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Isotope fractionation of volatile organic contaminants in porous media under unsaturated conditions: Mechanisms and applications

English title Isotope fractionation of volatile organic contaminants in porous media under unsaturated conditions: Mechanisms and applications
Applicant Hunkeler Daniel
Number 138208
Funding scheme Project funding (Div. I-III)
Research institution Centre d'hydrogéologie et de géothermie Université de Neuchâtel
Institution of higher education University of Neuchatel - NE
Main discipline Other disciplines of Environmental Sciences
Start/End 01.11.2011 - 31.10.2012
Approved amount 66'069.00
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All Disciplines (2)

Discipline
Other disciplines of Environmental Sciences
Geochemistry

Keywords (3)

Groundwater; Contamination; Isotopes

Lay Summary (English)

Lead
Lay summary
The same chemical substance produced by different manufacturers can have a different isotopic composition. Therefore, stable isotope analysis of contaminants can be used to distinguish between different sources of contamination. Furthermore, during degradation processes, molecules with light isotopes are degraded preferentially. As a result, the heavy isotopes accumulate in the remaining fractionation of contaminants. Hence, stable isotope analysis can also be used to follow the progress of reactive processes. However, for contaminant sources located in the vadose zone between soil and groundwater table, isotope ratios can also be modify by physico-chemical processes. For example, during diffusion, molecules with light isotopes migrate more rapidly than molecules with heavy isotopes, leading to a depletion of heavy isotopes with distance. Hence when applying isotope methods to differentiate contaminant sources or to trace biodegradation, the effect of physico-chemical processes such as vaporization, air-water partitioning or diffusion needs to be quantified. In this project, the effects of these processes on isotope ratios are investigated in detail using chlorinated hydrocarbons, which are common environmental contaminants, as model compounds. The project includes batch experiment to investigate the effect of partitioning of compounds between different phases (organic liquid, water, air), column experiment to evaluate the effect of transport processes and field studies. The project will provide insight into mechanisms of isotope fractionation and a solid basis for the application of isotope methods to study the origin and fate of contaminants in the vadose zone.
Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

Employees

Name Institute

Collaboration

Group / person Country
Types of collaboration
Dr Martin Elsner, Helmholtz Munich Germany (Europe)
- in-depth/constructive exchanges on approaches, methods or results
Prof. R. Aravena, University of Waterloo Canada (North America)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
Prof. D. Ronen, Ben Gurion University, Israel Israel (Asia)
- in-depth/constructive exchanges on approaches, methods or results
- Publication

Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
8th Intl Conference on Remediation of Chlorinated and Recalcitrant Compounds 21.05.2012 Monterey, CA, USA


Knowledge transfer events



Self-organised

Title Date Place
Detailuntersuchung von Altlasten 25.10.2012 Neuchâtel
Probenahme und Feldmessungen 13.09.2012 Neuchâtel

Associated projects

Number Title Start Funding scheme
121932 Isotope fractionation of volatile organic contaminants in porous media under unsaturated conditions: Mechanisms and applications 01.11.2008 Project funding (Div. I-III)

Abstract

Compounds-specific isotope analysis (CSIA) is increasingly used to evaluate the origin and fate of contaminants in the environment. In contaminant hydrogeology, the method has so far mainly been applied under water-saturated conditions. However, CSIA potentially also provides insight into processes controlling the fate of compounds in the unsaturated zone. As a basis for application of the method, it needs to be known how different processes fractionation isotopes. For volatile organic compounds, which are the focus of the project, the major processes include vaporization, transport by diffusion and biodegradation. While there is substantial information available about vapor pressure isotope effects and it is well established that diffusion and biodegradation of small molecules is associated with isotope fractionation, little is known on how isotope ratios evolve when these processes interact. For example, it is not known if and under what conditions the Rayleigh equation can be applied to quantify biodegradation when biodegradation and diffusion occur simultaneously in a porous medium. The main aim of the project is to investigate how isotope ratios evolve in an unsaturated porous medium when vaporization, diffusion and biodegradation occur simultaneously and to draw conclusions for the application of CSIA in unsaturated zone studies. The project relies on laboratory studies to identify and quantify isotope fractionation for isolated and interacting processes, analytical and numerical model to evaluate isotope trends for different processes, and field studies. Furthermore, a mathematical concept was developed to calculate effective isotope fractionation factors for interacting processes and is tested using the laboratory and field data. The project is carried out in the frame of a PhD thesis. This proposal corresponds to the year 4 of the PhD thesis. In year 4, the following activities are planned: a)Carry out laboratory experiments investigating the combined effect of diffusion and biodegradation on isotope ratios. b)Complete the field study that investigates the isotope evolution of organic contaminants diffusing upward from the water table to the atmosphere. c)Carry out numerical simulation of isotope fractionation. The simulations carried out so far have focused on the effect of diffusion on isotope ratios. In a next step, the isotope fractionation during vaporization and biodegradation will be incorporated in the model as well. d)Writing of scientific paper. Due to the novelty of the research and the number of different laboratory, field and modeling studies that were carried out, several papers will be completed during year 4.The laboratory and field experiments, combined with the mathematical framework for data evaluation will provide new insight into isotope fractionation under unsaturated conditions and provide a solid foundation for application of the isotope methods in the unsaturated zone to assess biodegradation and distinguish between different sources of vapor.
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