Is hydrotectonics influencing the thermal spring in Eisensteinhöhle (Bad Fischau, Lower Austria)?

Eisensteinhöhle is a 2 km long crevice cave that is significantly overprinted by hydrothermal karst processes. It was opened during quarrying in the Fischauer Vorberge, at the western margin of the Vienna Basin. This pull-apart basin cuts the eastern foothills of the Alps and is formed by a major NE-SW striking, sinistral transform fault. The western margin consists of NNE-SSW striking normal faults creating paths for thermal water to rise from the central basin. The deepest part of the cave, 73 m below the entrance, hosts a pond with 14.6 ±0.2 °C warm water that occasionally acts as a spring. The water level and temperature fluctuate and at a certain level, water visibly discharges into a nearby narrow fissure. As sporadic observations of the water level since 1992 gave no obvious connection to precipitation events, the connection to an aquifer and the origin of the water remained unknown. A pumping test, conducted on 13/7/2016, yielded a volume of the spring/pool of about 2.8 m³ that is fed by a very small inlet at the sandy bottom. At the time of the pumping test, the discharge was only 4.5 l/h but during previous overflow events, discharge values of up to 289 l/h were recorded.

Water temperature and hydrochemistry hint towards a mixture of an old thermal component and a young meteoric component. During continuous monitoring of water level and temperature from October 2015 until November 2018, the water level was almost stable with few periods of high level (almost at overflow) that lasted for about 3 to 4 weeks each. The water temperature increased during most high stands and is positively correlated with the water level. Correlation of the high-resolution data on water level and temperature fluctuations with precipitation measurements at the nearest meteorological stations show a relation of water level to certain rainfall events and the sporadically taken long time records show a correlation with annual precipitation sums. Long-term observations also indicate a connection to groundwater levels in the Vienna Basin with a delay of about 8 weeks in Bad Fischau. In July 2017, the water level dropped suddenly and then recovered simultaneously in the time of several weak earthquakes in the vicinity. The data suggest that the spring in Eisensteinhöhle is influenced by precipitation. For one seismic event, there is a correlation with unusual water level changes at Eisensteinhöhle, but the rareness of earthquakes demands for a longer time series to confirm this observation.