Hydrogeochemistry of thermal groundwaters in the Serbian crystalline core region

• Geochemical exploration has been applied for studying thermal water in Serbia.
• Based on the major elements all of the water samples are of HCO3–Na to SO4–Na type.
• Waters from schist and granitoid formations have high content of B, Be, Cs, F, Ga, Ge, and W.
• 222Rn activity concentration is high in groundwaters that are in contact with schist and along fault zone.

Geochemical exploration has been applied for studying thermal groundwater characteristics in the Serbian crystalline core region. Within this hydrogeological region, thermal groundwater with temperature higher than 20 °C occurs at seven locations. The maximum discharge temperature of the thermal groundwater is 105 °C which is the highest groundwater temperature encountered in Serbia. Geothermal reservoirs are present within andesite, schist, grus (the fragmental products of in-situ granular disintegration granite), marl and sandstone.Based on the concentrations of the major elements all of the water samples are of HCO3–Na to SO4–Na type. An elevated content of F, B, Ge and Rb is observed, while some samples also exhibit higher concentrations of As, Be, Cs, Ga, Ge, Li, V and W, and one has elevated concentrations of Nb, Zr and heavy rare earth elements (HREE). The groundwater is neutral to alkaline with TDS of 130–3822 mg/L.Making use of various geothermometers, it was estimated that the temperatures in the selected aquifers of the Serbian crystalline core range from 45 to 146 °C.Activity concentration of 222Rn ranges from 10.4 ± 0.9 to 104 ± 15 Bq/L and is higher in groundwater that is in contact with schist and along faults, while 226Ra has a smaller activity concentrations, within the range of 0.21 ± 0.09 to 0.48 ± 0.18 Bq/L.