Abstract
Crystalline aquifers of semi-arid southern India represent a vital water resource for farming communities. A field study is described that characterizes the hydrodynamic functioning of intensively exploited crystalline aquifers at local scale based on detailed well monitoring during one hydrological year. The main results show large water-table fluctuations caused by monsoon recharge and pumping, high spatial variability in well discharges, and a decrease of well yields as the water table decreases. Groundwater chemistry is also spatially variable with the existence of aquifer compartments within which mixing occurs. The observed variability and compartmentalization is explained by geological heterogeneities which play a major role in controlling groundwater flow and connectivity in the aquifer. The position of the water table within the fracture network will determine the degree of connectivity between aquifer compartments and well discharge. The presented aquifer conceptual model suggests several consequences: (1) over-exploitation leads to a drop in well discharge, (2) intensive pumping may contribute to the hydraulic containment of contaminants, (3) groundwater quality is highly variable even at local scale, (4) geological discontinuities may be used to assist in the location of drinking-supply wells, (5) modeling should integrate threshold effects due to water-table fluctuations.
Résumé
Les aquifères de socle des régions semi-arides du sud de l’Inde représentent une ressource en eau vitale pour les communautés agricoles. Cette étude sur un site expérimental a pour but de caractériser le fonctionnement hydrodynamique d’un aquifère de socle fortement sollicité par les pompages sur la base d’un suivi détaillé de forages durant une année hydrologique. Les résultats principaux montrent une fluctuation importante de la nappe causée par la recharge durant la mousson et les pompages, une grande variabilité spatiale des débits des forages, et une diminution des débits concomitante de la baisse du niveau de la nappe. La chimie des eaux souterraines est également variable spatialement avec l’existence de compartiments au sein desquels un mélange d’eau opère. La variabilité et compartimentation observées s’expliquent par la présence d’hétérogénéités géologiques qui contrôlent les écoulements et la connectivité de l’aquifère. La position de la nappe au sein du réseau de fractures détermine à la fois le degré de connectivité des différents compartiments et le débit des ouvrages. Le modèle conceptuel proposé de l’aquifère semble indiquer plusieurs conséquences : (1) la surexploitation conduit à une diminution des débits pompés, (2) le pompage intensif peut contribuer au confinement hydraulique des contaminants, (3) la qualité des eaux souterraines est très variable même à l’échelle locale, (4) les discontinuités géologiques peuvent être utilisées à bon escient pour l’emplacement d’ouvrages d’alimentation en eau potable, (5) les simulations doivent intégrer les effets de seuil générés par les fluctuations de nappe.
Resumen
Los acuíferos cristalinos en el sur semiárido de la India representan un recurso vital de agua para las comunidades agrícolas. Se describe un estudio de campo que caracteriza el funcionamiento hidrodinámico de los acuíferos cristalinos intensivamente explotados a la escala local basado en el monitoreo detallado de pozos durante un año hidrológico. Los principales resultados muestran grandes fluctuaciones del nivel freático causadas por la recarga monzónica y el bombeo, alta variabilidad espacial en la descarga de pozos, y un descenso de los rendimientos de los pozos a medida que los niveles freáticos se profundizan. La química del agua subterránea también es variable espacialmente con la existencia de compartimentos acuíferos dentro de los cuales existen mezclas. La variabilidad observada y la compartimentalización se explican por las heterogeneidades geológicas que juegan un rol significativo en controlar el flujo de agua subterránea y la conectividad en el acuífero. La posición del nivel freático dentro de la red de fracturas determina el grado de conectividad entre los compartimentos acuíferos y la descarga de pozos. El presente modelo conceptual de acuífero sugiere varias consecuencias: (1) la sobreexplotación conduce a una caída en la descarga de los pozos, (2) el bombeo intensivo puede contribuir a la contención hidráulica de contaminantes, (3) la calidad del agua subterránea es altamente variable aún a escala local, (4) las discontinuidades geológicas pueden ser usadas para ayudar en la ubicación de pozos de abastecimiento de agua potable, (5) el modelado debe integrar los efectos umbral debidos a las fluctuaciones de los niveles freáticos.
摘要
印度南部半干旱地区的结晶岩含水层是农业用水的重要水源。本文通过现场踏勘,依据一个水文年内的详细观测结果,在本地尺度上描述了密集开采下结晶岩含水层的水动力响应。主要结果表明,大的水位波动主要是由季风补给、开采、井排泄的高度空间变率及水位降低引起的出水量降低所致。有混合发生的含水层单元内地下水水化学也有空间变化。地质的非均质性是控制地下水流和含水层水力联系的主要因素,据此解释了观测到的变化和含水层划分。裂隙网络中水位的高低决定着含水层间的水力联系和井排泄量的大小。提出的含水层概念模型表明:(1)过量开采地下水导致井的排泄量下降; (2)强烈开采可能导致污染物集中;(3)即使在本地尺度上,地下水质的变化也较大;(4)地质不连续面有利于确定饮用水供水井的位置;(5)模型应该考虑水位波动导致的阈值效应。
Resumo
Os aquíferos cristalinos na região semi-árida do Sul da Índia representam um recurso de água vital para as comunidades agrícolas. Faz-se a descrição de um estudo de campo em que se caracteriza o funcionamento hidrodinâmico, à escala local, de aquíferos cristalinos intensivamente explorados, com base na monitorização detalhada de furos durante um ano hidrológico. Os principais resultados evidenciam grandes flutuações do nível piezométrico, provocadas pela recarga das monções e pela extração, elevada variabilidade espacial no caudal das captações e um decréscimo no caudal dos furos em função do rebaixamento dos níveis piezométricos. A química da água subterrânea também varia espacialmente com a existência de compartimentos no aquífero, dentro dos quais ocorre mistura. A variabilidade e compartimentação observadas são explicadas pelas heterogeneidades geológicas, as quais desempenham um papel fundamental no controlo do fluxo subterrâneo e na conectividade dentro do aquífero. A posição do nível piezométrico dentro da rede de fraturas vai determinar o grau de conetividade entre os compartimentos do aquífero e o caudal das captações. O modelo conceptual apresentado para o aquífero sugere várias consequências: (1) a sobre-exploração leva a uma diminuição na descarga das captações, (2) a bombagem intensiva pode contribuir para a contenção hidráulica de contaminantes, (3) a qualidade da água subterrânea é altamente variável, mesmo à escala local, (4) as descontinuidades geológicas podem ser utilizadas para auxiliar na localização de captações de abastecimento, (5) a modelação deve integrar os efeitos limite provocados pelas flutuações do nível piezométrico.
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Acknowledgements
We are indebted to the students (J. Cachera, E. Bougon, M. Ahmad) who contributed to the collection of field data and to Mr Wajiduddin for his continuous involvement in field activities. The community of Sangapur is warmly thanked for its support and interest in this research project. We are grateful to the Editors and two anonymous reviewers for their valuable comments on the original manuscript.
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Perrin, J., Ahmed, S. & Hunkeler, D. The effects of geological heterogeneities and piezometric fluctuations on groundwater flow and chemistry in a hard-rock aquifer, southern India. Hydrogeol J 19, 1189–1201 (2011). https://doi.org/10.1007/s10040-011-0745-y
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DOI: https://doi.org/10.1007/s10040-011-0745-y