Water from the ponor of the Bubovice Stream rapidly flows in depth beneath clastic fill of the Holyně-Hostim Syncline, feeding springs in the Loděnice Stream Valley
Abstract
The few positive results of tracer tests in the Bohemian Karst were achieved within a small area between the Arnika Ponor and Svatý Jan pod Skalou Spring. In the rest of the area, flow directions and velocities are generally unknown.
In 2021, five tracer tests with fluorescein dye were performed mostly at high water levels from the uppermost sinks of the Bubovice Stream. The tracer was repeatedly detected in two large springs in the Loděnice Stream Valley: the V nivě Spring and the Svatý Jan pod Skalou Spring.
The first arrival was after 27-38 hours to the former and after 50 hours to the latter. This means a surprisingly fast flow, as water needs to flow beneath a thick impervious clastic fill of the Holyně-Hostim Syncline, about 2 km in direct horizontal distance.
The tracer test demonstrated a fast flow velocity and relatively small diameters of karst conduits which probably exceed 400 m below ground surface and 200 m below the water table. The tracer test confirmed the assumption that water from the upper reach of the Bubovice Stream, which has a negative budget, is drained away from the Bubovice catchment.
This, together with the increased amount of sinking water at recent time, explains the unusually low amounts of water in the downstream part of the catchment in the last few years. The recovery of the tracer was only 10-20 %, the rest flows either to hidden springs in the major Berounka River or was more likely captured in the alluvium through which the sinking water passes to limestones and was subsequently diluted below the detection limit.
Tracer tests should be performed at other sinks to better understand hydrogeology of the Bohemian Karst.