Organic chemicals are among the most common groundwater contaminants. Often the same type of contaminants can orignate from different sources for example old industrial sites or landfills. By measuring only the concentration of these contaminants it can be difficult to find out which source contributes how much to groundwater contamination. An alternative approach is the measurement of the isotopic composition of the chemicals. Identifical chemical compounds can contain a different amount of heavy and light isotopes of an element, for example carbon with a mass of 12 and carbon with a mass of 13. Hence the measurement of the isotopic composition of the contaminant in addition to its concentration can help to determine from which source a contaminant originates. However, in order to apply the method, it is important to know if the isotopic composition changes as the contaminants migrate through the subsurface. In this project, we investigate how isotope ratios evolve during their transport in groundwater. The project includes laboratory experiments which simulate the migration of contaminants in sand columns and field measurements. The project will make the isotope method to distinguish different sources of contaminants more reliable.