Lisa D. Olsen

Natural attenuation of chlorinated volatile organic compounds in a freshwater tidal wetland: Field evidence of anaerobic biodegradation

Field evidence collected along two groundwater flow paths shows that anaerobic biodegradation naturally attenuates a plume of chlorinated volatile organic compounds as it discharges from an aerobic sand aquifer through wetland sediments. A decrease in concentrations of two parent contaminants, trichloroethylene (TCE) and 1,1,2,2-tetrachloroethane (PCA), and a concomitant increase in concentrations of anaerobic daughter products occurs along upward flow paths through the wetland sediments. The daughter products 1,2-dichloroethylene, vinyl chloride, 1,1,2-trichloroethane, and 1,2-dichloroethane are produced from hydrogenolysis of TCE and from PCA degradation through hydrogenolysis and dichloroelimination (reductive dechlorination) pathways. Total concentrations of TCE, PCA, and their degradation products, however, decrease to below detection levels within 0.15–0.30 m of land surface. The enhanced reductive dechlorination of TCE and PCA in the wetland sediments is associated with the naturally higher concentrations of dissolved organic carbon and the lower redox state of the groundwater compared to the aquifer. This field study indicates that wetlands and similar organic-rich environments at groundwater/surface-water interfaces may be important in intercepting groundwater contaminated with chlorinated organics and in naturally reducing concentrations and toxicity before sensitive surface-water receptors are reached.

Biodegradation of Trichloroethylene and Its Anaerobic Daughter Products in Freshwater Wetland Sediments

The wide range of redox conditions and diversity of microbial populations in organic-rich wetland sediments could enhance biodegradation of chlorinated solvents. To evaluate potential biodegradation rates of trichloroethylene (TCE) and its anaerobic daughter products (cis-1,2-dichloroethylene; trans-1,2-dichloroethylene; and vinyl chloride), laboratory microcosms were prepared under methanogenic, sulfate-reducing, and aerobic conditions using sediment and groundwater from a freshwater wetland that is a discharge area for a TCE contaminant plume. Under methanogenic conditions, biodegradation rates of TCE were extremely rapid at 0.30 to 0.37 d−1 (half-life of about 2 days). Although the TCE biodegradation rate was slower under sulfate-reducing conditions (0.032 d−1) than under methanogenic conditions, the rate was still two orders of magnitude higher than those reported in the literature for microcosms constructed with sandy aquifer sediments. In the aerobic microcosm experiments, biodegradation occurred only if methane consumption occurred, indicating that methanotrophs were involved. Comparison of laboratory-measured rates indicates that production of the 1,2-dichloroethylene isomers and vinyl chloride by anaerobic TCE biodegradation could be balanced by their consumption through aerobic degradation where methanotrophs are active in wetland sediment. TCE degradation rates estimated using field data (0.009 to 0.016 d−1) agree with the laboratory-measured rates within a factor of 3 to 22, supporting the feasibility of natural attenuation as a remediation method for contaminated groundwater discharging in this wetland and other similar environments.

Ground-water and surface-water quality data for the West Branch Canal Creek area, Aberdeen Proving Ground, Maryland

................................................................................................................................................................................. ̂ Inffoduction................................................................... Purpose and scope.........................................................................................................................................................2 Site history .....................................................................................................................................................................3 Description of study area...............................................................................................................................................3 Site investigations..........................................................................................................................................................3 Acknowledgments.........................................................................................................................................................3 Methods of investigation........................................................................................................................................................^ Ground-water and surface-water sampling networks....................................................................................................5 Ground-water 1-inch piezometers.........................................................................................................................5 Ground-water 0.75-inch drive-point piezometers.................................................................................................9 Ground-water 0.25-inch piezometers....................................................................................................................9 Ground-water multi-level monitoring system.......................................................................................................9 Ground-water profiler.........................................................................................................................................11 Ground-water porous-membrane sampling devices........................................................................................... 12 Ground-water passive-diffusion-bag samplers................................................................................................... 12 Surface-water network........................................................................................................................................ 12 Ground-water and surface-water sampling methods............................................................................................................ 14 Well and piezometer sampling methods...................................................................................................................... 14 1-inch piezometer sampling methods................................................................................................................. 14 0.75-inch drive-point piezometer sampling methods.......................................................................................... 14 0.25-inch flexible tubing and inverted-screen piezometer sampling methods.................................................... 14 Multi-level monitoring system sampling methods...................................................................................................... 14 Ground-water profiler sampling methods.................................................................................................................... 15 Porous-membrane sampling device sampling methods............................................................................................... 15 Passive-diffusion-bag sampler methods...................................................................................................................... 15 Surface-water sampling methods................................................................................................................................. 15 Ground-water and surface-water analytical methods........................................................................................................... 15 Field measurements..................................................................................................................................................... 15 Redox analyses............................................................................................................................................................ 16 Inorganic analyses .......................................................................................................................................................16 Organic analyses..........................................................................................................................................................17 Quality-assurance methods.................................................................................................................................................. 17 Field replicates and blind samples............................................................................................................................... 17 Blanks ..................................................................................................................................................................... Laboratory quality assurance....................................................................................................................................... 18 Matrix spikes and matrix spike duplicates ..................................................................................................................19 Ground-water data from wells, piezometers, and multi-level monitoring systems .............................................................19 Field measurements andredox constituents for wells, piezometers, and multi-level monitoring systems................. 19 Inorganic constituents for wells, piezometers, and multi-level monitoring systems...................................................20 Organic constituents for wells, piezometers, and multi-level monitoring systems .....................................................20 Ground-water profiler and supplemental piezometer data...................................................................................................24 Field measurements and redox constituents for profiler and supplemental piezometer samples................................24 Inorganic constituents for profiler and supplemental piezometer samples .................................................................24 Organic constituents for profiler and supplemental piezometer samples....................................................................25