Use of alternative conceptual models to assess the impact of a buried valley on groundwater vulnerability

A buried valley incised into a sequence of pre-Quaternary sediments is shown to seriously affect the vulnerability of groundwater. Often the existence of buried valleys is not known or is not described explicitly in a hydrogeological model. In the present study, two numerical groundwater models, representing two alternative conceptual models, were produced to help quantify the effect of the valley on groundwater vulnerability. One model included the buried valley and the other did not. Both models were subjected to calibration and were found to describe hydraulic head and river discharge equally well. Even though the two models showed similar calibration statistics; fluxes, travel paths and travel times were affected by the inclusion of the buried valley. The recharge area and the groundwater age of potential abstraction wells placed in the pre-Quaternary deep aquifers surrounding the buried valley were different for the two models, with significantly higher vulnerability when the valley was included in the model. Based on the results of the present study, it is concluded that a buried valley may not always be detectable when calibrating a wrong conceptual model. If reliable results should be obtained a good geological model has to be constructed.     

Evaluation of alternative conceptual models for groundwater modelling

This study evaluates the alternative conceptual models for groundwater modelling. A true model was created with a synthetic alluvial fan-plain hydrogeological framework. Various alternative conceptual models were evaluated for groundwater flow simulations. The first alternative model is a single aquifer layer model; the second alternative model is a 3-layer aquifer model; and the third model is a 5-layer model consisting of 3 aquifers separated by 2 aquitards. All models could fit very well to the observations with optimized values of hydraulic conductivities. However, the single aquifer layer model can only compute water balance components with good accuracy. The 3-layer aquifer model can be used for water balance computation and groundwater head simulation with small errors. The 5-layer model is capable of simulating water budget, groundwater head distribution and travel times with high accuracy. Multi-model analysis found only the 3rd alternative model superior.     

Karst groundwater: a challenge for new resources

Karst aquifers have complex and original characteristics which make them very different from other aquifers: high heterogeneity created and organised by groundwater flow; large voids, high flow velocities up to several hundreds of m/h, high flow rate springs up to some tens of m³/s. Different conceptual models, known from the literature, attempt to take into account all these particularities. The study methods used in classical hydrogeology—bore hole, pumping test and distributed models—are generally invalid and unsuccessful in karst aquifers, because the results cannot be extended to the whole aquifer nor to some parts, as is done in non-karst aquifers. Presently, karst hydrogeologists use a specific investigation methodology (described here), which is comparable to that used in surface hydrology.Important points remain unsolved. Some of them are related to fundamental aspects such as the void structure – only a conduit network, or a conduit network plus a porous matrix –, the functioning – threshold effects and non-linearities –, the modeling of the functioning – double or triple porosity, or viscous flow in conduits – and of karst genesis. Some other points deal with practical aspects, such as the assessment of aquifer storage capacity or vulnerability, or the prediction of the location of highly productive zones.

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Resumen Los acuíferos kársticos tienen características originales y complejas que los hacen muy diferentes de otros acuíferos: alta heterogeneidad creada y organizada por el flujo de agua subterránea, espacios grandes, velocidades altas de flujo de hasta varios cientos de m/h, manantiales con ritmo alto de flujo de hasta algunas decenas de m3/s. Diferentes modelos conceptuales que se conocen en la literatura tratan de tomar en cuenta todas estas particularidades. Los métodos de estudio usados en hidrogeología clásica- pozos, pruebas de bombeo y modelos distribuidos- son generalmente inválidos y no exitosos en acuíferos kársticos, debido a que los resultados no pueden extenderse a todo el acuífero ni a alguna de sus partes, como se hace en acuíferos no kársticos. Actualmente los hidrogeólogos kársticos usan una metodología de investigación específica, que se des cribe aquí, la cual es comparable con la que se utiliza en hidrología superficial. Puntos importantes permanecen sin resolverse. Algunos de ellos se relacionan con aspectos fundamentales tal como la estructura de espacios- solo una red de conductos, o una red de conductos más una matriz porosa-, el funcionamiento-efectos threshold y no-linealidades-, el modelizado del funcionamiento-porosidad doble o triple, o flujo viscoso en conductos- y génesis del karst. Algunos otros puntos se relacionan con aspectos prácticos, tal como la evaluación de la capacidad de almacenamiento del acuífero o la vulnerabilidad, o la predicción de la localización de zonas altamente productivas.

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Résumé Les aquifères karstiques présentent des caractères originaux complexes qui les distinguent profondément de tous les autres milieux aquifères : forte hétérogénéité créée et organisée par les écoulements souterrains eux-mêmes, vides de très grandes dimensions, vitesses découlement pouvant atteindre quelques centaines de mètres par heure, sources à débit pouvant atteindre quelques dizaines de m³/s. Différents modèles conceptuels tentent de prendre en compte ces particularités. Les méthodes détude de lhydrogéologie classique—forage, essai de pompage et modèles maillés – ne sont en général pas adaptés aux aquifères karstiques, parce que les résultats obtenus ne peuvent pas être étendus à laquifère tout entier, ou à certaines de ses parties, comme cela est fait pour les aquifères non karstiques. Actuellement les hydrogéologues du karst ont recours à une méthodologie détude spécifique décrite ici, comparable à celle utilisée en hydrologie de surface.Des points importants restent à résoudre. Certains concernent des aspects fondamentaux, comme la structure des vides – réseau de conduits uniquement, ou bien réseau de conduits et matrice poreuse –, le fonctionnement – problèmes de seuil et plus généralement les non-linéarités –, la modélisation du fonctionnement – double porosité ou écoulement visqueux en conduits – et de la genèse du karst. Dautres points portent sur des aspects pratiques, comme lévaluation de la capacité de stockage ou de la vulnérabilité de laquifère, et la prédiction des zones à haute productivité.

     

Developing conceptual hydrogeological model for Potsdam sandstones in southwestern Quebec,Canada

A hydrogeological study was conducted in Potsdam sandstones on the international border between Canada (Quebec) and the USA (New York). Two sandstone formations, arkose and conglomerate (base) and well-cemented quartz arenite (upper), underlie the study area and form the major regional aquifer unit. Glacial till, littoral sand and gravel, and marine silt and clay discontinuously overlie the aquifer. In both sandstone formations, sub-horizontal bedding planes are ubiquitous and display significant hydraulic conductivities that are orders of magnitude more permeable than the intact rock matrix. Aquifer tests demonstrate that the two formations have similar bulk hydrologic properties, with average hydraulic conductivities ranging from 2?×?10^?5 to 4?×?10^?5 m/s. However, due to their different lithologic and structural characteristics, these two sandstones impose rather different controls on groundwater flow patterns in the study area. Flow is sustained through two types of fracture networks: sub-horizontal, laterally extensive fractures in the basal sandstone, where hydraulic connectivity is very good horizontally but very poor vertically and each of the water-bearing bedding planes can be considered as a separate planar two-dimensional aquifer unit; and the more fractured and vertically jointed system found in the upper sandstone that promotes a more dispersed, three-dimensional movement of groundwater.     

Alternative conceptual models and the robustness of groundwater management scenarios in the multi-aquifer system of the Central Veneto Basin, Italy

A three-dimensional (3D) groundwater flow model of the deep multi-aquifer Quaternary deposits of the Po plain sedimentary basin, within a 3,300-km² area (Veneto, Italy), is developed, tested and applied to aid sustainable large-scale water-resources management. Integrated mathematical modelling proves significantly successful, owing to an unusual wealth of available geological, geophysical, and hydrologic data and to state-of-the-art numerical tools. Of particular interest is the evaluation of the influence of alternative conceptual models; that is, of reconstructed representations of the 3D geological model of the structure of the aquifers. The reference conceptual model is set up by means of extended geological sections and stratigraphic records, and is used to create a large, unstructured 3D finite element grid. By analyzing the effects on piezometric surfaces and on the overall water budget of geometrical perturbations from the reference structure, alternative geo-structural models, obtained by systematically shifting the pinch-out of the aquitards, are compared. Interestingly, the impacts of aquitard pinch-out prove far from negligible. The results suggest the critical importance of reliable geological models even for large, complex 3D models. The good practice of iteratively testing numerically the impact of surprises on the conceptual model, as more field information is collected, is thus supported.     

非饱和带裂隙岩体渗流的特点和概念模型

非饱和带裂隙岩体中的渗流和饱和带相比有其不同的特点,包括毛细管流、薄膜流、优先流和裂隙-基质相互作用等。这些特点导致非饱和带裂隙岩体中的渗流具有相当的非均质性。笔者描述了裂隙岩体中非饱和渗流的这些特点,并简单讨论了目前所存在的模拟非饱和渗流的概念模型。     

裂隙岩体渗流概念模型研究

裂隙岩体中的渗流和传统的多孔介质渗流在机理上存在本质的差别,这种差别主要表现为裂隙岩体在各种尺度上存在的非均质性。模拟裂隙岩体渗流的主要困难在于描述这种非均质性。目前的概念模型,包括等效连续体模型、离散裂隙网络模型和混合模型使用了不同的技术来预测裂隙岩体中的渗流。这些模型基于不同的假设和概念框架,有着各自不同的优缺点。在实际应用时,应当根据研究域的具体特点和所要解决的问题的要求对其选择,此外,还讨论了单裂隙的概念模型。     

Dryland salinity processes within the discharge zone of a local groundwater system,southeastern Australia

Detailed study of a localised saline discharge zone in southeastern Australia shows that the salinisation is mostly due to the shallow water table (<1–2 m from the surface). Direct evaporation, particularly in summer, leads to extremely high soil–water salinities at the surface, even though the underlying groundwater is moderately fresh. Groundwater discharge is localised at a break of slope, where the water table intersects the surface, and where the transition from permeable sands to clay-rich sediments inhibits lateral groundwater flow. Higher salt concentrations build up in the clays because of the long residence times during which soil-waters are exposed to evapotranspiration and the reduced potential for salts to be flushed from the sediments. As a result the saline discharge area does not correspond to the part of the site with the largest salt store. Results of the study demonstrate that for dryland salinisation to occur, the groundwater beneath the discharge zone need not be saline, and the presence of a large salt store does not necessarily lead to problems of dryland salinisation if, as in the clay-rich sediments at the site, the salt lies below the pasture root zone. Furthermore, mobilisation of salt stores within low permeability sediments by rising groundwater may be minor.

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Résumé L’étude détaillée d’une zone localisée d’émergence salée au sud-est de l’Australie montre que la salinisation est surtout due à la nappe phréatique (<1–2 m sous la surface). L’évaporation directe, particulièrement durant l’été, conduit à des salinités de l’eau du sol extrêmement élevées à la surface, même si l’eau souterraine sous-jacente est modérément douce. L’émergence de l’eau souterraine est localisée à la rupture de pente, là où la nappe phréatique rencontre la surface du sol et où la transition entre sables perméables et sédiments riches en argiles inhibe les écoulements d’eau souterraine latéraux. Les plus fortes concentrations en sel s’accumulent dans les argiles du fait de temps de résidence élevés, durant lesquels les eaux du sol sont exposées à l’évapotranspiration et à un lessivage réduit des sédiments. Il en résulte que l’aire d’émergence des eaux salées ne correspond pas à la partie du site rencontrant la réserve de sel la plus importante. Les résultats de l’étude démontrent que pour que la salinisation d’une zone aride devienne effective, l’eau souterraine sous la zone ne doit pas être forcément salée, et la présence d’une zone étendue de réserve de sel ne conduit pas forcément à des problèmes de salinisation de zones arides si, comme dans les sédiments argileux du site, le sel ne repose pas sous la zone de pature. De plus, la mobilisation des réserves de sel dans les sédiments peu perméables par la montée du niveau de l’eau souterraine devrait être mineure.

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Resumen El estudio detallado de una zona local de descarga salina en el sudeste de Australia, muestra que la salinización es principalmente debido a un nivel freático poco profundo (<1–2 m de la superficie). La evaporación Directa, particularmente en verano, conlleva a salinidades de suelo-agua sumamente altas en la superficie, aunque el agua subterránea subyacente es moderadamente dulce. La descarga de Agua subterránea se localiza en una interrupción de la ladera, dónde el nivel freático intercepta la superficie, y donde la transición de las arenas permeables a los sedimentos ricos en arcilla inhibe el flujo lateral del agua subterránea. Las concentraciones de sal más altas se forman en las arcillas debido a los tiempos de residencia largos durante los cuales se exponen el conjunto suelo-agua a la evapotranspiración y también por el potencial reducido para las sales de ser expulsadas de los sedimentos. Como resultado el área de la descarga salina no corresponde a la parte del sitio con el contenido de sal más grande. Los resultados del estudio demuestran que para que ocurra la salinización en terrenos secos, el agua subterránea bajo la de zona de descarga no necesita ser salina, y que la presencia de un almacenamiento de sal grande, no necesariamente lleva a los problemas de salinización en terrenos secos si, como en los sedimentos ricos en arcilla del sitio, la sal yace debajo de la zona de raíz de la pastura. Además, la movilidad de depósitos de sal dentro de los sedimentos de permeabilidad baja pueden ser menores, por causa del agua subterránea ascendente.

     

Performance-based landfill design: development of a design component selection matrix using GIS and system simulation models

Designing environmentally safe and economically feasible landfills can be a challenging task due to complex interactions that need to be taken into account between landfill size, waste and site characteristics. The main focus of this study is, by interfacing the geographic information systems (GIS) with system simulation models (SSM), to develop a methodology and a landfill design component selection matrix that can enable the determination of landfill design components providing the desired performance with minimal design details. In this paper, the conceptual framework and applications of the developed methodology demonstrating the selection of landfill design components that are suitable for the existing site conditions are presented. The conceptual model defines design variables, performance criteria and design components of a landfill. GIS and SSM are used to handle the site-specific data and to evaluate the landfill performance, respectively. Results indicate that the landfills having the same design characteristics show different performance under different site conditions; therefore, a landfill design that is technically and economically feasible should be selected on the basis of performance.     

Using turbidity dynamics and geochemical variability as a tool for understanding the behavior and vulnerability of a karst aquifer

In a karst system, the characterization of transport properties is based on the comparison of natural tracers observed at the inlet (a swallow hole on the karst plateau) and the outlets of the system (a spring and a well). At Norville, northwest France, electrical conductivity (EC) and turbidity (T) were used as natural tracers for characterizing dissolved elements (surface water geochemistry) and particulate matter transport, respectively. Two methods were used for this study: (1) a comparison of the relations between EC, T and spring discharge (Q) by means of normalized EC–T–Q curves, and (2) a principal component analysis (PCA) including water geochemistry data in addition to EC, T and Q. Three different characteristic flood events have been chosen for the analyses. EC–T–Q curves highlighted the direct transfer, resuspension and deposition of particles during their transport in the karst network. Transport from the swallow hole to both the spring and the well appeared to be dominated by karst-conduit flow. On the other hand, PCA results showed a diffuse source of contamination of groundwaters by nitrate and a point-source contamination of groundwaters by streaming/runoff surface waters with high turbidity and phosphate concentration infiltrated at the swallow hole.     

基于概念设计的长短桩复合地基典型工程应用模式

本文从长短桩复合地基各组成部分的功能角度,阐述了长桩、短桩、褥垫层的作用机理,提出长短桩复合地基概念设计应遵循的设计原则。应用概念设计思想,建立了长短桩复合地基的五种典型工程应用模式,并在各典型模式的分析说明中,提出设计与施工参数的选用建议。通过在北京某工程的实际应用,显示长短桩复合地基的典型工程应用模式具有较好的实用性。     

Birds as lines: The production of alternative regimes of environmental management in the aftermath of a toxic disaster

This paper analyses how in the aftermath of one of the worst environmental disasters ever to occur in Spain – the Aznalcóllar Disaster – various environmentalist and conservationist groups mobilised migratory birds to bring new insights and the need for new precautions to the controversy elicited by the spill. The case study, thus, revolves around how environmentalists established a “hybrid collective action” to draw attention to unconsidered risks and impacts of the disaster and thereby make the case for open debate. Building upon this, I engage with two different, though interrelated, theoretical debates that contribute to a rethinking of environmental management (EM) as a social and materially situated practice. Drawing on the idea of “tactic” (De Certeau, 1984), I draw attention to the devices, actions and procedures that environmentalists carried out to resist attempts to minimise the spill and to undermine administrations’ assumptions of control, coherence and singularity associated the idea of management. Drawing on Tim Ingold’s latest work (2007, 2008, 2011), I analyse environmentalists’ most successful tactic: the enactment of migratory birds as “lines”. Together with other authors in this special issue, I will use this notion to make an argument against some of the assumptions of the “hybrid ontology”. In contrast to more essentialists and static notions of non-human agency and politics, the idea of line is particularly useful as a way of understanding how nature(s) can be effectuated differently and how this leads to the imagining of new regimes of cohabitation, human and non-human management and intervention.     

流域坡面汇流研究现状述评

对当前坡面汇流计算方法的研究进展进行了较为系统的总结与分析,并对坡面汇流的非线性效应以及城市低影响开发中的雨水入渗与蓄集对坡面汇流的控制作用进行了简要分析。从模型简单实用的角度出发,认为以流域时间-面积关系与线性水库相串联的ModClark法等为代表的概念性分布式坡面汇流模型具有良好的发展前景;考虑到基于等流时单元的变动等流时线法在反映雨强非线性影响中存在的问题,认为根据水文响应单元在不同雨强条件下汇流时间的变化,调整其汇流参数以反映坡面汇流的非线性效应,对于流域坡面汇流的分布式模拟更具有实际意义;针对目前低影响开发设施长时间序列大空间尺度的室外降雨径流监测资料普遍较为缺乏的现状,给出了后期应积极选择合适的技术以加强低影响开发性能监测工作的建议。     

Deuterium composition and flow path analysis as additional calibration targets to calibrate groundwater flow simulation in a coastal wetlands system

A conceptual model of the Lake Warden coastal wetlands system, Western Australia, was developed using hydraulic, chemical and stable isotopic data, and formed the basis for a groundwater flow model using the finite element numerical code (FEFLOW). The system to be modeled is complex. The surface water and groundwater within the wetlands system show varying salinity and isotopic composition over short distances and time frames. As a first step, the flow model was calibrated to observed groundwater levels measured since 2001 for both steady state and transient stresses. Particle tracking analysis was conducted to test the source areas of water discharging to the lakes within the wetlands system. The analysis was able to delineate the connectivity between the lakes in the wetland and the flow path. Enrichment of isotopic concentration is evident along a NE–SW transect and the data set provides a means for calibrating a detailed transport model. The study incorporates the varying deuterium composition of the water bodies directly into a transport model and a good match between observed and simulated temporal variations along the transect indicates that the model closely simulated the dynamics of water exchange between the lakes and groundwater within the system.     

Glacio-lacustrine stratigraphy,aquifer characterization and contaminant transport: a case study in South Lake Tahoe,California, USA

A hydrogeologic model that has been used by many researchers and consultants to describe an area of South Lake Tahoe, California, USA impacted by MTBE contamination describes a relatively homogeneous unconfined aquifer comprised of poorly sorted glacial outwash deposits, within which water-supply wells are able to exert significant alteration in natural groundwater flow. A re-examination of the area’s hydrogeology is presented, which supports a layered heterogeneous aquifer system constructed of alternating fine and coarser-grained glacio-lacustrine depositional units. This re-evaluation was accomplished through a review of lithologic logs across an area of approximately 1 km², combined with observations of significant hydraulic head differences and knowledge of the depositional environments controlled by Pleistocene Lake Tahoe high stands. Many of the fine-grained units observed at depths from 6 to 15 m, although relatively thin, are generally continuous and serve as significant barriers to groundwater flow. The vertical migration of contamination across these fine-grained units to deeper groundwaters was facilitated by cross-screened monitoring wells installed as part of site investigation activities. This conclusion highlights the importance of geologic characterization and proper monitoring well construction at contaminated site investigations.

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Electronic supplementary material   The online version of this article (doi:) contains supplementary material, which is available to authorized users.

Performance of quasilinear elastic constitutive models in simulation of geosynthetic encased columns

Past numerical simulations of geosynthetic encased columns (GECs) using different versions of the quasilinear elastic hyperbolic model for the encased granular material have, in certain cases, yielded unrealistic results. In this paper the cause of such results is investigated by performing three-dimensional finite element analyses of GECs in soft clay, utilizing three common functional forms of the hyperbolic model for the encased granular material. Results indicate that one form of the hyperbolic model can predict an unrealistic lateral response for GECs during application of load to the column. In addition, the inability of hyperbolic models to properly account for soil behavior near failure compromises their ability to realistically capture the behavior of encased granular soil in GECs. Modeling the behavior of soil near failure is essential for properly simulating the behavior of GECs, as soil shear failure is necessary to mobilize the tensile stresses in the encasement and improve the stress–displacement response of the GEC. Although this type of hyperbolic model behavior was demonstrated for the specific case of encased soil in a GEC, the limitations of the hyperbolic model described herein apply equally to other geotechnical problems in which some portion of the soil mass is at or near failure.     

Variability of groundwater nitrate concentrations over time in arid basin aquifers: sources, mechanisms of transport, and implications for conceptual models

In groundwater of the Trans-Pecos region of West Texas, unexpectedly high levels of nitrate (NO₃ ^?) are documented in four basins: Red Light Draw, Eagle Flats, Wild Horse and Michigan Flats, and Lobo and Ryan Flats. NO₃ ^? concentrations are changing over time in the majority (82.8 %) of wells and are increasing in most (69.8 %). The temporal change raises questions about the potential sources of NO₃ ^? and about flow dynamics in these basins. Presence of NO₃ ^? and temporal variability in concentration has implications beyond contamination risk because it indicates relatively rapid recharge (<60 years) to the basin groundwaters which was not expected based on previous estimates from chloride mass balance models and groundwater age-dating techniques. This research combines existing data ranging back to the 1940s with data collected in 2011 to document a multi-decadal trend of overall increasing NO₃ ^? concentration in deep basin groundwaters. Chlorofluorocarbon analyses of groundwater collected during 2011 indicate the presence of young (<70 years) water in the basins. The authors infer from these data that there are mechanism(s) by which relatively rapid and widespread recharge occurs on the basin floors; that recharge is spatially and temporally variable and that it results from both anthropogenic (irrigated agriculture) and natural (precipitation) sources. In light of these observations, fundamental conceptual models of flow in these basins should be re-evaluated.     

A conceptual hydrogeological model of ophiolite hard-rock aquifers in Oman based on a multiscale and a multidisciplinary approach

Ophiolites are found all over the world: from the Alps to the Himalayas, in Cuba, Papua-New Guinea, New Caledonia, Newfoundland, etc. They are composed of hard rocks—basalt, dolerite, gabbro and peridotite, which are formed at the mid-oceanic ridges, with specific ridge-related tectonic fracturing and intense hydrothermal alteration. Their geological and thus their hydrogeological properties differ from those of both granite or classical gabbro and classical basaltic lava. A conceptual hydrogeological model of these hard-rock aquifers was developed based on the convergent results of a multidisciplinary approach at several spatial scales, from rock-sample (centimetre) to catchment (kilometre), on well-preserved ophiolite rocks in Oman. In ophiolite rocks, groundwater circulation takes place mostly in the fissured near-surface horizon (50 m thick), and, to a lesser degree, in the tectonic fractures. Hydrograph analysis (Water Resour Res 34:233–240, 1977), interpretation of numerous pumping tests using both classical Theis and dual porosity models [Water Resour Res 32:2733–2745, 1996; Comput Geosci J (in press)], and mercury porosity and hydraulic conductivity lab-measurements support the aquifer parameter estimates. The hydraulic conductivity K of the fissured horizon is estimated at 10^–5 to 10^–6 m/s for gabbro and dolerite, and 10^–7 m/s for peridotite. The storage coefficient S of the peridotite aquifer is estimated at 10^–3 and appears to be controlled mainly by microcracks (20 to 100 m wide). Tectonic fractures in the ophiolite have similar hydrodynamic properties regardless of lithology (10^–1<T<10^–4 m²/s and 10^–1<S<10^–3) though the probability of obtaining productive wells is two to three times greater in gabbro and dolerite than in peridotite. Some of the tectonic fractures produce small hydrothermal, hyperalkaline springs in the peridotite. The water budget and hydrochemistry of the Oman ophiolite are characterized and support the conceptual hydrogeological model. Despite low annual rainfall, a relatively low hydraulic conductivity and a significant storage coefficient explain why most of the streams in peridotite are perennial.

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Résumé Les ophiolites se trouvent partout dans le monde : des Alpes à lHymalaya, à Cuba, Papouasie Nouvelle-Guinée, Nouvelle Calédonie, etc. Les ophiolites sont composées de roches dures basaltiques, de dolérites, gabbros et péridotites, qui sont formées aux rides mi-océaniques, avec, en relation avec les rides, une fracturation tectonique et une altération hydrothermale intense. Leurs propriétés géologiques et hydrogéologiques diffèrent de celles des granites ou des gabbros classiques ou des laves basaltiques classiques. Un modèle hydrogéologique conceptuel des aquifères de hard-rock a été développé sur les résultats convergents dune approche multidisciplinaire à différentes échelles spatiales, de léchantillon de roche de quelques centimètres à la taille du bassin versant en kilomètres, des ophiolites bien conservées dOman. Dans les roches ophiolitiques, la circulation des eaux souterraines est surtout localisée dans les fissures dun horizon de surface de plus dune cinquantaine de mètres et à un degré moindre dans les fractures tectoniques. Lanalyse des hydrographes (Water Resour Res 34:233–240, 1977), linterprétation des nombreux essais de pompage utilisant des modèles classiques de Theis et des modèles à double porosité (Water Resour Res 32:2733–2745, 1996; Comput Geosci J (Soumis), la porosité au mercure et la conductivité hydraulique calculée en laboratoire supportent lestimation des paramètres de laquifère. La conductivité hydraulique K de lhorizon fissuré est estimée entre 10^–5 et 10^–6 m/s pour les gabbros et les dolérites, et 10^–7 m/s pour les péridotites. Le coefficient demmagasinement S de laquifère de péridotite est estimé à 10^–3 et semble être contrôlé par les micro-fractures (20 à 100 m de largeur). Les fractures tectoniques dans les ophiolites possèdent des propriétés hydrodynamiques en regard de la lithologie (10-1<T<10-4 m²/s et 10-1/S/10-3) bien que la probabilité dobtenir des puits productifs est trois plus grand dans le grabbro et la dolérite que la péridétite. Quelques unes des fractures tectoniques produisent de petites sources hydrothermales hyperalcalines dans la péridotite. Le bilan hydrologique et hydrochimique de lophiolite dOman caractérise le modèle hydrogéologique conceptuel. Malgré des faibles précipitations annuelles, une relativement basse conductivité hydraulique et un coefficient demmagasinement significatif expliquent pourquoi la plus part des cours deau sur les péridotites ne sont jamais taris.

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Resumen Las rocas ofiolitas se encuentran en muchas partes del mundo desde los Alpes a los Himilaya, Newfoundland, Nueva Caledonia etc. Son rocas duras compuestas de basalto, dolerita, gabro y peridotita que son formadas por los cordones medio-oceánicos y por fracturación relacionada con los cordones tectónicos específicos y alteración hidrotermal intensa. La geología y los propiedades hidrogeológicas varian entre las de granito y gabro clásico y lava de basalto clásico. Se desarrolló un modelo conceptual de los acuíferos de las rocas duras Basado en los resultados convergentes de un enfoce multidisciplinario a varias escalas a partir de muestras de roca (centímetro) a cuenca kilómetro) enfocado en las rocas ofilíticas bien preservados en Oman. En las rocas ofiolíticas la circulación de agua ocurre principalmente en el horizonte de fisuras cercanas a la superficie (>50 m de espesor) y en un grado menos importante en fracturas tectónicas. Los estimados de los parámetros de estos acuíferos se basan en análisis de hidrógrafos (Water Resour Res 34:233–240, 1977), en la interpretación de pruebas de bombeo usando los modelos clásicos de Theis y de porosidad doble (Hamm y Bidaux 1996; Lods y Gouze, [en prensa]), y porosidad de mecurio y conductividad hidraúlica medida en el laboratorio. La conductividad hidraúlica K del horizonte fisurado se estima en 10E-5 a 10E-6 m-s para gabro y dolerita y 10E-7 para peridotita. El coeficiente de abastecimiento S del acuífero peridotita se estima en 10–3 y parece ser controlado principalmente por microfracturas (20 to 100 m de ancho). Las fracturas tectónicas en la ofiolita tienen propiedades hidrodinámicas similares independientes de la litología (10–1<T<10–4 m²/s and 10–1<S<10–3) pero la probabilidad de obtener pozos productivos es dos o tres veces mayor en gabro que en dolerita o peridotita. Algunas de las fracturas tectónicas producen pequeñas descargas de agua hidrotermal hiperalcalina en la peridotita. El presupuesto e hidroquímica de las ofilitas de Omán se caracteriza y apoya en el modelo conceptual hidrogeológico. La baja lluvia anual, una conductividad hidraúlica relativamente baja y un coeficiente de abastecimiento importante explican porqué la mejoría de los drenajes en peridotita es peremne.

     

Effects of fracture lineaments and in-situ rock stresses on groundwater flow in hard rocks: a case study from Sunnfjord, western Norway

Systematic field mapping of fracture lineaments observed on aerial photographs shows that almost all of these structures are positively correlated with zones of high macroscopic and mesoscopic fracture frequencies compared with the surroundings. The lineaments are subdivided into zones with different characteristics: (1) a central zone with fault rocks, high fracture frequency and connectivity but commonly with mineral sealed fractures, and (2) a damage zone divided into a proximal zone with a high fracture frequency of lineament parallel, non-mineralized and interconnected fractures, grading into a distal zone with lower fracture frequencies and which is transitional to the surrounding areas with general background fracturing. To examine the possible relations between lineament architecture and in-situ rock stress on groundwater flow, the geological fieldwork was followed up by in-situ stress measurements and test boreholes at selected sites. Geophysical well logging added valuable information about fracture distribution and fracture flow at depths. Based on the studies of in-situ stresses as well as the lineaments and associated fracture systems presented above, two working hypotheses for groundwater flow were formulated: (i) In areas with a general background fracturing and in the distal zone of lineaments, groundwater flow will mainly occur along fractures parallel with the largest in-situ rock stress, unless fractures are critically loaded or reactivated as shear fractures at angles around 30° to σ_H; (ii) In the influence area of lineaments, the largest potential for groundwater abstraction is in the proximal zone, where there is a high fracture frequency and connectivity with negligible fracture fillings. The testing of the two hypotheses does not give a clear and unequivocal answer in support of the two assumptions about groundwater flow in the study area. But most of the observed data are in agreement with the predictions from the models, and can be explained by the action of the present stress field on pre-existing fractures.     

Accelerating strategies to the numerical simulation of large‐scale models for sequential excavation

In this paper, a novel combination of well‐established numerical procedures is explored in order to accelerate the simulation of sequential excavation. Usually, large‐scale models are used to represent these problems. Due to the high number of equations involved, the solver algorithm represents the critical aspect which makes the simulation very time consuming. The mutable nature of the excavation models makes this problem even more pronounced. To accomplish the representation of geometrical and mechanical aspects in an efficient and simple manner, the proposed solution employs the boundary element method with a multiple‐region strategy. Together with this representational system, a segmented storage scheme and a time‐ordered tracking of the changes form an adequate basis for the usage of fast updating methods instead of frontal solvers. The present development employs the Sherman–Morrison–Woodbury method to speed up the calculation due to sequential changes. The efficiency of the proposed framework is illustrated through the simulation of test examples of 2D and 3D models. Copyright © 2006 John Wiley & Sons, Ltd.