Groundwater from a MA site with a large plume contaminated with low concentrations (2 to 29 ppb) of vinyl chloride (VC) was treated with oxygen, ethene, and mineral nutrients under aerobic (ethenotrophic) conditions in laboratory microcosms. The results showed that the VC was completely biodegraded, well below the USEPA MCL of 2 ppb. This observation has led to a field remediation program in which oxygen and ethene are introduced into a groundwater plume using a gas delivery technique known as the iSOC® system. The site was originally contaminated with PCE, but has undergone incomplete reductive dechlorination, resulting in the formation of a low concentration plume of VC.
The formation of persistent low concentration VC plumes by reductive dechlorination of PCE or TCE is not unique. This topic has been addressed by Cupples, Spormann and McCarty (2004, ES&T 38:1102) who proposed that the anaerobic degradation of vinyl chloride under reductive dechlorination is subject to substrate limiting conditions. These authors concluded that dehalorespiring microorganisms will not grow when the electron acceptor concentration of cis-DCE and VC is below 0.7µM (44 µg /L).
Consequently, it appears unlikely that VC would be readily removed by reductive dechlorination at this site. We therefore decided to evaluate the aerobic metabolism of VC using ethene as the co-substrate in order to optimize biodegradation. Our approach was partly based on the work of Coleman et al. (2002, AEM 68:2726, 6162) at the Air Force Research Laboratory, and Coleman and Spain (2003, J. Bact.185:5536). These authors recently provided strong support for the view that ubiquitous soil Mycobacteria and related organisms are responsible for the aerobic natural attenuation of VC observed at many sites. Coleman et al. (2002) also showed that VC could be aerobically biodegraded at low concentrations. However, at very low concentrations less than Ks (around 1 µM [63 µg/L]), substrate limitations may result in less than optimum rates of VC degradation. Earlier studies (Hartmans & de Bont, 1992, AEM 58:1220) demonstrated that ethene supported growth of VC-degrading bacteria, and induced the enzymes involved in VC degradation.
Using microcosm testing, BCI evaluated the ability of ethene to stimulate the growth of VC-degrading bacteria in site groundwater. The addition of ethene, oxygen and minerals to site groundwater resulted in optimized biodegradation of vinyl chloride, achieving levels below the drinking water standard of 2 ppb. The successful biodegradation of 30 ppb of VC in site groundwater occurred by either direct microbial oxidation or by co-metabolism, and the VC was biodegraded to concentrations less than 0.5 ppb. Methane-oxidizing bacteria did not appear to be involved in the observed optimized biodegradation of VC. The field implications of this work will be discussed.