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Significance of transport dynamics on concentration statistics and expected mass fraction based risk assessment in the subsurface
KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Land and Water Resources Engineering.
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 [en]
Local scale dispersion, Heterogeneity structure, Concentration moments, Moment collapse, Expected mass fraction
National Category
Other Environmental Engineering Environmental Engineering
Identifiers
URN: urn:nbn:se:kth:diva-133455ISBN: 978-91-7501-900-0 (print)OAI: oai:DiVA.org:kth-133455DiVA: diva2:661510
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
List of papers
1. Impact of aquifer heterogeneity structure and local-scale dispersion on solute concentration uncertainty
Open this publication in new window or tab >>Impact of aquifer heterogeneity structure and local-scale dispersion on solute concentration uncertainty
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.

Keyword
heterogeneity structure, advection, local-scale dispersion, concentration uncertainty, skewness, kurtosis
National Category
Water Engineering
Identifiers
urn:nbn:se:kth:diva-126864 (URN)10.1002/wrcr.20314 (DOI)000322241300045 ()2-s2.0-84879249322 (Scopus ID)
Note

QC 20130828

Available from: 2013-08-22 Created: 2013-08-22 Last updated: 2017-12-06Bibliographically approved
2. Collapse of higher-order solute concentration moments in groundwater transport
Open this publication in new window or tab >>Collapse of higher-order solute concentration moments in groundwater transport
2013 (English)In: Water resources research, ISSN 0043-1397, E-ISSN 1944-7973, Vol. 49, no 8, 4751-4764 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, we use numerical simulations based on a Lagrangian framework to study contaminant transport through highly heterogeneous porous media due to advection and local diffusion (under local diffusion, we assume coupled effect of mechanical dispersion and molecular diffusion). The analysis of the concentration field is done for the case of a two-dimensional hydraulic conductivity domain representing the aquifer, with three log-conductivity structures that differ in spatial correlation. In addition to different conductivity structures, we focus our investigation on mild and highly heterogeneous porous media characterized by the values of hydraulic log-conductivity variance (σY2) being equal to 1 and 8. In the concentration moment analysis, we show that a linear relationship exists between higher-order to second-order normalized concentration moments on a log-log scale up to the fourth-order moment. This leads to the important finding that moments of a higher than the second order can be derived based on information about the first two concentration moments only. Such a property has been observed previously for boundary-layer water channels, wind tunnels, and turbulent diffusion in open terrain and laboratory experiments. Normalized moments are shown to collapse for different types of hydraulic conductivity structures, Peclet (Pe) numbers and σY2 values. In the case of local diffusion absence, a linear log-log relationship is derived analytically and is set as a lower limit. The deviation from the lower limit is explained to be predominantly caused by the local diffusion, which needs time to evolve. In the case of local diffusion presence, we define the moment deriving function (MDF) to describe the linear log-log relationship between higher-order concentration moments to the second-order normalized one. Finally, the comparison between numerical results and those obtained from the Columbus Air Force Base Macrodispersion Experiment (MADE 1) is used to demonstrate the robustness of the moment collapse.

Keyword
concentration moment collapse, moment deriving function, heterogeneity, advection, local diffusion, MADE
National Category
Environmental Engineering
Identifiers
urn:nbn:se:kth:diva-133333 (URN)10.1002/wrcr.20371 (DOI)000324838300018 ()2-s2.0-84880959817 (Scopus ID)
Note

QC 20131030

Available from: 2013-10-30 Created: 2013-10-30 Last updated: 2017-12-06Bibliographically approved
3. Solute concentration statistics and intermittency in groundwater transport
Open this publication in new window or tab >>Solute concentration statistics and intermittency in groundwater transport
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-133454 (URN)
Note

QS 2013

Available from: 2013-11-04 Created: 2013-11-04 Last updated: 2013-11-04Bibliographically approved
4. Risk characterization for toxic chemicals transported in aquifers
Open this publication in new window or tab >>Risk characterization for toxic chemicals transported in aquifers
2012 (English)In: Advances in Water Resources, ISSN 0309-1708, E-ISSN 1872-9657, Vol. 36, 86-97 p.Article in journal (Refereed) Published
Abstract [en]

The risk characterization resulting from the introduction of toxic chemicals in a subsurface flow field is presented. The concept of concentration threshold is used to quantify the risk associated with non-carcinogenic chemicals introduced to the population by ingestion of groundwater as the exposure pathway. The risk assessment methodology presented uses the expected mass fraction (EMF) concept with exposure duration to identify the distribution of dosage over different concentrations during the plume migration over the well location. The numerical simulation of the subsurface transport by advection and local diffusion is used to produce the concentration plume that passes different locations of interest. The EMF obtained presents the probability of the expected mass above some concentration threshold found at the location of interest. The risk formulation is defined with the risk reliability (safety) and its complement, the risk exceedance (failure) value. The risk characterization is obtained as a probability for exceeding the human reference dose which is considered uncertain due to the necessary extrapolation between concentration used in toxicological studies and the concentration to which humans could be exposed in nature. The final risk assessment expression is derived in a closed form by coupling the expected mass fraction with the safe human threshold concentration probability density function (pdf) inferred from the toxicological studies. The results indicate the importance of estimating the probability of a concentration mass found at locations of interest together with its exposure duration. The exposure duration was revealed to be an important parameter that needs to be estimated depending on the human concentration threshold selected and the distance from the source. The results in terms of the risk safety and risk failure also indicate the dilution effect on the passing concentration plume in the subsurface as a function of the distance and orientation from the source. Inclusion of uncertainty in the selection of the human concentration threshold shows the important effect on the risk quantification.

Keyword
Expected mass fraction, Exposure duration, Dilution effect, Risk assessment
National Category
Environmental Engineering
Identifiers
urn:nbn:se:kth:diva-133335 (URN)10.1016/j.advwatres.2011.04.009 (DOI)000299971900009 ()2-s2.0-84855223654 (Scopus ID)
Note

QC 20131030

Available from: 2013-10-30 Created: 2013-10-30 Last updated: 2017-12-06Bibliographically approved

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