Formatted Title
Evaluation of Chlorinated Ethene Degradation by an Integrated Approach in the Risk Assessment of a Contaminant Plume Influenced by Source and Barrier Remediation
Background/Objectives
Remediation of severe source zone contamination with chlorinated ethenes at a former gas regulator production site has included excavation, NAPL pumping and thermal treatment with steam and installation of a ZVI funnel and gate reactive barrier intercepting the plume at the site in early 2000s. However, the primary flow of groundwater and the plume pass by (not through the barrier) resulting in the plume heading for the town of Hedensted. The heating and ZVI installation has changed conditions in the plume and increasing levels of vinyl chloride (VC) reveals ongoing reductive dechlorination. The purpose of the current study is to evaluate the natural/stimulated attenuation of the plume for evaluation of the current risk associated with the contamination. An integrated approach including CSIA and molecular biological tools is applied to identify and document biogeochemical processes in the plume.
Approach/Activities
Four transects each of eight to 10 boreholes with depths until 10 m below surface have been established with a distance of 25, 50 and 60 meters, respectively, across the plume flow line downstream of the source area. Concentrations of chlorinated ethenes, degradation products, redox conditions and NVOC have been measured in groundwater samples from all boreholes. CSIA of C and Cl and molecular microbial tools have been used on selected samples both along the groundwater flow line in the plume and in representative samples in each of the transects. Groundwater flow rates and contaminant mass discharge across transects has been established.
Results/Lessons Learned
The source area is still highly contaminated with chlorinated ethenes. Downstream of the source area two plumes are observed in the groundwater; one where primary TCE is present representing water from the source area (thin plume 10 m) and another where degradation products are present representing back diffusion from the clay till matrix (wider plume 20 m). In the central part of the plume chlorinated ethenes and degradation products are observed up to 3 m into the clay till matrix. Groundwater will therefore be affected both by contaminants from the source area and by back diffusion from the clay till matrix for many years to come. The microbial analysis revealed widespread presence of the specific degraders Dehalococcoides and of the genes responsible for VC degradation. This indicates a good potential for complete dechlorination of the chlorinated ethenes in (parts of) the plume. C-CSIA (analytical results pending) is expected to document degradation of chlorinated ethenes, including VC, and dual CSIA may help in differentiating biotic and abiotic processes (potentially induced by ZVI). From transect 3-4 a significant reduction in the concentration of VC in the groundwater is observed, possibly caused by degradation and/or dilution as water from the top of the flow till layer seem to run off here. Degradation is documented based on microbial data and CSIA. Natural degradation processes stimulated by heating and ZVI have resulted in ongoing dechlorination of chlorinated ethenes in the plume. However, degradation processes in the low permeable clay till also influence the contaminant composition in the plume. Better understanding of the ongoing processes are expected to lead to understanding of the effect of current processes on the risk and to identify beneficial approaches to remediation of the plume. The study illustrates that an integrated approach combining concentration measurements, CSIA and molecular microbial tools is a valuable method to document complex processes at a site contaminated with chlorinated ethenes. In the present study documented processes are crucial in the planning of future plume remediation as documented processes have high impact on the risk assessment today but also how it will evolve in the future.