1 Introduction

Model assessment of reductive dechlorination as a remediation technology for contaminant sources in fractured clay: Case studies

1.1 Background
1.2 Project description

1.1 Background

Chlorinated solvents are wide spread subsurface contaminants and an important threat to groundwater quality. Chlorinated solvents are sparingly soluble dense non aqueous phase liquids (DNAPL) that can be long term sources of contamination to groundwater. Many contaminated sites occur in areas with fractured clay geology at the land surface (Chapman and Parker 2005), where the released DNAPLs penetrate into preferential flow pathways formed by fractures and can then rapidly dissolve and diffuse from the fractures into the matrix (Falta 2005). Even after the removal of the physical source from the site, the contaminant can back diffuse to the fracture network for hundreds of years, causing long-term contamination of an underlying aquifer (Harrison et al. 1992,Parker et al. 1997,Reynolds and Kueper 2002). In Denmark, clay tills are wide spread and this scenario is very common. It is important to characterize the behavior of chlorinated solvents sources in clay aquitards so as to be able to predict their impact on the underlying groundwater aquifers. The remediation of contaminated clayey till sites is very challenging, because of the complexity of the source, the processes taking place and the mass transfer limitations due to slow diffusion process in the low permeability clay matrix (Johnson et al. 1989).

Recent laboratory and field experiments have shown that bioremediation may be an attractive method for chlorinated solvents decontamination. Chlorinated solvents can be anaerobically degraded through sequential reactions to a non toxic end product (ethene). These sequential reactions are termed “reductive dechlorination”. This degradation is possible in an anaerobic environment, with the presence of both dechlorinating bacteria and electron donor (generally hydrogen).

Bioremediation, where an electron donor and/or bacteria are injected into the fracture system to enhance reductive dechlorination, is a promising remediation technology that may be able to reduce clean-up times.

1.2 Project description

The overall purpose of the project is to assess the effects and timeframes for remediation using enhanced reductive dechlorination in clay till. The first phase consists in gathering the different experiences for reductive dechlorination as a remediation technology in clay till in Denmark (Miljøstyrelsen 2008b). A modeling tool for assessment of the time horizons for remediation of low permeability media using reductive dechlorination has been developed during the second phase (Miljøstyrelsen 2008a).

In this third phase, the modeling tool is tested on three selected case sites each representing clay till systems with different occurrences of vertical fractures, horizontal sand lenses and sand stringers.

The initial model presented in (Miljøstyrelsen 2008a) has been further developed and Section 3 gives a short description of the model and the new features.