Natural attenuation of chloroethenes: identification of sequential reductive/oxidative biodegradation by microcosm studies

A different lines of evidence approach for investigation of biodegradation processes at a chloroethene contaminated site showed well corresponding results of pollutant profiles, redox zonation, characterisation of autochthonic microflora and microcosm studies. In particular microcosm studies allowed identification of the predominating degradation pathways. Perchloroethene and trichloroethene are reductively chlorinated to mainly cis-1,2-dichloroethene (cDCE) under anaerobic conditions. Further reductive degradation to vinyl chloride (VC) is restricted to a distinct strongly anaerobic zone in the plume. Addition of high amounts of sediment material (80 vol%) to groundwater microcosms enabled reductive dechlorination without amendment with further auxiliary substrates. Reductive dechlorination was not irreversibly hindered by initially high nitrate concentrations and initially high oxidation-reduction potential. The products of anaerobic degradation cDCE and VC are subsequently aerobically mineralised, even when only low oxygen concentrations are available. Anaerobic oxidative degradation could not be proven in this study.