Assessment of key transport parameters in a karst system under different dynamic conditions based on tracer experiments: the Jeita karst system, Lebanon | Evaluation des paramètres clefs du transport dans un système karstique sous différentes conditions dynamiques à partir des essais de traçage: le système karstique de Jeita, Liban Evaluación de parámetros de transporte claves en un sistema kárstico bajo diferentes condiciones dinámicas basadas en experimentos con trazadores: el sistema kárstico de Jeita, Líbano 根据示踪实验评价岩溶系统中不同动力条件下关键运移参数: 黎巴嫩Jeita岩溶系统 Avaliação dos principais parâmetros em um sistema cárstico sob diferentes condições dinâmicas baseado em experimentos com traçadores: sistema cárstico Jeita, Líbano
2018
Doummar, Joanna | Margane, Armin | Geyer, Tobias | Sauter, Martin
Artificial tracer experiments were conducted in the mature karst system of Jeita (Lebanon) under various flow conditions using surface and subsurface tracer injection points, to determine the variation of transport parameters (attenuation of peak concentration, velocity, transit times, dispersivity, and proportion of immobile and mobile regions) along fast and slow flow pathways. Tracer breakthrough curves (TBCs) observed at the karst spring were interpreted using a two-region nonequilibrium approach (2RNEM) to account for the skewness in the TBCs’ long tailings. The conduit test results revealed a discharge threshold in the system dynamics, beyond which the transport parameters vary significantly. The polynomial relationship between transport velocity and discharge can be related to the variation of the conduit’s cross-sectional area. Longitudinal dispersivity in the conduit system is not a constant value (α = 7–10 m) and decreases linearly with increasing flow rate because of dilution effects. Additionally, the proportion of immobile regions (arising from conduit irregularities) increases with decreasing water level in the conduit system. From tracer tests with injection at the surface, longitudinal dispersivity values are found to be large (8–27 m). The tailing observed in some TBCs is generated in the unsaturated zone before the tracer actually arrives at the major subsurface conduit draining the system. This work allows the estimation and prediction of the key transport parameters in karst aquifers. It shows that these parameters vary with time and flow dynamics, and they reflect the geometry of the flow pathway and the origin of infiltrating (potentially contaminated) recharge.
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