Chlorinated aliphatic hydrocarbons; an interdisciplinary study of degradation and distribution in complex environments

Abstract: In this thesis degradation of chlorinated solvents, e.g., tetrachloroethene (perchloroethylene [PCE]), has been examined, both natural occurring degradation and in situ remediation strategies. The projects included have focused on two field sites of former dry-cleaning facilities, which are contaminated with PCE in both the groundwater and the sediments. One site had only been exposed to natural degradation, whereas the other site has been targeted for enhanced in situ remediation, both abiotic and biotic. The aim of this thesis was to characterize the subsurface conditions and monitor changes at contaminated site with an interdisciplinary approach. The data included within the studies are contaminant concentrations, major and minor ions concentrations, Compound Specific Isotope Analysis (CSIA) of carbon, and DNA analysis describing microbial content. At the site with natural degradation geoelectrical resistivity and chargeability measurements were also performed. In addition, a laboratory study has been conducted to link microbial growth and activity to geoelectrical signals.The studies have shown that changes in geological setting with shifts in hydrogeological properties, i.e., transition units, proved to be important features associated with more efficient degradation. The transitions units showed larger variations in microbial communities, compared to the aquifer material. The microbial communities were similar when comparing samples from the sediment and the groundwater; however, more of the microbes associated with dechlorination were found in the sediment samples. Correlation between the amount of microbes and resistivity was possible, while the chargeability could only indicate to arise due to by-products. During monitoring of enhanced biodegradation PCE’s metabolites were needed to be included when evaluating the CSIA data, but for the natural degradation it was not required. Since the location of transition units has been shown to be an important hydrogeological setting, both with regards to localization of the contaminant and as a preferred environment for the microbial communities, high quality data of the geological conditions are required from contaminated sites to capture changes in the settings. The sediment matrix should be investigated during in situ bioremediations to fully examine the microbial communities and to be able to evaluate the degradation effect. The interdisciplinary approach made it possible to challenge the limitation of commercially available analysis and state-of-the-art methods, and have contribute to more nuanced interpretations and evaluations of the complexities at contaminated sites.