Assessment of in situ biodegradation of monochlorobenzene in contaminated groundwater treated in a constructed wetland

Abstract

The degradation of monochlorobenzene (MCB) was assessed in a constructed wetland treating MCB contaminated groundwater using a detailed geochemical characterisation, stable isotope composition analysis and in situ microcosm experiments. A correlation between ferrous iron mobilisation, decreasing MCB concentration and enrichment in carbon isotope composition was visible at increasing distance from the inflow point, indicating biodegradation of MCB in the wetland. Additionally, in situ microcosm systems loaded with ¹³C-labelled MCB were deployed for the first time in sediments to investigate the biotransformation of MCB. Incorporation of ¹³C-labelled carbon derived from the MCB into bacterial fatty acids substantiated in situ degradation of MCB. The detection of ¹³C-labelled benzene indicated reductive dehalogenation of MCB. This integrated approach indicated the natural attenuation of the MCB in a wetland system. Further investigations are required to document and optimise the in situ biodegradation of MCB in constructed and natural wetland systems treating contaminated groundwater.

Introduction

Monochlorobenzene (MCB) is encountered worldwide as a groundwater pollutant, and persists in the essentially anaerobic aquifer at the large-scale contaminated site in Bitterfeld, Germany (Heidrich et al., 2004, Wycisk et al., 2004). In recent years, interest has grown in using phytoremediation processes for the elimination of recalcitrant organic substances from waste- and groundwater (Macek et al., 1998, Schnoor et al., 1995; Shimp et al., 1993; Trapp, 2000) including chloroaromatics (Gilbert and Crowley, 1997). Wetland systems represent an effective and inexpensive option to treat groundwater polluted with organic compounds by taking advantage of the geochemical and biological processes (e.g. Baker, 1998, Dunbabin and Bowmer, 1992, Gumbricht, 1993). Indeed, rapid degradation of chlorinated organics has been observed in the rhizosphere (Anderson and Walton, 1995, Jordahl et al., 1997, Lorah and Olsen, 1999, Pardue et al., 1996).

While aerobic degradation of MCB has been well studied (e.g. Van Agteren et al., 1998), only some evidence for MCB transformation under anoxic conditions has been presented yet and the degradation pathway is unknown (Kaschl et al., 2005, Liang and Gribic-Galic, 1990, Nowak et al., 1996). Moreover, only very few studies focus on the anaerobic microbial transformation of MCB under field conditions. Recently, indications of anaerobic MCB degradation taking place in the Bitterfeld contaminated aquifer were provided on the basis of isotope fractionation patterns (Kaschl et al., 2005). Kinetic isotope fractionation processes have been employed to demonstrate the biological transformation of various contaminants (Richnow et al., 2003a, Richnow et al., 2003b, Sherwood Lollar et al., 2001, Song et al., 2002). A substantial enrichment of ¹³C in the non-degraded fraction in the course of a contaminant plume indicates microbial degradation, as dilution and sorption do not affect the isotope composition of contaminants significantly (Harrington et al., 1996, Schüth et al., 2003, Slater et al., 2000). Combining stable isotope composition analysis with information obtained from simple in situ microcosm experiments (BACTRAPs) using isotope labelled substrate may provide a suitable approach to qualitatively support in situ biotransformation and to monitor spatial and temporal natural attenuation processes. Previously, BACTRAPs were exclusively installed in groundwater monitoring wells (Geyer et al., 2005, Kästner et al., 2006, Stelzer et al., 2006) and were deployed in sediment for the first time in the framework of this study.

For the assessment of in situ biodegradation in constructed wetlands and wetlands treating contaminated groundwater, it may be necessary to use several methods providing more than one line of evidence. A combined approach may be of additional benefit in particular when systems are complex, possess several compartments and convincing evidence is required. Moreover, a better understanding of the controlling geochemical processes in wetland systems is necessary to reliably predict the retention and transformation of contaminant. In this study, we evaluated the natural attenuation of MCB in a constructed wetland treating MCB contaminated groundwater using a detailed geochemical characterisation, stable isotope composition analysis and in situ microcosm experiments. The spatial variations of geochemical parameter were studied with the help of multivariate statistics to investigate the main processes controlling the wetland system. The concentration and carbon stable isotope composition of MCB was analysed to monitor the in situ contaminant degradation and in situ microcosms were used to provide qualitative evidences of in situ biotransformation of MCB.