Hydraulic properties and scale effects investigation in regional rock aquifers,south-western Quebec,Canada

This paper reports on the characterization of hydraulic properties of regional rock aquifers carried out within a groundwater resources assessment project in the St. Lawrence Lowlands of south-western Quebec. To understand the aquifer behavior at both the fracture level and at field scale, hydraulic investigations were carried out using various aquifer tests. The groundwater flow at the local scale is controlled mostly by the fracture system. Results of the constant-head injection tests show a weak decreasing trend of hydraulic conductivity with depth indicating that a major part of the groundwater flow occurs in the first meters of the rock sequence. At the regional scale, the equivalent porous media approach is applicable. The hydraulic conductivity measurements were correlated to the scale of the aquifer tests expressed with the investigated aquifer volume. A simple interpolation procedure for the hydraulic conductivity field was developed based on the distance between field measurements and the tested aquifer volumes. The regional distribution of the hydraulic conductivity for the major fractured aquifer units indicates that dolostone is the most permeable whereas sandstone and crystalline rocks are the least permeable units.

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Resumen Este artículo trata de la caracterización de las propiedades hidráulicas en acuíferos regionales rocosos, la cual se llevó a cabo dentro del proyecto de evaluación de los recursos de agua subterránea en St. Lawrence Lowlands al suroeste de Quebec. Para entender el comportamiento del acuífero tanto a nivel de fractura como a escala del campo, se ejecutaron investigaciones hidráulicas usando varias pruebas de acuífero. El flujo del agua subterránea a escala local está controlado principalmente por el sistema de fracturas. Los resultados de las pruebas de inyección con cabeza constante muestran una tendencia decreciente débil de la conductividad hidráulica con la profundidad, indicando que la mayor parte del flujo de agua subterránea sucede en los primeros metros de la secuencia rocosa. A escala regional se puede aplicar la aproximación equivalente de medios porosos. Las medidas de la conductividad hidrálica fueron comparadas con la escala de las pruebas del acuífero, expresadas con el volumen investigado del acuífero. Se desarrolló un proceso de interpolación simple para la conductividad hidráulica de campo, con base en la distancia entre las medidas de campo y los volúmenes probados del acuífero. La distribución regional de la conductividad hidráulica, para las unidades acuíferas fracturadas mayores, indica que la dolomita es más permeable mientras que la arenisca y las rocas cristalinas son las unidades menos permeables.

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Résumé Larticle présente les propriétés hydrauliques dun aquifère régional rocheux qui ont été mesurées dans le cadre dun projet concernant lestimation de la ressource en eau de la plaine de St. Lawrence située dans la partie sud-ouest de Quebec. Affin de comprendre le comportement de l› aquifère tant à l› échelle de fracture qu› à l› échelle régionale on a mené des investigations hydrauliques en utilisant des essais differents. À l› échelle locale lécoulement est déterminé en principal par le système des fractures. Les résultats des essais dinjection au niveau constant ont montré une tendence de décroissance de la conductivité hydraulique avec la profondeur ce qui indique que l› écoulement ait lieu en principal dans les premiers mètres de la structure rocheuse. À l› échelle règionale on peut appliquer lapproche de milieux poreux equivalent. Les mesures de conductivité hydraulique ont été corrélées avec l› échelle des essais de pompage exprimée en termes de volume investigué de l› aquifère. On a mis au point une méthode simple dinterpolation pour le champ de la conductivité hydraulique basée sur la distance entre les essais in situ et les volume investigués de l› aquifère. La distribution règionale de la conductivité hydraulique montre que pour la majorité des unités de roche fracturé les plus perméable sont les roches dolomitiques alors que les grés et les roches cristalines presentent la plus faible perméabilité.

     

Numerical simulation of groundwater flow for a coastal plain in Japan: data collection and model calibration

Using a three-dimensional finite element model, this study characterizes groundwater flow in a costal plain of the Seto Inland Sea, Japan. The model characterization involved taking field data describing the aquifer system and translating this information into input variables that the model code uses to solve governing equations of flow. Geological geometry and the number of aquifers have been analyzed based on a large amount of geological, hydrogeological and topographical data. Results of study demonstrate a high correlation between the ground surface elevation and the groundwater level in the shallow coastal aquifer. For calibrating the numerical groundwater model, the groundwater flow was simulated in steady state. In addition, the groundwater level and trend in the transient state has also been elucidated. The numerical result provides excellent visual representations of groundwater flow, presenting resource managers and decision makers with a clear understanding of the nature of the types of groundwater flow pathways. Results build a base for further analysis under different future scenarios.     

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.     

A RAM-economizing domain decomposition technique for regional high-resolution groundwater simulation

A major limitation of the use of high-resolution groundwater models on a regional scale for resource management by water companies is the excessive RAM requirements of such models which surpass the capacity of today's PCs. A strategy is presented to overcome this problem by using overlapping domain decomposition techniques. Furthermore, because very long computing time is the bottleneck for the practical use of this technique for large groundwater models, an analysis is also presented of a number of methods implemented to increase calculation performance. The approach presented here is characterized by a fairly simple structure that represents a generalized relaxation algorithm. It can be adapted for use with finite element as well as with finite difference methods. Electronic Publication     

A regional-scale groundwater flow model for the Leon-Chinandega aquifer,Nicaragua

Groundwater flow in the Leon-Chinandega aquifer was simulated using transient and steady-state numerical models. This unconfined aquifer is located in an agricultural plain in northwest Nicaragua. Previous studies were restricted to determining groundwater availability for irrigation, overlooking the impacts of groundwater development. A sub-basin was selected to study the groundwater flow system and the effects of groundwater development using a numerical groundwater flow model (Visual MODFLOW). Hydrological parameters obtained from pumping tests were related to each hydrostratigraphic unit to assign the distribution of parameter values within each model layer. River discharge measurements were crucial for constraining recharge estimates and reducing the non-uniqueness of the model calibration. Steady-state models have limited usefulness because of the major variation of recharge and agricultural pumping during the wet and dry seasons. Model results indicate that pumping induces a decrease in base flow, depleting river discharge. This becomes critical during dry periods, when irrigation is highest. Transient modeling indicates that the response time of the aquifer is about one hydrologic year, which allows the development of management strategies within short time horizons. Considering further development of irrigated agriculture in the area, the numerical model can be a powerful tool for water resources management.     

Effects of intraborehole flow on groundwater age distribution

Environmental tracers are used to estimate groundwater ages and travel times, but the strongly heterogeneous nature of many subsurface environments can cause mixing between waters of highly disparate ages, adding additional complexity to the age-estimation process. Mixing may be exacerbated by the presence of wells because long open intervals or long screens with openings at multiple depths can transport water and solutes rapidly over a large vertical distance. The effect of intraborehole flow on groundwater age was examined numerically using direct age transport simulation coupled with the Multi-Node Well Package of MODFLOW. Ages in a homogeneous, anisotropic aquifer reached a predevelopment steady state possessing strong depth dependence. A nonpumping multi-node well was then introduced in one of three locations within the system. In all three cases, vertical transport along the well resulted in substantial changes in age distributions within the system. After a pumping well was added near the nonpumping multi-node well, ages were further perturbed by a flow reversal in the nonpumping multi-node well. Results indicated that intraborehole flow can substantially alter groundwater ages, but the effects are highly dependent on local or regional flow conditions and may change with time.     

Identifying trends in groundwater quality using residence time indicators: an example from the Permian aquifer of Dumfries, Scotland

The Permian sandstone and breccia aquifer of Dumfries has an important role in supplying water to the principal town in southwest Scotland. The area comprises mainly pastoral farmland with some industry and fish farming. Ongoing development of the aquifer has revealed the existence of complex groundwater flow through fractures and increasing nitrate concentrations. To further investigate these issues, the age and quality of groundwater throughout the aquifer has now been assessed using standard hydrogeochemical techniques together with CFCs and SF₆ as residence time indicators. The aquifer consists of sandstone- and breccia-dominated units: the Locharbriggs Sandstone in the east and the Doweel Breccia in the west. Groundwater throughout the aquifer is of Ca–Mg–HCO₃ type and moderately mineralised; pH is near neutral. The observed groundwater chemistry is the product of maritime rainfall modified by the dissolution of carbonate material in the breccia, sandstone and surficial deposits. CFC and SF₆ concentrations are interpreted on the basis of mixing between older (>50 years) and recent (1990s) components. Although there is generally a higher proportion of older water within the Locharbriggs Sandstone compared to the Doweel Breccia, stable isotope evidence suggests that the older water component in the interbedded sandstones of the breccia is of much greater antiquity, possibly containing an element of palaeowater. Concentrations of nitrate across the aquifer can be directly related to the amount of recent recharge. Modern groundwater contains approximately 9 mg l^–1 NO₃-N and pre-1950s groundwater has approximately 2 mg l^–1 NO₃-N. Nitrate concentrations measured at individual boreholes are explained by the relative proportions of modern and pre-1950s groundwater. If current practices continue, the concentrations of nitrate measured across the Dumfries Basin will rise as the proportion of pre-1950s groundwater diminishes.

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Resumen El acuífero de areniscas y brechas Pérmicas de Dumfries desempeña un papel importante en el abastecimiento a la ciudad principal del Sudoeste de Escocia. El área comprende sobretodo tierras de pastoreo y granjas, junto con algunas industrias y piscifactorías. El desarrollo iniciado en el acuífero ha revelado la existencia de un complicado flujo subterráneo a través de las fracturas y de un aumento progresivo en la concentración de nitrato. Con el fin de profundizar en el conocimiento de estos temas, se ha determinado la edad y calidad de las aguas subterráneas en todo el acuífero por medio de técnicas hidrogeoquímicas estándar y de indicadores del tiempo de residencia como los clorofluorcarbonados (CFCs) y el fluoruro de azufre (SF₆). El acuífero está formado por las areniscas de Locharbriggs, al Este, y las brechas de Doweel, hacia el Oeste. Las aguas subterráneas son de tipo bicarbonatadas cálcico-magnésicas y presentan una mineralización moderada, mientras que el pH es prácticamente neutro. La química de las aguas subterráneas es el resultado del aerosol marino modificado por la disolución de materiales carbonatados en las brechas, areniscas y depósitos superficiales. Las concentraciones de CFCs y SF₆ se interpretan a partir de una mezcla de aguas entre una componente antigua (más de 50 años) y una reciente (de la década de 1990). Aunque en general se da una mayor proporción de aguas antiguas en las areniscas de Locharbriggs que en las Brechas de Doweel, los isótopos estables sugieren que la componente de aguas antiguas en las intercalaciones de areniscas que aparecen en la formación de brechas es mucho más antigua, y que, posiblemente, se trate de paleoaguas. Las concentraciones de nitrato en el acuífero pueden ser relacionadas directamente con la aportación de recarga reciente. Las aguas subterráneas modernas contienen aproximadamente 9 mg l^–1 de nitrato, mientras que las aguas subterráneas anteriores a 1950 tienen unos 2 mg l^–1. Se puede explicar las concentraciones de nitrato en pozos individuales por las proporciones relativas de aguas subterráneas modernas y anteriores a 1950. Si las prácticas actuales continúan, se producirá un aumento de las concentraciones de nitrato en la cuenca de Dumfries debido a la disminución de la proporción de aguas antiguas.

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Résumé L'aquifère des grès et brèches du Permien de Dumfries joue un rôle important dans l'alimentation en eau de la principale ville du sud-ouest de l'Écosse. La région est principalement soumise à de l'élevage pastoral avec quelques industries et des piscicultures. La progression de l'exploitation de cet aquifère a révélé l'existence d'un écoulement souterrain complexe en fractures et des concentrations en nitrate croissantes. Dans le but d'étudier ces deux points plus en détail, l'âge et la qualité de l'eau souterraine dans tout l'aquifère ont été évalués en utilisant des techniques hydrogéochimiques conventionnelles avec les CFC et SF₆ comme marqueurs du temps de séjour. L'aquifère est constitué par des unités dominées par des brèches et des grès: les grès de Locharbriggs à l'est et les brèches de Doweel à l'ouest. L'eau souterraine dans tout l'aquifère a un faciès bicarbonaté calcique et magnésien et est moyennement minéralisée; le pH est proche de la neutralité. Le chimisme observé de l'eau souterraine résulte des apports marins par la pluie modifiés par la dissolution du matériau carbonaté des brèches, des grès et des formations superficielles. Les concentrations en CFC et SF₆ sont interprétées sur la base d'un mélange entre des composantes ancienne (>50 ans) et récente (années 1990). Bien qu'il y ait en général une plus forte proportion d'eau ancienne dans les grès de Locharbriggs que dans la brèche de Doweel, les isotopes stables suggèrent que la composante d'eau ancienne dans les grès interstratifiés des brèches est beaucoup plus ancienne, contenant probablement un élément d'eau fossile. Les concentrations en nitrate dans l'aquifère peuvent être directement reliées au taux de recharge récente. L'eau souterraine moderne contient approximativement 9 mg l^–1 en NO₃-N et l'eau souterraine d'avant 1950 environ 2 mg l^–1 en NO₃-N. Les concentrations en nitrate mesurées dans des forages individuels sont expliquées par des proportions relatives d'eaux souterraines moderne et antérieure à 1950. Si les pratiques actuelles continuent, les concentrations en nitrate mesurées dans le bassin de Dumfries augmenteront alors que la part de l'eau souterraine d'avant 1950 diminuera.

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Numerical simulation of groundwater flowing to horizontal seepage wells under a river

A horizontal seepage well, consisting of an interconnected vertical well, galleries, chambers and small-diameter radiating bores, is used to acquire relatively clean water that has been filtered through natural alluvial deposits in a riverbed. It has wide application, especially in arid and semi-arid areas. The lack of calculation formulae or models for horizontal seepage wells, up until now, has resulted in several false applications. Based on the analysis of groundwater flow characteristics, it has been concluded that several flow regimes coexist and hydraulic head loss exists in the horizontal seepage well. To avoid the difficulty of confirming the flux or head distribution in such a complex system, the model boundary of the whole horizontal seepage well has been moved to that of just the vertical well, and the well-aquifer system was treated as a heterogeneous medium, where the horizontal seepage well itself is a highly permeability medium. A mathematical model has been developed, based on the coupled seepage-pipe flow, by the introduction of equivalent hydraulic conductivity according to different flow regimes. Then a three-dimensional finite difference numerical model, based on the mathematical model, was developed and applied to a horizontal seepage well in China. The numerical model verified the groundwater flow characteristics of the horizontal seepage well. An erratum to this article can be found at     

Comparative analysis between analytical approximations and numerical solutions describing recession flow in unconfined hillslope aquifers

Recession flow of aquifers from a hillslope can be described by the non-linear Boussinesq equation. Under strong assumptions and for specific conceptual formulations, different authors derived analytical approximations or linearized versions to this partial differential equation. A comparative analysis between some analytical approximations of the Boussinesq equation and the numerical solution of the recession flow of an unconfined homogeneous aquifer (horizontal, inclined and concave aquifer floor) was carried out. The objective was to define the range where the analytical solutions approximate the numerical solution. The latter was considered in this study as the reference method, because it requires fewer assumptions. From the considered analytical approximations, exponential decay relationships were found to be mainly valid for fine domain materials when horizontal, mild slopes (less than 2%) and concave aquifer floors were considered, but failed to reproduce coarse aquifer numerical model outflows, in contrast to the quadratic decay relationship, which better reproduce outflows in such domains. On the basis of the comparative analysis, it has been found that recession flows obtained with the considered analytical approximations yield similar values only for certain ranges of aquifer properties and geometries.

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Résumé L’écoulement de décrue dans les aquifères de versant peut être décrit par l’équation non-linéaire de Boussinesq. En considérant d’importantes hypothèses et des formulations conceptuelles spécifiques, différents auteurs ont dérivé des approximations analytiques ou des versions linéarisées de cette équation partielle différentielle. Une analyse comparative entre des approximations analytiques de l’équation de Boussinesq et une solution numérique de la décrue d’un aquifère homogène libre (limite inférieure de l’aquifère horizontale, inclinée et concave) a été effectuée. L’objectif était de définir la plage pour laquelle les solutions analytiques approchent la solution numérique. Cette dernière a été considérée dans cette étude comme la méthode de référence car elle demande moins d’hypothèses. Parmi les approximations analytiques considérées, les relations de décroissance exponentielle se sont avérées être principalement valides pour des matériaux fins lorsque des couches aquifères horizontales, concaves et de pentes douces (moins de 2%) étaient considérées; en revanche, celles-ci n’ont pas réussi à reproduire les écoulements sortants d’un modèle numérique pour un aquifère à matériaux grossiers, contrairement à la relation de décroissance quadratique qui reproduit mieux les écoulements sortants dans de tels milieux. Sur la base de l’analyse comparative, il s’est avéré que les écoulements en décrue obtenus avec les approximations analytiques considérées présentent des valeurs similaires seulement pour certains critères géométriques et propriétés de l’aquifère.

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Resumen Puede describirse el flujo de recesión de acuíferos de una ladera mediante la ecuación no-lineal de Boussinesq. Bajo fuertes supuestos y para formulaciones conceptuales específicas, distintos autores han derivado aproximaciones analíticas o versiones lineales para esta ecuación parcial diferencial. Se realizó un análisis comparativo entre algunas aproximaciones analíticas de la ecuación de Boussinesq y la solución numérica del flujo de recesión de un acuífero homogéneo no confinado (piso de acuífero cóncavo, inclinado, y horizontal). El objetivo consistió en definir el rango donde las soluciones analíticas aproximan la solución numérica. En este estudio se consideró la solución numérica como el método de referencia debido a que requiere menores supuestos. De las aproximaciones analíticas consideradas se encontró que las relaciones de desintegración exponencial eran principalmente válidas para materiales de ámbitos finos donde se consideraron pisos de acuíferos horizontal, pendientes medianas (menores a 2%) y cóncavos pero fallaron en reproducir flujos de salida de modelos numéricos de ámbito grueso, en contraste con la relación de desintegración cuadrática la cual reproduce mejor los flujos de salida en tales ámbitos. En base al análisis comparativo se encontró que los flujos de recesión obtenidos con las aproximaciones analíticas consideradas aportaron valores similares solo para ciertos rangos de propiedades y geometrías de acuíferos.

     

Small scale study of groundwater flow in a fractured carbonate-rock aquifer at the St-Eustache quarry, Québec, Canada

A small-scale hydrogeological study was conducted in a fractured carbonate-rock aquifer located in a quarry to relate groundwater flow to the fracture network. The field study in the St-Eustache quarry, which integrates structural surveys, well logging and hydraulic tests, showed that the most important features that affect groundwater flow in the sedimentary aquifer are high hydraulic conductivity horizontal bedding planes. Vertical fractures are abundant in the quarry and throughout the region, but they have a minor effect on groundwater flow. To have a significant impact on the flow regime and lead to vertical groundwater flow, the permeability of all vertical joints need to be enhanced compared to what was generally observed at the site. Such an increase in permeability could potentially occur where dissolution and fracturing is more intense or at stress release locations such as near the surface in the quarry.     

Groundwater flow through Pleistocene glacial deposits in the rapidly urbanizing Rouge River–Highland Creek watershed,City of Scarborough,southern Ontario,Canada

The city of Scarborough lies on the eastern margin of the Greater Toronto Area of southern Ontario, Canada, along the northern coastline of Lake Ontario. The City has a population of 500,000 and is presently one of the fastest growing communities in Canada. The City is expanding northwards onto rural land on the south slope of the large Pleistocene glacial Oak Ridges Moraine system. The moraine system is underlain by a thick (150 m) succession of tills, sands and gravels and is a regionally-significant recharge area for three principle aquifer systems that discharge to numerous watercourses that flow to Lake Ontario. Protection of deeper aquifers from surface-generated urban contaminants is a particular concern. A groundwater flow model using Visual MODFLOW was developed for the 350-km² Rouge River–Highland Creek (RRHC) drainage basin using an extensive GIS-based collection of subsurface geological, geophysical and hydrogeological data, maps of land use and surficial geology. The RRHC model was calibrated against point water level data, known potentiometric surfaces of the principal aquifers and baseflow measurements from streamflow gauging stations and determined to be within acceptable limits. Water balance calculations indicate that 70% of the basin recharge (106,000 m³/day) enters the Upper Aquifer along the crest and immediate flanks of the Oak Ridges Moraine. To the south, Upper Aquifer water moving through fractured till aquitards accounts for more than 75% of recharge to deeper aquifers. Water quality data confirm previous observations that urban- and rural-sourced contaminants (chlorides and nitrates) present in Upper Aquifer waters are moving rapidly into deeper aquifers. Some 83% of total RRHC recharge water is ultimately discharged as baseflow to creeks draining to Lake Ontario; the remainder discharges to springs and along eroding lakeshore bluffs. Model results demonstrate that deeper aquifers are poorly protected from urban contaminants and that long-term protection of ground and surface water quality has to be a priority of municipal planners if the resource is not to be severely degraded. Electronic Publication     

The role of some regional factors in the assessment of well yields from hard-rock aquifers of Fennoscandia

The attributes of about 13,600 hard-rock water wells were arranged in three profiles parallel to gradients of typical regional factors. These factors are thought to be of importance to groundwater yield from wells in the Fennoscandian crust and were subjected to a statistical analysis. The regional factors comprise annual rate of postglacial crustal uplift and annual average precipitation, whereas soil type, soil depth, and bedrock type are considered sub-regional and local factors. There is no clear trend in well yield along the regional gradients. The results of the statistical analysis indicate that for the typical regional variables, the rate of postglacial uplift and average annual precipitation explain not more than 11% of the observed variation in well yields. Some of these factors, and other factors not included in this study, work in opposite directions and may cancel each other out. It is concluded that other, more local factors, such as well depth, proximity to fracture-related lineaments and topography, have a greater influence on well yields in this area.

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Resumen Se ha recogido los atributos de aguas subterráneas con 13.600 años de antigüedad en tres perfiles perpendiculares a los gradientes de los factores regionales típicos, las cuales son captadas por pozos perforados en rocas duras. Se cree que estos factores tienen importancia en el rendimiento de los pozos realizados en la corteza de Fenoscandia, y han sido analizados estadísticamente. Los factores regionales comprenden la tasa anual de alzamiento postglacial y la precipitación media anual, mientras que el tipo y la profundidad del suelo, y el tipo de roca madre, son tratados como factores sub-regionales y locales. No se aprecia una tendencia clara en el rendimiento de los pozos a lo largo de los gradientes regionales. Los resultados del análisis estadístico indican que, para las variables regionales típicas, la tasa de alzamiento postglacial y la precipitación media anual explican como mucho el 11,2% de las variaciones observadas en el rendimiento de los pozos. Algunos de estos factores, así como otros no incluidos en el estudio, son de signo opuesto y pueden llegar a compensarse. Se concluye que factores diferentes, de escala más local, tales como la proximidad a lineamientos correlacionados con fracturas o la topografía, pueden afectar de forma más importante al rendimiento de los pozos en esta zona.

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Résumé Les propriétés d'environ 13,600 puits d'extraction d'eau souterraine dans le socle ont été réparties selon trois profils disposés perpendiculairement aux gradients de facteurs régionaux typiques. Ces facteurs sont considérés comme étant importants pour les rendements des puits dans le bouclier fenno-scandien et ont été soumis à une analyse statistique. Les facteurs régionaux sont le taux de soulèvement postglaciaire et les précipitations moyennes annuelles, alors que le type et l'épaisseur de sol, le type de substratum sont des facteurs sub-régionaux et locaux. Il n'existe pas de tendance dans le rendement des puits selon les gradients régionaux. Les résultats de l'analyse statistique indiquent que pour les variables régionales typiques, le taux de soulèvement postglaciaire et les précipitations moyennes annuelles n'expliquent pas plus de 11,2% de la variation observée du rendement des puits. Certains de ces facteurs, ainsi que d'autres non pris en compte dans cette étude, jouent dans des directions opposées et peuvent s'annuler l'un l'autre. En conclusion, d'autres facteurs, plus locaux, tels que la proximité de linéaments corrélés aux fractures et la topographie peuvent avoir une grande importance pour le rendement des puits dans cette région.

     

Linking groundwater quality to residence times and regional geology in the St. Lawrence Lowlands,southern Quebec,Canada

The assessment of groundwater quality in shallow aquifers is of high societal relevance given that large populations depend directly on these water resources. The purpose of this study was to establish links between groundwater quality, groundwater residence times, and regional geology in the St. Lawrence Lowlands fractured bedrock aquifer. The study focuses on a 4500 km² watershed located in the St. Lawrence Lowlands of the province of Quebec in eastern Canada. A total of 150 wells were sampled for major, minor, and trace ions. Tritium (³H) and its daughter element, ³He, as well as radiocarbon activity (A¹⁴C) were measured in a subset of wells to estimate groundwater residence times. Results show that groundwater evolves from a Ca–HCO₃ water type in recharge zones (i.e., the Appalachian piedmont) to a Na–HCO₃ water type downgradient, toward the St. Lawrence River. Locally, barium (Ba), fluoride (F), iron (Fe), and manganese (Mn) concentrations reach 90, 2, 18, and 5.9 mg/L respectively, all exceeding their respective Canadian drinking water limits of 1, 1.5, 0.3, and 0.05 mg/L. Release of these elements into groundwater is mainly controlled by the groundwater redox state and pH conditions, as well as by the geology and the duration of rock–water interactions. This evolution is accompanied by increasing ³H/³He ages, from 4.78 ± 0.44 years upgradient to more than 60 years downgradient. Discrepancies between calculated ³H/³He and ¹⁴C water ages (the latter ranging from 280 ± 56 to 17,050 ± 3410 years) suggest mixing between modern water and paleo-groundwater infiltrated through subglacial recharge when the Laurentide Ice Sheet covered the study area, and during the following deglaciation period. A linear relationship between ³H activity and corrected ¹⁴C versus Mg/Ca and Ba support a direct link between water residence time and the chemical evolution of these waters. The Ba, F, Fe, and Mn concentrations in groundwater originate from Paleozoic rocks from both the St. Lawrence Platform and the Appalachian Mountains. These elements have been brought to the surface by rising hydrothermal fluids along regional faults, and trapped in sediment during their deposition and diagenesis due to reactions with highly sulfurous and organic matter-rich water. Large-scale flow of meltwater during subglacial recharge and during the subsequent retreat of the Laurentide Ice Sheet might have contributed to the leaching of these deposits and their enrichment in the present aquifers. This study brings a new and original understanding of the St. Lawrence Lowlands groundwater system within the context of its geological evolution.     

Hydrogeochemistry and groundwater origin of the Basses-Laurentides sedimentary rock aquifer system, St. Lawrence Lowlands, Québec, Canada

A comprehensive hydrogeochemical study was carried out in the Paleozoic Basses-Laurentides sedimentary rock aquifer system in Québec over a 1500 km² study area. Groundwater samples were collected at 153 sites, characterizing all geological and hydrogeological units to a maximum depth of 140 m. Groundwater was analyzed for major, minor and trace inorganic constituents, stable isotopes δ ²H, δ ¹⁸O, and δ ¹³C of dissolved inorganic carbon (DIC), and some samples were analyzed for ³H, and ¹⁴C of DIC. The regional distribution of groundwater types shows that the hydrogeological conditions exert a dominant control on the major ions chemistry of groundwater. Preferential recharge areas are characterized by tritiated Ca-Mg-HCO₃ groundwater, and confined conditions by submodern Na-HCO₃ and Na-Cl groundwater types. Two groundwater end-members are identified in the aquifer system, modern meteoric water and Pleistocene Champlain Sea water. The region displays significant variations of groundwater geochemistry and quality controlled by glaciation, Champlain Sea invasion, lithological rock diversity, and flow system scales. This situation leads to varied groundwater types and origins within a restricted area.     

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.     

Characterization of the shallow groundwater system in an alpine watershed: Handcart Gulch,Colorado, USA

Water-table elevation measurements and aquifer parameter estimates are rare in alpine settings because few wells exist in these environments. Alpine groundwater systems may be a primary source of recharge to regional groundwater flow systems. Handcart Gulch is an alpine watershed in Colorado, USA comprised of highly fractured Proterozoic metamorphic and igneous rocks with wells completed to various depths. Primary study objectives include determining hydrologic properties of shallow bedrock and surficial materials, developing a watershed water budget, and testing the consistency of measured hydrologic properties and water budget by constructing a simple model incorporating groundwater and surface water for water year 2005. Water enters the study area as precipitation and exits as discharge in the trunk stream or potential recharge for the deeper aquifer. Surficial infiltration rates ranged from 0.1–6.2×10^?5 m/s. Discharge was estimated at 1.28×10^?3 km³. Numerical modeling analysis of single-well aquifer tests predicted lower specific storage in crystalline bedrock than in ferricrete and colluvial material (6.7×10^?5–2.0×10^?3 l/m). Hydraulic conductivity in crystalline bedrock was significantly lower than in colluvial and alluvial material (4.3×10^?9–2.0×10^?4 m/s). Water budget results suggest that during normal precipitation and temperatures water is available to recharge the deeper groundwater flow system.     

Three-dimensional variable-density flow simulation of a coastal aquifer in southern Oahu, Hawaii, USA

Three-dimensional modeling of groundwater flow and solute transport in the Pearl Harbor aquifer, southern Oahu, Hawaii, shows that the readjustment of the freshwater–saltwater transition zone takes a long time following changes in pumping, irrigation, or recharge in the aquifer system. It takes about 50 years for the transition zone to move 90% of the distance to its new steady position. Further, the Ghyben–Herzberg estimate of the freshwater/saltwater interface depth occurred between the 10 and 50% simulated seawater concentration contours in a complex manner during 100 years of the pumping history of the aquifer. Thus, it is not a good predictor of the depth of potable water. Pre-development recharge was used to simulate the 1880 freshwater-lens configuration. Historical pumpage and recharge distributions were used and the resulting freshwater-lens size and position were simulated through 1980. Simulations show that the transition zone moved upward and landward during the period simulated.Previous groundwater flow models for Oahu have been limited to areal models that simulate a sharp interface between freshwater and saltwater or solute-transport models that simulate a vertical aquifer section. The present model is based on the US Geological Surveys three-dimensional solute transport (3D SUTRA) computer code. Using several new tools for pre- and post-processing of model input and results have allowed easy model construction and unprecedented visualization of the freshwater lens and underlying transition zone in Hawaiis most developed aquifer.

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Electronic Supplementary Material Supplementary material is available in the online version of this article at .

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Resumen La modelación tridimensional del flujo de agua subterránea y del transporte de solutos en el acuífero de Pearl Harbor, en la parte sur de Oahu, Hawaii, muestra que el reajuste de la zona de transición agua dulce–agua salada, toma un largo tiempo a partir de cambios en el bombeo, irrigación o recarga en el sistema acuífero. Le toma alrededor de 50 años, a la zona de transición, moverse el 90% de la distancia hacia su nueva posición estacionaria. Además, el estimativo de Ghyben–Herzberg, sobre la profundidad de la interfase agua dulce–agua salada, se encuentra entre el 10 y el 50% en los contornos simulados de concentración de agua salada, de una manera compleja, durante 100 años de la historia de bombeo del acuífero. Por tanto, no es este un buen predictor de la profundidad del agua potable. Se utilizó una recarga pre – desarrollo, para simular la configuración del lente de agua dulce en 1880. Fueron utilizadas las distribuciones históricas del bombeo y de la recarga y se simularon el tamaño y posición resultantes del lente de agua dulce hasta 1980. Esas simulaciones muestran que la zona de transición se movió tierra adentro y hacia arriba, durante el periodo que se simuló.Los anteriores modelos de flujo para agua subterránea en Oahu, han sido limitados a modelos areales, que simulan una interfase abrupta entre agua dulce y agua salada, o bien han sido modelos de transporte de solutos que simulan una sección vertical del acuífero. El modelo presente está basado en el programa de computador del US Geological Survey (3D SUTRA), para transporte de solutos en tres dimensiones. Mediante el uso de varias herramientas nuevas para pre – procesamiento y post – procesamiento de las entradas y resultados del modelo, se ha permitido una construcción fácil del mismo y una visualización sin precedentes del lente de agua dulce y de la zona de transición subyacente en el acuífero más desarrollado de Hawaii.

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Résumé La modélisation tridimensionnelle de lécoulement et du transport dans la partie sud de laquifère Oahu-Hawai montre que le temps de réajustement de la zone de transition entre leau douce et leau salée est assez long et dépend de la variation des pompages et des irrigations, ainsi que de la recharge du système aquifère. Il sont nécessaires 50 ans pour que la zone de transition parcoure 90% de la distance qui la sépare de sa nouvelle position. La profondeur du biseau estimée par le schéma Ghyben–Herzberg se trouve entre les contours de 10 et 50% de la concentration de leau salée. Ce résultat a été obtenu après la simulation de lhistoire du pompage de laquifère pendant une période de 100 ans. Donc le schéma Ghyben–Herzberg conduit aux valeurs erronées de la profondeur de leau potable. La valeur de la recharge davant lexploitation de laquifère a été utilisée pour simuler la configuration des lentilles deau douce en 1880. En utilisant lhistoire du pompage et la distribution de la recharge ont on a simulé les dimensions et le positions des lentilles deau douce jusqu› en 1980. Les simulation montrent que le mouvement de la zone de transition est ascendant et vers le continent.Les modèles antérieurs de la zone dOahu ont été des modèles locaux qui ont simulé une interface nette eau douce-eau salée ou des modèles de transport bidimensionnels, dans une coupe verticale. Le modèle actuel est basé sur le code 3D-SUTRA, réalisé par le Service Géologique des États-Unis. L› utilisation des différents techniques de traitement des données a permis une construction facile du modèle, ainsi qu› une visualisation sans précédent des lentilles deau douces et de la zone de transition sous-jacente dans le plus grand aquifère du Hawai.

     

Numerical simulation of groundwater and early warnings from the simulated dynamic evolution trend in the plain area of Shenyang,Liaoning Province (P.R. China)

Groundwater level is the most direct factor reflecting whether groundwater is in a virtuous cycle. It is the most important benchmark for deciding whether a balance can be struck between groundwater discharge and recharge and whether groundwater exploitation will trigger problems pertinent to environment, ecology and environmental geology. According to the borehole and long-term monitoring wells data in the plain area of Shenyang, a numerical groundwater model is established and used to identify and verify the hydrogeological parameters and balanced items of groundwater. Then the concept of red line levels, the control levels of groundwater is proposed, the dynamic evolution trend of groundwater under different scenarios is analyzed and predicted and groundwater alerts are given when groundwater tables are not between the lower limit and the upper limit. Results indicated: (1) The results of identification and verification period fitted well, and the calculation accuracy of balanced items was high; (2) with the implementation of shutting wells, groundwater levels in urban areas of Shenyang would exceed the upper limit water level after 2020 and incur some secondary disasters; (3) under the recommended scenario of water resources allocation, early-warnings for groundwater tables outside the range would occur in the year of 2020, 2023, 2025 respectively for successive wet, normal and dry years. It was imperative to reopen some groundwater sources and enhance real-time supervision and early-warning to prevent the occurrence of potential problems.     

Effects of groundwater flow on mineral diagenesis, with emphasis on carbonate aquifers

 This article provides a critical synopsis of the effects of groundwater flow on mineral diagenesis. Emphasis is placed on those aspects and processes that change porosity and permeability in carbonate aquifers, because they are of particular importance to human societies as sources of supplies of water for human consumption (drinking, irrigation) and of crude oil and natural gas. Diagenetic settings in carbonates as well as clastics are generally ill defined. This paper proposes a new comprehensive classification of diagenetic settings into near-surface, shallow-, intermediate-, and deep-burial diagenetic settings; hydrocarbon-contaminated plumes; and fractures. These settings are defined on the basis of mineralogy, petroleum, hydrogeochemistry, and hydrogeology. This classification is applicable to all sedimentary basins. Diagenesis is governed by various intrinsic and extrinsic factors that include thermodynamic and kinetic constraints, as well as microstructural factors that may override the others. These factors govern diagenetic processes, such as dissolution, compaction, recrystallization, replacement, and sulfate–hydrocarbon redox-reactions. Processes such as cementation, dissolution, and dolomitization require significant flow of groundwater driven by an externally imposed hydraulic gradient. Other processes, such as stylolitization and thermochemical sulfate reduction, commonly take place without significant groundwater flow in hydrologically nearly or completely stagnant systems that are geochemically "closed." Two major effects of groundwater flow on mineral diagenesis are enhancement and reduction of porosity and permeability, although groundwater flow can also leave these rock properties essentially unchanged. In extreme cases, an aquifer or hydrocarbon reservoir rock can have highly enhanced porosity and permeability due to extensive mineral dissolution, or it can be plugged up due to extensive mineral precipitation. Received, April 1998 · Revised, July 1998 · Accepted, September 1998