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Hydrogeology Journal

, Volume 27, Issue 1, pp 335–343 | Cite as

Laboratory and numerical study of hyporheic flow-mediated DNAPL dissolution in karst conduits

  • Yuexia WuEmail author
  • Daniel Hunkeler
  • Nico Goldscheider
Paper
  • 142 Downloads

Abstract

Dense non-aqueous phase liquids (DNAPLs) can easily penetrate into karst aquifers, may accumulate in the sediment-filled depressions of conduits, and constitute a long-term source of groundwater contamination. In contrast to porous media, DNAPL dissolution in karst settings has not received much attention. This study investigated the dissolution of sediment-entrapped DNAPLs using a laboratory analogue model corresponding to a syphon structure partly filled by sediments and using numerical simulations. The mass transfer rate increased with an increasing conduit Reynolds number (Re) and increasing conduit angle, as both of these factors led to a more intense hyporheic flow in the sediments. In addition, the mass transfer rate increased linearly with the average flow velocity in the sediment and was higher than expected for an infinite porous medium considered as a reference case. The enhanced mass transfer rate can be explained by the curved streamlines associated with hyporheic flow, which transport solutes away from the DNAPL interface. This study demonstrates that hyporheic flow through karst sediment is a possible mechanism for the mobilization of DNAPL components and might explain the persistent occurrence of such contaminants in karst springs.

Keywords

DNAPL dissolution Hyporheic flow Karst Mass transfer coefficient Laboratory experiments 

Etude numérique et en laboratoire de la dissolution de LDNA en contexte d’écoulement hyporhéique dans des conduits karstiques

Résumé

Les liquides denses en phase non aqueuse (LDNAs) peuvent facilement pénétrer dans les aquifères karstiques, peuvent s’accumuler dans les dépressions remplies de sédiments des conduits et constituer une source à long terme de contamination des eaux souterraines. Contrairement aux milieux poreux, peu d’attention n’a été porté sur la dissolution du LDNA dans des environnements karstiques. Cette étude a exploré la dissolution des LDNAs piégés au sein des sédiments à l’aide d’un modèle analogue de laboratoire correspondant à une structure d’un siphon partiellement remplie par les sédiments et de simulations numériques. Le taux de transfert de masse augmente avec un nombre croissant de Reynolds (Re) et un angle de conduit croissant, ces deux facteurs entraînant un écoulement hyporhéique plus intense dans les sédiments. De plus, le taux de transfert de masse augmente linéairement avec la vitesse moyenne de l’écoulement dans les sédiments et est plus élevé que prévu pour un milieu poreux infini considéré comme un cas de référence. Un taux de transfert de masse amélioré peut être expliqué par les lignes de flux incurvées associées à l’écoulement hyporhéique, qui transporte les solutés loin de l’interface LDNA. Cette étude démontre que l’écoulement hyporhéique à travers les sédiments karstiques est un mécanisme possible pour la mobilisation des composants de LDNA et pourrait expliquer la présence persistante de tels contaminants dans les sources karstiques.

Estudio de laboratorio y numérico de la disolución de DNAPL producida por flujo hiporreico en conductos kársticos

Resumen

Los líquidos densos en fase no acuosa (DNAPL) pueden penetrar fácilmente en los acuíferos kársticos, acumularse en las depresiones rellenadas de sedimentos de los conductos y constituir una fuente de contaminación del agua subterránea a largo plazo. A diferencia de los medios porosos, la disolución de DNAPL en ámbitos kársticos no ha recibido mucha atención. Este estudio investigó la disolución de DNAPL atrapados en sedimentos usando un modelo analógico de laboratorio que corresponde a una estructura de sifón parcialmente rellena de sedimentos y utilizando simulaciones numéricas. La tasa de transferencia de masa aumentó con un número creciente de Reynolds (Re) e incrementó el ángulo del conducto, ya que ambos factores condujeron a un flujo hiporreico más intenso en los sedimentos. Además, la tasa de transferencia de masa aumentó linealmente con la velocidad de flujo promedio en el sedimento y fue más alta de lo esperado para un medio poroso infinito considerado como un caso de referencia. La velocidad de transferencia de masa mejorada puede explicarse por las líneas de corriente curvas asociadas con el flujo hiporreico, que transportan los solutos lejos de la interfaz DNAPL. Este estudio demuestra que el flujo hiporreico a través del sedimento kárstico es un posible mecanismo para la movilización de componentes DNAPL y podría explicar la presencia persistente de dichos contaminantes en los manantiales kársticos.

岩溶管道中伏流-介导重质非水相液体溶解的实验室和数值研究

摘要

重质非水相液体可以很容易地进入岩溶含水层,可能会积聚在管道内沉积物填充的洼地中,成为长期的地下水污染源。相比于孔隙介质,岩溶环境下重质非水相液体的溶解并未受到人们很大的关切。本研究采用实验室类似物模型以及采用数值模拟调查了沉积物截获的重质非水相液体的溶解,这个类似物模型相当于弯管结构,部分充填沉积物。传质速率随着管道雷诺数的增加以及管道角度的增加而增加,因为这两个因素导致沉积物中更强烈的伏流。此外,传质速率随着沉积物中的平均水流速度线性增加,在被认为是一个参考案例的无限孔隙介质中高于预期。增强的传质速率可以由与伏流相关的弯曲河流线解释,伏流可以把溶质从重质非水相液体界面传送到别处。本研究说明,伏流通过岩溶沉积物是重质非水相液体成分活动的一个可能途径,可以解释岩溶泉中这种污染物长期存在的现象。

Estudo laboratorial e numérico da dissolução de DNAPL mediada por fluxo hiporreico em condutos cársticos

Resumo

Os líquidos densos de fase não aquosa (LDFNAs) podem facilmente penetrar em aquíferos cársticos, podendo se acumular em depressões cheias de sedimentos nos condutos, constituindo uma fonte de contaminação de longo prazo das águas subterrâneas. Em contraste com os meios porosos, a dissolução de LDFNA em ambientes cársticos não tem recebida muita atenção. Este estudo investigou a dissolução de LDFNAs aprisionados em sedimentados usando um modelo analógico de laboratório correspondente a uma estrutura de sifão parcialmente preenchida por sedimentos e usando simulações numéricas. A taxa de transferência de massa aumentou com o aumento do número de Reynolds (Re) no conduto e com o aumento do ângulo do conduto, pois ambos os fatores levaram a um fluxo hiporreico mais intenso nos sedimentos. Ademais, a taxa de transferência de massa aumentou linearmente com a velocidade média do fluxo no sedimento e foi maior do que o esperado para um meio poroso infinito considerado como um caso de referência. A taxa de transferência de massa aprimorada pode ser explicada pelas linhas de corrente encurvadas associadas ao fluxo hiporreico, que transportam os solutos para longe da interface de LDFNA. Este estudo demonstra que o fluxo hiporreico através do sedimento cárstico é um mecanismo possível para a mobilização de componentes de LDFNA e pode explicar a ocorrência persistente de tais contaminantes em nascentes cársticas.

Notes

Acknowledgements

The authors thank Prof. Chunmiao Zheng and Prof. Ligang Xu for their support, Christian Moeck for his comments, Xinchang Liu for his discussions on simulations, Huanhuan Qin for checking the derivation of the scaling analysis, and the reviewers for their helpful comments which greatly improved the manuscript.

Funding information

This work was financially supported by the National Natural Science Foundation of China. (No. 41501511, No. 41330632, and No. 41371121).

Supplementary material

10040_2018_1858_MOESM1_ESM.pdf (948 kb)
ESM 1 (PDF 947 kb)

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018
corrected publication September/2018

Authors and Affiliations

  • Yuexia Wu
    • 1
    • 2
    Email author
  • Daniel Hunkeler
    • 1
  • Nico Goldscheider
    • 3
  1. 1.Center for Hydrogeology and Geothermics (CHYN)University of NeuchâtelNeuchâtelSwitzerland
  2. 2.Key Laboratory of Watershed Geographic Sciences, Nanjing Institute of Geography & LimnologyChinese Academy of SciencesNanjingPeople’s Republic of China
  3. 3.Institute of Applied Geosciences, Division of HydrogeologyKarlsruhe Institute of Technology (KIT)KarlsruheGermany

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