Identification of chlorinated solvents degradation zones in clay till by high resolution chemical, microbial and compound specific isotope analysis

Abstract

a b s t r a c tThe degradation of chlorinated ethenes and ethanes in clay till was investigated at a contaminated site (Vadsby, Denmark) by high resolution sampling of intact cores combined with groundwater sampling. Over decades of contamination, bioactive zones with degradation of trichloroethene (TCE) and 1,1,1-trichloroethane (1,1,1-TCA) to 1,2-cis-dichloroethene (cis-DCE) and 1,1-dichloroethane, respectively, had developed in most of the clay till matrix. Dehalobacter dominated over Dehalococcoides (Dhc) in the clay till matrix corresponding with stagnation of sequential dechlorination at cis-DCE. Sporadically distributed bioactive zones with partial degradation to ethene were identified in the clay till matrix (thickness from 0.10 to 0.22 m). In one sub-section profile the presence of Dhc with the vcrA gene supported the occurrence of degradation of cis-DCE and VC, and in another enriched δ 13 C for TCE, cis-DCE and VC documented degradation. Highly enriched δ 13 C for 1,1,1-TCA (25‰) and cis-DCE (−4‰) suggested the occurrence of abiotic degradation in a third sub-section profile. Due to fine scale heterogeneity the identification of active degradation zones in the clay till matrix depended on high resolution subsampling of the clay till cores. The study demonstrates that an integrated approach combining chemical analysis, molecular microbial tools and compound specific isotope analysis (CSIA) was required in order to document biotic and abiotic degradations in the clay till system.

Reductive Dechlorination of Trichloroethylene (TCE) in Competition with Fe and Mn Oxides—Observed Dynamics in H2-dependent Terminal Electron Accepting Processes

“…Four previously characterized Fe(III) bearing subsurface materials from well described Danish field sites such as Grindsted (Heron et al, 1998),Vejen (Bjerg and Christensen, 1992;Heron et al, 1994;Pedersen et al, 1991), Farum (Andersen and Vikjaer Lassen, 1990) and Vadsbyvej (Damgaard et al, 2013) were also included. It has been shown in previous studies that the predominant terminal electron acceptors at the sampling location in the Grindsted landfill leachate plume were Fe(III), and in addition Mn reduction also appears to be taking place in more oxidized parts of the plume (Albrechtsen et al, 1999;Bjerg et al, 1995).…”

“…Different approaches have been used to assess the redox conditions during transformation of TCE such as the concentration of either the parent compound or electron donor or of reduced species such as Fe 2+ , HS -and CH 4 (Damgaard et al, 2013;Hunkeler et al, 2011). As described above, H 2 concentrations can also be used as an indicator of the dominant TEAP in natural or contaminated groundwater systems (Chapelle et al, 1996;Lovley and Goodwin, 1988).…”

Identification of abiotic and biotic reductive dechlorination in a chlorinated ethene plume after thermal source remediation by means of isotopic and molecular biology tools

“…Furthermore, assessing the presence of genes that encode for VC reductive dehalogenases (rdhA) known to catalyze VC reduction to ethene, such as vcrA, and measuring genes' mRNA level constitutes a stronger line of evidence to support complete reductive dechlorination. The vcrA and bvcA genes identified in Dhc are so far the only two functional genes described to encode VC rdhA (Krajmalnik-Brown et al, 2004;Müller et al, 2004) and their presence in field samples from sites contaminated with chlorinated ethenes was successfully related to complete dechlorination (Scheutz et al, 2008;van der Zaan et al, 2010;Damgaard et al, 2013a). It was moreover shown based on field samples that rdhA genes directly involved in dechlorination should be targeted in addition to Dhc, as different microbial species might harbour vcrA and bvcA genes due to horizontal gene transfer and are therefore also able to dechlorinate VC down to ethene (van der Zaan et al, 2010).…”

Carbon and chlorine isotopologue fractionation of chlorinated hydrocarbons during diffusion in water and low permeability sediments

“…During the further migration in aquifer systems, DNAPLs tend to accumulate on top of low permeability zones and diffuse into them (Feenstra et al, 1991;Johnson and Pankow, 1992;Parker et al, 2004;Falta, 2005;Seyedabbasi et al, 2012). In these zones, frequently more reducing conditions are encountered and (bio)degradation of chlorinated hydrocarbons possibly occurs (Damgaard et al, 2013).…”