Hydrogeology of the deepest underwater cave in the world: Hranice Abyss, Czechia

Little is known about water mixing in deep underwater cave shafts of hypogene karst. The Hranice Abyss (HA) in Czechia is currently the deepest underwater cave in the world. It shares a thermal and CO₂-rich water source with an adjacent spa. Based on chemical and isotope composition, water in the HA is a mixture of shallow and thermal groundwaters. The shallow local groundwater is distinctly different from the adjacent Bečva River water in its elemental chemistry and sulfate δ³⁴S values. The thermal water is mixed with 5–10% of modern water, based on tritium content and chlorofluorocarbons. Vertical profiling and deep sampling in the HA showed distinct changes with depth in temperature and TDS. Density-driven flow controls the mixing. In winter, the shallow water of the open HA lake is efficiently cooled; the denser surface water sinks to greater depths, which mixes the water column in the HA. During the summer the shallow water stagnates at the depth of 0–15 m. Periods of stagnation and of accelerated water flow and mixing in the HA perfectly fit with the periodic occurrence of CO₂ evasion in the lake and the overall characteristics of the microbial communities, which showed the absence of any functional stratification. Ferric oxyhydroxide precipitation is the major cause for turbidity in the HA. Elevation-specific hydraulic responses of the HA groundwater, caused by the adjacent river’s level pulses, enabled a determination of the points along the river course at which the river is connected to groundwater by karst conduits.