Thermal waters of the Western Bohemian Massif (Germany): geothermometry, origin and circulation

• Geothermal reservoirs of the Western Bohemian Massif are low-enthalpy (< 100 °C). • Reservoirs occur in crystalline basement at ∼2–4 km depth. • Thermal water circulation is focused along deep permeable fault systems. • Hydraulic connectivity exerts important controls on thermal water composition. The fault-bound basement-hosted geothermal systems of the Western Bohemian Massif (Germany) remain poorly constrained in terms of reservoir temperatures, circulation depths and recharge. Here, we present some of the first systematic constraints on these parameters based on a geochemical survey of regional thermal water, non-thermal groundwater and surface water, combining isotope techniques, a multi-method geothermometric approach and mixing calculations. Stable isotope ratios (δ 18 O and δ 2 H) indicate that all studied thermal waters are derived from meteoric precipitation. The geothermal reservoirs of the region are identified as low-enthalpy “hot water type” systems with temperatures generally < 100 °C. The occurrence of local warm springs is linked to radiogenic heating in primarily granitic reservoirs at depth and migration of thermal water to the surface along long-lived permeable fault systems. Despite having interacted with similar reservoir lithologies at comparable temperatures, thermal waters exhibit pronounced hydrochemical variability in terms of composition, mineralization and extent of water-rock interaction. These differences primarily reflect structural characteristics of the individual geothermal systems, such as hydraulic openness and fault connectivity, which in turn affect recharge rates, residence times, extent of dilution with non-thermal groundwater and the supply of dissolved CO 2 . Together, these results demonstrate that hydraulic connectivity and permeable fault architecture exert a first-order control on geochemical signatures in the geothermal systems of the Western Bohemian Massif and similar basement-hosted settings.