## 5 Discussion
The highly variable permeability within the landslide results in local aquifers that respond rapidly to surface recharge, while deeper, less permeable zones can retain water for longer periods, creating complex groundwater dynamics that are difficult to predict. Tracer-determined groundwater flow velocities therefore indicate a dual-flow system, where fast recharge occurs through preferential pathways, while deeper, slower groundwater flow provides long-term moisture retention in the landslide body. Groundwater infiltration significantly influences pore water pressure, a key factor in landslide movement. A rapid influx of water into the landslide mass due to intense precipitation events can temporarily increase pore pressure, reduce shear strength, and potentially trigger slope displacement. Previous studies have shown that when pore pressure exceeds a critical threshold, landslide acceleration is observed [41; 45; 46; 47]. In contrast, the slow-moving groundwater component contributes to prolonged saturation of the deeper layers, which can weaken failure surfaces over time, promoting gradual instability. This complexity poses significant challenges for groundwater management and the planning of measures to improve the stability of the landslide. Interpretation of the hydrogeological conditions, characterized by a combination of fast and slow flow systems, and the highly variable permeability requires a detailed understanding of the local geology and groundwater flow dynamics. Tracer test results can provide valuable supporting information for just such a purpose. Despite the valuable insights provided by tracer tests,certain limitations must be considered. The use of sodium chloride and fluorescein as tracers may be influenced by factors such as dilution, sorption, and variable flow velocities, which could impact tracer recovery rates and measured travel times [48; 49]. Additionally, the spatial resolution of tracer detection is constrained by the number and spatial distribution of monitoring points, potentially overlooking smaller-scale flow heterogeneities [50]. Seasonal variations in groundwater levels and flow rates further complicate tracer interpretations, as different hydrological conditions can affect groundwater flow paths and residence times [51]. The primary causes of the Urbas landslide are linked to its geological composition, hydrogeological conditions, and external triggering factors [52]. The landslide consists of alternating layers of coarse-grained and fine-grained material, allowing rapid infiltration through permeable layers while trapping water in less permeable zones, thereby increasing pore water pressure and reducing slope stability. High seasonal precipitation, combined with snowmelt, results in repeated cycles of saturation and drainage, leading to stress accumulation along failure planes. Several key factors contribute to the instability of the landslide. Long-term saturation, on the other hand, affects deeper zones, gradually weakening the failure surface and leading to slow deformations. Erosion at the base of the landslide further destabilizes the slope by removing material and reducing support, while seismic activity can induce additional stress along failure planes, increasing the likelihood of movement. Given the complex hydrogeological conditions characterized by fast and slow flow systems and highly variable permeability, effective landslide management requires a detailed understanding of local geology and groundwater flow dynamics. Tracer test results provide crucial supporting information for this purpose. The relationship between groundwater flow and landslide stability is determined by both the rate of infiltration and the distribution of water storage within the landslide mass. Rapid responses to precipitation events can lead to transient increases in pore water pressure, temporarily reducing shear strength and triggering slope movement [52; 53].