In recent years, an increasing number of naturally produced organohalogens has been discovered. Environmental studies on organohalogens have frequently focused on chloromethane, a naturally produced ozone-killer, and its release to the atmosphere. However, there is increasing evidence that organohalogens produced in soils, especially chloroform, can also be transported to groundwater. The relatively frequent detection of chloroform in ambient groundwater suggests that natural chloroform production may be widespread. However, it is difficult to demonstrate a natural origin of chloroform in groundwater given its widespread use as an industrial solvent. For long-term evaluation of groundwater quality and because anthropogenic chloroform may frequently be associated with other contaminants, it is important to know to what extent chloroform in groundwater originates from natural sources. This project is a continuation of the project n°200020-117860, which corresponds to the first two years of a PhD thesis. In the previous project, soil gas measurements were carried out at several sites with natural chloroform production. Carbon isotope ratios between -25 and -30‰ were observed which are significantly more enriched in 13C compared to anthropogenic chloroform (-50 to -70‰). This clearly demonstrates the potential of using carbon isotope data to distinguish between natural and anthropogenic chloroform. At the study sites, a strong correlation between the chloroform concentration in soil gas and the quantity of base-hydrolysable trichloroacetyl-groups in soil samples was observed suggesting that the latter could be a precursor of naturally formed chloroform. Using base-hydrolysis, the site-specific isotope ratio of the trichloromethyl position was determined and values of -10 to -15‰ were obtained. If this position is the chloroform precursor, a substantial carbon isotope fractionation has to occur during their release. This is indeed the case as demonstrated in experiments with model compound and humic acids, and as was also shown in a study recently published by another research group. A carbon isotope enrichment factor of about -15‰ was found which explains the isotope ratio of soil gas chloroform very well. The fairly stable isotope ratio of soil gas chloroform implies a fairly stable isotope signature of the trichloromethyl-position despite fractionation during their release. A stable isotope ratio can be achieved if the precursor pool is large compared to the rate of chloroform production or if a steady state between formation and release of this position is established. Based on these considerations a simplified mathematical process model was developed incorporating different carbon pools leading to chloroform that reproduces the observed isotope values and temporal trends well. In the next project phase (year 3 and 4 of the PhD), we plan to explore these hypotheses of chloroform formation in more detail and to “validate” the mathematical process model with laboratory experiments. In addition, we plan to include chlorine isotopes as well for two reasons. On the one hand, by measuring chlorine isotope ratios, it will be possible to further confirm the conceptual model of chloroform production in soils. On the other hand, chlorine isotope ratios could also be used to distinguish between natural and anthropogenic sources of chloroform. Indeed, an adaptation of the simplified process model to chlorine suggests that chlorine isotope ratios in chloroform should substantially deviate from those of inorganic chlorine in soil. Since part of the soil-produced chloroform escapes to the atmosphere, its isotope ratio might be influenced by diffusion in addition to reactive processes. This possible effect will also be explored using column experiments and, in case of chlorine, diffusion isotope effects will be evaluated on an isotopologue basis. In the further project, the well-established collaboration with Geological Survey of Denmark and Greenland will be continued and some of the studies will be carried out at their well-characterized research field sites. Moreover, field sampling and measurement will be carried out in different Swiss sites selected on the basis of the data of the national groundwater monitoring network (NAQUA) to study the dynamics and isotope evolution of the trichloroactyl-pool and chloroform under field conditions.