The origin of highly permeable flow paths in carbonate-siliciclastic rocks, such as large-aperture fractures in aquifers in the Eastern Bohemian Cretaceous Basin (EBCB), is poorly understood. The karst potential was assessed from the rock carbonate content and the degree of disintegration after leaching in HCl. Surprisingly, dissolution of calcite in EBCB usually did not lead to rock disintegration until calcite > 78%. Instead, porosity increased significantly. High-porosity rock is held together by microns-thick secondary silica cement with a foam-like structure and considerable tensile strength. Three types of conduits occur in the EBCB: (i) bedding-parallel conduits associated with calcite-rich layers, (ii) subvertical fracture swarm conduits that develop on damaged zones of fracture swarms, and (iii) conduits formed by dissolution of calcite veins by groundwater flow. These are ghost-rock karst features where calcite is leached from the rock in the first phase and the residue is washed out by conduits under steep hydraulic gradients in the second phase. Very similar features have been described in Minnesota and Wisconsin, USA. Research has shown that fractures with sharp-edged walls that give the impression of an extensional tectonics origin may actually be ghost-rock karst features in which dissolution and piping have played an important role in their enlargement.

Článek přibližuje projekt Technologické agentury ČR, který se zabývá problematikou starých podzemních vod v hydrogeologických rajonech 4410 a 4522. Cílem příspěvku je na základě rešerše předchozích prací podat stručnou hydrogeologickou charakteristiku zájmové oblasti, představit dosavadní výsledky po prvním roce řešení projektu a popsat nejistoty stávajících informací. Staré podzemní vody, jež mají zanedbatelné koncentrace tritia, se dají považovat za strategický zdroj, protože jsou méně náchylné k současné kontaminaci. Za účelem kvantifikace využitelného množství těchto vod je pravidelně měřena vydatnost pramenů s nízkou koncentrací tritia.

Tracer tests are the key technique to characterize karst conduits. Hundreds of thousands tracer tests done worldwide showing typical flow velocities in conduits in km/day. In contrast slow flow in fractions of m/day is typical for porous aquifers. This study reports surprisingly slow flow velocity obtained from karst conduit in small metamorphosed carbonate occurrence in Czech Republic. Factors responsible for exceptionally slow flow are discussed.Chýnov Karst is formed by several km long but few hundred meters wide carbonates strip inside gneiss in slightly undulated landscape. It hosts karst conduit traversed by small subterranean stream (5-13 L/s, observation since 2001), fed by diffuse recharge, no sinking streams occur in area. Karst conduit is accessible at two places: Chýnov Cave and 1200 m distant spring Rutice. The carbonate strip is traversed perpendicularly by two small streams, which valleys contain Miocene Mydlovary Formation. The river network and karst conduit could be dated back at least to Miocene, as demonstrated by Miocene sediments below the recent streams. There are no signs of hypogenic origin. Cave is formed by deep phreatic loops indicating low density of fractures available for conduit forming. No transport of sediment was ever observed, the water remains limpid and flow in conduit is nearly constant.Tracer tests from 1960 indicated residence time in days, but individual results strongly differed, therefore new tracer test was performed using uranine in 2021. Uranine concentration has been monitored by field fluorometers GGUN-30 (Albilia, Switzerland) and laboratory fluorimeter Perkin Elmer LS55 for more than 3 years (continues). Breakthrough curve was analyzed by Qtracer2 code. New tracer test demonstrated residence time 1.4 year between cave and Rutice spring, which is equal to mean flow velocity 3.6 m/day. Volume of mobile water in conduit is 252000 m3 which is equal to average cross section of conduit 140 m2 . Tracer recovery is currently 25%. Tracer did not arrive to any other spring or stream in wider surrounding. Large conduit volume and low flow rate are responsible for extremely slow flow and long residence time. Large volumes are probably primarily caused by deep phreatic loops in cave. Phreatic loops disable under gentle landscape the transport of sediment from streams to cave.Therefore, the localized sinks can never develop in the area. As a consequence, the corrosion over more than 20 million years ever increases the conduit volume and no sediment is transported inside to infill it. Existence of this conduit indicates that low fracture density resulting in deep phreatic loops can in combination with gentle morphology over long time period result in evolution voluminous conduits with very slow flow. If Chýnov cave is not known in the area the long residence time of water and low flow fluctuation will be considered as indication of non-existence of karst conduits.Funded by the GAUK No. 171624.