Abstract
Tracer concentration data from field experiments conducted in several carbonate aquifers (Malaga province, southern Spain) were analyzed following a dual approach based on the graphical evaluation method (GEM) and solute transport modeling to decipher flow mechanisms in karst systems at regional scale. The results show that conduit system geometry and flow conditions are the principal factors influencing tracer migration through the examined karst flow routes. Solute transport is mainly controlled by longitudinal advection and dispersion throughout the conduit length, but also by flow partitioning between mobile and immobile fluid phases, while the matrix diffusion process appears to be less relevant. The simulation of tracer breakthrough curves (BTCs) suggests that diffuse and concentrated flow through the unsaturated zone can have equivalent transport properties under extreme recharge, with high flow velocities and efficient mixing due to the high hydraulic gradients generated. Tracer mobilization within the saturated zone under low flow conditions mainly depends on the hydrodynamics (rather than on the karst conduit development), which promote a lower longitudinal advection and retardation in the tracer migration, resulting in a marked tailing effect of BTCs. The analytical advection-dispersion equation better approximates the effective flow velocity and longitudinal dispersion estimations provided by the GEM, while the non-equilibrium transport model achieves a better adjustment of most asymmetric and long-tailed BTCs. The assessment of karst underground flow properties from tracing tests at regional scale can aid design of groundwater management and protection strategies, particularly in large hydrogeological systems (i.e. transboundary carbonate aquifers) and/or in poorly investigated ones.
Résumé
Les données de concentration de traceurs provenant d’expériences de terrain menées dans plusieurs aquifères carbonatés (province de Malaga, sud de l’Espagne) ont été analysées en suivant une approche duale basée sur la méthode d’évaluation graphique (MEG) et la modélisation du transport de soluté pour déchiffrer les mécanismes d’écoulement dans les systèmes karstiques à l’échelle régionale. Les résultats montrent que la géométrie du système de conduits et les conditions d’écoulement sont les principaux facteurs qui influencent la migration du traceur par les voies d’écoulement karstiques examinées. Le transport de soluté est. principalement contrôlé par l’advection longitudinale et la dispersion sur toute la longueur du conduit, mais aussi par le partage d’écoulement entre les phases fluides mobiles et immobiles, alors que le processus de diffusion matricielle semble moins pertinent. La simulation des courbes de percée du traceur (CPT) suggère que l’écoulement diffus et concentré à travers la zone non saturée peut avoir des propriétés de transport équivalentes pour une recharge extrême, avec des vitesses d’écoulement élevées et un mélange efficace en raison des gradients hydrauliques élevés générés. La mobilisation des traceurs dans la zone saturée dans des conditions d’écoulement faible dépend principalement de l’hydrodynamique (plutôt que du développement du conduit karstique), ce qui favorise une advection longitudinale inférieure et un retard dans la migration du traceur, entrainant un effet de traine marqué des CPT. L’équation analytique de l’advection-dispersion permet une meilleure approximation de la vitesse d’écoulement effective et des estimations de dispersion longitudinale fournies par le MEG, alors que le modèle de transport sans équilibre réalise un meilleur ajustement de la plupart des CPT asymétriques et à longue traine. L’évaluation des propriétés de l’écoulement souterrain karstique à partir des essais de traçage à l’échelle régionale peut aider à concevoir des stratégies de gestion et de protection des eaux souterraines, en particulier dans les grands systèmes hydrogéologiques (c-à-d. les aquifères carbonatés transfrontaliers) et/ou dans des systèmes peu étudiés.
Resumen
Se analizaron los datos de concentración de trazadores artificiales procedentes de varios ensayos realizados en algunos acuíferos carbonáticos de la provincia de Málaga (sur de España), mediante una aproximación dual basada en el método de evaluación gráfica (GEM) y transporte de solutos, para descifrar los mecanismos de flujo en sistemas kársticos a escala regional. Los resultados muestran que la geometría de los sistemas de conductos y las condiciones de flujo son los principales factores que influyen en la movilización del trazador a través de las líneas de flujo kárstico estudiadas. El transporte de solutos está controlado principalmente por procesos de advección y dispersión longitudinal a lo largo de los conductos, pero también por la partición del flujo entre fases móviles e inmóviles, mientras que el proceso de difusión desde la matriz de la roca parece ser menos relevante. La simulación de las curvas de paso de trazadores (BTCs) sugiere que el flujo difuso y concentrado a través de la zona no saturada puede tener propiedades equivalentes de transporte en condiciones extremas de recarga, con altas velocidades de flujo y una mezcla eficiente de flujos de agua como consecuencia de los elevados gradientes hidráulicos que se generan. La movilización del trazador dentro la zona saturada en condiciones de aguas bajas depende principalmente de las características hidrodinámicas (en lugar del desarrollo de los conductos kársticos), que promueven una menor advección longitudinal y un mayor retraso en el movimiento del trazador, lo que se traduce en BTCs con colas pronunciadas. La ecuación analítica de advección-dispersión se aproxima mejor a las estimaciones de la velocidad efectiva de flujo y de la dispersión longitudinal proporcionadas por el GEM, mientras que el modelo de transporte de no-equilibrio consigue un mejor ajuste de la mayoría de las BTCs asimétricas y con colas pronunciadas. La evaluación de las propiedades de los flujos subterráneos kársticos a partir de pruebas de trazadores a escala regional puede ayudar al diseño de estrategias de gestión y protección de las aguas subterráneas, particularmente en grandes sistemas hidrogeológicos (por ejemplo, en acuíferos carbonáticos transfronterizos) y/o poco investigados.
摘要
遵循基于图形评估方法和溶质运移模拟的双重方法分析了(西班牙南部Malaga省)几个碳酸盐含水层野外实验获取的示踪剂浓度数据,以在区域尺度上解译岩溶系统是水流机理。结果显示,通道系统的几何结构和水流条件是影响示踪剂通过岩溶水流路径迁移的主要因素。溶质运移主要受到整个通道长度内纵向平流和离散的控制,但是也受到移动和固定相之间水流分区的控制,而基质弥散过程似乎关联性较小。示踪剂突破曲线模拟表明,由于产生的很高的水力梯度,在伴有很高流速和有效混合的极端补给条件下,通过非饱和带的离散和集中水流可能有相等的传输特性。低水流条件下饱和带内的示踪剂活动化主要取决于水动力学(而不是取决于岩溶通道的发育),水动力学促进示踪剂迁移中较低的纵向平流和延迟,导致示踪剂突破曲线有明显的拖尾现象。解析平流-离散方程可以更好地估计图像评估方法得到的有效水流速度和纵向离散估算结果,而非平衡运移模型能够更好地调整大多数不对称和长尾示踪剂突破曲线。通过区域尺度示踪实验得到的岩溶地下水流特性评价结果可支持谋划地下水管理和保护战略,特别是在大的水文地质系统中(如跨边界碳酸盐含水层)及/或欠调查的水文地质系统中尤其如此。
Resumo
Dados de concentração de traçadores de experimentos conduzidos em diversos aquíferos carbonáticos (província de Málaga, sul da Espanha) foram analisados seguindo uma abordagem dual baseada no método de avaliação gráfica (MAG) e modelagem de transporte de soluto para decifrar os mecanismos de fluxo em sistemas cársticos em escala regional. Os resultados mostram que a geometria do sistema de conduto e as condições de fluxo são os principais fatores que influenciam a migração do traçador através das rotas de fluxo cárstico examinadas. O transporte de soluto é controlado principalmente por advecção longitudinal e dispersão ao longo do comprimento do conduto, mas também por particionamento do fluxo entre as fases de fluido móvel e imobilizado, enquanto o processo de difusão da matriz parece ser menos relevante. A simulação das curvas de identificação (CI) do traçador sugere que o fluxo difuso e concentrado através da zona não saturada pode ter propriedades de transporte equivalentes sob recarga extrema, com altas velocidades de fluxo e mistura eficiente devido aos altos gradientes hidráulicos gerados. A mobilização do traçador dentro da zona saturada em condições de baixo fluxo depende principalmente da hidrodinâmica (e não do desenvolvimento do canal cárstico), que promove uma advecção e atraso longitudinal mais baixos na migração do traçador, resultando em um efeito de cauda marcado nas CI. A equação analítica de advecção e dispersão se aproxima melhor das estimativas de velocidade de fluxo efetivas e de dispersão longitudinal fornecidas pelo MAG, enquanto o modelo de transporte sem equilíbrio alcança um melhor ajuste da maioria das CI assimétricas e de cauda longa. A avaliação das propriedades de fluxo subterrâneo cárstico a partir de testes de traçadores em escala regional pode auxiliar na elaboração de estratégias de gestão e proteção de águas subterrâneas, particularmente em sistemas hidrogeológicos grandes (isto é, aquíferos carbonáticos transfronteiriços) e/ou em meios pouco investigados.
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Acknowledgements
This research was funded by the projects P06-RNM 2161 of Junta de Andalucía, CGL2008-06158 BTE, CGL2012-32590 and CGL2015-65858-R of DGICYT, as well as by the Research Group RNM-308 of the Junta de Andalucía. We sincerely thank Ute Lauber for her kind and selfless support in solute transport modeling tasks. The authors also thank two anonymous reviewers for their constructive criticism, which contributed to improving the original version of the manuscript. Improvements and suggestions by associate editor Chris Groves are also much appreciated. Finally, we would like to acknowledge Jean Sanders for careful review of the English text.
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Barberá, J.A., Mudarra, M., Andreo, B. et al. Regional-scale analysis of karst underground flow deduced from tracing experiments: examples from carbonate aquifers in Malaga province, southern Spain. Hydrogeol J 26, 23–40 (2018). https://doi.org/10.1007/s10040-017-1638-5
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DOI: https://doi.org/10.1007/s10040-017-1638-5