Polonium and uranium activities were measured in surface and karst water in the Kle?nica Stream catchment and the Nied?wiedzia Cave system (Sudety Mts., South-West Poland). The highest polonium concentration was recorded in surface water during flood events and in a sinter pool, located close to the touristic trail inside the cave. It points to a detrital source of the polonium and its transport in water absorbed by the clay fraction. Uranium activity increases downstream in the surface water of the Kle?nica Stream, indicating significant input of 234U and 238U from an area of outcropping marble. Water, autochthonous in the marble, that originated from precipitation and infiltration directly into the cave system is characterized by low uranium content. However, the uranium activity increases with the time of water percolation in the soil and epikarst zone and is higher at the lower cave level than at the upper and middle levels. Po and U activities were used as tracers for the area of water recharge and the mode of circulation. The results point to atmospheric precipitation as the almost exclusive source of water at the upper and middle cave levels, while water at the active level of the cave is a mixture of precipitation, infiltrating directly from the area above the cave passages, and horizontal flow, originating out of the marble outcrop.
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