Numerous industries have been using chlorinated ethenes (CEs), mainly tetrachlorethene and trichloroethene, as non-flammable solvents since the beginning of the 20th century. Massive usage, along with careless handling and storage, made CEs one of the most abundant classes of aquifer contaminants. Porous aquifers are dynamic ecosystems showing complex interactions between physical, chemical and biotic components. These environments are inherently extremely heterogeneous in their structure and composition in terms of lithology, grain size distribution and chemical nature, and thus provide a large variety of living conditions. A possible approach for studying reductive dehalogenation processes in such environments consists in the analysis of the correlations between bacterial communities and environmental variables. Among the latter, the grain size distribution is expected to have a major impact on the structuring and functioning of the communities. In this study, this hypothesis was tested on samples issued from a CE-contaminated aquifer in which the dehalorespiration process was effectively occurring. Previous studies showed that this aquifer was semi-confined and had a slow groundwater flow. The bacterial communities were analyzed at different locations on this site using T-RFLP profiling, together with the analysis of multiple environmental factors. The analysis of the correlations between environmental factors and T-RFLP profiles surprisingly revealed that grain size distribution had only a limited influence. This unexpected result led us to hypothesize that the possible impact of the grain size distribution was decreased due to the slow groundwater flow. Diffusion phenomena would then be the principal mechanism of mass transfer of nutrients to specialized bacterial micro-niches.