139 research outputs found
Interpretation of flowmeter data in heterogeneous layered aquifers
No full textWe analyze numerically the impact of key assumptions which are usually adopted to interpret borehole flowmeter test data. We base our work on a set of detailed numerical simulations of transient and pseudo-steady state convergent flow to a fully penetrating well that pumps at a constant total flow rate in a confined three-dimensional system formed by well demarcated facies. Our results allow identifying the effects of (i) the presence of a skin zone around the well, (ii) the contrast of hydraulic conductivities of the aquifer facies and the gravel pack, and (iii) the dynamics of the flow regime on the ability to characterize the local aquifer conductivities of a stratified system through the typical interpretation of downhole flowmeter tests. Vertical intervals along the wellbore, where reliable estimates of the aquifer conductivities can be obtained through classical formulations are identified in terms of a set of dimensionless parameters. These parameters involve the thickness of the facies constituting the stratified aquifer and the contrast of conductivities amongst different facies and between them and the skin zone
Theoretical analysis and field evidence of reciprocity gaps during interference pumping tests
No full textWe address the question of the reciprocity of drawdowns observed during a sequence of interference pumping tests performed within a porous medium, in the context of different conceptual models of subsurface flow. We provide a generalization of the work by Bruggeman (1972), extending his results obtained in 1972 for Darcian flows in an unbounded, heterogeneous porous medium. We then analyze reciprocity within a dual-continuum conceptualization, where the medium is viewed as being composed of two overlapping continua, representing the porous matrix and embedded fractures, respectively. We show theoretically, and demonstrate numerically, that only drawdowns associated with the fracture continuum display reciprocity under transient flow conditions. Conversely, non-flowing matrix and fracture continua display reciprocity behavior under steady-state conditions. We then provide field evidence of the insurgence of reciprocity gaps by analyzing interference test data from the karstic limestone aquifer of the Hydrogeological Experimental Site (HES) in Poitiers, France. On the basis of our theoretical results and experimental observations, we discuss different interpretations of the observed reciprocity gaps. These include (a) non-linear dependencies of local hydraulic parameters, (b) occurrence of internal boundaries within the domain, (c) inertial effects that develop through open conduits within the rock matrix, (d) modifications of the aquifer properties between subsequent pumping tests, and (e) significant contribution of the matrix pressure in monitored wells when the behavior of the aquifer is conceptualized by a dual-continuum approach
Identification of the unknown pollution source in the Alsatian aquifer (France) through groundwater modelling and Artificial Neural Networks applications
No full textOn the emergence of reciprocity gaps during interference pumping tests in unconfined aquifers
No full textWe present a theoretical analysis of the conditions under which reciprocity gaps between vertically-averaged (square) drawdowns can be observed during typical interference pumping tests performed in unconfined heterogeneous aquifers. This work expands on the analysis of Delay et al. [2011] and indicates that reciprocity gaps between vertically-averaged heads monitored during two consecutive tests can be due to vertical trends in aquifer properties (i.e., hydraulic conductivity and specific storage). These are then reflected in the vertical averaging procedure which is performed within a screened borehole and give rise to different observed dynamics of the averaged system responses. Spatially distributed recharge reflecting drainage from the unsaturated region during pumping can also significantly contribute to non-reciprocal behavior, especially for large test durations. Our theoretical findings are then illustrated through a suite of numerical simulations under a variety of vertical distributions of hydraulic parameters and in the presence of saturated/unsaturated flow conditions. We conclude that proper identification of the causes underlying reciprocity gaps can significantly enhance our conceptual understanding of the system response to a sequence of pumping stresses
ANN-based approach for the estimation aquifer pollutant source behaviour
No full textThe problem of identifying an unknown pollution source in polluted aquifers, based on known contaminant concentrations measurement, is part of the broader group of issues, called inverse problems. This paper investigates the feasibility of solving the groundwater pollution inverse problem by using artificial neural networks (ANNs). The approach consists first in training an ANN to solve the direct problem, where the pollutant concentration in a set of monitoring wells is calculated for a known pollutant source. Successively, the trained ANN is frozen and it is used to solve the inverse problem, where the pollutant source is calculated which corresponds to a set of concentrations in the monitoring wells. The approach has been applied for a real case which deals with the contamination of the Rhine aquifer by carbon tetrachloride (CCl4) due to a tanker accident. The obtained results are compared with the solution obtained with a different approach retrieved from literature. The results show the suitability of ANNs-based methods for solving inverse non-linear problems
Statistical Description of Calcite Surface Roughness Resulting from Dissolution at Close-to-Equilibrium Conditions
No full textLinking the evolution of the surface area (as quantified, e.g., through its spatial roughness) of minerals to their dissolution rate is a key aspect of mineral reactivity. Unraveling the nature of their main features requires relying on approaches yielding a quantitative characterization of the temporal evolution of surface topography/roughness. Here, a mechanically polished {104} calcite surface was dissolved at room temperature and at close-to-equilibrium conditions (ω = 0.6) with an alkaline solution (pH = 8) across a temporal window of 8 days. Surface topography images were acquired daily using vertical scanning interferometry, the ensuing topography data being then embedded within a statistical analysis framework aimed at describing comprehensively the surface roughness evolution. The strongest system variations were observed after 1 day: the probability density function of surface roughness was observed to transition from being approximately Gaussian to being left-skewed and leptokurtic, exhibiting a dramatic increase in the variance and a significant change in the semi-variogram structure. After a relaxation time of approximately 2 days, the reacting surface appeared to attain a steady-state configuration, being characterized by the values of the statistical moments characterizing surface roughness that become virtually independent of time. Attempting to unravel the underlying dissolution mechanism, an original numerical model able to reproduce satisfactorily the statistical behavior observed experimentally was developed and tested. Our results suggest that under the investigated conditions, dissolution may be characterized as a spatially correlated random process, with the areas most exposed to the flowing fluid being prone to preferential dissolution. The numerical model was also used to obtain insights into the influences of the initial surface roughness and of the fluid composition on the steady-state statistical characterization of the surface roughness. Our results suggest that the influence of the initial surface roughness is limited. The present study suggests that potential empirical relations linking the surface roughness of the reacted crystals to the saturation state at which they dissolved may be developed, which would allow to back-estimate the reacting conditions only based on topography data
Solute transport in random composite media with uncertain dispersivities
No full textCharacterization of dissolved chemical migration in porous media requires knowledge of the fluid velocity field and parameters governing solute dispersion within the diverse geomaterials constituting the internal architecture of the system. Several studies have been focused on the assessment of the impact on solute concentrations of an incomplete knowledge of the fluid velocity field, typically a result of the effects of uncertain hydraulic properties of the hosting media (e.g., permeability). Limited attention has been devoted to analyze propagation of the uncertainty associated with spatial distributions of local dispersivity values to solute concentration fields. Here, we address this issue by focusing on a random composite medium, where the location of the boundary between two distinct geomaterials is uncertain as well as their associated dispersivity values. We derive and solve the equations satisfied by the (ensemble) mean and variance of solute concentration and investigate the relative impact on these moments of the two sources of uncertainty considered. Our results suggest that, in the investigated set-up, the temporal and spatial evolution of ensemble moments of the solute concentration depends on (i) the overall dispersive length scales encompassed by the solute during its migration and (ii) the actual sequence of the materials traversed by the solute
2D benchmark experiments and simulations of density coupled flow problems
No full textVariations of fluid densities can alter flow patterns and transport processes, if solute concentration differences are high enough to cause relevant density contrasts. Since numerous environmental problems are related to these phenomena, the need for accurate process description and modeling continues to increase. The numerical simulation of such processes is challenging due to the strong non-linear coupling of flow and transport processes. Therefore, experimental studies are required to elaborate the basic principles and to test numerical codes in order to provide reliable tools for water resources management and planning. In this thesis, density-coupled flow processes under the influence of geometrical boundary conditions are studied and numerical codes are tested against high resolution experimental data.
Photometric methods were further developed to increase the accuracy of measurements in flow tank experiments. They directly related digitally measured intensities of a tracer dye to solute concentrations. This enabled an effective processing of a large number of images in order to compute concentration time series at various points of the flow tank and concentration contour lines. Perturbations of the measurements were lens flare effects and the image resolution. Transmissive and reflective intensity measurements were compared. The reflection images were more homogeneous in spatial illumination than the transmission images. Major perturbations of the transmissive images were lens flare effects and light dispersion within the bead-water-Plexiglas system which smeared the front of the plume. Based on the conducted evaluation of transmissive and reflective intensity measurements, the reflection data delivered more reliable intensity values to derive solute concentrations in intermediate scale flow tank experiments. The newly developed resistivity measurement system used two different input voltages at gilded electrode sticks to enable the measurement of salt concentrations from 0 to 300 g/l. The method was highly precise and the major perturbations were caused by temperature changes, which can be controlled in the laboratory. The two measurement approaches, photometric and resistivity methods, were compared with regard to their usefulness in providing data for benchmark experiments. Due to the unknown measurement volume of the electrodes, the photometric method was better to determine experiments in a series of laboratory-scale 2D porous medium tank experiments.
Various density-driven flow problems were investigated using well-defined experimental parameters and boundary conditions. The experiments were carried out both in a rectangular flow tank (158×100×4 cm3) and in a more complex geometrical setup aiming to study variable density flow in geological formations of aquifers and aquicludes connected via fault zones. An impermeable layer within the porous medium tank forced the solutes to pass through a channel to reach the outlet of the tank. The porous medium was homogeneous in both cases. The image analysis technique deliverd 2, 10, 50 and 80% salt concentration isolines at distinct times and breakthrough curves of the dyed saltwater. The experimental data were presented as benchmark problems to evaluate numerical codes. A numerical model based on Mixed Finite Elements for the fluid flow problem and a combination of Discontinuous Galerkin Finite Element and Multi-Point Flux Approximation methods for the transport turned out to be adequate for the simulation of the physical experiments. The high data availability made the proposed benchmark experiments a valuable tool for assessing the performance of density-coupled flow models. Heterogeneous porous medium experiments were conducted with a low permeability zone in the centre of the tank. Three different boundary conditions, corresponding to different localizations of the inflow and the outflow openings at the opposite edges of the tank, were applied and different flow scenarios are observed in the heterogeneous tank. The numerical model used for the simulations was based on efficient advanced approximations for both spatial and temporal discretizations. The Method Of Lines (MOL) was used to allow higher-order temporal discretization and the model adapted in both the order of approximation and time step to provide the necessary accuracy. The model was able to reproduce the experiments. The numerical results were improved by assuming a non-Fickian dispersivity for high density experiments
Polluted aquifer inverse problem solution using artificial neural networks
No full textThe problem of identifying an unknown pollution source
in polluted aquifers, based on known contaminant concentrations
measurement in the studied areas, is part of the broader group of issues,
called inverse problems. This paper investigates the feasibility
of using Artificial Neural Networks (ANNs) for solving the inverse
problem of locating in time and space the source of a contamination
event in a homogeneous and isotropic two dimensional domain.
ANNs are trained in order to implement an input-output relationship
which associates the position. Once the output of the system is
known, the input is reconstructed by inverting the trained ANNs.
The approach is applied for studying a theoretical test case where the
inverse problem is solved on the basis of measurements of contaminant
concentrations in monitoring wells located in the studied area.
Groundwater pollution sources are characterized by varying spatial
location and duration of activity. To identify these unknown pollution
sources, concentration measurements data of monitoring wells are used. If concentration observations are missing over a length of
time after an unknown source has become active, it is more difficult
to correctly identify the unknown pollution source. In this work, a
missing data scenario has been taken into consideration. In particular,
a case where only one measurement has been made after the pollutant
source interrupted its activity has been considered
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