The effect of tetrachloroethene on biological dechlorination of vinyl chloride: potential implication for natural bioattenuation

Whereas reductive dechlorination of tetrachloroethene (PCE) and trichloroethene to ethene (ETH) has been demonstrated, some laboratory and field studies indicate that partial dechlorination is often observed. Because partial dechlorination may produce compounds of equal or greater toxicity than starting compounds, it is important to identify factors that are responsible for complete dechlorination. A comparative study was completed to examine ethene production from a PCE-fed culture and a vinyl chloride (VC)-fed culture. Physiochemical conditions, nutrients and the electron donor (methanol) were identical for both cultures. PCE-fed cultures were able to sustain an ETH production rate of about 0.1 μmol d−1, whereas ETH production by a VC-fed culture declined to about 0.02 μmol d−1 over 115 d. PCE additions to the VC-fed culture improved ETH production. Results suggest that VC dechlorination to ETH may not be sustainable in the absence of a higher chlorinated ethene. This could mean that for sites capable of partial reductive dechlorination (via natural bioattenuation or stimulated bioattenuation), differential retardation of higher chlorinated ethenes may result in a contaminant plume with VC at the leading edge. In such cases, the degree of VC dechlorination under anaerobic conditions may decline with time.