Hydrogeology of alpine lakes in the Northern Calcareous Alps: a comparative study on the role of groundwater in Filblingsee and Eibensee

In the Northern Calcareous Alps (NCA) there are countless small lakes with small orographic catchments that are often located only slightly below the respective summit regions. On the one hand, the lakes are located in karstable aquifers and their existence is likely to be related to karstification. Then, they are expected to be directly connected to the karst water body. These lakes are classified as karst lakes. On the other hand, the alpine environment is also influenced by glacial processes and lakes might be related to glacial erosion and deposition. For these glacial lakes, the share of groundwater inflow and outflow is regarded as subordinate even within high permeable karst lithologies. Here we compare two alpine lakes of potentially different origin in the NCA in Salzburg with the aim to provide a basis for an aerial survey of the numerous small alpine lakes in the NCA region and their characterization using the guiding parameters elaborated here. We consider (a) the lake geometry, (b) potential inflow and outflow systems, and (c) physicochemical parameters and hydrochemistry of the Filblingsee and the Eibensee, both located in the Fuschlsee region. Filblingsee was initially considered as a typical karst lake and Eibensee as a moraine-dammed glacial lake. Some clear differences arise in lake geometry, which in the karst lake shows a nearly round surface and concentric depth profile, while the glacial lake is elongated in the direction of glacier flow and has the deepest areas just upstream of the moraine dam. Both lakes show very little to no surficial inflow. Inflow and outflow occur in groundwater in both cases but are not directly tied to a highly permeable karst system. The depth profiles of the field parameters of the two lakes differ only slightly and show a dominant groundwater inflow in mid-depth regions but no flow through at the lake bottom. Water chemistry in both lakes and their potential outflows correspond to the respective aquifer in terms of solution load. Filblingsee can be characterized as a hanging lake in a secondarily sealed doline, Eibensee lies in a glacially excavated depression sealed by glacial sediments. While the inflow and outflow conditions and the hydrochemistry of both lakes are very similar, the lake geometry is a clear distinguishing feature that can be attributed to the different genesis of the two lakes. This can therefore be used as a guiding parameter for the classification of the numerous small alpine lakes in the NCA.