Ten years of optimizing a multicomponent remedy for a TCE plume

Optimization of a multi-component remedy for a 3-km-long trichloroethene (TCE) groundwater plume with a 100 year remedial timeframe has been ongoing for 10 years. The three-component remedy (in situ bioremediation [ISB] for the residual source area, pump and treat in the medial zone, and monitored natural attenuation [MNA] in the distal zone) was established in the Record of Decision with planned transition of the ISB and pump and treat remedies to MNA as contaminant concentrations declined. This TCE plume is located in a deep, fractured basalt aquifer beneath the Test Area North facility of the Idaho National Laboratory. Within the residual source area, optimization of the ISB remedy has been performed to meet the following objectives: (1) cut off TCE flux in the downgradient direction, (2) cut off TCE flux in the crossgradient direction, and (3) reduce the residual source area. Optimization activities have included modifications in the types, volumes, and concentrations of amendments injected based on amendment distribution and anaerobic reductive degradation (ARD) efficiency. Examples of these modifications include changing from single well injections to two-well simultaneous injections to increase distribution in situ and the use of sodium lactate as a pH buffer during whey powder injections. A positive side effect of the complete reduction of TCE to ethene via ARD is the transport of ARD by-products (specifically methane) from the residual source area to downgradient locations within the medial zone where methane serves as a substrate for aerobic cometabolism of TCE. As a result, enhanced attenuation of TCE is occurring within the medial zone and can provide a bridge to transition from a pump and treat to MNA remedy. Lines of evidence for enhanced attenuation include the presence of methane, decreases in TCE concentrations, and determination that the mechanism for aerobic cometabolism is present and active (via application of enzyme activity probes). Additional evidence in support of a transition from a pump and treat to MNA remedy within the medial zone includes the results of a 24-month rebound test of the pump and treat system which demonstrate a decline in TCE concentrations to 63-75% lower than TCE concentrations prior to the start of pump and treat operations. The pump-and-treat system is currently operating under a pulsed-pumping operation strategy to maximize contaminant removal while minimizing system operations. Over the past 10 years TCE concentrations have significantly declined throughout the plume, particularly in the residual source area and the medial zone. These declining TCE concentrations prove the success of optimization activities implemented for the ISB and pump-and-treat remedies.