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Understanding groundwater–surface water (GW–SW) interactions is vital for water management in karstic catchments due to its impact on water quality. The objective of this study was to evaluate and compare the applicability of seven environmental tracers to quantify and localize groundwater exfiltration into a small, human‐impacted karstic river system. Tracers were selected based on their emission source to the surface water either as (a) dissolved, predominantly geogenic compounds (radon‐222, sulphate and electrical conductivity) or (b) anthropogenic compounds (predominantly) originating from wastewater treatment plant (WWTP) effluents (carbamazepine, tramadol, sodium, chloride). Two contrasting sampling approaches were compared (a) assuming steady‐state flow conditions and (b) considering the travel time of the water parcels (Lagrangian sampling) through the catchment to account for diurnal changes in inflow from the WWTP. Spatial variability of the concentrations of all tracers indicated sections of preferential groundwater inflow. Lagrangian sampling techniques seem highly relevant for capturing dynamic concentration patterns of WWTP‐derived compounds. Quantification of GW inflow with the finite element model FINIFLUX, based on observed in‐stream Rn activities led to plausible fluxes along the investigated river reaches (0.265 m3 s−1), while observations of other natural or anthropogenic environmental tracers produced less plausible water fluxes. Important point sources of groundwater exfiltration can be ascribed to locations where the river crosses geological fault lines. This indicates that commonly applied concepts describing groundwater–surface water interactions assuming diffuse flow in porous media are difficult to transfer to karstic river systems whereas concepts from fractured aquifers may be more applicable. In general, this study helps selecting the best suited hydrological tracer for GW exfiltration and leads to a better understanding of processes controlling groundwater inflow into karstic river systems., Karst aquifers represent an increased complexity when aiming to measure the interaction between groundwater and river water. Combining field‐based measurements on catchment scale and modelling, the applicability of ‘classical’ environmental groundwater tracers was compared to selected organic (micro)pollutants often considered as conservative and originally arising from a wastewater treatment plant. This study demonstrates that the choice of an appropriate tracer is crucial when either aiming to quantify groundwater exfiltration into karstic river systems, or indicating hydrological processes, applying (globally) omnipresent pollutants., German Research Foundation (DFG) http://dx.doi.org/10.13039/501100001659

Transient storage zones (TSZs) are located at the interface of rivers and their abutting aquifers and play an important role in hydrological and biogeochemical functioning of rivers. The natural radioactive tracer Rn is a particularly well-suited tracer for studying TSZ water exchange and age. Although Rn measurement techniques have developed rapidly, there has been less progress in modeling Rn activities. Here, we combine field measurements with the numerical model HydroGeoSphere (HGS) to simulate Rn emanation, decay and transport during steady state (riffle-pool sequence) and transient (bank storage) conditions. Comparing the HGS mean water ages with the conventional Rn apparent ages during steady state showed a systemic underestimation of apparent age with increasing dispersion and especially where large concentration gradients exist within the subsurface. A large underestimation of apparent water age was also observed at the advective front during bank storage where regional high Rn groundwater mixes with newly infiltrated surface water. The explicit modeling of radiogenic tracers such as Rn offers a physical interpretation of this data as well as a useful way to test simplified apparent age models.