Tetrachloroethene (PCE) and trichloroethene (TCE) are ideal solvents for numerous applications, and their widespread use makes them prominent groundwater pollutants. Even more troubling, natural biotic and abiotic processes acting on these solvents lead to the accumulation of toxic intermediates (such as dichloroethenes) and carcinogenic intermediates (such as vinyl chloride). Vinyl chloride was found in at least 496 of the 1,430 National Priorities List sites identified by the US Environmental Protection Agency, and its precursors PCE and TCE are present in at least 771 and 852 of these sites, respectively. Here we describe an unusual, strictly anaerobic bacterium that destroys dichloroethenes and vinyl chloride as part of its energy metabolism, generating environmentally benign products (biomass, ethene and inorganic chloride). This organism might be useful for cleaning contaminated subsurface environments and restoring drinking-water reservoirs.

The 16S rRNA gene provides insufficient information to infer the range of chloroorganic electron acceptors used by different Dehalococcoides organisms. To overcome this limitation and provide enhanced diagnostic tools for growth measurements, site assessment, and bioremediation monitoring, a quantitative real-time PCR (qPCR) approach targeting 16S rRNA genes and three Dehalococcoides reductive dehalogenase (RDase) genes with assigned function (i.e., tceA, bvcA, and vcrA) was designed and evaluated. qPCR standard curves generated for the RDase genes by use of genomic DNA from Dehalococcoides pure cultures correlated with standard curves obtained for both Bacteria-and Dehalococcoides-targeted 16S rRNA genes, suggesting that the RDase genes are useful targets for quantitative assessment of Dehalococcoides organisms. RDase gene probe/primer pairs were specific for the Dehalococcoides strains known to carry the diagnostic RDase gene sequences, and the qPCR method allowed the detection of as few as 1 to 20 and quantification of as few as 50 to 100 tceA, bvcA, or vcrA gene targets per PCR volume. The qPCR approach was applied to dechlorinating enrichment cultures, microcosms, and samples from a contaminated site. In characterized enrichment cultures where known Dehalococcoides strains were enumerated, the sum of the three RDase genes equaled the total Dehalococcoides cell numbers. In site samples and chloroethane-dechlorinating microcosms, the sum of the three RDase genes was much less than that predicted by Dehalococcoides-targeted qPCR, totaling 10 to 30% of the total Dehalococcoides cell numbers. Hence, a large number of Dehalococcoides spp. contain as-yet-unidentified RDase genes, indicating that our current understanding of the dechlorinating Dehalococcoides community is incomplete.Chlorinated solvents are a well-recognized class of groundwater (GW) contaminants (1,7,41,53). Substantial knowledge regarding diverse groups of bacteria that partially dechlorinate tetrachloroethene (PCE) or trichloroethene (TCE) to cis-1,2-dichloroethene (cis-DCE) (5,12,19,30,50) or trans-DCE (13) has been accrued over the past decade. Partial reductive dechlorination contributes to the formation of DCEs and, in some cases, vinyl chloride (VC) (1,26,45). Environmental accumulation of VC is particularly troublesome because this compound is a proven human carcinogen; therefore, incomplete dechlorination of PCE and TCE does not result in detoxification.Recently, Dehalococcoides organisms were discovered as a deeply branching group within the green nonsulfur bacteria (Chloroflexi) (44). This physiologically and phylogenetically distinct bacterial group requires hydrogen as an electron donor and specific chloroorganic compounds as electron acceptors to support their energy metabolism. Dehalococcoides ethenogenes strain 195 was the first isolate described to dechlorinate PCE to VC and ethene (35); however, the last transformation step from VC to ethene occurred slowly in a cometabolic process (34). Similarly, metabolic dechlorination of TCE...

ABSTRACT. The results of a field experiment designed to test the effectiveness of a novel approach for long-term, in situ bioimmobilization of toxic and soluble Cr(VI) in groundwater using

Sixty leading members of the scientific, engineering, regulatory, and legal communities assembled for the PFAS Experts Symposium in Arlington, Virginia on May 20 and 21, 2019 to discuss issues related to per‐ and polyfluoroalkyl substances (PFAS) based on the quickly evolving developments of PFAS regulations, chemistry and analytics, transport and fate concepts, toxicology, and remediation technologies.  The Symposium created a venue for experts with various specialized skills to provide opinions and trade perspectives on existing and new approaches to PFAS assessment and remediation in light of lessons learned managing other contaminants encountered over the past four decades. The following summarizes several consensus points developed as an outcome of the Symposium: Regulatory and policy issues: The response by many states and the US Environmental Protection Agency (USEPA) to media exposure and public pressure related to PFAS contamination is to relatively quickly initiate programs to regulate PFAS sites. This includes the USEPA establishing relatively low lifetime health advisory levels for PFAS in drinking water and even more stringent guidance and standards in several states. In addition, if PFAS are designated as hazardous substances at the federal level, as proposed by several Congressional bills, there could be wide‐reaching effects including listing of new Superfund sites solely for PFAS, application of stringent state standards, additional characterization and remediation at existing sites, reopening of closed sites, and cost renegotiation among PRPs. Chemistry and analytics: PFAS analysis is confounded by the lack of regulatory‐approved methods for most PFAS in water and all PFAS in solid media and air, interference with current water‐based analytical methods if samples contain high levels of suspended solids, and sample collection and analytical interference due to the presence of PFAS in common consumer products, sampling equipment, and laboratory materials. Toxicology and risk: Uncertainties remain related to human health and ecological effects for most PFAS; however, regulatory standards and guidance are being established incorporating safety factors that result in part per trillion (ppt) cleanup objectives. Given the thousands of PFAS that may be present in the environment, a more appropriate paradigm may be to develop toxicity criteria for groups of PFAS rather than individual PFAS. Transport and fate: The recalcitrance of many perfluoroalkyl compounds and the capability of some fluorotelomers to transform into perfluoroalkyl compounds complicate conceptual site models at many PFAS sites, particularly those involving complex mixtures, such as firefighting foams. Research is warranted to better understand the physicochemical properties and corresponding transport and fate of most PFAS, of branched and linear isomers of the same compounds, and of the interactions of PFAS with other co‐contaminants such as nonaqueous phase liquids. Many PFAS exhibit complex transport mechanisms, particular...