Fold and fault control on the drainage pattern of a double-karst-aquifer system, Winterstaude, Austrian Alps

Abstract

Lithostratigraphy and geologic structures are major controls on groundwater flow in alpine karst systems. Understanding these factors is important for the delimitation of drinking water pro­tection zones. The Winterstaude mountain chain, western Aus­tria, belongs to the Helvetic nappes and consists of Cretaceous sedimentary rocks, including two karstifiable formations: Örfla and Schrattenkalk Limestone (lower and upper karst aquifer), separated by 60 m of marl. Strata are folded and cut by faults withdisplacements of 40–70 m. Folded carbonate rocks con­tinue below the alluvial valley floor so that the karst system can be subdivided in shallow and deep phreatic zones. This area is suitable for studying the combined influence of folds and faults on groundwater flow in a double-aquifer system. A multi-trac­er test withseven injections aimed at characterising hydraulic connections and linear flow velocities. Results show that (i) plunging synclines form the main drainage pathways in the up­per karst aquifer, withmaximum linear velocities of 91 m/h, while anticlines act as water divides; (ii) recharge into the lower aquifer, whichforms the central ridge of the mountain chain, contributes to springs discharging from the upper aquifer near the foot of the mountain (local flow systems); (iii) the two aq­uifers are hydraulically connected, presumably via faults, be­cause their displacements are in the same order of magnitude as the thickness of the intervening marl; (iv) flow in the upper aquifer continues below the valley floor toward the river, withmaximum velocities of 22 m/h(intermediate flow system).