A Novel Dual-Compartment, Continuous-Flow Wetland Microcosm to Assess Cis-Dichloroethene Removal from the Rhizosphere

Abstract

The anaerobic biodegradation of tetrachloroethene commonly results in the accumulation of chlorinated intermediates such as cis-1,2-dichloroethene (cDCE). Frequently, groundwater contaminated with chlorinated ethenes discharges to natural wetlands. The goal of this study was to quantitatively evaluate the effects of wetland plants and microorganisms on the fate of cDCE in the wetland rhizosphere. To accomplish this goal, a novel dual-compartment wetland microcosm was designed. A Phragmites australis individual was maintained in the microcosm, which was operated with continuous flows of air and mineral medium through the foliar and rhizosphere compartments, respectively, to incorporate mass transfer/transport processes that are important in natural wetlands and allow steady-state assessment of changes in dissolved O₂ and cDCE or [1,2–¹⁴C]cDCE levels. Substantial amounts of [¹⁴C]cDCE were phytovolatilized through a healthy P. australis individual to the foliar chamber. Rhizodegradation by native microorganisms associated with P. australis roots also converted substantial amounts of [¹⁴C]cDCE to ¹⁴C-labeled CO₂ and non-volatile compounds, presumably through cometabolic reactions that could be enhanced by the release of O₂ and exudates by P. australis. These results suggest that, in some cases, the intrinsic capacity of native wetland plants and microorganisms to remove cDCE from the rhizosphere may be substantial.

KEY WORDS:

• rhizoremediation
• phytoremediation
• natural attenuation
• wetlands
• microcosm

ACKNOWLEDGEMENTS

This work was supported, in part, by a Maryland Agricultural Experiment Station research grant and a Larson Aquatic Research Support (LARS) Scholarship awarded to Ilisa Tawney by the American Water Works Association. The authors would like to thank Joel G. Burken (University of Missouri-Rolla) and Eric A. Seagren (University of Maryland) for providing useful information on bioreactor design and David L. Freedman (Clemson University) for helpful comments on the manuscript. The assistance of Gary Seibel (University of Maryland) in implementing various aspects of the microcosm design is also gratefully acknowledged.