Numerous studies presented in the general literature have shown that the key mechanism affecting the rate and extent of migration of a contaminant plume is biodegradation since it removes contaminant mass and reduces average plume concentrations. This paper attempts to address the importance of biodegradation for fuel and chlorinated solvent plumes and to present a comprehensive review of rates of biodegradation obtained from field and laboratory studies. Data from approximately 280 studies are statistically analyzed to determine ranges of biodegradation rates for various contaminants under different redox conditions. A review of 133 studies for fuel hydrocarbons has yielded first-order biodegradation coefficients up to 0.445 day−1 under aerobic conditions and up to 0.522−1 under anaerobic conditions in 90% of the cases. A median rate constant for benzene of 0.3% day−1 was estimated from all studies, while those for toluene, ethylbenzene, and xylenes were estimated to be 4, 0.3, and 0.4% day−1, respective...

Biodegradation Rates for Fuel Hydrocarbons and Chlorinated Solvents in Groundwater

Vinyl chloride (VC) is both a known carcinogen and a regulated chemical, and its production capacity has almost doubled over the last 20 years, currently 27 million tons/year worldwide. According to recent reports it is still a cause for concern. VC has been found as a degradation product of chloroethylene solvents (perchloroethylene and trichloroethylene) and in landfill gas and groundwater at concentrations up to 200 mg/m(3) and 10 mg/L, respectively. Worldwide occupational exposure to VC still seems to be high in some countries (e.g., averages of approximately 1,300 mg/m(3) until 1987 in one factory), and exposure may also be high in others where VC is not regulated. By combining the most relevant epidemiologic studies from several countries, we observed a 5-fold excess of liver cancer, primarily because of a 45-fold excess risk from angiosarcoma of the liver (ASL). The number of ASL cases reported up to the end of 1998 was 197 worldwide. The average latency for ASL is 22 years. Some studies show a small excess risk for hepatocellular carcinoma, and others suggest a possible risk of brain tumors among highly exposed workers. Lung cancer, lymphomas, or leukemia do not seem to be related to VC exposure according to recent results. The mutation spectra observed in rat and human liver tumors (ASL and/or hepatocellular carcinoma) that are associated with exposure to VC are clearly distinct from those observed in sporadic liver tumors or hepatic tumors that are associated with other exposures. In rats, the substitution mutations found at A:T base pairs in the ras and p53 genes are consistent with the promutagenic properties of the DNA adduct 1,N(6)-ethenoadenine formed from VC metabolites. Risk assessments derived from animal studies seem to overestimate the actual risk of cancer when comparing estimated and reported cases of ASL.

Vinyl chloride: still a cause for concern.

This protocol is designed to evaluate the fate in ground water of chlorinated aliphatic hydrocarbons and/or fuel hydrocarbons. The data collected under this protocol can be used to compare the relative effectiveness of other remedial options and natural attenuation. This protocol should be used to evaluate whether monitored natural attenuation by itself or in conjunction with other remedial technologies is sufficient to achieve site-specific remedial objectives. Understanding the contaminant flow field in the subsurface is essential for a technically justified evaluation of a monitored natural attenuation remedial option; therefore, use of this protocol is not appropriate for evaluating monitored natural attenuation at sites where the contaminant flow field cannot be determined with an acceptable degree of certainty (e.g., complex fractured bedrock, karst aquifers).

/pdf/technical-protocol-for-evaluating-natural-attenuation-of-catf5ob7ug.pdf

Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water

▪ Abstract A variety of microbial dechlorination mechanisms have been demonstrated in laboratory microcosms, pure cultures, and in situ sedimentary environments. New perspectives on in situ processes from these efforts allow the design of more realistic bioremediation strategies that complement natural processes regardless of whether the strategy used is one of engineered accelerated bioremediation or natural attenuation. Since 1994 the scientific community has acquired considerable knowledge regarding natural attenuation of organochlorine compounds. Natural attenuation of chlorinated solvents has been documented at a number of field sites. Reductive dechlorination driven by co-contaminants or naturally occurring organics as substrates in combination with aerobic or co-metabolic degradation contains certain chlorinated solvent plumes. Although natural attenuation is not a panacea, at sites where it is applicable, it offers a scientifically sound, cost-effective method to remediate groundwater contaminated...

NEW PERSPECTIVES ON MICROBIAL DEHALOGENATION OF CHLORINATED SOLVENTS: Insights from the Field

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.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999WR900116

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

The DuPont Necco Park Landfill in Niagara Falls, New York, is contaminated with numerous chlorinated solvents at concentrations of up to hundreds of mg/L in the groundwater. An extensive monitoring program was conducted to determine if intrinsic anaerobic biodegradation was occurring at the site, to determine what might limit this activity, and to characterize this activity with depth and distance away from the landfill. It was determined that anaerobic microbial activity was occurring in all zones, based upon the presence of intermediate products of the breakdown of the chlorinated solvents and the presence of final metabolic end products such as ethene and ethane. Aerobic, iron-reducing, manganese-reducing, sulfate-reducing, and methanogenic redox conditions were identified at the site. High levels of nitrogen and biodegradable organic compounds were present in most areas to support cometabolic anaerobic microbial activity against the chlorinated solvents. Intrinsic biodegradation is clearly evident and is effective in reducing the concentrations of chlorinated organic in the groundwater at the site. Groundwater modeling efforts during development of a site conceptual model indicated that microbial degradation was necessary to account for the downgradient reduction of chlorinated volatile organic compounds as compared to chloride, a conservative indicator parameter.

D.E. Ellis

Papers

Intrinsic in situ anaerobic biodegradation of chlorinated solvents at an industrial landfill