Groundwater pump and treat systems often provide a false sense of security to both responsible parties and their consultants. These systems have historically been installed to mitigate potential future risks associated with off-site contaminant migration and are considered by many to be an effective groundwater containment system. Environmental Resources Management (ERM) conducted high resolution site characterization (HRSC) programs at three contaminated sites with ongoing or historical pump and treat systems. In all cases, ERM found that only a portion of the total contaminant mass having the potential to flux across the downgradient site boundary had been captured by the containment systems.

The responsible parties in these situations were unfortunately convinced that the systems were capturing all (or nearly all) of the contamination that could have migrated off-site. In one case, the contaminant plumes migrated beneath a neighborhood that was later determined to include a number of active residential irrigation wells.

Implementation of a high resolution site characterization program on these sites not only identified these major data gaps but also enabled redesign of the two active pump and treat systems. One of the systems was completely redesigned using the updated conceptual site model (CSM) developed using high resolution site characterization. The redesigned system focused on minimizing the volume of groundwater pumped while maximizing the mass of contaminants removed.

Plume Cores

Research has shown, and our experience has validated, that at many sites the majority of the contaminant mass (greater than 80 percent) fluxes across a very small cross-sectional area (less than 20 percent) of the plume. This zone of concentrated contaminant mass is referred to as a plume core (Figure 1). By identifying plume cores through high resolution site characterization, it is possible to target groundwater recovery wells more accurately and design very efficient plume containment systems. Optimization of pump and treat systems not only reduces risk and costs but also satisfies a number of sustainability principles, including energy reduction, reduced impact to the watershed, and enhanced community and stakeholder satisfaction.

Improving Mass Transfer

At most sites, mass transfer limitations (e.g., dense non-aqueous phase liquids [DNAPL] dissolution, desorption or back diffusion) are responsible for the ongoing need for a pump and treat system. High resolution site characterization can be used to define the relationship between residual (i.e., trapped) and fluxing (i.e., mobile) contaminant mass, which is the driving force behind mass transfer limitations. In such cases, it may be more efficient and equally effective to operate the system using a pulsed-pumping scheme rather than a continuous-pumping scheme to ensure capture of contaminant plumes. In this manner, it is possible to promote mass transfer while the system is inactive and then conduct focused mass removal while the system is active. This approach has been shown to reduce energy usage and water withdrawal significantly while maintaining effective containment.

About the Author

Joe Fiacco is an environmental consultant and partner at Environmental Resources Management in Boston, U.S.A. He specializes in high resolution characterization of both overburden and fractured bedrock contaminated sites. The majority of his experience involves the characterization and remediation of sites impacted by chlorinated solvents, with additional experience at petroleum and landfill sites.

Photograph: Reflections by G. Schouten de Jel, Den Haag, Netherlands.

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