Identifying the hydrological behavior of a complex karst system using stable isotopes

Abstract

Karst systems can be generally characterised by their high hydrological heterogeneities related mainly to highly variable permeabilities, which can significantly change over small spatial scales. This makes tracing and quantification of water flow pathways an extremely demanding task. In this study we present an analysis of hydrological characteristics of a complex karst system, the Ljubljanica river catchment in central Slovenia. Spatially distributed data on stable isotope composition (δ18O, δ2H) of precipitation as inputs, and of several karst springs/sinks as outputs, were obtained. These data were used to identify spatial and seasonal patterns and hydrological behaviour of the karst system in contrasting hydrological conditions. The intensive mixing of continental and Mediterranean air masses over the Ljubljanica river catchment makes the precipitation source identification difficult. However, the results of the precipitation isotopic composition analysis indicate a spatial pattern that could be recognised also in the δ18O and δ2H values of the karst springs and sinks. Along the prevailing karst conduits, the spatial differences in the δ18O and δ2H values decreased. The mean transit time (MTT) estimates using δ18O or δ2H as tracers were similar, with those for the main karst conduits and tributaries ranging between 0.34 and 0.74 years. Such a relatively short MTT could be expected for karst catchments without extensive deep groundwater storage. The fraction of young water (Fyw) for the whole catchment was 0.28, meaning that more than one-quarter of the total discharge was younger than approximately 2.3 months (assuming that the catchment transit time is described by exponential distribution). Small differences in the MTT over different parts of the karst catchment area might indicate intensive mixing and homogenisation of water along the underground conduits. However, the catchment’s homogenisation strongly depends on the preceding hydrological conditions; the differences in the isotope composition can be identified during low-flow conditions, which might indicate the dominant influence of the local recharge of the karst springs.