Performance of Full‐Scale Enhanced Reductive Dechlorination in Clay Till

Abstract

At a low permeability clay till site contaminated with chlorinated ethenes (Gl. Kongevej, Denmark), enhanced reductive dechlorination (ERD) was applied by direct push injection of molasses and dechlorinating bacteria. The performance was investigated by long‐term groundwater monitoring, and after 4 years of remediation, the development of degradation in the clay till matrix was investigated by high‐resolution subsampling of intact cores. The formation of degradation products, the presence of specific degraders Dehalococcoides spp. with the vinyl chloride (VC) reductase gene vcrA, and the isotope fractionation of trichloroethene, cis‐dichloroethene (cis‐DCE), and VC showed that degradation of chlorinated ethenes occurred in the clay till matrix as well as in sand lenses, sand stringers, and fractures. Bioactive sections of up to 1.8 m had developed in the clay till matrix, but sections, where degradation was restricted to narrow zones around sand lenses and stringers, were also observed. After 4 years of remediation, an average mass reduction of 24% was estimated. Comparison of the results with model simulation scenarios indicate that a mass reduction of 85% can be obtained within approximately 50 years without further increase in the narrow reaction zones if no donor limitations occur at the site. Long‐term monitoring of the concentration of chlorinated ethenes in the underlying chalk aquifer revealed that the aquifer was affected by the more mobile degradation products cis‐DCE and VC generated during the remediation by ERD.

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

“…Only a few sampling locations (i.e. F4-3, B16-1, B17-1, B23-1, B23-2, B23-3, and B34-4) indicate Dhc quantities in the ranges previously reported for other sites where biotic reductive dechlorination has naturally occurred (Damgaard et al, 2013b) (from 10 5 to 10 6 gene copies⋅-L −1 ,Table S 4, SI) and are thus high enough to support some dechlorination potential. Yet, the detection of Dhc DNA is not sufficient to confirm the actual occurrence of dechlorination as DNA does not indicate bacterial activity.…”

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

“…These results suggest lower generation of VFA in borehole C compared to A and B. Compared to sites undergoing ERD (Damgaard et al, 2012;Manoli et al, 2011;Scheutz et al, 2010) the concentrations of VFA measured in boreholes A and B are surprisingly high. However, several alcohols (methanol, ethanol, iso-propanol and n-propanol), hydrocarbons and NVOC were found in the groundwater from wells A and B (Table 1).…”

“…Making sure the samples remained frozen; they were freeze dried along with sediment samples over night.~1 g 1.4 mm ceramic beads (MoBio, Carlsbad, USA) were added and DNA/RNA extraction was performed as described by Damgaard et al (2012) (see Appendix A). DNA was also extracted using commercial Powerlyzer soil kit (Mobio, Carlsbad, USA) following the manufacturers recommendations, except the samples were pretreated with G2 blocking agent (described in Appendix A).…”

“…The samples were pre-concentrated with a purge-andtrap concentrator (Tekmar Velocity XPT, USA) equipped with a VOCARB 3000 trap and connected to a cryogenic trap (ATAS GL, Netherlands) installed inside the GC oven. The analytical procedure reported by Damgaard et al (2012) was used (also described in Appendix A). Carbon isotope ratios are reported in the delta notation relative to VPDB as defined by δ=(R/R std −1)×1000 [‰], where R and R std are the isotope ratio of the sample and the standard, respectively.…”

A Bayesian belief network approach for assessing uncertainty in conceptual site models at contaminated sites

“…Clay tills are common in northern Europe and parts of North America [Christiansen et al, 2008;Parker et al, 2008] and can lead to a complex system of preferential flow paths (e.g. fractures or sand lenses) in a low permeability matrix [Damgaard et al, 2013a;Damgaard et al, 2013b;Gerber et al, 2001;Hendry et al, 2004;Kessler et al, 2012;Kessler et al, 2013]. Capturing a correct conceptual understanding of such sites is crucial in risk assessment and management where predictions of contaminant breakthrough, leaching time and remedial options are required.…”