An Evaluation of Aerobic Trichloroethene Attenuation Using First-Order Rate Estimation

Natural attenuation of trichloroethene (TCE) was evaluated for a groundwater plume at the Idaho National Engineering and Environmental Laboratory. Significant evidence demonstrated that reductive dechlori-nation is occurring, but is limited to a small area around the contamination source. In spite of this, the plume is relatively stable. Three first-order rate estimation methods were used to help understand transport processes affecting TCE in the large, aerobic portion of the plume. Two of the methods gave attenuation half-life estimates for TCE of approximately 8 years; however, these methods do not adequately distinguish between degradation and dispersion. The third method showed TCE attenuation relative to the co-contaminants, tritium and tetrachloroethene (PCE), and used these "tracers" to distinguish between dispersion and degradation. The estimated aerobic degradation half-life for TCE was between 13 and 21 years. Aerobic cometabolism of TCE has been identified as a potential mechanism for the apparent degradation. The importance of distinguishing between dispersion and degradation was shown using an analytical model. The model demonstrated that, in general, the rate of contaminant concentration decrease due to dispersion is not constant with time after the source is removed. This has important implications for predicting the long-term effectiveness of natural attenuation for groundwater restoration.