Chlorinated hydrocarbons are among the most common groundwater contaminants. For example, in Switzerland, in nearly a quarter of the drinking water wells traces of chlorinated hydrocarbons are detected. It is generally believed that these compounds originate from anthropogenic activities such as metal processing industries or dry cleaners. However, recently, it was found that some chlorinated hydrocarbons, especially chloroform, can also be naturally produced during degradation of organic matter by fungi in forest soils. While, the natural production of these compounds has clearly been documented at some locations, it is not known yet how widespread this process is. For planning effective remediation measures and risk assessment, it is important to know the source of theses compounds. The main aim of the project is to develop a method to identify locations where chlorinated hydrocarbons are of natural origin, to apply the method to evaluate how widespread natural chlorinated hydrocarbon production is and to identify conditions under which these compounds can reach groundwater. The method to identify the origin of chlorinated hydrocarbons relies on stable isotope analysis. Preliminary studies have demonstrated the chloroform from natural sources contains more of the heavy carbon isotope, 13C, compared to the anthropogenic chloroform. The difference can be explained by the different origin of the carbon in the molecule. An analytical method will be developed to measure the carbon isotopic composition of chloroform at low concentrations. The method will then be applied to identify locations where chloroform is of natural origin. Finally, at some of these locations, detailed studies will be carried out to investigate how natural chlorinated hydrocarbons can reach groundwater.