Identifying the flow systems in a karstic-fissured-porous aquifer, the Schneealpe, Austria, by modelling of environmental 18O and 3H isotopes

The Schneealpe karst massif of Triassic limestones and dolomites with the altitude up to 1800 m a.s.l., situated 100 km SW of Vienna in Kalkalpen, is the main drinking water resource for the city. The catchment area of about 23 km2 is drained by two springs: the Wasseralmquelle (196 l/s) and the Siebenquellen (310 l/s). This karstic aquifer is approximated by two interconnected parallel flow systems of: (a) a fissured-porous aquifer, and (b) karstic channels. The fissured-porous aquifer is of a high storage capacity and contains mobile water in the fissures and stagnant water in the porous matrix. The water enters this system at the surface and flows through it to drainage channels, which are regarded as a separate flow system, finally drained by both springs. The channels are also connected with sinkholes, which introduce additional water directly from the surface. Measurements of 18O and tritium in precipitation and springs were modelled by a combined application of lumped-parameter models. Modelling yielded information on the mean values of the following hydraulic parameters: (1) The volume of water in the whole catchment area is 255×106 m3, of which about 1.8×106 m3 are in channels and 253×106 m3 in the fissured-porous aquifer. (2) The total volumetric flow rate is 506 l/s, of which 77 l/s comprises direct flow from sinkholes to springs and 429 l/s are contributed to fissured-porous aquifer. (3) As the volume of the massif is 16.6×109 m3, the total water saturated porosity (fissures and micropores of the matrix) is 1.5% and the channel porosity is about 0.01%