Groundwater contamination with chlorinated ethenes (CE) is a widespread problem in industrialized countries. It poses direct risks to human population and long lasting threats to groundwater security. In recent decades bioremediation, managed natural attenuation and various in-situ methods, like injection of reactive nano-materials, have gained popularity. However , each remediation method has certain drawbacks and limitations, and therefore combined remediation methods, such as biological and abiotic degradation, have become increasingly popular and show high potential for fast and efficient clean-up. This thesis revolves around several aspects of CE remediation. Once a site has been identified as contaminated, the first step is an assessment of the contamination extent and distribution, as well as of the encountered hydrogeology, to yield a conceptual site model (CSM). Next, the type, extent and distribution of natural attenuation can be assessed. Based on these information, decisions for a site management strategy can be made. Active remediation using nanoscale zerovalent iron (nZVI) or dechlorinating microbial communities are both popular approaches for in-situ CE remediation. Novel sulfidized nZVI (S-nZVI) materials have recently overcome some prevalent drawbacks of nZVI but still lack characterization. The introduction of these nano-materials into the subsurface still poses severe challenges and limitations in terms of transport and distribution, but also uncertainties from inherent hazard and risk considerations. Aside from regulatory and public concerns,the latter is of particular importance for combined abiotic-biotic treatment scenarios, due to potential adverse impacts on microbial communities. In the first part of this thesis, investigation of natural attenuation using a multiple lines of evidence approach is showcased at a perchloroethene (PCE) contaminated field site in Denmark. We show that natural attenuation occurs in different forms under different geochemical conditions, often in close proximity to one another, and how isotopic analysis is critical for such distinctions. We further show that the extent of attenuation is not only limited spatially but also limited overall. Finally, using microcosm experiments, we demonstrate that microbial communities at this site are able to partly reduce PCE to vinylchloride, while full biological dechlorination seems less abundant. In the second part of this thesis, we investigate the adverse effects of nZVI and two sulfidized nZVI materials on bacterial viability. We first use Shewanella oneidensis MR-1 as a test strain to gain insights into toxicity mechanisms under aerobic and anaerobic conditions, using the colony forming units (CFU) method and adenosine triphosphate (ATP) measurements. Next we expose two mixed dechlorinating cultures to these materials, KB-1 R (a commercial bioaugmentation culture) and a culture enriched from the field site. Toxicity under anaerobic conditions seems lower in the early exposure stages and S-nZVI materials show generally lower toxicity compared to nZVI. KB-1 R exhibits exceptional resilience towards nZVI and S-nZVI, likely due to the yearlong cultivation in media containing reduced FeS solids. Based on these results, we made recommendations for reactant material selection in abiotic-biotic treatment scenarios. In the last part we use X-ray micro computed tomography (µ-CT) to visualize S-nZVI retention in porous media and resolve it spatially. Direct correlation of S-nZVI retention with pore-space features may enable a better understanding of these crucial processes and improve the prediction of S-nZVI transport, retention and distribution during the planning and implementation of in-situ remediation projects. Our data show that straining is the dominant process for initial S-nZVI retention and that ripening processes become increasingly relevant with decreasing injection velocity and increasing S-nZVI concentration. We highlight the potential of this new approach, as well as limitations and encountered difficulties, to make recommendations for future efforts.