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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.

Place, publisher, year, edition, pages
2012. Vol. 36, 86-97 p.
Keyword [en]
Expected mass fraction, Exposure duration, Dilution effect, Risk assessment
National Category
Environmental Engineering
URN: urn:nbn:se:kth:diva-133335DOI: 10.1016/j.advwatres.2011.04.009ISI: 000299971900009ScopusID: 2-s2.0-84855223654OAI: diva2:660638

QC 20131030

Available from: 2013-10-30 Created: 2013-10-30 Last updated: 2013-11-04Bibliographically 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.
Trita-LWR. PHD, ISSN 1650-8602 ; 1074
Local scale dispersion, Heterogeneity structure, Concentration moments, Moment collapse, Expected mass fraction
National Category
Other Environmental Engineering Environmental Engineering
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)

QC 20131104

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

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Srzic, Veljko
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