For more than 10 years, electron donor has been injected into the Snake River aquifer beneath the Test Area North site of the Idaho National Laboratory for the purpose of stimulating microbial reductive dechlorination of trichloroethene (TCE) in groundwater. This has resulted in significant TCE removal from the source area of the contaminant plume and elevated dissolved CH 4 in the groundwater extending 250 m from the injection well. The δ 13 C of the CH 4 increases from -56‰ in the source area to -13‰ with distance from the injection well, whereas the δ 13 C of dissolved inorganic carbon decreases from 8‰ to -13‰, indicating a shift from methanogenesis to methane oxidation. This change in microbial activity along the plume axis is confirmed by PhyloChip microarray analyses of 16S rRNA genes obtained from groundwater microbial communities, which indicate decreasing abundances of reductive dechlorinating microorganisms (e.g., Dehalococcoides ethenogenes) and increasing CH 4 -oxidizing microorganisms capable of aerobic co-metabolism of TCE (e.g., Methylosinus trichosporium). Incubation experiments with 13 C-labeled TCE introduced into microcosms containing basalt and groundwater from the aquifer confirm that TCE co-metabolism is possible. The results of these studies indicate that electron donor amendment designed to stimulate reductive dechlorination of TCE may also stimulate cometabolism of TCE.

Complex sites in the subsurface are defined as those with difficult access, deep, and/or thick zones of contamination, large areal extent, heterogeneities that limit the effectiveness of remediation, or where long‐term remedies are needed to address contamination (e.g., because of long‐term sources or large extent). The Test Area North at the Idaho National Laboratory, developed for nuclear fuel operations and heavy metal manufacturing, is a complex site that demonstrates the endpoints strategy for adaptive remediation. Liquid wastes and sludge from experimental facilities were disposed in an injection well, which contaminated the subsurface aquifer located deep within fractured basalt. The mixed wastes included organic, inorganic, and low‐level radioactive constituents, with the focus of this study on the remediation of trichloroethylene within a systems‐based, regulatory framework. The framework facilitates site, regulator, and stakeholder interactions during the remedial planning and implementation process by using a conceptual model description as a technical foundation for decisions identifying endpoints. Endpoints are defined as interim remediation targets or decision points on the path to an ultimate end, and maintaining protectiveness during the remediation process. Results demonstrate that the three‐component remedy used at Test Area North as an example of the structured framework is largely functioning as intended and is projected to meet remedial action objectives by 2095. The remedy approach is being adjusted as new data become available. The framework provides a structured process for evaluating and adjusting the remediation approach, allowing site owners, regulators, and stakeholders to manage contamination at complex sites where adaptive remedies are needed. ©2016 Wiley Periodicals, Inc.