| Summary: | In this study, a bioremediation approach was evaluated for the decontamination of an aquifer affected by the release of organohalides by an industrial landfill. After preliminary physicochemical and microbiological characterization of the landfill groundwater, the stimulation of natural organohalide respiration by the addition of a reducing substrate (i.e., molasse) was tested both at microcosm and at field scales, by the placement of an anaerobic permeable reactive bio-barrier. Illumina sequencing of cDNA 16S rRNA gene revealed that organohalide-respiring bacteria of genera <i>Geobacter</i>, <i>Sulfurospirillum</i>, <i>Dehalococcoides</i>, <i>Clostridium</i> and <i>Shewanella</i> were present within the aquifer microbial community, along with fermentative <i>Firmicutes</i> and <i>Parvarchaeota</i>. Microcosm experiments confirmed the presence of an active natural attenuation, which was boosted by the addition of the reducing substrate. Field tests showed that the bio-barrier decreased the concentration of chloroethenes at a rate of 23.74 kg d<sup>−1</sup>. Monitoring of organohalide respiration biomarkers by qPCR and Illumina sequencing revealed that native microbial populations were involved in the dechlorination process, although their specific role still needs to be clarified. The accumulation of lower-chloroethenes suggested the need of future improvement of the present approach by supporting bacterial vinyl-chloride oxidation, to achieve a complete degradation of chloroethenes.
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