Characterization of high-permeability feeding conduits of thermal springs in fractured crystalline rocks (Karlovy Vary Spa, Czech Republic)

Granite forms a hardrock hydrogeological environment, characterized by a shallow permeable zone and relatively low transmissivity. Cold springs from granite mostly do not exceed a few L/s. Interestingly, hot springs with yield up to tens of L/s are not uncommon (e.g. Idaho batholith, USA). However, little is known about the character and origin of these high-permeability features in granite. The Karlovy Vary Spa (western Czech Republic) is fed by a large hot spring granite with a temperature of 73 °C and yields 30 L/s. Since the 1970s, > 70 boreholes have been drilled in the area to capture water for the spa. This provides a unique opportunity to study the characterization of the granite and its alteration by well logging techniques, to investigate conduits by borehole camera, to analyze cores, and to perform the tracer test from conduits captured by boreholes into a hot spring.

The Karlovy Vary Spa is located on the outskirts of the Eger Graben structure. One major spring, Vřídlo, (30 L/s) and tens of small springs occur in area (total yield < 3 L/s). The water has a TDS of 6.4 g/L and is classified as a Na-HCO₃-SO₄-Cl type, probably derived from water of high-TDS tertiary paleolakes. The water is enriched with CO₂ (water-to-gas ratio 1:3). Granite is capped by a 10-16 m thick aragonite shield precipitated from hot water. Open conduits in granite filled by hot water were observed. In 1980, the geophysical log probe (1050 mm long and 36 mm in diameter) was incidentally lowered into an open conduit feeding the Vřídlo spring from the base of a 133 m deep borehole. The probe reached a depth of 370 m. It follows that hot spring conduits have a considerable diameter. Flow velocity in this conduit exceeds 0.3 m/s, as water carries granite grain fragments up to 2 mm in diameter to the surface.

Four tracer tests using Na-fluorescein were conducted under a constant injection rate of 0.9–1.1 L/s. The tracer was injected into boreholes in close surroundings of the Vřídlo hot spring (tens of meters, depth up to 160 m) and monitored in boreholes feeding the Vřídlo spring. Flow velocities varied between 100–400 m/day (first arrival) and 50–140 m/day (mean residence time). Mean flow cross-sections derived from tracer tests were 5–10 m², recovery was 8–15 %. In one test, where the tracer was injected directly below the aragonite shield, the tracer did not arrive to the Vřídlo spring or boreholes as close as 20 m away, despite injecting 500 m³ of water to mobilize the tracer. This indicates a very high effective porosity of strongly weathered granite in shallow depth, which can accommodate hundreds of m³ of water without allowing any tracer to reach the monitored boreholes.

Hot water enriched with CO₂ can create highly permeable conduits in granite transmitting flow of tens of L/s. Granite is weathered into disintegrating residuum in places. It is likely that the high flow velocity >0.3 m/s emptied some fracture zones into high permeability conduits.

Funded by the GAUK No. 164524.