Popular Science: What are the latest findings on water in the Hranice Abyss?

The speleologists-divers (from the Czech Speleological Society’s local unit, ZO 7-02 – Hranický kras) took water samples at regular monthly intervals from several specific points at different depths of the Hranice Abyss. The samples were analysed for selected chemical and physical parameters and subsequently, in a laboratory, for basic chemical composition and content of oxygen, hydrogen, carbon and sulphur isotopes. The water’s microbiological activity was also tested on several rock and water samples.

Hydrogeologists from the Czech Geological Survey used a special multiparametric probe to measure the temperature, pH, conductivity, redox potential, and dissolved oxygen content along with depth. This is the first time in the history of the Hranice Abyss that such a detailed technique has been used.

The divers conducted the research up to a maximum depth of 200 m below the surface – in the difficult conditions of a flooded abyss with very variable visibility. Due to such a specific environment, the researchers had to find new methodological procedures, including the design of a new special sampling device for deep water. Another methodological challenge was the water sampling for CFCs analysis used to determine the water age. This sampling is complicated even in easily accessible conditions on the surface as the water samples must not come into contact with the atmosphere. To take a sample from the exact depth, the team used stainless steel cylinders with two valves filled with pure argon and sampled the water by opening the valves.

Water flow in the Hranice Abyss caused by different temperature and water mineralisation. In the winter (left), the water at the surface is cooled by cold air causing the water to flow to greater depths, mixing the entire water column in the process. In the summer, however, there is almost no mixing as the water at the surface stays less mineralised.

Based on the results of the chemical and isotope composition analyses, the water in the Hranice Abyss to a depth of 180 m consists of a mixture of mineral waters (90–95%) rich in CO₂, which come from greater depths, and shallow groundwater from the surrounding groundwater sources (5–10%). Also, even though there is hydraulic connection between the nearby Bečva River and the Hranice Abyss, i.e., the oscillations of the Bečva are mirrored by the water in the abyss, the water from the Bečva does not flow into the abyss.

One of the features that distinguish the Hranice Abyss from similar locations elsewhere in the world are the significant changes in water properties – especially in temperature and conductivity. Moreover, it turned out that there are periods of increased water flow within the abyss and periods of partial stagnation. The flow occurs due to different densities caused by the changing temperature and conductivity (i.e., mineralisation) of water, evident especially in the winter. The water in the abyss is cooled by cold air at the surface, and this cooled water with higher density descends to greater depths, mixing the entire water column to a depth of at least 180 m in the process. In the summer, the water at the surface keeps a relatively constant temperature with lower mineralisation, i.e., with a density lower than the more mineralised water in the abyss at a depth of about 20–50 m. Therefore, there is almost no mixing during the summer.

This periodic process of water stagnation in the summer and water flow in the winter is consistent with the observations of CO₂ leakage near the surface, which is more intensive in the winter and contributes to the mixing of the water. The results of the study of microbial communities are also consistent with this theory, as aerobic organisms predominate in the Hranice Abyss in the entire water column all the way to a depth of 180 m. This has been a somewhat surprising finding in what is otherwise a prevalently anoxic environment. Water mixing and microbiological recovery are also associated with the occasional occurrence of water turbidity – a parameter important for diver activities. The occurrence of turbidity is most likely caused primarily by the transition of Fe²⁺ to Fe³⁺ during the precipitation of FeO(OH), which is catalytically assisted by the microorganisms present.

The formation of large, flooded areas of the Hranice Abyss is likely related to the mixing of warm mineral water rich in CO₂ with colder shallow groundwater. These conditions lead to stronger corrosive properties of the aquatic environment, which gradually dissolves calcareous rocks.

The research was conducted by a team led by Helena Vysoká and involving the members of the Czech Speleological Society’s local unit ZO 7-02 – Hranický kras – and experts from the Faculty of Science, Charles University and other institutions such as SG Geotechnika, Czech Geological Survey, Geological Institute of the Academy of Sciences, TU Liberec and CTU in Prague.

Tomáš Weiss

Vysoká, H., Bruthans, J., Falteisek, L., Žák, K., Rukavičková, L., Holeček, J., ... & Oster, H. (2019). Hydrogeology of the deepest underwater cave in the world: Hranice Abyss, Czechia. Hydrogeology Journal, 27(7), 2325–2345.