Hydrogeochemical research of thermal waters from Topusko, Croatia

A hydrochemical and geothermometric study of thermal waters in Topusko (Croatia) was conducted in order to improve the existing conceptual model of the Topusko hydrothermal system. The town of Topusko is located 80 km south of Zagreb at the SW edge of the Pannonian Basin System, which has favourable geothermal characteristics. The natural thermal springs of Topusko with temperatures up to 53°C and thermal waters from shallow wells with temperatures up to 65°C represent the second warmest natural thermal water source in Croatia. Water samples have been collected monthly since March 2021 from two natural thermal springs, i.e. Livadski izvor and Blatne kupelji springs, and the TEB-4 thermal well in the discharge area of the hydrothermal system. Furthermore, rainwater was sampled in the supposed recharge area. Chemical composition of groundwater is determined by the original composition of the infiltrated water as well as chemical reactions with various rocks along its circulation path. In-situ parameters, major anions and cations, stable water isotopes, radioactive isotope analysis of tritium (³H), and silica geothermometers were used to assess the origin of thermal waters in Topusko and their interaction with the thermal aquifer. Furthermore, a local meteoric water line was constructed and compared with the isotope ratio of the thermal waters. The results pointed to: i) the meteoric origin of thermal waters according to stable water isotopes; ii) Ca-HCO₃ hydrochemical facies suggesting that carbonate dissolution occurs in the aquifer, being consistent with the presence of carbonates in the stratigraphic logs of wells; iii) an equilibrium temperature in the reservoir of 88°C according to silica geothermometers; and iv) low tritium activity being consistent with sub-modern waters and a recharge before 1955. The monitoring of these thermal springs will continue for another year to collect a larger dataset and reinforce our conclusions. Additional analyses including radiocarbon dating and stable isotope composition from SO₄^2- will be conducted to provide a comprehensive hydrochemical characterisation of these thermal waters, which have been utilised since ancient Roman times, and intensively since 1980s, but have not been thoroughly investigated.

Acknowledgments: Presented research has been conducted in the scope of the project “Multidisciplinary approach to hydrothermal system modelling” (HyTheC) funded by the Croatian Science Foundation under grant number UIP-2019-04-1218.