Weber, 1991; Khandelwal et al., 1998; Krol and Rowe, 2004). Although the potential for diffusion across slurry walls has been long recognized by scholars, field studies to assess this scenario have not been performed. However, even if elevated contaminant concentrations are present in the immediate vicinity of a vertical barrier, diffusive contaminant fluxes are typically several orders of magnitude less than advective fluxes, and it is plausible that molecular diffusion would constitute a significant concern at only a very small number sites (e.g., sites with both large concentrations within a containment zone and a receptor located in close proximity to a vertical barrier).

Permeable Reactive Barriers

To function successfully, a permeable reactive barrier (PRB) must provide hydraulic control of the upgradient target capture zone, such that all contaminated water flows through the PRB rather than around or below it. In addition the PRB must have sufficient reaction capacity to sustain the necessary reduction in contaminant concentrations over the appropriate design time frame. Failure to achieve either or both objectives can occur because of inadequate design (e.g., improper wall placement or reaction zone thickness) or because of changes within the PRB that occur over time (loss of permeability and/or reactivity). In addition, if a PRB was placed downgradient of a source zone but within a region that previously contained dissolved contamination, it is possible that measurable downgradient concentrations will persist due to back-diffusion, even if the PRB is functioning as designed (Sale and Newell, 2010).

The vast majority of installed PRBs are constructed of zero-valent iron, which produces redox conditions and results in pH changes that are likely to promote precipitation of groundwater minerals. This phenomenon has long been recognized as a potential problem, and numerous laboratory and modeling studies have explored the potential consequences of these processes for PRB longevity (e.g., Yabusaki et al., 2001; Kohn et al., 2005; Johnson et al., 2008; Wilkin and Puls, 2003; Sass et al., 2002; Phillips et al., 2010). However, as noted by ITRC (2005a, 2011), no PRB has “failed” due to loss of permeability or reactivity. In the most detailed published evaluation of iron-based PRB performance (Henderson and Demond, 2007), a handful of active PRB projects reported situations where improper design (insufficient depth or width) resulted in incomplete hydraulic capture. Of the 40 projects, only three exhibited post-installation performance degradation involving the loss of permeability due to precipitation and/or deceased reactivity.

As with low-permeability barrier systems, the failure of a PRB system is likely to occur locally rather than across the entire plane of interest, and it is

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