Partenaires et organisations internationales
(Anglais)
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AT, BE, BG, HR, CY, CZ, DK, FR, DE, EL, HU, IE, IL, IT, LT, MT, NL, NO, SK, ES, CH, UK
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Résumé des résultats (Abstract)
(Anglais)
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Biodegradation in the unsaturated zone is a potentially important process limiting contaminant migration to groundwater. The aim of the project is to evaluate the feasibility of using compoundspecific stable isotope analysis to assess biodegradation of petroleum hydrocarbons in the unsaturated zone and to develop a soil probe to quantify the in situ biodegradation rates. Prior to field studies quantitative relationships between amount of an organic compound biodegradation and shift in 13C/12C or 2H/H were established using batch experiments. Results showed small 13C-isotope and large 2H-isotope enrichment factors during aerobic biodegradation for all the tested volatile organic compounds (VOCs) compounds under unsaturated conditions. Analytical simulations were performed to gain insight into how isotope ratios evolve for different contamination scenarios. The analytical simulations revealed a significant diffusion effect along with biodegradation as processes controlling carbon isotope fractionation in the unsaturated zone, especially during the initial phase after a spill. For hydrogen, biodegradation process quickly dominates the shifts in isotope ratios due to its large enrichment factor and fractionation by diffusion is negligible. To test the isotope approach at the field scale, carbon isotope ratios were measured during a highly-controlled field experiment conducted in Denmark as part of the GRACOS project. The experiment consisted of burying an artificial fuel source in the unsaturated zone. Concentration and isotope ratios of individual compounds were monitored using a dense network of sampling points. Significant shifts of the isotope ratios were observed for most of the compounds, with a depletion of heavy isotopes with distance during an initial phase followed by an enrichment of heavy isotopes indicating that both diffusion and biodegradation affect isotope ratios. To evaluate the effect of these processes in more detail, the concentration and isotope ratio evolution was simulated using a modified numerical code (MOFAT). The field and modeling studies demonstrated that after the intial phase, stable isotope ratios can be used to demonstrate biodegradation and, under steady state conditions, also to quantify it. Finally, a new soil probe approach was developed to estimate in situ biodegradation rates. The soil probe method relies on the injection of small amounts of organic compounds followed by quantification of biodegradation rates based on isotope ratios or concentration changes relative to nondegradable tracers. The use of the carbon isotope ratios provided identical in situ biodegradation rates compared to rates based on relative concentration changes, but with a smaller uncertainty and higher sensitivity. In summary, during the three year project, two new methods to assess biodegradation of VOCs in the unsaturated were developed. Field tests demonstrated that these methods do not only provide qualitative information but can also be used to quantify biodegradation.
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