Bioremediation of cis-DCE at a Sulfidogenic Site by Amendment with Propionate by T.P. Hoelen, J.A. Cunningham, G.D. Hopkins, C.A. Lebro´n, and M. Reinhard Abstract The technical feasibility of in situ reductive dechlorination of cis-dichloroethene (cDCE) and vinyl chloride (VC) was demonstrated at a sulfidogenic ground water site. Preceding laboratory studies had indicated that dechlorination at the site was limited by the supply of electron donors, that dechlorinating activity was sparsely and heterogeneously distributed, and that dechlorination was strongly enhanced by mixing sediments from multiple locations at the site. Based on these ob- servations, the remediation strategy consisted of amending the ground water with sodium propionate solution using a pair of recirculation wells as a mixing device. This strategy was able to overcome the sparse initial distribution of biological activity by creating a treatment zone. Dechlorination and sulfate reduction commenced within 10 and 4 d, respectively, after pro- pionate amendment began. Dechlorination efficiency increased during the 2 months of continuous operation. By the end of the 2 months, treatment converted ~1000 lg/L cDCE nearly stoichiometrically into ethene, with only low concentrations of VC remaining. More than 90% of chlorinated ethenes were removed as the ground water traveled from one well to the other (travel time of ~1 to 2 weeks). Near-complete removal of ~250 mg/L sulfate accompanied the rapid dechlorination, but no methanogenesis was observed. Aquifer clogging in the vicinity of the propionate injection wells became evident after ~40 d of propionate amendment and was attributed to the growth of sulfate-reducing bacteria and/or the formation of insoluble metal sulfides. Clogging was mitigated by pulsed amendment of the propionate solution. Introduction Natural attenuation is often the preferred method for cleaning up ground water that has been contaminated by chlorinated ethenes (MacDonald 2000). However, natural attenuation is not always a viable option. At some sites, reductive dechlorination of chlorinated ethenes may not occur, or dechlorination may be incomplete, if the necessary environmental factors are not met. Biologically mediated dechlorination requires the presence of dehalogenating bac- teria, fermenting bacteria, fermentable electron donors, and nutrients (McCarty 1997). Also, these must be in close spa- tial proximity, such that the relevant bacteria have access to the necessary donors and nutrients, and such that any requi- site mass transfer is possible. Furthermore, it has previously been observed that dechlorination can be inhibited or might occur very slowly when sulfate is present (Mazur and Jones 2001; Hoelen and Reinhard 2004), perhaps because sulfate- reducing bacteria (SRB) compete with dechlorinators for available hydrogen (McCarty 1997; Mazur and Jones 2001). Here, the term enhanced attenuation (EA), in contrast to natural attenuation, is used to describe the stimulation of indigenous organisms to degrade the targeted contam- inants. EA might be accomplished by providing necessary chemicals, by removing inhibitory conditions, and/or by mixing the ground water to allow contact between the bac- teria and the chemicals they need. For instance, electron donors or electron acceptors could be introduced through a well system that is designed to mix the ground water. In the case of a site contaminated by chlorinated ethenes, potential impediments to this approach include the follow- ing: (1) methanogenic bacteria (MB) or SRB could out- compete dechlorinating bacteria for the electron donor supply; (2) growth of MB or SRB could cause aquifer clog- ging; (3) methanogenesis could produce excess gas (beyond the solubility limit of methane) that could clog the aquifer; (4) the production of sulfide could lead to pre- cipitation of sulfide minerals, contributing to aquifer clog- ging; and/or (5) sulfate reduction could produce levels of sulfide that are inhibitory to dechlorination. The effect of electron donor amendment on dechlorination and aquifer clogging, particularly under methanogenic conditions, has been discussed in the literature (e.g., MacDonald et al. 1999; Fennell and Gossett 1999; Carr and Hughes 1999; Lutes et al. 2003; Yang and McCarty 2003), but there appear to be very few published cases where EA has actually been applied to sulfate-rich sites. Copyright ª 2006 The Author(s) Journal compilation ª 2006 National Ground Water Association. 82 Ground Water Monitoring & Remediation 26, no. 3/ Summer 2006/pages 82–91