Abstract
Groundwater flow in granitic bedrock is of major interest for underground projects such as radioactive waste disposal. It is generally accepted that granitic rocks of the upper crust are characterized as faulted low-porosity rocks showing fault-related permeability. In this study, the influence of existing faults on the present-day water flow in the Grimsel Test Site (Switzerland), an underground rock laboratory situated in granitoid rocks, was investigated by mapping water discharges. As a result, the link between water flow and faults considering slip-tendency analysis and fault intersections is evaluated. Water-conducting features were combined in a structural-permeability favorability map. Faults and dykes occur as three orientation groups, NE–SW, E–W, and NW–SE trending, all steeply dipping southwards with fault intersections also steeply plunging southwards. In total, 100 water discharges were mapped in summer 2014 and 85 in winter 2016, which are located along faults or fault intersections. A comparison of water discharges with structures showed that high-slip-tendency metabasic dykes and fault or dyke intersections represent the dominant flow paths. Further, it could be demonstrated that higher slip-tendency tends to lead to enhanced average hydraulic conductivity and therefore more constant water flow. Based on water fluxes, fault intersections are inferred to represent first-order locations of water percolation followed by high-slip-tendency metabasic dykes. The combination of all water-conducting features into a structural-permeability map results in covering all water discharges. Therefore, the structural-permeability favorability map can serve as suitable representation for constraining water inflow in fractured granitoid host rocks.
Résumé
L’écoulement des eaux souterraines dans le substratum granitique est d’un intérêt majeur pour les projets souterrains tels que le stockage des déchets radioactifs. Il est généralement admis que les roches granitiques de la croûte supérieure sont caractérisées comme des roches faillées de faible porosité montrant une conductivité hydraulique associée aux failles. Dans cette étude, l’influence des failles existantes sur l’écoulement actuel des eaux dans le site d’essai du Grimsel (Suisse), un laboratoire souterrain situé dans des roches granitoïdes, a été étudiée en cartographiant les sorties d’eau. Par conséquent, le lien entre l’écoulement d’eau et les failles en considérant l’analyse de tendance au glissement et les intersections des failles est évalué. Les caractéristiques conductrices vis-à-vis de l’eau ont été combinées dans une carte de la conductivité hydraulique structurale favorable. Les failles et les dykes se présentent selon trois groupes d’orientation, de tendance NE–SW, E–W, et NW–SE, tous avec un pendage fort vers le Sud avec des intersections de faille plongeant également de manière abrupte vers le Sud. Au total, ce sont 100 sites de sorties d’eau, localisés le long de failles ou d’intersection de failles qui ont été cartographiés à l’été 2014 et 85 à l’hiver 2016. Une comparaison des sorties d’eau avec les structures a montré que les dykes métabasiques à tendance élevée au glissement et les intersections de failles ou de dykes représentent les voies d’écoulement dominantes. De plus, on pourrait démontrer que la tendance au glissement la plus élevée tend à conduire à une augmentation de la conductivité hydraulique moyenne et donc à un écoulement d’eau plus constant D’après les flux d’eau, on déduit que les intersections des failles représentent des emplacements de premier ordre de percolation de l’eau, suivis des dykes métabasiques à tendance au glissement élevé. La combinaison de toutes les caractéristiques conductrices d’eau sur une carte de la conductivité hydraulique structurale permet de couvrir toutes les sorties d’eau. Par conséquent, la carte de conductivité structurale favorable peut servir de représentation appropriée pour limiter l’afflux d’eau dans les roches granitoïdes fracturées hôtes.
Resumen
El flujo de agua subterránea en el basamento granítico es de gran interés para proyectos en el subsuelo, como la eliminación de desechos radiactivos. En general, se acepta que las rocas graníticas de la corteza superior se caracterizan por ser rocas de baja porosidad con fallas que muestran permeabilidad relacionada con el fallamiento. En este estudio, la influencia de las fallas existentes en el flujo real de agua en el sitio de ensayo de Grimsel (Suiza), un laboratorio del subsuelo situado en rocas granitoides, se investigó mediante el mapeo de las descargas de agua. Como resultado, se evalúa el vínculo entre el flujo de agua y las fallas considerando el análisis de tendencias de deslizamiento y las intersecciones de fallas. Las características de la conducción del agua se combinaron en un mapa de favorabilidad de la permeabilidad estructural. Las fallas y los diques ocurren como tres grupos de orientación, NE–SW, E–W y NW–SE, todos con fuertes pendientes hacia el sur con intersecciones de fallas que también se hunden hacia el sur. En total, se asignaron 100 descargas de agua en el verano de 2014 y 85 en el invierno de 2016, que se ubican a lo largo de fallas o intersecciones de fallas. Una comparación de las descargas de agua con las estructuras mostró que los diques metabásicos de alto deslizamiento y las intersecciones de fallas o diques representan las trayectorias de flujo dominantes. Además, podría demostrarse que una mayor tendencia al deslizamiento tiende a conducir a una mejor conductividad hidráulica promedio y, por lo tanto, a un flujo de agua más constante. Con base en los flujos de agua, las intersecciones de fallas se infieren para representar ubicaciones de primer orden de percolación de agua seguidas por diques metabásicos de tendencia de alto deslizamiento. La combinación de todas las características de conducción de agua en un mapa de permeabilidad estructural da como resultado cubrir todas las descargas de agua. Por lo tanto, el mapa de favorabilidad de la permeabilidad estructural puede servir como una representación adecuada para restringir el ingreso de flujo de agua en las rocas granitoides fracturadas.
摘要
花岗岩基岩中的地下水水流是地下项目诸如放射性废物处理项目中主要关注的问题。普遍公认的是,上层地壳的花岗岩特征一般都是断裂的低孔隙岩石,其渗透性显示出与断层相关。在本研究中,在(瑞士)一个位于花岗岩岩石中的地下岩石实验室,即Grimsel试验场,通过绘制水排泄图研究了现有断层对目前水流的影响。因此,还评估了鉴于滑动趋势分析的水流和断层之间的联系以及断层交叉点。在结构性渗透性有利度图中,综合了导水特征。断层和岩脉以三个方向组出现:NE–SW、E–W和NW–SE走向,所有都是向南陡然倾斜,并且断层交叉点也是向南陡然俯冲。总共2014年夏季绘制了100次水排泄,2016年冬季绘制了85次的水排泄,所有这些水排泄都位于沿断层或者沿断层交叉点的地方。水排泄与结构的比较显示,高度滑动趋势的变质基性岩脉和断层或岩脉交叉点为主要水流通道。此外,可以证明,较高的滑动趋势易于导致平均水力传导率增大,因此,使水流更加恒定。根据水通量,推测了断层交叉点,以展示水渗透的一级位置,水渗透之后为高度滑动趋势的变质基性岩脉。把所有导水特征融入结构性渗透性图中就会覆盖所有的水排泄。因此,结构性渗透性有力度图可作为约束断裂花岗岩水流入的合适表现形式。
Resumo
O fluxo de águas subterrâneas em embasamento granítico é de grande interesse para projetos subterrâneos como disposição de rejeito radioativo. É comumente aceito que rochas graníticas da crosta superior são caracterizadas como rochas falhadas de baixa porosidade apresentando permeabilidade relacionada aos falhamentos. Neste estudo, a influência de falhas existentes no fluxo atual de águas no Local de Testes de Grimsel (Suíça), um laboratório subterrâneo situado em rochas graníticas, foi investigada mapeando-se as descargas de água. Como resultado, é avaliada a conexão entre o fluxo de água e o falhamento considerando a análise da tendência de deslizamento e intersecção das falhas. As feições condutoras de água foram combinadas em um mapa de favorabilidade da permeabilidade estrutural. Falhas e diques ocorrem em três grupos de orientações, direções NE–SO, E–O e NO–SE, todos com mergulho alto para sul, com intersecções de falhas também com alto ângulo de mergulho para sul. No total, 100 pontos de descarga de água foram mapeados no verão de 2014 e 85 pontos no inverno de 2016, os quais estão localizados ao longo das falhas ou intersecções de falhas. Uma comparação das descargas de água com as estruturas demonstrou que diques metabásicos com alta tendência de deslizamento e intersecção de falhas ou diques representam os fluxos preferenciais dominantes. Além disso, pode ser demonstrado que maiores tendências de deslizamento tendem a levar a uma maior média de condutividade hidráulica e assim a um fluxo de água mais constante. Com base nos fluxos de água, infere-se que as intersecções de falhas representam locais de primeira ordem para percolação de água, seguidas por diques metabásicos com alta tendência de deslizamento. A combinação de todas as feições condutoras de água em um mapa de permeabilidade estrutural resulta na cobertura de todas as descargas de água. Portanto, o mapa de favorabilidade da permeabilidade estrutural pode servir como uma representação adequada para restringir a entrada de água em granitóides fraturados como rochas hospedeiras.
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Acknowledgements
The authors thank the Nagra staff in the Grimsel Test Site for the excellent working environment. Furthermore, two anonymous reviewers and the anonymous associate editor are thanked for thorough reviews that greatly improved the manuscript. Jens Becker, Nagra, is thanked for reviewing of an earlier version of the manuscript.
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This study was part of the LASMO project run, jointly funded by NAGRA, RWM, and SURAO, which are acknowledged for funding the study.
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Schneeberger, R., Egli, D., Lanyon, G.W. et al. Structural-permeability favorability in crystalline rocks and implications for groundwater flow paths: a case study from the Aar Massif (central Switzerland). Hydrogeol J 26, 2725–2738 (2018). https://doi.org/10.1007/s10040-018-1826-y
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DOI: https://doi.org/10.1007/s10040-018-1826-y