Exploration von Trinkwasser aus geklüftetem Hauptdolomit in den Nördlichen Kalkalpen

The alpine Hauptdolomit represents an important fractured reservoir for groundwater in the Northern Calcareous Alps (NCA) due to its widespread occurrence and its special geohydraulic characteristics. A new exploration scheme tailored to the complex nature of the Hauptdolomit has been developed and is presented in this study, in order to ensure the production of safe and hygienic drinking water in this region. Multiple studies focusing on the structural geology, lithology and hydro-geochemistry were carried out to determine potential areas for an extensive exploration drilling program for drinking-water from fractured Hauptdolomit. The region of interest spans from Inzell in the East of the NCA to the Kochelsee in the West and the areas Rottach-Egern/Kreuth (Lechtal-Unit) and Reit im Winkl (Tirolikum) were chosen for extended research. Geophysical measurements and flow tests from numerous wells exploring the Haupdolomit, revealed a strong anisotropy of the fractured aquifer. A linked fractured flow system, which combines different orders of fracturing is suggested to provide connection of hydraulic and hydrostatic fissure water. Fissure water flow in the Hauptdolomit follows a regional indirect fracture system which is based on the connection of well flowing high-permeable hydraulic fracture systems with limited extend. The main carriers for water flow are large fractures (1st-order) which are bound to fault zones related to the most recent phase of alpine tectonics. Water influx into these 1st-order fractures is provided by a smaller fracture system (2nd-order) and also a close-meshed set of fine fractures and joints (3rd-order). These 2nd and 3rd-order systems enable water influx to the larger scale fracture system only on a very local scale and regional flow is therefore only provided if a well connected fracture system (2nd/hydraulic and 3rd-order/hydrostatic) is present. Large local variations in the efficiency of the fracture network are therefore characteristic for this type of groundwater reservoir. In order to estimate the groundwater recharge potential for the different exploration areas, digital terrain models have been developed to calculate the distribution of precipitation, evapotranspiration and temperature. Based on this data a climatic water balance which includes groundwater formation, surface water runoff and groundwater outflow was accomplished and a hydrogeochemical classification scheme combining phase diagrams (Ca-Mg and Ca-Mg-SO4) with the local the geological regime, allow to hydrogeochemically subdivide the groundwater in the investigated areas into three categories. Dolomite-rich water, calcium-rich water and sulphate-rich water were distinguished which can be utilised for the differentiation of groundwater mixtures from different catchment areas representing varied lithologies.