Introduction to PRB 2021 PRB Function and Design

PRB can remove and retain contaminants (such as dissolved metals) travelling in groundwater, degrade some contaminant compounds (such as chlorinated hydrocarbons) directly, or facilitate their degradation (for example through the retention and biodegradation of petroleum hydrocarbons). Barriers can be used to both arrest the migration of contaminant plumes as well as remediate contaminated sites. Barriers should be optimised for specific site settings and aquifer and contaminant chemistries in order to achieve these functions. Critically though, the barrier must at all times maintain a greater permeability than the aquifer material. As a consequence, there is a broad range of designs; however, PRB are typically installed in one of two configurations.

The simplest is a continuous wall (also known as a reactive wall) extending across the width of the contaminant plume. This design is either keyed into an aquitard or impermeable substrate such as permafrost, bedrock or clay (Fig. 20.1a), or forms a "hanging wall" where the aquifer flows freely beneath the barrier (Fig. 20.1b). These wall barrier designs would be employed where the reactive media is inexpensive, or where construction of a funnel and gate system is not possible.

The alternative configuration is a "funnel and gate" design (Fig. 20.1c) consisting of impermeable walls such as sheet piling, plastic sheeting or a cement/ soil-bentonite clay (+/- geofabric) slurry mixture, which direct groundwater through a permeable gate which is filled with reactive media (Starr and Cherry 1994; Gavaskar 1999). Multiple gate designs are possible, with up to four gates in a barrier in Colorado (Wilkin et al. 2002). Where soil water is focussed through a gate, the design of barrier permeability and residence times can assume great importance for the success of the remediation. Arrays of wells and injected media systems (particularly using nano-particle media) are also used (Naftz et al. 2002; Meggyes 2005). A range of PRB designs and case histories has been reviewed by Roehl et al. (2005a).

Plan View Section View --H

Flow

ground surface

e g. DNAPL plume --—>

1-10+ m

clean effluent ___^

q ro u n dwateF""^-

-*■

S

<2 m '

impermeable substrate (permafrost/bedrock)

□ Reactive media OS Permeable material

□ Reactive media OS Permeable material

Prb Groundwater

Fig. 20.1 Reactive barriers can consist of a continuous ("reactive") wall extending across the width of the contaminant plume. The wall is either keyed into an impermeable substrate (a) or forms a "hanging wall" where the aquifer flows freely beneath the barrier (b). Alternatively, a "funnel and gate" design (c) consisting of impermeable walls, directs groundwater through a permeable gate which is filled with reactive media

Fig. 20.1 Reactive barriers can consist of a continuous ("reactive") wall extending across the width of the contaminant plume. The wall is either keyed into an impermeable substrate (a) or forms a "hanging wall" where the aquifer flows freely beneath the barrier (b). Alternatively, a "funnel and gate" design (c) consisting of impermeable walls, directs groundwater through a permeable gate which is filled with reactive media

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