Documentation and quantification of in situ natural and enhanced degradation of chlorinated ethenes

Chlorinated ethenes, such as tetrachloroethene (PCE) and trichloroethene (TCE), are widespread soil and groundwater contaminants. Fortunately degradation, either naturally or enhanced, can provide in situ destruction of these contaminants. In order to apply degradation-based remedial strategies to treat chlorinated ethene plumes, documentation and quantification of degradation are required. No method alone can provide the necessary information, and an integrated approach is therefore recommended in order to obtain an overall assessment. An integrated approach combine knowledge obtained by multiple tools, to characterise and quantify degradation, with system understanding (e.g. flow and transport) and conceptualisation. Various tools exist, that can be used in the integrated approach, where molecular diagnostic techniques and compound-specific isotope analysis (CSIA) are the most recent additions to the toolbox. Molecular diagnostic tools, e.g. measurements of the abundance of specific bacteria and functional genes related to degradation of chlorinated ethenes, document the potential for biodegradation. CSIA can document the occurrence of degradation and quantify the extent. These novel techniques show great promise, but have only to a limited extent been applied in an integrated approach for the assessment of in situ degradation of chlorinated ethenes. Therefore, the advantages and challenges of the combined approach have yet to be assessed.
This PhD thesis aimed to address this knowledge gap on the applicability of the approach, by advancing and exploring the use of techniques to characterise and quantify in situ degradation in diverse settings and scenarios. Particularly focus has been on the application of the advanced isotopic and microbial tools. Three scenarios were investigated: (1) A biostimulated large-scale plume with several degradation pathways, (2) A naturally transient plume at the groundwater-surface water interface, and (3) A plume bioremediated with activated carbon and bioamendments.
In scenario 1, extensive sampling was conducted in a large-scale plume, tracking the lasting impact of a source remediation, which had released dissolved organic carbon that stimulated degradation down-gradient the source zone. Isotopic and microbial data were used to identify degradation pathways (biotic and abiotic), document biodegradation potential and quantify degradation rates throughout the plume. Integrated in a conceptual site model, it was demonstrated that the chlorinated ethenes underwent a destructive process, but documentation of the responsible mechanism(s) was constrained at the plume front.
In scenario 2, a near-stream system, where a groundwater plume consisting of primarily cis-DCE and VC discharged to the stream, was investigated through a multi-scale integrated approach. The study documented a behaviour not commonly observed: a change in dynamics from limited attenuation to significant bioattenuation. The significant bioattenuation was documented by a high abundance of relevant microbial targets, and through significant increases in δ13C values for cis-DCE and an isotope mass balance. Integrated in a conceptual site model, it could be determined that a short residence time in the near-stream system restricted complete dechlorination.
In scenario 3, a comprehensive investigation was conducted after bioremediation with activated carbon and bioamendments injected across a TCE plume. The injection caused transient dynamics, and only an integrated approach could be used to describe the complex, non-stationary system. Microbial and isotopic data provided understanding on dynamics that could not have been obtained otherwise; Microbial data revealed potential reasons for the degradation stalling at cis-DCE and isotope data enhanced the knowledge on the effect of bioremediation through increased process understanding. The integrated approach effectively documented degradation and described the factors controlling it, yet quantification of degradation was inaccessible with the applied methods, due to the high complexity of the system dynamics.
In conclusion, this PhD study has identified advantages and limitations of applying an integrated approach in order to document and quantify in situ degradation in new settings and scenarios. The investigations underlined the added value when including microbial and isotopic techniques, and identified constraints in the use of the tools. Furthermore, the importance of integrating the novel techniques in an overall system understanding and conceptualisation was highlighted. The obtained knowledge can improve future site-specific evaluations of in situ natural and enhanced degradation as well as holistic risk assessments.