Assessment of key transport parameters in a karst system under different dynamic conditions based on tracer experiments: the Jeita karst system, Lebanon

The system consists of a catchment ranging in elevation from 90 to 2000 meters above sea level, with a total area of approximately 150 square kilometers. The catchment is drained by the Jeita spring, which is the main domestic water source for the Beirut area and its northern suburbs, serving around 1.5 million inhabitants. The karst system is composed of limestone and dolostone of Jurassic age, overlain by formations of Middle to Upper Cretaceous age. The system's hydrology is influenced by the Dog River, an ephemeral river that constitutes a no-flow boundary to the south. The total annual precipitation in the Jeita catchment is estimated at approximately 1,200 millimeters on average, with the spring constituting the main outlet of the Jeita cave, which acts as an underground drainage channel developed in Jurassic limestone and dolomitized limestone. Twenty-nine artificial tracer experiments were conducted in the Jeita karst system to assess conservative transport parameters through different compartments in the system. The experiments involved the injection of two conservative tracers, uranine and sodium naphthionate, into a phreatic conduit and karst surface features. The recorded tracer breakthrough curves (TBCs) were systematically analyzed using a two-region nonequilibrium approach (2RNEM) to account for the observed skewness and pronounced tailing. The analysis of the TBCs revealed a discharge threshold in the system dynamics, beyond which the transport parameters vary significantly. The transport velocity was found to be related to the variation of the conduit's cross-sectional area, with a polynomial relationship observed between transport velocity and discharge. Longitudinal dispersivity in the conduit system was not a constant value and decreased linearly with increasing flow rate due to dilution effects. Additionally, the proportion of immobile regions, arising from conduit irregularities, increased with decreasing water level in the conduit system. The results of the tracer experiments also showed that longitudinal dispersivity values were large in the unsaturated zone, ranging from 8 to 27 meters. The tailing observed in some TBCs was generated in the unsaturated zone before the tracer actually arrived at the major subsurface conduit draining the system. This work allows the estimation and prediction of key transport parameters in karst aquifers, which vary with time and flow dynamics, and reflect the geometry of the flow pathway and the origin of infiltrating recharge.