Hydrogeological analysis of topography-driven groundwater flow in a low temperature geothermal aquifer system in the Julian Alps, Slovenia

Abstract Groundwater flow and heat distribution was investigated in the regional karstic-fissured aquifer-aquitard system near Lake Bled in the Slovenian, eastern Julian Alps. The area features thermal springs with temperatures of 19–23 °C which are exploited by abstraction wells. The occurrence of low-temperature geothermal systems, which are common in the Alps, are associated with specific hydrogeological conditions, such as vertical hydraulic connectivity between different geological formations, relatively large elevation differences along flow paths, and the concentrated upwelling of geothermal water to the surface. The occurrence of the low-temperature geothermal field is explained by the presence of a hydraulically conductive fault along with a regional groundwater flow pattern that supports deep groundwater circulation. Hydraulic measurements and temperature data were collected from springs and wells in the area to support the analysis of flow patterns, together with the construction of a basin-scale 2D numerical flow and heat transport simulation. The diverse topographic and geological conditions result in a multi-scale groundwater flow system. The discharge of thermal waters in the Lake Bled area is a consequence of the upwelling of deep groundwater induced by a combination of the ~ 650 m difference in hydraulic head and hydrogeological heterogeneity and anisotropy, related to faulting of the geological formations. In addition, individual flow subsystems were found to significantly affect the natural heat distribution and travel times within the basin-scale system. The study highlights the combination of a basin scale approach taking into consideration local to regional-scale heterogeneities and faults in order to better understand the hydrogeological behaviour of Alpine groundwater systems.