Enzyme probe degradation rate assessment for an aerobic TCE plume

A pump-and-treat system began operation in 2001 as the remedy for the medial zone (TCE concentrations between 1,000-20,000 μg/L) at Test Area North (TAN) of the Idaho National Lab. Operations resulted in a significant decline in TCE concentrations during the first year (from -500 μ/L to -100 μg/L). However, TCE concentrations changed little over the following two years of operation. Therefore, a two-year rebound study was undertaken in the spring of 2005 with the objective of assessing the contaminant concentration reduction within the pump-and-treat capture zone of the plume. TCE concentrations 16 months following shutdown suggest that TCE has reached equilibrium conditions between 150 to 250 μg/L. Therefore, enzyme probe evaluations to assess aerobic TCE cometabolism in field samples and laboratory studies evaluating the attenuation rate of TCE were conducted to determine the potential for monitored natural attenuation (MNA) to replace pump-and-treat as the medial zone remedy. Previous studies using enzyme probes demonstrated the potential for aerobic biodegradation of TCE in the distal zone (TCE concentrations between 5-1,000 μg/L) of the plume as part of the MNA assessment. These probes target the soluble methane monooxygenase (sMMO) and toluene oxygenases, and provide direct evidence of an active enzyme in a sample. An enzyme probe analysis was performed at eight wells across the medial zone, and both the sMMO and at least one of the toluene oxygenases were detected in groundwater from all of the wells sampled. The sMMO activity was substantially higher (factor of approximately 3) in the medial zone compared to that measured in the distal zone. This may be due in part to ISB operation in the source area (TCE >20,000 μg/L), which produces high concentrations of methane, which is then transported to some parts of the medial zone. Contrary to sMMO, toluene enzyme activity, as measured by the probes, was generally lower in the medial zone compared to the distal zone. Toluene activity was variable (5% to 25% of the total cells) with the highest toluene oxygenase activity at the downgradient edge of the medial zone. In addition, laboratory studies were conducted to assess the attenuation rate of TCE and the corresponding enzyme probe response using groundwater from the site. The degradation rate of TCE was determined and normalized to enzyme activity, previously determined for natural populations in the field. Degradation rates were estimated for field populations based on contaminant concentrations and the number of microbial cells with active degradative enzymes. Contaminant half-lives were estimated. Together, these results indicate that aerobic cometabolic degradation of TCE is an active component of the biological community present in TAN groundwater in the medial zone. Evidence of positive enzyme probe analyses and estimation of a biological degradation rate will be used to determine a strategy for transition of pump and treat to MNA for the medial zone at TAN.