Hydraulic, electrical, and tracer test results are presented for a natural fracture in granite. The hydraulic and electrical apertures of the fracture are similar and suggest minimal fracture surface‐to‐surface contact. Tracer aperture exceeds hydraulic aperture indicating transport at a rate less than that predicted on the basis of hydraulic aperture. Numerical simulation of tracer transport reveals that transport within the fracture is not explicable in terms of parallel plate flow and that transport occurs locally at rates in excess of the overall rate defined by tracer aperture.
Analytic relations are developed between the fractal parameters of a random, isotropic population of disc‐shaped fractures and the parameters of the corresponding population of fracture traces expressed in outcrop. These relations indicate that a fractal distribution of fracture diameters translates to a fractal distribution of trace lengths and that the parameters for diameter may be uniquely determined from the parameters for trace length. Probabilistic results demonstrate the accuracy of the analytic relations and identify nonideal behaviors at the limits of the computed trace length data. It is expected that these relations will be useful in the three‐dimensional characterization of fracture systems in rock from in situ trace length data.
Abstract Numerical simulation of ground‐water flow and transport is often impeded by a lack of information regarding the conceptual model and properties that best represent the water‐bearing formation. Inverse analysis assists in the estimation of the properties of the formation once a defensible form of the conceptual model is defined. In essence, inverse analysis reverses the direction of model application to determine the properties that best match data collected from the formation under investigation. This paper describes the development of an inverse analysis implementation of the well‐known and versatile SUTRA model of ground‐water flow and transport. The inverse analysis algorithm retains the extensive functionality of SUTRA in forward simulation and allows various forms of parameter constraints to be stipulated. Addition of inverse analysis functionality to an existing ground‐water model circumvents the development of redundant models, reduces development time, and ensures the consistent and concurrent development of predictive and interpretive capabilities. Two example analyses are presented to demonstrate a portion of the functionality of the novel inverse analysis algorithm.
Abstract This study evaluated the capability of four spatial hydrologic models to estimate summer low-flow stream discharge, as a surrogate for baseflow, and assessed the influence of land cover/land use on these flows, in small streams across the Oak Ridges Moraine. Low-flow discharge varied predictably with area of the upstream catchment, but also with reach slope and a measure of land cover disturbance (LDI). Low-flow volumes were lowest in streams with moderate agricultural and/or urban development (LDI of eight to 12%), and high over a range of development intensities. Each of Baseflow Index (BFI×Area), Darcy Index (DI), MODFLOW (MF) and a finer resolution MODFLOW model (FMF) were about equal in their capability to estimate low-flow discharge, with MF and FMF having a somewhat stronger relationship and Darcy Index having a somewhat poorer relationship, particularly in smaller catchments. Each of the models generally predicted low-flow discharge volumes to within about 400 L/s of the actual observed low-flow discharge. The models, therefore, were generally unable to predict whether a stream was flowing during periods of low-flow when the upstream catchment was smaller than about 17,800 ha. It was found that these methods cannot be reliably applied in small catchments as there is too much natural variability in flow conditions. This paper suggests that these methods do not reflect local conditions, but rather provide generalized information about water flows. As a result, it is recommended that until spatial model predictions are improved for local applications, water managers should invest in field surveys to confirm flow conditions in small catchments. La présente étude avait pour objectif d’une part d’évaluer quatre modèles hydrologiques spatiaux afin d’en dégager la capacité d’estimation du débit d’étiage d’été, en tant que substitut du débit de base, et, d’autre part, d’évaluer l’incidence sur ces débits de la couverture terrestre et de l’affectation des terres dans les petits cours d’eau à l’échelle de la moraine d’Oak Ridges. Les débits d’étiage variaient de manière prévisible selon la zone du bassin hydrographique en amont, mais également selon la pente du bief et selon une mesure de la perturbation de la couverture terrestre (PCT). Les volumes en période de basses eaux étaient plus bas dans les cours d’eau touchés par des activités agricoles et/ou un développement urbain modérés (PCT de 8 à 12 %) et ils étaient élevés en fonction d’un éventail d’intensités sur le plan du développement. Chacun des modèles suivants : indice de débit de base (IDB × surface), coefficient de Darcy (CD), modèle MODFLOW (MF) et modèle MODFLOW à plus haute résolution (MFHR) étaient à peu près égaux en ce qui concerne leur capacité d’évaluer les débits d’étiage, le modèle MF et le modèle MFHR ayant une relation un peu plus forte et le coefficient de Darcy ayant une relation un peu plus médiocre, en particulier dans les bassins plus petits. Chacun des modèles ont permis en général de prédire des volumes d’étiage dans des limites de 400 litres/seconde environ par rapport au débit d’étiage réel observé. Par conséquent, en général les modèles ne pouvaient pas prédire si un cours d’eau s’écoulait pendant les périodes d’étiage lorsque la superficie du bassin en amont était inférieure à 17 800 ha environ. Il a été constaté que ces méthodes ne peuvent être appliquées en toute fiabilité aux petits bassins car il existe trop de variabilité naturelle dans les conditions d’écoulement. Le présent article donne à entendre que ces méthodes ne reflètent pas les conditions locales, mais qu’elles fournissent plutôt des renseignements généralisés sur les processus d’écoulement de l’eau. Il est donc recommandé que, jusqu’à ce que les prédictions du modèle spatial soient améliorées pour les applications locales, les gestionnaires de l’eau auraient intérêt à investir dans les études sur le terrain afin de confirmer les conditions d’écoulement dans les petits bassins hydrographiques.
Numerical models of ground-water flow within the regional aquifer underlying Lambton County, Ontario, Canada, are constructed by the conjunctive application of methods of regression and inverse analyses. Regression analysis of physiographic and hydraulic head data reveals a distinct relation between ground-water levels and ground-surface topography that is used to condition the aquifer models that are subjected to inverse analysis. Inverse analysis determines the variation of hydraulic head along the perimeter of the region and the distribution of ground-water recharge and discharge within the region that optimally replicate the observed hydraulic head data. The use of physiographic data as a substitute for geologic data in the construction of the aquifer models is defended on the basis of the constraints that apply to the investigation and the opportunity to invoke hydrogeologic judgment in the evaluation of the results. Interpretation of the results of the analyses reveals important characteristics of the hydrogeology of Lambton County, including an area of elevated ground-water recharge and the partitioning of ground-water discharge to the Saint Clair River.
Hydraulic fracturing of a geologic formation induces fluid flow within the formation in response to fracturing fluid loss and poroelasticity effects. Thus, hydraulic fracturing displaces the fluids that are distributed within the formation prior to fracturing, and may mobilize contaminants if applied in conjunction with groundwater contamination remediation. Fluid displacement is determined by coupling relations for hydraulic fracture extension to the temporal and spatial superposition of fundamental solutions for point fluid injection and a point dilation. Expressions are developed for three two-dimensional hydraulic fracture models for the limiting cases of high and low fracturing fluid loss to the formation. These results indicate that fluid displacement decreases with distance from the fracture and approaches an axisymmetric variation at greater than two fracture lengths from the well bore. Displacements induced by fluid loss and poroelasticity converge at greater than one fracture length from the fracture surface. Determination of fluid displacement for a hydraulic fracture treatment that may be typical in groundwater contamination remediation returns displacement magnitudes that are not likely to hamper the remediation effort. La fracturation hydraulique d’une formation géologique provoque 1’écoulement des fluides dans la formation sous l’effet de la perte de fluides de fracturation et de la poroélasticité. Ainsi, la fracturation hydraulique déloge les fluides qui étaient répartis dans la formation avant la fracturation et, utilisée en conjunction avec la dépollution des eaux souterraines, pent mobiliser les contaminants. On établit le déplacement des fluides en mettant les rapports qui visent l’extension de la fracturation hydraulique en corrélation avec la superposition temporelle et spatiale des solutions fondamentales pour l’injection ponctuelle des fluides et une dilatation ponctuelle. Les auteurs formulent des expressions pour trois modèles bidimensionnels de fracture hydraulique pour les cas limites de forte et de faible perte de fluides de fracturation dans la formation. Les résultats obtenus indiquent que le déplacement des fluides diminue avec la distance par rapport à la fracture et se rapproche d’une variation axisymétrique à plus de deux longueurs de fracture du trou de sonde. Les déplacements provoqués par la perte de fluides et la poroélasticité convergent é plus d’une longueur de fracture de la surface de fracturation. La détermination du déplacement des fluides pour un traitement par fracturation hydraulique qui pent êitre représentatif dans la dépollution des eaux souterraines montre que cc déplacement est d’un ordre de grandeur qui est pen probable de faire obstacle aux efforts de dépollution.
Physical and numerical studies of transient flow in a model of discretely fractured rock are presented. The physical model is a thermal analogue to fractured media flow consisting of idealized disc‐shaped fractures. The numerical model is used to predict the behavior of the physical model. The use of different insulating materials to encase the physical model allows the effects of differing leakage magnitudes to be examined. A procedure for determining appropriate leakage parameters is documented. These parameters are used in forward analysis to predict the thermal response of the physical model. Knowledge of the leakage parameters and of the temporal variation of boundary conditions are shown to be essential to an accurate prediction. Favorable agreement is illustrated between numerical and physical results. The physical model provides a data source for the benchmarking of alternative numerical algorithms.
Abstract A revised approach to the calculation of baseflow using the method originally proposed by the United Kingdom Institute of Hydrology is presented. The revisions resolve two aspects of the method that lead to less than optimal results; that is, the calculation of values of baseflow that exceed the corresponding values of streamflow and the dependence of the calculated values on the origin of the five-day segmentation of the input streamflow data. The approach is illustrated using streamflow monitoring information that is typical for areas of southern Ontario, Canada, where baseflow is primarily the result of groundwater discharge. Key words / Mots clefs: baseflowgroundwaterstreamflowécoulement de baseeaux souterrainesécoulement fluvial Additional informationNotes on contributorsAndrew R. Piggott andrew.piggott@ec.gc.ca
