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

  • Joanna Doummar
  • Armin Margane
  • Tobias Geyer
  • Martin Sauter
Report

Abstract

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.

Keywords

Tracer tests Two-region nonequilibrium model Karst 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

Résumé

Des essais de traçage artificiels ont été réalisés dans le système karstique mature de Jeita (Liban) pour différentes conditions d’écoulement en utilisant des points d’injection de surface et souterrain, afin de déterminer la variation des paramètres de transport (atténuation du pic de concentration, vitesse, temps de transit, dispersivité, et proportion des régions d’eau mobile et immobile) le long de voies d’écoulement rapides et lentes. Les courbes de percée des traceurs (CBT) observées à la source karstique ont été interprétées à l’aide d’une approche de non équilibre comprenant deux régions (2RNEM) pour prendre en considération l’asymétrie des en utilisant une approche de nonéquilibre à deux régions (2RNEM) pour tenir compte de l’asymétrie des longs résidus des CBT. Les résultats de l’essai en conduit a mise en évidence un effet de seuil du débit dans la dynamique du système, au-delà duquel les paramètres du transport varient de manière significative. La relation polynomiale entre la vitesse de transport et le débit peut être associée à la variation de la surface de la section transverse du conduit. La dispersivité longitudinale dans le système de conduits n’est. pas une valeur constante (α = 7–10 m) et diminue de manière linéaire avec l’augmentation du débit à cause des effets de dilution. De plus, la proportion de régions d’eau immobile (résultant des irrégularités des conduits) augmente avec la diminution du niveau d’eau dans le système de conduits. A partir des essais de traçage injectés en surface, les valeurs de dispersivité longitudinale sont comprises entre 8 et 27 m. L’effet résiduel observé dans certaines courbes de percée est. généré dans la zone non saturée avant l’arrivée du traceur dans le conduit principal du système de drainage souterrain. Ce travail permet d’estimer et de prédire les paramètres clefs du transport au sein des aquifères karstiques. Il montre que ces paramètres varient avec le temps et l’hydrodynamique, et qu’ils sont le reflet de la géométrie des voies d’écoulement et de l’origine de la recharge (potentiellement contaminée) qui s’infiltre.

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

Resumen

Se realizaron experimentos con trazadores artificiales en el sistema kárstico maduro de Jeita (Líbano) bajo diversas condiciones de flujo utilizando puntos de inyección superficial y subsuperficial del trazador, para determinar la variación de los parámetros de transporte (atenuación de la concentración máxima, velocidad, tiempos de tránsito, dispersividad y proporción de regiones móviles e inmóviles) a lo largo de vías de flujo rápido y lento. Las curvas de avance del trazador (TBC) observadas en el manantial kárstico se interpretaron utilizando un enfoque de no equilibrio de dos regiones (2RNEM) para explicar la asimetría en los relaves largos de los TBC. Los resultados de la prueba del conducto revelaron un umbral de descarga en la dinámica del sistema, más allá del cual los parámetros de transporte varían significativamente. La relación polinómica entre la velocidad de transporte y la descarga puede estar relacionada con la variación del área de sección transversal del conducto. La dispersividad longitudinal en el sistema de conductos no es un valor constante (α = 7–10 m) y disminuye linealmente al aumentar la velocidad de flujo debido a los efectos de dilución. Además, la proporción de regiones inmóviles (que surgen de las irregularidades del conducto) aumenta con la disminución del nivel del agua en el sistema del conducto. A partir de las pruebas de trazador con inyección en la superficie, los valores de dispersividad longitudinal son grandes (8–27 m). El seguimiento observado en algunos TBC se genera en la zona no saturada antes de que el trazador llegue realmente al conducto subsuperficial principal que drena el sistema. Este trabajo permite la estimación y predicción de los parámetros claves de transporte en los acuíferos kársticos. Muestra que estos parámetros varían con el tiempo y la dinámica del flujo, y reflejan la geometría de la trayectoria de flujo y el origen de la recarga (potencialmente contaminada) de la infiltración.

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

摘要

利用地表和地下示踪剂注入点在(黎巴嫩)成熟的Jeita岩溶系统进行了各种水流条件下的人工示踪试验,以确定沿快速和慢速水流通道运移参数(最高浓度的衰减、速度、经过时间、弥散性以及非流动和流动地区的比例)的变化。采用两个地区不平衡方法解译了岩溶泉观测到的示踪突破曲线,以说明示踪突破曲线中长距离回线偏态的原因。管道试验结果揭示了系统动力学中的排泄阀值,超过这个阀值,运移参数变化很大。运移速度和排泄之间的多项式关系可与管道界面面积的变化相关。管道系统中的纵向弥散率不是一个常数值(α =7–10 m),由于稀释效应,随着水流量的增加而线性减少。此外,(管道不规则行为引起的)非流动区比例随着管道系统中水位的降低而增加。从地面示踪注入试验中看出,发现纵向弥散值很大(8–27 m)。在示踪剂到达主要地面以下系统排泄管道之前,在一些示踪突破曲线中观测到的回线就已经生成。这个行为可以估算和预测岩溶含水层中关键的运移参数。显示出这些参数随着时间和水流动力学条件而变化,反映出水流通道的几何学状态以及入渗(可能污染的)补给的成因。

Avaliação dos principais parâmetros em um sistema cárstico sob diferentes condições dinâmicas baseado em experimentos com traçadores: sistema cárstico Jeita, Líbano

Resumo

Experimentos com traçadores artificiais foram conduzidos no sistema cárstico desenvolvido de Jeita (Líbano) sob várias condições de fluxo usando traçadores com pontos de injeção superficiais e subterrâneas para determinar a variação de parâmetros de transporte (atenuação do pico de concentração, velocidade, tempo de trânsito, dispersão e proporção de regiões móveis e imóveis) ao longo de trajetos de fluxo rápido e lento. Curvas de identificação de traçadores (CITs) observadas na nascente cárstica foram interpretadas usando uma abordagem não equilibrada de duas regiões (2RNEM) para explicar a assimetria nas longas caudas das CIT. O teste de conduto revelou uma descarga limitante na dinâmica do sistema, além de que os parâmetros de transporte variam significantemente. A relação polinomial entre velocidade de transporte e descarga pode ser relacionada à variação da área de conduto transversal. A dispersividade longitudinal no sistema de condutos não é um valor constane (α = 7–10 m) e reduz linearmente com o aumento da taxa de fluxo por causa do efeito da diluição. Adicionalmente, a proporção de regiões imóveis (decorrentes de irregularidades dos condutos) aumenta com a redução do nível da água no sistema. Nos testes com traçadores com injeção na superfície, os valores de dispersividade longitudinal são grandes (8–27 m). As caudas observadas em algumas CITs são geradas na zona insaturada antes que o traçador realmente alcance o principal canal subterrâneo que drena o sistema. Este trabalho permitiu a estimativa e predição do principal parâmetro de transporte em aquíferos cársticos. Isto mostra que estes parâmetros variam com o tempo e dinâmica de fluxos, e eles refletem a geometria do trajeto do fluxo e origem da infiltração de recarga (potencialmente contaminada).

Notes

Acknowledgements

Data were collected within the context of the cooperation project “Protection of the Jeita Spring” funded by the Bundesanstalt für Geowissenschaften and Rohstoffe (BGR; Germany) in Lebanon.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  • Joanna Doummar
    • 1
  • Armin Margane
    • 2
  • Tobias Geyer
    • 3
  • Martin Sauter
    • 3
  1. 1.Department of GeologyAmerican University of BeirutBeirutLebanon
  2. 2.Bundesanstalt für Geowissenschaften und Rohstoffe (BGR)HannoverGermany
  3. 3.Geoscientific CentreUniversity of GöttingenGöttingenGermany

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