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.