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Tremendously flexible, permeable reactive barriers (PRBs) are a sustainable biological and chemical treatment approach in which subsurface “walls” are installed to intercept and passively treat contaminants in groundwater, thereby reducing risks to the receiving environment and downstream receptors. Thanks to their flexible attributes and adaptation to site-specific conditions, PRBs are designed to treat a variety of inorganic and organic contaminants in groundwater through their destruction or adsorption. For instance, PRBs have been successfully applied to mitigate groundwater impacts from metal leaching and acid rock drainage (ML/ARD) in numerous sites across the US and Canada where groundwater has been adversely impacted by mining-related operations.

PRBs intercept contaminated groundwater in situ using a reactive media, which then responds to or adsorbs contaminants such as copper, zinc, chromium, selenium and uranium as the groundwater flows under natural or induced gradients. Several installed PRBs have demonstrated typical metal removal rates greater than 99%. Different reactive materials can be used in PRBs to treat a wide variety of metal contaminants, as shown in the following table.

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SuStainable Groundwater ManaGeMent StrateGieS RemediaTing meTal ConTaminaTion Using PeRmeable ReaCTive baRRieRs

Michael Choi, B.Sc.; Ben Lin, B.A.Sc., P.Eng.; and Gabriel Viehweger, MS, P.Geo.

i n n o v a t i o n M a r c h/a p r i l 2 014 27

Samples of Reactive Media Used in PRBs for Metal Remediation

Reaction Type Reactive Media

Precipitation (Eh)Zero valent iron (ZVI), iron slag

Ferrous hydroxide, Ferrous carbonate, Ferrous sulphide

Precipitation (pH) Limestone

Precipitation (biochemical) Organic matter (leaf detritus, compost, wood mulch, sawdust, manure, hay, sludge)

Adsorption

Peat, Lignite (Brown coal), Fly ash

Organic matter (leaf detritus, compost, wood mulch, sawdust, manure, hay, sludge)

Aluminosilicates (Zeolite), Activated Alumina

Phosphates (Apatite)

Activated Carbon, Exchange Resins

The reactive media and treatment mechanisms are not mutually exclusive. For example, organic matter PRBs can also include limestone and zero valent iron (ZVI) to enhance biogeochemical precipitation of metal sulfides, while also promoting adsorption of metal contaminants onto both iron oxyhydroxides.

Organic Matter, Limestone and ZVI PRBsUsing sustainable, naturally occurring biogeochemical reac-tions, organic matter-based PRBs promote the growth of sul-phate-reducing bacteria. The bacteria biologically reduces sulphate to sulfide, which then immediately reacts with metal contami-nants to form insoluble metal sulfides. Maintaining an anaero-bic, low Eh, pH-neutral environment is essential for promoting growth of sulphate-reducing bacteria and proper functioning organic matter/ZVI PRBs.

Principles of PRB DesignPRB design is a successful remediation method since con-taminated water must flow through and contact the reactive media. Successful PRB design depends mainly on the bal-ance of media reactivity, longevity, structure, and perme-ability. While longevity can be thought of as the factor that unites the three other design factors, all are interdependent and require careful planning and site-specific consideration in order to develop a successful PRB solution for effective ML/ARD treatment.

An iterative approach can be applied to balancing technical requirements and cost-effectiveness during PRB design. First, individual design and site factors must be assessed for technical feasibility and cost. Critical design factors (described below) are then ranked for further study and consideration.

ReactivitySeasonal variations, maximum influent concentrations and potential future changes are considered when assessing mixture reactivity and longevity. To evaluate their suitability, different reactive media mixes are bench-tested using site groundwater.

Understanding the influent water quality for PRBs is critical. As a PRB matures, both intended and unintended precipitates begin to accumulate on the front face of the wall (this is known as wall face fouling), which causes permeability to decline. To address this, site water quality is evaluated during preliminary PRB design to identify local media that may be reactive competi-tion or fouling by other contaminants, cations or anions.

PRB design can address reactivity performance concerns by incorporating residence time safety factors (e.g. decreasing media density and/or increasing PRB wall thickness). Permeability safety factors and/or structural improvements (discussed below) can also be used to maximize PRB media reactivity and longevity.

PermeabilityTo avoid significant changes in groundwater flow patterns, the hydraulic conductivity of the PRB media is designed to be

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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.

StructureConceptually, 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.

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.

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.

InstallationInstallation 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.

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.

RejuvenationProper 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.

PerformancePRB performance monitoring includes three basic compon-ents: monitoring geochemistry in transects across the PRB, monitoring hydraulic conductivity, and monitoring mineral-ogy. For example, organic and ZVI-based PRBs require anoxic,

Contaminant Plume

Reactive Media

Groundwater flow direction

Contaminant Plume

Funnel (Sheet pile or Slurry wall)

Gate (Reactive Media)

Groundwater flow direction

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

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reducing conditions, therefore, redox potential would be monitored following installation (expected to start low and gradually increase over the lifetime of the PRB as treatment media is consumed).

As a PRB matures, organic carbon content decreases and ZVI surfaces degrade, while secondary wall face fouling begins. Collecting cores for analysis can be useful in inferring the geochemical environment in which the minerals were formed, and the magnitude of face fouling.

PRBs as an Effective TechnologyPRBs are a cost-effective, site-specific and sustainable treatment technology, particularly for managing ground-water metal contamination in such places as mine sites. Sustainably designed and often including locally sourced materials, PRBs can be structurally adapted to site features, further minimizing disturbance. This passive treatment method allows for on-site source and pathway removal of contaminants. With careful planning and attention to quality, PRBs can also offer significant cost savings compared to other technologies, such as chemical injections or pump and treat system.

The long-term performance of PRBs provides a sustain-able solution at remote sites where access may be difficult and power not available. Current estimates indicate that PRB

systems have low monitoring costs over a typical 10-year lifespan. The only typical long-term costs include routine compliance and monitoring. In contrast, active treatment systems require energy for groundwater pumping, pipes and water treatment infrastructure, in addition to imported chemicals. Although active treatment technologies are considered more reliable and definitive in their treatment capability, their capital and ongoing costs can be consider-ably higher than passive systems such as PRBs.

For more information or further reading, please contact Michael Choi at mchoi@hemmera.com.

Michael Choi is a Senior Environmental Scientist, Project Director, and Business Leader at Hemmera with 16 years of experience in the environmental consulting industry.

Ben Lin, P.Eng., is a Professional Engineer at Hemmera with 10 years of experience practising environmental engineering for contaminated site investigation and remediation at complex sites throughout BC.

Gabriel Viehweger, P.Geo., has 20 years of experience as a hydrogeologist and geologist, investigating subsurface condi-tions and both physical and contaminant groundwater flow in unconsolidated and fractured flow settings.