Water ages in thermal system of Podhale Basin, Inner Carpathians, southern Poland

Fissured and karstified Eocene and Mesozoic carbonate formations of the Podhale Basin represent the largest reservoir of renewable thermal waters in Poland. They outcrop in the Tatra Mts. at altitudes of 1 000-1 800 m and deep to the north under the flysch formations of the basin. The main direction of flow is to the north for abt. 15 km where the impermeable formations of the Pieniny Klippen Belt divides it and diverts to the west and east, and next to the south to the Danube watershed in Slovakia. The temperatures range from abt. 20° C near the outcrops to abt. 85° C at the most northern wells. For a better understanding of the flow pattern, environmental isotopes (δ¹⁸O, δ²H, ³H, ¹⁴C, δ¹³C) have been used since early seventies and recently also gaseous tracers (He, Ne, Ar and SF₆) under the grant No N 525 402334 from the Ministry of Science and Education.
   The C¹⁴ data of thermal waters change from 37 to 0 pmc with δ¹³C from abt. −5 to 0‰; exhibiting the influence of isotopic exchange with carbonate minerals, which makes the quantitative dating difficult. The δ¹⁸O and δ²H are similar to those of modern waters in springs and wells with cold water, with several exceptions characterized by shift of δ¹⁸O to heavier values, which are caused by isotopic exchange with carbonate minerals. The isotopic altitude effect was estimated form the data of springs and wells within the Tatras area. For δ²H, the mean altitude of recharge area reads: h2 (m a.s.l.) = −69.1⋅δ2H − 4054, with the uncertainty of about 100-200 m. The most negative δ²H values of thermal waters are similar to the values observed for large karstic springs in the Tatras, which may suggest their Holocene age. However, the spatial distribution of δ²H values indicates that close to the recharge area, the thermal waters are similar to those of medium springs discharging at the lowest altitudes. Thus, the most negative δ²H values of thermal waters observed far in the basin most probably result from recharge under cooler climatic conditions. Very high He excess contents and negative noble gas temperatures (NGT) derived from Ne and Ar
concentrations are in agreement with such interpretation. The lack of ¹⁴C and δ¹³C values close to 0‰ in these wells also confirms that hypothesis.
   Tracer data indicate the presence of the oldest waters in the north-eastern part of the basin whereas in the western part the exchange of water is faster by one to two orders of magnitude. That unexpected flow pattern most probably results both from the presence of some karstic channels in the western part, which enhance regional permeability, and from obstacles to horizontal flow caused by fault zones in the eastern part.