The geochemistry and origin of fluids in the carbonate structure of the Hranice Karst with the world's deepest flooded cave of the Hranicka Abyss, Czech Republic

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Applied Geochemistry


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The origin of fluids in the Hranice Karst containing the deepest flooded abyss in the world has been investigated using hydrogeological, hydrogeochemical, and isotopic data. At least a part of the CO2 gas originates in the mantle as indicated by very enriched δ13C(DIC) values and from existing He isotope analyses. The origin of groundwater in the karstic aquifer which is exploited at the Teplice nad Bečvou Spa is meteoric with a recharge area about 200 m above the Bečva River valley as indicated by depleted values of δ2H and δ18O compared to the river water. Based on detectable tritium, the groundwater is from 20 to 50 years old. Water in the Hranicka Abyss and in the Zbrašov Aragonite Caves is a mixture of carbonate aquifer groundwater with the river and/or shallow groundwater comprising variable proportions of both end-members. Water in Death Cave Lake seems to be affected by agriculture contaminated shallow groundwater as indicated by increased nitrate concentration. Inverse geochemical modeling of aquifer geochemistry suggested two scenarios: (1) reaction of Mg-rich calcite with deep hypogenic CO2 (about 30 mmol/l) plus dissolution of trace amounts of halite and sylvite and cation exchange; (2) reaction of Mg-depleted calcite and Mg-silicate (talc) as a source of Mg together with deep CO2. Both scenarios were calibrated using δ13C(DIC) values and gave satisfactory results. A conceptual model of the site has been developed which includes a gravity-driven flow system where meteoric water which has recharged in the surrounding uplands is heated at depth and acquires large amounts of hypogenic CO2, which preferentially dissolves Mg-carbonates along the pre-existing tectonic features. The Miocene transgression followed by the later incision of the Bečva Valley played an important role in groundwater circulation and the origin of fluids.


Carbonate aquifer, Recharge, Endogenic CO2, Carbon isotope modeling, Hranicka abyss

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