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Ethanol-based fuels are becoming more heavily used, increasing the likelihood of ethanol-based fuel spills during transportation and storage. Although ethanol is well-known to be readily biodegradable, very little is known about the effects that such a spill might have on an indigenous microbial community. Of particular concern is that ethanol contamination could stimulate the growth of organisms that can generate regulated compounds and/or produce explosive quantities of methane gas. A column-based study was performed to elucidate the potential impacts of ethanol-based fuel (E85) on the indigenous microbial community during a simulated fuel spill. A continuous dilute supply of E85 resulted in profound shifts in both the bacterial and archaeal communities. The shift was accompanied by the production of high concentrations of volatile fatty acids and butanol, a compound that is regulated in groundwater by some states. Results also indicated that a continuous feed of dilute E85 generated explosive levels of methane within one month of column operation. Quantitative PCR data showed a statistically significant increase in methanogenic populations when compared to a control column. The elevated population numbers correlated to areas of the column receiving a sustained carbon load. Toxicity data indicated that microbial growth was completely inhibited (as evidenced by absence of ethanol breakdown products) at ethanol levels above 6% (v/v). These data suggest that ethanol from ethanol-based fuel can be readily degraded, but can also produce metabolic products that are regulated as well as explosive levels of methane. The core of an E85 spill may serve as a long-term source of contamination as it cannot be degraded until significant dilution has occurred.

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This paper compiles a detailed set of in situ chemical oxidation (ISCO) lessons learned pertaining to design, execution, and safety based on global experiences over the last 20 years. While the benefits of a "correct" application are known (e.g., cost effectiveness, speed, permanence of treatment), history also provides examples of a variety of "incorrect" applications. These provide an opportunity to highlight recurring themes that resulted in failures. ISCO is, and will continue to provide, an important remedial tool for site remediation, particularly as a component of a multifaceted approach for addressing large and complex sites. Future success, however, requires an objective understanding of both the benefits and the limitations of the technology. The ability to learn from the mistakes of the past provides an opportunity to eliminate, or at least minimize, them in the future. Over the last 25 years of ISCO application, process understanding and knowledge have improved and evolved. This paper combines a thorough discussion of lessons learned through decades of ISCO implementation throughout all aspects of ISCO projects with an analysis of changes to the ISCO remediation market. By discussing the interplay of these two themes and providing recommendations from collective lessons learned, we hope to improve the future of safe, cost-effective, and successful applications of ISCO.

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Optimization of large‐scale injection‐based remedial systems requires engineering to intentionally capitalize on the biological, chemical, and physical mechanisms that occur within and between the zones of reagent application. These types of systems can be called hybrid designs as they employ multiple processes to achieve remediation endpoints (Figure 1), resulting in optimized system performance and a reduction in the overall life‐cycle cost. While all remedial applications incorporate these mechanisms to some extent, the importance of each of these processes is magnified in large‐scale applications. This column discusses the dominant mechanisms responsible for mass reduction within both source and distal plume footprints, with a focus on the application of “Hybridized Design” for enhanced reductive dechlorination (ERD) systems. 1 Diagram showing the hybrid design approach which encompasses physical (sorption, advection, diffusion), chemical (mass flux, abiotic degradation) and biological (metabolic and cometabolic degradation) processes.

Research and field experience from the past 15 years has allowed remediation professionals to purposefully design injection-based remediation systems with a high potential for success. Industry professionals can now claim a number of achievements that were unthinkable just a few years ago:(1) we have demonstrated that maximum contaminant levels (MCLs) can be achieved for multiple contaminants;(2) we have successfully targeted dense nonaqueous-phase liquid (DNAPL) source zones; (3) we have expanded our understanding of injection hydraulics to treat large plumes; and(4) we have collected sufficient data on rates of treatment to be more predictive regarding outcomes.The next decade will continue to evolve the design and execution of these types of systems for application to more complex problems. At this point on the timeline, questions regarding the mechanisms of treatment have largely been addressed, allowing a shift in focus to operational enhancements. Specific operational insights arising from the body of work to date that arguably will continue to shape and influence the design and execution of injection-based remediation systems include: (1) the fact that delivery does not always equal distribution, (2) treatment optimization requires aquifer tuning, and (3) life-cycle costs can be reduced with remedy-optimized investigation.The number of examples that support these concepts and their ramifications to future technology refinement is already increasing, demonstrating how the refinements that can be made around these areas of focus will enhance our ability to effectively tackle larger and more complicated plumes, and do so with maximum efficiency. O