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Impact of aquifer heterogeneity structure and local-scale dispersion on solute concentration uncertainty
KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Land and Water Resources Engineering.
KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Land and Water Resources Engineering.
Split University.
Split University.
2013 (English)In: Water resources research, ISSN 0043-1397, E-ISSN 1944-7973, Vol. 49, no 6, 3712-3728 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, we study the influence of high log-conductivity variance (sigma(2)(Y)) and local-scale dispersion on the first two concentration moments as well as on higher-order moments, skewness, and kurtosis, in a 2-D heterogeneous aquifer. Three different heterogeneity structures are considered, defined with one and the same global isotropic Gaussian variogram. The three structures differ in terms of spatial connectivity patterns at extreme log-conductivity values. Our numerical approach to simulate contaminant transport through heterogeneous porous media is based on the Lagrangian framework with a reverse tracking formulation. Advection and local-scale dispersion are two competing and controlling mechanisms, with a relative ratio defined by the Peclet number (Pe); hydraulic log-conductivity variance sigma(2)(Y) in the simulations is assumed to be one or eight. The term local-scale dispersion is used as a combined effect of molecular diffusion and mechanical dispersion. Uncertainty of the concentration field is quantified by the second-order moment, or the coefficient of variation (CVC) as a function of the sampling position along a centerline, Peclet number, and sigma(2)(Y), as well as by higher-order moments, i.e., skewness and kurtosis. The parameter sigma(2)(Y) shows a strong influence on the concentration statistics, while the three different structures have a minor impact in the case of low heterogeneity. The results also indicate that for sigma(2)(Y) = 8, the influence of local-scale dispersion is significant after five integral scales (IY) from the source for the connected (CN) field, while in case of a disconnected field, the local-scale dispersion effect is observed after 20IY from the source. In the case of unit sigma(2)(Y), local-scale dispersion acts very slowly affecting concentration uncertainty at distances higher than 20IY from the source. Our inspection of Monte Carlo concentration skewness and kurtosis with the ones obtained from the Beta distribution show the discrepancies for high sigma(2)(Y) and CN log-conductivity structure.

Place, publisher, year, edition, pages
2013. Vol. 49, no 6, 3712-3728 p.
Keyword [en]
heterogeneity structure, advection, local-scale dispersion, concentration uncertainty, skewness, kurtosis
National Category
Water Engineering
Identifiers
URN: urn:nbn:se:kth:diva-126864DOI: 10.1002/wrcr.20314ISI: 000322241300045Scopus ID: 2-s2.0-84879249322OAI: oai:DiVA.org:kth-126864DiVA: diva2:642562
Note

QC 20130828

Available from: 2013-08-22 Created: 2013-08-22 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Significance of transport dynamics on concentration statistics and expected mass fraction based risk assessment in the subsurface
Open this publication in new window or tab >>Significance of transport dynamics on concentration statistics and expected mass fraction based risk assessment in the subsurface
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis relies on a Langrangian framework used for conservative tracer transport simulations through 2-D heterogeneous porous media. Conducted numerical simulations enable large sets of concentration values in both spatial and temporal domains. In addition to the advection, which acts on all scales, an additional mechanism considered is local scale dispersion (LSD), accounting for both mechanical dispersion and molecular diffusion. The ratio between these two mechanisms is quantified by the Peclet (Pe) number. In its base, the thesis gives answers to contaminant concentration features when influenced by: i) different log-conductivity variance; ii) log-conductivity structures defined by the same global variogram but with different log conductivity patterns cor-related; and iii) for a wide range of Peclet values. Results conducted by Monte Carlo (MC) analysis show a complex interplay between the aforementioned pa-rameters, indicating the influence of aquifer properties to temporal LSD evolu-tion. A stochastic characterization of the concentration scalar is done through moment analysis: mean, coefficient of variation (CVC), skewness and kurtosis as well as through the concentration probability density function (PDF). A re-markable collapse of higher order to second-order concentration moments leads to the conclusion that only two concentration moments are required for an accurate description of concentration fluctuations. This explicitly holds for the pure advection case, while in the case of LSD presence the Moment Deriv-ing Function (MDF) is involved to ensure the moment collapse validity. Fur-thermore, the expected mass fraction (EMF) concept is applied in groundwater transport. In its origin, EMF is function of the concentration but with lower number of realizations needed for its determination, compared to the one point PDF. From practical point of view, EMF excludes meandering effect and incorporates information about exposure time for each non-zero concentration value present. Also, it is shown that EMF is able to clearly reflect the effects of aquifer heterogeneity and structure as well as the Pe value. To demonstrate the uniqueness of the moment collapse feature and ability of the Beta distribution to account for the concentration frequencies even in real cases, Macrodisper-sion Experiment (MADE1) data sets are used.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. xiii, 53 p.
Series
Trita-LWR. PHD, ISSN 1650-8602 ; 1074
Keyword
Local scale dispersion, Heterogeneity structure, Concentration moments, Moment collapse, Expected mass fraction
National Category
Other Environmental Engineering Environmental Engineering
Identifiers
urn:nbn:se:kth:diva-133455 (URN)978-91-7501-900-0 (ISBN)
Public defence
2013-11-05, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20131104

Available from: 2013-11-04 Created: 2013-11-04 Last updated: 2013-11-12Bibliographically approved

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