Seasonal recharge of spring and stream waters in a karst catchment revealed by isotopic and hydrochemical analyses

Quantifying and understanding recharge behavior of aquifers in complex hydrogeological systems is challenging, which limits our ability to manage water resources in karstic areas. In this study, we analyzed the seasonal recharge sources and processes of a stream, an intermittent spring, and a perennial spring in a small dolomitic catchment. Weekly monitoring of stable isotopes and chemical characteristics and daily hydrological data of these waters was performed in 2017 and 2018. There were broad seasonal variations in rainfall isotopes, with more negative values observed in the wet season and more positive values observed in the dry season, while narrow ranges were observed in spring and stream waters. Such values plotting on a LMWL represented a homogeneous mixing of rainfall without the effective evaporation effect. Hydrograph separation showed that the mean proportion of old water was approximately 94% for the springs and stream, which indicated a mixing mechanism in recharge processes. The mean residence time was approximately 23 weeks for spring 1, 201 weeks for spring 2, and 43 weeks for stream. The significant difference between springs was attributed to the combined effects of relatively higher proportions of rocky outcrops, thinner soil–epikarst, and better karstic development in the aquifer of spring 1, which enhanced the sensitivity to rainfall. The stream was recharged by waters from hillslopes, which mixed extensively in the depression, accompanied by soil–epikarst interaction. However, only approximately 1.5% of total stream flow was recharged by springs annually, and most of the stream was recharged by water through underground paths, based on the discharge analyses. Moreover, stream was recharged by subsurface flows, which were considerably affected by soil, leading to the fluctuating stream discharge characteristics during the wet season. The results suggest that greater attention should be paid to the roles of near-surface soil–epikarst architecture in hydrological processes.