###### Abstract  
This study investigates groundwater flow patterns in a landslide area above the settlement of Koroska Bela in NW Slovenia using a series of tracer tests with sodium chloride (NaCl) and fluorescein (uranine). The tracer experiments, using a combination of pumping tests and continuous groundwater observations, reveal two distinct groundwater flow horizons within the landslide body: a prevailing shallower flow within highly permeable gravel layers and a slower deep flow in the weathered low-permeability clastic layers. Uranine injections suggest longer retentions, indicating complex hydrogeological conditions. Groundwater is recharged by the infiltration of precipitation and subsurface inflow from the upper-lying carbonate rocks. In the upper landslide, highly permeable gravel layers accelerate flow, especially during heavy rainfall, while downstream interactions between permeable gravel and less permeable clastic materials create local aquifers and springs. These groundwater dynamics significantly influence landslide stability, as rapid infiltration during intense precipitation events can lead to transient increases in pore water pressure, reducing shear strength and potentially triggering slope movement. Meanwhile, slow deep flows contribute to prolonged saturation of critical failure surfaces, which may weaken the landslide structure over time. The study emphasizes the region's geological heterogeneity and landslide stability, providing valuable insights into the groundwater dynamics of this challenging environment. By integrating hydrogeological assessments with engineering measures, the study provides supportive information for mitigating landslide risks and improving groundwater management strategies.  
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water within landslides. Groundwater tracer tests are often used for hydrogeological characterization of groundwater flow [11], particularly in karst aquifers [12; 13; 14; 15; 16]. In the context of landslides, tracer tests help to track particles and fluids as they move through the landslide material, providing insight into the internal processes at work, such as groundwater movement and sediment transport, as well as the formation of forces. These tests are particularly valuable for predicting landslide behavior and improving risk models in landslide-prone regions.  
To our knowledge, there are only a few studies that have applied tracer tests in landslides, as they can be logistically complex and expensive [17]. The studies were conducted either to define rainfall infiltration processes [18; 19] or to define groundwater flow paths and their velocities (e.g., [17; 20; 21; 22; 23; 24]). According to [21], tracer experiments in landslides are mainly interpreted semi-quantitatively or qualitatively in order to assess local groundwater flow paths and verify hydraulic connections between different parts of a landslide body.  
Tracer tests were used in this study to determine the groundwater flow paths in the area of the Urbas landslide and its recharge characteristics. Several site investigations and monitoring projects have been carried out in recent years, which have helped to improve our knowledge about the landslide [25; 26; 27; 28; 29]. Recent studies on hydrogeological conditions have focused on stable isotopes, which were used to assess the main groundwater flow components and their residence times [30].  
Three series of tracer tests were carried out for this study: the first in spring 2020, the second in autumn 2020, and the last in spring 2022. Both uranium (fluorescein) and sodium chloride were injected into the first two series of tests, while only fluorescein was used in the last campaign. The fluorescent tracers were selected for their high detection sensitivity, low cost, and low toxicity [31]. To improve the hydrogeological conceptual model of the landslide, the results of a pumping test carried out in 2020 and the data from continuous hydrogeological monitoring were also included.