Natural Attenuation of Chlorinated Volatile Organic Compounds in a Freshwater Tidal Wetland, Aberdeen Proving Ground, Maryland

By Michelle M. Lorah and Lisa D. Olsen

Abstract

Field evidence collected along two ground-water flowpaths and laboratory evidence from microcosm experiments indicates that biodegradation naturally attenuates a plume of chlorinated volatile organic compounds as it discharges from an aerobic sand aquifer through anaerobic wetland sediments. A decrease in concentrations of two parent contaminants, trichloroethylene (TCE) and 1,1,2,2-tetrachloro-ethane (PCA), and a concomitant increase in concentrations of anaerobic daughter products occur along upward flowpaths through the wetland sediments. The daughter products cis-1,2-dichloroethylene (cis-12DCE), trans-1,2-dichloroethylene (trans-12DCE), vinyl chloride (VC), 1,1,2-trichloroethane, and 1,2-dichloroethane are produced from hydrogenolysis of TCE and from PCA degradation through hydrogenolysis and dichloroelimination pathways. Total concentrations of TCE, PCA, and their degradation products decrease to below detection levels within 0.15 to .30 m of land surface.

Microcosms constructed with wetland sediment and porewater from the site and incubated under methanogenic conditions confirmed field evidence of TCE and PCA degradation pathways and showed extremely rapid degradation rates. First-order anaerobic degradation rates for TCE and PCA ranged from 0.045 to 0.37 per day, corresponding to half-lives of 1.9 to 15 days. In aerobic microcosm experiments, biodegradation of cis-12DCE, trans-12DCE, and VC occurred only if methane consumption occurred, indicating that methanotrophs were involved. Aerobic biodegradation rates for cis-12DCE, trans-12DCE, and VC (0.05 to 0.21 per day) were within the same range as those measured for TCE and PCA under anaerobic conditions. Production of these anaerobic daughter products of TCE, therefore, could be balanced by their consumption where methanotrophs are active in the wetland sediment, such as near the land surface and near plant roots. This study indicates that natural attenuation can be a feasible ground-water remediation method where wetlands and similar organic-rich environments at ground-water/surface-water interfaces are discharge areas for plumes of chlorinated solvents.