NDLI: Diagnostic hydrogeologic characteristics of a karst aquifer (Kentucky, USA)

Content Provider	Springer Nature Link
Author	Worthington, Stephen R. H.
Copyright Year	2009
Abstract	Laboratory experiments and numerical modeling have shown that dissolution in carbonate aquifers results in high-permeability channel networks. However, the lack of techniques to assess the extent and significance of these channel networks presents a major problem in characterizing carbonate aquifers. This problem was addressed by identifying the differences between two simulations (with and without channels) of the intensely studied limestone aquifer at Mammoth Cave (Kentucky, USA). Long-distance tracer-test results and spring discharges were used for assessing model accuracy as well as head measurements in wells. The channel simulation provided a much better calibration than the homogeneous porous-medium simulation and revealed five important differences: (1) convergent flow to large springs, (2) equipotentials forming troughs, (3) decreases in hydraulic gradient and (4) increases in hydraulic conductivity in a downgradient direction, and (5) substantial scaling effects. These five characteristics are also common in other carbonate aquifers and provide a means of identifying whether a carbonate aquifer is more similar to porous-medium or to karst-aquifer end members.Les essais en laboratoire et les modélisations numériques ont démontré que la dissolution dans les aquifères carbonatés aboutit à la formation de réseaux chenalisés de très forte perméabilité. Cependant, le manque de méthodes permettant d’estimer l’extension et l’importance de ces réseaux de chenaux constitue un problème majeur pour la caractérisation des aquifères carbonatés. Ce problème a été abordé en identifiant les différences existant entre deux simulations (avec et sans chenaux) de l’aquifère calcaire de Mammoth Cave (Kentucky, Etats-Unis), déjà considérablement étudié. Les résultats de traçages longue distance et les débits des résurgences ont été utilisés pour tester la précision du modèle, de même que les mesures de niveau dans les puits. La simulation qui intègre des chenaux a produit une meilleure calibration que la simulation en milieu poreux homogène, et a mis en exergue cinq différences importantes : (1) un flux convergent en direction des sources majeures, (2) des équipotentielles formant des dépressions, (3) des décroissances du gradient hydraulique, (4) une augmentation des transmissivités vers l’aval hydraulique, et (5) des effets d’échelle marqués. Ces cinq caractéristiques sont également communes aux autres aquifères carbonatés, et fournissent un moyen d’identifier si un aquifère carbonaté est plus proche d’un milieu poreux ou au contraire du pôle constitué par les aquifères karstiques.Los experimentos de laboratorio y los modelados numéricos han mostrado que la disolución en acuíferos carbonáticos da como resultado redes de canales de alta permeabilidad. Sin embargo la falta de técnicas para evaluar la extensión y el significado de estas redes de canales es un problema mayor en la caracterización de los acuíferos carbonáticos. Este problema fue evaluado identificando las diferencias entre dos simulaciones (con y sin canales) del intensamente estudiado acuífero de calizas en Mammoth Cave (Kentucky, EEUU). Los resultados de pruebas de trazadores de grandes distancias y las descargas de manantiales fueron usados para evaluar la precisión del modelo así como las mediciones de niveles en los pozos. La simulación del canal proveyó una mucho mejor calibración que la simulación de un medio poroso homogéneo y reveló cinco diferencias importantes: (1) flujo convergente hacia los grandes manantiales, (2) equipotenciales que forman depresiones, (3) disminución en el gradiente hidráulico y (4) aumento en la conductividad hidráulica en sentido gradiente abajo, y (5) efecto sustancial de escala. Estas cinco características son también comunes en otros acuíferos carbonáticos y proveen un medio de identificar si un acuífero carbonático es más similar a un medio poroso o a un estado final de un acuífero kárstico.实验室实验和数值模拟表明碳酸盐岩含水层里的溶解作用导致了岩层里高渗透性的管道系统。但是, 缺少评价这些管道系统的发育程度和重要性的方法是目前刻画碳酸盐含水层特征中存在的一个主要问题。这个问题已在对广泛研究过的美国肯塔基州Mammoth Cave的灰岩含水层进行的两个模拟 (有管道和无管道) 的不同中表述出来。利用长距离示踪试验结果和泉的排泄量来评价模型的准确度和井中水头的测量值。管道模拟比均质多孔介质模拟能提供更好的校正, 并揭示出5个重要的不同之处: 1) 流向大泉的收敛流; 2) 等势线形成水槽; 3) 水力梯度的减小; 4) 梯度减小方向上水力传导度的增大; 5) 本质上存在的尺度效应。这五个特征在其它碳酸盐含水层中也同样很普遍, 且为判断一个碳酸盐岩含水层与多孔介质或岩溶含水层端元何者更为类似提供了一种方法。Experiências laboratoriais e modelação numérica demonstraram que a dissolução em aquíferos carbonatados dá origem a redes de canais de alta permeabilidade. Contudo, a inexistência de técnicas para caracterizar a extensão e o grau de significado destas redes de canais constitui o maior problema na caracterização de aquíferos carbonatados. Este problema foi abordado através da identificação das diferenças entre duas simulações (com e sem canais) no intensamente estudado aquífero calcário de Mammoth Cave (Kentucky, USA). Foram utilizados os resultados de ensaios com traçadores de longa distância e os caudais de nascentes na caracterização da precisão dos modelos, bem como os valores de potencial hidráulico em furos. A simulação considerando a rede de canais conduziu a uma calibração muito melhor do que a que considerou um meio poroso médio equivalente e revelou cinco diferenças importantes: (1) fluxo convergente em direcção às grandes nascentes, (2) equipotenciais formando depressões alongadas, (3) decréscimos no gradiente hidráulico, (4) aumentos na condutividade hidráulica no sentido do escoamento, e (5) efeitos de escala substanciais. Estas cinco características são também comuns a outros aquíferos carbonatados e constituem uma forma de identificar se um aquífero carbonatado é mais semelhante a um meio poroso ou aos termos finais de um aquífero cársico.
Starting Page	1665
Ending Page	1678
Page Count	14
File Format	PDF
ISSN	14312174
Journal	Hydrogeology Journal
Volume Number	17
Issue Number	7
e-ISSN	14350157
Language	Portuguese
Publisher	Springer-Verlag
Publisher Date	2009-06-09
Publisher Institution	International Association of Hydrogeologists
Publisher Place	Berlin, Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Carbonate rocks Conceptual models Karst Numerical models USA Waste Water Technology Water Pollution Control Water Management Aquatic Pollution Geology Hydrogeology
Content Type	Text
Resource Type	Article
Subject	Earth and Planetary Sciences Water Science and Technology

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