Treatment of chlorobenzene-contaminated groundwater in a pilot-scale constructed wetland
Introduction
Groundwater contaminations caused by chlorinated hydrocarbons are a global hazard particularly at industrial megasites. The Bitterfeld area in the middle-eastern part of Germany was characterized by extensive activities of the chemical industry and open-cast lignite mining for more than 100 years. This led to changes in the groundwater system and groundwater contamination affecting an area of 25 km2 and a volume of about 200 million m3 with the main contaminants being chlorinated aliphatic and aromatic hydrocarbons (Weiß et al., 2001). During recent years, groundwater flow conditions changed and the groundwater table rose due to the abandonment of mining water management. This development is expected to lead to the mobilisation of contaminant plumes and in the mid-term to the infiltration of contaminated groundwater from surface-near layers into the floodplains of the nearby river Mulde, a tributary of the river Elbe. The situation is exemplary as industrial megasites worldwide are often located in the vicinity of rivers and surface-near groundwater from contaminated aquifers may thus enter the river floodplains. Large-scale constructed wetlands represent a promising approach to protect river catchment areas against the impact of contaminated groundwater and to reduce environmental damage.
There are strong indications from several investigations that the interface between anaerobic groundwater and surface water can play an important role in the natural attenuation of organic contaminants (Bradley and Chapelle, 1998, Lendvay et al., 1998, Witt et al., 2002). Steep gradients of oxygen concentrations and other geochemical parameters greatly enhance the intrinsic biodegradation potential for the breakdown of many contaminants such as monochlorobenzene (MCB), dichloroethenes (DCEs) and vinyl chloride (VC). These substances are persistent or only slowly degraded under the anaerobic conditions usually found in aquifers (Lorah and Voytek, 2004) and are therefore widespread in the anaerobic aquifer at the large-scale contaminated site in Bitterfeld, Germany (Heidrich et al., 2004). They are dead-end products of the anaerobic dechlorination of highly chlorinated benzenes and ethenes in anaerobic environments. A promising way of taking advantage of interface phenomena is to utilize surface-near anaerobic/aerobic mixing zones in constructed wetlands (CW). CW can be constructed on a large scale, integrated into the landscape, and may lead to an efficient removal of organic contaminants (Haberl et al., 2003, Kassenga et al., 2003, Kassenga et al., 2004, Lorah et al., 2001, Lorah and Voytek, 2004).
During recent years, the phytoremediation of waters and soils contaminated by chlorinated hydrocarbons has moved into the focus of research, with most of the investigations being targeted on the removal of chlorinated volatile organic compounds (VOC), particularly ethenes and ethanes. Highly chlorinated compounds, e.g. perchloroethylene, require initial anaerobic microbial degradation steps and thus the high removal efficiencies of more than 90% were found mainly under anaerobic conditions (Mastin et al., 2001, Lorah and Voytek, 2004, Strand et al., 2005). However, under these conditions low-chlorinated compounds may accumulate if the process is not properly managed. Other authors showed the effectiveness of coupled anaerobic and aerobic systems (Kassenga et al., 2004, Richard et al., 2002) and the enhancing impact of plants on the removal of chlorinated VOC (Cho et al., 2005, Bankston et al., 2002, Wiltse et al., 1998).
In contrast to chlorinated ethanes and ethenes, only little is known about the relevant removal processes for chlorinated benzenes and the impact of the plants on removal efficiency in CW. Volatilisation was found to be a dominant elimination process for MCB in laboratory microcosms planted with reed (MacLeod et al., 1999). However, MCB mineralization was enhanced by the presence of plants and accounted for 27% of the added MCB in comparison to 16% in unplanted microcosms. Non-reversible sorption to wetland soils was shown to limit bioavailability and mineralization particularly of dichlorobenzenes (Lee et al., 2003).
The removal efficiency of a large-scale CW has so far only been evaluated for 1,4-dichlorobenzene with values ranging from 63 to 87% (Keefe et al., 2004). In general, most of the investigations on phytoremediation of waters and soils contaminated by chlorinated hydrocarbons have been carried out under laboratory conditions using micro- or meso-scale systems. Field investigations in pilot or full-scale treatment wetlands are lacking. Furthermore, potential impacts of undesirable processes such as volatilisation have not been addressed adequately. Volatilisation of toxic chlorinated hydrocarbons may be increased by technological problems such as clogging and subsequent flooding, and may lead to serious air pollution.
A pilot-scale horizontal subsurface flow CW was installed and operated at the Bitterfeld site for field investigations on the removal of chlorobenzenes as the main groundwater contaminants. In order to evaluate the processes involved, spatial concentration dynamics of MCB, 1,2-dichlorobenzene (1,2-DCB), 1,4-dichlorobenzene (1,4-DCB), oxygen and ferrous iron as well as the redox potential were investigated in relation to the distance from the inflow and the depth of the wetland bed. To provide further evidence for the hypothesis that plants display an enhancing effect on chlorobenzenes removal efficiency, investigations were carried out in parallel in a planted wetland and an unplanted reference plot.
Section snippets
Materials and methods
The SAFIRA (Remediation research in regionally contaminated aquifers) groundwater research site is located in the south-east of the city of Bitterfeld in Saxony-Anhalt, Germany. MCB is the predominant groundwater contaminant at the site. Averages and standard deviations of the main pollutant concentrations and geochemical characteristics as observed in June 2005 are given in Table 1. The groundwater passing through the wetlands was pumped from 22 m depth from the SAFIRA well no. 5.
Results and discussion
In order to investigate the pollutant removal and the influence of plants in the pilot-scale CW, the concentrations of MCB, 1,2-DCB and 1,4-DCB from the contaminated groundwater were measured at various distances from the inflow and depths of the wetlands. The mean concentrations of MCB and 1,4-DCB decreased with distance from the inlet in both the planted and unplanted system but the removal was more effective in the planted wetland, indicating the enhancement of contaminant removal by the
Conclusions
The presented data show that the planted CW is highly effective in removing MCB and 1,4-DCB from contaminated groundwater. However, concentrations of 1,2-DCB in the μg L−1 range were not affected. Aerobic microbial degradation of MCB and 1,4-DCB are considered to be dominant removal processes. Strong evidence for anaerobic microbial reduction of ferric iron was observed in the wetlands; this may be linked to chlorobenzene oxidation or to the decomposition of organic matter. Anaerobic microbial
Acknowledgements
The work was funded by a grant from the UFZ. The Department of Groundwater Remediation, the SAFIRA Project and the Department of Analytical Chemistry of the UFZ are acknowledged for assistance in the field and laboratory work. We are particularly grateful to S. Täglich, J. Ahlheim, O. Thiel, I. Mäusezahl and T. Nullmeyer. We thank A. Schulz and S. Herrmann for their contributions and K. Ethner and K. Puschendorf for their assistance in our laboratory. We thank the Deutsche Bundesstiftung Umwelt
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