1,721,064 research outputs found
Revisitation of the dipole tracer test for heterogeneous porous formations
No full textIn this paper, a new analytical solution for interpreting dipole tests in heterogeneous media is derived by associating the shape of the tracer breakthrough curve with the log-conductivity variance. It is presented how the solution can be used for interpretation of dipole field test in view of geostatistical aquifer characterization on three illustrative examples. The analytical solution for the tracer breakthrough curve at the pumping well in a dipole tracer test is developed by considering a perfectly stratified formation. The analysis is carried out making use of the travel time of a generic solute particle, from the injection to the pumping well. Injection conditions are adapted to different possible field setting. Solutions are presented for resident and flux proportional injection mode as well as for an instantaneous pulse of solute and continuous solute injections. The analytical form of the solution allows a detailed investigation on the impact of heterogeneity, the tracer input conditions and ergodicity conditions at the well. The impact of heterogeneity manifests in a significant spreading of solute particles that increases the natural tendency to spreading induced by the dipole setup. Furthermore, with increasing heterogeneity the number of layers needed to reach ergodic conditions become larger. Thus, dipole test in highly heterogeneous aquifers might take place under non-ergodic conditions giving that the log-conductivity variance is underestimated. The method is a promising geostatistical analyzing tool being the first analytical solution for dipole tracer test analysis taking heterogeneity of hydraulic conductivity into account
Baseflow Statistics in Aggregated Catchments
No full textThis paper employs stochastic analysis to investigate the combined effect of temporal and spatial variability on the temporal variance of baseflow in large catchments. The study makes use of the well-known aggregated reservoir model, representing the catchment as a network of parallel linear reservoirs. Each reservoir models a sub-catchment as an independent unit whose discharge temporal variation is characterized by a response time. By treating the rainfall-generated recharge and the sub-catchment response times as random variables, the statistical temporal moments of total baseflow are quantified. Comparisons are made between the temporal variance of baseflow in the aggregated reservoir model and that of a single homogeneous reservoir to define an upscaled response time. The analysis of the statistical moments of the random baseflow reveals that the number of reservoirs N has a weak impact on baseflow variance, with ergodic conditions achieved even with a small number of reservoirs. The study highlights that the ratio between the recharge correlation time and the geometric mean of the sub-catchment response times plays a critical role in baseflow damping and the upscaled response. The results indicate that the dynamics of baseflow generation depend not only on the catchment hydro-geological structure but also on the variability of the input signal. This research underscores the importance of understanding the combined influences of hydro-geological factors and recharge input variability for baseflow prediction under uncertainty. The present study should be regarded as a first step, setting the theoretical framework for future research toward incorporating field data.</p
Eine erweiterte Theis-Lösung: Berechnung und Implementierung einer Lösung der transienten Grundwassergleichung unter Berücksichtigung von Heterogenität im Coarse-Graining-Modell
Full text linkDie vorliegende Arbeit behandelt die Modellierung von Fließprozessen in Grundwasserleitern. Grundlage dafür ist die Grundwassergleichung, welche diese Prozesse mathematisch beschreibt. Die wichtigste hydraulische Eigenschaft von Untergründen ist hierbei die hydraulische Leitfähigkeit, welche die Fließgeschwindigkeit des Grundwassers angibt. Da man die Verteilung der Leitfähigkeit in einem betrachten Wasserleiter aber durch das Fehlen von Informationen nicht vollständig bestimmen kann, ist man auf vereinfachende Modelle und Versuchsszenarien angewiesen. In der Vergangenheit haben sich zur Untersuchung von Böden sogenannte Pumpversuche etabliert, wobei ein Brunnen gebohrt wird, welcher den Grundwasserleiter vollständig durchteuft und an dem mit konstanter Rate Wasser aus dem Boden gepumpt wird. Parallel beobachtet man an einem oder mehreren Referenzbrunnen die sich verändernde hydraulische Druckhöhe. Aus diesen Daten möchte man Informationen über den betrachteten Boden gewinnen. Dazu braucht es gewisse Modellfunktionen, welche für fest definierte Standardsituationen das Grundwasserverhalten beschreiben. Eine Möglichkeit, Böden zu klassifizieren ist es, die heterogene Struktur der Leitfähigkeit durch log-normal verteilte Zufallsgrößen zu modellieren. Dabei beschränkt man sich auf den Mittelwert, die Varianz und die Korrelationslänge dieser Verteilungen. Das hier zugrunde liegende Coarse-Graining-Modell generiert zu diesen Parametern eine effektive Leitfähigkeitsverteilung, welche nur vom radialen Abstand zum Pumpbrunnen abhängt. Damit wird die Grundwassergleichung zu einer radialsymmetrischen parabolischen Differentialgleichung. Zu dieser Differentialgleichung wurde in der vorliegenden Arbeit ein Lösungsalgorithmus entwickelt und implementiert sowie anschließend gegen verschiedene Parametereingaben getestet
Field investigations and reactive transport modelling of biodegrading coal tar compounds at a complex former manufactured gas plant
Full text linkGroundwater contamination by coal tar and other aromatic hydrocarbon mixtures is a global issue with significant environmental and public health implications. Coal tar, a dense liquid, can persist as a long-term source of groundwater pollution when it infiltrates the subsurface after being deposited on the ground's surface. Hydrocarbons dissolve from the coal tar into the groundwater, where they undergo complex physical, biological and chemical transformations that depend on site-specific conditions. This study focuses on the Griftpark, a former gas manufacturing site in Utrecht, that is severely contaminated with coal tar. The site features numerous chemical and physical complexities, that lead to a lack of comprehensive field data. Our research seeks to provide valuable knowledge for risk assessments and remediation strategies for such complex sites that are common in the real world, but not often studied in the academically. Our findings demonstrate that employing conventional investigation techniques yields valuable information about physical, chemical and biological conditions at the site. Subsequently, we developed a numerical reactive transport model that effectively captures and qualitatively describes significant subsurface processes. The model highlights the potential effectiveness of monitored natural attenuation, a method that relies on naturally occurring processes (including degradation facilitated by microbes) to reduce contaminant concentrations, as a management option for Griftpark. Success depends on factors like the volume of contaminated soil and degradation rates. This study underscores the value of combining field research and numerical modeling to gain insights into complex sites like Griftpark. Such insights support the transition from active management to natural attenuation, contributing to more sustainable and cost-effective groundwater remediation strategies
Evidence Based Estimation of Macrodispersivity for Groundwater Transport Applications
No full text: The scope of this work is to discuss the proper choice of macrodispersion coefficients in modeling contaminant transport through the advection dispersion equation (ADE). It is common to model solute concentrations in transport by groundwater with the aid of the ADE. Spreading is quantified by macrodispersivity coefficients, which are much larger than the laboratory observed pore-scale dispersivities. In the frame of stochastic theory, longitudinal macrodispersivity is related to the hydraulic conductivity spatial variability via its statistical moments (mean, variance, integral scales), which are generally determined by geostatistical analysis of field measurements. In many cases, especially for preliminary assessment of contaminant spreading, these data are not available and ad hoc values are adopted by practitioners. The present study aims at recommending dispersivity values based on a thorough analysis of tens of field experiments. Aquifers are classified as of weak, medium, and high heterogeneity and for each class a range of macrodispersivity values is recommended. Much less data are available for the transverse macrodispersivities, which are significantly smaller than the longitudinal one. Nevertheless, a few realistic values based on field data, are recommended for applications. Transport models using macrodispersivities can predict mean concentrations, different from the local ones. They can be used for estimation of robust measures, like plumes spatial moments, longitudinal mass distribution and breakthrough curves at control planes
Impact of aquifer heterogeneity on subsurface flow and salt transport at different scales: from a method determine parameters of heterogeneous permeability at local scale to a large-scale model for the sedimentary basin of Thuringia
Full text linkMore than 70% of the German population rely on groundwater as their daily drinking water supply. Hence, the understanding of processes like fluid flow and solute transport in porous media are of broad relevance. However, subsurface hydrology faces two problems. On the one hand, information about the subsurface structure and hydraulic properties is scarce. On the other hand, many aquifer parameters like the permeability which controls the flow velocity, are heterogeneously distributed in space. This thesis is dedicated to the question of how aquifer heterogeneity impacts on subsurface flow and transport processes at different scales. We first present a large-scale numerical model of the Thuringian basin in order to investigate the mechanisms of saltwater transport. The results underline the fact that permeability and its heterogeneous spatial distribution are decisive factors for the evolving flow and salt patterns. Our results show that the correlation structure and the degree of heterogeneity (variance) impact significantly on the amount of dissolved salt and the location where salt reaches near-surface regions. Thermally induced convection is not present. However, variations in fluid density due to dissolved salt can lead to significant changes in the distribution and amount of salt in the Thuringian Basin. We further focus on a method to determine the statistical parameters which describe the heterogeneous spatial distribution of permeability by analyzing well flow at local scale. This is of interest, because pumping tests are a widely used tool to infer on porous medium characteristics. We derive an analytical solution which describes the drawdown of a steady state pumping test in 3D heterogeneous anisotropic media by making use of upscaling theory. By combining the analytical solution with an inverse estimation strategy we determine the statistical parameters, like mean permeability, variance and correlation length from on-site pumping tests
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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