Organic compounds such as chlorinated compounds and petroleum hydrocarbons are among the most common groundwater contaminants. For risk assessment and in order to plan effective remediation methods, it is important to know from where these contaminants originate and to what extent they are removed by naturally occurring biodegradation. A recently proposed approach to gain insight into the origin and fate of organic compounds in groundwater is the use of stable isotope analysis. Organic contaminants from different sources (e.g. spills caused by different companies) often have a different isotopic composition. In other words, the naturally occurring stable isotopes (e.g. 13C and 12C) are present in different proportions. Furthermore, during biodegradation frequently the heavy isotopes become more and more abundant in the contaminant because molecules with light isotope are degraded faster by bacteria. This isotope enrichment trend makes it possible to identify and quantify biodegradation. The stable isotope method has so far mainly been investigated and applied in groundwater studies. In contrast little information is available so far on its use in the unsaturated zone between the water table and the land surface. The main aim of the project is to investigate whether the stable isotope method can also be used in unsaturated zone studies to link contaminants detected at the surface to subsurface sources and to demonstrate biodegradation. The project focuses on volatile organic compounds because these compounds diffuse rapidly through the unsaturated zone and hence can contaminate groundwater or impact air quality in buildings. Before the isotope method can be applied to solve practical questions, more knowledge is necessary on how isotope ratios evolve during transport and degradation of organic compounds under unsaturated conditions. The effect of these processes on isotope ratios will be investigated in laboratory column experiments. Such experiments have the advantage that key experimental parameters can be controlled exactly and mathematical approaches to quantify the phenomena tested rigorously. In addition, several field sites will be equipped with multilevel gas samplers and it will be investigated whether isotope ratios can be used to link contaminants to their source and/or to demonstrate biodegradation.