Adaptation of a Constructed Wetland to Simultaneous Treatment of Monochlorobenzene and Perchloroethene

Abstract

Mixed groundwater contaminations by chlorinated volatile organic compounds (VOC) cause environmental hazards if contaminated groundwater discharges into surface waters and river floodplains. Constructed wetlands (CW) or engineered natural wetlands provide a promising technology for the protection of sensitive water bodies. We adapted a constructed wetland able to treat monochlorobenzene (MCB) contaminated groundwater to a mixture of MCB and tetrachloroethene (PCE), representing low and high chlorinated model VOC. Simultaneous treatment of both compounds was efficient after an adaptation time of 2½ years. Removal of MCB was temporarily impaired by PCE addition, but after adaptation a MCB concentration decrease of up to 64% (55.3 μmol L⁻¹) was observed. Oxygen availability in the rhizosphere was relatively low, leading to sub-optimal MCB elimination but providing also appropriate conditions for PCE dechlorination. PCE and metabolites concentration patterns indicated a very slow system adaptation. However, under steady state conditions complete removal of PCE inflow concentrations of 10–15 μmol L⁻¹ was achieved with negligible concentrations of chlorinated metabolites in the outflow. Recovery of total dechlorination metabolite loads corresponding to 100%, and ethene loads corresponding to 30% of the PCE inflow load provided evidence for complete reductive dechlorination, corroborated by the detection of Dehalococcoides sp.

KEY WORDS::

• phytoremediation
• chlorinated solvents
• groundwater-surface water interface
• reductive dechlorination
• microbial degradation

ACKNOWLEDGMENTS

M. Braeckevelt thanks the DBU Deutsche Bundesstiftung Umwelt for a PhD scholarship (AZ 20006/824) and the Helmholtz Interdisciplinary Graduate School for Environmental Research (HIGRADE) for support. The UFZ Departments of Groundwater Remediation and Analytical Chemistry, the Projects SAFIRA and CoTra, and in particular S. Täglich, J. Ahlheim, O. Thiel, I. Mäusezahl, K. Ethner, K. Puschendorf, M. Schröter, M. Kirsten, and S. Dotzauer are acknowledged for their valuable support. We are very grateful to G. Imfeld for helpful advice in the molecular biology laboratory.