Intrinsic biodegradation of vinyl chloride in the vadose zone

Natural attenuation has become a critical component of many groundwater management strategies and remedial actions. The processes and methodology for evaluating natural attenuation in groundwater has been established by the US Air Force, the US EPA, and several state regulatory agencies. The application of natural attenuation to the vadose zone is not as well documented and often relies on a significantly different approach to document the processes acting to reduce contaminant mass and concentration. Historical and recent soil gas data were used to define the contribution that natural attenuation was having on vadose zone restoration. The area and volume of a vinyl chloride impacted vadose zone was determined geostatistically using data collected at four time points spanning seven years. Changes in the relative mass of vinyl chloride in the vadose zone and the change in the total area impacted by vinyl chloride were used to demonstrate soil restoration by natural attenuation. The mass of vinyl chloride had decreased by 96 percent during the past seven years. The results also indicated a 2-fold expansion in the vinyl chloride plume during the same time. The rate of mass reduction observed during the past six years indicated that the vinyl chloride plume was attenuating with a half-life of 1.2 to 1.8 years. Soil gas composition demonstrated that biodegradation was largely responsible for the change in mass. Evidence for aerobic and anaerobic biodegradation of vinyl chloride was observed in the soil gas. The oxygen concentration was depressed, the carbon dioxide, ethene, and ethane concentrations were elevated, and methane was being produced within the impacted area The results from this evaluation provided strong evidence that natural attenuation was effectively remediating the impacted vadose zone. The evaluation approach seems reliable for vadose zone plumes that have multiple sampling events containing enough sample locations to delineate plume dimensions and the changes that occur over time.