Abstract
Field evidence of hydraulic connections between a bedrock aquifer and an overlying granular aquifer in the Canadian Shield (Grenville Province) is presented. This issue is rarely considered and investigated despite its important hydraulic and chemical consequences and its widespread occurrence worldwide. The methodology employed is based on complementary field tests conducted at specific experimental sites instrumented both in the rock and in the overlying deposits. One of the bedrock sites revealed a natural hydraulic connection with the overlying granular aquifer caused by the weathered surface of the uppermost bedrock. Another site revealed an artificial hydraulic connection between the bedrock and the granular aquifer created by an improperly sealed casing. A regional study showed that hydraulic connections yield an erroneous interpretation of the true hydraulic properties of the tested aquifer. The detection of hydraulic connections is therefore essential to properly define well-capture areas and contamination conditions. It is recommended to practitioners that pumping tests be performed as well as hydrochemical comparisons of each existing aquifer unit. Falling-head permeability tests are also helpful in verifying the quality of the seal at the bedrock–casing contact. More effective procedural controls and better well-construction practices are necessary to reduce the risks of cross-contamination induced by defective seals.
Résumé
Des preuves de terrain de connexions hydrauliques entre l’aquifère de socle et l’aquifère sédimentaire détritique sus-jacent dans le bouclier canadien (province de Grenville) sont présentées. Cette question est rarement considérée et étudiée en dépit de ses conséquences importantes du point de vue hydraulique et chimique et de la présence de ce type de configuration très largement répandue dans le monde entier. La méthodologie utilisée est basée sur des tests complémentaires menés sur le terrain sur des sites expérimentaux spécifiques instrumentés à la fois dans les formations de socle et dans les dépôts sus-jacents. Un des sites en contexte de socle a révélé une connexion hydraulique naturelle avec l’aquifère sédimentaire détritique sus-jacent du fait de la surface altérée de la partie supérieure du substratum rocheux. Un autre site a révélé une connexion hydraulique artificielle entre le substratum rocheux et l’aquifère sédimentaire détritique créée par un tubage scellé de manière inappropriée. Une étude régionale a montré que les connexions hydrauliques conduisent à une interprétation erronée des véritables propriétés hydrauliques de l’aquifère testé. La détection des connexions hydrauliques est donc essentielle afin de définir de manière adéquate les zones de captage et les conditions de contamination. Il est recommandé aux praticiens d’effectuer des essais de pompage ainsi que des comparaisons hydrochimiques au niveau de chaque unité aquifère. Des tests de perméabilité à charge variable sont également utiles pour vérifier la qualité de l’étanchéité entre le substratum et le tubage du forage. Des contrôles de procédures plus efficaces et de meilleures pratiques de réalisation de forages sont nécessaires pour réduire les risques de contamination induits par des défauts d’étanchéité au niveau des tubages du forage.
Resumen
Se presentan evidencias de campo de las conexiones hidráulicas entre un acuífero de basamento y un acuífero granular suprayacente en el escudo canadiense (Provincia de Grenville). Este tema es raramente considerado e investigado a pesar de las importantes consecuencias hidráulicas e hidroquímicas y su amplia ocurrencia en todo el mundo. La metodología empleada está basada en ensayos complementarios de campo realizados en sitios experimentales específicos seleccionados tanto en la roca como en los depósitos suprayacentes. Uno de los sitios de la roca de basamento reveló una conexión hidráulica natural con el acuífero granular suprayacente provocado por la superficie meteorizada de la parte más alta del basamento. Otro sitio reveló una conexión hidráulica artificial entre el basamento y el acuífero granular creado por un caño camisa mal sellado. Un estudio regional mostró que las conexiones hidráulicas dan una interpretación errónea de las verdaderas propiedades hidráulicas del acuífero ensayado. La detección de las conexiones hidráulicas es asimismo esencial para definir adecuadamente las áreas de captura de los pozos y las condiciones de contaminación. Se recomienda a los operarios la realización de ensayos de bombeo, así como comparaciones hidroquímicas de cada una de las unidades acuíferas existentes. Las pruebas de permeabilidad con caída de carga hidráulica son también útiles para verificar la calidad del sello en el contacto entre la roca y el caño camisa. Se necesitan controles de procedimientos más efectivos y mejores prácticas de construcción de pozos para reducir los riesgos de contaminación cruzada inducidos por sellados defectuosos.
摘要
本文介绍了加拿大地盾(格林威尔区)一个基岩含水层和其上覆的粒状含水层之间水力联系的现场证据。尽管水力上和化学上具有重要影响以及世界上分布很广,但对这个问题很少进行研究和调查。所使用的方法基于基岩和其上覆的沉积层中特定试验场进行的互补性现场试验。一个基岩试验场揭示了由于最上层基岩的风化面造成的基岩含水层和上覆粒状含水层之间天然水力联系。另一个试验场揭示了由于套管密封不恰当而造成的基岩含水层和和粒状含水层之间的人为水力联系。区域研究显示,水力联系可对所实验的含水层真正水力特性给出错误的解译。因此,探测水力联系对于正确确定井的捕获区和污染条件非常重要。建议对每个现有的含水层单元进行抽水实验及水化学对比。变水头渗透试验对验证基岩套管接触面的密封质量也有帮助。减少由于密封不佳造成的交叉污染风险需要有更有效的过程控制手段和更好的成井实践。
Resumo
São apresentadas evidências de campo de conexões hidráulicas entre o substrato rochoso e um aquífero granular sobrejacente no Escudo Canadiano (Província de Grenville). Esta questão é raramente considerada e investigada, apesar das importantes consequências hidráulicas e químicas e da sua ocorrência generalizada em todo o mundo. A metodologia utilizada é baseada em testes de campo complementares realizados em sítios experimentais específicos, com instrumentação quer na rocha de base, quer nos depósitos sobrejacentes. Um dos locais do substrato revelou uma conexão hidráulica natural com o aquífero granular sobrejacente causada pela superfície mais alterada da parte superior do substrato rochoso. Outro lugar revelou uma ligação hidráulica artificial entre o substrato rochoso e o aquífero granular, criado por uma tubagem indevidamente selada num furo. Um estudo regional mostrou que as ligações hidráulicas podem conduzir a uma interpretação errónea das verdadeiras propriedades hidráulicas do aquífero testado. A deteção de ligações hidráulicas é, portanto, essencial para definir adequadamente os perímetrso de proteção e as situações de contaminação. Recomenda-se aos profissionais que os testes de bombeamento sejam realizados, bem como comparações hidroquímicas em cada unidade aquífera existente. Ensaios de permeabilidade são também úteis para verificar a qualidade da selagem no contacto com o substrato rochoso. São assim necessários processos de controlo mais eficazes e melhores práticas de construção para reduzir os riscos de contaminação cruzada induzidos por selagens defeituosas.
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Acknowledgements
This project was funded by the Fondation de l’UQAC (FUQAC), the Fonds de Recherche du Québec sur la Nature et les Technologies (FRQNT), and the Programme d’Acquisition de Connaissances sur les Eaux Souterraines of Quebec (PACES), with contribution from the Ministère du Développement Durable, de l’Environnement et des Parcs, UQAC, Ville de Saguenay and the four Municipalités Régionales de Comté of the SLSJ region. The authors would like to acknowledge the contributions to this report from team members of the PACES project at UQAC. The authors also thank Mustafa Afsin, an anonymous reviewer, the editor and the associate editor for their comments and advice that greatly improved the quality of this manuscript.
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Richard, S.K., Chesnaux, R., Rouleau, A. et al. Field evidence of hydraulic connections between bedrock aquifers and overlying granular aquifers: examples from the Grenville Province of the Canadian Shield. Hydrogeol J 22, 1889–1904 (2014). https://doi.org/10.1007/s10040-014-1183-4
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DOI: https://doi.org/10.1007/s10040-014-1183-4