Assessment of thermal water outflow plume in heterogeneous glaciofluvial deposits: a case study from Julian Alps, Slovenia

In Alps, a number of thermal springs are known, which represent the outflow of thermal water from low temperature geothermal systems in fractured rocks. Such dynamics is usually characterized with convection flow, derived either by fault intersection or hydrogeological barrier where the thermal water is uprising due to hydraulic pressure imbalance. When the water is uprising due to convection, it is very likely that the mixing processes between the deep thermal component and the shallow fresh groundwater are established. In Bled case study in Slovenia, the thermal water with average temperature of 21.5 °C, which is around 12 °C higher than average annual air temperature, is discharging from fractured carbonate rocks into glacial Quaternary sediments. Since they have relatively higher but heterogeneous permeability, the uprising thermal water drains into these deposits and, consequently, forms thermal plume which is extending parallel to prevailing fresh groundwater flow direction. Knowing the extent of the thermal plume is of crucial importance for sustainable exploration of the geothermal resource, since it provides answers also to the key issues related to its geothermal and hydraulic characteristics and the dynamics of the regional flow of groundwater, including its recharge area. By approximating the thermal water outflow as a planar source (since we assume it springs out from a fault zone), a planar advective heat transport model (PAHM) was used to evaluate its geometry and quantify the rates. Nine scenarios were applied accounting for different dimensions of the heat source. Each scenario was verified by calculating relative error between the analytical model results and measured borehole temperatures. The PAHM proved to be a useful tool in applying heat transfer as a planar source in groundwater flow. Still, it is necessary to consider or to introduce relatively rough assumptions (e.g. simple model geometry) leading to a very conservative approach. The heterogeneity of the medium has a significant influence on the temperature distributions obtained with different simulation scenarios. Therefore, the calculated temperature distribution within a thermal plume is a subject to uncertainty. In addition, some small portion of a relative error can be attributed to Lake Bled, since the thermal plume is extending in the zone of lake water temperature fluctuation influence. Nevertheless, the analytical model can be used as a tool for simulating spatial distribution of the observed values acquired from field measurements and thus more correctly evaluating the average natural conditions.