DOE OSTI2016
A field demonstration of enhanced attenuation (EA) was initiated in 2014 to expedite the remediation of industrial solvents in the groundwater impacted by the former landfill Operable Unit 1 (OU-1) at the US Department of Energy (DOE) Mound, Ohio, Site. EA uses active engineering solutions to alter the target site so that the contaminant plume will passively stabilize and shrink. For OU-1, the EA was implemented by targeted injection of an electron donor (vegetable oil) to create 'structured geochemical zones'. Volatile organic compounds (VOCs) in the altered subsurface system encounter alternating anaerobic and aerobic environments along the plume flow path. The effect of these structured geochemical zones is effective and rapid degradation of parent compounds such as trichloroethene (TCE) and tetrachloroethene (PCE), and daughter compounds such as cis-1,2-dichloroethene and vinyl chloride. EA at Mound OU-1 provides a transition to natural attenuation and is an alternative to the baseline pump-and-treat (P and T) system. Industrial solvents (primarily TCE) and other VOCs that originated from the former solid-waste landfill contaminated the groundwater in the Buried Valley Aquifer beneath the Mound site. A groundwater P and T system was initiated in 1996 to control the plume, and a soil vapor extraction system was installed and operated from 1997 to 2003 to remove the bulk of the VOC sources from the vadose zone. Physical removal of the landfill waste and contaminated soil was performed between 2006 and 2010. Approximately 76,100 cubic meters (99,500 cubic yards) of material were removed from the OU-1 landfill area. Operation of the groundwater P and T system continued throughout this period. The principles and technical basis of the ongoing EA field demonstration were developed by the Interstate Technology and Regulatory Council. The goal of the EA implementation is to provide a bridge between active groundwater treatments and monitored natural attenuation. EA relies on understanding and manipulating the contaminant mass balance. Technologies are applied to sustainably reduce the contaminant loading from the source and/or increase the rate of natural attenuation processes within the plume. In 2014, a core team that included DOE, the US Environmental Protection Agency (EPA), the Ohio EPA, and technical support organizations evaluated remedial options for OU-1. During the 2014 evaluation, the team projected a cleanup time frame for the baseline P and T system of 26 additional years and a cleanup time frame for the EA remedy of 13 years. On the basis of that evaluation, EPA approved the implementation of the EA remedy that required the shutdown of the P and T system. The combination of technologies implemented for OU-1 EA included (a) neat vegetable oil in the vadose zone in the former landfill and (b) emulsified vegetable oil within the footprint of the groundwater plume. In the first part of the deployment, neat oil spreads laterally and forms a thin layer on the water table beneath residual soil sources to intercept and reduce future VOC loading and to reduce oxygen inputs to the local groundwater. In the second part of the deployment, emulsified oil forms active bioremediation anaerobic treatment zones within the plume footprint to degrade existing groundwater contaminants (via reductive dechlorination, co-metabolism, or both) and stimulates long-term attenuation capacity in the down-gradient aerobic plume (via co-metabolism). The results from the first year of the field demonstration indicate that the dissolved TCE and PCE plumes have decreased in size and mass. Statistical tests indicate that the VOC concentrations in the majority of the source-area wells are decreasing. The VOC concentrations throughout the plume are projected to be below the target maximum contaminant level of 2 to 5 μg/L within 5 to 10 years-sooner than the design projection of 13 years. The biogeochemistry and microbial community within the structured geochemical zones are consistent with the design and appear to be stable. At Mound OU-1, EA has accelerated progress toward the remedial action objectives and reduced costs. Ongoing monitoring will determine whether the EA remedy has transitioned the site into monitored natural attenuation status and whether the EA will continue to be effective, timely, and sustainable. (authors)
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗