Use of artificial and natural tracers to assess groundwater transit-time distribution and flow systems in a high-alpine karst system (Wetterstein Mountains, Germany)

Groundwater in mountainous karst regions is vital for regional water budgets and freshwater supply. Owing to increasing water demand and climate change, detailed knowledge of the highly heterogeneous alpine aquifer systems is required. Multi-tracer analyses have been conducted in the steep karstic Wetterstein Mountains, which includes Germany's highest summit, Zugspitze (2,962 m asl). Results of artificial tracer tests demonstrate well-developed flow paths through the unsaturated zone (up to 1,000 m thickness). Flow paths cross topographic divides and contribute to deep drainage systems underneath alpine valleys. Cross-formational flow has been identified. Quantitative analysis of tailing-dominated breakthrough curves and stable isotopes (O-18) has enabled determination of the mean transit-time distribution. A fast-flow component with transit times between 3 and 13 days was found in karst conduits and open fissures, dependent on flow conditions. An intermediate-flow component, showing mean transit times of about 2.9-4.9 months, was found in well-drained fissures and fractures. A slow-flow component with mean transit times greater than 1 year is attributable to slow flow and low storage in the poorly drained fissures and rock matrix. The conceptual model enables a better understanding of drainage, water resources and vulnerability of the high-alpine karst system.