First bioreactor pilot study of in situ biogeochemical transformation in China for remediation of chlorinated ethenes contaminated groundwater and its microbial insights

In situ biogeochemical transformation (ISBGT) is an emerging remediation strategy for groundwater contaminated by chlorinated solvents. In this study, ISBGT was implemented, consisting of a reaction pit filled with remedial amendments coupled with recirculation pipelines to increase hydraulic retention time and sustain favorable geochemical conditions. Over 180 days, perchloroethylene (PCE) and trichloroethylene (TCE) declined by 98 %, with limited accumulation of cis -dichloroethene ( cis -DCE) and vinyl chloride (VC), suggesting near-complete dechlorination. ISBGT operation led to a substantial drop in redox potential from −50 to −420 mV, creating reducing conditions favorable for iron and sulfur cycling. Metagenomic analysis revealed a temporal shift in functional potential, beginning with carbohydrate hydrolysis (e.g., celA , 2194–842 RPKM), followed by enhanced protein fermentation ( pepA/N/P/T , 1126–9430 RPKM) and acetate/H 2 generation ( porABCD , 1872–9125 RPKM). Concurrently, genes associated with redox iron and sulfur cycling, including mtrCDE (8778–21,461 RPKM) and pilABC (4318–51,410 RPKM), increased notably. These processes facilitated formation of biogenic iron sulfide (Fe S) minerals, which likely played a central role in the abiotic reduction of PCE and TCE. Overall, ISBGT created a geochemically favorable environment where microbial metabolism continuously regenerated reactive Fe S minerals, enabling sustained and synergistic CVOC degradation under highly reducing conditions. • First bioreactor-style ISBGT in China achieves >98 % PCE/TCE removal sustainably. • Recirculation extends residence time and maintains strongly reducing redox conditions. • Small accumulation of cis-DCE/VC highlights predominance of abiotic pathway. • Metagenomics links fermentation, EET, sulfate reduction to robust dechlorination. • Biogenic Fe S minerals form in situ, mediating abiotic reduction of ethenes.