The Myth of “cis-Stall”: Using Isotopes and Concentration Trends to Evaluate cis-1,2-Dichloroethene Degradation

摘要

Background/Objectives. Numerous investigators have taken the presence of cis-1,2-dichloroethene (cDCE) in conjunction with the absence of vinyl chloride in subsurface environments contaminated with tetrachloroethene (PCE) and/or trichloroethene (TCE) to indicate that cDCE degradation is not occurring (“cis-stall”) and therefore that some form of enhancement is necessary to facilitate bioremediation. The assumption that degradation was “stalled” at cDCE came about because the reaction that results in the production of cDCE from TCE is reductive dechlorination and during complete reductive dechlorination vinyl chloride is produced from the degradation of cDCE if the reaction proceeds uninterrupted. The lack of VC at a site led to the conclusion that cDCE was not degrading, either because there was insufficient organic carbon or because the requisite bacteria were absent. If cDCE was not degrading at such a site, then the concentration and mass of this compound should be seen to increase. This is not the case at many, if not most, sites. However, the sale of bioremediation products that enhance biodegradation through either the addition of organic carbon or an inoculum of bacteria such as Dehalococcoides has been greatly enhanced by perceived “cis-stall.” As will be shown, the use of biostimulation and/or bioaugmentation may not be necessary in many cases because the natural treatment system is indeed working to remediate the site and cDCE is simply degrading via a mechanism or mechanisms other than biological reductive dechlorination. Potential degradation mechanisms for the degradation of cDCE that do not involve biological reductive dechlorination include oxidation and abiotic degradation. In some cases biostimulation and/or bioaugmentation may actually increase the risks posed by PCE, TCE, and cDCE contamination. This is particularly true where receptors could be exposed to the more mobile and toxic vinyl chloride that is not only produced, but accumulates, during complete reductive dechlorination.