What is a permeable reactive barrier?

A permeable reactive subsurface barrier can be defined as:

an emplacement of reactive materials in the subsurface designed to intercept a contaminant plume, provide a preferential flow path through the reactive media, and transform the contaminant(s) into environmentally acceptable forms to attain remediation concentration goals at the discharge of the barrier.

What is the history of remediation with iron metal?

History shows that the concept of using iron and other zero-valent or zero-oxidation-state metals for contaminant remediation in the subsurface environment is very recent, with most of the research occurring since 1987. Earlier research in non-environmental fields had noted phenomena, however, that would be rediscovered by environmental scientists.

What is the scope of this technology?

Permeable reactive subsurface barriers are currently being used in full-scale field applications for the treatment of plumes of chlorinated hydrocarbons and chromate. Research in both the laboratory and in the field is being carried out on a variety of reactive materials and contaminants. As of this writing, six full scale reactive barriers have been installed in the field. Table 1 provides some information about these sites.

How are these barriers designed?

Currently, two basic designs are being used in full-scale implementations of reactive barriers, (1) the funnel and gate and (2) the continuous trench. Other designs are being researched and evaluated.

The basic design of a funnel and gate system is shown in Figure 1a. Basically, an impermeable "funnel", typically consisting of interlocking sheet pilings or slurry walls, is emplaced to encompass and direct the flow of contaminated water to a "gate" or "gates" containing the permeable zone of reactive Fe metal. The design must prevent the contaminant plume from flowing around the barrier. Due to directing large amounts of water through a much smaller cross-sectional area of the aquifer, ground water velocities within the barrier will be higher than those resulting from the natural gradient. The continuous trench (Figure 1b) is simply a trench that has been excavated and simultaneously backfilled with reactive Fe, allowing the water to pass through the barrier under its natural gradient.

What site characterization do you need to do?

Both configurations require that information on contaminant concentration, contaminant degradation rate in the presence of the reactive substrate, and ground water flow rate through the barrier be known. This allows determination of the required residence time in the zone needed to achieve remedial goals, hence allowing calculation of the required thickness of the reactive zone. To accomplish this requires both laboratory studies and careful site characterization.

How do you monitor the success of the barrier?

Both regulatory compliance and treatment performance monitoring are necessary when using reactive barrier technology. The sampling program objectives and site conditions should be carefully considered when locating the wells, selecting the screen lengths, and designing other aspects of the monitoring well system.

Sources of Additional Information on Permeable Reactive Barriers

EnviroMetal Technologies Inc.

Remediation Technologies Development Forum (RTDF), Permeable Barriers Action Team