Abstract
Four bench-scale solute injection experiments were conducted in a natural undisturbed basaltic fractured core of the Serra Geral Formation (SGF) (Brazil) to assess the performance of the equivalent apertures on conservative solute transport. Fracture aperture is particularly important in controlling solute transport in fractured-rock aquifers. The spatial heterogeneity of fracture apertures complicates solute transport modeling. Scientific researchers and mainly hydrogeology professionals still adopt a single value for the fracture aperture field, i.e., an equivalent aperture. However, there is a lack of information about the most suitable equivalent aperture associated with solute transport phenomena, especially for basaltic fractured aquifers. The objective of this study was to evaluate the performance of three types of equivalent apertures on conservative solute transport in a basaltic aquifer sample. The fracture aperture field was measured using the advanced high-resolution and nondestructive technique, x-ray micro-computed tomography. The performance of the equivalent apertures was evaluated using an analytical solution of the one-dimensional advection-dispersion equation (ADE) compared to observed breakthrough curves. As a result, the arithmetic mean of the measured aperture field was 111.74 ± 54.26 μm. The analytical solution of ADE coupled with mass balance aperture was able to predict breakthrough behavior, especially for low volumetric flow rates. This study demonstrates that the mass balance aperture should be adopted as an equivalent aperture instead of the hydraulic and friction loss equivalent aperture when inertial forces of macroscopic flow are negligible. The findings provide a step forward towards better understanding of groundwater contamination risk in basaltic fractured aquifers.
Résumé
Quatre expériences d’injection de soluté ont été réalisées au laboratoire dans une carotte basaltique fracturée intacte de la formation Serra Geral (SGF) (Brésil) pour évaluer la performance des ouvertures équivalentes sur le transport conservatif de soluté. L’ouverture de fractureest particulièrement importante en ce qui concerne le contrôle du transport de soluté dans les aquifères de roches fracturées. L’hétérogénéité spatiale des ouvertures de fracture complique la modélisation du transport de solutés. Les chercheurs, et principalement les praticiens en hydrogéologie, adoptent toujours une valeur unique pour le champ d’ouverture de fracture, c’est-à-dire une ouverture équivalente. Cependant, on manque d’information à propos des ouvertures équivalentes les plus adaptées au processus de transport de soluté, particulièrement pour les aquifères basaltiques fracturés. L’objectif de cette étude était d’évaluer la performance de trois types d’ouvertures de fracture équivalentes sur le transport conservatif de soluté dans un échantillon d’aquifère basaltique. Le champ d’ouverture de fracture a été mesuré en utilisant la technique de pointe, haute résolution et non destructive, de tomographie à rayon-x par micro-ordinateur. La performance des ouvertures équivalentes a été évaluée par comparaison des résultats d’une solution analytique de l’équation d’advection-dispersion unidimensionnelle (ADE) avec les courbes de restitution observées. En termes de résultats, la moyenne arithmétique du champ d’ouverture mesuréest de 111.74 ± 54.26 μm. La solution analytique d’ADE couplée avec l’ouverture par bilan de masse a été capable de prédire le comportement en termes de restitution, notamment pour les faibles débits volumétriques. Cette étude démontre que l’ouverture par bilan de masse devrait être adoptée comme ouverture de fracture équivalente plutôt que l’ouverture équivalente par perte de friction et hydraulique, lorsque les forces inertielles du flux macroscopique sont négligeables. Ces résultats constituent un progrès en vue de la meilleure compréhension du risque de contamination des eaux souterraines dans les aquifères basaltiques fracturés.
Resumen
Se realizaron cuatro experimentos de inyección de soluto a escala de laboratorio en un núcleo basáltico natural fracturado y no perturbado de la Formación Serra Geral (SGF) (Brasil) para evaluar el funcionamiento de las aberturas equivalentes en el transporte de un soluto conservativo. La abertura de la fractura es particularmente importante para controlar el transporte de solutos en acuíferos de rocas fracturadas. La heterogeneidad espacial de las aberturas de las fracturas complica el modelado del transporte de solutos. Los investigadores científicos y principalmente los profesionales de la hidrogeología todavía adoptan un valor único para el campo de apertura de la fractura, es decir, una abertura equivalente. Sin embargo, se carece de información sobre la abertura equivalente más adecuada asociada a los fenómenos de transporte de solutos, especialmente en el caso de acuíferos basálticos fracturados. El objetivo de este estudio fue evaluar el desempeño de tres tipos de aberturas equivalentes en el transporte de un soluto conservativo en una muestra de acuífero basáltico. El campo de la abertura de la fractura se midió utilizando una técnica avanzada de alta resolución y no destructiva, la tomografía microcomputada de rayos X. El funcionamiento de las aberturas equivalentes se evaluó utilizando una solución analítica de la ecuación unidimensional de advección-dispersión (ADE) comparada con las curvas de avance observadas. Como resultado, la media aritmética del campo de abertura medido fue 111.74 ± 54.26 μm. La solución analítica de ADE junto con la abertura del balance de masa fue capaz de predecir el comportamiento de penetración, especialmente para caudales volumétricos bajos. Este estudio demuestra que la abertura del balance de masa debe ser adoptada como una abertura equivalente en lugar de la abertura hidráulica y la pérdida de fricción equivalente cuando las fuerzas inerciales del flujo macroscópico son insignificantes. Los resultados proporcionan un paso adelante hacia una mejor comprensión del riesgo de contaminación de las aguas subterráneas en los acuíferos basálticos fracturados.
摘要
为评估等效开度对保守性溶质运移的效果,利用Serra Geral构造(SGF)(巴西)天然未扰动的玄武岩裂缝岩心开展了四次实验室规模的溶质注入实验。裂缝开度对控制裂隙岩含水层中溶质运移尤为重要。裂缝开度的空间非均匀性使溶质运输模型复杂化。科学研究人员和大多数水文地质专业人员仍视裂缝开度为定值,即等效开度。然而,缺乏与溶质运移现象相关的最适合的等效开度信息,特别是在玄武岩裂缝含水层中。本研究的目的是评估玄武岩含水层样品中保守性溶质运移的三种等效开度的效果。采用先进的高分辨率和非破坏性技术(X射线计算机断层成像)来测量裂缝开度。使用一维对流弥散方程(ADE)的解析解与观察到的穿透曲线相比较来评估等效开度的效果。结果发现,测量的开度算术平均值为111.74 ± 54.26 μm。 耦合质量平衡开度的ADE解析解可以预测穿透行为,特别是对于低体积流量。该研究表明当宏观流动的惯性力可以忽略不计时,质量平衡开度应采用等效开度,而不是水力和摩擦损失等效开度。这些发现有助于更进一步理解玄武岩裂缝含水层中的地下水污染风险。
Resumo
Quatro experimentos de injeção de soluto, em escala de bancada, foram conduzidos em uma amostra basáltica fraturada, natural, indeformada da Formação Serra Geral (FSG) (Brasil) para avaliar o desempenho das aberturas equivalentes no transporte conservativo de soluto. A abertura da fratura é particularmente importante no controle do transporte de soluto em aquíferos de rocha fraturada. A heterogeneidade espacial das aberturas da fratura complica a modelagem do transporte de soluto. Pesquisadores científicos e, principalmente, profissionais da hidrogeologia ainda adotam um valor único para o campo de abertura, p.ex., uma abertura equivalente. No entanto, há uma falta de informação sobre a abertura equivalente mais adequada, associada ao fenômeno do transporte de soluto, especialmente para aquíferos basálticos fraturados. O objetivo deste estudo foi avaliar o desempenho de três aberturas equivalentes no transporte conservativo de soluto em uma amostra de aquífero basáltico. O campo de abertura da fratura foi medido usando uma técnica avançada de alta resolução e não destrutiva, a microtomografia de raios-x. O desempenho das aberturas equivalentes foi avaliado usando uma solução analítica da equação unidimensional de advecção-dispersão (EAD) comparada às curvas de passagem observadas. Como resultado, a média aritmética do campo de abertura medido foi 111.74 ± 54.26 μm. A solução analítica da EAD em conjunto com a abertura de balanço de massa foi capaz de predizer o comportamento das curvas de passagem, principalmente para baixas taxas de escoamento volumétrico. Este estudo demonstra que a abertura de balanço de massa deveria ser adotada como a abertura equivalente, ao invés da abertura equivalente hidráulica e de perda por atrito, quando as forças inerciais macroscópicas são negligenciáveis. As descobertas fornecem um passo à frente em direção à melhor compreensão do entendimento do risco de contaminação das águas subterrâneas em aquíferos basálticos fraturados.
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The first author would like to thank the National Council for Scientific and Technological Development (CNPq – Conselho Nacional de Desenvolvimento Científico e Tecnológico) for financial support (grant agreement numbers 141877/2012-0 and 481477/2012-8).
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The original version of this article was revised: There was an error during production which resulted in the alpha symbol (α) being omitted from the article in some locations.
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Lucas, M., Cantareira, G.D. & Wendland, E. Solute transport performance analysis of equivalent apertures in a single undisturbed basaltic fracture. Hydrogeol J 27, 1999–2010 (2019). https://doi.org/10.1007/s10040-019-01960-x
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DOI: https://doi.org/10.1007/s10040-019-01960-x