Young Water Fractions and Hydrogeological Controls in Alpine Springs

The young water fraction (Fyw) has become a widely used metric to characterize catchment transit time behavior while avoiding many of the aggregation biases inherent in mean transit time estimates. Although Fyw has been widely studied in rivers, little is known about its magnitude, variability, and controls in springs, despite their importance for mountain hydrology and water supply. Here, we present the first systematic, large-sample analysis of young water fractions in spring discharge.We quantify Fyw for 469 springs across Austria, spanning a broad range of hydrogeological settings including karst, talus, fractured, and alluvial aquifers. Fyw is estimated by comparing seasonal stable isotope cycles in precipitation and spring water to quantify the fraction of water reaching the spring within approximately 2–3 months. Across all springs, Fyw values are generally low and approximately log-normally distributed, with a mean of about 0.06, indicating a dominant contribution of older groundwater. However, pronounced differences emerge between spring types. Karst springs exhibit the highest young water fractions and the largest variability, reflecting rapid and dynamically activated flow paths. Talus springs show intermediate values, while fracture and alluvial springs display consistently low Fyw with limited variability, indicative of strongly buffered flow systems.We further analyze the sensitivity of Fyw to discharge, revealing contrasting responses to hydrologic forcing. Karst springs show the strongest discharge dependence, consistent with shifting proportions of fast and slow flow paths, whereas fracture springs exhibit near-invariant young water fractions across flow conditions. Comparison with a reference dataset of 565 rivers reveals a clear and systematic offset between surface- and groundwater-dominated systems. Springs consistently contain substantially lower fractions of young water than rivers, highlighting the dominant role of slow subsurface transport.By resolving young water fractions across a large and diverse population of springs, this study provides new quantitative constraints on groundwater transit time dynamics and demonstrates the diagnostic value of Fyw for conceptual modeling, groundwater protection, and contamination risk assessment in alpine environments.