Multi-scale evaluation of high-permeability hydrothermal conduits in crystalline rocks (Karlovy Vary Spa, Czech Republic)

Quantitative assessment of long-distance hydrothermal groundwater flow in crystalline rock is challenging, primarily due to low matrix permeability and extreme heterogeneity. However, sites exist worldwide where deep groundwater circulation occurs over long distances within crystalline rocks. For instance, the Idaho Batholith (USA) hosts dozens of hot springs within granitic formations, featuring discharges in the tens of L/s and recharge areas of at least 5 000 km². Moreover, such flow can be remarkably rapid, as observed in Karlovy Vary (Carlsbad) in the Czech Republic. The main spring, known as the Vřídlo hot spring, has a temperature of 73 °C and a discharge of 30 L/s. Considering the local terrestrial heat flow, a recharge area of at least hundreds of km² is required to heat the Vřídlo spring, assuming a circulation depth of ≈ 2.5 km.
This study evaluates over a century of research on the Vřídlo hot spring and addresses the question of why water is channelized from hundreds of square kilometers into a single discharge point. Radiometric dating of spring travertine accumulations reveals that the Vřídlo hot spring has been active for at least 230 000 years. Stable isotope analysis confirms the thermal water is of meteoric origin, having infiltrated after the last Ice age. Evidence of high-permeability fractures with large apertures exists both directly within Karlovy Vary and in the broader region. Extensive drilling campaigns have identified fracture apertures on the order of tens of centimeters, with these zones characterized by flow rates of up to 30 L/s. Tracer tests revealed high anisotropy in flow velocities both within the Vřídlo discharge conduit and in the surrounding granite massif, demonstrating the presence of relatively voluminous conduits. Notably, a well-logging probe inadvertently descended through the bottom of a 133 m deep borehole into the conduit feeding the Vřídlo hot spring, reaching a final depth of 370 m in the granite.
Highly permeable flow paths extend at least tens of km from Vřídlo within the granite. Hydraulic tests conducted 15 km from Karlovy Vary established a direct hydraulic connection with the Vřídlo hot spring. This repeatedly verified relationship manifests as a decrease in the Vřídlo discharge following increased abstraction at the distant site, with a time lag of approximately three months. Without elucidating the origin of highly permeable conduits in granite and understanding the effect of hot groundwater flow on porosity and permeability changes, the implementation of deep geological repositories (DGRs) for radioactive waste cannot be considered safe. This is particularly critical given that a considerable proportion of such facilities are planned within granitic formations.
Funded by the GAUK No. 356525: Character of the feeding vents of thermal springs.