INNOVATION March-April 2014

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For example, as groundwater contamination is rarely uniform in depth or lateral extent, different media mixes can be layered in a PRB or placed in separate PRB wall sections. Surface caps or a top layer of less permeable media can also be used to reduce atmospheric oxygen from entering through the ground surface. For areas of highest groundwater contamina- tion, tandem walls can be used to increase residence time or overcome limitations associated with geotechnical and equip- ment trenching widths. Installation Installation will directly affect a PRB’s reactivity, permeability and longevity. The successful installation of a PRB depends on the quality of materials, process and installation method. Careful control over material quality also ensures adequate PRB reactivity and permeability. In addition, thorough reac- tive media mixing and careful placement of the media into the ground is necessary to avoid mix separation (through unintended gravity sorting) and densification (which affects permeability). Time is essential during the installation process, as ZVI will continue to undergo undesirable aerobic reactions with atmospheric oxygen and mois- ture until it is placed in the ground.

higher than that of the surrounding soil. For example, where site soil is highly conductive sand and gravel, a PRB conductiv- ity design target may be approximately one order of magnitude higher than the surrounding soils. To achieve this desired permeability target, clean, sorted sand and/or gravel is mixed with the reactive media to reach desired permeability while supporting the reactive media in situ . A thorough understanding of site geology and groundwater hydrogeology is necessary to meet permeability requirements. Depending on the complexity and scale of groundwater con- tamination, as well as the site’s geological and hydrogeologi- cal features, different PRB media mixes can be customized to account for higher or lower contamination and diversity in soil permeability. The reactively and permeability of a PRB can be modified by adjusting the treatment media components (i.e. ZVI, compost, gravel) ratio. Structure Conceptually, PRBs can be installed as either a continuous wall or as a funnel and gate configuration, which includes a hydrau- lic cutoff wall to redirect groundwater toward the PRB.

Gate (Reactive Media)

When determining a suitable installation method for the PRB, decision factors need to include wall thickness, installation depth, availability and costs of specialized trench- ing equipment, and site-specific geotechni- cal conditions. PRB installation methods are diverse and can overlap with chemical pressure injection methods typically applied to source zone remediation. Each method has its own advantages and limitations in achieving site-specific objectives. Rejuvenation Proper PRB design should involve life-cycle planning, including methods for rejuvenation and projected cost estimates for replacement of reactive media. While reactivity, permeabil- ity and longevity design factors are typically considered when initially installing PRBs, the process of rejuvenating PRBs also depends on geotechnical stability of the surrounding soil, and geochemical stability for the rejuvenated wall. It is critical to understand the geochemical stability of the spent media and its long-term reactivity due to the addition of new media, mechanical agitation and oxygen ingress during reinstallation. If removal of the original PRB is necessary, it is also essential to carefully man- age its removal and disposal.

Funnel (Sheet pile or Slurry wall)

Contaminant Plume

Groundwater flow direction

Reactive Media

Contaminant Plume

Groundwater flow direction

Top: Funnel and gate PRB design. Bottom: Continuous wall PRB design. Credit:Hemmera.

PRB technology allows for design flexibility and direct, in situ placement of reactive media to remedy contamination. In addition to carefully choosing the appropriate reactive media, mix ratios and thickness of PRBs to intercept contamination, design dimensions and features should account for site topog- raphy and conditions.

Performance PRB performance monitoring includes three basic components: monitoring geochemistry in transects across the PRB, monitoring hydraulic conductivity, and monitoring mineralogy. For example, organic and ZVI-based PRBs require

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