made up of almost pure carbon dioxide (99.5%). The rest is made up of nitrogen, oxygen, and methane. The gas contains helium isotopes 3He and 4He in unusually high concentrations (1.8 to 4 ‰), which are otherwise very rare on Earth. This indicates that the gases emerge from great depths.Gas eruptions in the mofettas make a characteristic loud sound, as if the water in the mofettas is boiling, but the water temperature is low, not even reaching 10 °C. In the case of a high flow rate, the water column is abruptly pushed to the surface, creating a miniature geyser. You can rely on your hearing to find vents, mofettas really do bubble loudly. Geologists believe that the gases escape directly from the plastic asthenosphere in the Earth's upper mantle along the boundaries of geotectonic faults and pass through weakened zones in the Earth's crust.
Photo 1-3: Hartousov mofets