Quantifying the Role of Karst Groundwater on Mountain River Discharge

In snow-fed, mountainous karst regions, aquifers sustain and are connected to streamflow via springs and distributed groundwater exchanges (DGE) including inflows (DGI) and losses (DGL). Despite their significance, there is limited understanding of the complex flow paths linking aquifers and streams, particularly regarding flow path source and residence time, and thus system resilience to future shifts in recharge. This study advances a combined approach using discharge, solute, and solute isotope (⁸⁷Sr/⁸⁶Sr, ²³⁴U/²³⁸U) mass balances alongside spring environmental tracer data to quantify DGE and characterize DGI in a snow-fed karst watershed. The integration of DGI solute isotope composition and mineral saturation indices, along with a comparison to spring ³H-informed apparent ages, allowed for a novel assessment of flow path dynamics. Results reveal the dominance of karst-conduit-influenced contributions to DGI during high and low flow. Karst-conduit-influenced springs and DGI showed large increases in residence times from high to low flow periods. This is explained by a shift in the presence from flow in quick response conduit and shallow soil flow paths during spring runoff to carbonate matrix flow paths during baseflow. Because of the significant spring and DGI contribution to river baseflow, insight into these groundwater sources has important implications for system resilience and the potential storage volume available that can buffer impacts of climate variability.